COMPRESSORS & COMPRESSOR LUBRICATION
Compressors have a long history. The first compressors were probably bellows made of animal skins or bladders. They were used to aid fire starting at least as far back as 2000 B.C. Later they were used to increase the temperature of burning charcoal to the point where metals and glass could be melted and formed. Today, all gases are put under pressure for many reasons including transport, storage, purification, refrigeration,, power transmission, chemical reaction etc.
Compressors are useful in everyday lives in vacuum cleaners, refrigerators and air-conditioners, for filling tyres, etc.
HOW COMPRESSORS WORK
Compressors are really gas pumps. They move gas from one place to another. But they do more. They squeeze that gas into a smaller volume, and that results in an increase in pressure and temperature. The Ideal Gas Laws obtained by combining Boyle’s & Charle’s Laws
|P₁V₁/T₁ = P₂V₂/T₂
|PV = n RT
and other gas laws form the underlying basis for compression of gases.
There are two ways gases can be compressed. In one method, a quantity of gas is trapped in an enclosed space and then squeezed into a smaller volume until a desired pressure or a limiting temperature is reached. Machines that operate on this principle are called positive displacement compressors. Examples are the reciprocating piston in cylinder type and the rotary machines, such as the sliding vane and screw types.
The other way of compressing gas uses the ram effect. A high speed fan or impeller accelerates the gas to a high velocity and literally rams it into a container of fixed volume. The gas molecules are packed close together by being jammed into a confined space, and this results in a rise in pressure and temperature. If this gas is then fed into another high speed fan, still higher pressures and temperatures can be obtained. Each fan or impeller in series then would be called a step up in pressure or “stage” of compression. Gas pumps that work on this principle are known as “kinetic“ or “dynamic“ type compressors. Examples are centrifugal and axial flow compressors. Normally, they are built with 6-10 or more impellers or stages of compression on the same shaft.
Positive Displacement Compressors are Discharge-Temperature Limited
Positive displacement compressors take in gas one bite at a time. Each bite or quantity of gas taken in is trapped in an enclosed space and then squeezed into a smaller volume, thereby increasing its pressure and its temperature. It is this final discharge temperature that limits all practical compressor operation.
The “degree of compression” or pressure ratio is determined by the properties of the gas within the limits imposed by practical compressor construction and operation.
The basic elements of a reciprocating compressor are a cylinder with inlet and discharge valves and a moving piston. If gas is compressed in one end only of the cylinder, the operation is called single acting; and the piston is the regular trunk-type automotive engine style.
These are the most commonly used type of compressors. Sizes vary from < 1 hp to > 5000 hp.
Valves are Pressure Operated
|| If compression occurs in both ends of the cylinder, the compressor is double acting; and a stuffing box or piston rod seal must be added with the crosshead-type of connection to the crank shaft.
If the gas being compressed is passed through consecutively smaller cylinders, each cylinder is a stage. Normally, the gas is cooled between stages by passing through a heat exchanger (“intercooler”), Multistage compression with inter-stage cooling increase efficiency, limits the maximum gas temperature, assists lubrication, and prevents overheating the machine.
Many arrangements of cylinders are possible ranging from one cylinder single acting to multi-cylinder double acting, multistage in L,V,Y,W and radial configurations.
Valves are usually the self acting, pressure operated check valve type.
The intake valve opens when the pressure in the cylinder, due to the movement of the piston, drops below the pressure in the intake system. As the piston reverses travel and starts to compress the gas in the cylinder, the intake valve closes when the pressure in the cylinder is greater than the pressure in the intake system.
The discharge valve does not open until pressure in the cylinder exceeds the pressure in the compressed gas system. The discharge valve is closed by the discharge system pressure after the piston reaches the end of its compression stroke and starts back on its intake stroke.
A major cause of compressor trouble is valve leakage caused by deposits from unstable oil or dirty gas as discussed next. Over lubrication is a contributor to the deposit problem also.
