Heater and Coolers in Ventilation Systems

Classification, basic equations of heat transfer and selecting criteria of heaters and coolers

Classification of heater and coolers

It is often common to classify both heaters and coolers by the medium used to provide or remove the energy: 

• water heated

• steam heated

• electrical heated

• water cooled

• brine cooled

• evaporative medium cooled

Basic Heating Equations 

Heating Air 

The basic equation expressing the heating process of the air in an ventilation heater can be expressed as: 

H = ρ c_p q_v (t_o - t_i)  (1)

where

H = heat (W)

ρ = air density   (kg/m³)

c_p = air specific capacity (J/kg^oC)

q_v = volume flow air (m³/s)

t_o = temperature out of the heater (^oC)

t_i = air temperature into the heater (^oC) 

Heat Transfer through Exchanger Surface

The heat transfer through a heat exchanger surface can be expressed as:

H = A dt_m U    (2)

where

A = heat surface  (m²)

dt_m = logarithmic mean temperature difference (^oC)

U = heat transmission coefficient (W/m²K) 

The heat transmission coefficient - U - depends on the air speed and the water (fluid) speed through the heater. 

Water Heater

The heating process on the water (fluid) side of the heat exchanger can be expressed as

H =  ρ_w c_pw q_w (t_wi - t_wo)  (3)

where

ρ_w = density water (kg/m³) 

c_pw = water specific capacity (J/kg^oC)

q_w = water flow (kg/s)

t_wi =  temperature water to the heater (^oC)

t_wo = temperature water from the heater (^oC) 

Steam Heater

If saturated steam is used to heat the air, the heating process can be expressed as:

H = q_s h_e   (4)

where

q_s = steam flow  (kg/s)

h_e  = evaporating enthalpy water (J/kg)

 Selecting Heaters

Kind of heater	Criteria
Water heated	increase of temperature (^oC) air volume through the heater (m^3/s) air speed through the heater (front area) between 2 to 5 m/s water inlet temperature (^oC) water speed (m/s). Copper pipes 0.2 - 1.5 m/s. Steel pipes 0.2 - 3 m/s. maximum running and test pressure (Pa) maximum running temperature (^oC) material properties for pipes and lamellas horizontal or vertical installation
Steam heated	increase of temperature (^oC) air volume through the heater (m^3/s) air speed through the heater (front area) between 2 to 5 m/s steam temperature (^oC) steam pressure (Pa) steam properties (overheated or saturated) maximum running and test pressure (Pa) maximum running temperature (^oC) material properties for pipes and lamellas horizontal or vertical installation (Note! Remember condensate drain)
Electrical heated	increase of temperature (^oC) air volume through the heater (m^3/s) air speed through the heater (front area) according manufactures recommendation voltage (V) control system (on/off, step or modulating) safety switches according local rules (in general one safety switch at 110^oC for manual reset and one safety switch for automatic reset at 65^oC)  material properties for pipes and lamellas horizontal or vertical installation

Cooling Air 

When coolers are calculated, it is important to know if the surfaces are dry or wet.

• If the cooling medium has a temperature below the dew point temperature of the air - the surface may be wet

• If the cooling medium has a temperature above the dew point temperature of the air - the surface is always dry

Dry Surface Cooling

For a cooler with a dry surface the same equation used for the heating process (1) may be modified and used: 

H_c = ρ c_p q_v (t_i - t_o)  (5)

where

H_c = heat removed from air (W)

ρ = air density   (kg/m³)

c_p = air specific capacity (J/kg^oC)

q_v = volume flow air (m³/s)

t_o = temperature out of the cooler (^oC)

t_i = air temperature in to the cooler (^oC) 

Wet Surface Cooling

For coolers with wet surfaces, the water vapor in the air is condensed and it is necessary to use the enthalpy of air in the calculations:

H_c = ρ q_v (h_i - h_o)  (6)

where

h_o = enthalpy in the air out from the cooler (kJ/kg)

h_i = enthalpy in the air into the cooler (kJ/kg) 

Water (fluid) Flow through the Cooler

The water flow cooling process can be calculated with a modified formula similar as for the heating process.

Selecting Coolers

Kind of cooler

Criteria

decrease of air temperature and enthalpy in the cooler (^oC) air volume through the cooler (m^3/s) air speed through the cooler (front area) between 2 to 3 m/s. With speed above 3 m/s a drop separator should be installed. water inlet temperature (^oC) or evaporating temperature for the cooling medium water speed (m/s). Copper pipes 0.2 - 2.0 m/s properties for the cooling medium maximum running and test pressure (Pa) maximum running temperature (^oC) material properties for pipes and lamellas horizontal or vertical installation (Note! Wet surfaces must be drained)

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