Contrary to what many may think, compressed air is not cheap. It really is quite expensive to convert electrical energy into compressed air. Compressed air is one of the highest cost energy sources.
A single acting air cylinder only uses compressed air to move the piston and rod in one direction. The double acting air cylinder uses compressed air to both extend and retract the cylinder rod even if there is no load in one direction.
The compressed air ($$) savings on a high cycle cylinder application can be huge.
For example, a cylinder in a vertical - rod up - configuration, will use compressed air to extend the rod vertically. When the operation is complete, the valve shifts, and rather than using compressed air to retract the rod, the weight of the load on the rod is sufficient to retract the rod without use of compressed air. In this case, the single acting air cylinder operates without an internal spring as the load provides the force to retract the air cylinder.
In another example, a small stroke and bore single acting air cylinder is used to eject parts from a conveyor belt. Compressed air is used to drive the rod out to impact and, with sufficient force, eject the part. The spring inside the air cylinder is compressed as the rod extends under the force of the compressed air acting on the piston. Then, when the valve shifts, the air is exhausted from the back end, and now the spring has enough force to drive the piston back and retract the cylinder rod.
You can spot a factory-made single acting air cylinder, as it will have only one threaded air fitting port. One on the back end if it is a spring return, or one on the rod end if it is a spring extend single acting air cylinder.
In the other end of the cylinder barrel from the air fitting port will be a small hole. This allows the air that is in the non-ported end to exhaust to atmosphere as the piston moves toward that end. It also allows air back into the cylinder as the piston moves back. The hole is necessary and should not be blocked, Without it, the cylinder piston would alternately compress air in the blind end, or pull a vacuum as the cylinder cycled, reducing its effectiveness.
Since there is no spring inside a typical double acting cylinder, it can act like a singel acting cylinder if the load extends or retracts the rod when compressed air is not flowing to the cylinder. Typically, that will be with the load in a vertical orientation.
The air port on the end of the cylinder not being plumbed for compressed air is left open, or a filter/muffler can be installed to prevent dust from getting inside as the cylinder cycles.
You can get single acting air cylinders that have an NFPA footprint, typpically with a bore size from 1.5" and up. the link takes you to information about NFPA cylinders on this site.
Single acting air cylinders are either built to the standards of the North American manufacturer, or of European ISO-6432 standard. This link takes you to a page about ISO cylinders on this site.
Here are the things you will want to know to correctly size and select your single acting air cylinder:
Single acting cylinders are available with non-rotating rods, should your end-of-rod tooling need specific orientation. Non-rotating rods are necessary if tooling orientation is important, since in normal operation of an air cylinder with a round rod, over time, the piston and the rod will slowly and continuously rotate.
If you have a question about single acting air cylinders here is the cylinder forum on this website.