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.
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What are the advantages and disadvantages of using 4 tube passes instead
of just 2 of the same diameter?
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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. Calculatetherateofheattransferfromtheexhaustgastothecombustionair required to heat the combustion air to 850 K.
b. Whatistheoutlettemperatureoftheexhaustgasesundertheseconditions?
c. What is the total surface area of the heat exchanger required to heat the
combustion air to 850 K?
d. Whatwouldbethearearequiredifaoneshell-passcrossflowheat
exchanger were used under the same conditions?
e. Whatwouldbethearearequiredifatwoshell-pass,4tube-passheat
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?
Open page for Q1(1)
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. Whatistherateofheattransferredtotheblood?
b. Whatisthetemperatureofthebloodleavingtheheatexchanger?
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. Whatistherateofheatremovedfromthebloodundertheseconditions?
e. Whatisthenewoutlettemperatureofthebloodleavingtheheatexchanger?
f. Will this cause a problem for the patient?
Open page for Q2
3. Steam is used in a three shell-, 6 tube-pass shell & tube 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 470oK, and flows through at a rate of 0.5 kgs-1. Determine the following:
a. therateofheattransferredtotheoil
b. thetemperatureofsteamleavingtheheatexchanger
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. therateofheattransferredtotheoilunderthese¡°fouled¡±conditions
e. thetemperatureofsteamleavingtheheatexchangerunderthese¡°fouled¡±
conditions
f. the overall heat transfer coefficient UA (or U) of the heat exchanger under
these ¡°fouled¡± conditions
g. thethicknessofthefoulingdepositontheinnerpipewalloftheexchanger
h. summarisetheresults,andcommentonthefouling(e.g.howdoesitaffect
the rate of heat transfer? How could it be reduced?)
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