Heat pum p dryers have been known to be energy
efficien t when used in co njunction with drying ope r-
ations. The princi pal advantag es of heat pump dryers
emerg e from the ability of the he at pum ps to recov er
energy from the exhau st gas as well as their ab ility to
control the drying gas tempe ratur e and humidi ty.
Many research ers have demo nstrated the importance
of prod ucing a range of precise drying con ditions to
dr y a w ide r ang e o f p rod uc ts a nd i mpr ov e t he ir qua li ty .
At the same time, MacArthur (1984) has mentioned
the need to optimize component and system design to
increase energy efficiency in heat pump systems.

Any dryer that uses convecti on as the primary

mode of heat input to the dr yer (w ith or witho ut
supplem entary heat input by other mod es of he at
transfer) can be fitted wi th a suit ably de signed he at
pump (HP). Alth ough batch shelf, tray dr yers, or
kilns (for wood) are the most co mmonl y repo rted
dryers used in conjuncti on wi th heat pumps, other
types may also be used , e.g., fluid beds (Alves-F ilho
and Str fmmen, 199 6; Str fmmen and Jonassen, 1996)
and rotar y dryers . Ho wever, dr yers that requ ire large
amounts of drying air, e.g., flash or spray dryers , a re
not suited for HP operati on. Figure 47.1 displays a
general ized class ification scheme for heat pump
dryers based on the pro cessing mode, num ber of dry-
ing stages, number stage s of heat pum p, types of

auxiliary he at input, and he at pump dryer operatio n.
Many of these classes of heat pump dryer have been
proposed and repo rted over the last two dec ades.
However, this flowchart includes some classes that
are proposed here for the first time.

47.2 THE BASICS OF A HEAT PUMP

The basic components of the heat pump system com-
prise an expansion valve, two heat exchangers (evap-
orator and condenser), and a compressor. A schematic
diagram depicting the operation of a heat pump dryer
is shown in

Figu re 47.2

Figu re 47.3

and

Figu re 47.4

show the tempe rature–

entropy and pressure–enthalpy diagrams of the heat
pump cycle, respectively. The heat pump cycle works
as follows:

The cooling and dehumidification of the air
occurs at the evaporator. The refrigerant, mov-
ing from point 1 to 2, absorbs heat from the air
and undergoes a two-phase change from vapor–
liquid mixture to vapor. Evaporation of the
refrigerant is achieved by the gaseous escape of
the molecules from the surface of a liquid while
maintaining its temperature and pressure.

The refrigerant vapor enters the suction line of
the compressor at point 3. The electrical energy

Heat pump dryers

classification

Processing

mode of

dryer

Number

of drying

stages

Number of

stages of

heat pump

Auxiliary

heat input

Batch
dryer

Continuous

dryer

Single
drying

stage

Convection

Multiple-

stage heat

pump dryer

Cyclic

operation

Continuous

operation

Heat pump

dryer

operation

Conduction

Intermittent

operation

Multiple

drying

stage

SIngle-stage

heat pump dryer

Product

temperature

Above

freezing

point

Below

freezing

point

Others:
1. Radio-frequency
2. Microwave
3. Infrared

FIGURE 47.1 A generalized classification scheme for heat pump dryers.

2006 by Taylor & Francis Group, LLC.

input to the compres sor is conv erted to shaft
work to raise the pressure of the refr igeran t
vapor to that of the conden ser at point 4. By
increa sing the vapo r pressur e, the boili ng and
cond ensing temperatur es of the refriger ant is
raised to a level higher than that of the he at
sink tempe rature (the surround ing temperatur e).
At this stage , the vapor is in a superheat ed stat e.

After compres sion, the refr igerant vap or is
directed to the conden ser that is basica lly a
heat exchanger to carry out the con densing pro-
cess. The refriger ant first unde rgoes quick de su-
perheat ing ch ange from superheat ed vapor to
satur ated vapor and then undergo es conden sing
process in the conden ser. At the condenser, the
refriger ant undergoes two-ph ase con densatio n,
changing from vapor to liquid pha se. Duri ng
this pro cess, heat is rejected by the conde nser
to he at the surroundi ng air.

Heat reco very oc curs when the he at ene rgy
absorbed in the evaporat or and the work en ergy

from the compres sor is ‘‘pum ped’’ to the con -
denser side for sensib le heati ng of the air.

Once the vapor refriger ant exits the con denser,
it unde rgoes an a dditional stage of subco oling
(point 5 to 6) in another heat exchanger. There
are two advantag es of subcooli ng. First ly, ad d-
itional hea t ca n be reco vered for sensi ble heat-
ing of the air. Secondly, it reduces flashing
when the refriger ant pressur e is reduced in the
throttli ng de vice.

After t he con densing process, a throttling de-
vice such as a valve , orifice plate, or c apillary
tube is used to expand the liquid refrigerant in
order t o r educe the pressure of the refrigerant
liquid l evel to a boiling temperature below that
of the heat source. After the e xpansion pro-
cess, the refrigerant enters the e vaporator in a
two-phase state.

The entir e cycle repeat s its elf.

47.3 PRINCIPLE OF HEAT PUMP DRYER

Figure 47.5

repres ents a schema tic layout of various

refriger ation comp onents integ rated with the drying
chamber. The inlet drying air passes through the
drying ch amber at poin t 1 a nd pick s up mois ture
from the product. The mo isture-laden air a t poin t 2
is then directed to the evapo rator coil. Two types of
evaporat or systems exist. One is a direct expansi on
coil wher eby the refr igeran t unde rgoes a two-phase
change from liquid to vapor to co ol and dehumidi fy
the air. The other is a chille d water syst em wherein the
flow of chilled water to the coil is con trolled for cool-
ing and dehumidi ficati on. Duri ng the de humidifi ca-
tion process from point 2 to 3, the air is fir st cooled
sensibly to its dew point. Further cooling results in
condensation of water from the air. Latent heat of
vaporization is then absorbed by the evaporator for

Condenser

Evaporator

Condensate

Heated air

Recovered heat

Expansion valve

Cooled air Compressor

FIGURE 47.2 Schematic diagram of an air-to-air heat
pump cycle.

Temperature (K)

source

sink

Entropy (kJ/kg K)

FIGURE 47.3 Temperature–entropy diagram of the heat
pump cycle.

source

sink

Pressure (kPa)

Enthalpy

FIGURE 47.4 Pressure–enthalpy diagram of the heat pump
cycle.

2006 by Taylor & Francis Group, LLC.

boiling the refrigerant. The recovered heat is
‘‘pumped’’ to the condenser. The cooled and dehu-
midified air then absorbs the heat at the condenser
moving from point 4 to 1 for sensible heating to the
desired temperature.

The energy efficiency of a heat pump is defined

by the coefficient of performance (COP). COP is
given by

COP

useful heat output

power input

(47:1)

The maximum theoretical heat pump efficiency is

given by the Carnot efficiency as

COP

carnot

condenser

evaporator

(47:2)

The COP

carnot

cannot be realized physically but is

used as a gauge to determine how far a refrigeration
system is from the ideal system. In practice, the actual
efficiency of the heat pump is usually 40 to 50% of
the theoretical Carnot efficiency (Geeraert, 1976;
Strumiłło and Zyłła, 1985).

A performance indicator that is commonly used to

define the performance of the dryer is the specific
moisture extraction rate (SMER).

SMER is defined as

SMER

amount of water evaporated

energy input to the dryer

,kg=kWh

(47:3)

Alternatively, another parameter known as spe-

cific energy consumption (SEC) that is the reciprocal
of SMER, can be used to compare energy efficiencies
of different types of dryer. There is a relation between
SEC and COP (Strumillo and Zylla, 1985) that is
given by

SEC

(COP)

)

(COP)

)

(47:4)

47.4 ADVANTAGES AND LIMITATIONS

The key advantages and limitations of the heat pump
dryer are:
Advantages:

1. Heat pump drying (HPD) offers one of the high-

est specific moisture extraction ratio (SMER),
often in the range of 1.0 to 4.0, since heat can be
recovered from the moisture-laden air.

2. Heat pump dryers can significantly improve

product quality by drying at low temperatures.
At low temperatures, the drying potential of
the air can be maintained by further reduction
of the air humidity.

3. A wide range of drying conditions typically

–208C to 1008C (with auxiliary heating) and
relative humidity 15 to 80% (with humidification
system) can be generated.

4. Excellent control of environment for high-

value products and reduced electrical energy
consumption for low-value products.

Limitations:

1. Chlorofluorocarbons (CFCs) are used in refriger-

ant cycles, some of which are not environment-
friendly.

2. Requires regular maintenance of components

(e.g., compressor and refrigerant filters) and
charging of the refrigerant.

3. May incur higher capital costs.

Water

Refrigerant path

Air path

Condenser

Evaporator

Dryer

FIGURE 47.5 Schematic representation of heat pump drying system.

2006 by Taylor & Francis Group, LLC.

47.5 ENERGY EFFICIENCY

The ability of heat pum p dr yers to convert late nt heat
of conden sation into sensi ble he at at the hot cond en-
ser mak es them unique heat recover ing devices for
drying ap plications . The en ergy effici ency of HPD
can be reflect ed by the higher SMER values and
drying efficien cy when compared to other drying sys-
tems as sh own in Table 47.1. Conse quently , high er
SMER would then be translat ed to lower ope rating
cost, mak ing the payback period for initial capit al
consider ably shorter.

47.6 DRYING MODE AND DRYER

CONFIGURATION

H ea t pu mp dr y e r c a n o pe ra t e i n d if fe r e nt m od e s ,
e.g., the batch (parallel and cross-flow) and continu-
ous modes. Figure 47.6 shows a batch heat pump
dryer. The products are placed on trays that are
positioned in the drying chamber and removed
once the desired product moisture content is reached.
The drying air can flow parallel or perpendicular
to the product surface. A cross-flow dryer configur-
a ti on is s ho w n i n

Fi gu re 4 7 .7

. Ba t c h d ry i ng i s g e ne r -

ally suitable for smaller production rates but entails
higher labor cost.

