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Issue : October-December 2003
Air-Cooled Chillers : Myths & Facts
It is commonly assumed that Air-Cooled Chillers are ineffecient and not suitable for
the hot climate prevailing in north India. The factual situation is very different, as
explained in this article.
By Nirmal C. Gupta
Gupta Consultants
New Delhi.
Nirmal C. Gupta is a mechanical engineer from Oklahoma State University, USA.
He returned to India in 1957, worked 14 years for Blue Star Ltd. and then started his
consultancy business where he continues to remain the principal consultant. He is
currently chairman of ASHRAE India Trust.
Air-cooled chillers have been used in the USA for a long time with great
effectiveness, due to their simplicity of design and operation. In view of scarcity of water
in the Middle East, major manufacturers in the West, have modified the chiller design,
so that they can work satisfactorily even in the hot and humid climates prevailing there.
Thus, their use has become quite extensive in the Middle East, with great success, and
for a variety of applications.
Their use in India has been slow, because of the general perception that these
chillers consume too much power and hence are not suitable for Indian conditions. The
experience gained by their use in the Middle East, has been useful in deciding to use
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aircooled chillers in India. It has been found that these units can withstand both high
and low temperatures and monsoon rain without affecting their performance.
Further, detailed analysis which was carried out on some installations has thrown
up some interesting facts and has established their overall efficiency and suitability,
regardless of climate conditions. It is also a fact that the use of these chillers, results in
conserving water resources, which are becoming scarce in many places, and which does
not have the required quality, hence, requiring extensive treatment.
The operation of these chillers is simple, because there are fewer parts to operate.
Maintenance costs are also lower due to fewer components and saving in the water
treatment plant.
Power consumption (air-cooled system)
The main assumption against aircooled chillers is that they consume too much power
and hence are very inefficient. It is true that at an ambient temperature of 44C their
power consumption is quite high and is in the range of 1.3 to 1.5 ikW/ton, depending on
the type of compressors being used.
The lower figure of 1.3 ikW applies to reciprocating and scroll compressors and the
higher figure applies to screw compressors, which are otherwise efficient in watercooled
applications. However, the fact to be noted is that in an aircooled system the output of
the chiller increases and power consumption falls as the ambient temperature reduces.
Thus, the power required at 29C (85F) is as low as 0.94 ikW/ton. Overall, the
power required varies between 0.94 to 1.5 ikW/ ton. The power consumption at 35C is
quite favorable as compared to watercooled units.
This brings about two interesting facts :
The maximum temperature in north India varies from a low of 30C in March to a
maximum of 44C in May/June and again reduces to 27C in November, during which
month air conditioning is still required. It also varies from a minimum of 18C in March
to 31C in May/June and further drops down to 15C in November.
In the central plains of India, while the maximum temperature is similar to the
North, the minimum temperature is generally much lower. Also, in these places (such as
Nagpur, Pune, Indore, Hyderabad, Bhopal etc.) the maximum and minimum
temperature reduce considerably once the rains start.
Hence, it is clear that the power consumption will vary not only from month to
month, but also from day to day, as well as from morning to evening, everyday, and in
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all parts of the country.
Table 1: Sample hourly calculation
City :New Delhi
Operating Time :8AM to 6PM
Hours per Day :10
Days per Months 25
Days per months 25
F Max
Temp
Min.
Temp
Daily
Range
Hours of use upto given temprature
limits
C 85 90 95 100 105 110
Months 29 32 35 38 41 44 Total
Hours
March 90
32
65
18
25
14
75 175 -- -- -- -- 250
April 100
38
70
21
30
17
25 50 175 -- -- -- 250
May 110
44
80
26
30
18
-- -- 25 50 50 125 250
June 110
44
80
26
30
18
-- -- 25 50 50 125 250
July 9535
7524
2011
-- 25 175 50 -- -- 250
August 95
35
75
24
20
11
-- 25 175 50 -- -- 250
September 90
32
78
25
12
7
25 225 -- -- -- -- 250
October 90
32
72
22
18
10
75 175 -- -- -- -- 250
November 85
29
70
21
15
8
250 -- -- -- -- -- 250
A) Sub
Total
Hours
X X X 450 675 575 200 100 250 2250
B) Power
ikW/ton
X X X 0.94 1.00 1.10 1.20 1.3 1.4 Total
ikW/ton
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C) sub-Total
ikW/ton
A X B 423 675 632 240 130 350 2450
Mean
annual
ikW/ton
C(Total)A(Total) 24502250 1.09
WaterRequired
If cooling pads arefitted on chillers)
NAX
NAX
NAX
[top]
In an office application, the air conditioning system operates for 250 to 300 days in
a year, depending on the location of the city. Thus, the annual operating hours for
offices working 10 hours a day, are between 2,500 to 3,000 out of a total of 8,760 hours
in a year. An hourly chart in Table 1indicates the number of hours for which a system
operates annually at different ambient temperatures in north India.
