BY MANUCHEHR MEHDIZADEH, BSME LIFE TIME MEMBER ASHRAE, COMMITTEE OFFICER @ IIR, REFRIGERATION CHAIR @ ASHRAE HOUSTON CHAPTER
Ammonia Refrigeration Basics
Refrigeration systems use basic physics to transfer heat energy out of one area and into another, leaving the first area cooler than it was before as in the cold store systems. Various ice production systems and temperature controlled warehouses (TCW) use the same process on a larger scale. Massive industrial facilities like oil & gas and food processing plants rely on large-scale refrigeration systems for their day-to-day operations.
The most common type of industrial refrigerating system is a positive displacement (increase the pressure of refrigerant thru movement of piston by reducing its volume) vapor-compression (Perkins cycle) refrigeration.
This approach uses a fluid called a refrigerant as the means of moving heat around. Most of the time, the refrigerant is a vapor. At one point in the system, it’s compressed to become a liquid; later, it’s allowed to expand and vaporize again. The process repeats on a cycle. Each time the refrigerant vaporizes, it absorbs heat energy from its surroundings, and each time it condenses, it releases that heat to its new location.
The physical properties of the refrigerant determine the pressure and temperature ranges of the system, along with the rate of cycling required for a given cooling effect. In turn, those details determine the efficiency of the refrigeration system as a whole. The choice of refrigerant is important, and many synthetic materials were created for the purpose. During the 20th century many kind of chlorofluorocarbons (CFCs) gases such as CFC-12 &22 etc. were developed and widely used, until perhaps mid-1985 , when their destructive impact on the environment (ozone hole over the Antarctic in the South Pole) was discovered and announced by Laureates Sherwood Rowland & Mario Molina.
Why Ammonia Refrigeration?
Ammonia (NH3) is a Natural choice of refrigerant used on large cold chain industries. Its following salient features are reasons to consider for choosing ammonia as refrigerant of choice:
- Ammonia’s physical properties make it effectively reliable and efficient for large systems.
- It breaks down quickly in the environment and in the human body, minimizing potential environmental impact.
- Any spill or accidental release can be quickly identified, because of ammonia’s strong odor. Ammonia has a high volumetric efficiency, is naturally occurring, plentiful, and completely renewable. Made of one part Nitrogen and three parts Hydrogen, ammonia is one of the most abundant gases in the environment.
- While synthetic patented refrigerants are being phased out because of their harmful effects to the environment, ammonia has always had a Global Warming Potential (GWP) of Zero and Ozone Depletion Factor (ODP) of Zero.
- Besides since ammonia is not restricted by a patent, anyone can produce it. This keeps the supply bountiful, the price low and makes large-scale refrigeration accessible to both developed and developing countries.
- Ammonia is very efficient – a relatively small amount is needed to accomplish a large reduction in temperature. By using ammonia-based refrigeration systems, food producers and cold warehouses reduce both energy use and energy costs.
- Anhydrous ammonia is a clear liquid that boils at a temperature of -28°F(-33.3C). In refrigeration systems, the liquid is stored in closed containers under pressure. When the pressure is released, the liquid evaporates rapidly, generally forming an invisible vapor or gas.
According to the International Institute of Ammonia Refrigeration (IIAR), ammonia is 8 to 15% has superior thermodynamic properties (ammonia at 0.3bara vacuum = -42C, whereas HCFC 22 at 0.3 bara vacuum = -33C) than competitive refrigerants. This allows an ammonia-based refrigeration system to achieve the same cooling effect while using less power. As a result, where ammonia refrigeration is incorporated, its overall system can provide a Low Life Cycle Cost Analysis (LCCA).
This could lead to obtaining LEED certifications from USGBC’s local chapter and an eventual recipient to OSHA’s PSM (Process Safety Management) and EPA’s RMP (Risk Management Program) audit compliance and certifications. Thus OSHA & EPA compliances are mandatory inspections expedited by qualified inspectors on installations where 10 ton or more ammonia charge are required on any such refrigeration system installations.
Ammonia breaks down in the environment very quickly (lasting less than a week in the air). Unlike synthetic refrigerants like CFCs, it doesn’t damage the ozone layer. Most of ammonia’s potential for harm relies on there being too much of it in one place. Also, only 5% of globally produced NH3 are used as refrigerant, while ammonia is mostly used as a fertilizer and sprayed on fields for industrial farming to increase global crop production.
Also large quantities of ammonia should not be vented in enclosed area, near open flames or heavy spark. A proportion of 16 to 25% by volume in air in the presence of an open flame burns and could explode. Mains should be sized carefully to provide low pressure drop and avoid capacity or power penalties caused by inadequate piping. It is considered toxic even at low concentration levels of 35 to 5o mg/kg.
The pungent smell of ammonia can be noticed when it’s only about 20 parts per million (ppm) in the air. While some refrigerants have no noticeable smell, allowing small leaks to go unnoticed, that’s not the case with ammonia. Even a tiny amount in the air will be obvious. Importantly, the detectable concentration is much lower than the concentration that will cause immediate harm.
The ammonia’s physical properties are best suited to large refrigeration systems, there is likely to be a large amount of ammonia in any system that uses it. Any water in the system would freeze and obstruct piping. Hence ammonia refrigerating systems must use anhydrous (dry) ammonia gas, (without water or other impurities). The physics of vapor-compression refrigeration require the system to use enough pressure to compress the gas into liquid. Together, this means the refrigeration system uses required amount of pure ammonia under high pressure.
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