Fig ure 47.8

shows a drying chamb er designe d to

operate in continuous mode. The products are placed
on trays positioned on the conveyor belt system. The
speed of the conveyor can be varied with a controller-
gears system. Continuous systems involve faster load-
ing and unloading of the drying products and are less
labor-intensive. The selection of the drying mode

largely depends on the drying characteristics of the
product and the required product loading capacity.

47.7 MULTISTAGE HEAT PUMPS

Many of the commercial heat pump dryers consist of
a single-stage vapor compression cycle. In such sys-
tems, only one evaporator is used for cooling
and dehumidifying, and recovering the latent heat of
vaporization from the drying air. A mechanical con-
straint is then imposed on the amount of heat recov-
ered because of the physical area available for heat
transfer. Further, the single-stage heat pump cycle is
not able to produce several streams of drying air
with different drying conditions, both in terms of

Compressor

Air-cooled

Hot-gas condenser

Expansion valve

Product Tray

Main centrifugal
fan

Evaporator

Circulating
centrifugal fan

FIGURE 47.6 A parallel flow batch mode heat pump dryer.

TABLE 47.1
Comparing Heat Pump Drying with Other
Drying Systems

Parameter

Hot Air

Drying

Vacuum

Drying

Heat

Pump

Drying

SMER (kg water/KWh)

0.12–1.28

0.72–1.2

1.0–4.0

Drying efficiency (%)

35–40

Operating temperature

range (8C)

40–90

30–60

10–65

Operating % RH range

Variable

Low

10–65

Capital cost

Low

High

Moderate

Running cost

High

Very high

Low

Source: Adapted from Perera, C.O. and Rahman, M.S., Trends
Food Science Technology, 8(3), 75, 1990. With permission.

2006 by Taylor & Francis Group, LLC.

tempe rature and hum idity, to severa l independen t
drying ch ambers .

Figure 47.9

shows the impro vement

in ene rgy effici ency for different he at pump confi gur-
ations. It can be obs erved that there is a significa nt
impro vement in the he at pump en ergy effici ency from
a single to tw o-stage dryer.

M ultistag e vapor-c ompres sion syst ems can be

designe d to integrate wi th the dry ing chambers as
shown in

Figure 47.10

. One addition al advantag e of

the incorpora tion of a mu lti-stage heat pum p cycle is
that it allows a control mechani sm for regulating the
humidi ty of the air to be impl ement ed.

In a two-st age he at pump drye r, the refr igera nt

vapor is spli t into two stre ams at the exit of the
conden ser. One stream enters an expansi on va lve at
a higher dischar ge capacit y to be regula ted to the
‘‘low’’ e vaporat or tempe ratur e whi le the other enters
another expansi on valve to be expanded to a high er

tempe rature as shown in

Figure 47.11

. At both high-

and low-pres sure evaporat ors, the evaporat ion pro -
cesses take place. The pressur e of the refr igeran t
vapor at the exit of the high-pr essure evaporat or is
regula ted by a back pressure regula tor to that of the
low-pres sure evaporat or before mixin g takes place at
a v apor chamber. The pr essure of the mixe d vapor is
then rais ed by the compres sor to that of the con denser.
This two-stage cycle then rep eats itself .

47.8 REAL-TIME CONTROL OF DRYING

ENVIRONMENT

The complex chemical reactions involved in the destruc-
tion of heat-sensitive materials during drying are well
documented. Optimization based on the reduction of
quality degradation of such processes is difficult. The

Drying air in

Perforated plate

Axial fan

Air distribution
nozzle

Wet air out

FIGURE 47.7 A perpendicular flow batch mode dryer.

Outlet hopper

Inlet hopper

Conveyor belt

Condenser

Evaporator

Compressor

Expansion valve

Product to be dried

Fan

FIGURE 47.8 A continuous mode heat pump dryer.

2006 by Taylor & Francis Group, LLC.

traditional approach in food technology is based on
employing well-known technologists who conduct
trial and error tests. Quite often, the task is time con-
suming and arduous. In most competitive industries,
this method is no longer appropriate. Yet modern food
technology makes it imperative that solutions be found
that will allow the optimization of complex processes
with respect to complex quality factors (Karel, 1988).

On the ba sis of the state-of-t he-art technol ogy, the

direction tow ard so lution to this problem lies in the
combinat ions of line- sensors an d expert syst ems wi th
feedback respon se to allow immediat e quality-r elated
decision to be made. The sensors are placed in stra -
tegic locat ions to measur e real-ti me qua lity param-
eters. The signals are then fed to expert syst ems,
usually compri sing a softwar e system that has the
ability to recei ve an d trans mit decision signals to
control lers. It is wel l know n that the quality de grad-
ation of foo d produc ts, such as browni ng effects and
ascorbi c acid (AA) degradat ion, is mainl y due to the
thermal effect of the drying air. It is thus possible to
reduce these quality effects through a proper feedback
system to regulate the air or product temperature.

Figu re 47.12

shows an exampl e of a real-time

process control strategy for a heat pump dryer to
improve the product color and minimize surface
cracking through time variation of the drying air
temperature. A thermovision camera is used to cap-
ture the surface temperature profiles. On the basis of
predefined constraints on the surface temperature, a
signal from the computer is sent to the PID controller
to tune the temperature of the drying air. In this way,
the quality degradation of the product can be minim-
ized without compromising the drying rate excessively
to achieve the desired final moisture content.

Figu re 47.13

sh ows another exa mple of a real-ti me

process control for the heat pump dryer to reduce
nutrient degradation. Experiments have been carried
out with hypodermic thermocouple needles to measure

Single-stage, motor-driven heat
pump dehumidifier

Single-stage, motor-driven heat pump dehumidifier
with subcooling

Coefficient of performance

Single-stage, engine-driven heat pump
dehumidifier with subcooling

Single-stage, engine-driven heat pump
dehumidifier

Two-stage, engine-driven heat pump
dehumidifier with subcooling

FIGURE 47.9 Coefficient of performance of various heat
pump configurations. (Adapted from Perry, E.J., Inst.
Refrig. Mtg., 1, 1981.)

Condenser

Low-

pressure

evaporator

Drying

chamber

Condensate

Recovered heat

Compressor
work

Expansion
valve

High-

pressure

evaporator

Vapor
mixing
chamber

Evaporator pressure regulator

Air path

Condensate

Refrigerant path

Air path

Rejected heat

FIGURE 47.10 Two-stage heat pump system coupled with a drying chamber.

2006 by Taylor & Francis Group, LLC.

the transient temperature profiles of food products
(Chou et al., 1997). These measured values make it
possible to tune the drying air temperature to prevent
the internal product temperature from reaching a
threshold value, hence reducing thermally induced
nutrient degradation.

47.9 FIXED AND TIME-VARIABLE

OPERATING SCHEMES

In order to reduce the energy consumption per unit of
product moisture, it is necessary to examine different

methodologies to improve the energy efficiency of the
drying equipment. One possible method is to apply time-
dependent drying schemes to reduce the drying time
to obtain the desired product moisture content. Time-
dependent drying schemes which imply time-varying
supply of thermal energy for drying in the batch mode
can be classified into the following categories:

1. Intermittent drying whereby heat is supplied

intermittently rather than continuously. This
can be achieved by interrupting the airflow to the
product or by intermittently heating the drying air.

Air-on
coil

To vapor mixing
chamber then
compressor

Suction main

Evaporator
pressure regulator

From
condenser

High-pressure
evaporator

Low-pressure
evaporator

Check valve

Thermostatic
expansion valve
(high pressure)

Thermostatic expansion
valve (low pressure)

Pilot solenoid
valve

Dry bulb
temperature
feedback

From
condenser

Air-off
coil

FIGURE 47.11 The refrigeration flow of a two-stage heat pump cycle.

Air out

SP: set point

Refrigerant out

Thermovision camera

Computer with
thermovision
software

PID controller

Refrigerant in

From compressor

Product tray

Refrigerant out

Refrigerant in

Air in

Hot-gas condenser

3-Way
modulating valve

Air-cooled condenser

Product

FIGURE 47.12 An online feedback system for a heat pump dryer to control the color of the product.

2006 by Taylor & Francis Group, LLC.

2. Dryaeration which is a drying process involv-

ing a combination of high temperature, short
drying period, tempering, and slow cooling
concluded by final drying.

3. Air reversal drying which is reversing the direc-

tion of the airflow for a period of time and then
revert it back to its original direction. This is
applied to deep bed drying of particulates.

4. Cyclic drying which is a drying process

whereby the air temperature, humidity, or
even velocity undergoes a specified cyclic pat-
tern variation such as sinusoidal, square-wave,
or sawtooth patterns.

Several experimental studies have been carried out

to investigate various time-dependent drying schemes
and their impacts on dryer energy consumption and
produc t quality. Thes e studi es, summ arized in

Table

47.2

, have foun d severa l inter esting feat ures of tim e-

dependent drying. These features are:

Thermal energy savings

Shorter effective drying time

Higher moisture removal rates

Lower product surface temperature

Higher product quality; these include reduced
shrinkage, cracking and brittleness, improved
color, and nutrient detention

Studying the effect of regulating the airflow on the

drying process, Ratti and Mujumdar (1993) presented
a simulation study on batch drying of shrinking hy-
groscopic materials in a fixed bed using time-varying
flow rate. Their work has shown that the total air
consumption for drying is reduced with minor or no
increase in drying time. Later, Ratti and Mujumdar
(1995) extended their study to include the case of air-
flow reversal. The results showed that both moisture
and temperature profiles in the bed were flatter when
air reversal was applied to the drying process while the
mean drying curves remained practically unchanged.

Based on the above literature reviewed, it can be

concluded that intermittent and time-varying drying
have significant advantages in terms of reducing the
required drying energy and enhancing the product
quality of heat sensitive products.

In the next few sections, the recent results on HPD

will be presented to demonstrate the advantages of
employing time-varying drying schemes over fixed
operating drying conditions.

