The usual methods of determining residual oil content are only capable of establishing the remaining oil aerosols after the filter. The residual oil content of oil in vapour form is therefore not indicated. The latter can be obtained on the basis of calculations (see Section 2.5.5) for the maximum oil vapour content in compressed air. The residual oil content after a filter is given by the oil vapour content of the oil, as a function of temperature. For this reason, comparison between filter elements is only valid for conditions of equal temperature. With filter borosilicate-glassfibre material, residual oil contents down to 0.01mg/m3, given mineral based lubricants, at an operating pressure of p = 7 bar and a reference temperature of t = 20°C, can be achieved.
Diagram 4.8.4.1 - Residual content of mineral oil (after depth filter)
The concentrations regarding the residual oil content are based on the depressurised gas volume in m3 and apply to an oil with a molecular weight of around 300. Deviating residual oil values at higher operating temperatures and different operating pressures are shown in Diagram 4.8.4.1 for mineral oil and in Diagram 4.8.4.2 for synthetic oil, for comparison purposes. Using Dalton’s Law of Partial Pressures, the saturated vapour content of compressed air at other conditions or alternative gases can be determined. Synthetic oil, in contrast to mineral oil, has a much lower ability to evaporate. The respective partial pressure of the lubricant determines this. An accurate determination of the residual oil value can be carried out only on the basis of knowledge of the type of oil used and of the parameters particular to this oil. For the quality of the compressed air, the residual oil content indicated in mg/m3 or also ppm is the determining factor. Standards are set for various applications.
Diagram 4.8.4.2 - Residual Oil Content of Synthetic Oil (after depth filter)
At a given separating efficiency of the filter, the residual oil content in compressed air depends on the initial loading or content. Diagram 4.8.4.3 shows residual oil concentration as a function of initial content as far as a fine filter is concerned. Reduction of loading or content leads to an exponential drop in residual oil concentration down to the theoretical value of 0.01 mg/m3. Below this value, the influence of the vapour phase is larger than the concentration present in droplet form. In practice, a value of 0.001 mg/m3 is regarded as the limiting value for residual oil concentration. In order to achieve a residual oil concentration of 0.01 mg/m3, the original loading can amount to a maximum of 3 mg/m3 at a given separating efficiency. A number of methods are used for determining the residual oil content of compressed air. ISO 8573.2 and ISO 8573.5 are the preferred standards. In the majority of cases, the following filter integrity test methods are presently used, but are only preferred for operation of the filter at atmospheric pressure
- Methylene blue test
- Sodium flame test
- D.O.P. test (Dioctyl Pthalate)
Different methods of measurement inevitably lead to differing results, so that the residual oil contents established in the compressed air cannot be compared with each other, or with reservations only. This is why an International Standard has been produced. ISO 8573.2 contains a method for determining the residual liquid oil content in compressed air, and also offers the possibility of obtaining reproducible results to the filter manufacturer as well as the compressed air user. In the course of this, the compressed air is filtered and then led to an aerosol generator via a pressure regulator. The constant oil/air mixture formed in the generator flows through a filter, the pressure loss of which is monitored by means of a differential pressure gauge. Following the filter, the compressed air is divided into a main and a test flow. The test flow is conducted through a high-efficiency collection membrane with the aim of completely filtering out the oil droplets contained in the compressed air. Parameters such as temperature and pressure have to be monitored with appropriate instruments during the test, so that the operating state can be established and subsequently converted to the standard conditions by calculation.
