Rate Of Flow From A Tank For A Given Pressure Drop?

Hi bigg,

I am definitely dumber than I thought.

I see your figures but can't get my head around them.

I thought it would be all Boyle's Law 'ish, you know P1 V1 =.... but the time element threw me.

Your first calculation 150l*....... that is Litres x Atmospheres what units does that give you?

I was of the initial thought that it could be treated as "free air" but with the drop from 10 - 8 Bar I wasn't so sure.

I (probably wrongly) think that a tank of 150 litres capacity, filled with air at 10 bar, that is allowed to bleed off until the tank has 8 bar in in 2 minutes will have displaced a certain volume of air.

The equipment supplier said a compressor of about 3 x the machines air requirement (96 litres/ min.) would do the job BUT the compressor should have a Cast Iron head. His compressor has an aluminium head and I think the CI requirement means it will be working hard.

That still doesn't solve the problem of the number of stop starts/ hour.

Again many thanks for your input,

John

Plenty of compressors nowadays have unloading devices. When it reaches high set pressure it unloads (by holding the inlet valve open) but the motor doesn't stop. When pressure falls to low it restarts. If it hasn't reloaded, after 20 minutes (for max 3 starts/h) the motor stops. It restarts at low pressure. If you can retrofit that to your machine it should be cheaper than a bigger receiver.

Hi Wal,

Thanks for your input.

If the storage capacity is doubled to 300 litres that should give 4 minutes of compress off time and require the compressor to run twice as long to re-charge the tank from 8 bar to 10 bar. still 7.5 starts per hour. To get to to 3 stop starts per hour it would need a tank capacity of 750 litres!

The pipes to the machine are only 8 mm OD nylon which is capable of passing about 330 litres /min.

Best regards,

John

I guess your math is right.

6m of 4" pipe holds around 45litres. Stack up a bundle of them....

750 litres isn't that big.

If you slow the pump down (change the pulleys) it will run for longer intervals with fewer starts. If you slow it down enough it won't ever stop.

Can you increase the storage pressure?

More starts per hour can be tolerated by the motor if it is cooled better. Bleed off some compressed air to cool the motor core while it is off. Maybe a blower would be better.....

How hot does the motor get anyway?

Thanks for the interesting discussion topic.

Hi Wal,

A bundle of 17 4" x 6m pipes not an option, neither is a tank of 750 litres.

Slowing pump not an option either, It's an off the shelf compressor of not the most robust designs he has just bought.

Max. pressure of compressor 10 Bar.

Motor tripping out on it's thermal overload protection. The unit is in the back of a 20 foot container with the doors closed and no ventilation!!!

The first thing I said was either move the compressor to a better ventilated spot or cut vents in the container.

He has overcome the problem partially at present by connecting the Aeration unit into the air supply of a large filter press he has.

The air supply to the press is from a 10 HP, 10 Bar compressor on a 300 litre tank that fills an additional 500 litre tank. The air is only used at the end of the filter cycle to blow back to the holding tank the liquid slurry in the filter presses feed tube. This is so they only get dry "cakes" from the machine.

It only operates occasionally but when it does it drains the whole air system to the press. This causes the Aeration system to throw up a "low air pressure" alarm.

I suggested he tees both air systems into the aeration system with a non return valve in the line to the filter press so that the filter press air system works them both but the aeration system compressor will be standing by full charged to go when the filter press demands all the air.

Not the best solution for me as it would cost about £20 for the bits to do it.

He wasn't too keen on the idea for some reason, so I didn't push it.

I have quoted him for a reconditioned 10 HP rotary vane compressor on a 300 litre tank with modulating control and a run on timer to stop it if there is no air demand.

I have been advised that as the pipes are "open ended" this will be a problem if supplied from a "small" compressor.

As the "open ends" are fed through Flow Meters set at 25 litres/min. I feel that although it is a continuous air demand the Flow Meters give control to the "open end".

Thanks for your input,

John

Hi Codemaster,

Thanks for your input, as you are in the UK, you will be familiar with The Machinery Mart and their Clark's range of compressors?

This is what the customer purchased to replace an older version of the same model.

Not possible to to modify to Unloading type.

Thanks again for the input.

Best regards,

John

No prob John

I don't suppose it helps much, but I think your maths is slightly pessimistic. To get maximum start frequency f per hour, for compressor output Q lph and start/stop pressure change ΔP bar, receiver volume V litre = Q/(4*ΔP*f). You can show it by a bit of fairly easy calculus, which is where the 4 comes from.

Max start frequency is when demand = 1/2 compressor output, not surprising as when demand = 0, start frequency = 0 (never starts) and when demand = compressor output, start frequency = 0 (never stops) so max frequency is likely to be 1/2 way between.

For your case I make that about 640 litre. But 3 starts per hour sounds rather conservative. Most motors are OK with 6, specially small ones, I estimate 3 kW for yours.

Cheers...Codey

Hi Codey,

Thanks again for your input, the compressor is a 3 HP Single Phase unit with a Free Air delivery of approx 260 L/M and a maximum pressure of 10 Bar.

Still can't make out from the replies (thanks to all) the air flow figure I originally asked for.

