How Ammonia Refrigeration Systems Work

I often see questions about this and wonder what sort of answer someone is expecting.  Almost all industrial refrigeration use ammonia as a refrigerant. Although there is a renewed interest in CO2 refrigeration systems, ammonia is the dominant refrigerant by choice.  When compared to any other mechanical refrigeration system the basic fundamentals apply.

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Since the advent of the mandated Process Safety Management program (PSM) the industrial refrigeration sector has been faced with attempts to maintain lower refrigerant charges (re: inventories of refrigerant used in the system).  To date, most of the efforts expended focus on keeping the ammonia refrigerant charge under 10,000 pounds.  To a degree, this has largely centered around minimizing the quantity or use of gravity-flooded air-cooling evaporators or flooded shell and tube heat chillers.  While these methods will drastically reduce the total refrigerant charge, other methods can be applicable.  These typically involve re-thinking the basic premise for the use of a refrigerant and the application of specific sub-systems that are used in ammonia refrigeration systems.

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In the final entry on this series, the application of hand expansion valves (HEV) to liquid overfeed systems will be discussed.  The application of HEV’s in this type of system somewhat differs from those of the prior discussions.  When selecting HEV’s for the evaporators where liquid overfeed is being utilized the hand expansion valves take on a secondary function, which may not be obvious at first glance.

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We have been somewhat busy the last few weeks completing a new project.  The next few blog entries should be more timely.

These valves are used for multiple purposes in ammonia refrigeration systems.  However, they are commonly designated by a single description.  This common term of hand expansion valve (HEV) is sometimes misleading. It may help to consider the actual function the valve provides; throttling of liquid or vapor and the mass flow requirements for the application.

The common applications of this valve are:

• Liquid make-up to maintain a relatively constant liquid level in a vessel.
• Throttling liquid from a higher pressure to a lower pressure (control pressure systems).
• Balancing upstream liquid pressures to evaporators in a liquid overfeed system.
• Throttling vapor flow in float columns to minimize rapid liquid level fluctuations.

The next few blog entries will discuss the multiple applications of these valves.

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