What emerges is that, the hours of operation in excess of 35C ambient are only
between 500 to 600 or 20% of the total operating time. The system, therefore, operates
between 27C to 35C for 80% of the time and the power consumption at these points is
quite favorable. The average power consumption thus varies between 1.0 to 1.1 ikW/ton
and not 1.3 to 1.5 as it would appear at first glance.
In cases where a system operates for 12 or 24 hours per day, the percentage of hours
above 35C will be much less than 20%. This will result in an even lower average ikW/
ton. This is quite favourable as compared to the water-cooled system as can be seen later
in this article.
As the demand reduces the ikW of the chiller compressor reduces and also the
power used by condenser fans, reduces proportionally because some of the fans shut
down on reduced demand and also at lower ambient temperatures. Thus, the ikW
remains nearly constant even at part loads conditions.
Added benefit
There is another advantage of aircooled chillers in night application. During night, while
the building load falls by 20 to 30%, the output of the chiller increases by 20 to 25%.
Thus, while one air-cooled unit may meet the night requirement, it may be necessary to
run 1 water-cooled units for the same load.
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Cooling pads on condenser
Another method which can be adopted to improve the efficiency of the air-cooled units,
is to provide cooling pads over the condenser. This method has not been tried out
sufficiently, but there is no reason for any apprehension or doubt about the unit
performance with pads. The water flow on the cooling pad could be activated, only when
the ambient temperature crosses 35C (95F).
The result would be that the maximum ikW/ton will not exceed the ikW obtained at
35C and in fact will reduce further. This will reduce the Mean ikW/ton by an additional
5% to 1.04.
The water required for 500 to 600 hours on the pads, is approximately 10% of the
water required for a water-cooled system. The increase in generator capacity will only
be 5% instead of 20% and this can make the air-cooled systems more attractive than
water-cooled systems in certain cases.
Power consumption (watercooled systems)
The power consumption of a water-cooled system is given here for the purpose of
comparison. The power consumption in a watercooled system is dependent on the
ambient wet bulb temperature and not the dry bulb temperature. The daily and monthly
variation in wet bulb temperature is much less than the dry bulb temperature.
Further, the variation in water temperature leaving the cooling tower is even lower
than the variation in wet bulb temperature. Hence, the output and power consumption
(ikW) of a watercooled chiller does not change appreciably in different ambient
conditions, throughout the year.
The total ikW of a water-cooled system also has to include the power consumed by
the condenser water pumps and the cooling towers. Therefore, the result is that while
the ikW of the chiller itself reduces proportionately to the reduction in the requirement,
the kW of pump and cooling tower remains constant. This results in a higher overall ikW
for the system at part loads.
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Normally an air conditioning system does not operate at full load conditions for
more then 10% of the operating hours. The system loading usually varies between 40%
to 90% of the full load for most of the operating time. It is obvious, that the overall ikW
of the water-cooled system is higher than the ikW of the compressor. In view of the
above, the net ikW of the two types of system are close and not very different, as is
generally assumed.
[top]
Hourly calculation chart
An hourly calculation chart has been devised and prepared, to be able to analyze and
calculate the Mean ikW/ton for an air-cooled chiller. A sample calculation has been
carried out for the Delhi climate to arrive at the Mean annual ikW per ton. Similar
calculations can be carried out for any other city using the weather data and daily
variation chart.
