Question

A shell-and-tube heat exchanger similar to the one shown in the Figure below is used to recover energy from waste water at 40°C to heat fresh water entering at 18°C. The mass flow rate of the waste water is 4 kg/s which is the same as that of the fresh water. Using the data given, calculate: (i) the optimum rate of energy recovery; the required heat transfer area; (iii) the temperature of the fresh water at exit. Shell fluid inlet Shell Tuhes Baffle plates Tube fluid intet Tube fluid outlet Shell fluid outlet Data Specific heat of waste water and fresh water, 4.1 kJ/kg K. Overall heat transfer coefficient, 2700 W/m K. For a single shell-pass, two tube-pass heat exchanger take the following characteristic when R= 1. Effectiveness o NTU 0 0.46 1.0 0.53 1.5 0.56 2.0 0.57 2.5 0.58 3.0 0.58 3.5 0.6 4.0

Answer 1

S)» Guven Data Tiwwa 40° c T44% = 18°C : це • ks رأ) 4 4 4x4 ni ww = nifw = ungle U= 2700 W/m²K R=1 Өркін хоч боur%, халы when effectiveness is maxime from given table, Є. О. 6 € = Teww - Te ww Taww - Tefw ү. Чь - Teww 19 Tww. ac, 8 °C me. ( Т: wwд - 1.45 ) чкц 11 (цо - 26 •8) R16 •48 kW ( ії ) for - б• 6, Now, NTUS NTO= 4 (from Table) UA ( ). 25ook A 43 4100 ач. 3 ми

2 \li ) 1) 246 48 - m + ( те Ru -15 ) 4*4 11 (тед» - 18 ) 46-4 ( Tee - 14) Te fue 16:48 — 34:2°C 12 Temperature of fresh water at exit = 31.2°C.