I am reviewing methods of assessing systems in heat transfer. One of the method I have run across for looking at single pass heat exchangers is log mean temperature difference (LMTD). What is the advantage of using LMTD instead of the basic/arithmetic difference in temperatures?

Describe the three mechanisms for Heat transferre?

a. An air stream passing through a 2-inch (1/6 ft) diameter, thin-walled tube is to be heated by high- pressure steam condensing on the outer surface of the tube at 320 °F. The overall heat transfer coefficient, h between steam and air can be assumed to be 25 Btu/(ft2.hr °F) with the air entering at 100 ft/sec, 10 psia, 40 °F. The air is to be heated to 150 °F. Determine the tube length required. Assuming Rayleigh Line flow, calculate the static pressure change due to heat addition. Also, for the same inlet conditions, calculate the pressure drop due to friction, assuming Fanno flow in the duct with f = 0.018. b. c. d. To obtain an approximation to the overall pressure drop in this heat exchanger, add the two results. Discuss the accuracy of this calculation.

EXAMPLE 20 A heat exchanger is being used to transfer heat between water and a 20% ethylene glycol solution. The ethylene glycol enters the heat exchanger at 150 gpm with a temperature of 20°F and exits at 40°F. The water enters the heat exchanger at 130 gpm at a temperature of 80°F. A heat transfer analysis reveals that the overall heat transfer coefficient of the heat exchanger is 400 Btu/h-ft2-°F. Determine the heat transfer area required for (a) a parallel-flow heat exchanger and (b) a counterflow heat exchanger. www. www www www mTho T Parallel-flow HX T Counterflow HX Thi Tho Tho tco Heat exchanger area A Heat exchanger area A