It will be seen that the maximum and minimum temperatures have been worked out
for each month based on ISHRAE weather data for monthly variations and the daily
range each month, based on the Carrier Handbook.
The daily range is then used in conjunction with the Carrier Handbook chart to
arrive at the number of hours for which the system is likely to operate upto a given
ambient temperature which represents the air entering temperature to the chillers. A
copy of Carrier Chart is given in Table 2.
Table 2 : Daily temperature variation Ref.: Handbook of Air Conditioning System
Design by Carrier Air Conditioning Corp., USA.
Daily Range
of
Temperature
(F)*
Dry or
Wet-Bulb
SUN TIME
AM+ PM
8 10 12 2 3 4 6 8 10 12
10 Dry-BulbWet-Bulb
-9-2
-7-2
-5-1
-10
00
-10
-2-1
-5-1
-8-2
-9-2
15 Dry-Bulb
Wet-Bulb
-12
-3
-9
-2
-5
-1
-1
0
0
0
-1
0
-2
-1
-6
-1
-10
-3
-14
-4
20 Dry-Bulb
Wet-Bulb
-14
-4
-10
-3
-5
-1
-1
0
0
0
-1
0
-3
-1
-7
-2
-11
-3
-16
-4
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25 Dry-Bulb
Wet-Bulb
-16
-4
-10
-3
-5
-1
-1
0
0
0
-1
0
-3
-1
-8
-2
-13
-3
-18
-5
30 Dry-Bulb
Wet-Bulb
-18
-5
-12
-3
-6
-1
-1
0
0
0
-1
0
-4
-1
-10
-3
-15
-4
-21
-6
35 Dry-Bulb
Wet-Bulb
-21
-4
-14
-4
-7
-2
-1
0
0
0
-1
0
-6
-1
-12
-3
-18
-5
-24
-7
40 Dry-Bulb
Wet-Bulb
-24
-7
-16
-4
-8
-2
-1
0
0
0
-1
0
-7
-2
-14
-4
-21
-6
-28
-9
45 Dry-Bulb
Wet-Bulb
-26
-7
-17
-5
-8
-2
-2
0
0
0
-2
0
-8
-2
-16
-4
-24
-8
-31
-10
*The daily range of dry-bulb temperature is the difference between the highest and
lowest dry-bulb temperature during a 24 hour period on a typical design day.
Equation: Outside Design temperature at any time = Standard Outside Designtemperature + correction from above table.
The power consumption and output at different air entering temperatures are
available from chiller manufacturers. The ikW/ton and hours at each condition have
been multiplied to arrive at the total ikW/hours at each temperature. This data has been
used to work out the Mean ikW/ton for the whole year.
It will thus be seen that the average ikW/ton is only 1.09 which is not very high. In
the case of a water- cooled system, the net ikW/ ton even at full load works out to 0.95
for reciprocating, scroll and small size screw compressor and at part load this figure
increases to 1.0 ikW.
Design of air-cooled chillers
Air-cooled chillers usually contain more then one compressor, except in units of very
small capacity. This is true for all units with different types of compressors i.e. scroll,
reciprocating and screw. The compressors are either connected to separate chillers or a
single chiller with upto three independent circuits.
Thus, the units operate as multiple separate circuits, in which case the problem or
failure in one circuit does not affect the performance of the other circuit or circuits. This
provides for built-in redundancy and in many cases, in small installations a single unit
can provide a certain margin of safety.
The condenser coils are also divided in the same number of circuits as the
compressor to provide total separation. There are multiple propeller/axial fans for
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removing condenser heat.
The whole system is operated by either a microprocessor or advanced
electro/mechanical control system. This cycles the operation of fans in proportion to the
demand and helps in maintaining the lowest possible discharge pressure in relation to
the prevailing ambient dry bulb temperatures. This process saves on condenser fan
power and also tries to achieve optimum ikW/ton based on the prevailing ambient
conditions.
All the components are mounted on a sturdy composite steel frame work. The
weight of the components is distributed evenly for nearly equal loading on the structure.
The frame work is large enough, so that the overall loading of the unit does not exceed
500 kg/m2. This loading suits all standard RCC construction.