The lubrication of the reciprocating air compressor is similar to that of the internal combustion engine. Similar machine elements are involved: pistons, piston rings, cylinder walls, valves, piston pins, connecting rods, and crankcase bearings. Minimizing wear of these parts is just as important in the compressor as it is in the engine. Combustion products are not present, but exposure of the lubricating oil to hot oxidizing conditions can be severe in the compressor. Also, the pressure-operated compressor check-type valves are more sensitive to deposits than the mechanically operated engine valves that tend to be self-cleaning by rotation.
Since some oil oxidation is inevitable, particularly in the discharge valve area, the resultant sticky and carbonaceous deposits must be reckoned with. Adherence of these oxidized residues to the valve surfaces can be minimized and often prevented entirely by including a stable detergent/dispersant in the lubricating oil. From these considerations, it would appear that the ideal lubricating oil for a reciprocating air compressor would be made from a well-refined stable base oil to which are added good high temperature dispersants and oxidation inhibitors along with anti-wear additives and rust inhibitors for protection during shutdown.
Large (over 150mm cylinder diameter) compressors use mechanical lubricators to pump oil directly to the cylinder walls. The rate of oil feed required depends on the size of the cylinder and the speed of the machine. Oil feed rates are recommended by OEMs are often exceeded. However, the oil supplied beyond that needed for lubrication contributes to deposits in the discharge systems, particularly in and around the valves.
Rotary Sliding Vane Compressors are Compact
The positive displacement vane compressor consists of a slotted rotor turning inside an eccentric housing.
Vanes or blades are free to slide in the radial slots extending the length of the rotor. Centrifugal force causes the vanes to be held out against the inner surface of the eccentric casing thus forming a seal between the compartments formed by the vanes. The ends of the rotor and vanes are sealed also.
||The eccentric location of the rotor with respect to the enclosing cylinder forms a clearance space that is crescent shaped in cross section. As the rotor turns, gas enters through ports into the compartments formed by the vanes on the side of the crescent whether the volume is increasing. The gas is discharged on the opposite side as the volumes of the compartments are reduced.
While there are a small number of moving parts in this type of compressor, all of them must be lubricated. The oil must lubricate and seal
(1) the outer edge of the vanes where they slide along the surface of the enclosing cylinder, (2) the faces if the vanes where they slide back and forth in contact with the rotor slot surfaces,
(3) the ends of the vanes where they slide along the end seals and
(4) the same oil also lubricates the rotor support bearings, drive gears, and shift seals.
Modern design and usage places a further essential function on the oil – that of removing the heat of compression from the air. For saving in cost, space, and weight, portable one- and two-stage vane compressors have cooled oil injected directly into the air being compressed. In the two stage units, this system of cooling allows the first stage to discharge directly into the second stage. No external interstage cooling is required.
Obviously an oxidation inhibited oil is required. The intimate mixture with air that occurs suggests that a foam inhibitor would be beneficial. A rust inhibitor should provide some protection against rusting during shutdown and for intermittent operation.
While the oil ought to be able to rapidly separate from water that may condense from the compressed air, water condensation and accumulation in the oil is not a problem because the compressed air is not cooled to the dew point at any time while it is in contact with the oil. (Except possibly for a brief period during a cold start).
In addition to rust and oxidation inhibition, the oil should have good detergent/dispersant properties to maintain a deposit-free circulating system and prevent vane sticking. Note that such oils are relatively slow to separate from water.
The usual design is to separate the oil and air while hot as discharged from the compressor. The hot oil passes through a cooler on its way back to the compressor. Moisture from subsequent cooling of the air is trapped out of the air system, not the oil system.
Screw or Lobe Type Compressors
In its simplest form, the straight lobe compressor consists of a casing containing a matched set of symmetrical rotors (lobes), having a figure-of-eight profile. Such Roots Blowers are available in Twin lobed, Tri-lobe & Multi-lobe variants.
||The rotors do no actually touch. They are driven by timing gears. Since there is no contact between rotors or between rotors and casing, no internal lubrication is required. Lubrication is required for the shaft bearings and precision timing gears.
