Air and other noncondensible gases may enter a system through leaks in seals, gaskets, or uncapped valves. Air may also be present because of imperfect evacuation before the initial charging of the system or due to impurities in the refrigerant or oil. Another way that air gains access to the system is when an evaporator coil or a compressor is opened. Air could be drawn into the system through leaks in the low-pressure portion of the system when operating with refrigerant pressures below atmospheric, which will occur at evaporating temperatures shown in Table 7.3. Air drawn into the system on the low-pressure side is eventually pumped to the condenser where the liquid seal prevents it from traveling further.
The presence of noncondensables in condensers penalizes the system performance through the artificial elevation of the condensing pressure. As pictured in Fig. 7.22, the noncondensables add their partial pressure to that of the refrigerant vapor and thus increase the pressure against which the compressor must work. A further penalty is the reduction in the heat-transfer coefficient by requiring the refrigerant to diffuse through the noncondensables on its way to the tube surface where it condenses.
A test of the need for purging is to compare the actual pressure to the saturation pressure at the temperature of liquid at a location where liquid and vapor are in equilibrium, such as in the receiver of Fig. 7.22. If the actual pressure p is significantly higher than the saturation pressure corresponding to t, purging is warranted. Purging may be performed on rare occasions in small systems, but is often done frequently by automatic purgers on large systems. These automatic purgers proceed from one purge point to another to extract gas.
There are preferable locations for purging, and basically these are (1) on the high-pressure side of the system, (2) where only vapor exists, and (3) where the vapor velocity is low. Air at a given pressure and temperature is more dense than ammonia, and not as dense as the halocarbon refrigerants, but no appreciable settling of one of the constituents can be aniticipated. The air diffuses quite uniformly throughout slow-moving refrigerant.
The three principal concepts available for purging are
– direct venting of the air-refrigerant mixture
– compression of the mixture, condensing as much as possible of the refrigerant, and venting the vapor mixture that is now rich in noncondensables
– condensation of refrigerant using a small evaporator, followed by venting of the air-refrigerant mixture
Figure 7.23a shows the first method, a primitive, manual technique. Vapor is released from a high-pressure vessel, such as the receiver, and this vapor is mostly refrigerant but also contains a small amount of the noncondensables that are the target. In the case of ammonia, the discharge bubbles through a container of water to absorb the ammonia. As venting proceeds, more refrigerant liquid vaporizes, so the concentration of noncondensables decreases, but never drops to zero. This method wastes considerable refrigerant to expel a small amount of noncondensables.
The second method of purging, as shown in Figure 7.23b, consists of drawing a sample from the vessel with a small compressor that elevates the pressure and condenses some refrigerant on a water-cooled coil. The vapor vented from this after-condenser is higher in noncondensable content than at the original sampling position. This purging concept is widely applied in centrifugalcompressor water-chiller systems using such low-pressure refrigerants as R-123, but sees limited application in industrial refrigeration.
The third concept in purging (Figure 7.24), which is widely used in industrial refrigeration, avoids the need of a separate compressor, and instead uses a low temperature developed in a small evaporator. The air- refrigerant mixture from the condenser or receiver bubbles through cold liquid and condenses most of the refrigerant. This concept is embodied in automatic purgers which move from one purge point to another allowing enough time at each for a satisfactory purge. Commercial models of refrigerated purgers employ refinements in handling the vented stream leaving the after- condenser. The proper procedure is to purge one point at a time, because if one solenoid control valve serves two or more purge points, the pressure at these positions will be equalized during purging. In the later sections of this chapter, the need to properly regulate pressure differentials will be emphasized.
A manual technique is sometimes used for a massive purge that would require a long time for automatic purgers to handle. The method applies to a multiple-coil condenser, such as shown in Fig. 7.25, that is equipped with individual valves from the discharge gas header and individual vents to atmosphere. If one coil is to be purged, the valve in the gas line is closed, but its fan and spray water continue to operate. Other parallel coils continue to operate normally, so the temperature in the coil being purged drops and ammonia vapor in the coil condenses. As the vapor condenses, the vapor volume decreases and liquid is drawn from the condensate line. After a period of time most of the ammonia vapor has condensed, leaving the small volume of mostly noncondensables which can be vented to the atmosphere or to a vessel of water.