All the electrical components are encased in a weather-proof housing. In addition,
all components, frame work etc. are treated with protective processes to be able to
withstand all types of extreme weather conditions including heat, dust, sea breeze, rain
etc. All the operational components are connected to an electro-mechanical control
system or a microprocessor- based control system.
The function of the control system is many fold as given below :
Pre-start check up of all components and system.
Sequential start of condenser fans and compressors.
To maintain design head pressure at varying ambient temperature.To maintain design chilled water temperature.
Protect the components from damage due to high/low pressure, freeze up etc.
The operation of the multiple condenser fans is decided by the ambient
temperature so that the head pressure of the system remains as low as possible at
a given ambient temperature and is consistent with the safety of the compressors.
Generally, the head pressure is not allowed to fall below the pressure
corresponding to an ambient temperature of 28C.
The control of head pressure is achieved by cycling the number of condenser
fans.
The operation of the number of these fans also allows the ikW to remain
proportional even at part load, unlike water-cooled units, where the power of
condenser water pumps and cooling towers remains constant even at part loads.
[top]
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Heat pumps
The use of air cooled chillers offers another advantage in places where both cooling and
heating are required. These units can efficiently provide both cooling in extreme hot
weather and heating in extreme cold weather, when the machines are configured in a
"Heat Pump" mode.
In winter, the air-cooled condenser becomes the evaporator to extract heat from the
atmosphere. The unit chiller then acts as a shelland- tube condenser, to produce hot
water, which can be used for providing heating in the premises. The outlet temperature
of the hot water can be as high as 55C, but is usually designed for 50C to achieve a
reasonable balance between cooling efficiency and heating demand.
The advantage of the heat pump is that it produces the same amount of hot water as
an electric boiler by using less then approximately 40% of electricity as compared to an
electric boiler. This can reduce the overall cost of winter heating by 50% as compared toan electric hot water system.
A further provision in the aircooled chiller or heat pump is that of a superheat (or
auxiliary) condenser to generate hot water. This auxiliary condenser removes the excess
superheat from the discharge gas, before it goes to the condenser without affecting the
performance of the refrigeration cycle. This heat from the discharge gas is available both
in cooling cycle and the heating cycle. The bonus hot water thus becomes available both
in summer and winter or for nearly 10 months in a whole year.
The hot water produced by this extra condenser can be used to meet the hot water
requirements of the kitchen and toilets without any additional operating cost. This
bonus heat improves the overall performance of the air-cooled system.
Heat pumps for winter
Heat Pumps are also available for providing maximum efficiency during the heating
cycle. These heat pumps represent an ideal solution for places which require heating
only in winter and where electricity is the main source of energy.
The addition of an auxiliary condenser can meet the other hot water requirements
during winter months.
As stated earlier, the use of heat pumps for winter heating would reduce energy
consumption by over 50% and thus pay back the extra cost within a short time.
Cost of air-cooled system
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The cost of this system is generally quite favourable, for capacities of upto 1,000 ton.
This is evident from the fact that in an aircooled system, one single unit (the chiller)
replaces the following components of a water-cooled system:
Induced draft cooling tower.
Condenser water pump.
Condenser water piping, valves and fittings.
Electricals i.e. panel, cables etc. to connect the cooling tower and the condenser
water pump.
Water treatment plant.
Underground and overhead storage tanks for the treated water and its pumping
system.
Several comparative analysis have found that the cost of an air-cooled system varies
between 95% to 105% of a similar water-cooled system. The variation in the cost of the
two types of system depends on the size of the plant and whether indigenous or
imported chillers are selected.
There is however, an additional cost due to a bigger D.G. set capacity, but this is not
significant in the overall context.
Comparative chart
A chart showing the advantages and disadvantages of an air-cooled and water-cooled
system is given in Table 3.
Table 3 : Comparison of the Air-cooled and Water-cooled Chilling Units. (Based on
100 TR)
S. No. Description Water-Cooled Air-Cooled
1. Water Requirement Water consumption will
be @ 12 LPH / ton or
12,000 litres / hour for
225 days in a year.
Not Required
2. Power Consumption Lower power
consumption at full load
capacity. However, the
difference reduces at part
load as the pumps and
cooling tower consume
Slightly higher by
approximately 9%
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constant power.
