Ask Away! with Jeff Smith: How to Upgrade the Capacity of Your Existing Shop Compressor

This is a little different question but maybe you can help me with it. I have a small oil-free, 115-volt air compressor with a 20 gallon tank. It will make 150 psi but because the tank is small, it can’t keep up with some of my air tools. I was thinking of relocating it outside my garage under an awning. Moving the compressor to a convenient spot means adding about 30 feet of pipe from the compressor to the shop. If I use 1½-inch PVC pipe from the compressor to the shop and then neck it down to ¾-inch inside the shop, this should increase the capacity of the compressor, but I’m not sure how much. How would I figure this out?

K.S.

Jeff Smith: First we’ll calculate the increase in-tank volume and then we’ll take a look at the PVC pipe. Calculating the volume in the large pipe is the same as computing the volume of a engine’s cylinder. The formula we learned in high school geometry class is to multiply the area of a circle by the length of the cylinder. So first let’s calculate the area of a circle with the formula Pi x radius x radius or Pi R squared. Pi, as we all know, is represented by the number 3.1417. So if we have a 1½-inch diameter pipe, then the formula is:

 3.1417 x 0.75 x 0.75 = 1.76 square inches

Now we have to multiply this times the length of the pipe (or cylinder) in inches to find the volume of the pipe in cubic inches.

Area of bore x length of pipe = Cu. In. Volume

First we need to convert the length in feet to inches: 30 feet x 12 inches = 360 inches

1.76 inches x 360 = 636 cubic inches.

We now want to compare that number to the volume of the air compressor which we know is 20 gallons. Air compressors are rated in wet gallons instead of cubic inches or cubic feet because the volume of large cylinders or tanks is most commonly rated in gallons of liquid.

We had to research this one, but one U.S. gallon equals 231 cubic inches. If we divide 1 by 231, the decimal equivalent is 0.0043 gallons for one cubic inch. So now we multiply our 636 cubic inches times 0.0043 = 2.74 gallons.

So with an air compressor with a 20 gallon capacity, adding 30 feet of 1 ½-inch pipe will increase the capacity by only 14 percent. This isn’t a huge increase, but it will help. Anytime you can increase the capacity of the system, it will take the air compressor longer to build pressure the first time, but after that, the compressor won’t run as often to maintain the pressure. Obviously, if you doubled the capacity from 20 to 40 gallons (like buying a used compressor tank for additional storage) this would really help. But you are certainly on the right track.

But now let’s talk about your choice of PVC pipe. You won’t find any place that supplies compressor parts and accessories that will recommend PVC pipe. But let’s look at why. First, the burst pressure of a Schedule 40 1½-inch PVC pipe is 330 psi at 73 degrees F. The temperature rating is important because the biggest concern is heated air from the compressor reducing the burst pressure of the tubing. We looked that up and at 100 degrees F demands multiplying the 73-degree burst pressure by 0.62. This lowers the burst pressure to 204 psi.

Clearly, running a 175 psi compressor with Schedule 40 PVC very close to the compressor where the air will be well over 100 degrees F is a bad idea. I know that all of us can point to many shops that run PVC pipe for compressed air and never seem to have a problem. So rather than just say you shouldn’t run it, how about this for a suggestion: If you are going to run PVC line, such as along a wall running into your shop, protect it from sunlight and mechanical damage. Protecting it means covering the PVC with a durable covering that would prevent damage in case of a bursting pipe.

Here’s why protecting the PVC tubing it is a good idea. PVC fails by splintering – especially when subjected to high pressure. So imagine you drop a heavy object like an iron cylinder head on the line where it is plumbed along the work bench. The head hits the PVC and the pipe explodes. PVC splinters – creating jagged projectiles just like a plastic grenade. These shards will be sharp – just like metal shrapnel from a grenade. Then, when you end up at the hospital, they will inform you that plastic doesn’t show up on an x-ray. So now the doctor has to dig around in your face or chest to dig out all the pieces. It’s too scary to even think about those sharp pieces entering your eye. Have we sufficiently scared you yet?

By the time the PVC pipe is fully protected and shielded, you’ve probably spent more money than it would cost to at least plumb your shop with aluminum or copper piping. When I built my shop, I spent the money on copper tubing even though it was going to be routed through the wall studs and covered with dry wall. Yes, it was more expensive, but I think it was a good plan and worth the added expense. The burst pressure of Schedule 5 (0.058-inch wall thickness) 1½-inch 6063 T832 aluminum tubing is a very safe 2,110 psi! I also looked up 1½-inch copper tubing at 100 degrees F which is around 850 psi – still very safe. I then found that Summit actually sells this 1½ -inch diameter, 0.058-inch wall thickness aluminum tubing in 6-foot lengths for a mere $13.97. The expense might come in joining these sections properly but at least you have another option.

Another friend suggested using aluminum-lined PEX – which is plastic but with an aluminum lining that still allows it to be flexible and isn’t as expensive as copper. I found some for around $1.25 per foot but you had to buy 300 feet!

The other thing to consider is that these burst pressures are for a length of straight tubing but the greatest potential for a failure will be at the connection or junction. All of these are not just potential leak paths but they’re also where the failure will probably occur.

All of this is just to give you some things to think about to keep everyone from a trip to emergency room. Look at it this way, if you’re badly hurt because of an exploded air line, you won’t be able to work on your car for awhile. And that’s the real crime. Be safe out there, even if you have to spend a little more money to pull it off. It will be worth it.