Page 161 - Design and Operation of Heat Exchangers and their Networks

CHAPTER 4

Thermal design of evaporators

and condensers

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

Evaporators, boilers, and reboilers are widely used in power plants, process
industries, and air-conditioning and refrigeration industries. One of the most
important ways for energy conversion from primary energy to useful energy
(thermal, mechanical, and electrical) is via heat engines. In a thermal power
station, water is pumped into a boiler, where it is heated and turns into
steam. The steam spins a steam turbine, which drives an electrical generator.
After it passes through the turbine, the steam is condensed in a condenser and
recycled to the boiler. In a heat pump, the cycle runs in the opposite direc-
tion. Liquid refrigerant enters an evaporator, absorbs heat at a lower temper-
ature level from the hot medium, and evaporates into vapor. The vapor is
compressed by a compressor to a higher pressure, and then, it is condensed in
a condenser at higher temperature level and rejects heat from the system by
either the water or the air. In these systems, evaporators (boilers) and con-
densers play an important role.

4.1 Evaporation heat transfer and evaporators

Evaporators and boilers are used to boil liquid into vapor phase. The name
evaporator or boiler is a conventional name. Usually, boilers use a burning
heat source and work at high temperature, while evaporators use a hot fluid
or electric heating and work at relative low temperature.
4.1.1 Pool boiling

The well-known Nukiyama boiling curve (Nukiyama, 1934) illustrates dif-
ferent regimes of pool boiling: natural convection boiling, nucleate boiling,
transition boiling, and film boiling. As is shown in Fig. 4.1, the experimental

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