Lect_19_Shell.Tube_Xchanger

Lect_19_Shell.Tube_Xchanger - Shell and Tube Heat...

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Shell and Tube Heat Exchangers

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Goals: By the end of today’s lecture, you should be able to: describe the common shell-and-tube HE designs draw temperature profiles for parallel and counter-current flow in a shell-and-tube HE calculate the true mean temperature difference for a shell-and-tube HE (use F G chart) make heat transfer calculations for shell-and-tube HEs describe how the inside and outside heat transfer coefficients are determined for shell-and-tube HEs use the Donohue equation to estimate h o in a multiple pass heat exchanger make heat transfer calculations for multiple pass shell-and-tube heat exchangers
Outline: I . Review II. Shell-and-tube equipment III. Rate equation and T TM IV. Ten Minute Problem - F G for multiple pass HE V. Heat transfer coefficients VI. Example Problem - Handout

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I. Review Last time, we reviewed heat transfer in double pipe (concentric pipe) heat exchangers. We considered cases of parallel and countercurrent flow of the hot and cold fluids in the concentric pipe design. The basic equations required in the design of a heat exchanger are the enthalpy balances on both fluid streams and a rate equation that defines the heat transfer rate.
Enthalpy balances for fluids without phase change: (hot stream) and (cold stream) If a phase change occurs (the hot stream is condensed), then the heat of condensation (vaporization) must be accounted for: In most applications, the heat gained by the cold stream can be assumed to equal the heat lost by the hot stream (i.e., q h = q c ). h ph h h T C m q = c pc c c T C m q = . . cond h cond m q λ =

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The rate of heat transfer for a concentric pipe heat exchanger with parallel or countercurrent flow can be written as: or where T TM is the true mean temperature difference. For concentric pipe heat exchangers, the true mean temperature difference is equal to the log mean temperature difference ( T LM ). TM i i T U L D q = ) ( π TM T U A q i i =
Recall, the overall heat transfer coefficient is written as: 1 U o A o = 1 U i A i = 1 h i A i + x kA LM + 1 h o A o

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II. Shell-and-Tube Equipment Concentric Pipe vs. Shell & Tube Heat Exchangers: The simple double pipe heat exchanger is inadequate for flow rates that cannot readily be handled in a few tubes. Double pipe heat exchangers are not used for
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