# Report

• Category:
Physics
• Document type:
Assignment
• Level:
High School
• Page:
2
• Words:
1089

Cooling Tower Report

Exercise A: Observation of the processes occurring within a forced draught cooling tower

Evaporative cooling is the main principle by which the cooling tower works based on thermodynamic processes. As air come into contact with cooling water, some water molecules evaporate. This reduces the temperature of the remaining water significantly, thus, cooling the water. The amount of thermal energy lost by water equals the amount of energy gained by cooling air through heating (Armfield Ltd). The Armfield UOP6-MKII Water Cooling Tower was used to demonstrate forced draught cooling system. The variables measured are as below:

Temperature of water entering at the top of packing,
= 40oC

Temperature of water leaving at the bottom of packing,
= 30.4oC

Temperature of air leaving at the top of packing (dry bulb),
= 23.3oC

Temperature of air entering the bottom of packing (dry bulb), = 22.3oC

Relative humidity of air entering at the bottom, RH2 =

Relative humidity of air leaving at the bottom, RH1 =

Power required to heat water (cooling load)

Temperature of air in,
= 12.68oC

Temperature of air out,
= 21.07oC

Calculations:

()

=

The purpose of the cooling tower is to cool water. The effectiveness of the tower determines its performance, and this is affected by cooling range and approach. Increasing the range and reducing the approach will achieve a higher effectiveness. Evaporation can also be maximized by using a different packing materials, varying flute sizes or using a more efficient fan.

Exercise B: Effect of water inlet temperature on the performance of the cooling tower

In this test, the temperature of water in the slump was varied accordingly and then passed through the cooling tower to observe the effect on the performance of the tower. The variables of interest are Approach, Range, and cooling load. The table below shows all the measured values, and calculated values of Approach, Range, and Effectiveness.

Table 1(a): Effect of water inlet temperature on effectiveness

 Temperature () For 30 For 35 For 40 For 45 T2 = Tw out T3 = Ta out RH1 = Rh out (%) RH2 = Rh in

Table 1(b): Comparing performance at each water temperature

 Water temperature () 30 35 40 45 Approach value Range Cooling load kW Effectiveness %

Various indicative graphs can be plotted to depict the effect of temperature.

1. Graph of range against water inlet temperature

Figure 1: Graph of Range against water inlet temperature

From the graph in figure 1 above, it can be see that the Range increases as the water inlet temperature increases. This means that the range of the cooling tower increases as the temperature of the inlet water increases.

1. Graph of cooling load against water inlet temperature

Figure 2: Graph of cooling load against water inlet temperature

From the graph in figure 2 above, we see that the cooling load increases as the temperature of water inlet increases. This is because more energy would be required to raise the temperature of water.

1. Graph of Approach against water inlet temperature

Figure 3: Graph of Approach against water inlet temperature

The graph in figure 3 depicts that Approach increases with increasing water inlet temperature. Increase in temperature increases the rate of heat transfer between air and water due to larger temperature differences.

1. Effectiveness against water inlet temperature

Figure 4:
Graph of Effectiveness against Water inlet temperature

From the figure above, it can be observed that effectiveness increases with increasing water inlet temperatures. This is due to increased heat transfer associated with increase in temperature difference.

1. Graph of Approach against cooling load

Figure 5: Graph of Approach against cooling load

The graph in figure 5 above depicts that, for a higher Approach a higher cooling load is required. Increasing Approach requires higher Range, which in turn requires more energy to increase the temperature.

Exercise C: Effect of air flow rate on the performance of the cooling tower

Air flow rate is one of the factors that affects the performance of the cooling tower. In this exercise, the rate of air flow was varied by changing the speed of the centrifugal fan located at the bottom of the tower to study the effects of air flow rate. The table below shows the results of various measurements

Table 2 (a): Effect of air flow rate on effectiveness

 Air flow l/sec Pressure (mbar) T2 = Tw out T3 = Ta out RH1 = Rh out (%) RH2 = Rh in

Table 1(b): Comparing performance at each Air flow rate

 Air flow l/sec Approach value Range Cooling load kW Effectiveness %

Figure 6: Graph of range against approach

Figure 7:
Graph of cooling load against flow rate

Figure 8:
Graph of approach against flow rate

Figure 9:
Graph of effectiveness against flow rate

Figure 10:
Graph of approach against cooling load

From the graph in figure 9, it can be deduced that, generally, the effectiveness of the cooling tower increases as rate of air flow is increased from 15.89L/sec. to 60L/sec. More water is evaporated as more air passes through the fills, increasing the cooling effect. Approach decreases as airflow rate increases, depicting increase in the overall performance of the cooling tower.

Exercise D: Effect of water flow rate on the performance of the cooling tower

In this exercise, the effect of water flow rate on the performance of the cooling tower was examined by varying the speed of water flow using the electric pump. The table below shows various measurement parameters that were taken.

Table 3 (a): Effect of water flow rate on performance

 Water flow rate l/min T2 = Tw out T3 = Ta out RH1 = Rh out (%) RH2 = Rh in

Table 3(b): Comparing performance at each Air flow rate

 Water flow rate l/min Approach value Range Cooling load kW Effectiveness %

Figure 11: Range against water flow rate

Figure 12:
Cooling load against water flow rate

Figure 13:
Approach against water flow rate

Figure 14:
Effectiveness against water flow rate

Figure 15:
Approach against flow rate

From these results, it can be noticed that the Range and Effectiveness drops as the rate of water flow increases. In overall, the performance of the cooling tower reduces with increase in water flow rate. Increase in flow rate reduces heat transfer as the surface area of the cooling tower is limited and has a fixed volume for water. This reduces the range and increases the Approach, leading to reduced performance.

Works Cited

Armfield Ltd. Basic Water Cooling Tower Instruction Manual (UOP6-MKII). Vol. Issue 3. United Kingdom, July 2015.