The screw-compressor operates on a similar principle but the lobes are lobes comprise of two intermeshing rotors with helical or spiral contours that rotate on parallel axes in a close fitting casing. The lobes can be identical or have a mating configuration, usually male-female type. The rotors may be thought of as a series of pistons and cylinders.
|For the male-female type, the male rotor has convex lobes corresponding to pistons. Having a circular arc cross-section, these lobes from helices along the length of the rotor like the ridges of a screw thread. The corresponding female rotor has concave flutes or inter-lobe spaces equivalent to cylinders that have the same circular-arc cross-section to accept the mating male rotor lobe.
||The turning rotors produce a three phase cycle. In the first of suction phase, the “cylinder” spaces pass the inlet port at one end of the casing and are filled with air or gas. By the time the interlobe space is completely filled, rotation of the cylinders has caused the space to pass beyond the inlet port trapping the gas between the rotors and the casing.
Further rotation of the cylinders brings on the compression phase. Here the male helical lobes or pistons roll into the female rotor flutes or cylinders. The point of intermeshing moves along the length of the rotor, progressively reducing the volume of gas, thereby increasing its pressure.
The final discharge phase occurs when the inter-lobe space, filled with compressed gas, arrives at the outlet port.
|Compressors of this type have several lobes on each rotor. Thus, by the time one inter-lobe spaces is completely discharged, the next inter-lobe space has begun its discharge. A smooth, continuous flow of compressed gas can be obtained with a four lobe male rotor and a six lobe female rotor running at high speed.
||Lubrication requirements and oil circulation rate of wet screw-type compressor are identical to those of the oil cooled rotary vane-type compressors for the same reasons as discussed in the preceding section. Enough oil must be circulated to hold the final air discharge temperature down to the range desired. Quality criteria are the same for both types of compressors.
Dry screw compressors can use the same oils as the wet screw type, but requirements are not as severe because no lubricant is in contact with the hot gas being compressed. Regular rust and oxidation inhibition turbine-type oil should be entirely satisfactory. Generally, both types of compressors will operate best on oils of low viscosity because of the high rotating and circulation speeds involved.
Dynamic or Kinetic-Type Compressors
Dynamic-type compressors are machines that cause the gas to flow by imparting motion in a given direction to the impinging gas molecules by means of a high speed impeller or bladed rotor turning in a close fitting shroud. The fit, while close, is not a contact clearance or seal as it is in the positive displacement compressors. The velocity pressure thus generated is converted into static pressure when the gas is slowed by discharge into the stationary diffuser passage. Then the compressed gas either may be discharged into the receiver or redirected to the next compression. Each additional compression is a stage.
Dynamic compressors include jet blowers or ejectors and axial and centrifugal compressors.
||In the axial flow compressor the gas flows along the axis of the compressor from one end to the other.
|Radial Flow (Centrifugal)
The radial flow compressor causes the gas to flow from the centre of rotation of the impeller to the outside in a radial direction.
High discharge pressures can be achieved with dynamic-type compressors if they are operated at high enough speeds to compensate for the relatively low density of gas. Speeds up to 20,000 rpm are used but require special bearing designs to prevent “oil whip” or whirl” and resultant vibration.
||Oil whip is defined as that condition of vibration and occurs when the center of the shaft rotates around the central axis of the bearing. Normally, the center of the loaded rotating shaft maintains a fixed displacement in distance and angle relative to the center of the bearing.
Dynamic compressors do not require internal lubrication; so rust and oxidation inhibited-type oils are used customarily for lubrication and cooling of the outboard bearings. Because of the high speed, relatively low viscosity oils are used. Low viscosity oils also tend to minimize “oil whip”. Care should be taken to ensure that the shaft seal oil is compatible with the gas being compressed if the design is such as to allow contact between the two.
Vacuum Pumps are Gas Removers
Vacuum pumps are compressors that operate with an intake pressure below atmospheric and discharge at atmospheric pressure or slightly higher. Both positive displacement and dynamic-type compressors can be used.
