SCHOOL of ENGINEERING

BENG2 APPLIED THERMODYNAMICS (THER 205)

Heat Transfer : Heat Exchangers - Performance

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1. It has been ascertained that under certain operating conditions a heat exchanger has the following parameters:
thermal capacity ratio C=0.83, number of transport units NTU=3.2

Determine the effectiveness of the heat exchanger if it is operated as (a) a counter flow heat exchanger, and (b) as a parallel flow heat exchanger. [0.810, 0.545]

2. Show that the effectiveness of a counter-flow heat exchanger where the hot and cold streams are the same fluid at equal mass flow rates (e.g in a heat recovery situation)
i.e. C=1 is given by:-

3. An enterprising home-owner decided he could reduce his heating bill for domestic hot water by running the cold water supply to his hot water cylinder through a heat-exchanger situated in the chimney of his central heating boiler. The arrangement he devised is shown at right.

Assuming an overall heat transfer coefficient of 25 W/m²K, C_{p water} = 4.185 kJ/kgK,
C_{p flue gas} =1.1 kJ/kgK and given the data shown:-

(a) Determine the water outlet temperature from his heat-exchanger. [34.8°C]

(b) Give at least two reasons why he would be most unwise to install such a heat-exchanger.

4. A proprietary design of single pass counter-flow air-to-water heat exchanger of the shell-and-tube type passes water through the tubes and air through the shell. The required water exit temperature is 85ºC. The specifications of the heat-exchanger are given below.
Show that the heat-exchanger will not achieve the required water exit temperature.

Data: Tube ID: 6mm, Tube length: 1.25m; Number of tubes: 10; Water flow 12 kg/min; Water inlet temperature: 12ºC; Air flow: 30 kg/min; Air inlet temperature 300ºC;
Air side surface heat transfer coefficient: 1100 W/m²K.
Assume the heat-exchanger tubes are thin and made from a high conductivity material. [T_c2=66.7°C ]