Pressure and Flow

OK Tornado, but it's worth adding that the compressor cfm is at free air conditions (i.e. at atmospheric pressure). So if flow is calculated from swept volumes and operating frequencies, need to multiply by absolute pressure at use point divided by atmospheric pressure, = in English units (gauge pressure + 14.5)/14.5. If the demand is stated by equipment supplier, it will be at some standard conditions. Conditioons should be stated, but they don't differ much so unlikely to make a difference in practice.

Cheers............Codey

Typically, an air compressor is rated for X SCFM at some defined rating point, usually its peak pressure. The key word (letter) here is "S".

Compressors are ALL rated in "Standard Air" at the inlet- 68F/40C, 0% RH- which does not exist except in calculations. When selecting a compressor, the rated capacity should be AT LEAST 25% higher than the required capacity to allow for losses due to compression of water vapor into water- removed by compressor and storage tank drains and by dryers downstream of the compressor. NET DELIVERED AIR will be 80-85% of the actual compressor rating because of this, depending on the amount of ambient humidity present. The 125% of rating factor makes up for these losses. IF the compressor is rated at, say, 100 PSIG and the required flow is only 80 PSIG, that will provide another slight safety factor indicated in your formula.

sr dru,

Hello, you have already received some good information related air systems, but reading you original post and follow-up I am wondering if your question is more basic or entry-level in nature. If it's not, please forgive my assumption. If it is, in layman terms...

Look at pressure as force and flow as speed.

The solenoid valves to which you referenced, I'm assuming to electrically control the movement and/or the lifting force of your pick and place, will be rated as to what kind of pressure they can withstand based on the size and type of material used to construct them, and they will also be rated on what kind of flow they can allow through them by nature of the size of their orifices.

You'll want to look at your working components on your pick and place to determine what their ratings are. You will want to select your solenoid valve based on similar ratings. For example, if you have a pneumatic cylinder rated at 100 psi, then you would want all other components in that circuit to also be rated at 100 psi, minimum.

As for flow, or the speed of movement of your pick and place, you'll want to oversize the flow capability on all component of your circuit and then use a flow control valve to regulate the flow to the circuit. A common practice is that all orifice openings on each leg of the circuit should be the same size. If, for example, your solenoid control valve is under sized, it will typically cause 4 problems:

1. It will be the bottleneck in your system and the performance of your entire system will be limited by this component.
2. Fluid systems are designed to work with a laminar flow, restrictions or irregularities in the flow path will cause a turbulent flow condition which will affect system performance and maintenance frequency of components in the turbulent flow area.
3. Maintenance frequency will increase on this component as it will likely operate continuously at 100% of its designed capability.
4. Most importantly, especially for lifting devices, the spool inside the valve is likely to drift out of position. Research "Bernoulli Principle" for additional information. This could cause a safety hazard.

In summary, good design practice is that the working part of the circuit (motor, cylinder, venturi, etc.) be sized large enough to accomplish the task, plus a safety factor, but small enough to not break your machine should things go awry. Then, for each component upstream of the working part (hoses, valves, tubing, solenoids), they should be sized to the next level up of capability as you don't want your infrastructure to be your weak link. Finally, you control the flow and pressure of your air to the system by means of pressure and flow regulators.

Again, I apologize if you were not looking for an entry-level response.

JavaHead

sr dur,

You are close, but not quite there. There are a few things you're missing:

I am assuming that you're getting your SCFM and PSI requirements for each machine from summing the requirements of all "working" components on the machine. However, there are additional components in your circuit "consuming" (for lack of a better term) your PSI and your SCFM

We'll start with piping and hoses. It takes less force to move air/liquid through 1 foot of 1/2 tubing than it does to move the same amount of air, at the same speed, through 100 feet of tubing. Also, each fitting, coupling, valve creates and additional pressure drop. For example, at 100psig 16scfm of compressed air traveling through a 3/4" Sch 40 pipe will loose about 12psi for every 100 feet of length. Contact your pneumatic supplier for assistance in calculating the head pressure of your existing piping runs.

If your piping infrastructure is too restrictive it will decrease efficiently of your compressor. Back pressure in a system, any system, causes the device supplying the power to work harder. And in some cases, possibly yours, your piping system may require you to upsize your compressor, add accumulators, etc.

Look at a car. The engine is capable of supplying X amount of HP and achieving X amount of MPGs... the exhaust system, although not part of the drive-train, has a large impact on engine performance due to back pressure. The more restrictive your exhaust system the less HP and MPG the engine will be able to produce. The more restrictive your piping system, the less CFM and PSI your compressor will be able to get to your working components and it will increase the reaction time it will have to make up pressure losses when things start to move.

Speaking of when things start to move and pressure losses, my next point is that even though your component may require 100psi to perform its job, doesn't mean that's what you supply it. Using a cylinder as an example again. When pressuring one side of the cylinder, the other side is exhausting to atmosphere. There is back pressure in this exhaust, especially if you are metering the exhaust to control movement rate. In addition to the back pressure there is also flow loss as soon as the cylinder starts to move and pressure differentials across each port. It is common, in order to makeup for both the back pressure and the flow loss to provide 25% more PSI in your feed line then is required by the component.

Some solutions to piping/supply issues (which it sounds like is your problem) are revisions to pipe sizes, addition of accumulators adjacent to your large consumption components, increase in compressor HP.

In closing, based upon the description of your problem, I do not think you will find your final solution here, in CR4, without providing a detailed schematic of your system. My suggestion is to contact your pneumatic component supplier and ask for some technical assistance.

Start with asking for assistance in locating the best place to locate pressure gauges for troubleshooting purposes, throughout your system. This will help you identity your trouble spots. Then, through consultation with your supplier you can determine, based on your system design and budget, the best method of addressing the issue.

Good luck and may I suggest a set of reference books for you as you learn about fluid power? These were invaluable to me as a young designer and they still occupy a place on my bookshelf... well, they use to, the new designer we hired seemingly appropriated them for his reference.

JavaHead

I am going to add to javahead's excellent response.

If you go back to discussion #5- You will see that your compressor needs to be rated for at least 70 x 1.25, or about 88 SCFM at 100 PSIG.

One other point is that you need to research the ACTUAL operating requirements of your devices. Very few devices have a pressure requirement of 100 PSIG. That may be the pressure that everyone BELIEVES is the requirement, but- check with the device manufacturers to get the actual pressure needs. Running a device with an overpressure condition will greatly lower its active life due to the internal wear on the components.

Are there any additional air outlets- nozzles, connections to controls, etc. These loads must also be accounted for.

I believe that another discussion also mentioned leakage to overcome- depending on you original piping conditions, this can easily be 25% (or more) of the compressor capacity. Since you mentioned that your company is small, your budget for testing devices is also likely small. If you put a small bowl of mint extract close to the intake of your compressor, you will be able to identify any serious leaks by walking around the plant along the piping and smelling for mint.

Either way, you must also allow for line leakage losses. That would be a minimum of 88 x 1.25 or 110 SCFM.

You did not indicate whether the pressure losses were at the main storage tank, or at the end-users. If it is at the end users, the leak test may help, but you may have undersized distribution piping. Javahead's comments were right on track. Additionally the pressure drop through HOSES is likely at least twice that of rigid pipe or tubing, often 3 to 4 times as high.

Also- if there is any low points in the piping, be sure that there are drain valves to be sure that any possible water build-up is not restricting flow at a low point.

Hope this helps.