How does air behave in the daisy chaining of compressors?

[Dan Lannan]

I'm working on a project involving compressing air to a desired high pressure. So far the only equation I've been paying attention to is the ideal gas law. My question is if an air compressor pump is rated to create a maximum pressure of 100 psi based on its inlet being at atmospheric pressure, does this mean if the inlet is at 100 psi already the pump will be able to raise the outlet pressure to a 200 psi maximum? In other words, is the difference between inlet and outlet pressure the only thing to consider here? It seems too simple that daisy chaining low-cost compressor motors together could achieve incredibly high air pressures. If anyone could explain or drop a link or reference to any good resources which explain the mechanisms at play here I would greatly appreciate it. Thanks!

 

[.Scott]

The total pressure generated is important.
If too much pressure is generated, the pipes and/or fixtures will break - in potentially hazardous ways.

Also, the output pressure generated will be regulated relative to ambient pressure - not input pressure.

 

[anorlunda]

In a PM conversation, the OP assured me that his tubing is rated for 1000 psi, and that his plan is to not exceed 200 psi. So this topic is not too dangerous for PF.

 

[.Scott]

You haven't described your specific setup.
But in general, what keeps the output pressure to 100psi is not the physical limitation of the pump, but some sort of regulation device that keeps the pump from reaching its physical limit. That regulation device is generally measuring the air pressure relative to ambient pressure. So if you had two 100psi pumps in series with pump A feeding pump B, what would normally happen is:
1) Both pumps would start up.
2) The outlet pressure of pump B would reach 100psi and turn off pump B.
3) The outlet pressure of pump A would then reach 100psi and turn off pump A.
The pumps would then remain off until leakage or usage caused a drop in the air pressure.

Alternatively, their may simply be a relief valve. In that case, the pump B relief valve would open when it reached 100psi and both pump A and B would continue pumping.

Once again, you have not described your specific setup, but in general, you don't want the pump to reach its limit.
Should that happen, the pump or the engine driving it could be damaged. For example, an electric pump could stall and create a high-current condition that could overheat the motor. A gasoline-driven pump could result in either the motor or the pump becoming physically destroyed.

There are pumps that can survive by simply loosing their effectiveness in an over pressure situation - but that result in a waste of fuel or electricity.

 

[Dan Lannan]

@.Scott my current setup prototype is a single stage compressor system comprised of a 24V swing piston compressor capable of achieving 100 psi feeding into an air storage cylinder. My goal is to raise the pressure in that cylinder to around 150 psi without using a larger sized compressor pump. My design constraints within the scope of this problem include not increasing my storage cylinder size and not increasing the size of my pump due to prohibitive costs. My new design includes a sort of "russian doll" daisy chain where I encase my current setup in a pressurized outer cylinder. I suppose the root of my question is if a swing piston compressor is completely enclosed in a pressurized vessel at say 80 psi, will that same swing piston compressor behave the same as it would if it was at atmospheric pressure? My initial assumption is that the compressor's behavior, specifically a swing piston compressor due to its architecture, would only depend on the differential between its inlet and outlet pressure. I plan to have a constant flow of pressurized air into the encasing cylinder in order to maintain it at 80 psi while the inner compressor pulls air from this 80 psi enclosure into the inner cylinder. Being that the line of pressure switches I am using are designed relative to ambient pressure as you pointed out, I plan to use a pressure switch designed to open at 150 psi.

Also, how much of a factor does the increase in temperature play in a system like this? If anyone could I would really appreciate a good reference to a book or website explaining the mechanisms at work between how temperature and pressure behave as air is compressed.

 

[.Scott]

So what you have isn't really "daisy chaining" anymore. It's more like "nesting".
Your inner compressor will work - but heat is a concern.

But there is another concern: the operation of the inner motor in an environment with a high partial pressure of O2.

The compressor would normally generate heat both from the motor and the adiabatic air compression. So your outer cylinder is going to need to conduct that heat to the outside.

It is likely that your motors have temperature protection - to shut them down in case they overheat. If they don't, I would highly recommend that for the inside motor.

This is especially critical because you don't know how that inner motor is going to react to overheating in an environment where the partial pressure of the O2 is at 24psi (21% x (100+14.7)psi) instead of the normal 3psi. The thin winding insulation that normally just smokes may oxidize - then ignite the metal - then explode.

 

[anorlunda]

The other members gave good advice. Let me add something more general.

Let's say we have a pump and motor designed to compress room temperature air from 0 PSIG to 100 PSIG. Now you want to use it to compress air starting at substantially different pressure and temperature. In other words, for something other than what was designed for. To say that is OK, means that you know all the considerations that went into the original design. So the general answer where health and safety may be involved is, "No. It is imprudent to use devices for purposes different than what they were intended to do."

It is likely true that your laboratory has the facilities and supervision to do what you want safely. But the full circumstances of your lab are not a part of the archival record of this public thread. Nor would others who read this thread in the future necessarily have the same facilities and supervision. Therefore, we need to be even more conservative when answering.

