Intelligent Compressed Air: Double-Acting Reciprocating Compressor

Evaluating all of the different types of compressors and which is right for you can seem like a daunting task. Today, I’d like to take some time to talk about the Double-Acting Reciprocating type of air compressor.

double acting compressor
Cut-out of a double-acting reciprocating compressor

Double-Acting Reciprocating compressors are a subset of the larger family of positive displacement compressor types. In positive displacement compressors, air is drawn into a chamber where the volume is then mechanically reduced. The energy used to displace the air volume is converted to an increase in air pressure. Dynamic compressors operate a little differently. They utilize an increase in air velocity to create the change in pressure. Air is accelerated to a high velocity through an impeller. The kinetic energy of the air is converted to an increase in potential (pressure) energy.

The Double-Acting Reciprocating compressor is a close relative to the Single-Acting Reciprocating compressor. In these types of compressors, an “automotive-type” piston driven by a crankshaft provides the compression. In a Double-Acting Reciprocating compressor, air is compressed as the piston moves in each direction. Hence the name, “double-acting”. In a Single-Acting Reciprocating compressor, air is only compressed on each full revolution of the piston. This makes the Double-Acting Reciprocating compressor much more efficient than its brethren.

Double Acting Recip
Double Acting Reciprocating Air Compressor

Double-Acting Reciprocating compressors are also available in much larger sizes. While Single-Acting compressors can be found up to 150HP, generally they’re much less common any larger than 25HP. Whereas a Double-Acting compressor is available from 10HP-1,000HP, making it a better choice for larger plants that require a significantly greater volume of compressed air. While they’re a bit more expensive due to the added mechanisms to produce the double-action compression, this cost is quickly offset by the increase in efficiency. At a performance of 15-16 kW/100 cfm, they’re 32% more efficient than a single-acting reciprocating compressor.

If you’re in the market for a new compressor and are struggling to determine the most suitable compressor, talk with your local compressor sales representative. Once you’re up an running, EXAIR has a wide-range of products that’ll make sure you’re using your compressed air safely and efficiently!

Tyler Daniel
Application Engineer
E-mail: TylerDaniel@exair.com
Twitter: @EXAIR_TD
Image courtesy of Best Practices for Compressed Air Systems – second edition

 

About Double Acting Reciprocating Air Compressors

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.

Compressor Types

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 Reciprocating Air Compressor

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