Design and operating characteristics of evaporative cooling systems

S. Datta, P. N. Sahgal, S. Subrahmaniyam, S. C. Dhingra and V. V. N. Kishore* Department of Chemical Engineering, Indian Institute of Technology~ New Delhi, India

*Tata Energy Research Institute, New Delhi, India Received 28 June 1986

Evaporative cooling systems are commonly used in countries where the climate is hot and dry, as found in most zones of India and Australia. The potential energy savings envisaged by replacing conventional refrigerated systems by evaporative systems is ~ 75°. Indirect systems can achieve comfort conditions similar to refrigerated systems in climatic zones where the wet bulb temperature is usually < 25C. The comfort afforded by indirect evaporative systems is superior to that achieved by direct evaporative systems. An 8.5 ton indirect--direct evaporative cooling system has been fabricated and tested and its performance compared with a computer prediction. The system's scope for use in India and Australia is analysed. (Keywords: cooling; evaporative cooling systems; design)

Caractbristiques de la conception et du fonctionnement des syst6mes frigorifiques bvaporatifs

Les systbmes friqori[iques bvaporatifs sont utilisbs couramment dans les pays [~ climat chaud et set', tels que ceux qu'on rencontre dans la plupart des r~oions de I'Inde et de l'Australie. Le remplacement des systbmes [rigorifiques traditionnels par des svstbmes, bvaporat![s conduit h des ~eonomies d'~neryie possibles h 7~5 o/,, environ. Les systbmes indirects peuvent assurer des conditions de conJbrt semblables h celles des systbmes .~i.qorifiques dans des zones climatiques oh la tempbrature de bulbe humide est habituellement inferieure /I 25"C. Le eon[brt que permettent les systbmes bvaporat!~ indirects est supbrieur a celui que permettent les systbmes bvaporatiJJ directs. Un systbme bvaporatif indirect~irect de 30 k W environ a ~tb construit et essay~" et sa perfi~rmanee a bt~, comparbe h celle prbvue par ordinateur. On analyse la portbe du systbme h utiliser en lnde et en Australie. tMots cles: refroidissement: systbmes frigorifiques/t evaporation: conception)

Historical background

Evaporative cooling has been used for many centuries in tropical countries, for comfort cooling. The earliest systems used were the fountains and underwater storage tanks found in many palaces in India, Egypt, Iran etc. Ancient buildings combined the advantages of natural ventilation with evaporative cooling, by the use of domes, high roofs, ponds and fountains.

Evaporative cooling has gained popularity more recently for cooling process streams in the chemical industry and in air conditioning systems; by the use of cooling towers.

Advantages over other systems Evaporative cooling systems are of two types: indirect and direct. Indirect evaporative coolers lower the inlet ambient air temperature at a constant absolute humidity level; thus the cooling is sensible (see the left-hand side of Fiyure 1). Direct coolers introduce moisture into the inlet air stream, and cool the air stream adiabatically.

An indirect system can be coupled with a direct system to give a maximum cooling effect (Figure 1). The main advantages of evaporative cooling systems over conventional air conditioning systems are:

1. large ventilation rate; about 10 times higher than current mechanical refrigerated air conditioning systems;

2. low energy consumption: an energy saving of the order of 75 ,Qo can be achieved; 3. low maintenance: substantially lower installation and maintenance costs; and 4. easy fabrication, installation etc.

Evaporative cooling systems are thus rapidly gaining popularity over conventional mechanical air con- ditioning equipment in most zones where the climate is hot and dry, and where the wet bulb temperature rarely exceeds ~ 25'~'C.