3. Installed Power capacity Less then air cooled by
20%.
20% higher capacity is
required.
4. Covered space
requirement
Covered space is required
for all equipments e.g.
chiller, pumps, electricalpanel etc. approximately
150 m2.
Covered space is required
only for pumps and
Electrical panelapproximately 15 m2.
5. Space for Cooling Tower
on ground or Terrace
Space for cooling tower is
required, approximately
80 m2.
No cooling tower is
required. Space to
accommodate chillers
will be 120 m2.
6. D.G. Set size D.G. set of lower capacity
is required.
D.G. set of higher
capacity is required as
the power demand of the
system is higher at full
load.
7. Staff for Maintenance
and Operation
More number of staff is
required because of more
number of equipments.
50% less staff is required
because of less number of
equipments to operate
and maintain.
8. Descaling of condenser
tube
Descaling of condenser
tube is
required twice a season.
Descaling of condenser is
not required.
9. Water Softening plant
requirement
Water softening plant is
required.
Water softening plant is
not required.
[top]
Water saving
The water-cooled system requires approximately 12 litres of water per hour per ton
capacity. This water has to be of a reasonably good quality as per factors given below:
It should not be turbid or contain suspended impurities.
The PH value must be between 6.5 and 7.5 to prevent salt deposition when it is
alkaline or corrosion when PH value is acidic.
The total permanent, CaCo3 hardness should be below 120 PPM, otherwise the
scaling on condenser tubes will require very frequent cleaning.
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Since most available water has impurities and high hardness level, the water has to
be filtered and softened, with a water softener for the make-up water system. If the
water is not softened, it will cause deposition of salts on the tubes of the water-cooled
condenser, which reduces heat transfer and raises the ikW of the chiller.
As the air-cooled system does not use water, all the above processes of descaling etc.
become unnecessary and irrelevant. In addition, there is a saving of water of nearly 120
litres per day per ton or 12,000 litres per day for a small 100 ton system.
Simple operating system
The high side equipment in an air-cooled system consists of only two main components
i.e the air-cooled chillers and chilled water pumps. This is, in contrast to the high side
equipment of the water-cooled system which not only requires water-cooled chillers and
chilled water pumps, but also requires:
Condenser water pumps
Cooling towers
Additional condenser water piping
Additional electrical equipment
Water storage for cooling towers
Water treatment plant
Pumping system for make up water
It is obvious that the additional equipment requires more steps to put the plant in
operation. It also requires greater effort to make sure that there is adequate quantity of
suitable quality of water in the cooling tower at all times and that the water treatment
system is always functional.
If there is a shortage of water, the system may stop in mid-operation, and poor
water quality will lead to scaling in the pipes, condenser etc.
It is clear that to operate the aircooled system, no such care is required. The system
can simply be put in operation in just two steps of starting the water pumps and the
chiller at any time. The conclusion of simplicity of operation is obvious.
Ease of maintenance
Air-cooled systems are also simple to maintain, as there are fewer components, in view
of the fact, that the condenser water pumps and cooling towers are not required.
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The cooling towers require periodic cleaning to remove the sludge which
accumulates in the sump. It is necessary to make sure that there is sufficient quantity of
water in the cooling tower at all times to ensure proper functioning of the system.
The water treatment plant requires constant attention, so that the makeup water is
always below the required hardness. There is also a need for regular descaling of the
condenser tubes, so that the condenser efficiency is maintained near its peak.
In comparison, the air-cooled condenser does not scale and therefore, it only
requires annual cleaning with hot water to function properly. It is quite clear that there
are very few components or functions which need to be checked in an aircooled system.
Therefore, fewer number of staff are required for the operation and maintenance of
the air-cooled system.
Space saving
The air-cooled chiller has to be installed in an open space without any roof cover on top.
Therefore, these units are installed either on the roof of a building or in an open space,
outside the building. The space required for them is 50% to 80% more than the space
normally required for installing cooling towers of the water-cooled systems.
