Introduction to Heat Exchangers

Sections 11.1 to 11.3

CH EN 3453 Heat Transfer

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includes answers to more complex (c) and (d) as well, but those arent assigned.

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Heat Exchanger Types Concentric-tube Cross flow Shell-and-tube Compact

Concentric-Tube Heat Exchangers Simplest configuration Superior performance associated with counter flow

Parallel Flow Counterflow

Cross-Flow Heat Exchangers For cross-flow over the tubes, fluid motion, and hence

mixing, in the transverse direction (y) is prevented for the finned tubes, but occurs for the unfinned condition

Heat exchanger performance is influenced by mixing

Finned - Both FluidsUnmixed

Unfinned - One Fluid Mixedthe Other Unmixed

Shell-and-Tube Heat Exchangers Baffles are used to establish a cross-flow and to induce turbulent

mixing of the shell-side fluid, both of which enhance convection

The number of shell and tube passes may be varied:

1 shell, 2 tube passes 2 shell, 4 tube passes

Compact Heat Exchangers Widely used to achieve large heat rates per volume,

especially when one or both fluids is a gas Characterized by large heat transfer surface areas per unit

volume, small flow passages and laminar flow

Fin-tube (flat tubes, continuous plate fins)

Fin-tube (circular tubes, plate fins)

Fin-tube (circular tubes, circular fins)

Plate-fin(single pass)

Plate-fin(multipass)

Overall Heat Transfer Coefficient Essential requirement for heat exchanger design and

performance calculations Contributing factors

Convection between the two fluids and solid Conduction of the solid separator Potential use of fins in one or both sides Time-dependent surface fouling

General expression (c and h = cold and hot)

Fouled Heat Exchanger

Fouled Heat Exchanger

Fouled Heat Exchanger

Log-Mean Temperature Difference

Cocurrent flow (parallel flow) Countercurrent flow

Special Operating Conditions

"heat capacity rate"

Example Book Problem 11.5Transfer of energy from hot flue gases passing through an annular region (od=60 mm) to pressurized water flowing through inner tube (id=24 mm; od=30 mm). Eight struts each 3 mm thick connect the tubes. Made of carbon steel (k = 50 W/mK). Water at 300 K flows at 0.161 kg/s through inner tube while flue gas at 800 K flows through annulus, maintaining a convection coefficient of 100 W/m2K on both struts and outer surface of inner tube.

What is the heat transfer rate per unit length of tube?