Simulation of indirect and direct cooling systems

To assist in the assessment of the performance of indirect and direct evaporative cooling systems appropriate computer programs have been prepared. Upon loading the input parameters, such as ambient weather conditions, room load, type of system etc., the program calculates the air flow rate, water rate, cooling tower size, heat exchange area, etc. Obviously, the programs are specific to the units used. Two systems were chosen for computer simulation. First, an indirect system using a natural draft cooling tower with a finned tube heat exchanger, coupled with a direct contact evaporative air cooler. Air is cooled in two stages, an indirect sensible stage followed by an adiabatic stage as in Fiyure 1. The cooling path is shown as A B ~ on a psychometric chart

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Evaporative cooling systems. S. Datta et al.

(Figure 2). Second, an indirect system using a bank of tubes with air flow inside the tubes, the outsides being cooled by water sprays and forced draft air (path A-D, Figure 2). The simulation programs were written in FORTRAN IV and implemented on an ICL 2909 computer.

Direct evaporative cooler

Table 1 shows the levels ol cooling that can be theoretically achieved by a desert cooler compared with a few sample experimental results. The theoretical design predictions are within 5'I~ of the experimental results obtained.

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Figure 1 Indirect-direct cooler. 1, Cooling tower; 2, blower; 3. heat exchanger; 4, mist eliminator; 5, water supply to heat exchanger; 6, water tank; 7, spray nozzles Figure 1 Refroidisseur indirect~lireet. 1, Tour de refi'oidissement: 2, ventilateur; 3, bchangeur de chaleur: 4, pibge h gouttelettes: 5. alimentation en eau; 6, rgservoir d'eau: 7, ajutages de pulv&isation

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Dry bulb temperature p

Figure 2 Comparison of direct and indirect systems. , Direct cooler; - - , indirect cooler. A, Inlet, dry bulb = 45.0°C; B, coil exit, dry bulb=29.0°C, wet bu lb= 18.8°C; C, outlet, dry bulb=20.0°C, wet bu lb= 18.8°C; D, outlet, dry bulb = 24.0°C, wet bulb = 2 3 C Figure2 Comparaison des syst&nes directs el indirects . . . . . Refroidisseur direct; - -, refroidisseur indirei-t. A, Entree, 45~C bulbe sec; B, sortie de l'bchangeur, 29,0°C bulbe sec, 18,8°C bulbe humide; C. sortie, 20,0°C bulbe sec, 18,8°C bulbe humide; D, sortie, 24,0~C bulbe sec, 23°C bulbe humide

Experimental test rig

An indirect~lirect space cooling system following the schematic diagram in Figure I of 8.5 tonnes capacity is under development at IIT, Delhi, India. The natural draft cooling tower is 4.881n hmg by 2.44m wide by 2.44 m high. The cross-flow air-to-water heat exchanger is a six-row finned tube (1 × l × 0.15m), with four fins per centimetre. The blower is an axial fan type having a free air delivery of 2 4 0 0 0 m 3 h 1 {14000f t3min- t ) and a power rating of 1.2 kW. This indirect stage is followed by an adiabatic cooling stage of a spray chamber (1 × 1 x 1 m) followed by a mist eliminator.

A comparison of some experimental results with those predicted by the simulation program is given in Table 1. If should be noted than an observed outlet air temperature of about 1.5-2.5°C lower than the prevalent ambient wet bulb temperature could be achieved. The 22 24:C outlet air is suitable for comfort cooling,

Further design improvement,~

In an attempt to improve perlbrmance and reduce costs, a small size rig is being built in which the cooling tower and air/water heat exchanger are constructed as a single unit Preliminary laboratory experiments mdicate air temperature approaches to within 2 3°C of the ambient wet bulb temperature, Further cooling is then possible by direct evaporative cooling.

