1. Chilled-Water Systems
Fundamentals of H V A C R
Third Edition
Chapter 81
Chilled-Water Systems
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2. Objectives (1 of 3)
81.1 Describe the difference between a flooded and a dry (direct expansion) type chiller Explain why low-pressure chillers need purge recovery Explain why some chillers are better suited for air- cooled condensers while others operate with water-cooled condensers.

3. Objectives (2 of 3)
81.4 List the different types of compressor arrangements found on chillers Describe the function of a chiller economizer Describe the operation for an oil-return system for a chiller Transfer refrigerant to and from a chiller.

4. Objectives (3 of 3)
81.8 Explain how an absorption chiller operates.

5. Introduction
(a) Chilled-water cooling loop from central unit to multiple zones; (b) glycol and chilled- water piping connections to air handler
The more common way to deliver refrigerant to individual
air handlers is to use a secondary refrigerant such as
chilled water. The chilled water is cooled in a central station
chiller by a primary refrigerant. The chilled water is then
delivered throughout the building to individual air handlers
by circulating pumps.

6. Flooded and Dry-Type Water Chillers
Chilled-water cooling loop for cooling coil
Centrifugal compressor chiller
The water in a chiller is cooled to approximately 43–45°F.
The chilled-water pump circulates the chilled-water supply
(CHWS) to the cooling coil in the air handler. The
chilled-water loop will have an expansion tank and fill
connections for makeup water, and it will also have air
vents, balancing valves, and cooling coil flow controls,

7. Purge Recovery Units and Air-Cooled Condensers
Centrifugal compressor suction-side rupture disk
A low-pressure chiller will operate with a negative suction
pressure that is below atmospheric. Any leaks in the system
will draw in air. This air typically collects in the top section of
the condenser. The air will increase the discharge pressure
of the unit and lead to higher compressor power consumption.
The air will also contain water that will oxidize the oil
in the system and lead to the formation of sludge and acids.
Chillers of this type generally have some type of purge
recovery unit.
Never allow a low-pressure chiller to reach or exceed the
rupture disk setting when testing for leaks. The rupture
disk setting for low-pressure chillers is normally set at
15 psig, but it is advisable not to exceed 10 psig. If this
happens, then the chiller rupture disk will release and
all of the refrigerant in the unit will be lost.

8. Air-Cooled Condensers (1 of 2)
Package chiller with reciprocating compressor
Control sensor: pressure transducer
Starting at the lower capacities, packaged water chillers
use one or more reciprocating compressors.
This type of unit would be mounted directly outside
or on a rooftop with supply and return connections for the
chilled water piped into the building. For head (discharge)
pressure control, the condenser fan control receives input
from a pressure transducer. The fans are maintained
at the lowest condensing pressure and temperature
possible to maintain the highest unit efficiency.

9. Air-Cooled Condensers (2 of 2)
(a) Air-cooled condenser installation with a package chiller; (b) remote air-cooled condenser
The larger package chillers with reciprocating compressors
can be air cooled or water cooled and range upward
from 40 to 200 tons. The cooler and compressor can
be located indoors and the air-cooled condenser located on
the rooftop or outside the building.

10. Scroll Compressor Chillers
Cutaway view of a scroll compressor
In addition to reciprocating compressors, the scroll-type
compressor is being utilized increasingly in packaged chillers.
These compressors use a pair of mating scroll-shaped
surfaces to compress the refrigerant with pure rotary motion.

11. Centrifugal Compressor Chillers
Hermetic centrifugal compressor
Centrifugal compressor impeller
Compressor discharge through casing volute to condenser
Hermetic centrifugal compressors vary
in design and refrigerant type. This unit has a hermetically
sealed motor that will drive the compressor through a set
of transmission-speed-increasing gears. The compressor
speed is typically much higher than the motor speed. The
refrigerant vapor is drawn from the cooler into the suction
eye of the rotating impeller. The pressure of
the refrigerant is increased as it passes through the discharge
of the spiral compressor casing (volute) and into
the condenser.

