Question

he chiel engineer at a university that is constructing 1. large number of new student dormitories decides to install a counterflow concentric tube heat exchanger on each of the dormitory shower drains. The thin-walled copper drains are of diameter D 50 mm. Wastewater n the shower enters the heat exchanger at T-38 C while fresh water enters the dormitory at T 10°C. The 11.4 wastewater flows down the vertical wall of the drain in a thin, falling film, providing hi- 10,000 W/m2.K Hot waste water Warm fresh water Copper tube Falling film Annulus Cold fresh water, Teii m Cool waste water (a) If the annular gap is d= 10 mm, the heat exchanger length is L = 1 m, and the water flow rate is 705 m10 kg/min, determine the heat transfer rate and the outlet temperature of the warmed fresh water. (b) If a helial spring is installed in the annular gap so the fresh water is forced to follow a spiral path from the inlet to the fresh water outlet, resulting in he 9050 W/m2 .K, determine the heat transfer rate and the outlet temperature of the fresh water. (c) Based on the result for part (b), calculate the daily savings if 15,000 students each take a 10-minute shower per day and the cost of water heating is $0.07/kW h

Answer 1

TY-8.81 Reynolds murnboc is Re-i444- The loo is lamimavri 0T1 octh Table 8-2 = 536 XO .591 0 02 Theoverall heat 158 1o0OD 60 698

NTU 0,035 一 0.035 FAITU1+0.035 =0034x698 (53-10) ご66 l 698 b) -1 905010000 m in 698 141,01 0 511x698 (38-o 10)100 648 The cost the wante watex ramijerrectd savings = 10-1乂(1DX609メ