There are sometimes good reasons for connecting two air compressors.
A single air compressor may not offer enough flow to supply a higher demand air tool.
Another, single air compressor, can have the same problem with that same air tool. But if you connect two air compressors into one line to the air tool, you may have enough air flow from the two air compressors to use that higher demand air tool.
How Compressors Run
A quick review of how compressors work (also covered in detail on a page on this site) is that they have an air tank into which air is pumped. When the tank pressure reaches the cut out setting of the pressure switch, the pump stops, and nothing else happens until you begin to use the compressed air that’s stored in the compressor tank.
As you use air, the tank air pressure drops. When the tank pressure drops to the pressure switch cut in level, the pump will start, and add more air to the tank until the tank reaches the cut out pressure setting, and the pump stops.
The Compressor Pump May Not Stop
However, if your air tool has a greater demand for compressed air than the compressor can deliver you will use air in the tool at a greater rate than the compressor can supply it, and even though the compressor is running, since it cannot keep up with the demand of the air tool, the tank pressure will keep dropping until there is not enough compressed air to run the air tool.
To use a higher demand air tool you will need a bigger compressor. Or, connecting two air compressors to the same air line to the tool, if you have a couple of small air compressors around, might work.
Connecting Two Air Compressors
If you have two air compressors, select which will be the primary air compressor and plumb a tee into the discharge coupler on that one.
One leg of that tee is the supply from the primary air compressor, one leg of this tee is the supply from the secondary air compressor, and the last leg is where you install the air line to your air tools.
If we assume that both air compressor tanks are full of compressed air, when you start using your air tool, compressed air will be drawn from both tanks… at least, in theory.
The reality is, compressed air always flows through the easiest path from high pressure to low. As air is drawn from the two compressor tanks to the air tool, compressed air from one tank will flow into the other tank and vice-versa, rather than all of it flowing down the air line to the tool.
By installing a check valve in the line between the secondary air compressor and the tee of the primary compressor, this will prevent air from the main tank from flowing back into the secondary, improving the efficiency of the air flow.
Once the pressure in the hose to the air tool drops due to air use, air will be drawn from the secondary tank.
However, once again, air from the secondary tank may flow back into the tank of the primary air compressor rather than down the line to the air tool.
Solution? Another non-passing valve before the tee in the discharge coupler of the primary compressor, allowing air out of the tank, but not back into it from the other compressor.
The image above is of a simple, in-line, check valve for compressed air. It’s also known as a non-passing valve. If you Google air check valve you will find many sources with a variety of attachment options.
With the male thread on either end of the check valve in the image, installation is not complex. Note the arrow on the body. That shows the direction that the compressed air can flow through this valve. Take a care not to install it backwards!