Comparison with c u r r e n t e v a p o r a / i t : e roolrt c o o i e r s

To illustrate the advantages of the systems under development, a comparison with a typical currently marketed direct evaporative room cooler may be made. The cooler tested was the conventional wetted pad type. The pads consist of a mat of wood shavings, and the structure is made of painted galvanized iron sheeting. The air to the room is drawn through the wetted pads using a 180W exhaust fan. A 2 - 3 C approach to the wet bulb temperature was normally observed at the cooler, and the average temperature in the room was maintained at

Table 1 Indirect cooling system: comparison of experimental (A) and theoretical (B} results Tableau 1 Syst&ne de refroidissement indirect: comparison des rbsuhats thi, oriques et experimentaux

Run 1 Run 2

No. Parameters A B A B

1 Inlet temperature, dry bulb (~C) 42 42 42 42 2 Temperature, wet bulb (°C) 24 24 26 26 3 Inlet relative humidity (%) 26 26 28 28 4 Coil exit temperature, dry bulb ( C ) 28 30 32 32 5 Coil exit temperature, wet bulb (°C) 22 20.8 23.5 23.5 6 Coil exit relative humidity (%) 50 43 48 48 7 Outlet temperature, dry bulb (°C) 23 21.8 24 24 8 Outlet temperature, wet bulb (°C) 22 21 23.5 23.5 9 Outlet relative humidity ( ~ ) 90 90 95 95

10 Cooling tower water temperature (°C) 25 27 28 29 l I Temperature of water at coil exit (°C) 28 30 32 32

Experimental and theoretical readings

Run ~ Rutl 4.

A B A l~

40 4(.1 40 40 ~5 ~ 25.5 ~ < 23.5 32 32 24 24 30 31.5 28 29.5 22.8 23.3 20 20.3 55 48 45 43 ~'~ 8 ~__ ,3 22,8 23.3 22 20.~

100 I00 95 95 27 28.5 255 26.5 30.5 31.5 28 29.5

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5-6°C above the wet bulb temperature. Since the room temperatures were of the order of 25-30°C, this sort of cooling would be classified as relief cooling rather than comfort cooling.

Economic analysis

Comparison with mechanical compression systems

Evaporative cooling has been classified by the California Energy Commission as a non-depletable energy source: thus it is also commonly called free cooling. This term refers to the production of cooling without the use of mechanical compression. It is not literally 'free' as it requires a cooling tower and pump and fan operation. Evaporative cooling does, however, provide a super- efficient method of comfort cooling at a much lower power and initial investment.

The advantages of indirect~lirect systems over mechanical compression systems are summarized below. This system offers:

1. low power consumption; it consumes one-twelfth of the power used by mechanical compression systems; 2. low initial investment; indirect~tirect evaporative systems cost 20 ?'o of the cost of mechanical compression systems; 3. reductions in installation and maintenance costs; 4. larger fresh air flow rates: thus the room environment is cleaner: 5. air flow rate as a gain component for cooling, as opposed to mechanical compression systems where air flow rate is a load component; 6. reductions in insulation requirements as evaporative cooling systems have a very high air flow rate and provide low cost cooling; and 7. evaporative cooling used in conjunction with refrigerated cooling for specialized applications such as cold storage etc.

Scope of applications

Evaporative air conditioning can be used for all types of homes, offices, cinemas, hospitals, schools, factories etc. where comfort cooling is required.

Climatic conditions

Evaporative systems have been found to be economical in all zones where the wet bulb temperatures are below 24~ 26~'C for most of the hot season. These conditions are readily available in many parts of the USA, Australia, India and the Middle East. These systems are particularly attractive in rural areas.

Table 2 Direct evaporative cooler (wetted pad type): comparison of experimental (A) and theoretical (B) results Tableau 2 Syst~me i,t, aporat i f direct (type ~t matelas mouill~): comparaison de.s" ri~sultats thi'oriques et experimentaux

Run I Run 2

Parameters A B A B

Inlet temperature, dry bulb CC) 34 34 37 37 Inlet temperature, wet bulb CC) 25.3 25.3 24 24 Inlet relative humidity (%) 48 48 34 34 Outlet temperature, dry bulb (°C) 27 26.5 24 25.5 Outlet temperature, wet bulb (cC) 25.3 25.3 24 24.5 Outlet relative humidity (%) 87 90 100 90

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Enerqy conservation

In many countries a restriction has been placed on the use of air conditioning systems due to power shortages. In these areas, evaporative air conditioning is the only solution.

