Quick Exhaust

Quick exhaust, a nifty little device that can speed up the cycle speed on an air cylinder where necessary.

On some equipment, the compressed air power valve is sometimes located quite a distance from the cylinder it is powering. This has negative ramifications for the speed of the air cylinder.

Distance Is The Issue

Whether the air cylinder is single or double acting, if the power valve for that air cylinder is too far away, even though the compressed air moves very quickly it still takes time for the air to flow from the power valve to the cylinder when the valve is shifted.

If it is a double acting cylinder, not only has the new air got to travel from the valve to the cylinder, but the air presently in the air cylinder has to travel back down the exhaust air line to atmosphere.

Since the cylinder exhaust air is being driven from the double acting air cylinder by the inrushing air pushing the cylinder piston to the opposite end, the out flowing air may not move as quickly as the incoming air, and this retards or slows the piston movement a bit, and reduces the cylinder cycle speed.

Quick Exhaust Improves Speed

Enter the quick exhaust.

Installing a quick exhaust at the cylinder ports will allow the air, exhausting from the cylinder, to exit the cylinder and the air line right at the cylinder port, rather than having to travel back down the air line to the valve exhaust.

In a single acting cylinder, it is the internal spring that is pushing the piston back to home position when the valve shifts, and air starts to bleed from the cylinder. It can benefit from a quick exhaust, as does the D.A cylinder. The moving piston has to force the compressed air out of the cylinder and back down the air line to the exhaust on the valve. With a quick exhaust installed in the single cylinder port, when it is time to exhaust the air from the cylinder, the air exits right there. The compressed air does not have to travel back to the valve.

How Quick Exhausts Work

The quick exhaust is nothing more than a vented check valve.
Quick Exhaust Schematic

When air is flowing into the air cylinder through the quick exhaust, the ball or flap inside the q.e. valve seats on the exhaust port preventing air escaping from there, and the compressed air has full flow access to the cylinder port to begin moving the air cylinder piston.

When the air pressure in the supply line that travels through the Q.E. to the air cylinder begins to lose pressure, the quick exhaust goes to work.

The power valve supplying compressed air to that line has shifted. That same air line is now open to the valve exhaust port. The pressure in the line starts to drop. As the line pressure drops, the ball or flap inside the Q.E. is shifted by the now higher pressure inside the cylinder, the path to the valve exhaust is closed and the path to the quick exhaust is now opened, and this allows the air in the cylinder to flow rapidly out to atmosphere right there at the cylinder port, rather than flowing all the way back down the line to the valve exhaust.

Your air cylinder can cycle much faster as a result.

Downside Of Q.E.

The downside of a Q.E. is that, if you are using an in-line flow control, or valve-exhaust flow controls, to control the speed of the air cylinder rod, the quick exhaust renders them useless, as the exhausting compressed air will never reach them.

With a functioning quick exhaust in place, the air cylinder piston will stroke very quickly, and that cylinder piston will impact the inside of the cylinder end caps during cycling. This might cause damage. It becomes necessary to reduce impact of the load / piston by stopping it externally with a tooling stop or an external shock absorber.

Quick Exhaust Options

Quick exhausts come in a variety of plumbing options, depending on the manufacturer.

Some come with instant fittings on the supply port. Some come with integral mufflers, others with a FNPT thread to screw in your own muffler.

Quick exhausts are sold by a huge number of manufacturers, some making theirs out of cast metal, others from composites, or a blend of both.

Your fluid power distributor will likely stock one or two brands of quick exhausts, with fittings to suit most applications.