Water Chillers: Fundamentals Application, and Operation VAPOR COMPRESSION CYCLE WATER CHILLERS

 Water Chillers: Fundamentals
Application, and Operation
VAPOR COMPRESSION CYCLE WATER CHILLERS

VAPOR COMPRESSION CYCLE WATER CHILLERS

VAPOR COMPRESSION CYCLE WATER CHILLERS:

As introduced in the section “Vapor Compression Refrigeration,” a secondary
refrigerant is a substance that does not change phase as it absorbs heat. The most
common secondary refrigerant is water and chilled water is used extensively in
larger commercial,
institutional, and industrial facilities to make cooling available
over a large area without introducing a plethora of individual compressor
systems. Chilled water has the advantage that fully modulating control can be
applied and, thus, closer temperature tolerances can be maintained under almost
any load condition.
For very low-temperature applications, such as ice rinks, an antifreeze component,
most often ethylene glycol or propylene glycol, is mixed with the water and
the term brine (left over from the days when salt was used as antifreeze) is used
to describe the secondary refrigerant.
In the HVAC industry, a chiller using the vapor compression cycle consists of
one or more compressor(s), evaporator(s), and condenser(s), all packaged as a
single unit. Where multiple compressors are used, it is typical to provide multiple,
separate refrigeration circuits so that the failure of one compressor will not impact
on the operation of the remaining compressors. The condensing medium may be
water or outdoor air.
The evaporator, often called the cooler, consists of a shell-and-tube heat
exchanger with refrigerant in the shell and water in the tubes. Coolers are designed
for 3–11 fps water velocities when the chilled water flow rate is selected for a
range of 10–20°F.
For air-cooled chillers, the condenser consists of an air-to-refrigerant heat
exchanger and fans to provide the proper flow rate of outdoor air to transfer the
heat rejected by the refrigerant.
For water-cooled chillers, the condenser is a second shell-and-tube heat
exchanger with refrigerant in the shell and condenser water in the tubes.
Condenser water is typically supplied at 70–85°F and the flow rate is selected for

a range of 10–15°F. A cooling tower is typically utilized to provide condenser
water cooling,
but other cool water sources, such as wells, ponds, and so on, can
also be used.

Scr oll Compressors:

Scroll compressors are positive-displacement orbital motion compressors that use
two spiral-shaped scroll members, one that is fixed and the other that rotates, to
compress refrigerant gas.
Scroll members are typically a geometrically identical pair, assembled 180°
out of phase. Each scroll member is open on one end and bound by a base plate on

the other. The two are fitted to form pockets between their respective base plates
and various lines of contact between their walls. The flanks of the scrolls remain
in contact, but the contact point moves progressively inward, compressing the
refrigerant gas, as one scroll moves. Compression occurs by sealing gas in pockets
of a given volume at the other periphery of the scrolls and progressively reducing
the size of the pockets as the scroll relative motion moves them inward toward
the discharge port.
Two different capacity control mechanisms are available. The most common
approach to capacity control is variable-speed control, utilizing a variable
frequency
drive to control the rotational speed of the moving scroll. The cooling
capacity, then, varies directly as a function of its speed. Another control
method is called variable displacement, which incorporates “porting” holes in
the fixed scroll. Capacity control is provided by disconnecting or connecting
compression chambers on the suction side by closing or opening these porting
holes.
Scroll compressors are available in capacities from 1.5 tons to about 40 tons
and are applied in both single and multiple compressor configurations. The maximum
chiller size typically applied is 80–160 tons, using a multiple compressor
and usually with air-cooled condensing.

Rotary Scr ew and Centrifugal Compressors:

For larger chillers (150 tons to over 10,000 tons), rotary compressor water chillers
are utilized. There are two types of rotary compressors applied: positive-displacement
rotary screw compressors and centrifugal compressors.
Figure 1.5 illustrates the rotary screw compressor operation. Screw compressors
utilize double-mating helically grooved rotors with “male” lobes and “female”
flutes or gullies within a stationary housing. Compression is obtained by direct
volume reduction through rotary motion. As the rotors begin to unmesh, a void is
created on both the male and the female sides, allowing refrigerant gas to flow into
the compressor. Further rotation starts the meshing of another male lobe with a
female flute, reducing the occupied volume, and compressing the trapped gas. At a
point determined by the design volume ratio, the discharge port is uncovered
and
the gas is released to the condenser.
Capacity control of screw compressors is typically accomplished by opening
and closing a slide valve on the compressor suction to throttle the flow rate of
refrigerant gas into the compressor. Variable-speed control can also be used to
control the compressor capacity.

