The high pressure receiver is a vessel that is used to store liquid ammonia refrigerant. More than 98% of refrigeration systems rely on the liquid from the condenser to flow freely to the receiver by gravity. With proper piping design between the condenser and receiver it will assist in the removal of liquid from the condenser maintaining as much condensing surface as possible. The more surface available for condensing, the higher the efficiency will be. The direct result is lower head pressures and a savings of energy.
In a typical industrial refrigeration system the load variations can be fairly great. The high pressure receiver should be sized to handle the pump down of the entire system. Typically, good engineering calls for the receiver to be no more than 85% full when the system is totally pumped down. This allows for hydraulic liquid expansion and reduces that chance of a release.
Systems that have flooded accumulators will usually experience a lower liquid level in the high pressure receiver under light loads. During heavier loads much of the liquid refrigerant in these systems is displaced be refrigerant vapor. As less liquid is required in the coils more ends up back in the receiver. For an unsuspecting operator this may be counter to what he would expect.
The high pressure receiver is an ASME coded vessel that is pressure tested and registered with the National Board. Typically, the working pressure of most high pressure receivers is 250 psig. This value is well below typical operating levels of 90 to 185 psig.
The high pressure receiver is usually equipped with the following accessories:
Dual Relief Assembly
Each receiver must be equipped with a dual relief assembly with a relief pressure setting no greater than the nameplate working pressure of the vessel. The assembly consists of a three-way valve with two relief valves mounted on top. Only one of these valves is supposed to be in operation at any given time. With the stem turned all of the way "in" the relief valve closest to the stem will be in service to protect the vessel from rupturing. If this valve blows it may not reseat and the stem can be turned all the way "out" to put the other relief online.
Gage Glass Column
The receiver has been equipped with a gage glass to determine the quantity of ammonia in the receiver. Typically, these liquid level indicators are liquid level "bulls eyes" with a reflex lens or an armored glass assembly. Caution should always be taken when working around any of these liquid indicators as a breakage of any of the glass parts can cause a disaster. Isolation valves have been installed to allow the operator to isolate this part of the system. It is good practice for the operator to run the system with the bottom valve opened no more than about two turns. The bottom valve should always be the one shut off first in the event of a leak or ruptured to stop the flow of liquid. A purge valve should be installed on the column to remove pressure on the column before repairs are made.
This valve is located near the high pressure receiver. Typically, systems that have a charge greater than 1,000 lbs of ammonia will have the first charge brought in by a special tanker truck that will pump the ammonia into the receiver even if the pressure is greater in the receiver than the truck.
The King Valve is the main shut off valve feeding liquid ammonia from the high pressure receiver to the system. In the event that the flow of liquid needs to be stopped for an emergency or for pump down purposes, this valve may be closed by the operator to stop the flow. This valve must be labeled so that the Fire Department, Hazmat or qualified operating personnel can easily identify it.
This valve is used to isolate the high pressure receiver from the condenser system. Its only purpose is to provide the operator with a single point of closure to isolate the condenser from the receiver. This valve is often closed after pump down of the system to prevent the migration of gas back into the discharge headers of the compressors and other traps in the high side system. If this valve is located above the receiver, the only liquid left in the system will be in the “P” traps at the condenser or the thermosyphon vessel if the system is equipped with this unit.
Oil Drain Valve
This valve is located at the bottom of the receiver at the lowest point of this part of the system. To keep the inside clean and dry it should remain plugged until it is necessary to remove excessive compressor oil from this vessel. The oil removal process should only be done by a qualified and experienced operator or service mechanic. Upon completion, this valve should be closed and the plug replaced. Used compressor oil is considered to be a contaminant and must be disposed of as required by the EPA and other related agencies.