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COP improvement of air conditioning system using evaporative condenser.

Introduction

In the original experimental test set up, air cooled condenser is used. In the modified set up, experimental condenser is used. In the modified experimental set up, instead if air-cooled condenser, evaporative condenser is used. The method selected to increase the refrigeration effect is subcooling method. In technical terms, subcooling is the process of removing heat from the refrigerant after condensation. Hence the temperature of liquid is below the condensing temperature at a specified pressure. In terms of performance, sub cooling unit are much more efficient compared to regular condensing units since the refrigerant coming out of the condenser is in complete liquid state.

The evaporative condenser consists of condenser tubes, which are already in aircooled mode. Cold water is circulated all over the tubes carrying the refrigerant. The refrigerant within the tubes is desuperheated and then it quickly reaches the saturation temperature where the gas is condensed into liquid.

The original air conditioning test rig had air cooled condenser. In order to increase the refrigeration effect, it is converted into evaporative condenser by circulating water along with air being blown by fan.

Methods to improve coefficient of performance of refrigeration and air conditioning systems

The most commonly used methods of improve coefficient of performance of systems are:

(1) Sub cooling or Under cooling

(2) Liquid Pressure Amplification

(3) Adding additives to the refrigerant.

Out of these methods, subcooling or under cooling method is used in the experimental analysis of Improvement of coefficient of performance of the system[13]. The process of under cooling is carried out by circulating more quantity of cooling water over the condenser tubes [4]. Sometimes, sub cooling can be carried out by employing a heat exchanger. In actual practice, the refrigerant is desuperheated after compression and under cooled before throttling.

Experimental method selected to improve COP of air-conditioning system

Evaporative condenser is used to improve the COP of the system. It consists of a coil in which refrigerant is flowing and condensing inside, and the outer surface is wetted with water and exposed to steam of air to which heat is rejected principally by evaporation of water. Fig. 1 and Fig. 2. shows typical arrangement of the components of evaporative condenser. The coils are generally made of copper or steel in multiple circuits and passes. The external surfaces are sometimes finned to increase heat transfer surface. The coil has arrangement for cleaning water under fouling water conditions.

The wetting of coil is done by re-circulating system comprising water pan, a pump and water distribution system. The water distribution system mainly comprises of nozzles for spray of atomized water on the coils. The pan catches the drainage of all coils. There is a float valve to admit make up water and maintain the correct level in the pan. Centrifugal pumps of moderate head are necessary; such pumps are not affected much by extraneous matter found in such open reciprocating systems.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Evaporative condenser employed utilizes forced circulation of air with a fan to either blow or to draw air through the unit. Effective elimination of moisture from the leaving air stream by eliminators is essential to prevent projection of rust, which can deposit moisture on the surrounding surfaces. The eliminator plates work on the simple principle of abrupt changes in flow direction Moisture particles being heavier get deposited on these eliminator plates and get drained back to the sump or the pan.

Explanation of P-h diagram for experimental subcooling method

The P-h diagram (Fig.3) shows that the refrigeration effect is improved by subcooling method. Subcooling is attained when the refrigerant is cooled through some more degrees before entering the throttle valve.

The figure shows the refrigerant, after condensation process. 2'-3', is cooled below the saturation temperature T3' before expansion by throttling. Such a process is called under cooling or sub cooling of the refrigerant and is generally done along the liquid line. The ultimate effect of under cooling is to increase the value of coefficient of performance under the same set of conditions.

[FIGURE 3 OMITTED]

Specifications of existing experimental set up

Figure 4 shows the original experimental set up with following specifications:

(I) Compressor--Kirloskar AHRI3, R22 gas 1.2 HP, 1 ton

(II) Evaporator--Cooling coil, fin type, Air / Freon Heat exchanger

(III) Condenser--Coil type, Air cooling system

(IV) Expansion Valve--Thermostatic type, imported (Italy make)

[FIGURE 4 OMITTED]

Modifications done in existing set up for analysis of system

Figure 5 shows the modified experimental set up. The modifications are as follows:

* Water cooling system: Evaporative condenser

* Water Tank: It is 60 cm x 50 cm leak proof tank, 15 cm in height. It is made of galvanized plate of aluminium.

* Pump: A water pump is attached to the base of water filled tube to circulate water over the condenser coils. This water cools the refrigerant on account of the latent heat of vaporization.

* PVC Pipes: 2nos, Length- 2 ft, Diameter -1.5 inches.

* Flexible Pipes: These are two separate pieces of them. The first is to transfer water from the pump to the PVC pipe. The second one connects two PVC pipes.