This means that these plants do not require a covered plant room either within the
building or adjacent to the building, thereby saving usable space and the cost of
constructing such spaces. A small pump room is however required for the air-cooled
system, which is built near the place where the air-cooled chillers are installed. The size
of pump room requires approximately 10 to 15% of the size of a regular plant room.
Thus, the total space required is much less and usually does not represent prime
usable space.
[top]
Case studies
A few case studies are given to illustrate various applications. These are :1. Videocon Plaza, New Delhi.The largest air-cooled installation in north
India.(950 tons)
2. Apartment Building, Laburnum, Gurgaon. A system providing summer
cooling, winter heating and hot water throughout the year. (120 tons)
3. Tala Hydel Authorities Gedu, Bhutan. Winter heating only for offices and
club house building. (6,75,000 Kcal/hr.)
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Videocon Plaza, New Delhi-(capacity-950 tons)
This is a 16-story office complex located in New Delhi near Karolbagh. It has a built up
area of 19,500 m2and is air conditioned by nine, air-cooled chillers each of 106 tons
capacity at 44C.
The total plant capacity is 954 tons. All the chillers are installed on the terrace of the
building. The pumps and electric hot water boilers are also installed on the terrace in a
room of 50 m2.
This system has saved 1,20,000 litres of water per day and approximately 200 m2of
space in the basement. The system requires a staff of only three people to operate.
Laburnum Apartments (capacity-120 tons) Gurgaon, Haryana
This building has 17 high end apartments varying in size from 1-bed studio to 5-bed
room set. It is located as part of a housing complex in Gurgaon, a satellite town of Delhi.
All apartments are centrally air conditioned . There is provision for equitable
payment through a Building Automation System, so that each user pays according to the
quantum of air conditioning that is used. The use of air-cooled heat pumps, with
provision for hot water, was chosen for the complex, since a simple, reliable,
maintenance-free system was required.
Accordingly, two air-cooled units of 60 TR each cooling capacity (total 120 TR) were
installed. In winter, the total heat output is 500 kW which is three times the
requirement. The auxiliary condensers provide hot water for toilets and kitchens of all
apartments. Standby heaters are provided in the hot water calorifiers to heat the water
during mild season when neither heating or cooling is required.
The system is controlled by a BAS, so that start-up is instant and operation is
automatic with minimum operational problems.
Tala Hydel project - 750 kW heating output, Bhutan
This installation is located at Gedu, Bhutan at an elevation of 2,700 m above M.S.L.
Summers are comfortable and winters are long and cold (for nearly 5 to 6 months).
Hence, only a heating system is required.
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The requirements called for central heating of an office complex measuring 7,000
m2and club house and auditorium facility measuring 3,000 m2i.e. a total of 10,000 m2.
Electricity was the referred medium since these offices were meant for a new 1,000 mW
hydel project and Bhutan is surplus in electricity.
The requirement was assessed at 6,75,000 kcal/hr requiring electric hot water
boilers of 750 kW or 780 kW including pumps, AHUs etc. The use of heat pumps was
suggested as an alternative, even through the initial cost increase was nearly 50% over aconventional system.
The advantage of the above system was that it reduced the electric demand from
780 kW to 280 kW which meant a release of 0.5 mW electricity for better use else
where.
The system is operated with five, air-cooled heat pumps each of 150 kW heat output
totaling 750 kW. All the units together require input electricity of 250 kW. The heat
pumps will give the design output of 50C at 5C ambient and are equipped with a
defrost arrangement for the evaporator.
The system is in its first year of operation and is functioning satisfactorily. This is the
first installation in this region, where pure heating is being done using heat pumps
resulting in considerable saving of electricity.
Conclusion
It will be seen that the air-cooled chillers represent a fairly efficient and cost effective
alternative to the watercooled system. These chillers can be used very effectively for
installations upto 1,000 tons.
In the milder climate of south India, there is no practical limit to the overall plant
capacity.
They are very useful for heating /cooling applications as well as pure heating
applications.
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ooled Chillers : Myths & Facts - Issue Oct-Dec 2003 http://www.ishrae.in/journals/2003oct/artic
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ooled Chillers : Myths & Facts - Issue Oct-Dec 2003 http://www.ishrae.in/journals/2003oct/artic
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