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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 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.
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 44°C 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 29°C (85°F) 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 35°C 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 30°C in March to a maximum of 44°C in May/June and again reduces to 27°C in November, during which month air conditioning is still required. It also varies from a minimum of 18°C in March to 31°C in May/June and further drops down to 15°C 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 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 | 95 35 |
75 24 |
20 11 |
-- | 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 |
| C) sub-Total ikW/ton | A X B | 423 | 675 | 632 | 240 | 130 | 350 | 2450 | ||
| Mean annual ikW/ton | C(Total)÷A(Total) | 2450÷2250 | 1.09 | |||||||
| Water Required | If cooling pads are fitted on chillers) | NA X |
NA X |
NA X |
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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 1 indicates 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 35°C ambient are only between 500 to 600 or 20% of the total operating time. The system, therefore, operates between 27°C to 35°C 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 35°C 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.
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 1½ water-cooled units for the same load.
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 35°C (95°F).
The result would be that the maximum ikW/ton will not exceed the ikW obtained at 35°C 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.
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.
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.
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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-Bulb Wet-Bulb |
-9 -2 |
-7 -2 |
-5 -1 |
-1 0 |
0 0 |
-1 0 |
-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 |
| 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 Design temperature + 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.
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 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 :
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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 55°C, but is usually designed for 50°C 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 to an 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 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.
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:
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.
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 constant power. | Slightly higher by approximately 9% |
| 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, electrical panel etc. approximately 150 m2. | Covered space is required only for pumps and Electrical panel approximately 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. |
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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:
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.
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:
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.
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.
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.
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.
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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.)
This is a 16-story office complex located in New Delhi near Karolbagh. It has a built up area of 19,500 m2 and is air conditioned by nine, air-cooled chillers each of 106 tons capacity at 44°C.
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 m2 of space in the basement. The system requires a staff of only three people to operate.
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.
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.
The requirements called for central heating of an office complex measuring 7,000 m2 and club house and auditorium facility measuring 3,000 m2 i.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 a conventional 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 50°C at –5°C 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.
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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