Systems of air conditioning using fan coil units, induction units or chilled ceilings require a supply of chilled water throughout the year. As an alternative to operating a vapour compression system in the conventional manner during the winter, it is possible to switch the compressor off and to rely on a natural circulation of refrigerant between the evaporator and the condenser, under favourable operating conditions, the performance then being like that of one form of heat pipe. See Figure 9.9. The condenser is located above the evaporator and the compressor and expansion valve are temporarily by-passed. The water being chilled by the evaporator causes the liquid refrigerant to evaporate and migrate upwards to the condenser, under the pressure difference corresponding to the higher temperature in the evaporator (water chiller) and the lower temperature in the condenser. The vapour is condensed by cold airflow or waterflow over the outer heat transfer surface of the condenser and the liquid refrigerant flows by gravity back to the evaporator. The three-port, two — position valves at A and В must be switched to the appropriate positions.
Thermosyphon cooling was first developed in 1984, according to Pearson (1990) and
Liquid refrigerant returns to the evaporator by gravity
Evaporator (water chiller)
Fig. 9.9 Diagrammatic arrangement to achieve thermosyphon cooling. The broken line shows the path followed when normal vapour compression is taking place.
Blackhurst (1999), and has been successfully used with conventional refrigerants. Since 1997 it has also been applied for use with ammonia as the refrigerant, essential attention being paid to safety, in accordance with BS 4434, 1989.
Pearson (1998) points out that heat transfer at the surfaces of cupro-nickel tubes can be enhanced by providing a coating that increases the number of nucleation sites on the surface. It is claimed that overall heat transfer can be increased by a factor of over three. When properly designed, installed and commissioned, thermosyphon cooling provides a very efficient way of operating refrigeration plant in air conditioning systems. The compressor does not run when the local weather conditions allow free cooling and the changeover from using the compressor to free cooling can be automatic.
Posted in Air Conditioning Engineering