Basically, there are two ways to purge a system of air: manual or automatic. To purge manually, a properly positioned valve is opened by hand, allowing the air to escape. It is a common misconception that when a cloud of refrigerant gas is seen being discharged to atmosphere, the system has been purged of air. Air can still be trapped in the system.
Therefore, many refrigeration system users prefer automatic purging. Refrigeration systems include the compressor, condenser, receiver, evaporator and purger (see Figure 3.35). Of these components, the purger is perhaps the least understood and appreciated. The purger’s job is to remove air from the system, thus improving compressor and condenser operating efficiency.
Two types of automatic purgers are used as follows : (i) nonelectrical mechanical and (ii) automatic electronic purgers. Determining the type of automatic purger to use is a matter of whether electricity is available at the purger location and if it safe to allow electrical components to be used. The nonelectrical mechanical units are used primarily in applications where electricity is not available at the point of use or in hazardous applications where electric components are not allowed. They remove air by sensing the density difference between the liquid refrigerant and gases. An operator opens and closes valves to start and stop the purging operation and ensure its efficiency.
Electronic automatic refrigeration purgers are classed as single-point and multipoint purgers. The single-point electronic refrigerated purger has a mechanical-purge operation with a temperature/gas level monitor that controls the discharge to atmosphere. The purging sequence is performed manually. A multipoint refrigerated purger will purge a number of points using the same unit. However, each purge point is purged individually, and the multipoint purger offers total automation, including startup, shutdown and alarm features. With this purger, it is important to choose a purger designed for the total tonnage of your system. Undersized purgers may cost less initially but may adversely impact the system’s efficiencies and payback period. Some multipoint purgers include a microprocessor-based programmable controller rather than a clock timer. The fuzzy logic controller can ‘learn’ as it cycles through the system. As the purger accumulates air and purges, the controller records and prioritizes each purge point in its memory, thus removing air more efficiently.