That is how engineers earn the reputation as being conservative. Where health and safety are concerned, we have a moral obligation to be conservative. An engineering education should teach safety culture just as much as technical prowess.

 

[jrmichler]

It will be far easier, cheaper, and faster to buy a 150 PSI air compressor than to kludge together the system described by the OP. Here's one from Northern Tool for less than $100.00: https://www.northerntool.com/shop/tools/product_200405815_200405815.

 

[Asymptotic]

An air pressure amplifier/booster is an option where flow rate at the higher pressure is low enough. Haskell comes to mind, but other manufacturers are also in that market segment.

 

[jonnie45]

I'm working on a project involving compressing air to a desired high pressure. So far the only equation I've been paying attention to is the ideal gas law. My question is if an air compressor pump is rated to create a maximum pressure of 100 psi based on its inlet being at atmospheric pressure, does this mean if the inlet is at 100 psi already the pump will be able to raise the outlet pressure to a 200 psi maximum? In other words, is the difference between inlet and outlet pressure the only thing to consider here? It seems too simple that daisy chaining low-cost compressor motors together could achieve incredibly high air pressures. If anyone could explain or drop a link or reference to any good resources which explain the mechanisms at play here I would greatly appreciate it. Thanks!

Hi - the thought experiment I would suggest is that you imagine placing the second compressor in a sealed pressure vessel and raise the pressure of that vessel using the first compressor. The point of this thought experiment is to highlight the differential pressure between the interior of a compressor and the ambient exterior.

Does your second compressor "know" that the 'ambient' pressure around it is infact already raised considerably above atmospheric?

The air around the second compressor will have increased density but the long hand expression is more useful here, the air will have higher mass per unit volume - will that affect any components ?...they will certainly do more work since they are shifting more mass. Also consider if any of the heating effects will be radically different. Are the components more likely to fail because they are exposed to an ambient pressure that is higher than usual or are they only sensitive to the differential pressure - ie consider components that have the same (high) pressure on all sides and then compare with components that have a high pressure on one side and a low pressure on the other side - it is these latter components that are exposed to both the interior pressure and the external pressure that are more likely to fail.

I suspect that as long as each daisy chained compressor is contained in a pressure vessel that is fed by the preceding compressors that you could go a long way - that is until...

1. Components fail simply because the construction materials change when exposed to a sufficient pressure - I am not referring to differential pressure - for instance a plug that has one surface external and one surface internal - we have already eliminated differential pressure by using pressure vessels. Here I mean simply that a material will change physical properties when immersed ( on all sides ) in sufficient pressure. Many solids are very resistant to pressure if on all sides - its differential pressure ( different on one side to the other ) that causes stress and failure.

2. The gases being compressed have changed physical properties sufficiently to cause damage, overloading or fatigue to components. Again I suspect you can go a long way here.

3. Liquification of gases - stay above critical temperature.

In short I suspect that what kills a compressor in a daisy chaining experiment is not "absolute pressure" but rather differential pressure - as long as the ambient pressure is only 100psi lower than the internal pressure then I suspect you could successfully chain quite a few compressors at room temperature.

The thought experiment places responsibility of withstanding the pressure of atmospheric to final output pressure on the pressure vessel which surrounds each compressor and is pressurised by the output of the preceding compressors.

Is this a cop-out - just shifting responsibility onto the pressure vessels?

In a way yes...

But it does in turn lead to thought experiment - what if the pressure vessels were spheres and placed one inside the other - each pressure vessel containing both a compressor and a set of pressure vessels one inside the other - babushka doll style - that way each compressor sees only a 100psi differential but so does each pressure vessel.

That's why I suggested a pressure vessel that is a sphere so that the sheet metal or whatever is acting more like a very strong soap bubble ( tensile ) rather than say a steel box which will not scale very nicely as a pressure container.

 

[Dullard]

@Dan:
I think that you (and others) have a fundamental misunderstanding of how a positive displacement compressor works. Don't think of it as a device that 'adds 100 PSI.' It is a device that 'multiplies' 14.7 PSIA up to 114.7 PSIA. The motor on your compressor is very unlikely to survive the nesting exercise.

This is a little over-simple, but:

When used in 'normal' mode (atmospheric inlet), the piston initially has no pressure difference.
When the volume is cut in half (by piston motion), the cylinder pressure is 29.4 PSIA (14.7 PSID across the piston)
when the volume is cut in half again, the pressure is 58.8 PSIA (44.1 PSID across the piston)
cut it in half again: 117.6 PSIA (103.2 across the piston)

The instantaneous speed and pressure across the piston determine how hard the motor has to work.

Repeat this exercise with an 80 PSIG air charge (94.7 PSIA) (and 80 PSIG 'atmospheric' pressure. You'll note that the pressure difference across the piston is 94.7 PSID after only 1 'halving' of the volume - that motor is going to be working a lot harder unless you have the ability to shorten the stroke or reduce the speed.