Air out

SP: set
point

Refrigerant out

PID controller

Air in

Refrigerant
in

Product

Air-cooled condenser

3-Way
modulating valve

Product tray

Refrigerant out

From compressor

Hypodermic
thermocouple needles

Refrigerant in

Hot-gas
condenser

Fixture for internal
temperature measurements

FIGURE 47.13 An on-line feedback system for a heat pump dryer to reduce nutrients degradation.

2006 by Taylor & Francis Group, LLC.

47.9.1 D

RYING WITH

IXED

PERATING

ONDITIONS

Most of the indust rial dr ying operati ons are carried
out unde r fix ed drying conditio ns, i.e., the tempe ra-
ture, hum idity, an d veloci ty of the airflow are ke pt
relative ly constant throughou t the entire drying pr o-
cess until the desir ed pr oduct moisture content ha s
been atta ined. Many of these fixed drying co ndition s
have been obtaine d from several trails until the pro d-
uct dryness and desired quality have been obt ained.
Prescrib ing a fixed drying cond ition would indeed
simplify the drying process . Often the optimal set of
fixed ope rating parame ters can not be obtaine d and
since the intrinsi c propert ies of the material change as
moisture is remove d and the mate rial gets heated up,
what may be optim um drying co nditions for initial
drying stage s may not be optim um for later stage s.
With the de velopm ent of advan ced control lers and
process co ntrol techni ques that can incorpora te dif-
ferent prod uct qua lity c onstraints in the feedback
control strategy, drying condition s used in HPD can
now be ‘‘tu ned’’ to produ ce a few ‘‘constant ’’ drying
conditi ons at diff erent stages of the drying. Ther efore,
severa l fix ed operatio ns can be incorpora ted resul ting
in a series of fixed drying operations with favorable
results in drying kinetics as well as product quality.

47.9.2 D

RYING WITH

NTERMITTENT

PERATION

Three intermittent HPD patterns were used in the
present study and their prescriptions are shown in

Figure 47.14

. Thes e inter mittent profiles are pre -

scribed by raising the inlet air temperature for a de-
fined period (t) and dropping the air temperature
back to its original level until the periodic interval.
The intermittency, a, is defined as the fraction of time
during which the inlet air temperature is raised to the
defined cycle time, i.e.,

þ t

off

(47:5)

47.9.3 D

RYING WITH

YCLIC OR

RBITRARY

IME

-V

ARYING

PERATION

Devahastin and Mujumdar (1999) have demonstrated
by a mathematical model the feasibility and advan-
tages of operating a dryer by varying the temperature
of the inlet drying air in terms of reducing drying time
by up to 30%. As technology advances, more options
are available to improve the quality. One potential
avenue in improving quality degradation in food
products during drying is to employ time-varying
temperature profiles that minimize quality degrad-
ation and dry the products to the desired moisture
content within an allowable production time. Several
researchers have studied the degradation of quality of
dried products under sine or square wave tempera-
ture fluctuations (Wu et al., 1974; Kamman et al.,
1981) during storage. However, little work has been
reported to study the effect of temperature pro-
files on quality during convective drying process.
In the following section, we examine several time-
varying drying operations and their impacts on
drying kinetics and product quality.

47.9.4 D

RYING

INETICS

47.9.4.1 Intermittent Operation

The limited study on intermittent drying of materials
in a batch dryer have confirmed the potential advan-
tages of time-dependent supply of energy for drying,
e.g., reduced energy consumption, reduced air con-
sumption, and enhanced quality of heat-sensitive
products.

TABLE 47.2
Summary of Different Time-Dependent Drying Studies

Study

Material and Dryer Type

Drying Scheme

Sabbah et al. (1972)

Corn (thin layer)

Dryaeration: tempering periods: 0–4 h

Troger and Butler (1980)

Peanuts

Intermittent drying: airflow interrupted at 1 in 4 h

Harnoy and Radajewski (1982)

Maize (bin dryer)

Intermittent drying: aeration periods: 1–6 min; rest periods: 3–90 min

Giowacka and Malczewski (1986)

Wheat (fluidized bed)

Sinusoidal heating

Ha¨llstrom (1986)

Compound fertilizer

(fluidized bed)

Intermittent drying: drying periods: 2.5–6 s; rest periods: 4.5–6 s

Zhang and Litchfield (1991)

Corn (thin layer)

Intermittent drying: drying period: 20 min; rest periods: 0–120 min

Hemati et al. (1992)

Corn (flotation fluid bed)

Intermittent drying: drying period: 20 min rest; periods: 0–60 min

2006 by Taylor & Francis Group, LLC.

For HPD systems, intermittent drying can be

classified into three categories:

1. Intermittent regulation of the air temperature
2. Intermittent supply of airflow
3. Intermittent regulation of the air humidity

Among the three, the intermittent regulation of

air temperature is considered to have the most signifi-
cant influence on the product drying kinetics and
various quality parameters and is the focus of the
following sections.

47.9.4.2 Mean Moisture Content

Figure 47.15 shows the mean moisture content of the
potato samples conducted under each intermittent

drying profiles and continuous drying process of tem-
perature 358C. It is evidenced that employing inter-
mittent temperature drying profile can result in saving
drying time by about 25, 48 and 61% for a

¼ 1/4,

¼ 1/2, and a ¼ 2/3, respectively. Longer product

tempering period occurs for a

¼ 1/4, resulting in a

flat region in the drying curve. It can also be observed
from Figure 47.15 that, during each tempering pro-
cess, i.e., during t

off

period, some form of partial

drying occurs, particularly during the earlier stage
of drying. However, toward latter stage, little or no
drying occurs as the mean moisture content curve
tapered following an asymptotic value. Therefore, it
can be inferred from these curves that as substantiate
amount of moisture is removed during the last phase
of drying, product tempering enables to bring about

off

t (min)

(

⬚C)

ΔT

Drying Period,

(min)

Tempering Period,

off

(min)

Cyclic Time,t

(min)

Intermittency

Process Conditions

1/4

= 30

⬚C, ΔT

= 5

⬚C

1/2

= 2.5 m/s, RH = 30%

2/3

= 4.2 kg/kg dry basis

FIGURE 47.14 Intermittent varying of inlet air temperature profiles. (Adapted from Chou, S.K., Chua, K.J., Hawlader,
M.N.A, Mujumdar, A.S., and Ho, J.C., Transactions of the Institution of Chemical Engineers, 78(C), 193, 2000. With permission.)

0.5

1.5

2.5

3.5

4.5

120

180

240

Drying time (min)

Mean moisture content,

, (kg/kg dry basis)

1. Continuous (35

⬚C)

2. Intermittent (a

= 1/4)

3. Intermittent (a

= 1/2)

4. Intermittent (a

= 2/3)

180 min

FIGURE 47.15 Time-evolution of the average moisture content for various drying profiles. (Adapted from Chou, S.K.,
Chua, K.J., Hawlader, M.N.A, Mujumdar, A.S., and Ho, J.C., Transactions of the Institution of Chemical Engineers, 78(C),
193, 2000. With permission.)

2006 by Taylor & Francis Group, LLC.

unifor m moisture distribut ion within the produ ct
rather than reducing mois ture con tent.

47.9.4 .3 Su rface Te mperature

Figure 47.16 shows that intermittent heati ng abo ut
the mean tempe rature, T

¼ 30 8 C, can produce

lower surface tempe rature than the con tinuous one .
This might be of interest in the drying of sensi tive
material s, particu larly those of biologi cal na ture
(e.g., marine and agricultural products). Gentle time-
vary ing he ating u sing the se hea ting schemes o ffe r
grea te r flexibility in the control of the surfac e tempera -
ture and minimize many product quality p roblems such
as non enz ymatic brown in g (N EB ) an d surface cra cks. It
is inte resting to n ote that surface tempe rature due to
con t inuo us tempe rature dry ing env el ops tho se surface
tempe rature p rofiles d ue t o t ime-temp erature v arying
sche mes espe cially during the i nitial stage of drying .
It is also expected that a longe r surfac e tempering
period a t cooler temperature is o bser ved for a

¼ 1/4.

A comb i nation of lower sur fa ce temp erature and lo nge r
tempering period might be the method to enhan ce the
produc t quality related to product su rface conditio ns.

47.9.4 .4 Su rface Mois ture Cont ent

Figure 47.17

shows the evolut ion of the surface mois -

ture con tent unde r differen t intermit tent tempe rature
drying. On the onset of each tempering pe riod, the
surface moisture content increa ses. The increm ent is
particu larly pro minent for the second tempering
period. For the first tempering period, the produc t
surface is sti ll moisture saturated and vapor flux
continues to be transferr ed from the surfa ce to the

air. Once the surface becomes partiall y dried, intern al
moisture moves to the surfa ce and an imm ediate
incremen t in surface moisture is obs erved. There
may be an increa sed moisture flux from the intern al
to the surfa ce as drying pro gresses. When the tempe r-
ing period is longer, the allowance tim e for the new
surface mois ture to form is also longer. Therefor e, a
gentle rais e in surfa ce mois ture con tent can be ob-
served when compared with the sampl es with surfac es
subjected to a longer heating period (a

¼ 1/2 and a ¼

2/3). Once tempering is over, the surface moisture re-
sumes its drying process and moisture is rapidly re-
moved again. According to Jumah et al. (1996), the
effect of introducing cooler air, in addition to partial
drying, is able to repeat the initial steep drying curve.
Consequently, high drying rate follows after each heat-
ing period. This favorable supply of more surface mois-
ture during the tempering period enhances the removal
of mo isture from the produ ct and , eventual ly, reduc es
the tim e to obtain the de sired mois ture con tent.

47.9.5 C

YCLIC OR

RBITRARY

IME

-V

ARYING

PERATION

The ev olution of the mea n moistur e conte nt with time
for drying of banana samples in a heat pump dryer
is shown in

Figure 47.18

. It can be observed that step-

do wn te mp era tur e p rofi l e i s ab l e t o re du ce t he dry ing
time to reach desired moisture content. Taking the mois-
ture content of each product at 240 min as the basis for
comparison, it can be observed that the step-down tem-
perature profile was able to reduce the drying time for
banana samples by 180 min. A noteworthy point is that
step-down temperature profile was more effective in
reducing in the drying time for banana samples.

