An oil heater keeps the oil warm in the oil sump of
the compressor during plant shutdown. If the oil is not
kept heated, it absorbs large amounts of refrigerant. This
results in excessive oil foaming when the unit is started.
The heaters in most plants are connected so that they are
automatically turned on when the compressor is off, and
off when the compressor is on..
Figure 10-20 shows a centrifugal compressor
air-conditioning unit. This particular plant has a 150-ton
capacity and uses R-114 as the refrigerant. The gauges
and controls for the plant are on the other side of the unit.
Lithium Bromide Absorption Unit
Water is used as a refrigerant in the lithium bromide
absorption cycle. The absorption system differs from the
compression-type refrigeration machines. The
absorption cycle uses heat energy instead of mechanical
energy to cause the change in conditions necessary for
a complete refrigeration cycle. In other words, the
compressor is replaced by steam heat.
The following are the two principles that form the
basis for the lithium bromide absorption refrigeration
cycle:
1. Lithium bromide has the ability to absorb large
quantities of water vapor.
2. When under a high vacuum,
(vaporizes) at a low temperature and,
absorbs heat.
water boils
in doing so,
To understand the lithium bromide absorption
cycle, follow along on figure 10-21 during as you read
the following explanation. Notice that the
EVAPORATOR and ABSORBER sections are in a
common shell. The sections are separated by the
refrigerant tray and baffles. This shell is under a high
vacuum of about 29.8 in.Hg. Water boils at 35°F (1.7°C)
at this pressure. (Note that this is only 3°F above the
freezing point of water.) The refrigerant pump
circulates the refrigerant (water) through the evaporator.
The water is sprayed out over 88the chilled water tubes
through a spray header. This causes the water to
vaporize (or flash) more readily. As the water vaporizes
around the chilled water tubes, it removes heat from the
circulating chilled water. The water vapor is floating
about in the evaporator/absorber shell. Now, the
absorber comes into play.
Lithium bromide solution is sprayed out from a
spray header in the absorber. The absorber pump
provides the driving head for the spray. As the lithium
bromide solution is sprayed out, it absorbs the water
vapor, which is in the shell from the evaporation
process. As the lithium bromide absorbs more and more
water vapor, its ability to absorb decreases. This is
47.217
Figure 10-20.-R-114 centrifugal air-conditioning plant.
10-22
