Heat Exchangers

As noted above, a boiler is a type of heat exchanger, a device in which energy exchange occurs between two different fluids. There are three basic heat transfer arrangements: parallel flow, counterflow, and crossflow. In parallel flow, both fluids enter at the same relative location and flow in parallel paths over the heating surface. In counter-flow, the two fluids enter at opposite ends and flow in opposite directions over the surface. In crossflow, the paths of the two fluids are generally perpendicular to one another. Counterflow is the most efficient heat exchanger arrangement, producing the greatest temperature difference and requiring the least heating surface.

The heat transfer rate, Q, can be related to the characteristics of the heat exchanger by the equation


U = Overall heat transfer coefficient

A = Surface area

F = Arrangement factor

ATlmtd = Mean temperature difference

The overall heat transfer coefficient, U, expressed in Btu/h • ft2 • °F (W/m2 • °K), is the coefficient used to express the aggregate effect of all the conductances. The U value of a pane of glass, for example, is about 1. The U value for a water-to-water heat exchanger typically ranges from 150 to 300 (850 to 1,700). The U value for steam-to-gas ranges from 5 to 50 (28 to 280) and for water-togas from 10 to 20 (57 to 114).

The log mean temperature difference is defined as:

atLMTD =


Initial temperature difference between the hot and cold fluids

Final temperature difference between these media Natural logarithm

The log mean temperature difference depends on the relative directions of flow of the fluids as they pass through or over the surface; that is, fluids may flow counter to, parallel to, or across one another. Figure 7-10 shows the three flow arrangements and presents Equation 7-9 written specifically for each case.

Fouling factors must be considered in the determi


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