Oil-free reciprocating compressors for very small units and oil-free integrally-geared centrifugal compressors for very large packages have been used. For more on reciprocating compressors and considerations for their maintenance and reliability, see the article from the Plant Services July 2020 issue.
Air compressors are most often positive displacement compressors. Integrally-geared centrifugal compressors have sometimes been used for very large applications. But, the majority of air compressors are dry screw compressors for medium-sized applications. There should be no lubricant or oil within the compressor chambers regardless of what type of compressor is used. On this basis, dry screw compressors are employed for a wide range of small- and medium-sized (including some large-sized) applications.
Dry screw compressors
In a dry screw compressor (also known as an oil-free screw compressor), helical lobe screws use no liquid or oil for sealing the screw rotor clearances and driving the non-coupled screw rotor. The rotor-to-rotor relationship is maintained by timing gears on each screw rotor, and the non-coupled screw rotor is driven by the coupled rotor through the timing gears with no rotor-to-rotor contact. These compressors have commonly been used to replace reciprocating compressors for medium-sized packages.
In a dry screw compressor, the air is compressed entirely through the action of the screws. Multi-stage dry (oil-free) compressors, where the air is compressed by several sets of screws, can achieve pressures of more than 10 Barg.
Dry screw compressors have been used in many applications for instrument air services. Contaminants ingested from the ambient air should also be removed prior to the compression though the filtration and dryer. Subsequently, sophisticated air treatment is still required to ensure a given quality of compressed air.
For process gas compressors, dry screw gas compressors are usually as per the latest API-619 (rotary-type positive displacement compressors). However, API-619 type screw compressors are not commonly specified for air applications. Usually, manufacturers’ standard dry screw compressors are used in such applications. Therefore, great care is needed for the specification and requirements of dry screw compressors for air compression services.
As rough indications, oil-free dry screw gas compressors have been produced for very small capacities to 80,000 m3/h (sometimes 130,000 m3/h); the pressure capability range is up to 35 Barg using multistage machineries. Dry screw compressor rotors should operate at speeds lower than the first lateral critical and are practically insensitive to imbalance.
Screw compressor components
Compressor casings can be of suitable grades of cast steel or cast iron. Permanent anti-corrosion coating is usually applied to the internal surfaces of the casing. Although some special machines have used alloy steels for the casing.
Screw rotors should be of one-piece construction, usually from properly-selected high-quality forged steels. Internal cooling is usually not acceptable, nor used. Screw rotor stiffness should be adequate to prevent contact between the rotor bodies and the casing and between gear-timed screw rotors at the most unfavorable specified conditions.
Screw rotors should ideally be supported on each end by rolling-element type bearings, even for medium and large compressors. Hydrodynamic bearings are not suitable for screw compressors because of relatively large gaps (compared to internal tight clearances between components); they are only used for very large screw compressors.
Timing gears are important parts in the dry screw compressors. These gears should be made of high-quality forged steel. They are usually of the helical type. ISO/AGMA service factor should be high, ideally 2.5”, 3” or even more. The meshing relationship between gear-timed rotors should be adjustable with proper positive locking devices.
Dry screw machineries still need lubrication oil for the bearing systems and timing gears. The lubrication system should usually include a directly-driven oil pump. For critical, large packages, a standby electric motor driven oil pump is also required. Duplex oil filters are needed.
Compressor air considerations
Air coming to the compressor should be very clean; although some tiny (around 1 micron or less) particles might enter. Clearances between the components are very tight. However, a very low quantity of tiny particles might not be harmful for dry screw compressors.
Air piping and connection parts at the suction of after-coolers (and inter-coolers) might be suitable grades of carbon steel, although many critical packages have stainless steel ones. Air piping and connection parts at the discharge side of the inter-cooler and after-coolers should be stainless steel to protect the components from corrosion caused by condensation.
Air-coolers have been used in many small- and medium-sized compact air compressor packages as oil cooler, inter-cooler(s) and after-cooler. Air-coolers should be fabricated from proper materials considering corrosion caused by condensation. Air-coolers are usually fabricated from stainless steel or aluminium alloy. Stainless steel is usually preferred by many engineers and operators because of excellent corrosion resistance. However, proper grade of aluminium alloy is often used because of better thermal performance and lower costs. The grade and details of the selected aluminium alloy need great care.
After-coolers (and often inter-coolers) with moisture separator and automatic moisture removal are needed for air compression packages. If the cooling water is available, water-cooled heat-exchangers would be used. Otherwise, forced-draft air-cooled ones are good options.
A high-quality air inlet filter is an excellent investment, payback by trouble-free air compressor operation. Dry-type multistage high-efficiency air intake filter capable of removing 98% of particles 1 micron (or larger) over the inlet capability range is commonly specified and provided. To save energy and for proper operation, ideally, the maximum clean filter pressure drop should not exceed 0.01 – 0.02 Bar.
Air inlet filters should be provided with a weather hood or louvers. For plant locations subject to unusual conditions, such as heavy snow or sandstorms, the inlet to the air filter may be elevated some distance above the compressor. Each air filter should be provided with a differential pressure for the monitoring. Ideally the first-stage (pre-filter) elements can be changed while the unit is operating.