120

180

240

Drying time (min)

Product surface temperature (

⬚C)

1. Continuous (35

⬚C)

2. Intermittent (a

= 1/4)

3. Intermittent (a

= 1/2)

4. Intermittent (a

= 2/3)

FIGURE 47.16 Time-evolution of product surface temperature for various drying profiles. (Adapted from Chou, S.K.,
Chua, K.J., Hawlader, M.N.A, Mujumdar, A.S., and Ho, J.C., Transactions of the Institution of Chemical Engineers, 78(C),
193, 2000. With permission.)

2006 by Taylor & Francis Group, LLC.

The drying rate curves for the banana samples are

portra yed in

Figure 47.19

. It can be observed from

this figure that stepw ise variation of the air tempe ra-
ture produ ces unusual drying rates. For the step-
down tempe ratur e profi le, tw o con ventio nal, i.e. ,
first and second, falling drying rate curves exist in
tandem . Such findin gs maybe attribut ed to rapid re-
moval of moisture from the satur ated surfa ce dur ing
the initial drying stage , follo wed by the high tempe ra-
ture gradie nt be tween product surface and intern al
once a drop in air tempe ratur e was init iated. Two
drying rate peak s can be de tected for the step- up

tempe rature pro file. Due to the slow initial drying
resulting rate from reduced sensi ble heat trans ferre d,
the sampl es with high mois ture co ntent were still a ble
to experi ence increa sed drying rate when tempe rature
step increm ent of 58 C was implement ed.

47.9.6 P

RODUCT

UALITY

SPECTS

The impact of c onstant temperatur e drying on pro d-
uct quality is wel l known . Mo st of the product quality
par am e ters such as NEB a nd AA cont ent a re oft en
manifested by a progressive loss with increasing

0.5

1.5

2.5

3.5

4.5

120

160

200

240

Drying time (min)

Surface moisture content,

(kg/kg dry basis)

1. Continuous (35 8C)

2. Intermittent (a

= 1/4)

3. Intermittent (a

= 1/2)

4. Intermittent (a

= 2/3)

FIGURE 47.17 Time-evolution of product surface moisture for various drying profiles. (Adapted from Chou, S.K., Chua,
K.J., Hawlader, M.N.A, Mujumdar, A.S., and Ho, J.C., Transactions of the Institution of Chemical Engineers, 78(C), 193,
2000. With permission.)

1.5

2.5

3.5

120

180

240

300

Drying time (min)

Moisture content (kg/kg dry basis)

25 8C

Step-up profile

Step-down profile

FIGURE 47.18 Moisture content of banana samples versus drying time. (Adapted from Chua, K.J., Mujumdar, A.S.,
Hawlader, M.N.A, Chou, S.K., and Ho, J.C., 2001, Food Research International, 34, 721, 2001. With permission.)

2006 by Taylor & Francis Group, LLC.

temperature. The following sections will discuss the im-
pact of employing the different time-varying drying pro-
files on some product quality parameters during HPD.

47.9.6.1 Color Change due to Nonenzymatic

Browning

Chua et al. (2000a) have demonstrated that a two-
stage heat pump dryer can be controlled to produce
prescribed time-varying air temperature profiles to
study the effect of nonuniform temperature drying
on color change of food products. They have also
shown that by subjecting food products to different
temperature profiles in a heat pump dryer, it is
possible to reduce the change in individual color
parameters as well as in the overall color change in
the food products. High-sugar content product such
as banana favors a time-varying profile with a cold
starting temperature of 308C while high-moisture
product like potato with low sugar content enabled
the use of higher temperature profiles to yield higher
drying rates without any pronounced change in the
overall color change. Prescribing the appropriate
cyclic temperature variation schemes, Chua et al.
(2000a) have shown that the percentage reductions
in overall color change for potato, guava, and banana
were 87, 75, and 67%, respectively.

47.9.6.2 Ascorbic Acid Content

On the basis of an extensive experimental study of the
kinetics of batch drying and AA degradation of guava

pieces under isothermal as well as time-varying drying
air temperature, Chua et al. (2000b) have shown that
with proper selection of the temperature schedule, the
AA content of the guava pieces can be up to 20%
higher than that in the isothermal drying without
significant enhancement in drying time. Results from
Chua et al. (2000b) indicate that employing reduced
air temperatures at the onset of drying followed by
temperature elevation as drying proceeds yield a bet-
ter quality product. Recently, Pan et al. (1999) have
clearly demonstrated the advantage of intermittent
drying as far as product quality is concerned. They
have shown that in vibrated bed batch drying of
carrot pieces the retention of beta-carotene in the
dried product is higher in intermittent drying while
at the same time the net energy consumption is re-
duced and even the actual drying time is shortened
somewhat.

47.9.6.3 Other Quality Parameters

Ginger dried in a heat pump dryer was found to
retain over 26% of gingerol, the principal volatile
flavor component responsible for its pungency, com-
pared with the rotary dried commercial samples that
have only about 20% (Mason et al., 1994). The higher
volatile retention in heat pump-dried samples is prob-
ably due to reduced degradation of gingerol when
lower drying temperatures are employed compared
with higher commercial dryer temperatures. Since
HPD is conducted in a closed chamber, any compound
that volatilizes will remain within it, and the partial

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

120

180

240

300

Drying time (min)

Drying rate (kg/kg)/min

Drying temperature (

⬚C)

25 8C

Step-up

Step-down

Step-up temperature profile

Step-down temperature profile

Peak 1

Peak 2

FIGURE 47.19 Drying rate of banana samples undergoing stepwise varying of drying air temperature. (Adapted from Chua, K.J.,
Mujumdar, A.S., Hawlader, M.N.A, Chou, S.K., and Ho, J.C., 2001, Food Research International, 34, 721, 2001. With permission.)

2006 by Taylor & Francis Group, LLC.

pressur e for that compou nd will gradual ly build up
within the ch amber, retard ing furt her volatilizati on
from the pro duct (Perera an d Rahm an, 1 990).

The color an d aroma herb s (e.g ., parsley, rose-

mary, and swe et fennel ) can be impr oved when com-
pared with the co mmercial produ cts. The sensory
values were ne arly doubl ed in case of heat pump -
dried herbs compared with commer cially dried pro d-
ucts. There was no signifi cant difference in the quality
of herbs dried below mo isture con tent of 0.04 for
experi menta l drying tempe ratures (40 and 5 08C) and
relative humidity (0.30 and 0.40) .

The use of modified atmos pheres for drying sen-

sitive mate rials such as food products is an other im-
portant potentia l aspect of the HPD techno logy.
Drying with oxygen -sensitive mate rials such as flavo r
compou nds and fatty acids can undergo oxidat ion,
giving rise to poor flavor, color, and rehyd ration
propert ies. Use of modified atmos pheres to replace
air woul d allow new dry prod ucts to be developed
without oxidative reactions occurri ng (Perera and
Rahm an, 1990).

47.10 SELECTED INDUSTRIAL

APPLICATIONS

Heat pump dryers a re gaini ng reco gnition as ene rgy
efficien t drying devices for producti on of quality
produc ts. The followin g section s are devoted to
selected exampl es of indust rial ap plications of the
heat pum p dryer.

47.10.1 H

EAT

UMP

RYERS FOR

IMBER

RYING

HPD can provide effici ent and cost- effective dry ing
of timber, pa rticular ly wher e quality is a key issue . In
the last 10 y, the industry emphasis has been on increas-
ing throughpu t throu gh higher tempe ratur e process -
ing. W ith increa sing concerns about the ability of the
interna tional market to absorb lower grade tim ber,
the cycle is retur ning to exami ne the need to produce
a wid er range of prod uct types of accepta ble qua lity.
In this marke t, HPD can be made compet itive, as it
has distinct c ost advan tages over conven tional heat-
and-vent kilns at lower operati ng tempe ratures and
where higher humidity levels need to be maintained
(Bannis ter et al., 1999). Furtherm ore, heat pump
dryers operate at higher energy effici ency when the
amount of water remove d increases. Table 47.3 shows
the perfor mance of a hea t pump dr yer for timbe r
drying co nducted at the Forest Educa tion Center in
Rotoru a (Bannis ter et al., 1999). It can be observed
that the dr yer performed with better en ergy efficiency
(measu red by SMER) when more water was remove d.
It is expected because of the higher amount of latent

heat recovered when higher amount of water is re-
moved. This would translate to a shorter payback
period for higher volume of timber dried.

The advantages of HPD for timber include efficient

utilization of recovered heat, and slow and controlled
drying rates resulting in reduction of physical defects.
It is also possible to accomplish simultaneous drying
of different wood species in the same kiln in low-
tempe rature drying.

Fi gure 47.20

shows the schema tic

of a heat pump dryer designed for timber drying.

According to Bannister et al. (1999), timbers that

are physically slow to dry, tend to warp, split, or
collapse during drying. These timbers are classified
under ‘‘hard-to-dry’’ woods. Many of these timbers
have high commercial value and so there is a need to
establish a drying schedule that is able to produce
consistent high quality results. Examples of hard-to-
dry timbers in Australia include Red Beech (Notofa-
gus fusca), Hard Beech (Notofagus truncata), and
many Eucalyptus species (Bannister et al., 1999).

Bannister et al. (1999) proposed the following

schedules to improve timber quality by HPD:

Initial drying at low temperature (less than
308C) and high humidity (85% and higher)

A ramping period during which air temperature
is increased and its humidity is lowered

A finishing period at a reasonable temperature
(~508C) and lower humidity (40 to 50%)

Potential benefits of HPD of timber that can be

reaped include (Bannister et al., 1999):

1. Improved quality—this is important since the

present timber market is now driven by quality

2. Reduced drying time resulting in enhanced

productivity.

