Lubricants play a vital role in the compressor’s performance and longevity, as they function as a sealing medium, lubricate and cool the rotating components. The most common types of lubricants are PAO, PVE, and PAG. These will be analyzed in detail in a dedicated section.
When selecting lubricants, several factors must be taken into account:
Before an oil is used with a refrigerant, it is examined in laboratory conditions where it is mixed, pressurized, and tested at high temperatures to ensure chemical compatibility. It is also evaluated for chemical breakdown and sludge formation. Furthermore, the refrigerant–oil mixture is tested for material compatibility with all compressor components to ensure that no adverse chemical reactions occur. In addition, because the refrigerant–oil mixture is continuously exposed to varying high- and low-pressure and temperature cycles, the oil must maintain its viscosity and provide an adequate lubricating film under all operating conditions. It must also exhibit sufficient miscibility with the refrigerant to ensure proper oil return to the compressor and prevent oil from becoming trapped in the heat exchangers. Although the type of lubricant is critical for compressor and system performance, the oil charge also plays a vital role. Excessive oil can prevent proper cooling and lead to high oil carry-over, while insufficient oil reduces bearing life. Furthermore, high-viscosity lubricants increase drag during operation, resulting in higher input power and internal friction, which generates additional heat. Therefore, when selecting a lubricant, parameters such as viscosity and solubility must be carefully considered for the specific operating conditions.
The most popular oil cooling methods for screw compressors are liquid refrigerant injection and an external oil cooling.
The purpose of liquid injection is to control the discharge and oil temperature, thereby protecting the compressor from thermal stresses, material wear, and lubricant breakdown.
The liquid injection system draws high-pressure, sub-cooled refrigerant from the condenser outlet line before the intermediate or main expansion valve. When this liquid refrigerant enters the compressor, it immediately flashes and evaporates, absorbing heat from the gas. That extra mass flow exiting the compressor carries away additional heat. According to the law of energy conservation, this absorbed heat must be accounted for, and it is ultimately rejected in the condenser, increasing the overall heat load.
However, there are trade-offs. Liquid injection increases the power input because the extra vapor added in the flutes raises the compressor load, resulting in a drop in efficiency. Furthermore, liquid injection causes the suction and discharge mass flow rates to differ. In isentropic efficiency calculations, this additional mass flow is disregarded, and only the suction mass flow rate is considered.
ṁSuction + ṁLiq.inj = ṁDischarge
A metering valve regulates the liquid injection flow when the discharge temperature exceeds a predetermined threshold, such as 100 °C, maintaining the temperature below that limit. This protects the oil and the gate rotors from overheating. If the discharge temperature rises above 100 °C, the lubricant may break down, and the gate rotors could soften, potentially leading to premature failure.
In double-gate rotor screw compressors and in heat pump applications, double liquid refrigerant injection may used at the top and bottom of the screw rotor when external oil cooling is not available. Typically, during high-suction and high-discharge temperature conditions, only one injection port is used. However, at low-suction and high-discharge temperatures, double liquid injection is required because the refrigerant density is reduced, and so is the mass flow rate.
The high-pressure liquid should be injected as late as possible in the screw to avoid re-compression in the early stages, which would increase power consumption. Conversely, placing the injection port too close to the discharge can create a back-pressure risk, as the pressure in the flute may exceed the liquid injection pressure.
For this reason, the injection port is positioned at the intermediate-pressure flute. As illustrated by the blue dashed line in the schematic below, the liquid injection line is extracted from the condenser outlet—before the intermediate expansion valve—and is injected into the compressor in liquid form.
Figure 11. Cycle Schematic for a two-stage compressor with vapor using a heat exchanger and liquid injection that is extracted upstream of the economizer
Screw compressors operate with oil injected into the refrigerant and rotating members during operation. This oil absorbs heat from the compression and leaves the compressor with the refrigerant at a discharge temperature. After separation from the refrigerant, the oil is cooled in a heat exchanger before it returns for injection into the Compressor. The method of cooling oil with an external heat exchanger rejects the heat to cool water or antifreeze. This method of oil cooling is preferred because it offers better performance compared to liquid refrigerant injection, which can lead to a reduction in efficiency. At high discharge temperatures, a significant amount of liquid refrigerant must be injected to cool the oil, further impacting performance. Additionally, screw compressors do not require an external oil pump, as oil circulation is driven by the compressor’s pressure difference.
Figure 12. Oil cooling using an external heat exchanger, rejecting heat to water or antifreeze.
Vapor injection or economizer uses part of the total condenser sub-cooled liquid refrigerant to cool down the rest of the refrigerant flow, providing an increase in the system's capacity and performance.
The refrigerant undergoes 2 phase changes during operation. During the liquid-vapor phase change, the liquid refrigerant absorbs heat as it evaporates. That effect provides the refrigeration effect; however, effective refrigerant utilization is reduced because a portion of the refrigerant flashes in the expansion valve before entering the evaporator, leading to reduced refrigeration effect as shown in Figure 13a.
This loss percentage is contingent on the refrigerant type and the pressure differential. To address the lost refrigeration effect, vapor injection is used. With a low investment cost, adding an extra evaporator in the single vapor cycle makes it possible to increase the refrigeration effect as illustrated in Figure 11.
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Figure 14a. Enthalpy |
Figure 14b. Enthalpy |
Figure 14b illustrates the schematic of a two-stage vapor injection cycle. The single-stage and two-stage vapor cycles will be discussed in detail in a dedicated section, and the associated calculations will also be provided.
The two stage vapor cycle works as such. The subcooled liquid refrigerant leaving the condenser at point 5 splits into two lines, 6 and 5*. The refrigerant in line 6 goes into the intermediate-pressure expansion device, flases into vapor before it enters the intermediate heat exchanger, and exits at point 3. The vapor at this stage is at intermediate pressure and enters the compressors through an economizer port at point 2.The economizer gas must be at a higher pressure than the intermediate compressor pressure to enter. The location of the economizer port is positioned between the suction and discharge and the intermediate pressure can be calculated by the formula below:
P_intermediate=√(P_(Suction )·P_Discharge )
Although the extra vapor that enters the compressor increases the power, the system's performance is far greater than the motor power increase. The total mass flow rate is calculate the by the formula below:
ṁSuction + ṁinj = ṁDischarge
The rest liquid refrigerant enter at 5* which is further sub-cooled by the economiser line and exits at point 7. After the exist, the liquid refrigerant enter the main expansion valve, where the refrigerant flashes and enters the main evaporator at point 8.
Figure 15. Vapour Injection Schematic