Performance and Optimisation of Cooling Tower

Proceedings of the National Conference on Recent Trends in Renewable Energy Technology (NACOEE 05) 09-10 December 2005 National Engineering College. K. R. Nagar, Kovilpatti-628 503. Tamil Nadu. pp-131-138

131

PERFORMANCE AND OPTIMISATION OF COOLING TOWER REVEALS ENERGY SAVING OPPORTUNITY

Sharan.Shegedar 1, B.M.Shrigiri 2, S.M.Nagure3, M.B.Shetti 1, Chandrashekhar.R.Tamburimath 4

1Department of Mechanical Engineering,College of Engineering, Ambajogai 431517

2Department of Mechanical Engineering, Appa Institute of Engg & Technology, 585103 3Department of Mechanical Engineering,T.B.G. Polytechnic, Ambajogai 431517.

4Department of Mechanical Engineering, K.C.T College of Engg, Gulbarga 585103 E-mail: [email protected], [email protected]

ABSTRACT The present paper will outline the main aspect of energy saving opportunity in cooling tower by

studying its performance and optimization. Cooling tower is a specialized heat-exchanging device, which dissipates low grade unusable heat to atmosphere. So far very less attention has been paid to cooling tower. With a deeper insight study performance and optimization of cooling tower can increase the efficiency of the entire power plant and reveals energy saving proposals.

To generate electricity in thermal power station fossil fuels are used. As the study shows that fossil fuels are diminishing in nature and will be exhausted within hundred years, so efficient use of this low grade energy is necessary to increase the efficiency of the plant. In this paper an attempt is made to analyse the energy saving opportunities in cooling tower. This paper has exhaustive data taken during operation of cross flow induced draft cooling tower unit-III at Parli (V) thermal power station. As cooling tower performance depends on wet bulb temperature, range, discharge, inlet water temperature, refrigerating effect and NTU. Cooling tower performance mainly depends on wet bulb temperature and weather conditions. Considering the above parameters cooling tower efficiency and performance were determined and there is enough scope for energy saving opportunities by using sump thermostat and by replacing solid blades of induced draft fan with hollow plastic blades. 1. INTRODUCTION

Cooling tower is an artificial device used to dispose of waste that from industrial processes and from refrigeration or air conditioning systems, where it has been cheaper or more convenient to reject heat to the atmosphere rather than to water in a near by River, Lake or Ocean.

In a fossil fire thermal power plant nearby 30 percent of the heat energy produced in the boiler is rejected in condenser, large quantity of cooling water is required for condensing the exhaust Steam. There are two basic types of cooling system. They are Direct cooling system (DCS) and closed loop recirculatory cooling system (RCS).

In the closed loop recirculatory cooling system with the evaporative cooling tower, the hot water that comes from the condenser is cooled in the tower by direct contact with ambient air and this cooled water is recirculated through the condenser. Most of the heat transfer that occurs in the tower is due to evaporation and it is estimated that about 2.5 to 3 percent of the circulating cooling water is required as make up to compensate the loss due to evaporation, drift and blow down requirement. Water availability for power generation is becoming continuously scare due no many other important uses of water like drinking, irrigation and other industrial requirements and to the power plant.

Performance and Optimisation of Cooling Tower Reveals Energy Saving Opportunity

132

This report covers the thermal performance of cooling tower of unit III of the Parli Thermal Power Station and energy saving by using sump thermostats. The plant has installed one cross flow induced draft cooling tower of 10 cells with a design flow of 31000 m3/h and 9.5 0C range. The plant has replaced all the metallic aluminum blades of the cooling tower fans with fibre - reinforced blades the detailed specifications of the cooling tower are given in appendix 1.

