Six Measurements of Cooling Tower - Lab Report Example

Cooling Tower A total of six measurements were carried in a cooling tower. This was possible, via the changing ofset-up or conditions, for example, via changing of the air flow, water flow rate, and the cooling load in the analysis of coefficient of performance (COP) of the cooling tower, values of measured make-up water flow, and heat removed. Two methodologies were used in the make-up water flow calculation. The directly measured values in the make-up water flow calculation were found to be more accurate than the first methodology. The cooling load had the greatest change on the coefficient of performance (COP) and the amount of heat removed (QOUT) from the cooling tower. The setup’s QOUT ranged from 607.23-1121.38 Watts, while the COP values ranged were from 5.10 to 9.17; the make-up water flow was 0.417 to 0.750 g/s. The uncertainty analysis carried on set-up F, the set up that experienced the highest make-up water flow rate, showed error values of approximately 24.97 % when the make-up flow is calculated from the first method, while 19.92 % when directly measured make-up flow are used; the two methodologies were within required value limits. Introduction The main experimental setup objective was to gain an understanding on both the usage and functions of cooling towers; analysis of coefficient of performance (COP) of the cooling tower, values of measured make-up water flow, and heat removed. The following lab experiment was aimed at the cooling water circuit in order to find the cooling load, energy used in pumping water and air. . Engineers work on transferring heat from the cooling towers into the atmospheric air with the consideration of other metrics, for example, cost and water retention. The cooling towers are used in the rejection of the plants’ waste heat into the atmosphere; in addition, they cool the circulating water through the plant. Power plants, or heat engines, utilize the second law of thermodynamics in rejecting waste heat to the environment when producing work. The cooling process is carried via reduction or taking away of hot liquid from the plant’s condenser, and then directing or transferring the heat from the hot liquid to the atmosphere; mostly water is utilized as the thermal heat sink. Enthalpy Balance and mass balance on the cooling tower is obtained: mw-mw1=ma (Y-Y1)………………(i) dmw = madY……………………..(ii) The mass velocity for the dry air remaining constant via the cooling tower: Take enthalpy balance over the same differential section mwH + mah1 = mw1H1 + mah……………………(3) mw(H-H1) = ma (h1 – h)…………………………(4) The equation 4 can be changed into heat balance term: mwCpwdT=madh………………………………...(5) Taking the integral of the equation 5. mwCpw(T2 – T1)=ma (h2 – h1)……………………(6) mw is the mass flow of the water; Cpw is the specific heat of the water, (h2 – h1) is the latent heat of vaporization for water and (T2 – T1) is the cooling range. Figure 1. Enthalpy balance and mass balance on the cooling tower Figure 2. The cooling tower’s operating line Experimental set up and procedure Figure 2. The bench top cooling tower The load tank has two electrical heaters for the simulation of the cooling load for the power plant. Heated water in the load tank is pumped via water flow meter and control valve to the water distributing system at the top. Distilled water is used when filling the makeup tank. Water evaporation amount was monitored and record during the test operations in the cooling tower. The process was done via the measurement of the time needs to utilized in adding quantity of water to the make-up tank. The wet temperature in the water reservoir was checked. When the system reached its study rate, all temperatures were recorded, the wet and dry bulb temperatures of the water and air temperature of all sections. Results and discussion Cooling range : Cooling range is obtained from (T1­­ - T6) T1 =Temperature of the liquid, i.e. water, at the inlet T6=Temperature of the liquid, i.e. water, at outlet Set-Up Cooling Range Temperature Ideal Cooling Range Temperature Difference (Error) A 9.00 13.47 4.47 B 4.38 9.48 5.10 C 4.28 7.29 3.01 D 8.07 10.72 2.65 E 13.97 17.80 3.83 F 7.45 12.20 4.74 Table 1: The measured and ideal cooling temperature range The ideal cooling range is obtained from (T1−T5) T1= Temperature of the liquid, i.e. water, at the inlet. T5= The wet bulb temperature at the inlet. Figure 3: The graph of measure cooling range versus water flow rate for setup A up to D. Figure was