Compressed Air Receivers

[Josh2008]

I'm new to compressed air systems but I've done some research online to learn as much as I can for them.

I'm designing a system for an underground mine application to power pneumatic machinery and tools on different sub-levels of a mine. I have calculated pressure losses for hoses, fitting, and pipe friction so I know the pressure required at the outlet of the receiver.

The air receiver is where I'm a bit stumped. I understand that the receiver plays the role of providing surge capacity to the compressed air system. It also separate moisture and reduce pulsations.

Knowing the pressure inside the receiver, the volume of the receiver, and the atmospheric pressure, we can determine what capacity of air is inside the receiver.

If for instance I know that the pressure at the outlet of the receiver has to be at least 100psi to achieve 90psi at the work tool underground, does that mean it is safe to size the system for a 100psi compressor? Is there any reason to size it at a higher pressure? IE say there where two compressors on the market @ 100 psi and 110 psi to choose from. Obviously you want to size the compressor to the minimum required pressure rating so that there is more cfm available.

http://www.scalesair.com/assets/pdfs/sizing_to_save.pdf

The link above sizes a receiver using 100psig from the compressor and 90psi at the outlet of the receiver. What are the circumstances here?

My assumption is that if one compressor is running a tool and the system surges because somebody requires so many cfm for a short period of time that the pressure drop due to flow demand creates flow from the receiver to compensate and avoid a second compressor turning on (therefore saving you energy).

Am I anywhere near correct? If for instance I'm correct, and I don't know what the exact short term surge is going to be, how do I size the receiver? Using the 'for every cfm = 1 cu.ft. rule of thumb'? How does the article above know to use a 10 psi band? Is this something preset?

Thanks everyone.

 

[PEDARRIN2]

I would recommend two approaches.

Call a couple manufacturer's rep and ask what they would do. They will have the experience to know what to provide.

Also, use the internet.

A few organizations/companies that I have found:

Air and Mine Equipment Institute of Autstralia
Norgren
Kaeser Compressors

Good luck

 

[crjones]

You really want your compressor to be able to go 10% above your need pressure wise.

The starting point for sizing a receiver is to take your capacity requirement times standard pressure divided by your working pressure absolute (not gauge). The multiply this by a load factor 1.5 for constant and 3.0 for variable.

 

[BigInch]

As long as you can fit the volume you need to have in the receiver on the skid, don't raise the pressure any higher than you need to hold that volume and still fit the receiver on the skid.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[davsy]

The link above sizes a receiver using 100psig from the compressor and 90psi at the outlet of the receiver. What are the circumstances here?

What do you mean by "what are the circumstances here?"
As the article mentions there has to be a differential pressure otherwise there is zero storage of "useable air" no matter how big the receiver is. The amount of differential pressure is what determines the storage volume. You could have a relatively small receiver with a large differential pressure or you could have a large receiver with a smaller differential pressure. Both could give you exactly the same "useable air" storage volume but would have greatly different Capital and Operating costs.

As PEDDARIN2 mentions above, why not get a couple of compressed air system vendors to discuss your application and have them show you the economics so that you can evaluate what they would likely offer and run that in parallel with your research and calculations.

How does the article above know to use a 10 psi band? Is this something preset?

10psi is a good starting point especially when trying to illustrate a calculation in a published article. The 10psi is quite common but it also makes the maths a lot easier to follow just like the 100psi compressor and the 1000 gallon receiver.
Having said that the example is pretty real and it indicates that using the 10psi differential pressure across the 1000 gallon receiver gives only 91cu.ft of "useable air" storage.

Am I anywhere near correct? If for instance I'm correct, and I don't know what the exact short term surge is going to be, how do I size the receiver?

This is where your experience and engineering judgement is going to come in, you will need to start with something to be able to size the whole system. If you can't calculate or estimate the capacity you require then you are likely to oversize your system with the increased costs associated or more likely undersize it with the associated process implications.

 

[CRG]

The way I see this problem is that the calculations are simple once you determine a diversity factor and utilization factor. Without experience, it is difficult to determine these factors. Add in redundancy, first cost, and life cost of equipment and you can start to make a choice on the compressor and receiver. Once you get a rough estimate for sizing the compressors, you may want to include an electrical engineer in the mix to ensure that you are not creating problems with the available power supply.

 

[Josh2008]

Just so I get this straight, let me give a simplified example.

Assuming that the pressure loss from the compressor to the receiver is negligible, and that the pressure in the receiver is the max pressure of the compressor.

If X psig is required at the outlet of the receiver to power a drill (say X psig is the max pressure for drill), do you try to choose a compressor of X + 10 psig or X + 5 psig so that when flow @ max pressure is demanded, that when intermittent surging is required, there is a suitable pressure differential (for the receiver) to provide suitable time for intermittent surging?

 

[gmax137]

I think maybe what you're missing is the idea that the compressor loads on a low pressure setpoint and unloads when the pressure is restored to a high setpoint. So even though the machine runs constantly, it cycles. The pressure measurement is taken at the receiver. More sophisticated controls have both half and full load. The unloading is done by holding the valves open. At least, this is the the kind of scheme I have seen. I think the advice (repeated) above to get input from the compressor vendors is good advice. The only downside is they may continue to bug you with sales calls. Can't blame them tho,it's how they make their living.

 

[Josh2008]

Right, I was told there is two modes of operation, load/unload (like you have mentioned), and modulation.

Thanks everyone for the input! I guess the hard part for me is trying to explain my question by typing it out.

All I mean is that if you don't account for the ability for the receiver to have some pressure differential at the peak pressure demand of the system, then essentially there is no intermittent ability to supply more cfm.

So therefore, I was trying to ask... if you only take into account sizing a compressor for the piping and fittings losses, then the receiver isn't able to provide a pressure differential at the peak pressure demand of the system.

So should size the compressor a little bit larger?

Anyways, I will talk to the salesman and get there input. Thanks to everyone who took the time to comment and share their knowledge.

Josh