APPENDIX 1.Specifications of the Cooling Tower [1]

Design Date

Type and Design of Tower

Induced Draft, R.C.C, one block, ten cells, direct contact, double cross floor, packed type manufactured by M/s Paharpur Cooling Tower Pvt. Limited

Design Flow of water 31000 m3/hr Design Heat Load 294. Mcal Design wet bulb temperature 25 0C Hot water inlet temperature 39.5 0C Cold water outlet temperature 30 0C Cooling Range 9.5 0C Highest dry bulb temperature 44 0C Drift loss 0.05% (15.5 m3/hr) Evaporation loss 1.53% (474.3m3/hr) Flow rate per cell 3444m3/hr Fan Details Make Paharpur Marley HP 4 8 No. of fans per cell One Type Axial Flow propeller Material of fan blades and hub Fibre Reinforced plastic. Dia. of fan 8.53m Number of blades per fan 8 Blade angle Adjustable (22.50 to start) Fan air flow rate per cell 21,46,000m3/hr

Power consumption at motor inlet kW per fan

92.32 kW

Motor Details Make Siemens Rated output 101 kW Full load Speed 1485 rpm

2. WORKING OF COOLING TOWER A cooling tower (CT) is a specialized heat exchanger in which two fluids (air and water) are

brought in to direct contact with each other to affect the transfer of heat. In the " Spray-filled" tower as shown in fig. 1 below is accomplished by spraying a flowing mass of water in to a rain like pattern through which an upward moving mass flow of cool air is induced by the action of a fan [2].

National Conference on Recent Trends in Renewable Energy Technology (NACOEE 05)

133

Fig 1. Cross flow induced draft fan.

Ignoring any negligible amount of sensible heat exchange that may occur through the walls of the

tower, the heat gained by the air must be equal to the heat lost by water. A portion of the water absorbs heat to change from a liquid to vapour at constant pressure. This heat

of evaporation at atmospheric pressure is transferred from the water remaining in the liquid state into the air stream.

However, because of the evaporation that takes place within the tower is less than that entering it and a proper heat balance must account for this slight difference. Hence the rate of evaporation must be equal to the rate of change in the humidity ratio (absolute humidity) of the air stream.

The below Fig 2 shows the temperature relationship between water and air as they pass through a counter flow cooling tower. The curve indicates the drop in water temperature (point A to point B) and the rise in the air wet bulb temperature (point C to point D) in their respective passages through the tower. The temperature difference between the water entering (tw1) and leaving (tw2) the cooling tower (A minus B) is the range (tw1- tw2). The range is determined by the heat load and water flow rate, not by the size of the cooling tower.

The difference between the leaving water temperature and the entering air wet bulb temperature (twb), B minus C is the approach (tw2-twb) of the cooling tower. The approach is the function of the cooling tower capability, and a larger cooling tower produces a closer approach (colder leaving water) for given heat load, flow rate and entering air condition. Thus the amount of heat transferred to the atmosphere by the cooling tower is always equal to the heat load imposed on the tower, while the temperature level at which the heat is transferred is determined by the thermal capability of cooling tower [3].

The entering air wet bulb temperature affects the thermal performance of cooling tower. Entering air dry bulb temperature (tdb) and the relative humidity have an insignificant effect on thermal performance, but they do affect the rate of water evaporation.

2.1 Terminology

The following terms are commonly used when referring to the performance of a cooling tower. [3] Approach: It is the difference between the temperature of the cooling water leaving the tower and the wet bulb temperature of the air entering to tower.

Range(R): Temperature difference between temperature of cooling water entering the tower and the temperature of water leaving the cooling tower.

Blow down: Water discharged to the drain periodically in order to avoid build up of dissolved solids.


134

Fig 3. Temperature relationship between water and air passing through cooling tower.

Fill: The structure that forms the heat transfer surface within the tower cooling water from the condenser or coil is distributed along the flow passage of the fill through the nozzles down to the water basin.

Make - up: Water added to the circulating water to compensate for the loss the water to evaporation drift and blow down.

Capacity: Average rate of flow of water circulating in the system and being handled by the cooling tower (m3/hr)

Drift: Loss of water in the form of air borne particles carried way by the exhaust air expressed as percentage of circulating water flow rate.

