Group 3

• An air cooler consisting of a bundle of 1.5” Sch 40 steel pipes enclosed in a well baffled shell is being built to cool 200 kg/hr of air from 80oC to 28oC. The air flows under pressure through the tubes of a 1-shell 2-tube pass heat exchanger. Cooling water at 18oC is under sufficient pressure to force it through at any desired rate, flows through the shell. The heat transfer coefficient based on the inside fluid is 50 W/m2K and for the outside fluid, 2100 W/m2K. Cooling water costs PhP 0.57/m3 and the annual fixed charges is 22% with cooler cost at PhP 24,000/m2. Calculate the Optimum exit temperature of cooling water and optimum length of tubes.

Given Ei=0 Eo=0 t1p = 80 t2p=28 t1=18 Δt1 = 10 Δt2 = 80 – t2 Uo = 50 Ui = 2100 Cpu = 1 Hy = 8760 Cu=5.67X10-4 pesos/kg H20 Di = .4089 Ca = 22400 Uo=Ui (Di/Do) A = pi/4 (Di)^2 Do = .00009735Assume Ft = 1

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Δt2= -10.11oC t2= 90.11oC Check for Ft R = 0.1944 S = 0.8387 From graph Ft = 0.968

Ft = 0.968 Δt2= -12.5 t2 = 92.5 R = 0.1054 S = 0.8387 Ft = 0.95 t2opt = 92.5

q = UoπDoLΔTlm=wCpu(t2-t1) L = 747.64 m