The ‘fine line’ of heat rejection

Selection of heat rejection equipment has traditionally entailed a choice between the higher energy consumption of an air-cooled solution, and the high water consumption of a water-cooled solution. This paper examines advancement in heat rejection technology and the way it can be applied to air conditioning and refrigeration plant in healthcare and other facilities. It also examines field difficulties encountered in pipework design as the knowledge and experience levels of engineers designing systems with remote condensers diminish. With plant larger than 1,000 kW, the only option previously has been watercooled solutions using an array of cooling towers, or perhaps an evaporative condenser, since air-cooled plant involved massive volumes of chemical refrigerant, which posed a problem ecologically. An additional hurdle was problems associated with limitations on pipe lengths for refrigeration plant. The advent of adiabatically pre-cooled closed circuit coolers and air-cooled condensers has introduced an alternative to cooling towers that offers the potential for “water-cooled performance” from an air-cooled solution with no serious threat of Legionella contamination. However, each application needs to be considered on a case-by-case basis. The paper examines, in detail, the impact of adiabatic pre-cooling, with recent examples of its application in sub-tropical Brisbane providing evidence of the potential performance achievable.

It is important to understand what heat rejection is before discussing the selection of equipment or piping it in the field. Heat rejection is the energy removed from a refrigerant in the condensing process. Hot gaseous refrigerant enters the condenser, where it loses its latent heat of evaporation to become hot liquid refrigerant. The process occurs regardless of the method used to absorb the heat rejected. In typical terms the heat rejection is some 18% to 28% greater than the cooling effect in the evaporator, as it includes the heat of compression, which is the energy input from the compressor motor. The actual percentage that occurs depends upon a number of factors, including the suction pressure/ temperature, the discharge pressure/ temperature, and the refrigerant used. However, the main factor is pressure/ temperature. The relationship between temperature and pressure in any refrigerant is proportional, and can be read from any chart provided by refrigerant suppliers, or from textbooks. Low suction temperature and high discharge temperature increase the percentage. As an example: heat of compression taken from the catalogue of a 12 cylinder reciprocating compressor using HCFC22 refrigerant is as shown in Table 1. Standard selection for air conditioning is usually based upon 4.4°C saturated suction in all cases, and 40.6°C saturated discharge with heat rejection through a cooling tower, or between 10°K and 15°K above critical ambient temperature for an air-cooled solution. Obviously the closer to a 10°K rise, the better the system will perform in extremes of ambient temperature, and the energy efficiency will be greater. The selection of refrigerant, or the type of compressor, have little impact on the percentage result.

Superheat and sub-cooling