The condenser normally drains its liquid refrigerant into the high-pressure receiver, which is a vessel providing storage space for the condensate. The considerations that surround the condenser/receiver subsystem are:
– piping arrangements of the receiver
– connecting the vapor space in the receiver to vapor locations in the condenser
– the role of refrigerant pressure drop in the condenser
The two typical piping configurations of the receiver are shown in Figure 7.27. Figure 7.27a shows the top inlet or mixing-type receiver where all the condensate passes through the receiver. Usually the outlet of this receiver is as shown, with the tip of the outlet line not quite touching the bottom of the receiver. This construction avoids passing solid contaminants on to the system.
In the bottom inlet receiver of Figure 7.27b, most of the liquid passes directly to the evaporators. The only liquid flow in or out of the receiver is associated with the rise or fall in liquid level. The liquid level may shift because of transient differences in the condensation rate and the rate of liquid flow to the evaporators. Some designers prefer the bottom inlet receiver over the top inlet for two reasons: (1) the ability to use subcooled liquid when it is available from the condenser, and (2) inherent trapping of the liquid line which, as explained later, is a necessity for good drainage in multiple condenser installations. In the top inlet receiver of Figure 7.27a, the liquid and vapor are in equilibrium, so saturated liquid passes on to the low side of the system. Even if subcooled liquid enters the receiver, it will quickly assume the temperature of the stored liquid—a temperature heavily influenced by the machine room temperature. Because the machine room temperature will be higher in most cases than that of the liquid coming from the condenser, the available subcooling is lost in the top inlet receiver.
One of the considerations mentioned above is to provide a connection from the vapor in the receiver to vapor space in the condenser. The liquid level in the receiver of an industrial refrigeration system is almost constantly rising or falling because liquid flow rate from the condenser is not precisely the same as the rate demanded of the low side of the system. When an excess flow enters the receiver, there is a tendency to compress the vapor, which builds up the pressure in the receiver and temporarily restricts the flow of condensate from the condenser. If the flow rate demanded by the system exceeds temporarily the rate provided by the condenser, the pressure in the receiver drops and some of the liquid vaporizes. The requirement of a connection between the vapor in the receiver with vapor in the condenser will be a thread running through recommended procedures for drain piping that will be presented in the next