PRACTICAL 6: Heat exchangers
Concept questions:
C1. Under what conditions is the thermal resistance of the tube in a heat exchanger negligible?
C2. Draw a 2-shell-pass, 4-tube-passes shell and tube heat exchanger. What are the advantages and disadvantages of using 4 tube passes instead of just 2 of the same diameter?
Copyright By PowCoder代写 加微信 powcoder
________________________________________________________________
1. Exhaust gas from a furnace is used to preheat the combustion air supplied to the furnace burners (both the gases have a specific heat of 1075 J kg-1K-1). 15 kg s-1 of exhaust gas with an inlet temperature of 1100 K enters the pre-heater, and flows through a bundle of tubes in a single shell pass, U tube (2 tube pass) heat exchanger. The combustion air has a flowrate of 10 kgs-1, and inlet temperature of 300 K. The tubes are thin-walled and the overall heat transfer coefficient for the heat exchanger is 100 Wm-2 K-1.
a. Calculate the rate of heat transfer from the exhaust gas to the combustion air required to heat the combustion air to 850 K.
b. What is the outlet temperature of the exhaust gases under these conditions?
c. What is the total surface area of the heat exchanger required to heat the combustion air to 850 K?
d. What would be the area required if a one shell-pass cross flow heat exchanger were used under the same conditions?
e. What would be the area required if a two shell-pass, 4 tube-pass heat exchanger were used under the same conditions? Compare with c-d.
f. If water is present in the combustion air, how will that affect the above results?
2. In open-heart surgery under hypothermic conditions, the patient’s blood is cooled before surgery and rewarmed afterwards. A concentric tube countercurrent heat exchanger of length 0.5 m is proposed. The thin-walled innter turbe has a diameter of 55 mm. Water enters the heat exchanger at a temperature of 60 oC and flow rate of 100 gs-1 and is used to heat the blood, which enters at 18 oC and a flow rate of 50 gs-1. The overall heat transfer coefficient is 500 Wm-2K-1. The heat capacity of blood is 3500 J kg-1 K-1.
a. What is the rate of heat transferred to the blood?
b. What is the temperature of the blood leaving the heat exchanger?
c. What is the outlet temperature of the water?
After cleaning, the heat exchanger is put back together incorrectly, such that is operates with cocurrent flow:
d. What is the rate of heat removed from the blood under these conditions?
e. What is the new outlet temperature of the blood leaving the heat exchanger?
f. Will this cause a problem for the patient?
3. Steam is used in a counter flow double-pipe heat exchanger to pre-heat 100kg/hour of oil (Cp=1700Jkg-1K-1) from 20oC to 80oC for use in a biodiesel production plant. The oil flows through the inside the tubes, which have inner diameter 15 cm. Steam (Cp=2.8 kJkg-1K-1) enters the heat exchanger at 470K, and flows through at a rate of 0.5 kgs-1. Determine the following:
a. the rate of heat transferred to the oil
b. the temperature of steam leaving the heat exchanger
c. the overall heat transfer coefficient UA (or U) for the heat exchanger
After the heat exchanger has been operating for some time, it is noted that it is not working as efficiently. For the same throughput of oil and steam, the outlet temperature of the oil has dropped to 64oC. Fouling (oil deposits on the inner pipe wall) is suspected.
Assuming that the conductivity of such fouling is 0.2 Wm-1K-1, determine the following:
d. the rate of heat transferred to the oil under these “fouled” conditions
e. the temperature of steam leaving the heat exchanger under these “fouled” conditions
f. the overall heat transfer coefficient UA (or U) of the heat exchanger under these “fouled” conditions
g. the thickness of the fouling deposit on the inner pipe wall of the exchanger
h. summarise the results, and comment on the fouling (e.g. how does it affect the rate of heat transfer? How could it be reduced?)
程序代写 CS代考 加微信: powcoder QQ: 1823890830 Email: powcoder@163.com