air-cooled chillers myths & facts - issue oct-dec 2003

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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

ooled Chillers : Myths & Facts - Issue Oct-Dec 2003 http://www.ishrae.in/journals/2003oct/artic

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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.

[top]


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air-cooled chillers myths & facts - issue oct-dec 2003

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