An economizer is a type of sub-cooler that uses part of the total refrigerant flow from the condenser to cool the rest of the refrigerant flow (see Figure 10.13). The evaporated refrigerant then enters the compressor at an intermediate pressure level. The cold gas from the economizer can also be used to provide extra cooling for the compressor.
The sub-cooling of the main refrigerant flow (m2) reduces the quality of the inlet vapor to the evaporator, which increases the cooling capacity. In addition, the work required by the compressor is decreased due to the lower flow rate in the first compression step (g) to (f) in the figure as well as the mixing of the flows in the intermediate step. The saved energy is illustrated by the shaded area in the figure. The high efficiency of the SWEP brazed plate heat exchanger economizer minimizes the required temperature difference between the sub-cooling and the evaporating streams, which in turn increases the overall efficiency of the system.
Economizer systems require extra components, such as piping and a compressor with an "economizer" entrance. The additional capital expense makes this system solution viable only for large refrigeration systems.