Question

Design a heat exchanger for a gas turbine exhaust

A paper mill has just purchased a 5.7 MW gas turbine for electrical power generation. To increase the overall plant efficiency and reduce the payback time it has been decided to use the exhaust gases to heat water in a heat exchanger. The company requires 12.7 tonnes/hr of steam at a pressure of 18.9 Bar and at a temperature of 315^oC. The temperature and pressure at inlet to the heat exchanger was 29^oC and 0.04 Bar.

The gas turbine exhaust has the following properties:

Exhaust mass flow rate (kg/s)	27.3
Temperature (oC)	475
Specific heat capacity (kJ/kgK)	1.088
Minimum stack temperature (oC)	120

Table 1: Gas Turbine Exhaust Properties

Design a heat exchanger to fulfil this requirement using the LMTD method. For design purposes assume that the heat exchanger comprises of an economiser, boiler and super heater section.

The maximum heat transfer coefficients that can be obtained for this type of heat exchanger design are:

Fluid Type	U (kW/m2K)
Gas - Liquid	1.362
Gas - Water Boiling	1.476
Gas - Vapour	1.249

Table 2: Maximum Heat Transfer Coefficients

Initially calculate the product UA for each section, determine the number of tubes and their diameter. The overall tube length in each section should not exceed 4 m.

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A pump has been chosen to pump the water and steam around the circuit. The test results are shown below in Table 3.

Flow rate (l/s)	0.0	0.2	0.6	1.0	1.4	1.8	2.2	2.6	3.0	3.4	3.8	4.2	4.6
Head (km)	8.10	8.07	8.04	8.01	7.98	7.95	7.80	7.65	7.50	6.90	6.00	5.10	3.90

Table 3: Pump Test Results

The circuit has a static head of 100 m. Using the flow resistance through the super heater and the boiler only, plot the load characteristic in the form H = Z + kQ² and hence determine the pump operating point. Find the power required by the pump at this operating point.

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The operating point of the pump does not co-inside with the flow rate required in the heat exchanger. What can one do to ensure that they do match?

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Describe briefly what is meant by a compact heat exchanger.

                                                                                                                                     (Need to answer)

Answer 1

What is meant by a compact heat exchanger?

Answer: They allow for an increased heat transfer rate, require less volume and weigh less compared to other heat exchanger types. The disadvantage is the higher pressure drop. They are clearly the preferred choice for applications which require a high heat transfer rate in a limited volume such as air conditioning devices, heat pumps, automotive radiators, etc. In these applications the main thermal resistance is located on the airside. To improve the heat transfer rate different strategies are used such as the addition of fins at the airside (e.g. plain fins, louvered fins, slit fins, offset strip fins, ...), the use of vortex generators or the application of novel materials (e.g. metal foams, polymer heat exchangers,).

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