The design of a centrifugal compressor for refrigeration duty originated with
Dr. Willis Carrier just after World War I. The centrifugal compressor raises the
pressure of the gas by increasing its kinetic energy. This kinetic energy is then
converted into static pressure when the refrigerant gas leaves the compressor and
expands into the condenser. Figure 1.6 illustrates a typical centrifugal water
chiller configuration. The compressor and motor are sealed within a single casing
and a refrigerant gas is utilized to cool the motor windings during operation.

Cutaway of a typical centrifugal water chiller.

Low-
pressure gas flows from the cooler to the compressor. The gas flow rate is
controlled by a set of preswirl inlet vanes and/or a variable frequency speed
controller
that regulates the refrigerant gas flow rate to the compressor in response
to the cooling load imposed on the chiller.
Normally, the output of the chiller is fully variable within the range 15–100%
of full-load capacity. The high-pressure gas is released into the condenser, where
water absorbs the heat and the gas changes phase to liquid. The liquid, in turn,
flows into the cooler, where it is evaporated, thus cooling the chilled water.
Centrifugal compressor chillers using R-134a are referred to as positive-pressure
machines, while those using R-123 are considered to be negative-pressure
machines, as defined by the evaporator pressure condition. At standard
Air-
Conditioning and Refrigeration Institute (ARI) rating conditions and using
R-134a, the evaporator pressure is 36.6 psig and the condenser pressure is
118.3 psig, yielding a total pressure
increase or lift provided by the compressor of
81.7 psig. However, for R-123, these pressure conditions are −5.81 psig in the
evaporator and 6.10 psig in the condenser, yielding a total lift of 11.91 psig.
Mass flow rates for refrigerants in both positive- and negative-pressure chillers
are essentially the same at ~3 lb/min ton. However, due to the significantly higher
density of R-134a, its volumetric flow rate (cfm/ton), which defines impeller size,
is over five times smaller than R-123 volumetric flow rate.

\Rotary screw compressor operation.

 Early compressors using R-123 typically used large-diameter impellers (~40″
diameter) and direct-coupled motors that (at 60 Hz) turn at 3600 rpm. These large
wheel diameters required by R-123 put a design constraint on the compressor and,
to reduce the diameter, current designs typically utilize two or three impellers in
series or stages to produce an equivalent pressure increase. In practice, the flow
paths from the outlet of one stage to the inlet of the next introduce pressure losses
that reduce efficiency to some degree.
Compressors using R-134a typically use much smaller impellers (about 5″
diameter) that are coupled to the motor through a gearbox or speed increaser and
can operate at speeds approaching 30,000 rpm.
Since the evaporator in positive-pressure chillers is maintained at a pressure
well above atmospheric, any leaks in the refrigeration system will result in a loss
of refrigerant and the effect of any leaks is quickly evidenced by low refrigerant
levels in the chiller. However, any leaks associated with a negative-pressure
machine result in the introduction of atmospheric air (composed of noncondensable
gases and water vapor) into the chiller.

 Noncondensable gases create two problems:

1. The compressor does work when compressing the noncondensable
gases, but they offer no refrigerating effect.
2. Noncondensable gases can “blanket” evaporator and condenser tubes,
lowering heat exchanger effectiveness.
Noncondensable gases can lower the efficiency of the chiller by as much as
14% at full load.
Moisture introduced with atmospheric air is a contaminant that can allow the
formation of acids within the chiller that can cause serious damage to motor
windings (of hermetic motors) and bearings.
To remove potential noncondensable gases and moisture from negative-pressure
chillers, these chillers are furnished with purge units. While purge units
are very efficient at separating and venting noncondensable gases and moisture
from the refrigerant, it is not 100% efficient and some refrigerant is vented to the
atmosphere each time the purge unit operates. Additionally, to reduce the potential
for leaks when chillers are off, the evaporator should be provided with an
external heater to raise the refrigerant pressure to above atmospheric.
The energy requirement for a water-cooled rotary compressor chiller at peak
load is a function of (1) the required leaving chilled water temperature, and (2) the
temperature of the available condenser water. As the leaving chilled water temperature
is reduced, the energy requirement to the compressor increases, as summarized
in Table 1.4. Similarly, as the condenser water temperature increases, the
compressor requires more energy (see Chapter 10). Thus, the designer and owner
can minimize the cooling energy input by utilizing a rotary compressor chiller

Rotary Chiller Input Power Change as a Function
of Chilled Water Supply Temperature

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