Wet bulb temperature (twb): Temperature as indicated by a thermometer the bulb of which is kept moist by wick over which air is circulated. This is theoretically, the lowest temperature to which water can be cooled and it depends on the dry bulb temperature and relative humidity of the ambient air.

Dry bulb temperature (tdb): External out door temperatures as indicated by a dry bulb thermometer in 0C.


135

Recirculation: Tendency for the hot air to be sucked along with the ambient (cold) air in to the tower this takes place in mechanical draft cooling towers and this has an adverse effect on the tower performance if spacing of cells and orientation of the tower with respect to prevailing winds are not proper. 3. ENERGY AUDIT OF COOLING TOWER

The energy audit of cooling tower is performed to assess the present level of approach and range against their designed value and also to identify the areas of energy wastage. The instrumented energy audit was carried out to know the various parameters such as, wet and dry bulb temperature, Cooling water (CW) inlet and outlet temperature, water and air flow rate and electrical parameters of cooling tower fan and motor.

4. EXPERIMENTATION

The first objective of study was to evaluate present heat removal load of cooling tower within the working condition keeping in mind the draw back and plant problem. The second objective was to suggest an optimum low cost solution to management towards energy saving.

By using the parameters of circulating water conditions such as inlet and outlet temperature of cooling water, wet bulb temperature (WBT),dry bulb temperature (DBT), atmospheric pressure, water flow rate, fan consumption and pump consumption, the performance and optimization of the cooling tower is conducted. These readings are taken directly from the inlet and outlet of the cooling tower with the help of measuring instruments such as dry bulb thermometer, wet bulb thermometer and hygrometer. Unit load, fan consumption (voltage and current) and flow rate of cooling water are taken at unit control board.The General observations made at cooling tower site are, Channeling of water due to choking of distributor holes, algae and fungi formation on structural and the water flow rate is reduced to 27000 m3/hr.The observations are drawn by, taking constant flow rate of 27000m3/hr and with all 10 fans working on 11th & 12th Aug 2004 on 13th Aug 2004 one fan is switched off. See the following Table 1.

Table1: Experimental observation of cooling tower unit III

The following observations have been drawn for fan consumption for three consecutive at 2300hrs see Table2.

Table 2: Experimental observation of fan consumption

Date Time In hrs

Load In

MW

DBT In 0C tdb

WBT In 0C tWb

CW Inlet In 0C tW1

CW Outlet In

0C tW2

Humidity in %

11-08-2004 2300 185 25 23 41 28.75 84 12-08-2004 2300 150 24.5 23 40 28 87 13-08-2004 2300 170 24.6 22.9 40.5 29.5 84

Voltage V in Volts 415 415 415 415 415 415 415 415 415 415 11th Aug

Current I in Amps 120 110 105 105 110 100 90 100 90 105


Current I in Amps 120 110 105 105 110 100 90 100 90 105


Current I in Amps 120 110 105 105 110 -- 90 100 90 105


136

4.1 CALCULATIONS AND RESULT SHEET

Cooling tower efficiency = 1001

21 xtttt

wbw

ww

Refrigerating effect (R.E) = mw Cp R/210

Where, mw is mass of cooling water kg/min, Cp is specific heat of water in KJ/Kg K. L and G are water loading and gas loading in Kg/hr From below Table3 and Fig 3.where, Ha enthalpy of air water mixture at wet bulb temperature

and Hw enthalpy of water [4].