Stated another way for the sake of comparison, the vacuum pump is a compressor that tries to maintain a constant pressure below atmospheric or ambient by gas removal. The compressor on the other hand tries to maintain a constant pressure above atmospheric or ambient by putting gas into a system.
The vacuum pump is controlled by inlet pressure; the compressor is controlled by outlet (discharge) pressure. Both have discharge temperatures limited by the ratio of compression as discussed in the section on Positive Displacement Compressors.
For high vacuums, internally lubricated compressors use narrow boiling range, low volatility, high viscosity oils to minimize vaporization or distillation of the oil. The maximum attainable vacuum is limited by the vapor pressure of the lubricating oil. Also in these systems the lubricating oil must be compatible with the gases being drawn through the vacuum pump.
Rust-inhibited detergent/dispersant compounded oils can be very effective in prolonging vacuum pump life where wet and/or dirty air with unstable contaminants is being processed. For example, animal or vegetable fats or oils are often present in air drawn from vacuum packed food products. These tend to form gummy, sticky deposits in the vacuum pumps unless alkaline detergent/dispersant compounded lubricating oil is used. Alkaline detergents are highly effective in neutralizing and preventing deposits from acidic residues resulting from oxidation of animal and vegetable fats and oils.
EXPLOSIONS AND FIRES IN COMPRESSED AIR SYSTEMS
Compressed gas is stored energy. Sudden, rapid release of energy is an explosion. The energy stored in compressed gas is released suddenly and violently when its container breaks. A pressurized container ruptures when a weak spot develops just as a balloon bursts from a pin prick. Weak spots in a compressed gas container can develop from corrosion, outside mechanical damage, or a localized hot spot due to an internal fire. Oily pipe lines containing compressed air have sustained single or a series of ruptures following the introduction of a shock wave that caused ignition of the oil on the pipe walls.
Fires have been known to occur in compressed air systems without explosions and, as already stated, explosions can occur in any compressed gas system without fire. However, the odds are that an explosion will occur in a compressed air system if there is a fire. Thus, if we can prevent the fire, we can eliminate this source of explosion hazard.
Fire anywhere always requires three components; oxygen, combustible material, and a source of ignition. In a compressed air system oxygen is, of course, plentiful. Oil and oxidized oil residues from the compressor lubricant can be present in greater or lesser amounts depending on the nature of the lubricant and its rate of feed to the compressor. Only the source of ignition is absent.
Oxidized oil residues can react with hot air even at atmospheric pressure to generate incandescent hot spots. These can serve as sources of ignition for the rest of the carbon deposit and any oil that is in the vicinity. It is easy to imagine how this could rapidly build up to an explosion in a closed compressed air system.
Theoretically, compressed air temperature reaches nearly (260C) with inlet air at (24C) and discharge pressure of 8 Bar. Even higher temperatures are reached with higher inlet air temperature or when discharge valves malfunction, allowing hot compressed gas to leak back and be recompressed.
COMPRESSING GASES OTHER THAN AIR
The same types of compressors that are used for air are also used for other gases. From the standpoint of compressor lubrication, gases can be divided into three classifications.
• Chemically reactive –oxidizing
• Chemically reactive-nonoxidizing
• Inert or reducing
Chemically Reactive-Oxidizing Gases
Gases in the first classification are oxygen, nearly pure, or at higher concentrations than in air (20%), fluorine, chlorine, bromine, and nitrogen oxides. These gases tend to react chemically with mineral oil with the release of heat. Under pressure, mixtures of these gases with hydrocarbon oil can be explosive. Hydrocarbon oils should not be used in this service. Although concentrated sulphuric acid is sometimes used as a lubricant in conventional compressors handling such reactive gases, it is best to use the so-called non-lubricated compressors fitted with Teflon or Graphite rings to prevent metal-to-metal contact. However, these do give higher compressor wear rates than oil.
Hydrogen Sulphide and Nitrogen Oxide compressors are lubricated with dry compounded oil as used for compressors for moist air. H₂S is corrosive in the presence of moisture. Nitrous oxide may react with engine oil additives.
