Using the waste heat generated from natural gas combustion to calculate the heat exchange area
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It is known that 25 cubic meters of natural gas are burned per hour, and the temperature of the exhaust gases is 180°C. Using circulating water at 20°C and a flow rate of 300 kg/h for heat exchange, the exhaust gas temperature is reduced to 80°C. The finned tubes are DN25 stainless steel finned tubes. I need the formulas for calculating the circulating water outlet temperature and the finned tube area. Thank you. Heat exchanger equipment technology community
First, to determine the outlet temperature of the circulating water, the formula Q=mcΔT is used, where Q represents the energy exchange, m is the mass, c is the specific heat capacity, and ΔT is the temperature difference. In this problem, we assume that all the heat generated by the combustion of natural gas is transferred to the circulating water. Therefore, Q = mcΔT can be written as Qg = mw cw(Tw2 – Tw1), where Qg represents the heat generated by the combustion of natural gas, mw is the mass flow rate of the circulating water, cw is the specific heat capacity of water, Tw2 is the outlet temperature of the circulating water, and Tw1 is the inlet temperature. Since the heat Qg generated by the combustion of natural gas is known, we can use this formula to calculate the outlet temperature of the circulating water, Tw2, namely Tw2 = Qg / (mw cw) + Tw1. Secondly, to determine the area of the finned tube, the formula Q = U * A * ΔT is used, where Q represents the energy exchange, U is the heat transfer coefficient, A is the heat transfer area, and ΔT is the temperature difference. In this problem, we assume that all the heat generated by the combustion of natural gas is transferred to the circulating water; thus, Q = U * A * ΔT can be written as Qg = U * A * ΔTg, where Qg represents the heat generated by the combustion of natural gas, U is the heat transfer coefficient, A is the surface area of the finned tube, and ΔTg is the decrease in temperature of the natural gas. Thus, we can obtain the formula A = Qg/(U * ?Tg) to calculate the heat exchange area of finned tubes. .
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