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Ammonia Refrigeration in Cold Storage Facilities
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Wednesday, 03 July, 2019, 08 : 00 AM [IST]
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Satish Kumar
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India is a leading producer of milk and second-largest producer of fruits & vegetables in the world, but nearly 20-30% of this goes waste (worth around $10 billion) due to lack of proper cold storage and cold supply chain facilities.
India has 7,129 cold storages with installed capacity of 32 million MT (NCCD, 2015) against the total horticulture and non-horticulture produce of around 500 million MT/year. Only 4-5% of Indian fresh produce is transported through cold chain comparing 80-90% in developed countries, hence, facing bigger challenge on doubling of cold storage/supply chain facilities.
2. Refrigeration
Process of keeping food products below room temperature by storing them in a system designed to cool or freeze. The most common form of refrigeration is provided by systems (i.e. refrigerators) generally use refrigerant chemical to remove heat from commodities stored inside the system.
First vapour compression refrigeration working model was built by Jacob Perkins, using ether, which was based on four basic parts of refrigeration system i.e., compressor, condenser, expansion valve, evaporator. This evolved into modern bulk refrigeration facilities in cold storages. For bulk cold storage facilities, NH3 has become refrigerant of choice since it produces greatest net refrigerating effect (btu/lb), and often having lowest brake horsepower/ton of refrigeration (BHP/TR) of any industrial refrigerant.
3. Ammonia (R-717) as a refrigerant & its properties
R-717 is being used in industrial applications since 1930s and generally acknowledged as being most efficient refrigerant chemical. It has a low boiling point and is favoured since i) highly energy-efficient refrigerant, having minimal impact on environment ii) having zero Ozone Depletion Potential (ODP) iii) having zero Global Warming Potential (GWP).
4. Work flow
i. Compressor: Ammonia (NH3) gas is compressed, resulting into heated or pressurised gaseous vapour.
ii. Condenser: Pressurised heated gas moves towards coils, located in the back of the refrigeration unit. In the coils, heat is dissipated, causing ammonia (NH3) to condense and turn into a liquid, which remains at high pressure.
iii. Expansion Valve: Pressurised NH3 then travels through expansion valve, a small hole that opens into a lower pressure area. When this occurs, liquid NH3 quickly begins to boil at -270F, at temperature colder than surrounding temperature.
iv. Evaporation: Cold ammonia cools air around it, begins to warm, the surrounding air gets colder. Ammonia continues to travel through the refrigerated area slowly becoming warmer. Finally, dragged back into the compressor where it will begin the cycle again.
5. Ammonia leakage-cum-hazard management in cold storage
Ammonia is a toxic irritant may be fatal at higher concentrations.
Based on humidity & temperature, humans can detect as low as 20ppm of ammonia concentration. Moderate irritation at 100 ppm, with major irritation at 400 ppm. Quick evacuation is recommended where ammonia leak or spill has occurred before it reaches to immediate danger to life and health (IDLH) levels. Following measure shall be taken:
I. Refrigerant piping of American Society of Mechanical Engineers (ASME) Code B31.5 be preferred. Vessels used should be as specified in ASME Section VIII – Unfired Pressure Vessel Code.
II. Well-trained operators can ensure the system operates efficiently and often will recognise problems before a breakdown occurs.
III. Each employee must be trained for Emergency Response Plan, and effective leak identification, should always be ready to properly implement the plan when needed.
IV. Emergency response personnel should be familiar with the Material Safety Data Sheet (MSDS) for ammonia.
V. Ammonia sensors may be chosen based on susceptibility of product to contamination (low, moderate, high) and isolating products that may be contaminated.
VI. Ammonia sensing cards based on colour change may be useful in detecting ammonia levels in the 1-5 ppm range.
6. Advantages & disadvantages of ammonia refrigerant over other refrigerants i. Ammonia-based refrigeration systems cost 10-20% less comparing CFCs because narrower dia piping can be used.
ii. Ammonia is 3-10% more efficient refrigerant comparing CFCs, so requires less electricity, resulting in lower operating costs.
iii. Ammonia is safe for environment, with an Ozone Depletion Potential (ODP) rating of 0 and a Global Warming Potential (GWP) rating of 0.
iv. Ammonia-based refrigeration is not compatible with copper, brass bronze and zinc.
v. Ammonia is poisonous at high concentrations. Two factors help identifying any leakage i) distinctive smell is detectable at concentrations well below those considered to be dangerous ii) ammonia is lighter than air, it will rise and dissipate in the atmosphere if case leakages.
vi. Ammonia may be explosive if present 16 to 25% by volume in air.
7. Conclusion
In context of India, several incidences of leakages or breakdown have been reported resulting in harmful and lethal effect of NH3. Preliminary measures such as systems with a high degree of mechanical integrity may reduce incidences of leakages. Having a well planned & documented emergency plan, that also routinely practiced and improved, can minimise effect of any hazard. An automated ammonia detector with alert system and complete isolation of system can avoid accidental releases; it may cost less than 1% of product value stored at your facility.
In most of cases reliable operation without incident is performed efficiently & effectively since decades. Having well-trained ammonia machine/unit operators can help promote efficient operation of your ammonia refrigeration system and identification of problems before any kind failures occur.
Disclaimer
(This general article is written by the author based on experience, interactions with food industries and knowledge gained from various sources available for information of public at large)
(The author is technical officer at (RCD)-FSSAI-HQ. He can be contacted at satish.fs2011@gmail.com)
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