A-A-54525A
2.7 Refrigeration systems.
2.7.1 The freezer shall be equipped with two complete, independent, cascaded, air cooled, refrigeration systems (4 motor/compressor combinations). Each cascaded system shall contain a high stage and a low stage. The evaporator of each low stage refrigeration system shall contain its own copper tube circuit, cooling all four sides of the chamber (and the bottom of the chamber if copper tube circuit is also used on the bottom). Each circuit shall be designed to uniformly cool the chamber; therefore, regardless of which cascade refrigeration system is operating, the chamber shall be uniformly cooled. Each cascaded system shall have an “on/off” switch. A key switch shall also be provided to enable start-up of refrigeration systems.
2.7.2 The four freezer compressors shall be of the reciprocating accessible semehermetic type; each compressor shall be rated a minimum of one horsepower. Oil level sight glasses and brass service valves shall be provided in order to visually observe oil level, and facilitate the use of gauges and the addition of refrigerants during servicing. Non-flammable refrigerants shall be used in the refrigeration systems. The four motors driving the compressors shall operate at 1750 rpm and shall be rated at least one horsepower each.
2.7.3 The refrigeration system shall have a minimum net thermal capacity of 3,830 BTU/HR empty at -121°F(-85°C) with both cascade systems operating in a temperature of 90°F(32°C).
2.7.4 The refrigeration system’s cooling design shall be independent air cooled cascades to allow sufficient reserve capacity to maintain an internal chamber temperature as specified herein.
2.7.5 The total refrigeration system shall have the capability to maintain an internal chamber temperature of at least -121°F (-85°C), ±2°F/±1C° (both refrigeration systems operating), with ambient air temperature 110°F(43°C) ±2°F.
2.8 Back-up system. The freezer shall be provided with a liquid nitrogen back-up system if desired by customer. This system shall automatically initiate cooling in the event of compressor system failure or power failure. Freezer chamber shall be maintained at the operating temperature automatically for at least 24 hours with the use of this backup system. Electrical input shall be 208/230 volts, 50-60 Hz three phase. Batteries shall be provided as back-up to operate the system.
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