Humid climates and special application

A combination of an indirect evaporative air conditioner with a mechanical compression type system has been found to provide comfort conditions in very humid climatic zones. This sytem consumes much less power than the conventional type of mechanical compression system.

Acknowledgements

The authors wish to acknowledge the financial support and co-operation extended by the staffof the Tata Energy Research Institute, New Delhi, India, for this work. The authors would also like to thank Michael Woolridge of the Commonweal th Scientific and Research Organisation, Melbourne, Australia, and his group, for their encouragement and assistance in obtaining data in this field.

References 1 Pescod, D. Energy savings and performance limitations with

evaporative cooling in Australia, Technical Report no. 5, Division of Mechanical Engineering, CSIRO, Victoria, Australia (1976)

2 ASHRAE Handbook: Fundamentals and Equipment (1973) 100-150 3 Eskra, N. Indirect/direct evaporative cooling systems ASHRAE .1

(May 1980) 21 4 Supple, R. G. Evaporative cooling for comfort ASHRAE J (August

1982) 36 5 Kays, W., London, A. I. Compact Heat Exchangers McGraw-Hill

(1955) 1-100

Appendix Design criteria for indirect-direct cooler

A design algorithm is given and is illustrated with a sample calculation.

Design outdoor dry bulb temperature, DB~ = 4 5 C Design outdoor wet bulb temperature, WB~ = 2 3 C Cooling tower water temperature, TC:

TC = WB~ + 3 = 26°C

Air temperature leaving coils (dry bulb), DB2:

DB z = TC + 3 = 29°C

Wet bulb temperature for air leaving coils, W B 2 = 18.8C

Wet bulb depression for air leaving coils, WBD~p :

WBD¢p = DB 2 - W B 2 = 11 ~C

Dry bulb temperature out of evaporative cooler, DB 3 = 20.2~C

DB3 = DB2(0.8 × WBDep )

Design indoor temperature, dry bulb, DB¢:

DB4 = DBt - 19c~C = mixing temperature = 26 C

Temperature rise of cooling air, a~:

~1 = D B 4 - DB3 = 5.8~'C

Specific volume of dry air at cooler discharge= 0.86 m 3 kg - 1 (Room load)= Q~ = 100000 Btu 11 -~ = 25 000 kcal h-

Cp = 0.25 kcal kg

~i,r=Q 1 × Specif_lC volume = 15(X)0m3 h i Cp x 6+

Where K~,~=volumetric flow rate of air and Q ~ - r o o m heat load

Enthalpy of entering air, H ~ = 16,0 kcal kg

Enthalpy of air leaving coil, Hz = 12.3 kcal kg Enthalpy change, flu:

6 u = H 1 - t t 2 = 3 . 7 k c a l k g ~

Total heat transfer in coils, Qz

Q2(6, x K,,0/Specific volume = 64 500 kcal h

Temperature rise in cooling water, 6w = 3 ( ' Water required, V,:

l'~ = (Q2 /6~ )/60 = 350 dm 3 min i

The details of cooling tower and heat exchanger design are outside the scope of this paper. Coil selection (from Reference 5): heat transfer a r ea= 120m 2 Cooling tower selection (from Reference 2): tower size = 8 × 8 × 8 f t ; water loading I (US gallon m i n - l ) f t - 2 : pump horse power = 2.0 Estimation of room heat load (from Reference 2): load = 100000 Btu h-~ or 8.5 tonnes refrigeration

The theoretical results obta)ned from the computer program based on the above algorithm have been summarized in Table 3. This table assumes inlet temperatures over the ranges which are commonly found in India and Australia, and predicts the outlet temperatures, heat transfer area, air flow rate, room load and water flow rate for a given set of inlet ambient weather conditions.

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