Civil Engineering Reference
In-Depth Information
temperature. This temperature must be compatible with the surrounding or warm-
region temperature, as already explained.
Compressors for refrigeration can be classified mainly as reciprocating
compressors, screw compressors, and centrifugal compressors. The choice among
them depends on the power range, the working temperatures, the range of demand
from end users, etc. Detailed information on compressors can be found in
specialized textbooks.
As a general rule, reciprocating compressors are used up to a maximum
refrigerating capacity of 500-600 kW; centrifugal compressors have practically
no upper limits and their application starts from a lower limit of a few hundred kW;
screw compressors usually cover a range between 200 and 4,000 kW. Reciprocating
and screw compressors can easily be adapted to different working temperatures and
pressures. The centrifugal compressor adapts poorly to variations in pressure.
Centrifugal and screw compressors can regulate the useful power continuously;
attention must be paid to the low operating limit, which always exists in centrifugal
compressors, established by the surge region where instability may occur. If this
limit is reached, either additional load must be placed on the machine by recycling
vapor from discharge to suction or one compressor must be taken out of service.
Screw compressors can regulate capacity in a wide range, between 10 and 100 %,
by regulating the quantity of gas at suction.
Reciprocating compressors can regulate the useful power step by step by
operating the suction valves sequentially.
Control of speed by a variable-speed drive is always possible.
12.4.3 Condenser
The condenser is basically a heat exchanger. Its main function is to
condense the refrigerant from a vapor state to a liquid state (saturated
or subcooled) by means of a surrounding medium (warm region) at lower
temperature. With reference to Fig.
12.1
the rate at which heat (
Q
out
)is
subtracted from the refrigerant is
Q
out
¼
Q
in
þ
P
c
ðÞ
W
where
mechanical power at
the compressor shaft or electric power in hermetic compressors (W).
Q
in
¼
refrigerating capacity (W), and
P
c
¼
Three main modes of operation occur in the condenser depending on the
different states through which the refrigerant passes: from superheated to saturated
vapor, from saturated vapor to saturated liquid, and from saturated liquid to
subcooled liquid. The sum of the three terms mentioned above is
Q
out
, already
introduced.