3. Low energy costs—with energy consumption

of around 2 to 2.5 kg/kWh. This is comparable

TABLE 47.3
Energy Performance of a Heat Pump Dryer
for Timber Drying

SMER (kg/kWh)

Water Removed (kg)

2.2

1300

2.6

1750

2.9

5200

3.2

7200

Source: Bannister, P., Carrington, G., Chen, G., and Sun, Z.,
Energy Group’s Heat Pump Dehumidifier Research Programme
Report, EGL-RR-02, ed. 1.1, 1999.

2006 by Taylor & Francis Group, LLC.

with efficie ncies for de humidifi ers drying easy-
to-dry timbers.

4. Increased throughput by using air with low er

humidity to enh ance drying rate with little
impact on drying- induced stresses .

47.10.2 H

EAT

UMP

RYERS FOR

OOD

RODUCTS

There has been a growing inter est in recen t years in
applyi ng HPD technol ogy to foods and biomateri als
where low-temp erature drying an d well- controlled
drying conditio ns are required to en hance the quality
of food pro ducts. High-val ue pro ducts, whi ch are
extremely heat- sensitiv e, are often freez e-dri ed. Thi s
is an extre mely expen sive drying process (B aker,
1997). Recently, there has been great inter est to look
at the HPD system as a substitute system for freeze-
dried prod ucts.

Table 47.4

presents a summ ary of

recent work on HPD of selected food products.

In most of the research studies presented in Table

47.4, the common conclusion was that the HPD offers
products of better quality with reduced energy con-
sumption. This is particularly true of food products
that require precisely controlled drying atmosphere
(temperature and humidity). Heat-sensitive food prod-
ucts, requiring low-temperature drying, can take the
advantage of HPD technology since the drying tem-
perature of HPD system can be adjusted from

20 to

608C. With proper control, it is also possible for HPD
to produce freeze-drying conditions at atmospheric
pressure (Prasertsan and Saen-saby, 1998b). As far as
food drying is concerned, HPD offers an alternative
to improve product quality through proper regula-
tion of the drying conditions. Chua et al. (2000a)
have demonstrated that HPD can produce prese-
lected cyclic temperature schedules to improve the

quality of various agricultural products dried in the
two-stage HPD. They have shown that with appro-
priate choice of temperature–time variation, it is
possible to reduce the overall color change and AA
degradation by up to 87 and 20%, respectively.

The ability of HPD to regulate drying conditions

quickly is another advantage that it offers for food
drying. In countries where the level of the air humid-
ity is high, high spoilage rates occur during the rainy
season when the drying air is very moist. Clearly,
HPD can reduce product spoilage by maintaining
the humidity of the drying air through the regulation
of latent heat removal at the evaporator.

Besides yielding better food quality, Rossi et al.

(1992) has reported that onion slices dried by HPD
used less energy in comparison to a conventional hot
air system. Food products with high water content
can be dried efficiently with HPD. As the drying air
absorbs more of this available energy, this latent heat
energy can be transferred at the evaporators for
higher heat recovery. Lower energy input is then
required at the compressor to enable sensible heating
of the air when it passes through the condenser.

To summarize, when the quality of dried food

products is paramount, HPD offers an attractive op-
tion to enhance product quality and reduces spoilage
through better regulation of the drying conditions.

47.11 HEAT PUMP DRYER

INCORPORATING OTHER
TRANSFER MECHANISMS

Thus far, the research work has been primarily aimed at
the performance of heat pump dryers for different prod-
ucts. Little work has been reported on the performance

Auxiliary
heater

Drying chamber

Dry air

Wet
air

Condensate

Compressor

Expansion
valve

Condenser

Evaporator

Timber
rack

FIGURE 47.20 Schematic diagram of a heat pump dryer for timber drying.

2006 by Taylor & Francis Group, LLC.

o f he at pu mp d ry er s u sin g ex t er na l e ne rg y s ou rc es to
complim ent it . HPD i s p rim arily a c onvect ive d ryi ng
process. Use o f i nf rar ed (I R ) or radio frequency ( RF)
sources can be incorporated along with the usual HPD to
en h an ce t he dr yin g ra te s w hil e re du c i ng the t he rm al lo ad
on the h eat pump itself.

In the follo wing section s, we look at severa l po-

tential areas of supplem entary heat sou rces to furth er
enhanc e the perfor mance of the conven tional HPD in
terms of lower energy consumpt ion, better prod uct
quality, an d en able the remova l of bound water in
thick mate rials.

47.11.1 F

LUIDIZED

EAT

UMP

RYER

Fluidi zed be d dry ing (FBD) has found many applic a-
tions for drying of granu lar soli ds in the food, cer-
amic, pharmac eutical, and agric ulture indust ries. For
drying of powder s in the 50 to 2000 mm range, FBD
compet es success fully with oth er more tradi tional
dryer types, e.g. , ro tary, tunnel, conveyor, and co n-
tinuous tray. FBD has the followin g adva ntages
(Mujum dar and Devahas tin, 1999):

1. High drying rates due to excellen t gas–par ticle

contact leadin g to high heat an d mass trans fer
rates

2. Smaller flow area
3. Higher therm al effici ency
4. Lower capital and maint enance co sts compared

with rotar y dryers

5. Ease of con trol

Howeve r, FBD suffers from certa in lim itations such
as:

1. High power consump tion due to the need to

suspend the entire bed in gas phase leading to
high pressure drop

2. High potential of attrition, in some cases

granulation or agglomeration

3. Low flexibility and potential of defluidization if

the feed is too wet

A schematic layout of a fluidized bed heat

pump dryer developed at the Norwegian Institute
of Tec hnology (N TN U ) i s s ho wn in

Fi g ur e 4 7 .2 1

(Strfmmen and Jonasen, 1996; Alves-Filho and
Strfmmen 1996). The drying chamber receives wet
material and discharges dried product through the
product inlet and outlet ducts. The desired operat-
ing temperature is obtained by adjusting the con-
denser capacity, while the required air humidity is
maintained by regulating the compressor capacity
by frequency control of the motor speed. According
to Alves-Filho and Strfmmen (1996), this setup can
produce drying temperatures from

20 to 608C and

air humidities from 20 to 90%. With these features,
heat-sensitive materials can be dried under convect-
ive air or freeze-drying conditions. It is also pos-
sible to sequence these two operations (convective
and freeze drying). It will be advantageous for
drying of food and bioproducts since freeze drying
causes minimal shrinkage but produces low drying
rates while convective air drying can be applied to

TABLE 47.4
Recent Work Conducted on Heat Pump Drying of Selected Food Products

Researchers

Application(s)

Conclusions

Chou et al. (1997, 1998)

Agricultural and marine products

(Mushrooms, fruits, sea-cucumber,
and oysters)

The quality of the agricultural and marine products can be

improved with scheduled
drying conditions

Chua et al. (2000a)(Singapore)

Prasertsan and Saen-saby

(1998a) and Prasertsan et al.
(1997) (Thailand)

Agricultural food drying (Bananas)

HPD is suitable for drying high moisture materials and

the running cost of HPD is cheap making them
economically feasible

Theerakulpisut (1990)

(Australia)

Grain

An open cycle HPD performed better during the initial

stage when the product drying rate is high

Meyer and Greyvenstein (1992)

(South Africa)

Grain

There is a minimum operating period that makes the

HPD more economical than other dryers

Rossi et al. (1992) (Brazil)

Vegetable (Onion)

Drying of sliced onions confirmed energy saving of the

order of 30% and better product quality due to shorter
processing time

Strfmmen and Kramer (1994)

(Norway)

Marine products (Fish)

The high quality of the dried products was highlighted as

the major advantage of HPD and introducing a
temperature-controllable program to HPD makes it
possible to regulate the product properties such as porosity,
rehydration rates, strength, texture, and color

2006 by Taylor & Francis Group, LLC.

e nh a nc e dr y in g r a te s . Th e r e fo r e , a c om bi na t io n of
dr ying pr oc esses, e.g. , freeze dr ying at

5 8 C

fo l l ow e d by c on v e c t iv e d ry i ng a t 2 0 to

30 8 C,

e na bl e s t he c on t r ol of qu a li t y pa r am e t e rs s uc h a s
po r os it y , r e hy dr a ti on ra t e s, s t re ng t h, te x t ur e, c ol or ,
a nd ta s t e (A lv e s -F il ho a nd St r f mm en, 1996). Ex -
pe r im e nt s pe rf or me d a t N TN U on v ar i ou s he a t -
s en si ti v e m a t er i a ls su c h a s ph a r m a ce ut i ca l pr od u ct s ,
fr ui ts , an d ve g e t a bl e s ha v e sh ow n th at he a t pu m p
FB D of fe r s a b et t er pr od uc t qu a li ty , bu t at hi g he r
c os t. Si nc e th i s te c hn i qu e pr od uc e s a pr e m iu m
qu a li t y pr od uc t , th e in cr e m e nt a l i nc r e a se in dr y in g
c os t ma y be of fs e t by th e hi g he r m a r ke t v a lu e of
th e pr o du c t.

47.11.2 I

NFRARED

-A

SSISTED

EAT

UMP

RYING

Infrared drying helps to reduce the drying time by
providi ng additional sensib le heati ng to expedit e the
drying process . IR en ergy is trans ferred from the
heatin g elem ent to the produ ct surfa ce withou t heat-
ing the surroundi ng air (Jones, 1992). Several re-
searchers have demonstrated the significant advantages
of IR drying. These advantages (Navarri et al., 1992)
include:

1. High heat trans fer rates (up to 100 kW m

paper industry) can be obtaine d with co mpact
heaters.

2. Easy to direct the heat source to drying surfa ce.
3. Quick response times, allowi ng easy and rap id

process co ntrol (if needed).

4. Incorporat ing IR into an existing heat pump

dryer is simple and capital cost is low.

IR drying has be en the subject of invest igations of

recent resear chers. W orks by Paa kkone n et al. (1999)
has shown that IR drying impr oves the qua lity of herb s
and Dontign y et al. (1992) have de monst rated that IR
drying of graph ite slurry signifi cantly increa ses drying
rate. Zbicins ki et al. (1992) invest igating co nvective air
drying and IR drying hav e suggested intermittent ir-
radiation drying mode co upled with co nvective air
drying for heat-sensi tive material s.

