Compressed Air Storage

[wk390107]

Afternoon all,

We currently have three air compressors at our plant. A 100HP (load-unload), 60HP (modulation), and a 50HP (modulation) - the 50HP is rarely used due to it's age. I had an air survey conducted to evaluate the system for possible improvements. I found that the flow is less than 250CFM 60% of the time, with large swings in demand from 50 to a max of 440CFM. With this information the 60HP should be able to carry the plant load the majority of the time and the 100HP should only come on during high demands. However both of them have to be kept running all the time because if there is a spike in demand there is not enough time for the 100HP to turn on before the header pressure drops to a critical low - which in turn starts the 50HP as well. 210HP of compressors running for 300CFM.

So to me it seems that there is a compressed air storage issues. Each compressor has a 400 gallon wet tank, but there is no dry tank. Adding a dry tank seems to be the solution, but I am hung up on a few things. One is the size, I want to size it so the 100HP (newest compressor with brand new air end) can run the plant by itself all the time and not cycle too much. So I was thinking that enough storage to allow the compressor to unload for ~2 minutes at a time would be appropriate since it takes the sump ~40 seconds to depressurize.

Now I am confused however on how to create/estimate the tank size and pressure differential across a new tank. I know is doesn't matter how big a tank is if there is no pressure differential - Is a flow-controller needed to create this differential?

If I was to add a flow controller I would still only be able to create a ~10psi control band since our compressors together only create about a 95psi header pressure. Which would mean I would need 5000 gallons of storage to create the needed 500CFM of storage so the 100HP machine can unload for ~2 minutes at a time? Is this correct? Do I subtract wet receivers from this total?

Thank you everyone, and Merry Christmas.

 

[TD2K]

Look at the capacities of the compressors versus the demand versus the time period you want the system to ride out a peak demand.

Basically, the dry and wet tanks provide air as the system pressure drops. Currently, you have 1200 gal of wet tanks or 160 ft3. Assuming the system pressure drops 15 psi (ie. about 1 atmosphere) and all three tanks float on the system pressure, those three tanks would provide about 160 scfm of air. Air temperature is another factor but I'm assuming your air is about 100F or less and that effect is minor.

If you have a significant volume of piping and line losses are minor, that can supply more 'surge' volume.

 

[nfinit]

There are a lot of questions and variables. Is the system designed in a loop? Do you have volume bottles at any of the extreme distances or the high users? Do you know the pressure requirements and flows for all of the users? What are the individual discharge pressures of the compressors? What causes the demand swings? What kind of plant?

not trying to set you back as you may have already reviewed these things, but I am curious and starting from the beginning is easier than trying to make assumptions on your assumptions.

 

[wk390107]

It is a polymer production plant. It is not looped, and would be impractical to do so. There are some small <50 gallon reciever near some users, but the largest one does not have any. There are big swings during a certain process in which a large amount of air is used for about 1-2 seconds at header pressure. The minimum pressure that we try to keep the plant at is around 85psi. Major process upsets are possible in the centrifuge if pressure goes below 80psi.

 

[nfinit]

what is the cfm size of your 100hp? 440CFM doesn't seem like a very high required flow for a 100hp driver at the pressures you are suggesting. Are you using a desiccant air dryer? are the air compressor discharge pressures significantly different?