Introduction to Heat Exchangers

Sections 11.1 to 11.3

CH EN 3453 – Heat Transfer

News Flash…

• Project experimental section due Friday

• Project theory section due a week from Friday

• Homework #8 due Friday– Problem #5 has only parts (a) and (b). Solution

includes answers to more complex (c) and (d) as well, but those aren’t assigned.

• Help session today at 4:30 pm

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/m·K). 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/m2·K on both struts and outer surface of inner tube.

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