Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
299
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 pressure is the pressure difference between the
compressor inlet and outlet and is the primary factor affecting
chiller efficiency. For a given load, reducing refrigerant pressure
improves efficiency. Evaporation and condensation
temperatures establish these pressures and are determined by
chilled water temperature and condenser water temperature.
Refrigerant pressure 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. It is often 70 to 80 percent, but can
be 100 percent.
RECIPROCATING COMPRESSOR
The reciprocating compressor is a positive displacement
device consisting of several cylinders and piston .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
pressure. However, power required per unit of cooling is directly
related to refrigerant pressure. 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.
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 capacity 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 of 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
PREROTATION
VANES
IMPELLER
VOLUTE
M11417
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.
In the absorber (Fig. 7) the absorbent, also called strong
absorbent at this point, assimilates the refrigerant vapor when
sprayed through it. The resulting weak absorbent is pumped by
the generator pump through the heat exchanger, where it picks
up some of the heat of the strong absorbent, then into the
A B
A
B
BYPASS
VIEW B-B
OUTLET
VIEW A-A
PISTON
CYLINDER
PORT AREAS
MIN MAX
INLET
SLIDING VALVE
M10507