Handbook of air conditioning and refrigeration / Shan K

Air-Cooled Screw Chillers
REFRIGERATION SYSTEMS: RECIPROCATING, ROTARY, SCROLL, AND SCREW 11.55
Minimum Efficiency Efficiency as of 10/29/2001
Sizes COP* IPLV COP IPLV
� 150 tons 3.8 3.9 4.45 4.5
� 150 tons and
� 300 tons
4.2 4.5 4.9 4.95
�300 tons 5.2 5.3 5.5 5.6
* For a COP of 5.2, the equivalent is 0.68 kW/ton.
One of the U.S. leading HVAC&R manufacturer offered an energy use of as low as 0.59 kW/ton
(5.96 COP) for their 150- to 360-ton (527- to 1266-kW) water-cooled screw chillers at full-load and
0.52 kW/ton (6.76 COP) at part-load. These energy consumption indices were even less than the
values projected as of October 29, 2001.
Air-cooled screw chillers are widely used in locations where water is scarce and the use of city water
as makeup water for cooling towers is prohibited by local codes. Because the air-cooled screw
chillers are more energy-efficient and more compact than air-cooled centrifugal chillers, and more
energy-efficient than recprocating and scroll chillers, air-cooled screw chillers become the dominant
air-cooled chillers in a cooling capacity range of 50 to 400 tons (176 to 1400 kW).
System Components. An air-cooled screw chiller consists of mainly two, three, or four screw
compressors, dry-expansion liquid cooler, air-cooled condenser, expansion valve, and a DDC unit
controller. There are two independent refrigerant circuits; each has its own compressors, expansion
valve, and refrigerant distributor and liquid cooler. This kind of setup provides greater reliability in
case there is a compressor problem.
Electric Expansion Valves. Some manufacturers offer electric expansion valves that can work
over a wider operating range, conditions such as at low ambient air temperature and at a high refrigeration
load, causing thermostatic expansion valve (TXV) hunting. Operating at a lower ambient always
saves energy. An electric expansion valve provides more precise control and gives a better
part-load performance than does TXV.
Microprocessor-Based DDC. A DDC unit controller and the correponding functional, optimizing,
and safety controls as well as diagnostics similar to that discussed for water-cooled screw
chillers are always equipped in air-cooled screw chillers.
System Performance. Air-cooled screw chillers are often manufactured at a capacity of 50 to 400
tons (176 to 1400 kW). They are rated at a chilled water leaving evaporator temperature at 44°F
(6.7°C) and a cooling air temperature entering the air-cooled condenser at 95°F (35°C). In
ASHRAE/IESNA Standard 90.1-1999, the minimum performance of air-cooled chillers with a
cooling capacity � 150 tons (527 kW) is the same as that for air-cooled scroll chillers of the same
cooling capacity (see Sec. 11.10). For air-cooled chillers in a cooling capacity � 150 tons, COP �
2.5 and IPLV � 2.5 (EER � 8.5 and IPLV EER � 8.5). In the early 1990s, the EER and IPLV EER of
an air-cooled screw chiller of a U.S. leading HVAC&R manufacturer were 10.0 (2.93 COP) and
13.2 (3.87 IPLV COP), respectively.
Location of Installation. Air-cooled screw chillers are usually located on the roof outdoors. The key
point for efficient operation is to have sufficient clearance between the condensing coils and the surroundings,
and between units, according to the manufacturers’ specifications. If the chiller is shut
down during winter, water should be drained from the evaporator in areas where outdoor temperatures
fall below 32°F (0°C). Otherwise, glycol should be added to the chilled water for freeze protection.