obtained from table 1, when the experiment used a fixed cooling load. From figure 3, values for the cooling range, at both low and high air flow, are inversely related to that of the water flow rate, giving a linear graph; in addition, the values of the measured cooling temperature range are reduced at higher air flows when compared to the lower air flows. Figure 4: The measured cooling range versus water flow rate for the setup C to F The Table 1 is used in plotting the graph, for the fixed air flow. From figure 4, values for the cooling range, at both low and high air flow, are inversely related to that of the water flow rate; the values of the measured cooling temperature range are reduced at higher air flows when compared to the lower air flows. Approach: Approach= T6−T5 T6 = The temperature of water, liquid, at the outlet of the tower T5 = The wet bulb temperature at the inlet Figure 5: Approach temperature against water flow rate for setup A to D. Figure 5 represents the approach plot with fixed cooling load. The graph is a mirror image of figure 3 and 4, hence cooling range is an inverse plot of the approach. Therefore, approach directly is proportional to the water flows, when water flow increases, the approach increases too. From Figure 5 above, when lower air flows, then the approach is greater when compared to the high air flows. Figure 6: Approach against water flow rate for setup C to F Figure 6 represents approach against water flow in the fixed air flow. The low cooling loads exhibit low approach when compared to the higher cooling loads. In setup D, the prediction for the ideal condition is closer as compare to other setups; predicted setup condition that approach closer to the ideal of the zero condition. Setup D exhibits these properties due the fact that it was operated at low water flow with cooling load, in addition, high air flow; hence setup D has the lowest approach value when compare to other setups. Make-up water flow rate: methodology 1 In the first method, make-up water flow rate is obtained from the equation below: =  specific heat of water = Cooling range Hv= Latent heat of vaporization =Mass flow of water =Mass rate of evaporating water The equation is changed, in order to obtain the directly measured, into: = =Area of the make-up tank =density of water =Time spent =Change in water level in the make-up tank Figure 7: The Make-up water versus the water flow rate for set-ups A to D for the fixed cooling load Figure 7 avails comparison between the directly measured values for the make-up water with their values observed in the first method. There are two plots for the values observed from the first set-up, with their high and low setting line plot used in finding make-up water; and the remaining two represent plots for the directly measured values, consisting of both low and high setting lines. Figure 7 utilises the first method in calculation for the make-up flow rate; the method has one demerit, from the graph the make-up water flow is reduced when compared to the direct measured values. This discrepancy emanates since evaporation is only considered process during cooling of water, other factors and processes are ignored. From Figure 7, the measured values for the make-up water flow increases with the increase in water flow, since water is added into the system for evaporation. Moreover, it is clear that higher air flows result into high make-up water flows for the measured values; this is because the low air flows avail enough time for water to evaporate. The above discussed observations are not indicated in the first method; this is brought by the errors in the thermocouples. Figure 8: A graph of make-up water versus water flow rate for the conditions C to F for the fixed air flow. Figure 8 utilized the first method in calculating the make-up flow rate. There are two plots, high and low setting line plot, and the plots for the directly measured values; from the graph the make-up water flow is reduced when compared to the direct measured values. The measured values for the make-up water flow indicate increases with the increase in water flow; moreover, the higher air flows result into high make-up water flows for the measured values. The make-up water flow increase faster in the measured values when compared to the calculated values. The higher cooling loads results in high make-up water flows, when compared lower cooling loads. From the graphs 7 and 8, set up E experienced the highest make-up water flow rate while set up A and B had