Cooling Tower

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Cooling tower

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3.0

OBJECTIVES

The main objectives for this experiment are: 1. To determine the correlation of water to air mass flow ratio with increasing water flow rate. 2. To determine the cooling load effect. 3. To know the effect of different flow rates on the wet bulb approach. 4. To estimate the evaporation rate of water (water loss) for the tower.

4.0

THEORY

A cooling tower is a specialized heat exchanger that has been altered in which air and water are brought into direct interaction for the transfer of heat to take effect. In order to accomplish that, it is spraying a flowing mass of water by the spray-filled tower into a rain-like pattern, through which an upward moving mass flow of cool air is encouraged by the action of a fan. The principle of evaporative or „wet-bulb‟ cooling is used in cooling tower in order to level-headed the water. It has some advantages over a conservative heat-exchanger such as it can attain water temperatures below the temperature of the air used to cool it. Besides that, it is also lesser and cheaper for the identical cooling load. The heat increased by the air must equivalent to the heat lost by the water by equilibrium, by ignoring any insignificant amount of sensible heat exchange that may happen through the walls or exterior of the tower. Within the air stream, the rate of heat gain is identified by the expression G (h2 – h1), where:    G = Mass flow of dry air through the tower—lb/min. h1 = Enthalpy (total heat content) of entering air—Btu/Ib of dry air. h2 = Enthalpy of leaving air—Btu/Ib of dry air.

Within the water stream, the rate of heat loss would appear to be L (t1 – t2), where:    L = Mass flow of water entering the tower—lb/min. t1= Hot water temperature entering the tower—°F. t2 = Cold water temperature leaving the tower—°F.

This derives from the fact that a Btu (British thermal unit) is the amount of heat gain or loss necessary to change the temperature of 1 pound of water by 1°F. Still, the mass flow of water leaving the tower is less than that entering it, because of the evaporation that takes place within the tower and an appropriate heat balance must be take into account for this minor difference.

Since the rate of evaporation must equal the rate of change in the humidity ratio (absolute humidity) of the air stream, the rate of heat loss represented by this change in humidity ratio can be expressed as G (H2 - H1) (t2 - 32) where:   H1 = Humidity ratio of entering air—lb vapor/lb dry air. H2 = Humidity ratio of leaving air—lb vapor/lb dry air.

The notation (t2 - 32) equals to an expression of water enthalpy at the cold water temperature - Btu/Ib. (The enthalpy of water is zero at 32°F) Including this loss of heat through evaporation, the total heat balance between air and water, stated as a differential equation, is: G dh = L dt + G dH (t2 - 32)  (1)

The expression “L dt” in equation (1) signifies the heat load imposed on the tower by whatever process it is serving. Nonetheless, because pounds of water per unit time are not straightforwardly measured, heat load is usually expressed as:

Heat Load = gpm x R x 81⁄3 = Btu/min.  (2) where:    gpm = Water flow rate through process and over tower— gal/min. R = “Range” = Difference between hot and cold water temperatures—°F. 81⁄3 = Pounds per gallon of water.

Note from formula (2) that heat load forms only a required temperature differential in the process water, and is undisturbed with the real hot and cold water temperatures themselves. Therefore, the mere indication of a heat load is meaningless to the Application Engineer attempting to properly size a cooling tower. More information of a specific nature is required.

Optimal operation of a process usually occurs within a relatively narrow band of flow rates and cold water temperatures, which establishes two of the parameters required to size a cooling tower—namely, gpm and cold water temperature. The heat load established by the process creates a third parameter - hot water temperature coming to the tower. For example, let‟s assume that a process developing a heat load of 125,000 Btu/min performs best if supplied with 1,000 gpm of water at 85°F. With a slight transformation of formula (2), we can determine the water temperature elevation through the process. Therefore, the hot water temperature coming to the tower would be 85°F + 15°F = 100°F.

Equation (1) would classify enthalpy to be of prime concern, but air enthalpy is not something that is routinely measured and recorded at any geographic location. Wet bulb temperature is the only air parameter needed to properly size a cooling tower, and its relationship to other parameters is as shown in the Figure 1 diagram.

Figure 1

5.0

APPARATUS

 Water cooling tower MODEL: HE-152

6.0

PROCEDURE

General start-up procedure.

1. Valve V1 to V6 were ensured to be closed while valve V7 was partially closed. 2. The load tank was filled with deionised water. 3. The make-up tank was filled with deionised water up to zero mark on the scale. 4. Deionised water is added to the wet bulb sensor reservoir to the fullest. 5. The appropriate cooling tower was installed for the experiment. 6. All appropriate tubing to the differential pressure sensor was connected. 7. The temperature set point of temperature controller was set to 45˚C. The 1.0 kW water heaters is switched on and the water is heated up to approximately 40˚C. 8. The pump was switched on and the control valve V1 was slowly opened. The water flow rate was set to 2.0 LPM. A steady operation where the water was distributed and flowing uniformly through the packing was obtained. 9. The fan damper was fully opened and the fan was switched on. Check that the differential pressure sensor is giving the reading : a. To measure the differential pressure across the orifice, open valve V4 and V5; close valve V3 and V6. b. To measure the differential pressure across the column, open valve V3 and V6; close valve V4 and V5. 10. The unit was being let to run for 20 minutes for the float valve to correctly adjust the level in the load tank. Refill the make-up tank as required. 11. The unit is now ready to use.

Experiment 1 1. The heater was switched on and set to 0.5 kW. 2. Pump and blower is then been switched on. 3. The blower damper was fully opened. 4. The water flow rate was set to 1.5 LPM. 5. The water cooling tower was being let to operate for 20 minutes. 6. The reading was taken when the float valve is correctly adjusted. 7. Step 1-6 is being repeated with 1.0 kW heating load.

Experiment 2 1. The heater was switched on and set to 0.5 kW. 2. The blower damper was semi opened. 3. Set the water flow rate to 1.5 LPM. 4. The unit was being let to run for 20 minutes. 5. The reading was taken after steady operation achieved. 6. Step 1-5 is being repeated with 1.0 kW heating load.

General shut-down procedure 1. The heater was switched off to let the water to circulate through cooling tower for 3-5 minutes until the water is cooled down. 2. The blower was switched off and the blower damper was fully closed. 3. The pump and power supply was switched off. 4. The water in the reservoir tank was retained. 5. The water from the unit was completely drained off.

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