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Question 4 The exhaust air from a building is at a temperature of 22 °C and has a flow rate of 4 kg/s (specific heat capacity of 1.005 kJ/kg-K). A thermal wheel is proposed to recover energy from this exhaust air to preheat the incoming fresh air at a flow rate of 4.5 kg/s and temperature of 10 °C (specific heat capacity of 1.005 kJ/kg-K). (a) Draw a schematic of a typical air-to-air thermal wheel along with the temperature distribution of both the air streams across the wheel in counter flow arrangement. (10%) (b) Given the information determine: i) The effectiveness of the thermal wheel (40%) ii) The actual heat transfer rate (10%) iii) The exit temperature of the fresh air leaving the thermal wheel (10%) (c) The exit temperature of the fresh air leaving the wheel if the speed is doubled. (20%) d) State the significance of your answers b(iii) and C. (10%) Data Specific heat matrix material = 0.4 kJ/kg-K Wheel speed = 15 rev/ min Width of the wheel = 0.2 m Face area for the wheel = 1.2 m2 Mass of the thermal wheel = 26 kg Heat transfer coefficients for each air streams = 20 W/m²K Matrix surface area to volume ratio = 3000 m²/m3 1 1 1 + UA hAh. A NTU = UA (mCp) min Assume that the effectiveness of a recuperative heat exchanger Eis: 1 E = E 9x[(m xCp)marix /(mxCp) min Where E is the equivalent effectiveness for a counter flow heat exchanger; 1-expl-NTU(1–2)) EC 1- Rx expl-N7T „I-NTU (1-R)] R is the ratio of thermal capacities (mxCp)min R= (m xCp)max