Thread: Help choosing an air compressor!

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I am going to start building pneumatic props and am interested in which air compressor I should get. I would rather spend the extra money now and get a good size compressor to run several different props,rather then finding out later that i need another compressor or i should have gotten a bigger one. I will want to probably run AT LEAST 3 props if not more off it. Preferably around 5 or so. Things such as a coffin jumper, trash can trauma, simple pop up, maybe a lynching or electric chair as well. Any suggestions would be greatly appreciated!

You need to maximise the CFM and size of reservoir, adding a reservoir at a later date is easy, but more CFM means another compressor.

CFM = cubic feet per minute.

to calculate the CFM you need to survey all the cylinders you have and estimate their volume (diameter*diameter* length is a good approximation, you can forget the 3.142/4 bit because it works out at a little below 1) and estimate the number of times per minute you activate that cylinder, to calculate its air consumption, in cubic feet per minute, don't forget to account for forward and backward strokes.

when you have summed up all your cylinders you get your total consumption.

if you have any free air blowers then you need to make an estimate, free air blowers are very hungry and you can quickly use a lot of CFM if you are not careful

Wow. Good information Phoenix.
Is it safe to say that a larger reservoir will supply more props more efficiently?

I have a 30 gallon compressor that I bought from Craigslist for $25, then had to invest $75 for a new auto-switch. The problem is that the compressor motor draws close to 30 amps on startup, so I have to keep it in close proximity to the power outlet in my garage; no extension cords. My garage is a couple of hundred feet from my haunt...so I need a lot of hose.

I've seen 20 gallon compressors at Harbor Freight that occasionally go on sale for around $100.

Originally Posted by Abunai

Wow. Good information Phoenix.
Is it safe to say that a larger reservoir will supply more props more efficiently?

I have a 30 gallon compressor that I bought from Craigslist for $25, then had to invest $75 for a new auto-switch. The problem is that the compressor motor draws close to 30 amps on startup, so I have to keep it in close proximity to the power outlet in my garage; no extension cords. My garage is a couple of hundred feet from my haunt...so I need a lot of hose.

I've seen 20 gallon compressors at Harbor Freight that occasionally go on sale for around $100.

wow, 30 amps is a little serious for a home haunt!

Do you know the CFM rating of the pumps. 20 gallons is just the amount of air it stores?, the larger the capacity the better the system is able to handle spikes in consumption. a large tank will allow a low CFM compressor drive some very meaty cylinders, but if you use the cylinders too often you will eventually drain the tank...... think of it like the old algebra question about a full bath with the taps on and the plug out, how long will it take to drain the bath.

Originally Posted by Phoenix_UK

wow, 30 amps is a little serious for a home haunt!

Do you know the CFM rating of the pumps. 20 gallons is just the amount of air it stores?, the larger the capacity the better the system is able to handle spikes in consumption. a large tank will allow a low CFM compressor drive some very meaty cylinders, but if you use the cylinders too often you will eventually drain the tank...... think of it like the old algebra question about a full bath with the taps on and the plug out, how long will it take to drain the bath.

Hey Phoenix, thanks for the info. I kind of understand what your talking about but since I havent built the pneumatic props I have no idea what kind of CFM im going to need. Harbor Freight has a 21 Gallon compressor on sale for 129.99. Originally 199.99. These are the stats that it has on it. Let me know if you think It would be good for a home haunt, supplying air for 3-5 props. Thanks!

Heavy duty 3 HP motor
120V 60 Hz
7.0 SCFM @ 40 PSI
4.9 SCFM @ 90 PSI
4.3 SCFM @ 115 PSI
Oil lubricated
CSA Listed

please note this is all theoretical figures and should only be used as a guideline

4.3 cfm is 121 liters per minute.... forgive me for converting to metric, but the math is easier

A 2" (50mm) cylinder with a 10" (250mm) mm stroke has a capacity of 0.625 litres (50*50*250/1000000), or 1.2 litres forward and back, which means you can theoretically activate it 100 times per minute before you suffer air starvation.

if you had 5 props each with 2 cylinders as above then you could activate each prop 10 times per minute......

.... however, watch out for the duty on the compressor, many compressors don't like being run constantly, so if you had a target duty cycle of 50% then your maximum activations for each prop would be 5 time per minute

Next watch out for air leaks, you could easily lose half your CFM by ignoring that hissing noise.

CFM at compressor is not an indication of CFM at the prop, often its a good idea to fit each prop with a local reservoir to ensure it has enough air to activate, the larger your supply lines from the compressor to the prop the better as this gives a better air supply to the prop and actually acts as a kind of resovoir as well.

I have a 7hp 30 gallon compressor and two 10 gallon accumulator tanks for this years haunt I want to minimize the amount of time the compressor is running. I am running four 2'' diameter 11'' stroke cylinders for different props and while testing them it seems to be adequate!

I use a Ingersol Rand SSL3 - 60 Gallon Single Stage 240Volt compressor

Heres a link to it...
http://www.aircompressorsdirect.com/...roducts_id=688

As a general rule you should never run long extension cords when using any sort of power tools or equipment. It creates more work for the tool or machine and can cause unexpected results!

If you do need your air from a location that is not so close more then 10' from a plug or source ihighly recommend increasing the length and size of your main air feed . Run a 1/2" or 3/8" air line to a splitter and then distribute your air from there. The cost of running 100' of the correct size electrical wire is far more expensive then running 100' of air line!

Since we are on the topic of compressors already....

I always recommend a good air dryer when using pneumatic clyinders to keep the moisture out of them.

As well emptying your tank with the relief valve at the bottom on as a DAILY occurance!
The bigger the tank the more the moisture forms inside which can rust the tank and lead to early tank failure causing ruptures of the tank!

Becareful tho, the water is normally laced with oil from the compressor! So don't do it inside the house on the carpet...wife mad...very mad...

Originally Posted by Crypt Keeper

As well emptying your tank with the relief valve at the bottom on as a DAILY occurance!

this made me laugh.... remembering the time I went and stripped a compressor down trying to figure why it wasn't reaching pressure.... boy did I feel stupid when I remembered the drain valve!

Can't say I haven't done that myself......

Thankfully smaller tanks these days have automatic returning valves on them