Oil/Water mixtures as compressor condensates occur in various forms:
- Coarsely dispersed mixtures
- Finely dispersed mixtures
- Emulsions
- Solutions
In the case of coarsely dispersed mixtures, individual oil droplets in the condensate are sufficiently large, by nature of their difference in density compared with water, to rise to the water surface if in a calmed zone of low flow velocity. In practice, these coarsely dispersed mixtures are in a frequently occurring form of oil containing compressor condensate which is treated by gravity separation followed by selective adsorption through activated charcoal in an Oil/Water Separator connected downstream of the equipment producing the condensate. In finely dispersed mixtures, separation through gravity takes place to a limited extent only. Given an appropriate impingement surface, these can, to some extent, be coalesced through coalescing pre-filters, provided that adequate attention is paid to effective presedimentation.
The residual oil content present in such mixtures is trapped by means of activated charcoal. Emulsions are significantly different to dispersed mixtures. For a better understanding, further details about the chemical structure of compressor oils are given here. Oil usually consists of long chain hydrocarbons without a polar centre. As water is partly polar, there are no significant interactions between oil and water molecules. Oil behaves hydrophobically (water repellant) in principle. Part of the function of a compressor oil consists of binding condensate water which is being formed. For this purpose, the oil is supplemented with additives containing polar groups with a view to binding water. Such characteristics are provided by amine compounds or sodium salts of fatty acids. These substances promote the formation of stable emulsions with the oil via long chain constituents. These polar constituents form bridges to the water molecules. Emulsions arise through water binding additives or through the application of mechanical energy. Both forms occur in the case of compressor condensates, the major cause of the emulsions being due to mechanical generation.
The water phase of such emulsions still contains finely distributed oil droplets with diameters of less than 50 microns. These oil droplets are electrically charged with the same polarity and this prevents coalescing of the droplets. Precleaning through gravity and coalescing filters is thus excluded, and a cleaning phase with activated charcoal is also not economical. The input of energy takes place not only in the compression of the compressed air as the draining of the condensate also adds considerable energy. Timed condensate drain valves for example are in practice set with a safety margin in such a manner that the maximum possible quantity of condensate is pushed out with a blast of compressed air. Given a low level of condensate and constant time setting, the blast of compressed air grows proportionately and unnecessarily. 1reverse osmosis on the other hand p = 20 - 80 bar When the timed condensate drain valve opens, the condensate is, in the first instance, pushed into the condensate pipe network and highly turbulent currents are formed because of the small cross-section. Upon impingement, the oil droplets are microscopically fragmented and dispersed, as the compressed air flowing out of the solenoid valve with high kinetic energy accelerates the droplets to a high velocity. Friction against the internal walls of the piping and in the air stream provides additional electric static charges on the oil droplets. The result is a highly stable emulsion. The mode of distribution of the smallest oil droplets in the condensate phase decides whether an emulsion is formed or not. Given an oil droplet diameter of 100 microns or smaller, a limiting value has been reached at which only poor phase separation by purely physical means is possible.
Stable emulsions can be specific to a compressor under certain conditions only. Fresh oil lubricated rotary compressors of older design, reliable and robust as far as their construction is concerned, are in practice commonly lubricated by means of engine oil containing high levels of dispersants. In conjunction with very high temperatures of compression, conventional oil/water separation is practically impossible. Almost the same circumstances apply to large air cooled piston compressors, as well as high pressure compressors, as strong thermal input has a negative influence on the demulsification characteristics of highgrade compressor oils. Oils with a strong inclination towards emulsion are fundamentally those engine oils described by the classification HD or SAE as the requirement to emulsify is of great importance for the original fields of application of these oils. Also, oils capable of demulsification may behave similarly to HD oil for the following reasons:-
- Excessive oil change intervals, followed by too high mechanical.shear, combined with intensive exposure to oxygen from the air and to ageing. Antioxidation additives, foam formation preventors and demulsifying agents lose their capabilities. Within hours, stable emulsions can be formed.
- Effects upon the oil through drawn in emulsion forming agents, where this symptom can, under certain conditions, occur after a short period of oil use. One has to investigate whether emulsifying substances are drawn in with the inlet air and whether these change the characteristics of the compressor oil. Examples could be emissions of combustion plant (catalytic effort) or PVC processing (formation of hydrochloric acid).
Describing compressor condensate is never straightforward. The question is: Is this solely an oil/water mixture or a genuine emulsion? Oils contained in condensate are only to a very limited extent bio-degradable and endanger life in standing or flowing water, interfere with the operation of cesspits and impede oxygen penetration as well as compost putrefaction. For this reason, the law demands appropriate treatment of the oil containing water where the maximum limiting value of hydrocarbons is <20 mg/L (or even less in some areas). For example, the German water treatment law prohibits dilution in order to diminish toxicity, therefore the oil and water have to be separated. Low viscosity liquid separators to DIN 1999, which are usually employed in drainage, or simple sedimentation basins with some diverting baffles, are not suitable for the treatment of air compressor condensate. The water emerging after treatment in such equipment usually contains an unacceptably high content of residual oil of about 30 - 200 mg/L.
