heatexchanger

heatexchanger - University of Texas at Arlington MAE 3183...

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University of Texas at Arlington MAE 3183, Measurements II Laboratory Concentric Tube Heat Exchanger 1 Experiment #4 Concentric Tube Heat Exchanger
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University of Texas at Arlington MAE 3183, Measurements II Laboratory Concentric Tube Heat Exchanger 2 Introduction A heat exchanger is a device by which thermal energy is transferred from one fluid to another. The types of heat exchangers to be tested in this experiment are called single-pass, parallel-flow and counter-flow concentric tube heat exchangers. In a parallel-flow heat exchanger, the working fluids flow in the same direction. In the counter flow exchanger, the fluids flow in parallel but opposite directions. (See Figure 1) Figure 1 , Concentric Tube Heat Exchangers The variables that affect the performance of a heat exchanger are the fluids’ physical properties, the fluids’ mass flow rates, the inlet temperature of the fluids, the physical properties of the heat exchanger materials, the configuration and area of the heat transfer surfaces, the extent of scale or deposits on the heat transfer surfaces, and the ambient conditions. Theory Heat transfer by conduction and convection between two fluids separated by a cylindrical tube (Figure 2) is described by Q. Q AU T T h c = - ( ) (1) Further manipulations can be made to find that: 1 1 1 1 1 2 1 UA U A U A h A R A Lk r r R A h A i i o o o o f o o o i f i i i i = = = + + + + , , ln π (2) Where h is the heat transfer coefficient, U is the overall heat transfer coefficient, and R f is the fouling factor. The fouling factor is merely a term used to account for the additional thermal resistance caused by rust formation, scale buildup, etc. For fully developed, turbulent flow in tubes where the Reynolds number is between 2300 and 5 × 10 6 and the Prandtl number is between 0.5 and 2000, an empirical correlation to determine h proposed by Gnielinski (1976) is widely used.
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University of Texas at Arlington MAE 3183, Measurements II Laboratory Concentric Tube Heat Exchanger 3 ( 29( 29 ( 29 ( 29 Nu hD k D H D = = ƒ - + ƒ - 8 1000 1 127 8 1 1 2 2 3 Re Pr . (3a) where, for smooth tubes, the friction factor is given by ( 29 [ ] ƒ = - - 079 164 2 . ln Re . D (3b) All fluid properties are taken at the average fluid temperature. Thus, the convective coefficients are determined from equation (3a,b) and used in equation (2) to find the overall heat transfer coefficient, U. The heat transfer rate, Q, in equation (1) is not suitable for this case since the temperature difference between the hot and cold fluids varies between inlet and outlet. The
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This note was uploaded on 06/15/2009 for the course MAE 3183 taught by Professor Staff during the Spring '08 term at UT Arlington.

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heatexchanger - University of Texas at Arlington MAE 3183...

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