EVAPORATIVE COOLING SYSTEMS (CT + BAC)

Cooling systems utilising water evaporation are usually the most cost-effective heat rejection systems. In a cooling tower hot water from a plant is brought into contact with ambient air and heat is transferred from the water to the air by evaporation (mass transfer) and convection (heat transfer). The cooled water is then returned to the plant. Generally, evaporative cooling systems can achieve lower re-cooled water temperatures and a lower condenser vacuum than dry cooling systems. In cooling tower terminology, range is the difference between the water inlet temperature and the re-cooled water outlet temperature. The difference between the re-cooled water temperature and the ambient air wet bulb temperature is known as the approach. Generally a cooling tower becomes very large if the approach is reduced below 4 °C. Evaporation rates are approximately 1.5 to 2 m³/hr per MW of heat rejection or 1 to 2% of flow rate. In order to keep the dissolved solids in the circulating water at an acceptable level, a fraction of the circulating water is discarded, i.e. the blowdown stream. A stream of fresh makeup water is required to replace the water lost by evaporation and blowdown. Main activities in evaporative cooling field: Design and construction of new turnkey evaporative cooling systems (custom built units), including pipework, valves, pumps, intermediate heat exchangers, variable speed drives, sound attenuators, etc. Cooling tower inspections Cooling tower performance testing and system evaluation Improvement/modification of existing systems Cooling tower repacking and refurbishment Maintenance. MECHANICAL DRAFT COOLING TOWERS (MDCT) Mechanical draft cooling towers make use of a fan to generate the air-flow through the tower. Depending on the cooling load, multiple cells, each with its own fan, can be employed. Fan diameters of up to 30 ft are commonly used. The relative flow direction of the water and the air streams can be arranged in either cross or counterflow. Generally, counterflow cooling towers are preferred nowadays, due to their higher thermal efficiency. For a given duty, the initial cost of mechanical draft cooling towers is lower than that of natural draft cooling towers. Mechanical draft cooling tower   NATURAL DRAFT COOLING TOWERS (NDCT) In a natural draft cooling tower, the natural buoyancy of the hot air moves the air upward through the tower, drawing in fresh cool air through the air inlet at ground level. No fan is required. Due to the layout of this type of cooling tower the possibility of hot air recirculation (and the resultant performance drop) is negligible. The tower shell is usually constructed in reinforced concrete, and can be as high as 200 m. Due to the high cost of the large concrete structures, natural draft cooling towers are usually only employed for large heat duties. Natural draft cooling tower   OTHER EVAPORATIVE COOLING SYSTEMS In South African mines bulk air coolers (BAC's) are often employed to provide large quantities of cold air for mine ventilation. Basically, a bulk air cooler is an inverted mechanical draft cooling tower in which cold water from a refrigeration unit is used to cool the incoming air and to provide a stream of cold air into the mine. Bulk air coolers are constructed in crossflow and counterflow layouts. Crossflow units are usually used underground where the available height is restricted. Closed circuit evaporative coolers and condensers employ heat exchanger bundles inside the cooling tower. This allows the cooling of any liquid by means of evaporative cooling. It is also possible to achieve this by using an intermediate heat exchanger, such as a plate heat exchanger or shell and tube heat exchanger in conjunction with an evaporative cooling tower. GEA is continuously improving the quality and the cost-effectiveness of its evaporative cooling products. With the dramatic improvement in the quality of plastic materials, we have in the recent past replaced nearly all our cooling tower components with plastic components, which have proved to be superior to those used previously. All these improvements have enabled us to supply our clients with cooling towers, which will not only require less maintenance, but which will also have a longer operational life. GEA Aircooled Systems recently developed a new low-cost pre-cast concrete panel type cooling tower (PTCT) as part of its drive to offer our clients increased quality at reduced prices.

Mechanical draft cooling tower at Eskom's Majuba Power Station.       Mechanical draft cooling tower at Belle Vue Power Station - Mauritius.

Features of PTCT: 2.1 m wide pre-stressed, pre-cast concrete panels Pre-cast concrete roof slab Pre-cast concrete or GRP fan stacks Cable tensioning after assembly results in a very rigid structure compared to other pre-cast concrete cooling towers. Advantages of PTCT: Low cost Rapid construction No civil work on site except for floor slab and ground works, for fast-track projects Can be disassembled and relocated High grade concrete (60 MPa) Very rigid structure compared to other pre-cast concrete cooling towers.

Pre-cast concrete panel type cooling tower (PTCT)

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