2.5.1 Basics of the Screw Compressor
The screw compressor is a positive displacement rotary compressor. As shown in Figure 2-3 Compressor Mechanism, the refrigerant (gas) is continuously compressed by changing the volume between the casing and the male and female meshed screw rotors, which have different profiles. The rotor with 4 protruding lobe sections is called the M rotor (male rotor), and the rotor with 6 lobe depressions is called the F rotor (female rotor). Throughout this manual they are referred to as the M rotor and F rotor. The compressor M rotor shaft is driven by the two-pole or four-pole motor via the speed increaser gear.
2.5.2 Suction Process
As shown in Figure 2-4 Suction Process, the rotors’ different profiles mesh together. Also the volume enclosed between the M and F rotor lobes and compressor casing increases from the suction side as the rotors turn. As rotations continue, at a certain point the volume reaches its maximum, the rotors start to trap the gas between the lobes and compressor casing thereby isolating the gas from the suction port.
Figure 2-4 Suction Process
2202L5JE-DA-C5-N_2015.05.2 Compressor Specifications and Structure Compound 2-stage Screw Compressor 2.5 Mechanisms 1612LSC Speed Increaser Type 2-7
2.5.3 Compression Process
As the rotors further rotate, the sealing line between them moves toward the discharge side and the volume between the rotor lobes decreases and compresses the trapped gas. Figure 2-5 Compression Process Figure 2-6 Discharge Process
2.5.4 Discharge Process
Through the compression process, the volume between rotor lobes decreases to a predetermined value at the discharge port. Following rotor rotation, the compressed refrigerant gas is pushed out of the discharge port.
2.5.5 About Volume Ratio (Vi)
Volume ratios (Vi) of C-series screw compressors are indicated in performance tables or catalogs by using port symbols L and M. The volume ratio represented by each symbol is as follows: L=2.63, M=3.65.
Which volume ratio (L or M) should be used is decided according to operating conditions. If the compressor is used with a volume ratio that does not match operating conditions, operation will go inefficiently wasting the power. The relationship between volume ratios and generally used compression ratios is as follows: (Vi) κ= πi = Pd/Ps κ= Cp/Cv of refrigerant gas Vi = designed volume ratio πi = designed compression ratio The constant of the refrigerant gas also a factor, and the Vi value for the compression ratio will change according to the refrigerant gas used.
Figure 2-7 Volume Ratio Explanation
2202L5JE-DA-C5-N_2015.05.2 Compressor Specifications and Structure Compound 2-stage Screw Compressor 2.5 Mechanisms 1612LSC Speed Increaser Type 2-8
(A) When Vi matches operation conditions The required compression ratio and Vi are both low The required compression ratio and Vi are both high
(B) When Vi does not match operation conditions Vi is too low compared to the required compression ratio Vi is too high compared to the required compression ratio Figure 2-8 Relationship between volume ratio (Vi) and operation conditions 2202L5JE-DA-C5-N_2015.05.2 Compressor Specifications and Structure Compound 2-stage Screw Compressor 2.5 Mechanisms 1612LSC Speed Increaser Type 2-9
2.5.6 Capacity Control Mechanism
The capacity control structure involves the moving of a slide valve, bypassing suction gas just before compression on the suction side, which shortens the portion of the rotor used for compression. The slide valve is at the bottom of the casing where the rotors mesh together, and is constructed to move parallel to the rotor’s axis. This movement is changed by a cam mechanism into rotation movement, and as the position (capacity control ratio) is indicated externally, the electrical resistance value changes to provide feedback to the automatic control circuit.
Figure 2-9 Capacity Control Mechanism
The 1612LSC speed increaser type has capacity control on the low-stage block only. 2.5.7 Bearings and Balance Piston For the load created on the rotor perpendicular to the axle, a white metal-lined sleeve-type bearing is used. The bearing uses surface fitted ball bearings with angular contact for loads along the axis direction. In particular, axial load for the M rotor, which has one type of helical gear, is comparatively larger than that of the F rotor because of the thrust load from discharge pressure. This load for the M rotor is reduced by the use of a thrust bearing, along with a balance piston providing opposing hydraulic pressure.
2.5.8 Shaft Seal
To prevent refrigerant gas and oil leakage, a reliable mechanical seal assembly is used for the shaft seal of the speed increaser gear spindle. Mechanical seal assembly is mainly composed of "rotating ring" installed on the rotor shaft and "stationary ring" installed in the seal cover. Rotating ring rotates with the shaft, and slides each other with the stationary ring while maintaining a micron class gap. The sliding each other place is called as the sliding surface. As an example, for the BBSE (Balanced Bellows Single Seal)-type, which is a standard seal currently in use, the fixed ring (mating ring) is cast iron, and the rotating ring is carbon, with an O-ring for the packing.
Figure 2-10 Slide Valve in the
Rotor Casing 2202L5JE-DA-C5-N_2015.05. 2 Compressor Specifications and Structure Compound 2-stage Screw Compressor 2.6 Gas and Oil Flow 1612LSC Speed Increaser Type 2-10 2.6 Gas and Oil Flow The screw compressor’s compression process is described earlier in this manual. Gas for the compound speed increaser type 1612LSC compressor passes from the evaporator and through the suction strainer and check valve, and is sucked into the center part of the compressor ①, and it is compressed at the low-stage ② .Then the compressed gas is discharged at ③. ③ and ④ are connected by piping through which gas used for super cooling is mixed in from the liquid cooler. Lubricating oil injected at the low-stage is, while kept mixed with gas, suctioned from ④ into the high-stage. After being compressed at ⑤, the gas mixed with lubricating oil is discharged from ⑥ to the oil separator, and then sent to the condenser. The oil is cooled even without intermediate gas cooling, so the high-stage discharge temperature can be maintained at below 90 °C. Oil Supply Route Lubricating oil is split into 4 flows as shown in Figure 2-12, and after providing lubrication, it is mixed with discharge gas and leaves the compressor. In standard configurations, oil injection is not performed at the high-stage.