Selecting a Twin Rotary Screw Compressor
The geometric profile of the rotating rotors is difficult to visualize. If you consider the male rotor as the pistons and the female rotor as the cylinders, it is easier to relate the compression process to a reciprocating compressor. As the male lobes and female flutes unmesh, an empty cylinder is created, drawing in suction gas through the synchronized opening on the rotor suction face. As rotation continues, the suction and discharge rotor faces are sealed off, trapping the gas in the cylinder and transferring the rotor wrap position. As rotation continues further, the male lobe rolls into the female flute and internally reduces the contained volume. The location (including size and shape of the discharge port) of completed compression at which the gas is allowed to exit is called the volume index ratio (Vi).
The final compressed volume of gas prior to exiting occurs beneath the rotors and produces radial bearing loads at the 10 o'clock position (female rotor) and 2 o'clock position (male rotor) from the internal discharge gas pressure. Internal discharge gas pressure forces the rotors toward the suction end of the compressor, producing an axial-thrust-bearing load. Hydraulic oil pressure is utilized to counteract axial-thrust loads generated by compression through the use of internal balance pistons. Internal balance pistons are connected to the rotating rotors, providing the surface area that converts hydraulic pressure to a shaft force that opposes the compression force and minimizes the axial-bearing thrust loads. Proper operating oil pressure and oil qualities are the primary factors to extending service intervals.
Compressor SizingSelecting a screw compressor size for a particular cooling requirement can be simplified by converting the evaporator cooling duty requirements to inlet volume flow rate or actual cubic feet per minute (cfm). The required inlet volume must be less than the compressor displacement cfm to allow for compressor volumetric efficiency. Suction-line pressure losses should be accounted for due to the decrease of refrigerant inlet density. Volumetric efficiency reduces at higher gas pressure differentials, lower rotor peripheral speed and lower oil viscosity. Screw compressor displacements vary directly with rotating speed, so electric motor frequency and engine drive speeds must be specified for the compressor selection process.
Inlet volume flow requirements can be reduced further with an economizer cycle. Economizer cycles subcool the liquid-line refrigerant temperature by vaporizing a portion of the liquid and compressing the generated flash gas through a side port on the compressor. The net result is a significant increase of evaporator refrigeration capacity with only modest compressor power increase.
An economizer cycle is most effective at compression ratios that dictate a multistage system design. The side port exposes a female flute (cylinder) after the suction gas is trapped. External economizer system pressure forces the additional flash gas into the female flute. At high-frequency compression cycles, the compressor will displace more gas at higher side-port feed pressures. An economizer cycle will generate higher flash loads at lower feed pressures. The end result will balance the compressor and economizer system at a steady-state interstage pressure.
Independent side-cooling loads in addition to economizer cycle flash gas may be combined into the compressor side port providing the balanced interstage pressure is within a reasonable range for the overall design operating conditions. Maximum side-port loading is limited to the compressor side-port flange size.
Oil ManagementAfter the oil exits the discharge oil separator and retains in the oil reservoir, the diluted pressurized lubricant is cooled, filtered and supplied to the compressor radial bearings, axial thrust bearings, balance pistons and mechanical shaft seal, and injected into the compression chambers.
The heat of compression or shaft horsepower kinetically goes to the gas and oil. The energy distribution to gas and oil heat depends on the compression ratio and the specific refrigerant thermodynamic property profile. At higher compression ratios, more heat of compression is distributed as oil cooler heat rejection. When balancing condenser heat of rejection requirements, an external oil cooler heat load may be deducted from the total condenser requirements.
A fundamental consideration when applying oil-flooded screw compressors for a particular cooling duty is the lubricant evaluation for the refrigerant being compressed. Because the oil reservoir is located at discharge pressure, the oil viscosity will dilute by absorbing a percentage of the gas. The amount of gas dilution entrained in the oil is dependent on three factors.
Molecular Weight of the Refrigerant. The molecular weight of the gas compressed is the primary factor of oil dilution characteristics (figure 1). Lighter molecular weight refrigerants like ammonia have low dilution characteristics of 3 to 5% where heavier molecular weight refrigerants like propane, R-22 and R-134a result in higher dilution characteristics of 15 to 20%. Low molecular weight refrigerants also have a higher dependency on injection oil to seal leakage paths to maintain peak volumetric efficiency.
Operating Discharge Pressure. The higher the operating discharge pressures, the higher the oil dilution rates (figure 2). When operating screw compressor systems, sudden reductions of discharge pressure will release the dilution effect too quickly, causing the oil to foam and disrupting the oil management system. High or abnormal dilution rates will tend to swell the oil operating level in the reservoir.
Operating Discharge Superheat. The operating discharge temperature plays a key role when striving for a steady-state normal oil dilution rate. Adequate discharge superheat measurements are an excellent indicator of stabilized oil dilution. Discharge oil separators should operate above 30oF (-1oC) discharge superheat. If the discharge temperature has an insufficient margin from the condensing temperature, refrigerant dilution can become extreme and detrimental to the bearing oil supply viscosity. Extreme low ambient temperature exposure to the oil separator/reservoir can have a detrimental impact to the oil dilution rate. Abnormally sustained liquid slugging into the suction of a screw compressor due to unbalanced evaporator heat loads will disturb the discharge superheat and may jeopardize the resultant bearing oil supply viscosity.
Lubricant SelectionThe type of lubricant selected must be compatible with the refrigerant and the compressor O-rings. The selected lubricant's viscosity index must allow for the refrigerant and oil dilution characteristics to ensure adequate bearing oil supply viscosity. The design oil supply viscosity exiting the oil cooler and feeding the compressor typically is between the range of 10 and 100 centistokes, including the refrigerant dilution effect. In order to maintain prescribed lubrication to the bearings, the oil supply temperature should remain constant under all operating conditions. Refrigerants with a higher specific heat ratio (k) require lower design oil supply temperatures from 104 to 130oF (40 to 54oC). A lower refrigerant specific heat ratio uses an oil supply range of 140 to 160oF (60 to 71oC). Compressor discharge temperatures should not exceed the selected lubricant's maximum allowed temperature to insure optimum operating hours before oil replacement.
An important refrigeration system consideration is the lubricant miscibility, lubricant vapor pressure and pour point. The lubricant vapor pressure influences how much oil may carry over from the oil separator/reservoir and circulate through the refrigeration system. Lubricant miscibility and pour point become crucial to return circulating oil from the cold evaporator back to the compressor lube oil system.
Design RecommendationsDesign specifications should consider all aspects of oil management and lubrication support for the compressor to maximize long-term reliability.
Oil reservoirs should have enough volume for approximately one-minute retention time. Oil retention allows the oil to stabilize refrigerant dilution with changes of discharge operating conditions.
Oil separators should be designed to minimize oil carry over at the maximum suction pressure and/or capacity that the drive motor and compressor are capable of producing. In addition, insulating the oil separator for outdoor installations may be necessary; therefore, outdoor installations should consider low ambient influences to the oil separator.
Oil lines that feed the compressor from the oil manifold should be sized for minimal pressure loss. Pressure drop on oil feed lines can release refrigerant dilution causing the mechanical shaft seal face to operate at an excessive temperature. Oil feed line pressure drop may also reduce the balance piston thrust load compensation. Minimum pressure drop in compressor lube feed lines will result in optimum long-term service intervals.
The compressor manufacturer and system designer should both endorse lubricant selection. Lubricant compatibility and viscosity grade along with refrigeration system characteristics must all
be considered. Proper operating oil pressure and oil qualities are the primary factors to extending service intervals for twin rotary screw compressors.