Air Consumption Rate?

Doesn't seem like i can edit my post but I wanted to update this. The tank we are looking at is actually 240 gallons at 200 psi.

Our max requirement (accounting for future growth) is 200 cfm @ 90 psi. The system we are looking at is a 30 hp screw compressor that is rated for 125 cfm @ 125 psi. Doing the calculation and reducing psi downward to 90 psi I get 173 cfm @ 90 psi.

However, I don't know how the tank factors into this, would the tank allow us to meet this occasional requirement?

Wow, that's not even close to the first posted requirements.....anyway...Your compressor should be able to supply between 1.25 and 1.50 of your max cfm requirement, so if you want 200 cfm @90 psi, your compressor should be sized to provide more than that, not less....the tank size of 240 gal seems ok...Depending on the use of the compressed air you may want to consider various driers and filters...

Not quite sure what you're trying to do, but a few comments.

Are your air flow figures at standard conditions (scfm)? That's how air flow is usually expressed, but just to check.

You appear to have calculated flow at 90 psi from Boyle's Law as 125cfm*125psi/90psi = 173cfm. But that requires pressure in absolute units, and I'd bet yours are psig. Add 14.7psi to get psia. But that only works for flow in actual cfm, ie at the actual pressure and temperature. In scfm, compressor output usually reduces with increasing discharge pressure, but not because of Boyle, it's because there is more slip and effect of any dead volume is greater at the higher pressure.

Obviously the compressor output has to be > average air demand. You can perhaps use a tank to provide higher flow for short times, but you need to think it through. Years ago it was normal to start/stop the compressor on tank pressure, and to size the tank to give acceptable start frequency (say 6 - 10 per hour depending on supplier) with a selected start/stop ΔP. Nowadays it often unloads/reloads, acceptable frequency much higher, and stop the motor if unloaded for a set time.

What do you want the cycle time to be, give or take??

Either tank will work, but with a variable output the cycle time will be in a range, from days to minutes...The larger the tank the more time between cycles, I would go big, 80 gal....

Yeah, I had the numbers wrong then realized I couldn't edit the numbers. As for the cycle time I am honestly not sure, I don't care if it runs the entire time we are using the machines, however i would assume this causes unnecessary wear from start/stop cycles but I am not sure.

Are there advantages to having a tank and having fewer start stop cycles?

What is the best place online to buy compressors? I found this one but not sure of whether it is a good buy: http://www.compressorworld.com/30-hp-rotary-screw-air-compressors-complete-air-system-lifetime-airend-warranty.html

You need to look hard at what the price gets you. There may be at lot of extra that need to be added on. In comparing a 50 hp VSD found that was standard on one company was extras on others. Went with Kaeser. What sold me is their sump size. Which helps It to run cooler. Heat is a factor on wear and tear on equipment.

I'd contact three suppliers and get proposals from them.

No sense in asking non-experts who have no stake in the outcome.

Or,

Compressed Air Basics Air Compressor Sizing Part 2

N2102 Receiving Surge Tank Sizing According to PSA ...

There are about 7.48 gallons in a cubic foot. Therefore, a 60-gallon tank will contain roughly eight cubic feet; a 40-gallon tank will contain about 5.35 cubic feet; a 200-gallon tank will contain about 26.74 cubic feet (about 25 seconds' worth at 64 CFM); and so on.

A twenty-gallon tank will contain about 2.67 cubic feet; this will not make a great deal of difference in your compressor's duty cycle, even considering that the air is supplied at 125 psig and used at 80 psig. Ultimately, the ballast tank won't matter over the long run; the compressor will run until the need is satisfied. With a large tank, it will start a bit later, but it will run longer after the demand stops or lessens.

Water is incompressible. A gallon of air can be compressed.

So, a gallon isn't always a gallon.

True, but the air is being used at 80 psig, therefore, if the ballast tank is at 80 psig, the numbers "work".

The more air receiver (tank) storage capacity a system has, the less cycling of the air compressor occurs. (Within physical footprint size limitations and economic reasoning.)

The minimum size of receiver/tank I would use in your application would be 80 gallon capacity however I would recommend a 100 gallon tank if you want to optimize the decrease in compressor cycling. (If size and budget are not a limitation and there is ample room for installation, I would step up to a 200 gallon receiver.)

To maximize compressor life and minimize maintenance I would run the system pressure at the upper end of the compressor range but allow at least 10-20 psi of headroom which in your case would be 100-105 psi.

With the "storage" tank, How far will your equipment be from the compressor?

If your equipment is spread out over a large factory, then you should get a few more storage tanks to be placed at different areas in the factory so you don't starve a piece of equipment that may take a lot of CFM and is the furthest away from the compressor. Or you could place smaller tanks close to the equipment?

It all depends on your requirements! Maybe talk to a few compressor companies and they can size the compressor and tanks for you?

Some good thoughts here and there, and I would like to add my $0.02 worth.

One of the first questions I would ask is whether your new compressor is a piston type reciprocating or a screw type, one of the two most common. If it is a reciprocating type it needs time to cool between compression cycles, using a basic rule of thumb, 1 minute of compression time requires 4 minutes at idle. This is an important factor when it comes to sizing a receiver.

A screw type does not require this down time and can run continuously, and its flow can be modulated using a VFD or a few other tricks.

A very basic comparison between screw and reciprocating is this. Reciprocating compressors can require annual valve maintenance as well as other PM work, and they do require idle or downtime between compression cycles. On the hand they can last for many years and are proven reliable. Screw type compressors can be quieter and perhaps more efficient, but they also require oil changes and separator maintenance. They also tend to carry more oil over into the plant system due to separator issues. I believe general maintenance costs are lower, but if and/or when the air end crashes it can get pricey!

I am including three basic formulas for sizing a receiver, based on the situation at hand. I will leave it to you to fill in the blanks.

Definition of terms:

V = Receiver size in cubic feet (Ft3)

Q = compressor output in cubic feet per minute (CFM)

Pa = 14.7 PSI, atmospheric pressure

P1 = Compressed air pressure in PSI, (PSIG)

General sizing, constant demand at constant pressure:

V=(Q * Pa)/(P1 * Pa) It is recommended that this result be multiplied by a fudge factor of 1.5 to 3 for sufficient capacity.

Scenario #2:

Sizing a receiver for variable demand, where usage exceeds compressor output

Definition of terms:

V = Receiver size in cubic feet (Ft3)

t = times in seconds, to charge or discharge

Qout = air usage in cubic feet per minute (CFM)

Qin = Air flow into the receiver (compressor output)

Pa = 14.7 PSI, atmospheric pressure

P1 = Initial or maximum working pressure in PSI, (PSIG)

P2 = Final or minimum working pressure in PSI, (PSIG)

V = (t * (Qout - Qin) x Pa) / (P1 - P2)

Lastly you calculate the recharge time on a receiver using the following:

t = (V * (p1 - P2)) / (Qout -Qin) * 14.7

Hopefully these will allow you to make an informed decision regarding the size of the receiver. Ultimately, as everyone else has stated, a good compressor house can help you choose the right compressor, receiver, dryer and any other accessories you need, as well as giving good advice on airline layout in the shop. I would start but that is a topic for another thread, or at least a different reply.

Have a great day!