the least; therefore set up E lost most water while A and B lost the least amount during the cooling process. Make-up water flow rate: methodology 2 In this method, the make-up water is determined using equation: =humidity ratio. Figure 9: Graph of make-up water versus water flow rate for set-up A to D for the fixed cooling load In figure 9, the data plotted is obtained from the second method. The above graph, figure 9, has calculated values approaching the measured when compared to Figure 8 and 7; hence the second method avails accurate make-up flow data when compared to the first method. Figure 10: Graph of make-up water versus water flow rate for the set-up C to F for the fixed air flow. Figure 10’s graph uses the second method when calculating the make-up flow rate. The second method has slightly lower values for the measured values in all conditions. Coefficient of performance (COP): The Coefficient of performance (COP) is calculated from: COP= =0.1 kW from the manual Both set-up E and F were run under the high cooling range; from Table 2 below, E resulted in higher COP, set-up F had less COP when compared to E since it utilised less amount of water. Other set-ups from A to D were under low cooling range. Heat removed: The heat removed is calculated from: QOUT= =Mass flow of water =Specific heat of water =The cooling range Set-up F had the most heat removal, as seen in Table 2. Set-up was operated under in the high cooling load, hence the large amount heat removed. The set up E also was under high cooling load, its heat removal amounts were approaching that of F. Set Up COP QOUT (W)  (g/s) A 6.51 677.13 0.417 B 6.33 658.22 0.417 C 5.27 643.69 0.583 D 5.10 607.23 0.583 E 9.17 1051.18 0.750 F 8.82 1121.38 0.667 Table 2: Values of measured make-up water flow, heat removed, and COP for various set-ups. Conclusion Setup E had the highest cooling range, i.e. 13.97 ℃, while setup C had the lowest cooling range, i.e. 4.28 ℃; all of the data presented, setup A to E, were non-ideal cooling range. Table 1 indicates setup D conditions having the lowest approach; this is because its actual range is closer to its ideal range. The uncertainty analysis carried on set-up F, as shown in index 1 for the set up that experienced the highest make-up water flow rate, showed error values of approximately 20.10 % when the make-up flow is calculated from the first method, while 19.92 % when directly measured make-up flow are used; the two methodologies were within required value limits. Therefore the experiment was successfully conducted, since there are rear mass/heat transfer experiments with errors of less than 10%. The experimental set up’s objectives were accomplished. Three setup factors were varied, that is, air flow, water flow rate, and the cooling load. Works Cited Moran, J.M. Engineering thermodynamics. Queensland: John Wiley & Sons, 2010. Appendix 1 Data from manual: Dimension of the column=0.15 m X 0.15 X 0.6 m high Discharge Coefficient of Orifice= 0.616 Density of water=1000 kg/m3 Diameter of the Orifice=0.08 m Density of dry air= 1.1987 kg/m3 Cooling tower water flow rate bias error= 0.1 g/s Thermocouple readings bias error= 1.1 K Water latent heat of vaporization=2257 kJ/kg Average Temperatures for set-up A: T1= Temperature of liquid, water inlet=29.40 C T2 =Dry bulb temperature for the outlet of the air-water vapour mixture=21.33C T3=Wet bulb temperature for the outlet of the air-water vapour mixture=21.97 C T4=Dry bulb temperature for the inlet of the air-water vapour mixture =27.80C T5=Wet bulb temperature for the inlet of the air-water vapour mixture=16.67 C T6=Temperature of the liquid water outlet= 20.10 C T7=Temperature of the liquid make-up water= 24.70 C TAvg= = 24.70 C Cp= 4.18 kJkg-1K-1 The Make-up Flow: First Method Calculations for A TAvg= = 21.25 C Cp= 4.179 kJkg-1K-1 The Make-up Flow: Measured Method =T1-T6 =0.3 g/s Make-up Flow: Second Method A psychometric calculator is used = 0.0034 =0.01375 For A = =0.0382 g/s The directly measured make-up flow for A: =5 mm =/t ==0.417 g/s Heat removed for A: QOUT= =18*4.179*9 =677.13 W COP for set-up A: Wwater = =18*1/1000*9.81*0.6= 0.105948W Wpump=100 W Wair= ( =3.883 W COP==6.51 The uncertainty analysis for first method of calculating the make-up flow for set-up F: = =-0.000244444 kJ/kgC = = =0.104022577 g/s  *100% =(0.104022577/0.417)*100%= 0.249654184 *100 =24.97 % The uncertainty analysis for the directly measured make-up flow for set-up F ==0.083 g/s  *100% = (0.083/0.417)*100%=0.1992*100 =19.92% Read More