Table 3: Calculation of test characteristic curve on 11-08-2003

Water

Air side

Enthalpy difference

Description tw in

C Hw in kJ/kg

Description Ha kj/kg

Hw-Ha

1/(Hw-Ha)

tw2+(0.1xR) 29.97 125.75 Ha+(0.1xL/GxR) 87.92 37.83 0.0264 tw2+(0.4XR) 33.65 140.3 Ha+(0.4xL/GxR) 91.97 48.33 0.0207 tw2+(0.6XR) 36.1 151.24 Ha+(0.6xL/GxR) 94.67 56.57 0.0177 tw2+(0.9XR) 39.77 166.58 Ha+(0.9xL/GxR) 98.72 67.86 0.0147

Sum of 1/(Hw-Ha) 0.0795

Tower demand (NTU) = sum of 1/(Hw - Ha) x 4

Range

0.2435

Tower demand (NTU) = 0.2435

Log mean enthalpy difference Hm =

83.3786.67ln

83.3786.67 = 51.391

KaV/L (NTU) = R/ Hm= 0.238

Hence 0.238 is less than 0.2435 performance is acceptable [5].

Fan power consumption. = 3 VIcos watts = 3 x 415 x 0.85 x (120+110+105+105+110+100+90+100+90+105) = 632365.25 watts


137

Figure 3. Test characteristic curve of cooling tower on 11-08-2003

020406080

100120140160180

29.97 31.97 33.97 35.97 37.97 39.97 41.97

Water temperature tw in C

Enth

alpy

in k

j/kg

Hw Ha

5. RESULT AND DISCUSSION On the basis of observations and calculations conducted on cooling tower, the following results are

obtained and shown in below Table 4. Table 4: Result sheet of cooling tower performance.

Date Range C

Approach C

L/G Kav/L CT Eff. %

R.E Tons

Power KW

Designed 9.5 5 1.264928 0.223289 65.52 97862.82 923.2 11-08-04 12.25 5.25 1.1017 0.2435 62.36 100188.92 643.051 12-08-04 12 5 1.1017 0.2784 70.58 107665.71 635.42 13-08-04 11 6.6 1.22412 0.213128 62.5 98693.57 571.26

On the basis of results on 13-08-2003 at 1200hrs to 2400 hrs one fan is switched off. The cooling tower performance is satisfactory during night hours. The NTU (diffusion units), Refrigerating Effect, Cooling tower Efficiency, outlet temperature of cooling tower are nearly matching with the designed values even though one fan is switched off. During this period the performance is satisfactory. So there is scope for energy saving when wet bulb temperature is low

Comparing the results obtained on 12-08-2003 and 13-8-2003 at 2400hrs (the weather conditions are almost same). Therefore here is enough opportunity for energy saving by switching off one or two fans by using sump thermostats, when WBT fall below designed value. The energy saved is equal to Fan consumption on 11-8-2003(10fans working) minus Fan consumption on 13-8-2003(9 fans working), i.e. 72.05KW. Approximate energy saved per annum when on fan is switched off for 6 hours, the energy saved is 157789.5 KWH.

The cooling tower has 10 cells, fans in cell No. 1,2,3,4 are solid blades and the fans in cell No.5, 6,7,8,9 are hollow blade fans. Therefore the power consumption of fan in cells 1,2,3,4 is more than the cells 5,6,7,8,9,10. The energy saving per annum by replacing hollow blades to cell No 1,2,3 and 4 is approximately 347894.64 KWH.


138

6. CONCLUSION

The field study established the scope of energy saving by stopping the cooling tower fan. Moreover, there exits a potential for savings on fan and replacing solid blades with hollow blade fans.

7. REFFERENCES [1] Performance guarantee provided by Paharpur Cooling Tower Pvt. Limited.

[2] Cooling tower performance: Basic Theory and Practice, Issue 1,

[3] www.marleyct.co/pdf_forms/CTll-1pdf

[4] Veeresh Angadi : Performance Analysis of Cooling Tower, C.P.R.I, Trivendrum 1998-99.

[5] Cooling Tower Technical Site of Daeil Aqua Co, Ltd for cooling tower Engineers,

[6] Operators and Purchasers, 2000-2001.

[7] www.coolingtowertechnicalsite.com

[8] D.Q.Kern, Process Heat Transfer, TATA Mc Graw-Hill, 1997.

Performance and Optimisation of Cooling Tower

Documents

Transcript of Performance and Optimisation of Cooling Tower