A finned-tube heat exchanger has to be designed to transfer heat from the humidified hot air to cold water. It is important to note that when the humid air temperature becomes lower than the dew-point temperature, water vapor in the air will start to condense. It is required that the heat exchanger effectiveness is 90% and has as minimum (possible) volume. The design conditions for air and water are,
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AIR
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WATER
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Inlet Temperature
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60 °C
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25 °C
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Mass flow rate
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0.17 kg/s
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0.34 kg/s
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Air inlet relative humidity
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95 %
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Air outlet relative humidity
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95 %
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The main objective of this project is to increase your ability to solve real world engineering problems which are characterized to be not well defined. That is, the given data is not enough to solve the problem. Thus, one has to make many decisions and careful assumptions such that it can be solved to meet the desire needs.
Some Considerations
Please note that sizing the heat exchanger involves determining the required heat transfer area to meet the heat duty (tube diameter, length, number of tubes and fins), and the associated overall heat transfer coefficient. In this regard you may follow the following steps:
(1) Select a configuration (e.g. finned-tube HX where fins are on the air side) and calculate the required UA product using the epsilon - NTU or LMTD methods
(2) Apply the energy balance on the hot and cold fluids and assume that the condensate (water vapor condensed from the air) is at a temperature of 40 °C.
(3) Calculate the overall heat transfer coefficient (U) for the assumed tube diameter, fin shape and material. Use correlations to estimate hot-and cold-side convection coefficients, conduction resistance, contact resistance (if any), and fouling resistance.
(4) Determine the area from the UA product and U value and check whether the HX volume can be reduced by assuming different dimensions.