Question

25 PRR HEAT TRANSFER NAME: UNIT SIX AND SEVEN EXCHANGERS BOILING AND CONDENSATION, He- Solve the following problems: 1. In a gas-faired boiler, water is boiler at 150 C by hot gases flowing through 59 m long, 5 cm outer diameter mechanically polished stailess-stell pipes submerged in water, If the outer surface temperature of the pipes is 180 °C. determine (a) the rate of heat transfer from the hot gases to water. (b) the rate of evaporation, (c) the ratio of the critical heat flux to the present heat flux, and (d) the surface temperature of the pipe at which critical heat flux occurs. 2. Ethylene glicol is heated from 25 °C to 40 "C at a rate of 2.5 kg/s in a horizontal copper tube (, k 386 W/m K) with a inner diameter of 2.0 cm and outer diameter of 2.5 cm. A saturated vapor (To 110 °C) condenses on the outside-tube Surface with the heat transfer coefficient (in W/m2 K) given by he 9200(Tg- Tw.25 where T 81.5 °C is the average outside-tube wall temperature What tube length must be used? Take the properties of ethylene glicol to be p 1109 kg/m, Cp 2.428 kJ/kg K, k 0.253 W/m K, 0.01545 kg/m s and Pr 148.5 3. In a 1-shell and 2- tube heat hexchanger, cold water with inlet temperature of 20 °C is heated by hot water supplied at the inlet at 80 °C. The cold and hot water flow rates are 5000 kg/h and 10,000 kg/h, respectively. If the Shelll-and-tube heat exchanger has a UAs value of 17,600 W/K, determine the cold water and hot water outlet temperatures. Assume Cpe 4.178 kJ/kg K and Cph 4.188 kJ/kg K.

Answer 1

Shell tube Heat Cchonger Thy しTha TOH Tc, COLD Cold warer Hotwart man flarcie m410000 kglh 2.71 Kife mO=3000 1kh - 1-38 K9lse Tc, = Flaw rate mun Femp inleT 2 o'c tenp inle トemp The 2 Ter outle 2 4.123 Kil CPh Gpc=4.118 1k = 17.6Kw/k UA ()Heat cupein- my crn (Cs) Heat capacin mcpe 2.17x 41& 8 1.38 x 4-118 - ll. 6 Kilr S.165 Kjse Mm C me Spe knaw U A NTU Cmin Cmin S.165 0.496 Cmm IT. 6 R = Cmat S.165 3.053 So at inle cold thrd Parrellel constder h ave we -NTU CITR Effectvnen 1+ R e-3o53 (1t 0.416 I+ 0.496 2019-6-7 07:43 0.661

E = 0. 661S myax mf C&ac hut N/7 6 CR marc Comax in (T,, - 6 Chl Th, - The) Cmin (Th, T) 1 1 680-The) O. 6615 S.765 (P02 0) 228.8 i1. 6 (80 - The) The 60.2c CO caeual Ca may Ccl TE2-Te wget 0.66IS Cmtn 1Th Tei) T4-T6 S765 (T-20) O. 661T 20) SA6S (80 T -70 39.69 S9. 69 °C Ter ANS 60.27°c Th2 S1.61 2019-6-7 07:43