Surprisingly enough, the screw compressor was invented in 1878. However, commercial application developed slowly because of its inability to match tight tolerances with existing manufacturing equipment of the time. Over the past 10 years, several manufacturers have introduced chillers with screw compressors and have moved away from older reciprocating technology. Screw compressor technology offers many benefits over reciprocating types, including higher reliability and improved performance. In addition to these benefits, some noteworthy characteristics make the screw compressor the compressor of choice for future chiller developments and designs (Duncan, 1999).
Screw compressors are also positive displacement refrigeration system components. Both single screw and twin screw compressors are widely used in refrigeration applications. A single screw compressor consists of a single helical rotor (shaft) and a pair of gate rotors, that then mesh together and with the casing form a sealed volume wherein compression takes place. There are two different rotary screw compressor designs. One is a twin rotary screw design, in which there is a male and a female rotor that mesh together (see Figure 3.9a). The other is a single rotary screw design, in which two gate rotors are placed on both sides of the main compressor rotor (see Figure 3.9b).
Reciprocating compressors have, until now, carried the workload in applications requiring temperatures below -35°C. This was the technology of choice, mainly because cascading refrigeration systems was the only choice. The screw compressors were developed specifically for use in applications of -40°C and below (down to -50°C). Originally designed for larger applications, this technology is now available in chambers requiring only a single 15 hp or larger compressor. The development of this advanced screw-style refrigeration system offers the following benefits:
– better performance per hp,
– improved reliability,
– reduced costs,
– fewer moving parts,
– less vibration, and
– less refrigerant loss.
By design and function, the screw compressor has far fewer moving parts than the reciprocating style. Engineered with no valves and rolling element bearings, the total number of parts is drastically reduced as well. This reduction of parts is important because it dramatically improves the compressor’s reliability rate and increases its expected lifespan.
Note that screw compressor technology greatly reduces the risk of refrigerant loss because of the decrease in vibration within the entire system. Any structural breakdown within a refrigeration unit may cause loss of its valuable refrigerant. With the accelerating costs of R-22, R-134a, and R-507/404A, product loss becomes a crucial operating factor.
A twin-screw compressor consists of two helically grooved rotors (containing a pair of intermeshing screws) and operates like a gear pump (Figure 3.10). The male screw is directly coupled to the electric motor and this drives the compressor. With the absence of the suction or discharge valves, the gas is drawn into the compression chambers between the gear teeth and the cylinder wall and the helical movement of the gears forces the gas to travel parallel to the rotor shaft. Single-screw compressors are also available and these consist of a helical gear on the main rotor shaft and a pair of planet wheels, one on either side to separate the high and low pressures. Oil flooding provides lubrication and restricts leakage of refrigerant gas. These compressors are used mainly in heat reclaim and heat pumping applications.
A screw compressor (Figure 3.10) has a male rotor (with four lobes) which drives a female rotor (with gullies) in a stationary casing with the inlet port at one end and the outlet port at the other. The rotating elements open a void to the suction inlet of the vane compressor, take in a volume of gas, and then seal the port. More rotation decreases the volume between the rotors and compresses the refrigerant gas. The gas is discharged at the low-volume, high-pressure end of the compressor through the outlet port. In general, the male rotor is directly driven. On the other hand, the female rotor rotates along with the male rotor, either through a gear drive or through direct rotor contact.
For industrial refrigeration applications, such as process chillers, the high-temperature compact screw compressors provide an ideal solution. The integrated oil separator and oil reservoir significantly reduces the installation time, complexity, cost and space required. Such compressors are available in sizes from 50–140 hp which are equipped with the dual capacity control system and auto-economizer and can be used with the common refrigerants R-134a, R-407C, and R-22 (R-404A, R-507A in special applications). The operation with or without economizing is possible.
Figure 3.11 shows the cutaway view of a hermetic rotary type screw compressor which is commonly used in small-scale refrigeration applications, particularly in household and commercial units.