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fundamentals of heat and mass transfer solutions manual phần 8

Ví dụ 9,4, Thảo luận 2. Giả định: (1) ống bề mặt nhỏ so với môi trường xung quanh, (2) Phòng không khí tĩnh lặng. TÀI SẢN: Bảng A.4, không khí (đánh giá sử dụng công cụ Pad Properties của IHT). Phân tích mô hình thích hợp được cung cấp tại Bình luận 2 | PROBLEM 11.14 KNOWN A shell and tube Hxer two shells four tube passes heats 10 000 kg h of pressurized water from 35 C to 120 C with 5 000 kg h water entering at 300 C. FIND Required heat transfer area As. SCHEMATIC ASSUMPTIONS 1 Negligible heat loss to surroundings 2 Negligible kinetic and potential energy changes 3 Constant properties. PROPERTIES Table A-6 Water Tc 350K cp 4195 J kg-K Table A-6 Water Assume Th o - 150 C Th - 500 K cp 4660 J kg-K. ANALYSIS The rate equation Eq. 11.14 can be written in the form As q UATLm 1 and from Eq. 11.18 _ _ AT -AT2 ATLm FATLm CF where ATLm CF Ln AT At2 . 2 3 From an energy balance on the cold fluid the heat rate is q mc Tc.o -Tc.i 10 000 kg h X4195- - 120 - 35 K 9.905 X105 W. c p o c i 3600s h kg -K From an energy balance on the hot fluid the outlet temperature is Th o Th i - q m h cnh 300 C - 9.905 X105 W -5000 X 4660- - 147 C. Iko IIJ I h p h 3600 s kg - K From Fig. 11.11 determine F from values of P and R where P 120 - 35 C 300 - 35 C 0.32 R 300 - 147 C 120-35 C 1.8 and F - 0.97. The log-mean temperature difference based upon a CF arrangement follows from Eq. 3 find . 300-120 __ . ATim 300-120 - 147-35 K Ln 47J357 143.3K. As 9.905 x105W 1500W m2 - K X 0.97X 143.3K 4.75m2 COMMENTS 1 Check Th 500 K used in property determination Th 300 147 C 2 497 K. 2 Using the NTU-e method determine first the capacity rate ratio Cmin Cmax 0.56. Then _ q Cmax Tc o -Tc i 1 120-35 C 0 57 e õmax Cmn Th i -Tc i 056x 300-35 C . . From Fig. 11.17 find that NTU AU Cmin - 1.1 giving As 4.7 m2. PROBLEM 11.15 KNOWN The shell and tube Hxer two shells four tube passes of Problem 11.14 known to have an area 4.75m provides 95 C water at the cold outlet rather than 120 C after several years of operation. Flow rates and inlet temperatures of the fluids remain the same. FIND The fouling factor Rf. SCHEMATIC ASSUMPTIONS 1 Negligible heat loss to surroundings 2 Negligible kinetic and potential energy changes 3 Constant properties 4 Thermal resistance for the