12. Centrifugal Compressor Capacity Control and Screw Compressor Chillers
Inlet guide vanes in the closed position
Inlet guide vanes in the open position
Screw compressor, end view
Variable-speed prime movers
may also have limitations in speed reduction and only operate
within a specific range. To accommodate this, most centrifugal
compressors will have variable inlet guide vanes located
at the compressor suction. These vanes will be closed at low
load and at compressor start-up,
and open at high load.
An important type of packaged water chiller uses the helical
rotary compressor, commonly known as a
screw compressor because of the appearance of the rotors.

13. Screw Compressor Chillers
Diagram of the rotors of a screw compressor
Cooling water tower for closed-loop water-cooled condenser
The twin rotor screw compressor uses a mating pair of rotors with lobes that rotate much like a pair of gears.
Screw compressors are used because of their high
capacity for a small unit and their continuously variable
(stepless) capacity control, typically modulating from 100
to 10 percent of full capacity. Being positive displacement,
screw compressors have piping flexibility to accommodate
remotely located condensers and can be used with high-pressure
refrigerants. They can operate with both water- and
air-cooled condensers. Cooling water towers may be used
for closed-loop water-cooled condensers.

14. Screw Compressor Chiller Operation (1 of 2)
Screw compressor refrigerant flow without economizer
An hermetic screw compressor
drawing suction from a flooded cooler. The refrigerant
vapor is compressed and discharged into the condenser.
A float valve will control refrigerant flow and allow for
the pressure drop required for the refrigerant to expand
in the cooler. A hot-gas bypass solenoid-operated valve
is used to recirculate refrigerant at low loads.

15. Screw Compressor Chiller Operation (2 of 2)
Screw compressor refrigerant flow with economizer
Many large commercial centrifugal and screw-type chillers
utilize economizer sections. Their purpose is to subcool the
liquid refrigerant before it enters the cooler.

16. Chiller Oil Return Screw compressor oil flow A typical oil return
system for a hermetic screw-type compressor
chiller. A metered amount of oil collecting in the bottom of
the cooler is allowed to pass to the vaporizer section. Hot
gas from the condenser passes through a coil located in
the vaporizer. This will vaporize any liquid refrigerant that
has been carried over with the oil. The oil then passes from
the vaporizer to the oil sump, where an electric heater is
utilized to further vaporize any refrigerant liquid entrained
in the oil. Mist eliminators are used to remove any entrained
oil in the refrigerant vapor before returning to the compressor
suction. This reduces the possibility of any oil carryover
that would slug the compressor.

17. Chiller Refrigerant Recovery and Transfer
Liquid recovery first
Vapor recovery follows liquid recovery
This system is permanently installed and can
be used to transfer refrigerant from multiple chiller circuits.
Chillers that do not have permanently installed transfer
systems will require a portable unit. This is different from
conventional portable recovery units because of the valve
arrangement.

18. Absorption Chiller Operation
Absorption-cycle operation
Absorption-cycle components and operational cycle
The refrigerant used is simply water,
and the absorbent is lithium bromide (LiBr). The evaporator
shell pressure is reduced to 0.12 psia (29.7 in Hg vacuum).
At this low pressure, water will evaporate at 41°F.

19. Summary (1 of 3)
Air-conditioning chillers are typically designed for indirect expansion.
Chilled water is circulated to individual air handlers or fan rooms rather than the primary ingredient.
Chillers come in various sizes, with smaller tonnage units utilizing reciprocating or scroll compressors with electric motor drives. 19

20. Summary (2 of 3)
It is important to understand the refrigerant cycle and operation of a chiller before transferring refrigerant.
Always remove liquid refrigerant from a chiller first before recovering the vapor or charging a unit.
Failure to do so could result in chiller freeze-up and damage. 20

21. Summary (3 of 3)
Never charge liquid refrigerant to a chiller that is under a deep vacuum.
Never raise the pressure in a chiller above the rupture disk burst pressure.
Follow all manufacturer’s recommendations and procedures when servicing a chiller.

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