Figure 47.22

shows an IR-assist ed HP D system.

To dry heat-sensitive materials, a combined r adia nt–

convecti ve drying method or an inter mittent drying
mode may be app lied. An IR-au gmented he at pump
dryer co uld be used for fast remova l of surface mois -
ture during the initial stage s of drying, foll owed by
intermittent drying over the rest of the drying process .
This mode of ope ration ensures a fast er initial drying
rate. Ther efore, an IR-a ssisted HPD woul d offer the
advantag es of compact ness, simp licity, e ase of con -
trol, and low equipment costs (Mujum dar, 2000a) .
Also, there are the possibili ties of signifi cant energy
savings and enhanced product quality due to reduced
residence time in the drying chamber. On the flip
side, the high heat flux may scorch the product and
cause fire and explosion hazards (Mujumdar, 2000a).
Clearly, good control of IR operation is essential to
achieve the desired results in terms of drying kinetics
and product quality, as well as to ensure safe oper-
ation. So, a good feedback control is the one that
enables the IR power source to cut off if excessi-
vely high temperatures are measured in the chamber,
which may lead to overheating of the product.
No work has been reported on this concept for
HPD to date.

External
condenser

Fluidized bed

Condenser

3-Way valve

Evaporator

Expansion valve

Compressor

Centrifugal fan

Liquid receiver

Wet material

Dry
material

FIGURE 47.21 A schematic layout of a fluidized bed heat pump dryer. (Adapted from Alves-Filho, O. and Strfmmen, I.,
Drying ’96, Strumillo, C. and Pakowski, Z. (Ed.), Krakow, Poland, 1996.)

2006 by Taylor & Francis Group, LLC.

47.11.3 R

ADIO

REQUENCY

-A

SSISTED

EAT

UMP

RYING

A lim itation of heat transfer in co nventi onal dry ing
with hot air alone, parti cularly in the falling rate
period, can be ov ercome by combini ng RF heating
with con vention al HP D (Marsh all and Metaxa s,
1998). RF generate s he at volume trically wi thin the
wet mate rial by the co mbined mechani sms of dip ole
rotation and co nduction effects that speedup the
drying pro cess (Meta xas and Meredit h, 1983).
A typic al RF-as sisted heat pum p dryer compri ses a
vapor compres sion he at pump syst em retr ofitted wi th
a RF gen erating system capable of impar ting RF
energy to the drying mate rial at various stage s of
the drying pro cess.

Figu re 47.23

shows a schema tic

diagra m of the RF-assis ted hea t pump dryer.

Materials that are difficult to dry with convection

heating alone are good candidates for RF-assisted dry-
ing. Materials with poor heat transfer characteristics,
e.g., ceramics and glass fibers, are traditionally the prob-
lem materials when it comes to heating and drying. RF
he ats a ll pa rts of t he pr oduc t m as s s imul t a n e o u s l y a n d
evaporates the water in situ at relati vely low temper-
atures usually not exceeding 1808 F or 828 C ( Thomas,
1996). Since water m oves through the prod uct in
the form of a gas rather than by capillary action ,
migrati on of solids is avoided. Warping , surfa ce dis-
colorat ion, and cracki ng associ ated wi th con ventio nal
drying methods are also a voided (Thomas , 1996).

The foll owing are some of the charact eristics of

RF-as sisted HPD:

1. RF-assis ted drying impr oves the color of the

products especially those that are highly sus-

ceptible to surfa ce color change since RF dry-
ing star ts from the inter nal to the produ ct
surface, minimiz ing an y surfa ce effect.

2 . Cr acking, ca use d b y the stre sse s of une ven shrink-

age in drying, c an be eliminate d by RF-a ss is te d
drying. This is ach ieved in the dryer by even
heating through out the produ ct maint aining
moisture unifor mity from the center to the
surface during the drying pro cess.

The potenti al for direct applic ation of the RF-

assisted HP D in the indust ries is appreci able for the
followin g reason s:

1. Simultaneou s extern al and inter nal drying sig-

nificant ly reduces the drying time to reach the
desired moisture co ntent. The pote ntial for im-
proving the throughput of produ ct is good. For
example, in the bakery ind ustry, the throughp ut
for cracker s and coo kies can be impr oved by as
much as 30 and 40%, respect ively (Clark, 1997).

2. By great ly reducing the mois ture varia tion

throughout the thickne ss of the produ ct, differ-
ential sh rinkage can be mini mized. This pr o-
motes RF-assis ted heat pump dryer for drying
materials with high shrinka ge propert ies.

3. Closer toleran ce of the diele ctric heati ng fre-

quency, (1) 13.56 MHz + 0.05%, (2) 27.12
MHz + 0.60%, and (3) 40.68 MHz + 0.05%
(Clark, 1997), significantly improves the level
of control for internal drying and thus has
potential in industry that produces products
that require precision moisture removal.

4. The moisture-leveling phenomenon of RF

drying ensures a uniform level of dryness

Condenser

Evaporator

Expansion valve

Compressor

Fan

SP: set

Thermocouple

Infrared light tube

Infrared-emitting

systems

Light
concentrate
reflector

PID controller

Product tray

Product

FIGURE 47.22 Schematic diagram of IR-assisted heat pump dryer.

2006 by Taylor & Francis Group, LLC.

throughout the product. In dustries that have
products requir ing unifor m dr ying, such as
the ceram ics, can consider RF drying as a
good alternati ve.

47.11.4 S

OLAR

-A

SSISTED

EAT

UMP

RYING

WITH

NERGY

TORAGE

YSTEM

In places where very rich sources of solar energy are
availab le, the incorpo ration of a solar heating syst em
to HPD may furth er impr ove the en ergy efficiency of
the ov erall drying syst em. Such a syst em may also be
approp riate for higher drying tempe rature. Inste ad of
using the conventi onal he ating system to provide for
auxiliary heati ng, the storin g of solar energy in a
phase-c hange mate rial such as paraffin wax for dis-
chargi ng sensi ble en ergy to the drying air leads to a
cheaper means of employ ing higher drying tempe ra-
ture. Fur ther, such a system offers the flexibi lity of
operati ng with the heat pump, solar syst em, or wi th
both syst ems complem enting each oth er. Tr oger and
Butler (1980) hav e experi menta lly evaluated a solar
collector- cum-r ockedbed stora ge syst em for peanu t
drying. Chauhan et al. (1996) have studi ed the dry ing
charact eristic s of co riander in a stationar y 0.5 t/ba tch
capacit y deep-bed dryer coupled to a solar air heater
and a rockbed storag e unit to recei ve hot air during
off sun shine hours. They found that to reduce the
average mo isture of coriander grains from 28.2 (dry
basis) to 11.4% (dry ba sis) requires 27 cumula tive
sunshine hours. Usin g the store d heat from the
rockbed energy storage system, the remova l of

the same moisture can be acc omplished with just
18 cumula tive sunshine hours.

The solar energy su pply syst em prop osed in this

section consis ts of solar collectors , blow ers, pha se-
change storage tank , air-valv es, an d pipes as sho wn
in

Figure 47.24

. Depending on the type of drying

material which determ ines the air tempe rature, the
air may be flown with ope n full parti al dischar ge
circulati on or full discharge circul ation mod e.

The advantag es of a solar -assisted HP D can be

summ arized as follows :

1. Easy co nversion of natural energy for storage

resulting in signi ficant saving of energy

2. Environme nt-frie ndly process
3. Easy to impl ement con trol strategy
4. Higher operati ng drying tempe ratur e

How ever, the disadva ntages of suc h syst em can

also be summ arize d as follo ws:

1. Higher capital costs are incurr ed for addition al

solar panels, blow ers, stora ge tank , and valves .

2. Implementation of the system is practical and

cost effective provided the average annual sun-
shine time (approximately greater than 2600 h
[Zhang et al., 2000]) is high and the annual
total quantity of radiation is sufficient (more
than 6

KJ/m

3. The amount of stored solar energy is greatly

subjected to the weather conditions.

Evaporator

Axial
fan

Load

Condenser

Compressor

Expansion
valve

Heat pump drying

chamber

RF generator

Metallic
perforated plate

FIGURE 47.23 Schematic diagram of RF-assisted heat pump dryer.

2006 by Taylor & Francis Group, LLC.

Table 47.5 provides the energy cost figures of

three methods for drying 1 m

lumber. It is apparent

that steam drying consumes more energy and has the
highest cost. Even though the solar-assisted HPD
consumes more energy than the pure solar system
the energy cost is only slightly higher.

47.11.5 M

ASS

RANSFER

ODE

—V

ACUUM

TMOSPHERIC

RESSURE

A very useful way to enhance the quality of heat-
sensitive products and yet achieve a fairly dried

product is through the use of a pressure-regulatory
system. The operating pressure range is usually from
vacuum to below atmosphere. A total vacuum system
may be very costly to build because of the need for
stronger materials and better leakage-preventing fa-
cility. Therefore, the system that is proposed here is
recommended to operate above vacuum condition.
The period of operating at lower pressure may be
continuous at a fixed level, intermittent, or a cyclic
prescribed pattern. The suitability of employing the
appropriate type of pressure-swing pattern depends
chiefly on the drying kinetics of the product and its
thermal properties.

Drying chamber

Phase-change-material
storage tank

Dry hot air

Wet air

Solar collector

Blower

Heat pump system

Charging
process

Discharging
process

FIGURE 47.24 A solar-assisted heat pump system incorporating energy storage system.

TABLE 47.5
The Consumed Energy and Cost of Three Methods for Drying 1 m

Lumber

Drying Method

Wood Species

Plate Thickness

of Lumber (cm)

Initial Moisture

Content (%)

Final Moisture

Content (%)

Energy

Consumption

(W/m

)

Energy

Cost ($/m

)

Solar

P. Koraensis

5.0

10.5

31.5

Solar-heat pump dehumidifier

P. Strobus

5.0

59.4

Steam

P. Koraiensis

5.0

65.5

Source: Adapted from Zhang et al., 2000.

