Handbook of air conditioning and refrigeration / Shan K

11.48 CHAPTER ELEVEN
System Characteristics
Scroll Chillers
opening of the porting holes, the cooling capacity of the scroll compressor is varied. If all porting
holes are closed, the compressor is operating at full capacity. If all the porting holes are open, the
cooling capacity drops to the minimum.
Takebayashi et al. (1994) showed the isentropic efficiency of a variable-speed scroll compressor
at a pressure ratio range between 2 and 5 during part-load operation, cooling, and heating mode operation.
Isentropic efficiency drops about 15 percent when the pressure ratio increases from 2 to 5.
Compared to reciprocating and rotary compressors, the system characteristics of air-cooled, DX
scroll refrigeration systems are as follows:
● Scroll compressors tolerate liquid refrigerant and debris during compression. For most of the aircooled
DX scroll systems, the crankcase heater and suction line accumulator are often not
required.
● A scroll compressor has less than 50 percent of the parts of a reciprocating compressor of the
same size. The fewer the parts of a compressor, the higher the reliability.
● Most scroll compressors provided an EER of 11.5 (3.37 COP) in the late 1990s, which is higher
than that of a rotary compressor and is similar to that of a high-efficiency reciprocating compressor.
● A scroll heat pump shows a flatter capacity curve when outdoor air temperature T o varies than a
reciprocating heat pump does. Its cooling capacity drops less when the outdoor ambient temperature
increases during summer, and the heating capacity of a scroll heat pump reduces less when
the outdoor ambient temperature decreases during winter. The advantage of a flatter capacity
curve is discussed in the next chapter.
● A scroll compressor has a lower noise than a reciprocating compressor and is about 5 dBA lower
than the reciprocating compressor of the same size and operated in the same environment.
On the other hand, scroll compressors need computer-aided precision manufacturing and have
been used in the field of air conditioning in the United States since 1987 only. There will be new
problems to be solved and potential to be improved and developed.
Scroll compressors are gradually replacing reciprocating and rotary compressors in room air
conditioners, heat pumps, and packaged units for residential and commercial buildings.
Scroll chillers are often available with a cooling capacity of 10 to 60 tons (35 to 210 kW). They are
gradually replacing reciprocating chillers in new and retrofit installations. Scroll chillers can again
be divided into air-cooled scroll chillers and water-cooled scroll chillers.
Air-Cooled Scroll Chillers. An air-cooled scroll chiller consists of mainly an air-cooled condenser,
two or more scroll compressors, a single or dual refrigerant circuit, a thermostatic expansion
valve, a distributor, a direct-expansion liquid cooler, and controls. A direct-expansion liquid
cooler can be easily connected to dual refrigerant circuits and multiple compressors. An air-cooled
scroll chiller is usually located on the rooftop of an air conditioned building. Chilled water freeze
protection should be provided during winter operation in locations where the outdoor temperature
falls below 32°F (0°C).
An air-cooled scroll chiller always has a microprocessor-based control of the chilled water temperature
leaving the liquid cooler. There are usually two capacity control steps for small air-cooled
scroll chillers and four capacity steps for large scroll chillers. Air-cooled scroll chillers are designed
to be operated between outdoor temperatures of 40 and 115°F (4.4 and 46°C); with an additional
head pressure control damper assembly, they are permited to operate at a lower ambient temperature
down to 0°F (�17.8°C).