Compressed air pipe sizing calculation

So, you want to know the correct pipe size for your compressed air installation?

It’s easy, I’ll explain how.

I still see too many places where the pipe size of the compressed air system is too small. It’s either because the factory or workshop has grown over time, and the old system became too small (quite understandable), or they just installed a too small pipe to begin with!

What’s the problem with a too small compressed air pipe?

Pressure drop!

If too much air needs to pass a too small pipe, it will have trouble passing through this pipe. The result is a pressure drop between the beginning of the pipe and the end of the pipe.

Now, what’s the problem with pressure drop you ask?

Money!

If the pressure drop becomes too high, you will need to set your compressor on a higher setpoint. The higher the setpoint of your compressor, the more energy (and money) it will use.

Therefore, the pressure drop should be maximum 0,1! This means that the pressure at the point-of-use should be maximum 0.1 bar lower than the pressure at the compressor outlet. For example 6.9 bar at the point-of use and 7 bar at the compressor.

What influences pressure drop?

In short, every obstruction creates a pressure drop. The pipes themselves of course, but also bends in the pipe, couplings, flexible hoses, quick-connect coupling, they all create pressure drops.

And, the longer the pipe, the bigger the pressure drop will be.

The amount of air passing through the pipe is also a factor. The more air needs to pass through a pipe at once, the bigger the pressure drop. This also means, that when no air is used at all (at night, in the weekends), there is no pressure drop. That’s why you always need to measure the pressure drop at full air consumption (all machines/air tools running, worst case scenario).

In short, the information we need to calculate pressure drop are:

• Diameter of pipe
• Length of pipe
• Number of bends, couplings, etc
• Air flow through pipe

Air flow

To start, you need to know the air flow through your system. The easiest way to find out the (maximum) air flow, is too look at the specs of your compressor (look in the manual or search online).

There will always be one line that tells you the maximum output of the machine in liters/second, m3 per minute or hour, or cubic feet per minute (cfpm).

This is the maximum amount of air the compressor is able to pump out, at the rated pressure.

But be careful, there is one important thing to look out for…

l/s vs. Nl/s (or cfpm vs Scfpm).

The air flow that is stated in the compressor specs, is most of the time Nl/s (or S cfpm), which means “Normal liters per second” (or standard cubic feet per minute). It means that the values are given at standard or reference conditions, which are 1 bar, 20 degrees Celsius and 0% relative humidity.

Often, the flow is stated as FAD, which means “Free Air Delivery”, which means the same thing: calculated back to reference conditions (more or less atmospheric air, like you and me breathe).

So in fact, the FAD (Normal liters per second, or Scfpm), is actually the amount of air that is sucked in by the compressor per minute.

It is compressed, and then transported through the piping system. So at 7 bar pressure, the liters per minute (without the ‘normal’ ) is about 8 (7 bar relative is 8 bar absolute) times smaller compared to the normal liters per second.

This difference is so often overlooked; most people don’t know about it and use the wrong terminology (even in compressor specifications sometimes!).

Compressed air pipe size table

Now instead of giving you complicated formulas to calculate the pressure drop, here is a simple table that will answer all your pipe sizing questions.

Look up your compressors maximum flow rate in the left column. Now, measure or calculate the total length of your compressed air pipes and look it up in the top row.

Now you can read the correct pipe size (in mm diameter) in the table.

This table is for 7 bars and maximum 0.3 bar pressure drop.

The value given is for a straight pipe without any bends, couplings or other restrictions. How to calculate the influence of those can be found in the next paragraph.

Table 1: Compressed air pipe sizing table (in millimeters).

Influence of bends, couplings and other stuff to pressure drop

As said before, bends, couplings and other kinds of restrictions will increase the pressure drop.

A pipe with one bend in it will have a greater pressure drop compared to a pipe with no bend. A pipe with a bend and a coupling will have an even greater pressure drop.

Now, I could give you all sorts of difficult formulas, but I know an easier way.

Below is a table to lookup what is called the ‘equivalent pipe length‘ for a generated pressure drop. It is simply a way to express the pressure drop for a certain bend or coupling will create, but not in bars (or psi) but in ‘virtual’ added pipe length.

Simply add extra ‘virtual’ meters of pipe to your pressure drop calculation (table 1 above) for every bend or valve in your system.

Equivalent pipe length table

Below (table 2) is the equivalent pipe length table. The value depends on the pipe diameter. A valve in a small diameter pipe will have a different influence compared to a valve in a big diameter pipe.

To find out the equivalent pipe length for the valve or bend in your system, simply look under the pipe diameter of your compressed air system to find the equivalent pipe length of the valve or bend.

Table 2. Equivalent pipe length table (values in meters).

For example a knee-bend in a 25mm pipe has an equivalent pipe length of 1.5 meters. This means that this knee-bend will create the same pressure drop as 1.5 meters of straight pipe.

Example calculation of required pipe diameter.

Here’s an example calculation using the compressed air pipe sizing table (table 1) and the equivalent pipe length table (table 2).

Let’s say we have a rotary screw compressor of 30 kW that can supply 250 Nm3/hour (normal cubic meters per hour). 250 Nm3/hour is the same as 4200 Nl/min (normal liter per minute) or 150 scfpm (standard cubic feet per minute).

We think that a 40mm diameter pipe should be ok, be we want to be sure by using the above tables.

Let’s say we have 20 meters of pipe of, with a 90 degrees bend (R = 2d, which means the radius of the bend is 2 times the diameter of the pipe) and a check valve, and then again 4 meters pipe.

The equivalent pipe length for this kind of bend is 0.25 meters. The equivalent pipe length for a check valve is 10 meters.

Our total meters now become: 20 + 0.25 +10 + 4 =34.25 meters.

Now we can look up the required pipe diameter in table 1 (above), with a pipe length of 34.25 meters. Looking in table 1 at 34.25 meters (which isn’t listed, but we’ll take the next value) and 250 Nm3/hour, we get 40 mm pipe diameter.

Of course, one bend or coupling doesn’t change the pressure drop much. But with a large system with many bends, valves and couplings, the pressure drop adds up quickly.

For a new system, if you’re not sure how many bends, couplings and other stuff will be installed in the system, multiply the estimated meters by 1.7 for the pressure drop calculation. This is a basic rule of thumb.