2006 by Taylor & Francis Group, LLC.

Most heat-sensitive materials often undergo a

freeze-drying process to minimize any quality degrad-
ation that may arise due to temperature effects. Gen-
erally, freeze drying yields the highest quality product
of any dehydration technique (Mujumdar, 2000b).
However, the cost of freeze drying has been found
to be at least one order-of-magnitude higher than
other drying system such as spray dryer.

According to Nijhuis et al. (1996), freeze drying

(known as a suitable dehydration process for pharma-
ceutical and food products) is not suitable for the
production of homogeneous films because the films
obtained are generally very spongy. Also, a freeze-
dried product tends to be porous and the problem of
rapid rehydration may arise once the product is ex-
posed to a more humid environment. Moreover freeze
drying is very energy intensive. The equipment is also
more expensive than atmospheric pressure dryers. It
is best suited for heat-sensitive materials, or when
solvent recovery is required, or if there are risks of
fire and/or explosion.

Maache-Rezzoug et al. (2001) have recommended

a pressure-swing drying mechanism to produce
homogeneous thin sheets. The experiments they con-
ducted recently to dry a collagen gel in order to obtain
a homogeneous film were carried out using a new
process: dehydration by successive decompression.
Their process involves a series of cycles during
which the collagen gel is placed in desiccated air at a
given pressure and then subjected to an instantaneous
(200 ms) pressure drop to a vacuum (7 to 90 kPa).
This procedure is repeated until the desired moisture
is obtained. A comparative study between this new
drying process and conventional methods indicated
that the drying time was reduced from 480 and 700
for vacuum and hot air drying, respectively to 270 min

for the process (Ph

¼ 600 kPa; th ¼ 18 s; pv ¼ 5 kPa,

and tv

¼ 5 s).

Integrating such a pressure-swing system to a heat

pump dryer would significantly improve product
quality, by the use of lower drying temperature, and
at the same time reduce the drying time that would
result in a smaller drying chamber to obtain similar
product throughput.

47.12 AIRFLOW DISTRIBUTION

IN DRYING CHAMBER

The distribution of the drying air within the drying
chamber influences the heat and mass transfer process
between the air and the drying product. It affects the
drying kinetics of the products placed at different
locations in the drying chamber. When the drying
air absorbs moisture from the product, it becomes
gradually saturated. Its potential to remove moisture
from the product placed downstream in the direction
of the airflow diminishes. If the distribution of the
drying air in the drying chamber is not properly con-
sidered then nonuniform drying occurs within the
chamber. Sections of the product that first contact
the conditioned air would be drier than other sections
that interact with a more saturated air. Therefore, it is
essential that the drying air be uniformly distributed
within the drying chamber to minimize the problem
of ‘‘uneven’’ drying within the drying chamber.

Floor racks, sidewall flues, and special ceiling

ducts are common items used to facilitate air circula-
tion. Figure 47.25 shows the example of a cross-flow
air circulation in the drying chamber. The air is first
distributed to a ceiling duct, which discharges the
conditioned air perpendicularly to the product trays.

Product trays

Evaporator

Condenser

Chamber partitions

Centrifugal fan

FIGURE 47.25 Air distribution within the drying chamber.

2006 by Taylor & Francis Group, LLC.

The ceiling duct, wall ducts, and a special floor duct
together form a completely closed envelope around
the drying chamber. The fan draws air through the
evaporator and condenser and discharges into a ceil-
ing duct above the trays. From here, it is distributed
to the product on either sides of the wall. It then goes
into the flow duct, which carries the air back to the
heat exchangers. When the wall dividers are in place,
dividing the chamber into three sections, the air cir-
culation pattern is essentially the same because air-
flow is from ceiling to floor and the wall dividers do
not interfere with the circulation pattern. Further,
heat and mass transfer is enhanced for a cross-flow
pattern when compared with parallel flow. Therefore,
less airflow may be required to achieve the amount of
moisture removal resulting in the saving of operating
fan cost.

47.13 REFRIGERANTS

The heat pump dryer depends on the properties of
CFCs and hydrochlorofluorocarbons (HCFCs) to en-
able: (1) heat recovery; (2) sensible heating and cool-
ing; and (3) dehumidification. CFCs and HCFCs are
widely used throughout the food industry as de-
scribed in Table 47.6. The principal CFC refrigerants
in current use are R502 and R12, while the principal
HCFC in use is R22. It is estimated that the total
usage of these refrigerants in the food sector in
Canada is about 1800 t/year (TCEA, 1995).

Issues of the impact of emissions on the Earth’s

stratosphere and its protective ozone layer has led to
the imminent schedule of CFC and HCFC phase-out.
The phaseout of CFCs (and HCFCs in the long-term)

will force the food industry to evaluate their refriger-
ation systems for refrigerant replacement. The most
important challenge is the need to develop refrigerant
management plans to convert CFCs to new ozone-
friendly refrigerant alternatives. On the flip side, this
challenge presents an opportunity for the food indus-
try to examine the energy efficiency of their refriger-
ation equipment and to introduce new energy savings
methods at the time of CFC (and HCFC) contain-
ment, conversion, or replacement. There are a large
number of alternative refrigerants on the market.
Table 47.7 lists some of these alternatives.

The present direction in refrigerant management

is to look at naturally occurring fluids as potential
replacements. Hydrocarbons and naturally existing
fluids, e.g., propane and ammonia, are among future
environment-friendly refrigerants with zero global
warming and ozone depletion potentials. The heat
pump dryer with hydrocarbon and natural work-
ing fluids can save significant amounts of energy
while playing the role of an environment-friendly
refrigerant (Strfmmen et al., 1999). Strfmmen et al.
(1999) have also compared the performance of several
refrigerants, e.g., R717, R290, R22, and R134a, in
their heat pump dryers. They found that for lower
air temperature, propane has about 3% higher SMER
than ammonia. However, above 10 to 308C ammonia
has higher SMER than R290, R22, and R134a. Am-
monia was found to be the most favorable refrigerant
for heat pump dryers in the temperature range of
30 to 808C.

Other natural working fluids, e.g., steam and air,

have been proposed for heat pumps. Steam and air
are

readily

available,

cheap

and

environment-

friendly. Steam was proposed as a heat pump work-
ing fluid for paper impingement drying (Nassikas

TABLE 47.6
Application of CFC/HCFC in the Food Industry

Application in Food Industry

CFC/HCFC

Drying

R12 and R22

Transport refrigeration

R502

Retail unitary refrigeration–

display/storage

R12, R502, and R22

Retail central refrigeration–

display/storage

R12, R502, and R22

Cold storage

R502

Refrigerated storage

R12 and R22

Refrigerated vending machines

R12 and R22

Source: TCEA, Capitalising on the Energy Saving Opportunities
Presented by CFC and HCFC Phaseout in Nondomestic
Refrigeration, E.A. Technology for the Canadian Electrical
Association, Montreal, 1995.

TABLE 47.7
Some Alternative Refrigerants

Alternative
Refrigerants

Molecular

Weight

Critical

Pressure

(bar)

Critical

Temperature

(8C)

R12 Alternatives
R134A

102.0

40.7

101.1

R409A

97.5

46.0

107.0

R22

86.48

49.9

96.1

R502 Alternatives
R404A

97.6

36.9

72.4

R507

98.9

37.9

76.1

Natural refrigerants
Ammonia (R717)

17.03

111.5

132.4

Propane (R290)

44.10

42.4

96.8

2006 by Taylor & Francis Group, LLC.

et al., 1992). An air cycle heat pump was proposed by
Mckay (1993). He showed that the condenser tem-
perature was as high as 2068C and the evapora-
tor temperature was as low as

408C. This wide

range of operating temperature makes the air cycle
heat pump versatile for freeze drying, hot air drying,
and process heating (Prasertsan and Saen-saby,
1998b).

47.14 VERSATILITY OF HEAT PUMP

DRYING SYSTEM

Commercially

available

heat

pump

dryers

are

designed with both the refrigerating components and
the drying chamber integrated as one complete drying
unit. For such dryers, the required refrigeration cap-
acity of the heat pump to cover the heat duties, both
latent and sensible, is sized according to the size of the
drying chamber and the maximum product loading.
These dryers are inflexible for further scale-up of the
refrigerating equipment to deal with increase in product
drying capacity. This results in the partial or complete
replacement of the refrigerating components. Further-
more, in the event of a component breakdown, the
drying operation has to be terminated, resulting in
downtime of the dryer with significant loss in the
production capacity.

The design of a versatile heat pump system is

proposed here whereby the refrigerating equipments
are assembled exclusively from the air-handling
chamber before they are interfaced through the use
of industrial air couplers. These couplers allow the air
to be received for conditioning and discharge the

conditioned air to the chamber for drying. The con-
ditioning of the air refers to the air temperature,
velocity, and absolute humidity.

The principal advantage of such a versatile heat

pump system is the flexibility to allow the heat pump
components, assembled as a modular unit with port-
able or transportable features, to be coupled and
decoupled to any portable or transportable chamber
for drying and general air-conditioning applications.
Also, this heat pump design enables easy scale-up of
dryers with varying chamber size and configuration to
meet higher production demands. Lastly, easy main-
tenance and repair work can be carried out on the
heat pump components with little or no production
downtime in the drying process, which minimizes
product spoilage in the storage chambers.

The present design comprises:

1. A heat pump system assembled in a modu-

lar unit comprising two internal evaporators,
one external evaporator, one internal conden-
ser, one air-cooled condenser, two expansion
valves, one backpressure-regulating valve, one
external evaporator, one compressor, and one
circulating fan

2. A chamber incorporating two or more ports to

enable air communication between heat pump
and chamber

3. Two or more industrial air couplers to link the

heat pump system to the chamber

Figure 47.26 shows the proposed design of the

heat pump field dryer. This design is unique because
it allows further scale-up of the drying chamber

Containerized drying chamber

Heat pump assembled in chassis

Food trolleys

Air distribution fan

Axial fan

Condenser

Air couplers

Evaporators

FIGURE 47.26 The proposed design of a versatile heat pump field dryer.

