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Question 4 A thermal wheel is used to recover energy from the extract air leaving a building at 5 kg/s mass flow rate and 23 °C dry bulb temperature. The heat is used to preheat the fresh air supply coming in at 5.5 kg/s and temperature of 12 °C. The thermal wheel speed is 20 rev/ min. (a) Draw a schematic of a typical thermal wheel with the temperature distribution of the air across the wheel - assume counter flow. (20%) (b) Using the data below calculate; i. The effectiveness of the matrix wheel. (40%) ii. The rate of actual heat recovery. (10%) iii. The temperatures of the air leaving the wheel. (10%) c) Calculate the temperature of the air leaving the wheel if the speed is increased to 25 revs/ min. (20%) Data Assume mass of the thermal wheel is 25 kg Specific heat capacity of Air, Cp air = 1.005 kJ/kg-K Specific heat capacity of Matrix, Cp matrix = 0.4 kJ/kg-K Heat transfer coefficient for extract air stream = 20 W/m2K Heat transfer coefficient for supply air stream = 25 W/m2K Face area for the wheel = 1.2 m2 Width of the wheel = 0.2 m Assume the matrix has a surface area to volume ratio of 3200 m2/m3 1 UA + NTU = hiAi hodo Assume that the effectiveness of a recuperative heat exchanger & is: 1 1 = UA (mcp) min eft 1.93 1 E = Ep 1 [(mcy) matrix/(mcp) min) Where &c is the equivalent effectiveness for a counter flow heat exchanger. 1 - exp[-NTU(1–R)] Ec 1-Rx exp[-NTU(1-R)] R is the ratio of thermal capacities and NTU = Number of Transfer Unit (mcp) min R = (mcp) max Total marks for Question 4 = 100%