Defrosting of
Air Coolers
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Various Methods I
The evaporator being the coldest surface in the cold room, attracts moisture from the air. This
moisture condenses on the evaporator surface, and when the surface temperature is below
o0c frost is formed. If this frost is not removed, the performance of the evaporator deteriorates
since the frost acts as resistance to heat flow, as also increases the air side resistance reducing
the air flow. If the frost is not removed in time and plant is allowed to operate, the evaporator
may become totally ineffective, as there will be no air flow or heat transfer and un-evoporated
liquid ammonia coming back, is likely to damage compressor.
I
t is therefore essential to it again is estimated to be nearly 1.5 method can be adapted to any
defrost the coolers in time to kW/Kg of ice (iiar condenser temperature in the cold room. It
maintain efficiency levels and magazine May 2010 issue). One can requires ducting and personnel to
avoid damage to components. thus estimate, based on condensate carry out this defrosting. In colder
Improper and incomplete defrosting water amount collected as to how climates this method is either
can damage compressor and much extra energy and money one ineffective or less efficient. The
evaporator coil to the extent that is spending. outside air brings moisture and
irreparable refrigerant leaks develop There are various methods of additional heat load on the system.
when ice is allowed to build up and defrosting the coolers and these are Electric Defrost
crush one or more coil tubes. The described below with their This is one of the popular
fan blades are also likely to be advantages and disadvantages as methods for small size air coolers
damaged if ice builds up on the fan well as which method is more particularly for HFC/HCFC
ring. The drain pan gets totally suitable for the application under refrigerants. The method can be
blocked by ice slab and water spills consideration. applied to any cold room application
over to floor and ice is formed on operating at any temperature. While
the floor as well. Air Defrost manufacturing and assembling of
Defrosting is therefore necessary Off cycle defrost coils; the dummy tubes are inserted
but not in excess also. Defrosting is In the cold stores operating in the coil blocks in a particular
doubly expensive procedure because above 2°c, the evaporator coils can pattern and these tubes contain
energy is used to pump heat into be defrosted by simply turning off electrical heating elements.
cooler and its surroundings, after the refrigerant flow to the evaporator In some designs the heating
which further energy is used to while maintaining fans running and elements are strapped to the outside
extract the heat from the cooler and allowing room air to pass over the of the fin/tube assembly. The
its surroundings before the system evaporator, thus melting of frost. advantage of this method is the
gets back to its operating The disadvantage of this process is manufacturer does not have to
temperature. The energy is thus it is very slow; however it is of provide extra dummy tubes.
consumed twice, once for forming lowest cost and requires no additional The advantage of electric defrost
ice and second time for melting ice. controls or energy. This method also is - it does not interferi:! with the
Defrosting as the name suggests does not help in removing refrigerant circuit, and chances of
should be activated when frost is accumulated oil in the cooler. liquid coming to compressor or
formed and not wait till ice is formed Warm outside air hydraulic hammer are eliminated.
on the coil surface. The total energy outside warm air can be ducted The system is low in initial cost
required to form the ice and defrost inside to defrost the coil. This but high in running cost since it
( oollll� lndl<l 78 II December 2012 --------------------------
This valve opens first and allows
the pressure in the coil to reduce
depending on size of evaporator and
available quantity of hot gas.
The timings can be adjusted to suit
particular evaporator model and size
-)>
slowly. This eliminates system The steps 1 to 10 are all built into since adjustable electronic timers -,
disruptions, which would occur if the control circuit of the controller. are provided in the controller. • 0
warm refrigerant were released 0
0
quickly in to suction piping. This
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also reduces vapour propelled Ramesh Parashuram Paranjpey, Fellow Life Member ASHRAE, ""'2
liquid, and prevents sudden Member IIAR USA is MTech (Refrigeration), IIT Bombay and Fellow-Life c.n
loading of compressor if suction Member, Institution of Engineers, India, and a Chartered Engineer. He was
pressure rises quickly. Ex-President, ISHRAE, Pune Chapter; Ex-President, ASHRAE Western
• Fan Delay time: The fan is not India Chapter and published more than 45 Papers in International/national
yet energized. Instead, the coil journals. he conducted corporate training on National level at various
temperature is allowed to drop, companies and ISHRAE, ASHRAE forums and on International level
freezing any water droplets that in US, Singapore, China, Germany, etc. Earlier he was Chief Executive
might remain on the coil surface Officer, Valtas Air International Ltd., Pune; Managing Director, Carrier
after the hot gas defrost phase, Refrigeration Ltd. Bangalore & Singapore; and Vice President, AC&R
thereby preventing the possibility Division, Kirloskar Pneumatics Ltd. Pune. He was also Examiner and
of blowing water droplets off the Paper setter in University of Pune for Master in Engineering and was in
coil in to refrigerated space. syllabus formulating committee. He was invited by German Chamber of
• Start of cooling cycle: After the Commerce to deliver lecture in Germany on "Ammonia Pump Circulation
fan delay has elapsed, the fan Systems for Low Temperature Applications" in 2008. He is winm!r of
gets energized automatically "Bry-Air Systems Design Award" for 2009 and published book on Cold
based on time setting. The room solutions for Emerson Climate Technologies. He is Refrigeration
refrigeration phase continues Specialist on NHB panel and has executed more than 700 projects for
until the next defrost cycle is cold storages, as well as air conditioning such as international air ports,
initiated. office buildings, and shopping malls. In last 5 years after retirement
The entire process can take from Valtas Air International, he is doing consultancy and is consultant
maximum 15 to 30 minutes for many projects/companies.
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------1 Decemb er 201211 83 Coolin� Indid