2006 by Taylor & Francis Group, LLC.

without changing the position of the coils and other
refrigerating components. For reasons pertaining to
simplicity and cost, a conventional transport con-
tainer can be used as the chamber. The drying cham-
ber is partitioned into two segments by three
partitions. These partitions create a U-channel duct
for the drying air to flow from the ports to the loading
trolleys with product trays before the drying air is
returned to the heat pump by the return ports.
A recirculation fan is installed to provide additional
airflow to the products. The products are first laid in
the product trays with fixed wheels before they are
pushed through the loading door. The product, after
being dried to the desired moisture content, leaves the
drying chamber by the unloading section.

It should be mentioned that the application of this

modular design is not exhaustive, it extends beyond
drying and cold storage applications. The concept of
a coupled and decoupled process, for easy interface
between equipment and air-handling unit, can be
used in many air-conditioning applications through
appropriate selection of expansion valve, compressor,
condensers, evaporators, and refrigerant.

47.15 ECONOMICS OF HEAT PUMP-

ASSISTED DRYING SYSTEM

Potential investors in drying technology need to have
information pertaining to the capital investment, total
operating cost, and breakeven period. Equally im-
portant is the knowledge of the various drying param-
eters that can affect the drying process. Such
information can help them to reduce the operating
cost required to remove per unit of moisture from
the product. The following section examines the
techno-economics of the heat pump dryer.

The total cost of a HPD system is made up of two

kinds of basic costs:

1. Fixed cost elements. These are items unrelated

to the amount of moisture removed from the
product over the years. The dryer cost is
the primary element because once the dryer is
purchased, interest has to be repaid whether or
not the dryer is actually in use. This item also
includes the maximum demand charges for the
electricity supply.

2. Variable costs. These are costs that progres-

sively increase as the dryer operates. Its main
element is the energy used but it could also
include the dryer maintenance element. The
actual cost is dependent on the product of the
energy cost and the effectiveness of the dryer.

Therefore, the equation for total cost is

Total cost of removing one liter of water

total variable cost

þ total fixed cost

total water removed

where
total variable cost

¼ dryer power (kW) running

time (hours)

energy cost ($/kWh)

and total fixed cost

¼ interest or amortization of the

capital cost.

The total cost of removing 1 L of water from the
product can be portrayed in graphical form as shown
in Figure 47.27. It can be readily observed that if the
dryer is operating for only a short period in a year,
say below 2000 h, then the total cost of extracting a
liter of water from the product is critical. However, if
the operating hours are longer at 8000 h/y, the total
cost of removing a liter of water from the product is
significantly lower. This trend is typical of most heat
recovery systems because greater amount of heat is
recovered for longer operating periods. In the case of
the dryer, the direct translation is then an offset of the
operating cost resulting in lower cost for each unit of
water removed.

Several factors are expected to influence the over-

all economic viability of a heat pump dryer. Some
of these are process or design parameters while
others are economic parameters. Zylla et al. (1982)
in evaluating the potential for heat pumps in drying

Capital cost of drying plant

£400/kW

£200/kW

£100/kW

Total cost per liter of water extracted (£)

8000

4000

Running cost of dryer (h)

FIGURE 47.27 Total cost of drying. Assumption: effective-
ness

¼ 1 l water/kWh. (Adapted from Brundrett, G. W.,

Handbook of Dehumidification Technology, Butterworths,
London, 1987.)

2006 by Taylor & Francis Group, LLC.

systems found that he at pum p dryers can reduce the
operati ng cost as much as 70% in compari son to a
recuperat ive heatin g dryer. Pendy ala et al. (1986)
have fou nd that for typical drying conditio ns with
air tempe ratur e and relative humidi ty in the respect -
ive range of 25 to 65 8 C a nd 40 to 100%, a heat pump
dryer de signed for a drying cap acity of 200 kg/h has a
payback period of 2 to 3 y. Judging from the humidi ty
of the exh aust air, it was furt her observed that pa y-
back period can be shorte ned from 3.2 to 2.0 y if
relative humidi ty of exh aust air increases from 0.4 to
0.7. It can then be dedu ced that the payb ack period
for initial investment is general ly reduced if more
produc t mois ture is avail able for heat recover y.
Also, the payback period is sensitiv e to the operating
pressur e of the evapo rator and cond enser. The min-
imum effecti veness factor sh ould be more than 0.55 if
the pa yback period is less than 3 y.

47.16 FUTURE TRENDS IN HEAT PUMP

DRYING—MULTIPLE DRYERS

Althou gh not repo rted heretof ore, it is possibl e to
design a HP D system that uses a singl e low capacity
HP to supply drying air to severa l different ch ambers
accordi ng to a preprogr amme d schedule. This is feas-
ible be cause many food prod ucts ha ve long falling rate
periods . Ther e a re severa l ad vantage s of operati ng the
heat pump with mult iple drying chamb ers. They are:

Improv ed qua lity of products such as su rface
color a nd reduced case hardening

Improv ed energy effici ency with more laten t
load for heat recover y

Reduced capital cost and floor space requirement

Easy tempe ratur e schedule co ntrol for diff erent
produc ts in diff erent drying chambers

W hen only a marginal amou nt of convecti on air is

needed to evap orate moisture, the drying chamb ers
are operate d in sequence. The air from the heat pump
can be direct ed sequ entially to tw o or more cha mbers
or can be divide d accord ing to a preset schedule to two
or more drying chambers , whic h may dry the same or
different products . Thus , the heat pump can be ope r-
ated at near optimal level at all tim es. Even if the
drying tim es for each chamber may increa se due to
the inter mittent he at input, the overal l econ omics
should improve considerably. A smaller HP can
double or triple the drying capacity, especially with
the help of supplementary heating by IR, MW, or RF.

Figure 47.28 shows a schematic diagram of a

multiple-chamber HPD process. When the drying rate
of the product approaches the second falling rate, the
drying air is channeled to the second chamber to dry
the freshly changed product of higher moisture con-
tent. Auxiliary heating may then be used to provide
the thermal requir ement for dr ying.

Figu re 47.29

shows the scheduling sequence. Such a scheduling
enables reduction in product temperature, resulting

Heat pump

Dryer I

Dryer III

Dryer II

Auxiliary
heating

Axial fan

Air damper

Air exhaust back

to heat pump

FIGURE 47.28 Schematic layout of multiple-chambers heat pump drying process.

2006 by Taylor & Francis Group, LLC.

in improved quality and reduces case hardening. Im-
proved energy efficiency of the heat pump system is
also expected because higher latent loading in the
second chamber in the form of product moisture is
available for heat recovery.

From an economic perspective, the most attractive

aspect of multiple-chambers HPD is the reduction in
capital cost, because one heat pump system is capable
of accomplishing the drying task of two or more sep-
arate HPD units. Further, a control strategy can be
easily implemented through control of air dampers.

47.17 CONCLUSION

HPD has evolved to become a mature technology
over the last two decades. However, it is not applied

as widely as it should or could be. Initial costs as well
as operating costs remain a problem, since few major
vendors of drying systems offer HPD systems. Effi-
cient use of energy in such energy-intensive oper-
ations as drying is crucial to the reduction of net
energy consumption and hence emissions of green-
house gas. With the eventual acceptance of a
carbon/energy tax around the world energy, conser-
vation will become a key concern in many industrial
operations. Heat pumps appear to have attracted
interest as a means of energy recovery since this idea
was first proposed by Lord Kelvin in 1852. Commer-
cial heat pumps based on the vapor compression cycle
or the absorption cycle are operational in numerous
applications. New HP technologies such as the ad-
sorption cycle or the chemical reaction cycle are emer-
ging rapidly, although they have yet to find major

Product temperature,

⬚C

Drying time, min

Constant rate

falling rate

2nd

falling rate

Cut-off HPD

Auxiliary heating bank activated

Drying time, min

Dryer II

Drying time, min

Dryer I

Drying time, min

Dryer III

Air temperature,

⬚C

Air temperature,

⬚C

Air temperature,

⬚C

Drying time, min

Drying rate

min

kg / kg

FIGURE 47.29 Sequence for multiple heat pump batch drying.

2006 by Taylor & Francis Group, LLC.

industrial applications. In future, the need to develop
cost-effective heat pumps using environment-friendly
refrigerants (e.g., CO

and NH

) to replace CFCs

will also impact HPD systems. With more versatile
designs of heat pump dryers in the pipeline, their
applications can be extended to include other air-
conditioning applications, resulting in effective in-
vestment of capital costs. While enough is known
about heat pumps and various dryers, optimal inte-
gration of the technologies remains a challenging
R&D task. It is hoped that more attention will be
paid to this development in the coming decade.

ACKNOWLEDGMENT

The authors wish to acknowledge the contributions
made by Dr. Arun Sadashiv Mujumdar, Dr. Ho Juay
Choy, and Dr. Mohammad Nurul Alam Hawlader in
the writing of this book chapter on HPD.

NOMENCLATURE

(COP)

actual coefficient of performance

enthalpy of drying air, J/kg dry air

heat of evaporation, J/kg K

HGC

hot gas condenser

heat pump

HPD

heat pump drying

HPE

high pressure evaporator

low pressure evaporator

mass flow rate of drying air, kg/s

PID

Proportional–Integral–Differential

power, KW

energy, kJ

SC1

subcooler 1

SC2

subcooler 2

SEC

specific energy consumption, kWh/kg mois-
ture

SMER

specific moisture extraction rate, kg mois-
ture/kWh

temperature, 8C

REEK

YMBOL

intermittency cycle ratio

humidity ratio of air, kg water/kg dry air

profile periodic time, min

UBSCRIPTS

100%

saturation level

actual

condenser

evaporator

off

nonheating period

active heating period

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Document Outline

Table of Contents
Chapter 047: Heat Pump Drying Systems

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