A common accessory used on many refrigeration systems is the liquid receiver. It is basically a storage vessel designed to hold excess refrigerant not in circulation. Refrigeration systems exposed to varying heat loads, or systems utilizing a condenser flooding valve to maintain a minimum head pressure during low ambient temperatures, will need a receiver to store excess refrigerant.
Liquid receivers are installed in the liquid line as close as possible to the outlet of the condenser. The piping between the condenser and the receiver should be arranged to allow free drainage. The piping should also not cause excessive friction pressure loss or gas binding and must have adequately sized valves and connection fittings.
The location of the receiver should not cause excessive heat to be added to the refrigerant, such as from direct solar radiation when located outdoors or near building heating equipment when installed indoors. Excess heat added to a receiver will reduce the operating efficiency of the system. However, if the receiver is installed outdoors and the system is required to operate during low ambient temperatures, it may be necessary to install trace heaters to maintain adequate pressure in the receiver in order to avoid system problems at startup.
A receiver’s storage capacity is based on 80 percent of its internal volume at a refrigerant temperature of 90°F per ARI Standard 495. Generally, a receiver is selected to hold 90 percent of the total system charge to provide adequate reservoir during high loads and to allow the refrigerant to be isolated between the condenser and the receiver during repairs.
A receiver generally has a service valve installed at its outlet, typically referred to as the king valve. A receiver will sometimes also have a service valve at its inlet. The king valve is very useful for service technicians as it allows them to read the system’s pressure at the receiver and is the valve used to trap the system’s refrigerant charge in the condenser and receiver during repairs.
A receiver may also have some type of relief valve installed, such as a fusible plug, ruptured disc, or a pressure relief valve. This provides the controlled release of abnormally high pressure within the receiver before that pressure causes an uncontrolled eruption of the refrigerant from the receiver or another part of the system.
A common question asked when discussing receivers is, “How can the refrigerant leave the receiver in a subcooled state?” That answer is because in the receiver, there is both liquid and vapor refrigerant. By definition, that refers to a saturated refrigerant.
So, is the refrigerant in the receiver saturated or subcooled? The answer to this question is, both. At the liquid vapor interface — where the liquid and vapor are in contact — the refrigerant is saturated, but the refrigerant below the interface can exist at a lower temperature. And, since the refrigerant’s pressure is constant, the refrigerant below the interface is subcooled. Normally, the refrigerant leaving the receiver is picked up below the interface toward the bottom of the receiver, allowing the refrigerant to leave in a subcooled state.
Publication date: 12/1/2014