Chris_Hamilton
Trying to be the best me, I can be
Lots of folks ask what gun should I get or what compressor should I buy. Many ask how much CFM do I need but I never see people asking what I think is the more important question which is, what size air piping should I have and how long should it be? Incorrectly sized piping or not enough piping between the compressor and your filters/regulator air the cause of more problems than folks realize.
First let's talk about the relationship between CFM and air pressure. They are not the same. Although this is an oversimplification CFM for our purposes is cubic feet per minute. This is measured at atmosphere or 0 psi. (again an oversimplification atmosphere is 14.7 psi)
When you increase air pressure CFM decreases. An illustration of this is try breathing through a straw for a few minutes. You can do it but you need to breathe in very hard to get enough air (cfm) into your lungs. The speed that you draw the air into your lungs is very high (pressure). Now get a larger pipe, say 1 1/2". Try breathing through it for a few minutes. It is much easier to fill your lung (cfm) and the speed/velocity at which you draw and expel the air is much lower. (pressure)
This is what happens with an air compressor with incorrectly sized piping. You may have plenty of pressure but you have no volume(cfm) as the piping is to restrictive.
There was a post on here recently talking about how when he used a 1/2" air hose at the gun it changed how the gun sprayed and made it much better. No gun needs that much air so why did it do better when he used the 1/2 inch hose? The air piping in his system is bound to be too small for the CFM that he has. So attaching a 1/2" hose acted as a reservoir of sorts, allowing the air to slow down and the volume to increase enough to let the gun spray as it was intended.
So you say, what is the correct size? That depends on a couple of factors.
1. What size air compressor do you have?
2. How great a distance is the air compressor from where you want to place your filters and regulators.
First lets examine air compressor size and CFM versus piping size. This assumes that the rated output is the actual output. Here's a chart that sums it up well.
For the type of work we do you should consider 3/4 piping to be the minimum, especially considering you should have at least 50 feet between your air compressor and your filters/regulator. Never mount your filters close to your compressor as they cannot filter out water that hasn't condensed yet. If you are using a refrigerated dryer you can use less length but you should still consider 50 foot as the minimum because most dryers require the inlet air temp to be no more than 200 degrees Fahrenheit. Your dryer will work better and last longer the cooler the inlet air is. Here is a picture of ideally how you should set up your piping.
Notice how the outlets go up and away from the main line. Each outlet also has a drain line.
I imagine some of you are saying my shop is tiny how do I run 50 feet of line? There are several ways to do that. Easy way is to get a 3/4" coil of copper. You can buy coils in various lengths 25-50 foot would be ideal, attach at your air compressors outlet and then try and emulate the above pic for running your line to your filters and regulators. Or you can run the line up and down the wall until you have sufficient length. If this method is used you should mount a drain valve at the bottom of each line that is going up or down in order remove as much condensed water s possible.
With a well thought out system you will get maximum benefit from your filters, water will not be an issue, even without a dryer. You will get the maximum amount of CFM that your compressor makes available to you at your regulator.
Hope this helps, feel free to ask any questions and if I remember anything else that might be relevant I'll add to this.
First let's talk about the relationship between CFM and air pressure. They are not the same. Although this is an oversimplification CFM for our purposes is cubic feet per minute. This is measured at atmosphere or 0 psi. (again an oversimplification atmosphere is 14.7 psi)
When you increase air pressure CFM decreases. An illustration of this is try breathing through a straw for a few minutes. You can do it but you need to breathe in very hard to get enough air (cfm) into your lungs. The speed that you draw the air into your lungs is very high (pressure). Now get a larger pipe, say 1 1/2". Try breathing through it for a few minutes. It is much easier to fill your lung (cfm) and the speed/velocity at which you draw and expel the air is much lower. (pressure)
This is what happens with an air compressor with incorrectly sized piping. You may have plenty of pressure but you have no volume(cfm) as the piping is to restrictive.
There was a post on here recently talking about how when he used a 1/2" air hose at the gun it changed how the gun sprayed and made it much better. No gun needs that much air so why did it do better when he used the 1/2 inch hose? The air piping in his system is bound to be too small for the CFM that he has. So attaching a 1/2" hose acted as a reservoir of sorts, allowing the air to slow down and the volume to increase enough to let the gun spray as it was intended.
So you say, what is the correct size? That depends on a couple of factors.
1. What size air compressor do you have?
2. How great a distance is the air compressor from where you want to place your filters and regulators.
First lets examine air compressor size and CFM versus piping size. This assumes that the rated output is the actual output. Here's a chart that sums it up well.
For the type of work we do you should consider 3/4 piping to be the minimum, especially considering you should have at least 50 feet between your air compressor and your filters/regulator. Never mount your filters close to your compressor as they cannot filter out water that hasn't condensed yet. If you are using a refrigerated dryer you can use less length but you should still consider 50 foot as the minimum because most dryers require the inlet air temp to be no more than 200 degrees Fahrenheit. Your dryer will work better and last longer the cooler the inlet air is. Here is a picture of ideally how you should set up your piping.
Notice how the outlets go up and away from the main line. Each outlet also has a drain line.
I imagine some of you are saying my shop is tiny how do I run 50 feet of line? There are several ways to do that. Easy way is to get a 3/4" coil of copper. You can buy coils in various lengths 25-50 foot would be ideal, attach at your air compressors outlet and then try and emulate the above pic for running your line to your filters and regulators. Or you can run the line up and down the wall until you have sufficient length. If this method is used you should mount a drain valve at the bottom of each line that is going up or down in order remove as much condensed water s possible.
With a well thought out system you will get maximum benefit from your filters, water will not be an issue, even without a dryer. You will get the maximum amount of CFM that your compressor makes available to you at your regulator.
Hope this helps, feel free to ask any questions and if I remember anything else that might be relevant I'll add to this.