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Air & Gas Filtration
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Filtration - parts per million Vs mg per cubic metre
Even some filter manufacturers have switched between the term parts per million (ppm) and milligrammes per cubic metre (mg/cu.m) as though they meant one and the same thing. Pick up an old price list or technical paper and you might see a filter performance quoted as '0.1 mg or 0.1 ppm' as though they were identical. This rough and ready calculation is based upon the weight of contaminant being compared to the weight of one cubic metre of air. Curiously the basis of this comparison is not used much outside the filter business - and it can be misleading when you talk to chemists or biologists. If we go back to the trick question 'which is heavier, one pound of feathers or one pound of lead', we can start to visualise the problem.
Let's take two extreme examples, both are metals that you don't often come across. If the contaminant was 0.1 mg of uranium per cubic metre of air, apart from killing you it might not be easily detected, because uranium is so dense that 0.1 mg occupies an incredibly small volume. On the other hand 0.1mg of lithium (a metal so light that it floats on water), would occupy a volume almost 35 times greater than its counterpart - and this might be noticeable.
The same is true for oils. Volume for volume, a heavy oil can be more than 4 times the weight of a light oil.
To overcome this, the scientific community observe that ppm is the ratio of the number of different molecules of substances that are contained within a Molar Volume. A known volume may contain 100 million molecules of one substance and only 100 molecules of another substance. The latter substance therefore represents a trace of approximately 1 ppm in the first substance. The exact figure is of course [100 / (100,000,000 +100)] * 1,000,000 ppm.
On the face of it we should all be using the scientific version of ppm - however as an engineer, you are probably not really interested in the number of molecules of oil that may be present in every 1,000,000 molecules of your air stream. On the other hand, if you are given a figure in terms of weight, you can visualise and even calculate the amount of oil contamination that may be present in the system. With an oil carryover of 20 mg per cubic metre and a compressor producing 10,000 cu.m per hour, you can calculate that 200 grammes of oil is being introduced into your factory every hour. If you know the specific gravity of the oil you can convert this into litres per day or litres per week. If the compressor is running 24-7 and the oil has a specific gravity of 0.9, this equates to [(200/1000) / 0.9] x 24 x 7 litres per week, or 37 litres per week. You might just notice this!
So how do we convert mg per cu.m into ppm? It's quite easy really, but it does involve a slight understanding of the Ideal Gas Laws, a little bit of maths - and we also need to know a little bit of information regarding the contaminant. This little bit of data is known as the Molecular Mass.
Petroleum oils have a molecular mass somewhere between 75 & 500, whilst typical compressor oils are in the region of 300 to 460. However, some special synthetics can be much higher or much lower than this, therefore a quick chat to your oil supplier may be needed before the calculation can proceed.
Environmentalists convert mass contaminant to ppm, all day long. However they tend to talk about microgrammes per cubic metre and parts per billion (as in one thousand million). The equation they use is:
Concentration (microgrammes per cu. metre) = Concentration (parts per billion) x Molecular Mass (grammes) / Molar Volume (litres)
If we turn this equation around, we get:
Concentration (parts per billion) = Concentration (microgrammes per cu. metre) x Molar Volume (litres) / Molecular Mass (grammes)
Molar Volume for air is derived as follows:
Molar Volume = RT/P (Ideal Gas Law) = 22.414 (at 273.13 deg K and at 1013 millibar pressure).
The actual molar volume will differ as actual temperature and actual atmospheric pressure change. We use the Ideal Gas Laws to make a slight adjustment to these parameters.
True Molar Volume = 22.414 x (T/273.13) x (1013/P) where T is the actual temperature expressed as deg K and P is the actual pressure expressed as millibar.
Let's have a look at an example:
Lets consider 1 cubic metre of air at 20 deg C (approx. 293 deg K), with an absolute pressure of 1000 millibar. The air contains 20 mg/cu.m of mineral oil that has a molecular mass of 360. What is this concentration of oil expressed as ppm?
The true molar volume will be = 22.414 x (293/273) x (1013/1000) = 24.368
20 mg of oil is the same as 20,000 microgrammes.
The oil concentration = (20,000 x 24.368) / 360 = 1354 ppb = 1.354 ppm
In this instance, an oil carryover of 20 mg/cu.m can be expressed as 1.35 ppm. If we go back to our earlier example of a 10,000 cu.m/hr compressed air system, a 1.35ppm oil carryover doesn't sound quite as bad as 37 litres of oil a week.