Reciprocating Compressor; Screw Compressor; Absorption Refrigeration; Absorption Cycle - Honeywell AUTOMATIC CONTROL Engineering Manual

PREROTATION
VANES
INLET
Reprinted by permission: The Trane Company,
LaCrosse, WI 54601
Fig. 5. Cutaway of Single Stage Centrifugal Compressor.
Centrifugal compressors are driven by turbines, electric motors,
or internal combustion engines. Inlet vane control or speed control
varies the capacity. Each method has different performance
characteristics. A combination of speed and inlet vane control
provides the highest operating efficiency. Multiple stage direct
drive type compressors are available in many configurations.
Refrigerant head is the pressure difference between the
compressor inlet and outlet and is the primary factor affecting
chiller efficiency. For a given load, reducing refrigerant head
improves efficiency. Evaporation and condensation
temperatures establish these pressures and are determined by
chilled water temperature and condenser water temperature.
Refrigerant head is reduced by the following:
– Reducing condenser water temperature.
– Raising chilled water temperature.
– Reducing load.
– Decreasing design differential temperature of evaporator
and condenser heat exchangers by increasing the size of
the heat exchangers.
The load for maximum chiller efficiency varies with chillers
and chiller manufacturers, but is often 70 to 80 percent.

RECIPROCATING COMPRESSOR

The reciprocating compressor is a positive displacement
device consisting of several cylinders and pistons. The
crankshaft is driven by a motor or engine. Spring loaded valves
allow low pressure refrigerant vapor to enter the cylinder on
the downstroke and high pressure refrigerant vapor to exit on
the upstroke. Because the compressor is a positive displacement
device its capacity is not greatly influenced by refrigerant head.
However, power required per unit of cooling is directly related
to refrigerant head. Keeping condenser temperature as low as
possible also reduces energy requirements, therefore,
compressors with water cooled condensers use less power than
air cooled condensers. However, condenser water temperature
must not be allowed to go too low or there will not be enough
pressure difference to circulate the refrigerant.
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
IMPELLER
VOLUTE
M11417
Reciprocating chiller capacity is controlled in stages (steps).
Methods of capacity control include the following:
– Unloading cylinders
– On-off cycling of multiple compressors
– Hot-gas bypass
– Hot-gas through evaporator
Cylinder unloading or multiple compressor on-off cycling is
sequenced by automatic controls. The cylinder inlet valves are
held open so no compression takes place during cylinder
unloading. Capacity control mechanisms and controls are
usually packaged with the chiller.
Step control of refrigeration must provide a compromise
between to frequent cycling and to wide temperature swings.
Use of chilled water return temperature as controlling variable
lengthens the compressor on and off cycles. When cylinder
unloading is used, the minimum off time after the compressor
is cycled off on low load, is normally less than for multiple
compressors. Off time is critical because the refrigeration
system must have time to equalize the pressure between high
and low sides so that the starting load will not be too great for
the motor.

SCREW COMPRESSOR

A screw compressor is a positive displacement device which
uses two meshed helical rotors to provide compression. It is
also known as a helical rotary compressor. Basic construction
of a helical rotary twin screw compressor is shown in Figure 6.
The capacity of a screw compressor can be modulated by speed
control or a sliding valve that varies the length of compression
area of the helical screws and bypasses some gas back to the
inlet of the compressor.
INLET
MAX MIN
VIEW A-A
PISTON
CYLINDER
PORT AREAS
Fig. 6. Helical Rotary Twin Screw Compressor.

ABSORPTION REFRIGERATION

ABSORPTION CYCLE

The absorption cycle uses a fluid called an absorbent to absorb
evaporated refrigerant vapor in an "absorber" section. The
resulting combination of fluid and refrigerant is moved into a
"generator" section where heat is used to evaporate the
refrigerant from the absorbent.
301 ENGINEERING MANUAL OF AUTOMATIC CONTROL
A
B
BYPASS
OUTLET
A B
VIEW B-B
SLIDING VALVE
M10507