Many thanks & best regards,

John

Hi again John

150 l receiver falling from 10 bar to 8 bar means 150*(10 - 8) = 300 l free air outflow. In 2 mins that's 150 lpm.

If the compressor output = 260 lpm (free air) the net input when running = 110 lpm, so it "ought" to take 300/110 = 2.7 mins to get back to 10 bar, so something doesn't add up, though not vastly out. Can you recheck the times?

Codey

Hi Codey,

You've nailed it.

Your figures look good, I isolated the compressor from the aeration system just so I could get some idea of the compressors performance.

So I charged the tank until the compressor stopped, 10 Bar, drained the tank until it re-started, 8 Bar. each part of the test took 2 minutes.

The times were near to yours when the machine was connected.

Thank you very much for your time and effort.

John

Clarke compressors look like they are belt driven. Fit a smaller pulley on the motor to slow the pump down. This reduces the air output (permanently) and the load on the motor (less heat) that allows more stop/starts per hour.

Or a retro-fit off-loading system that is a non-return backflow valve in the main output line, with a solenoid valve in a tee (to dump the excess air to atmosphere). This valve is controlled by a pressure switch, either the existing one (if it is adaptable) or an additional one. As a refinement fit a delay timer that then stops the motor if it is off-load for 15 (or 20) minutes.

Hi Horace40,

You are right they are belt driven, not really an option to change things as it is a brand new unit and according to the Aeration systems supplier should do the job.

The customer is more likely (I hope) to buy a new, bigger compressor off me.

Many thanks for the contribution,

John

Hello John

You don't need a bigger compressor for this particular duty. You need a bigger receiver - as has been suggested by most contributors. But as you have found this is not a practical solution.

There is nothing wrong with a bigger compressor but it must have an off-loading valve facility. The motor runs continuously but does not pump any air. A delay timer as an economy device would also be required to stop the motor when there is no demand. You have already suggested this - so don't let your customer talk it down.

In terms of air flow, your 150L cylinder at 10bar will hold the equivalent of 1650L of free air. At 8 bar it holds 1350L. So you have used 300L in 2 mins. This agrees with previous posts. You really need to count the cycle time in seconds to get a more precise figure.

Not that it matters much, because being more accurate will not alter the fact that you need a compressor with off-load control. Your vane compressor sounds a better bet.

Hi horace40,

Thanks again for the input.

The Aeration system supplier said the machine needed 96 l/m, the figure of 150 l/m outflow is 50% more. Our initial checks on the system were done to try and prove/disprove the Aeration peoples figures and confirm the output of the compressor.

The figures seem to point to a 5 HP compressor. I think it's time to let my customer decide. I've spent enough time (at no charge) looking into this not to mention the 1,000's of $ and £'s of consultants time (you are all consultants aren't you) not charged for (thank the lord).

Many thanks to you and all the "gang" who have helped me no end.

John

As someone else has pointed out, a larger compressor with the same size accumulator will actually increase the frequency of cycling, since the pump-up time will decrease while the draw-down time will stay the same. One other thing to consider is increasing the spread between the cut-in and cut-out pressures. If your users don't require 8 bar, 5 for example, you could reduce the cut-in pressure setting to 6 and leave the cut-out pressure at 10. This would increase both the draw-down and the pump-up times. The compressor your client has has adequate capacity. I can't speak for its overall suitability. If its the same build as my Clarke 2HP pancake from Tractor Supply, I wouldn't consider it for a continuous duty.

Perhaps an intermediate 2 to 1 accumulator and a second air tank. This could provide longer cycle times limiting the cycles per hour problem. This would not be a very expensive option, very low maintenance and would be very easily controllable as well.

Hi OldTooly,

By accumulator do you mean a 2:1 Ratio Pressure Booster? I think the compressor would have to work much longer to get the 3 stop starts per hour I was hoping it would run a bit longer but be stopped longer as well. Past experience of these compressors shows they can be a bit vulnerable if pushed hard.

I meant to say to Codey that the range of compressors we sell advise 6 stop starts per hour so you can see his unit is more utilitarian.

Best regards,

John

I see, just a thought Jes. It worked very well for a situation I had a while back. It also provided an additional high pressure line not included in our original design to solve a problem with our customers using our machine with larger products than it was originally designed for. By the way, the Pressure Booster/Accumulator we were using, which was a 1 to 3 ratio never did fail in 13 years of operation, before the machines were retired for some newer electronically controlled models. And yes, the main compressor ran longer per cycle, and was off longer as well. But it never presented any additional maintenance issues. Good luck to you sir.

Hi OldTooly,

Thanks for the good luck wishes. I had a phone call yesterday from the customer saying they were considering my proposal (which included having the compressor on sale or return to prove it will do the job) but he asked how much we would offer him for his "As new" compressor.

Not really interested as we stock and sell competitors units. He said he paid £750 for it (they have a list price of £439) so he was "stitched up" or "trying it on". My offer was too low so I suggested he try and sell it himself.

Once again thanks to all for the support, it helped a lot,

John

Hope you make a sale mate.

Good luck!

Thanks Wal