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How does Ammonia Refrigeration Work?

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Ammonia Refrigeration Systems

 

Large refrigeration systems are just that - large, often containing thousands of cubic feet of internal volume. Most of this volume is contained within heat exchangers (evaporators, condensers). Next on this list come vessels - on both the high and low system sides. These vessels and heat exchangers must be interconnected with piping - this too adds volume. So after we add up the volumes of all connected components, we have to put refrigerant into the 'whole'. As a rough average, somewhere around 25% to 33% of the total system volume is taken up with ammonia in its liquid state. The balance of the system has vapor in it.

Compressors and condensers are, in effect "vapor recovery devices"- they permit reuse of ammonia over and over. This statement is also true of nearly all other refrigerants as well - only a few are "wasted" to the atmosphere - those being liquefied nitrogen, air and carbon dioxide. Ammonia is lightweight - having a molecular mass of just under 17, therefore when this gas is compressed, it becomes hot in the process - very hot - one quickly learns not to place their hands on a compressor discharge line. This limits our compression ratio steps, depending upon the compression technology we choose to use. Years ago, we used rotary compressors as our boosters - a large swept volume compressor making it ideal for low temperature service but very noisy and very limited in its discharge pressures. The rotary was normally catalogued for 2.5:1 compression ratio, but if one wanted long life out of one of these compressors, 1.5:1 would yield far fewer broken and overly worn vanes.

Reciprocating compressors, depending upon how they are built (ammonia machines all have rifle-drilled crankshafts and pushrods with forced lubrication) can run with a compression ratio of up to 8:1. These two technologies - rotary vane and reciprocating - necessitate two stages of compression in order to achieve suction temperatures below 0 F because of discharge gas temperature limitations. When oil-injected screws came into the industrial market mix (mid 1970's), compression ratios achievable with these machines rose considerably - now 18:1. It now becomes possible to reach -40 F suction in one step (with a 95 F condensing temperature), although this is about tops for screws and one would not want to design for a continuous process at these conditions. When rolling energy consumption into the design process, -20 F is about the lowest suction temperature for a single stage system, again discharging to 95 F condensing.


Fig 1 Compressor – it is here that cold ammonia vapor returning from the freezer evaporators is recovered, compressed and sent off to condensers for reliquification. Fig 2 Condenser – hot ammonia vapor from the compressors is cooled and condensed back into a liquid. From here, liquid travels on to a liquid expansion device shown in the next figure. Fig 3 Throttle (Liquid Expansion) – the actual valve that throttles liquid ammonia from a high pressure to a low pressure isn’t shown in this photo but the little vessel that controls the opening and closing of the liquid expansion valve is shown (upper right-hand quadrant).  High pressure liquid refrigerant drains by gravity from the condenser into the vessel on the left-hand side.

 

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