Question

The ‘Condensate pre-heater’ shown in the process circuit for the Killingholme combined cycle is a counter flow shell and tube heat exchanger heated by a heat transfer oil. In reality this would utilise waste heat from the heat recovery boiler but this has been simplified for the purposes of setting an assessment question, to just using hot oil. The heat transfer oil flows through the tubes and the condensate flows through the shell of the heat exchanger. The oil enters at 250 OC and leaves at 90 OC. Assume that the condensate remains in the liquid state and enters and leaves at the temperatures and flowrate shown on the process circuit diagram. The condensate has a specific heat capacity cp of 4.25 kJ/kg K. The heat transfer oil has a specific heat capacity, cp of 2.20 kJ/kg K, dynamic viscosity of 1.6 x 10-3 N/m2 s and density ρ of 1083 kg/m3. The heat exchanger has 1078 carbon steel tubes with an inside diameter of 0.015 m and an outside diameter of 0.019 m. The conductivity of carbon steel is 36 W/m K. The outside (shell side) convection heat transfer coefficient hO = 6.61 W/m2 K. a)What is the required total mass flow rate of the oil? b)Assuming that the flow is fully developed inside each tube, calculate the overall heat transfercoefficient based on the outer tube diameter Uouter for the heat exchanger, justify and give areference for any convection heat transfer coefficient correlation used. c)What is the required heat transfer area?

Answer 1

0 T-L diagram Passuming Ikausid temperature at the Condensor is soc Ti=250€ Most Ore Ti-Tsat 2250-50 = 200°c To-Gotc 82-90-:50 That 250€ Cliquid) O2 = 40°c (conquid (CP) onl hoa 6.61w, 6.61 W/mik 4.25 KJ/kgle KJ/kg k r/min 2.2 kJ l 1.64153 m. Poil = 1083 kg/m3 ho. of tubes = 1078 Di = 0.015 m Dos 0.019 m Ksteel = 36 W/ Wymia To determine the mass low rate of oil for flow in a pipe of diameter, experimental Observation show that for fully developed flow, laminar when Re<2300 @ Orcars

46.62 Wys so, Rec 2 300 (for fully developed) puoi 2300 M 2300 10838 V X 0.015 1068 103 v= 0.22€ mys Savon (n = no. of tubes mass flow rate of oil where A = A x Di 1376X164 Az ģx 0.015? 1083X1,767104x0.226 X 1078 mail Imoil AN b over all heat tranyfer coefficient Po determine the based on outer diameter In (Pola t when I ho Ao 1 U Ao 1 hi Ai (Di/21 RAKL Sto= Tool Ai=Adil Do. 1 ho + Do hi Di + Do In DI 2 ksfeed To determine the hi use the Nusselt relationship for laminar for in Pipe for constant wall temperature Nu = 3.66 kojs = 0.180 Wave hixdi = 3.66 Koil from property table]

hixorol 5 Oo180 3.66 Thi = 43.92 W/ 2k ooolg en 0.019 1 1 t + OOLS 6.61 0,019 43.92X0.015 2X36 che ci 1 0.180 U= 5:54 mek Po determine the required heat transfer area Q= UA MTO where P= Mail Croix@T) 46.62 X 2.2x102x (256-90) LMTD= uo 200 O1-0 99.41 € In Color) In (2000) A= surface are area 46.62x2,2x103x(250-90) 5.54xAxGg.ul A=29797.17 m² total surface area for each tube surface area А [na z nor of tubes a= n 27.64 m2 29797.17 1078 la = 27. G4 m² for each tube surface area
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