مقاله انگليسي

CHAPTER

CONDENSERS

7.1 TYPES OF CONDENSERS USED
IN INDUSTRIAL REFRIGERATION
The three main types of condensers used in general refrigeration systems are:

air-cooled

water-cooled

evaporative
All of these serve the industrial refrigeration field as well. In comparison to
the air-conditioning industry, however, a lower percentage of air-cooled
condensers and a higher percentage of evaporative condensers are operating in
industrial refrigeration plants. In industrial refrigeration practice, it is common
to connect the evaporative condensers in parallel—a concept not normally used
in air conditioning.
The three types of condensers are shown schematically in Fig. 7.1a, 7.1b,
and 7.1c. The air-cooled condenser in Fig. 7.1a condenses refrigerant vapor by
rejecting heat to ambient air blown over the finned condenser coil with the aid
of a fan, usually a propeller type.
Most all water-cooled condensers (Fig. 7.1b) condense refrigerant in the shell and
on the outside of tubes through which water passes. The condenser cooling water
picks up heat in passing through the condenser and this warm water is cooled by
circulating through a cooling tower (Section 7.6). While the shell-and-tube

construction predominates for water-cooled condensers, plate-type condensers,
sister of the plate-type evaporator explained in Sec. 6.31, are now appearing.
The evaporative condenser of Fig. 7.1c might be considered a cooling tower,
with the condenser tubes washed by the water spray. Ultimately, the heat
rejected from the refrigeration plant is discharged to ambient air, except where
the condenser is cooled by water from a well, lake, or stream.
This chapter first explores the condensing process outside and inside tubes.
Next, the overall performance of water-cooled condensers and the translation
of performance to noncatalog ratings is examined. An explanation of the
performance of cooling towers, the constant companions of water-cooled
condensers, is given. Because of their prevalence in industrial refrigeration
plants, the emphasis of this chapter is on the performance, selection, application,
and operation of evaporative condensers.
7.2 THE CONDENSING PROCESS
Nearly a century ago, heat-transfer pioneer, Willhelm Nusselt, proposed a model
to predict the magnitude of a condensing coefficient for a special geometric
situation1. Nusselt envisioned the condensation of vapor on a cold vertical plate,
Fig. 7.2, as a process where vapor condenses on the plate and the condensate
drains downward, with the condensate film becoming progressively thicker as
it descends. The local condensing coefficient is taken to be the conductance
through the condensate film—the conductivity of the liquid divided by the film
thickness at that point. Nusselt developed the expression for the mean
condensing coefficient as

The immediate question is where, if at all, does condensation occur on a
vertical plate in industrial practice? Actually, a very old condenser design
oriented the tubes vertically and water flowed by gravity down the inside of the
tubes to ease their cleaning. The refrigerant in the shell condensed on the outside
of the vertical tubes.
A slight modification of Eq. 7.1 applies to the widely used horizontal shelland-
tube condenser, Fig. 7.1b. The product of the number of tubes in a vertical

FIGURE 7.2
Condensation of a vapor on a cold vertical surface

row multiplied by the diameter of the tubes replaces the vertical length of the
plane L. White2 found by experimental tests that the coefficient is 0.63 and Goto3
measured 0.65, so the equation for N tubes of diameter D in a vertical row is

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