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Principles of Cooling Towers


All cooling towers operate on the principle of removing heat from water by evaporating portion of the water that is recirculated through the unit. The heat that is removed is cause latent heat of vaporizaton. Each one pound of water that is evaporated removes approximately 1,000 BTU'S in the form of latent heat.

COOLING TOWER TERMS AND DEFINITIONS

BTU (British Thermal Unit)
A BTU is the heat energy required to raise the temperature of one pound of water one Fahrenheit in the range from 320 F to 2120F

Cooling Range

The difference in temperature between the hot water entering the tower and the cold water leaving the tower is the cooling range.

Approach

The difference between the temperature of the cold water leaving the tower and the water temperature of the air is known as the approach. Establishment of the approach fixes operating temperature of the tower and is a most important parameter in determining the tower size and cost.

Drift

The water entrained in the air flow and discharged to the atposphere. Drift loss does not include water lost by evaporation. Proper tower design can minimize drift loss.

Heat Load

The amount of heat to be removed from the circulating water within the tower. Heat load equal to water circulation rate (gpm) times the cooling range times 500 and is express in BTU/hr. Heat load is also an important parameter in determining tower size and cost.

Ton
An evaporative cooling ton is 15,000 BTU's per hour/3000 kcal/hr.

Wet-Bulb Temperature
The lowest temperature that water theoretically can reach by evaporation. Wet-Bulb temperature is an extremely important parameter in tower selection and design and structure measured by psychrometer

Pumping Head
The pressure required to pump the water from the tower basin, through the entire system return to the top of the tower.

Makeup
The amount of water required to replace normal losses caused by bleed off, drift, and evaporation.

Bleed Off

The circulating water in the tower which is discharged to waste to help keep the dissolved solids concentration of the water below a maximum allowable limit. As a result of evaporation dissolved solids concentration will continually increase unless reduced by bleed off.


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Article 2

Introduction; Cooling tower is totally misunderstood and is orphan products. As cooling word is associated with it, HVAC people have taken position of its good father.

Principle of Cooling Tower : Cooling Tower does not work on principle of refrigeration. It works on adiabatic/Evaporation principle. Hence it works on mass transfer, heat transfer is just by product of mass transfer. If there is no mass transfer, no evaporation, there is no heat transfer of all and hence no cooling.

Water cannot be cooled with less than 30C approach. Hence cold water temperature achievable shall be 30C and more than wet bulb temperature of air. Wet bulb temperature considered should be actual wet bulb temperature entry of entering air into cooling tower, not atmospheric wet bulb.

Actual wet bulb entering in cooling tower i effected due to many factors. Heat source near cooling tower, air movement of cooling tower is hindered; recycling because of size of tower and orientation of tower in respect to wind flow.

We shall discuss above points in details now:

1. What is Cooling tower?
Basically cooling tower is process water recycling equipment, where hot water is cooled to a atmospheric temperature, due to evaporation of fraction of circulating water (1 to 2%) which comes in contact with circulating air and remaining (99 to 98%) water cools to cold water temperature, and is recycled in system. Water lost is added as make up water.

2. Does Cooling tower cools the water?
Cooling tower is not a refrigeration machine; it cannot cool water if there is no heat input in system or temperature rise of system. Cooling tower will cool hot water to atmospheric temperature only. Fall in cooling tower temperature shall be same as rise in system.

3. Cooling tower utilizes fan power, and then it must cool better than cooling tower without fan, natural draft or fanless one?
Fan is not acting as compressor. It gives mechanical movement to air. It increases G/L ratio. Hence efficiency of fan type tower may be more than natural draft tower. But it can not cool water less than atmospheric conditions.

4. Up to what temperature cooling tower can cool water?
Cold water temperature from cooling tower depends upon mainly wet bulb temperature and cooling range.
Wet bulb is the lowest theoretical temperature, which the water can be, cooled. Practically the cold water temperature approaches but does not be equal to wet bulb temperatures, because some drive is required for evaporation of water. This drive i.e. temperature difference between cold water temperature and W.B.T. is called approach. Minimum approach (drive) is 30C.
Real approach shall depend upon cooling tower design, mainly air to water contact and contact time. This approach, hence cold water temperature depends upon range also. Approach is proportions to range. As for higher range (more heat to be released per kg of water) more evaporation rates are required. Hence more drive is required. Hence more approach is required.

Range and approach relation for same cooling tower shall be as below: (wet bulb temperature: 24-280C)

Range (0C)

4 6 8 10 12 14 16
Approach (0C) 4 5 6 7 8 9 10




5. Can cooling tower with fan cool water below wet bulb temp.?
Now W.B. is theoretical lowest temperature which can be achieved b adiabatic cooling on which cooling tower works. If we want lower temperature than W.B., we have to go for refrigeration and compressor shall be needed. In cooling tower fan does not work as compressor, and cooling tower with fan is not a refrigeration machine.

6. By increasing fan power can we get opproaches below 30C
Approach depends upon contact and contact time. There is limiting L & G ratio to achieve above as optimum. By increasing fan power no finer result shall be achieved only fan power and and draft loss shall increase.

7. How to get finer approaches?
Contact and contact time are proportional to height of tower. We have to increase height to achieve finer approaches. This will increase pumping power as well fan power and cost of tower. Following table give relation between height of tower and approaches.

Range : 8-100C

Approaches (0C)

Height of Tower (Ft.)
8-10 15-20
5-8 25-30
3-4 35-40






8. What other factors should be kept in mind to select proper tower.
Other factors which effect cooling tower is recirculation and interference, recirculation is part of discharge air entering back into suction of air. Large towers have more recirculating than smaller one. Rectangular tower has more recirculation than round one. Longitudinal wind direction will have lesser recirculation than perpendicular one. Lower wind velocities have lesser recirculation, greater wind velocity more recirculation. By increasing fan cylinder height. Recirculation can be decreased.

Heat source up wind of the cooling tower cab elevate the wet bulb temperature of the air entering, the cooling tower, there by affecting its performance. Such heat source can be heat/steam releasing equipment in vicinity of tower or another tower in the immediate vicinity. Restriction to the free flow of air to cooling tower also increases recirculation and interferences.

9. What should be ideal location of cooling tower?
Cooling tower should be location in separate corner of factory where there should be no restriction to free flow of air to cooling tower. Sufficient space should be left (1/3 to 1) length between two larger cooling tower say more than 100' in length to avoid interference. Noise or vibration may not create problem to vicinity. Hot and humid, air will be discharged from cooling tower. Hence location should such that it should not create problem due to dampness to vicinity. Larger cooling tower should be placed longitudinal to wind direction to avoid effect of recirculation. Ideal position to rooftop.

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