Industrial Ammonia Refrigeration Systems

 

 

 

 

 

As a refrigerant, ammonia offers three distinct advantages over other commonly used industrial refrigerants. First, ammonia is environmentally compatible. It does not deplete the ozone layer and does not contribute to global warming. Second, ammonia has superior thermodynamic qualities, as result ammonia refrigeration systems use less electricity. Third, ammonia’s recognizable odor is it’s greatest safety asset. Unlike most other industrial refrigerants that have no odor, ammonia refrigeration has a proven safety record in part because leaks are not likely to escape detection.

Industrial Ammonia Refrigeration Systems – Ways They Differ Probably the number one difference between a typical DX unit and an ammonia refrigeration system centers on this: oil. In a DX halocarbon unit, oil is continuously returned to the compressor. Oil is not returned to the compressor in ammonia refrigeration systems but is instead drained out of the system periodically. Oils used in ammonia refrigeration systems are essentially insoluble in NH3 (some slight solubility exists at high pressures); oil is heavier than liquid ammonia, making it easy to drain out.

On the other hand, oil solubility is absolutely essential with the halocarbons in order to facilitate oil return. This makes oil management in an ammonia system a relative piece of cake. It’s easy to manage – just drain it out. We don’t have to maintain minimum gas velocities in order to bring it back through dry suction piping. Paraffinic-based oils are commonly used with ammonia. These oils do a good job of cleaning out old welding slag and dirt, hence another reason why it can’t be returned to a compressor – it is too dirty to reuse. Oil, once drained from the system, is no longer usable. The next difference lies in the choice of piping materials. Copper, brass, bronze cannot be used with ammonia – your metal choices are mild steels, stainless steels, nickel, but absolutely no copper. The next difference lies in refrigerant management. In nearly all halocarbon packaged units (air-conditioning, commercial refrigeration), the refrigerant charge is critical. This means that the system has no alternate place(s) to store refrigerant not in use by the system at some particular point in time. Stated differently, refrigeration units are built using the simplest designs – no pressure vessels, no solenoid valves. Any excess liquid becomes stored inside the condenser – and any excess liquid decreases system refrigerating capacity. Generally speaking, a critically-charged unit should be charged within ±1-2% of the listed charge. If it isn’t, the unit will fail to produce its stated capacity. Over-charge also increases the risk of liquid carryover to the compressor. A few systems will have suction traps (a small vessel) to detain a liquid surge; some traps have an internal heat exchanger to facilitate liquid boil-off. Have you ever noticed how a critically-charged system (or unit) behaves when its compressor initially starts? A critically-charged system takes a few minutes to “get going” – that is, start to produce some cooling effect. During this interval, the compressor is evacuating refrigerant vapor from the low side and sending this now higher pressure vapor off to the condenser to be reliquified.

Then this liquid must start to back up in the high pressure liquid line so that it can seal off the opening into the liquid capillary (the expansion device). Now your unit will start to produce cooling. This time delay isn’t acceptable with most ammonia systems, especially when process cooling or freezing is involved. I know of only one ammonia refrigeration system designed for critical charging. The owner of this system wasn’t very happy with it either and has probably removed it by now. And for a good reason – the time delay associated with a critical charge just isn’t acceptable. Hence, another difference with ammonia – the need for pressure vessels.

Compressors

The major difference between the halocarbon series and ammonia with respect to compressors has to do with the motor – open drive versus hermetic.  With only one exception (Japan), all ammonia compressors are open-drive design due to the incompatibility of copper and NH3. The most commonly applied compressor design is twin rotary screw in today’s industrial marketplace.   Reciprocating compressors are still applied and have an inherent advantage of better part-load efficiency over a screw at off-load (slide valve positioned). With only one exception that I am aware of, centrifugal compressors are not used with ammonia.  The low mol weight of the NH3 molecule (17) makes this particular refrigerant a poor candidate for turbo compression which relies on a high density gas. Another factor that distinguishes ammonia systems from their halocarbon counterparts has to do with compressor capacity unloading.  One of the most energy-wasteful practices ever to come about in refrigeration is using hot gas bypass as a means of unloading a compressor.  When the hot gas bypass solenoid opens, hot gas is sent directly back into the compressor suction.  While effective at reducing system suction cfm, it does little or nothing for a corresponding reduction in compressor energy use.  Fortunately, hot gas bypass cannot be applied to ammonia systems, unless it is injected into a side-inlet distributor and then into a DX evaporator.  Any attempt at sending compressor discharge vapor directly back to suction will either shut the machine down on high oil temperature or high discharge temperature (screw compressors) or blacken the discharge heads of a reciprocating compressor.  You may also coke up your compressor oil as well.

Refrigeration Ammonia refrigeration systems operate in a similar manner to fluorocarbon systems, but have several key differences. While you will not usually find an ammonia-based system inside a home (ammonia is a very toxic substance, and the refrigerators are very expensive), they are used in factories that need large refrigeration devices that can cool substances very quickly. A refrigeration system is based on a type of refrigerant gas, which is constantly run through the system to gather and disperse heat. These gases are made of many different substances–most household refrigerants are actually a synthetic mixture designed for efficiency, but ammonia-based version simply use ammonia. Whatever the type of gas, it is passed through various devices, including a compressor, condenser, expansion device and evaporator. Ammonia as a Refrigerant Each part of a refrigerator is designed to change the state of the gas in some way. By changing the state of the gas, the system also changes its temperature and how much heat it carries. The compressor, for instance, makes the gas hot and raises its pressure, enabling it to hold larger amounts of heat. The condenser changes the gas to a liquid, allowing it to lose some of its heat in the process, while the expansion device turns the liquid back into a cold gas, releasing most of the heat it held. The evaporator cools the gas into a chilled vapor that is ready to be circulated back through the system. This is how all refrigerators remove heat from their compartments and disperse it. Ammonia refrigerators in factory settings need to cool very quickly. While household refrigerators take a few minutes to begin cooling after they are started, this delay is not acceptable in a manufacturing environment. To begin refrigeration immediately, the ammonia is distributed among pressure vessels that separate liquid ammonia from the gas, store the refrigerant and send it to different parts of the system when it is needed. By contrast, a hydrocarbon refrigerator has no pressure vessels. Ammonia Benefits Ammonia-based refrigerators do not need to constantly recombine oils into their systems to function properly, so oils are quickly drained rather than dissolved back into the gas. Ammonia systems are also able to deal with the accidental collection of water in some pipes, something hydrocarbon systems cannot work with. The water will still need to be drained, but ammonia can function even with water present. Of course, these positive factors also come with some drawbacks. Ammonia is a very caustic chemical and ammonia systems must be made with steel or nickel. No copper or copper-based piping can be used.