* Pipe 1--Length: 2 fts, Diameter: 1 inch

* Pipe 2--Length: 1 Ft., Diameter: 1.5 inches

* Clamps: Various fittings like clamps, nuts, bolts were used to hold the set up in place

Several pipes of flexible wires were needed along with switches to provide proper electrical supply to the pump. The connections were made in such a way that the energy meter in addition to compressor and the blower calculated the power consumed by pump. A separate switch was provided to switch the pump independently of the other electrical devices.

[FIGURE 5 OMITTED]

Comparative analysis of COP

The comparison of COP of the system with air cooled condenser and evaporative condenser is as follows

The comparative analysis shows that the COP increases by nearly 8% for the system with evaporative condenser, which is quite appreciable.

Future scope of work

The limitations of evaporative condenser are:

* Since the construction of water cooled condenser is complicated, therefore the initial cost is high. The maintenance cost is also high.

* The water-cooled condensers are difficult to handle.

* The pipes are required to take water to and from the condenser.

* There is a problem of disposing the used water unless a recirculation system is provided.

* Since corrosion occurs inside the tubes carrying the water, therefore fouling effects are high.

However, there can be overcome by taking due precautions and alternative methods can be worked out in the future.

Results and discussion

The primary objective of any set up is to obtain maximum possible output using least possible input. In the experimental analysis, evaporative condenser is used instead of air-cooled condenser. The comparative analysis shows that there is an increase in COP of the machine by nearly 8%, which is quite appreciable.

The concept of subcooling or under cooling can be used more effectively to improve the refrigeration effect and consequently the COP of the system. However, future work can still be carried out further to improve the performance of the system.

References

[1] Heydari A., "Miniature vapor compression refrigeration systems for active cooling of high performance computers," Proceedings of the Inter Society Conference on Thermal Phenomena, IEEE, 2002, 371-378.

[2] Phelan P.E., Swanson J., Chiriac F., Chiriac V., "Designing a mesoscale vapor-compression refrigerator for cooling high-power microelectronics," Proceedings of the Inter Society Conference on Thermal Phenomena, IEEE, 2004,218-223.

[3] Bhattacharyya S., Mukhopadhyay S., Kumar A., Khurana R.K., Sarkar J., "Optimization of a CO2-C3H8 cascade system for refrigeration and heating," International Journal of Refrigeration, 28 (2005), 1284-1292.

[4] Incropera F.P., DeWitt D.P., Fundamentals of Heat and Mass Transfer. (John Wiley and Sons, Inc, 2002), 670-690.

[5] Coggins C., Gerlach D., Joshi Y., Fedorov A., "Compact, low temperature refrigeration of microprocessors," Proceedings of the International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006, West Lafayette, IN.

[6] Peeples J., "Vapor Compression Cooling for High Performance Applications," Electronics Cooling 7, 3 (2001).

[7] ASHRAE Handbook Fundamentals, SI Edition .1993. American Society of Heating, Refrigeration and Air-Conditioning Engineers Inc., Atlanta, USA

[8] Whitman, W.C., Johnson, W.M.1995. Refrigeration and Air condition Technology. 3rd Edition, Delmar Publishers Inc., USA.

[9] ARI Guideline.1990, Air conditioning and Refrigeration Institute, USA

[10] Dossat, R.J.1997, Principles of Refrigeration, 4th Edition, Prentice-Hall International, USA

[11] Zhaolin Gu, Hongjuan Liu, Yun Li, "Thermal energy recovery of air conditioning system--heat recovery system calculation and phase change materials development" Applied Thermal Engineering, 24 (2004), 2511-2526.

[12] Vineyard, E.A. "The Alternative Refrigerant Dilemma for RefrigeratorFreezers: Truth or Consequences" ASHRAE Transactions, 1991, Vol. 97, Part 2,955-960.

[13] Vineyard, E.A.; Sand, J.R.; and Miller, W.A. "Refrigerator-Freezer Energy Testing with Alternative Refrigerants." ASHRAE Transactions, 1989, Vol. 95, Part 2,295-299.

[14] Clark, Earl M. et. al. "Retro-fitting Existing Chillers with Alternative Refrigerants." ASHRAE Journal, April 1991. Vol. 33, No.2, 38-41.

U.S. Wankhede

Department of Mechanical Engineering,

G. H. Raisoni College of Engineering,

Digdoh Hills, Hingna Road, Nagpur (M.S.) -440016, India

Email: udaywankhede@yahoo.co.in
Table 1: Comparative analysis

Sr. No.   COP (Air cooled Condenser)   COP (Evaporative Condenser)

  1                1.25                        1.59
  2                1.48                        1.56
  3                1.61                        1.735
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Title Annotation:coefficient of performance
Author:Wankhede, U.S.
Publication:International Journal of Applied Engineering Research
Article Type:Report
Date:Mar 1, 2009
Words:1543
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