Handbook of air conditioning and refrigeration / Shan K

Purge Unit
FIGURE 13.3 Orifice plates.
REFRIGERATION SYSTEMS: CENTRIFUGAL 13.5
Flash Cooler. A flash cooler is sometimes called an economizer. It flashes a small portion of
refrigerant at the intermediate pressure to cool the remaining liquid refrigerant to the saturated
condition, in order to provide a greater refrigeration effect, as described in Secs. 9.13 and 9.14. For
a two-stage compressor, a single-stage flash cooler is used. For a three-stage compressor, a twostage
flash cooler is used.
Orifice Plates or Float Valves. Orifice plates (arranged in series) or float valves are used as throttling
devices in centrifugal chillers that employ flooded refrigerant feed. The use of multiple orifices
as a throttling device is shown in Fig. 13.3. This device controls the amount of refrigerant feed
to the liquid cooler according to the pressure of the liquid refrigerant in the condenser.
During full-load operation, a certain liquid pressure is maintained before the first orifice plate.
When liquid refrigerant flows through the first orifice plate, it encounters a pressure drop. However,
the fluid pressure between the two orifice plates is still higher than the saturated pressure of the
liquid. No flash of vapor occurs in the region between the orifice plates. The second orifice plate
meters the maximum refrigerant flow.
As the refrigeration load in the centrifugal chiller drops, less refrigerant is delivered to the condenser
and evaporator, resulting in a lower condensing pressure and a higher evaporating pressure.
This reduces the pressure exerted on the liquid refrigerant at the condenser due to lower condensing
pressure and hydrostatic head. Because of the pressure drop across the first orifice plate, the fluid
pressure between the orifice plates is less than the saturated pressure of the liquid refrigerant. Flashing
of vapor occurs. Both liquid and vapor flow through the second orifice plate. Consequently, the
mass flow rate of the refrigerant flow is reduced.
There are no moving parts in an orifice plate throttling device. It is simple and reliable in
operation.
At an evaporating temperature T ev � 40°F (4.4°C), HCFC-123 has a saturated pressure of 5.8 psia
(40 kPa abs.) which is far lower than atmospheric pressure. Noncondensable gas such as air and
carbon dioxide may diffuse and leak into the evaporator through cracks and gaps. Because