Chemically Reactive-Nonoxidizing Gases
Examples are hydrogen chloride and sulfur dioxide. They can be compressed in oil lubricated compressors providing a pure acid-treated or hydrogen-treated paraffinic white oil is used as the lubricant. Sludge may be formed with other oils. Synthetic Polybutenes are also used with such gases.
Inert or Reducing Gases
The third classification includes nitrogen, hydrogen, helium, carbon dioxide, ammonia, carbon monoxide, halogenated hydrocarbon refrigerants (Freons), natural gas, and other hydrocarbon gases such as methane, ethane propane, butane, etc., and ethylene, acetylene, and many others. These gases have no chemical effect on the oil; therefore, straight mineral oils can be used.
However Hydrocarbon gases such as Methane, Ethane, Propane, Butane, Propylenes, Natural Gas, etc under pressure may dissolve in the oil thus reducing its viscosity. Cylinders handling such gases should use an oil one or two viscosity grades higher than air for comparable pressures..
For this reason and also because of the possibility of liquid hydrocarbon carry-over, high viscosity steam cylinder-type oils are used commonly as cylinder lubricants for compressing hydrocarbon gases. High viscosity synthetic polymeric oils such as the polybutenes are ideally suited for this service. They have the added advantage of minimizing contamination of the compressed gas because they are also hydrocarbons of high purity.
Gases such as nitrogen, helium, carbon monoxide, and ammonia have no effect on well refined mineral oils and no compounding is required.
The Freon refrigerants also have a thinning effect on the oil; but they tend to enhance the
lubricating value of the oil, thus at least partially offsetting the reduction in viscosity. Small, sealed-for-life refrigeration compressors tend to run hot, and special oils with metal deactivators are used to meet manufacturer specification requirements. These are regarded as special factory fill-type products.
Compressor lubrication requirements depend upon the type of compressor : positive displacement or dynamic. Positive displacement compressors often possess lubricated surfaces undergoing rolling or sliding contact as well as bearings & sealing components. Dynamic compressors often contain hydrodynamic journal and thrust bearings or rolling element bearings for supporting the shaft, but there are no bearing and sealing parts in the compression chamber. These requirements are further influenced by operating conditions such as working pressures & temperature and by the nature of the gas being compressed.
Rotary screw compressors may operate in the 80 to 115 C range which may cause formation of deposits that block filters, produce varnish on bearings, etc. Operating temperatures range of Vane compressors could be 80 to 150 C, which may lead to formation of deposits that block filters, varnish and cause excessive vane wear. Operating temperatures of reciprocation air compressors may be 270 C for a single stage and 160 to 210 C for a multi-stage. Varnish and carbon deposits on exhaust and inlet valves and piston ring wear increase leakage and deposits. Selecting the right lubricant is essential to minimize friction & wear, reduce internal leakage, protect against rust and corrosion, and minimise deposits on hot discharge surfaces.
Some compressors, such as reciprocating compressors do not circulate lubricating oil back to the reservoir for reuse. For such once-through lubrication systems, gas solubility in the lubricant and its impact on reducing viscosity & viscosity-pressure coefficient is an important issue.
The solubility of natural gas and other hydrocarbons is much higher in mineral oil and PAO based products than in other synthetics like diesters and PAGs. This is because both hydrocarbon gas and mineral ( or PAO) molecules are similar molecules consisting of C-H bonds unlike diesters and PAGs which are relatively polar. In a typical PAG molecule every third atom in the polymer backbone is an oxygen atom making it quite polar. Therefore hydrocarbons are less soluble in PAGs. In wet sump reciprocating and rotary screw compressors. The compressed gas and the lubricant come into contact with each other. Hydrocarbons are infinitely in mineral oil and PAO based lubricants whereas solubility increases with increasing pressure at constant temperature in less compatible fluids. Conversely, increasing the temperature at a constant pressure will result in lower gas solubility. Because increase in gas solubility decreases viscosity, at some point viscosity reduction of the lubricant may be too much, and failure may result because of loss of hydrodynamic lubrication.
In refrigeration compressors, shaft sealing is an important consideration for semi-hermetic & open compressors .