Oil-less compressors

Compressed air is used for various purposes. Air which might come into contact with the product must be free of odor, particles, and oil, have a pressure dew point at less than 2°C, and be sterile. The air is normally compressed to 1000 kPa (8—10 atm) and cooled direcdy after the compressor. If needed, the air is then further dried by an absorbtion unit and the small particles and germs are removed through a set of filters of declining pore si2e. 

For gas turbines, especially the more advanced high-temperature gas turbines, the oil of choice should be synthetic oil, since synthetic oils have a high flash point. Gas turbine lubrication systems should be run for about 20 minutes after shutdown since maximum temperatures are reached after 10 minutes of shutdown especially in the bearing area. Most gas turbines are also on turning gear to avoid sagging in the shaft. Mineral oils can be used for the compressor. It is not uncommon to have two types of oil in a petrochemical plant. Mineral oil costs much less than the synthetic oil. 

If oil buffered seals are used on the compressors, the seal leakage toward the process side of the compressor must be carefully measured, as it is (and should be) a small value. While five gallons per day doesn t sound too small, in a four-hour run, this is less than two pints, making the hold-up time at the inner seal chamber and in the lines to the drain pots a significant value. This makes exact measurement quite difficult. 

Ratio of the actual flow rate (entering compressor) to the displacement rate is the volumetric efficiency T va- The volumetric efficiency is less than unity due to reexpansion of the compressed vapor in clearance volume, pressure drop (through suction and discharge valves, strainers, manifolds, etc.), internal gas leakage (through the clearance between piston rings and cylinder walls, etc.), valve inefficiencies, and due to expansion of the vapor in the suction cycle caused by the heat exchanged (hot cylinder walls, oil, motor, etc.). 

The work extracted from the expanding uid (HA-HC), may be utilized to generate electricity, compress gas, or just be dissipated in an ambient blower or oil friction brake (see Section 3.2.2.4). The net result is that as refrigeration is generated across the turbine, less refrigeration is needed from the incoming air and the air compressor discharge pressure can be run at much lower pressures. Cycles termed low-pressure cycles ensued from this development. 

Phosphate esters have been produced commercially since the 1920s and now have important applications as plasticisers, lubricant additives and synthetic-based fluids for hydraulic and compressor oils. Their first use in lubrication was as anti-wear additives. Later developments in aircraft hydraulic control systems, particularly during the Second World War, introduced phosphate esters as less flammable hydraulic fluids. As esters of orthophosphoric acid they have the general formula OP(OR)3, where R represents an aryl or an alkyl group or, very often, a mixture of alkyl and/or aryl components. The physical and chemical properties of phosphate esters can be varied considerably depending on the choice of substituents [59, 60], selected to give optimum performance for a given application. Phosphate esters are particularly used in applications that benefit from their excellent fire-resistant properties, but compared to other base fluids they are fairly expensive. 

Low-temperature coolants, at less than 0 °C, include brine, which is a solution of a salt (for example, calcium chloride or sodium chloride) in water and alcohols (for example, methyl alcohol), glycerine, and oils. The source of the low temperature is anunonia, sulphur dioxide, or carbon dioxide from a compressor. Again this lends itself to centralized production in a service area, if the demand is sufficient and, in all cases, the refrigerant is recirculated. 

Conventional lubricants based on mineral oil tend to increase in viscosity on oxidation or absorption of gases. On decomposition, troublesome sludge formation may occur. The PEGs are free from this drawback and owing to their character have less tendency to absorb methane, ethane, ethylene, propylene, and so forth. They can therefore be used to advantage for lubricating compressors employed in... 

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