what is the reasoning to install a liquid suction heat exchanger on a chiller system?
There are several reason for employing a liquid to suction heat exchanger. For instance, the efficiency of the refrigeration cycle will be partially improved due to increase in sub-cooling. This is especially true for low temperature applicaitons.The evaporator performance (or the temperature approach) will be improved due to partial/complete elimination of super heat.Flashing at expansion valve will be eliminated to additional sub-cooling for systems with low condenser sub-cooling.Due to additional super heat, liquid droplets from the evaporator (if they exist) can be eliminated, thus improving the compressor saftey. However, when relatively large liquid suction heat exchangers are used the volumetric efficiency of the compressor needs to be considered due to lower gas density. Issues with system control exist depending on where the super heat measurement is performed (i.e, before or after the liquid - suction heat exchanger)
The primary reason is to reduce the amount of refrigerant needed to meet the chiller duty. The colder the liquid refrigerant is the greater the latent heat of vaporization per pound of refrigerant circulated. You can see this if you look at a Mollier diagram for the refrigerant in question. You can also see it if you have a table of refrigerant properties at various temperatures. For very large refrigerant systems is it common to use a different approach. The liquid refrigerant is flashed at a pressure approximately half way between the vapor pressure and the chiller pressure. The vapors from the flash stage are directed to the interstage of a two stage compressor and the liquid is then fed to the chiller.
