Page 83 - Design and Operation of Heat Exchangers and their Networks

P. 83

CHAPTER 3

Steady-state characteristics

of heat exchangers

a
b
Wilfried Roetzel , Xing Luo , Dezhen Chen c
a
Institute of Thermodynamics, Helmut Schmidt University/University of the Federal Armed Forces Hamburg,
Hamburg, Germany
b
Institute of Thermodynamics, Gottfried Wilhelm Leibniz University Hannover, Hannover, Germany
c
Institute of Thermal Energy and Environmental Engineering, Tongji University, Shanghai, China

The design procedure of a heat exchanger consists of problem specification,
structural determination, thermal and hydraulic design, mechanical design,
cost estimation, environmental impact analysis, equipment optimization,
and process optimization. The steady-state characteristics of heat exchangers
represent their fundamental features in heat duty, effectiveness, outlet tem-
peratures, and pressure drops under the steady-state operation conditions. In
this chapter, we will concentrate on their thermal performance and calcu-
lation methods for some typical kinds of heat exchangers.

3.1 Thermal performance of heat exchangers

In a recuperative heat exchanger, two fluids are separated by a heat transfer
surface (wall). To analyze the heat transfer in the exchanger, we make the
following assumptions: (1) The heat exchanger operates under steady-state
conditions. (2) Heat losses to or from the surroundings are negligible.
(3) There are no heat sources or sinks in the exchanger walls and fluids.
(4) The flow pattern of each fluid is a plug flow in its flow direction.
(5) The thermal conduction in the exchanger wall and fluids along the
flow direction is negligible. (6) The cross-sectional area, properties of the
fluids, wall thermal resistance (including fouling thermal resistance), heat
transfer coefficients (or overall heat transfer coefficient), and heat transfer
area are constant along the flow direction.

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