Question

The duct of the heating of a house passes through the space not heated in the basement. The cross section is 0.050 m2. Hot air enters the duct at 100.00 kPa and 60.00 ℃ and a speed of 5m / s. The temperature drops to 54℃ as a result of the loss of heat to the cold space in the basement. Determine the reason for the heat loss of the air in the duct to the basement, plus the cost if the heating equipment has an efficiency n = 0.80 and 0.60 (Usd / kW-hr). Rair = 0.287 (kJ / kg℃)

---d 5 m 20 cm Aire caliente 100 kPa 60 C 5 mis 35 C 25 cm www.ww an

Answer 1

The constant pressure specific heat of sin at the average temperature of = (54 + 60)/2 = 57 degree C is 1.007 KJ/kg degree C. We know, Q = m C_p delta T m is mass flow rate. delta T is temperature The density of air at inlet condition of rho = P/RT = 100KPa/(0.287 KPa.m^3/kg) (60 + 273)k = 1.046 kg/m^3 The cross-sectional area of is, A_c = (0.20m) (0.25)m = 0.05m^2 m = rho gamma A_c = (1.046) (5) (0.05) = 0.2615 kg/s Q_less = m C_p (T_in - T_out) = (0.2615) (1.007) (60 - 54) = 1.580 kJ/s. = 5688 kJ/h The cost of heat loss = (Rate of heat loss) (unit cost of energy i/p)/terrace efficiency = (5688 KJ/h) ($ 0.60/them)/0.80 (1/105.500k) = $0.04/h therefore The heat loss from the heating ducts in the basements is costing the home area 4 cents/hr.