In screw compressors, the flow of air is actually in a series of flow pulses which are superimposed upon the steady (average) flow. The characteristics of the flow pulses are determined by the operating details of the compressor (displacement, speed, rotors or pressures). The mechanical and acoustical response from the piping system and equipment is a function of the amplitude and frequencies of the pulses, and the characteristics of equipment and piping system (layout, supports or natural frequencies).
Screw compressors generate pulses that often are three-dimensional. Moreover, high frequencies combined with large diameter vessels, equipment or piping make circumferential modes important to consider.
For large dry screw compressors, pulsation vessels or silencers might be needed. Their primary function is to provide the maximum practical reduction of pulsations in the frequency range of audible sound without exceeding the pressure drop limits. As rough indications, pressure drop through the pulsation vessels or silencers should be below 1% and 2.5% of the absolute pressure at the suction and discharge, respectively. Based on experience, maximum vessel/silencer efficiency results from mounting the pulsation vessel, suppressors, or silencers directly on the compressor flanges, with a very short piping connection just to facilitate the access and maintenance (if needed). Long piping lengths between these vessels and the screw compressor can be the source of new resonance cases, operational issues, and complicated problems.
Many types of air dryer packages have been used to generate instrument air. The dryer package is usually induced three stages: a pre-filter (most often coalescer filters), a core dryer, and the after-filter (usually cartridge type filter). Air should pass through a filter package before entering the dryer to prevent the contamination in the dryer and instrument air systems. After-filters are provided to prevent carryover of materials from the dryer (such as desiccants) in to the instrument air network. Each filter package should be in duplex configuration (one operating, another standby).
A common dryer type, which has been used in many dry packages, is desiccant air dryer assembly with external heater and regenerative blower assembly (for purge air reduction). The redundancy concept of 2×100%, for example, 2×100% Filter, 2×100% Tower, or 2×100% Heater, is desired. The automatic switching to standby is preferred.
Preferably an air compressor package should be capable of operating at any demand flow-rate from zero to maximum capacity. However, this is just theory and practical limitations should always be considered for an optimum solution. Often several air compressors are supplied in “N+1” or “N+2” arrangements (with one spare or two spares, respectively) for critical plants. In this respect, an air compressor should be suitable for single compressor operation and parallel operation with an already operating air compressor. The fact that two or three compressors will supply the required instrument air of the plant eases the capacity control situation, because one machine can be stopped to reach the part-load. For instance, some plants use “2+1” arrangement: two operating (each 50% of the total required air) with another 50% package as a standby. In this case, 50% part-load can be achieved when only one package is operating.
A good option for large dry screw compressors is the variable speed drive (VSD). While an air compressor powered by a VSD can offer the lowest operating energy cost without reductions in service life over a properly maintained load/unload compressor, the VSD system typically adds significant cost to such a compressor package, negating its economic benefits. This is particularly true if there are limited variations in demand. However, theoretically a VSD provides for a nearly linear relationship between the compressor power consumption and the air delivery. In real-world applications, many air compressors do not use VSDs due to the high initial cost and complexity.
Monitoring and testing
For medium and large dry screw compressors (say above 90 kW), provisions should be made for mounting two radial vibration sensors (X-Y vibration probes) on each bearing, one axial position probe on each rotor and a one-event per revolution probe (key-phasor). For small compressors, if these sensors cannot be located inside the screw compressor (because of space limitation or other reasons), accelerometers can be provided on the casing. The author has received many such complaints from vendors and manufacturers of small compressors and then accelerometers, for instance, two per bearing, two on gear set casings, and one axial sensor for the compressor, may be specified for small units. For many screw compressors, axial sensors were not provided due to some excuses (for example, they might not be so useful). Axial monitoring can be very useful and it should be included even if required in the form of axial accelerometer.
At the manufacturer shop, the complete air compressor unit should be fully assembled and performance tested prior to the final inspection and delivery to the job site. A heat run test, four hours of mechanical run testing, and a complete performance test should be done.
For dry screw compressors, a heat run should be performed prior to the four-hour mechanical test run. The compressor should be run at the maximum continuous speed, with the discharge temperature stabilized at the maximum operating temperature at any of the specified operating conditions plus 10 °C for a minimum of 30 minutes. The excessive internal clearances required for higher temperature operation result in decreased volumetric efficiency under operating conditions. This is often a challenge for a screw compressor. In fact, for some dry screw compressors, the heat run test cannot be successfully performed with the above-mentioned procedure and a lower temperature (the rise lower than 10 °C) or a modified procedure should be considered. High discharge temperature shutdown point should usually be set below the heat run temperature.
The case study is a two-stage dry screw compressor with air flow of around 550 m3/h and discharge pressure of 9.5 Barg. The maximum pressure is about 11 Barg and the allowable pressure of the compressor casing is 20 Barg. The electric motor driver is induction type 120 kW. It drives two screw rotors with speeds of around 6,800 rpm and about 10,600 rpm through gear set. The first screw rotor is made from forged steel with proper coating. The second stage screw rotor is made of stainless steel (with suitable coating) due to condensation at the intercooler. Radial bearings and thrust bearings are roller bearings and ball bearings, receptively. They are supplied with the lubrication oil system of 4 Barg supply pressure.
This story originally appeared in the November 2021 issue of Plant Services. Subscribe to Plant Services here.