ASSIGNMENT 5 BMM3513_1415/2

FACULTY OF MECHANICAL ENGINEERING

BMM3513 HEAT TRANSFER

Assignment 5 (CO5)

NAME MATRIC NO.

1. How does the log mean temperature difference for a heat exchanger differ from the arithmetic mean

temperature difference (AMTD)? For specified inlet and outlet temperatures, which one of these two

quantities is larger?

2. The temperature difference between the hot and cold fluids in a heat exchanger is given to be ∆T1 at

one end and ∆T2 at the other end. Can the logarithmic temperature difference ∆Tlm of this heat

exchanger be greater than both ∆T1 and ∆T2? Explain.

3. What does the effectiveness of a heat exchanger represent? Can effectiveness be greater than one? On

what factors does the effectiveness of a heat exchanger depend?

4. Water (Cp=4180 J/kg.°C) is to be heated by solarheated hot air (Cp= 1010 J/kg.°C) in a double-pipe

counterflow heat exchanger. Air enters the heat exchanger at 90°C at a rate of 0.3 kg/s, while water

enters at 22°C at a rate of 0.1 kg/s. The overall heat transfer coefficient based on the inner side of the

tube is given to be 80 W/m2.°C. The length of the tube is 12 m and the internal diameter of the tube is

1.2 cm. Determine the outlet temperatures of the water and the air.

5. Air (Cp=1005 J/kg.°C) enters a cross-flow heat exchanger at 10°C at a rate of 3 kg/s, where it is

heated by a hot water stream (Cp= 4190 J/kg.°C C) that enters the heat exchanger at 95°C at a rate of 1

kg/s. Determine the maximum heat transfer rate and the outlet temperatures of the cold and the hot

water streams for that case.

6. A cross-flow air-to-water heat exchanger with an effectiveness of 0.65 is used to heat water (Cp=

4180 J/kg.°C) with hot air (Cp=1010 J/kg.°C). Water enters the heat exchanger at 20°C at a rate of 4

kg/s, while air enters at 100°C at a rate of 9 kg/s. If the overall heat transfer coefficient based on the

water side is 260 W/m2.°C, determine the heat transfer surface area of the heat exchanger on the water

side. Assume both fluids are unmixed.

/100 UMP