My colleague, Lee Evans, wrote a blog “About Single Acting Reciprocating Compressors”, and I wanted to extend that conversation to a more efficient relative, the double acting reciprocating compressor. As you see in the chart below, this type of compressor falls within the same family under the category of positive displacement compressors.
Positive displacement compressors increase air pressure by reducing air volume within a confined space. The reciprocating type of air compressor uses a motor that turns a crank which pushes a piston inside a cylinder; like the engine in your car. In a basic cycle, an intake valve opens to allow the ambient air into the cylinder, the gas gets trapped, and once it is compressed by the piston, the exhaust valve opens to discharge the compressed volume into a tank. This method of compression happens for both the single and double acting reciprocating compressors. With a single acting compressor, the air is compressed only on the up-stroke of the piston inside the cylinder. The double acting compressor compresses the air on both the up-stroke and the down-stroke of the piston, doubling the capacity of a given cylinder size. This “double” compression cycle is what makes this type of air compressor very efficient. A single acting compressor will have an operating efficiency between 22 – 24 kW/100 cfm of air while the double acting compressor has an operating efficiency between 15 – 16 kW/100 cfm. Therefore, electricity cost is less with a double-acting reciprocating air compressor to make the same amount of compressed air.
To explore the internals a bit closer, the mechanical linkage used to move the piston is slightly different as well as the additional intake and exhaust valves. Instead of the connecting rod being attached directly to the piston as seen inside a single acting compressor, a crosshead is added between the compression piston and the connecting rod (view picture below). The rod that connects the crosshead to the compression piston can be sealed to keep the cylinder completely encapsulated. For every rotation of the electric motor, the air is being compressed twice. With the added heat of compression, the double acting compressors are generally water-cooled. Also, with the added mechanism between the crank and the piston, the rotational speeds are typically less. Because of the larger size, water jackets, and added parts, the initial cost is more expensive than the single acting compressor, but the efficiency is much higher.
Double acting compressors are generally designed for rugged 100% continuous operations. Dubbed the work horse of the compressor family, they are also known for their long service life. They are commonly used in high pressure services in multistage styles and can come in lubricated and non-lubricated configurations. With the dual compression, slow speed and inter-cooling, it makes this type of air compressor very proficient in making compressed air.
John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb
Photos: used from Compressed Air Challenge Handbook