Ammonia: It's Good Stuff
Right up front, I want to say I like ammonia and make no apologies for it. Often, I hear people refer to ammonia as an alternative refrigerant, like they are settling for second best. I don't understand that kind of thinking. After all, ammonia is economical, energy efficient and environmentally friendly. Ammonia is the least expensive of all the commonly used industrial refrigerants. The cost of ammonia (cost/lb) is significantly less than HCFC-22, and its cost advantage when compared to HFC-134a and the newer blends used today is even more favorable. Ammonia was one of the first widely used mechanical refrigerants. Other industrial refrigerants to come along since actually are alternatives to ammonia.
Ammonia has been used as a refrigerant for more than 160 years; therefore, surprises are not likely. In most cases, a company using an ammonia refrigeration system will not have to write off the investment due to environmental concerns and high costs. Companies using chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) will have to face these issues as these refrigerants are phased out of production under the Montreal Protocol and become even more expensive.
Thermodynamically, ammonia is the most efficient refrigerant - about 3% more efficient than HCFC-22 and 10% more efficient than HFC-134a. As a single component refrigerant, ammonia is thermodynamically stable, condensing and evaporating at constant temperatures. Many new refrigerant blends are not as thermodynamically stable, complicating heat exchanger and system design.
Ammonia refrigeration systems tolerate considerable amounts of moisture. How-ever, in CFC, HCFC and hydrofluorocarbon (HFC) systems that operate in a vacuum, air can be drawn into the system through flanges, strainers or relief valves. When these systems operate below 32°F (0°C), moisture must be removed; otherwise, it can freeze and block control valves.>p> Water in an ammonia refrigeration system forms aqua ammonia, which is heavier than ammonia. As a result, the moisture migrates to the coldest spot in the system (evaporator) and stays there until it is drained. I do not mean to imply that water is good for an ammonia refrigeration system. It should be re-moved because it reduces ammonia's thermodynamic qualities. In an ammonia refrigeration system, dryers are not necessary, but they are mandatory in halocarbon systems - water in the system will freeze, especially in expansion valves.
The fact that ammonia is not miscible with paraffinic, napthanic and certain semi-synthetic lubricants is another plus for ammonia refrigeration, making lubricant management fairly simple. The small amount of oil that finds its way into a well-designed and well-operated system is not difficult to remove. Oil return from evaporators to compressors can be either automatic or semi-automatic. And, in situations where lubricant miscibility is necessary, there are lubricants available to handle it.
System ComponentsGenerally speaking, ammonia system components are smaller than those used in CFC, HCFC and HFC systems. Ammonia's high latent-heat capacity means that less refrigerant circulates in the system; thus, the piping, stop valves and control valves are smaller. And, because of ammonia's heat transfer characteristics, heat exchangers usually are smaller. Based on system component costs alone, ammonia refrigeration system installation costs typically are 10 to 20% less than an identical HCFC-22 system.
Ammonia refrigeration system components are reliable. They are built to industrial standards and must be designed to operate continuously (8,760 hr/yr). Most CFC, HCFC and HFC systems have components built to commercial-grade standards, which means they are designed to run only 2,000 hr/yr. When used in industrial cooling applications, these commercial-grade components will wear out or break down more frequently.
More often than not, there are complaints about ammonia's strong odor, but its smell is one of the best things about it. An ammonia release - large or small - cannot sneak up on you like a CFC, HCFC or HFC leak can, where large quantities may escape without detection.
Ammonia, like any other chemical, must be managed with respect, and that is what this column is about. In future issues, I will discuss ammonia refrigeration - old and new applications, system efficiencies, system operation, costs and safety.
As I said at the outset, there is no need to apologize for ammonia. Many poultry processing plants in the country use it, and many of those that do not are gradually converting to it. Cold storage warehouses and many food processing plants, wineries and breweries use ammonia. You could say that ammonia helps feed the world. There's nothing second best about that.