Specifying Rotary Screw Compressors for Refrigeration Duty
Knowing the specifications of compressor design features can help you select safe, reliable and profitable process cooling systems.
Refrigeration applications can be demanding for rotating equipment. When designing and specifying a screw compressor for your process cooling application, you must evaluate compressor components such as drive train capability, pressure containment ability, serviceability and process compatibility.
Rotors. The electric motor or natural gas engine is connected to the male rotor. The male rotor transmits the brake horsepower (bhp) to the compressed gas. Rotor materials differ with manufacturers and models (figure 1). To pick the right rotor material and construction, first evaluate the drive rotor’s torsion capability and long-term reliability (table 1).
Casing Construction. Casing pressure containment and construction are your next considerations. Casing construction design, connections, casing material and manufacturing tests are specified to meet the application, safety and casing-pressure requirements.
The casing of oil-injection screw compressors can be constructed of a single or double wall. The casing segments can be sealed with grooved O-ring compression or gaskets, when permitted, where they bolt together (figure 2). The casing sealing material must be compatible with both the refrigerant and the lubricant.
The suction and discharge gas connections may be ANSI-Class rated or refrigeration-definite purpose-type connections. Both suction and discharge class ratings should allow for equalized suction/discharge pressure conditions when the equipment is shut down.
Oil connections may be pipe thread, straight thread or, on steel casings where line size allows, flanged-type connections. It is good packaging practice to size the compressor oil supply lines that feed from the oil manifold with minimal pressure drop. Oil pressure is the lifeblood, feeding the compressor balance pistons, bearings and shaft seal. Oil pressure always should be maintained at the manufacturer’s recommendation.
Casing material is selected for design working pressure (DWP) and for safety requirements when compressing flammable refrigerants such as propane and ethylene. Pressure containment castings typically are hydrostatically pressure tested before assembly at 1.5 times the DWP.
The vast majority of ammonia and HCFC refrigeration screw compressors are gray cast-iron casing material (table 2). Gray cast iron has been successfully applied as low as -76oF (-60oC) suction temperatures during the last decade.
Nodular iron casings allow for both higher casing DWP and higher bearing load design operating pressure applications. Nodular casings are emerging as an economical way to apply CO2 refrigerant into higher operating pressures.
Cast steel casings offer the highest strength, field reparability and durability in the harshest environments. Cast steel casings are compliant to API 619 specifications.
Bearings. Radial bearings serve to absorb some of the gas compression forces. The rotor compression geometry transfers into radial shaft loads at about 10 o’clock and 2 o’clock vector on the female and male rotor, respectively. The rotors are contained into radial position by either rolling-element or sleeve (tilting pad) bearings. Rolling-element radial bearings can hold a tighter rotor position but may be limited by the L-10 calculated lifecycle. This calculated lifecycle must be specific to the design operating condition. In addition, rolling-element radial bearings are more difficult to remove from the rotors for service. Sleeve radial bearings have an infinite lifecycle and offer ease of rotor and bearing removal for service.
Axial Bearings and Load-Compensating Balance Pistons. Unlike air compressors that operate at atmospheric suction pressure, refrigeration compressors require axial-thrust-bearing load capability in both directions. Axial-thrust-bearing load capability safeguards the compressor across the entire range of suction and discharge pressure combinations the compressor could experience in process cooling (figure 3).
In most design operating cases, the rotor compression geometry transfers into axial shaft loads toward suction. Axial bearings may be back-to-back angular contact, rolling-element or sleeve bearings. Angular-contact bearing balls are typically submerged about one-third oil level inside the compressor. Tilting-pad thrust bearings are flooded to the top in lubricant.
Balance pistons, internally connected to the driveshaft, convert hydraulic energy from the central lube system to counterbalance the axial-bearing loads generated from compression. This counterbalance of axial-thrust loads extends the bearings’ mean time between overhaul. The number of balance pistons ranges from one to four among models and manufacturers.
Compressor Lubrication System Requirements
Compressors may be applied to operate with a single or several lubrication menus.
100 Percent Pumped Lubrication System. Oil pressure is supplied hydraulically to the compressor at a level above gas-discharge pressure with a full-sized gear-type oil pump. The lube system can be configured with a lube warmup recycle before prelube and start. Positive hydraulic pressure is available to position slide valve capacity control, feed-balance pistons, shaft seal, bearings and rotor injection. API 619 generally requires a force-fed lubrication system. The gas-flow control slide valve typically is a double-acting hydraulic control.
Partially Pumped Lubrication System. In this system, the oil pressure is force-fed to the shaft seal, balance pistons and bearings. The main oil injection -- the highest percentage of the total oil flow -- feeds into the rotor cavity controlling the temperature rise of compression. The main oil injection is supplied to the compressor at a level below the generated gas-discharge pressure while critical components such as the shaft seal, bearings and balance pistons are force-fed continuously.
Pumped Prelube/Nonpumped Lubrication System. In this system, a reduced-size, positive-pressure oil pump supplies oil at the startup cycle only. The system switches to a compressor-gas-differential-fed, nonpumped system at a predetetermined gas-operating pressure. All oil-connection feeds -- shaft seal, balance pistons, bearings and main oil injection -- are supplied to the compressor at a level below generated-gas-discharge pressure. The compressor-gas differential pressure circulates oil through the oil system cooler, filter, valves and then to the compressor supply. The flow-control slide valve typically is spring loaded to return to an unloaded position.
Totally Nonpumped Lubrication System. This type of lubrication system requires close control of the gas startup operating pressures. The compressor depends completely on the operating-system gas pressure to establish oil pressure and oil circulation at startup and continuous operating conditions. Suction and discharge gas- pressure regulators typically are incorporated into the design to facilitate the oil pressure actuating the spring-loaded flow-control slide valve and to facilitate oil flow.
Compressor Quality Testing
Compressor manufacturing requires essential inspections and testing prior to being incorporated into a process cooling system. Material certification documents ensure that the specified materials were used in the manufacturing plan. Hydrotesting certificates prove casing integrity. Mechanical run and performance test certificates prove the compressor meets the acceptance level to achieve the process cooling requirement. Leak testing is one of the final checks on the assembled compressor.
Individual compressor quality plans are established just prior to compressor manufacture. Most compressor suppliers stock standard production compressors that are manufactured with a standardized quality plan to allow them to meet the needs of most ammonia and HCFC process cooling applications. Production compressors can be tailored to meet special contract/build options or expanded test specifications that are incorporated into the manufacturer's quality plan prior to build.