Analysis Of Heat Exchangers

Heat Exchangers

5.1 Introduction

Heat exchangers are devices used to transfer heat between two or more fluid streams at different temperatures. Heat exchangers find widespread use in power generation, chemical processing, electronics cooling, air-conditioning, refrigeration, and automotive applications. In this chapter we will examine the basic theory of heat exchangers and consider many applications. In addition, we will examine various aspects of heat exchanger design and analysis.

5.2 Heat Exchanger Classification

Due to the large number of heat exchanger configurations, a classification system was devised based upon the basic operation, construction, heat transfer, and flow arrangements. The following classification as outlined by Kakac and Liu

The shape factors for a number of useful wall configurations are given below in Table 1. Additional results will be presented for some complex doubly connected regions.

Equation (5.7) is for clean or unfouled heat exchanger surfaces. The effects of fouling on heat exchanger performance is discussed in a later section. Finally, we should note that

UA = UoAo = UiAi (5.9) however, Uo 6= Ui (5.10)

Finally, the order of magnitude of the thermal resistances in the defintion of the overall heat transfer coefficient can have a significant influence on the calculation of the overall heat transfer coefficient. Depending upon the nature of the fluids, one or more resistances may dominate making additional resistances unimportant. For example, in Table 2 if one of the two fluids is a gas and the other a liquid, then it is easy to see that the controlling resistance will be that of the gas, assuming that the surface area on each side is equal.

74 Mechanical Equipment and Systems

Table 1 - Shape Factors

Geometry S

Plane Wall A