Reciprocating Compressors

Once the governor or pressure control device senses that the air in the receiving tank has reached the high pressure cut-out threshold, it signals the compressor to unload. Unloading may be full or partial, depending on the reciprocating compressor design.

As down-stream machinery uses the newly compressed air, the pressure level of the tank will gradually reduce. Once it falls to the pre-determined load point, the control device signals the compressor to re-start the compression cycle and build the tank pressure up once again.

One of the important reciprocating compressor basics to be aware of is the duty cycle. The duty cycle is determined by taking the time the compressor spends loaded and comparing it to the time the machine runs while completely unloaded or turned off. Reciprocating compressors are only designed for a 20% to 30% full-load time and should be unloaded the rest of the time.

Ensuring your compressor is operating within duty cycle limits is essential to maximizing its service life. Choosing an undersized compressor for your application or artificially increasing the load by ignoring air leakage will push the system beyond its capabilities and result in costly premature wear and tear in multiple components of the compressor.

Not every reciprocating compressor design works this way, but it is possible for the drive engine pump to share some of its lubrication with the compressor. In this design, the supply from the sump is essential to keeping the whole system lubricated and functioning properly.

In this configuration, you would need to alter the lubricant change-out intervals recommended by the engine supplier, as they would not take into account the extra needs of the compressor. The new schedule must consider the heat load that the compressor adds to the lubricant if you want to get an accurate gauge of the lubricant’s lowered life expectancy.

For most air compressors powered by engines, the primary cooling source for the system is the lubricant. The engine oil cooler cools this lubricant, which is then recycled through the compressor. Airflow from the fan in the engine oil cooler can also eliminate a small portion of the heat the compressor body releases, removing it from the system along with discharged exhaust air.

The cooling methods in a reciprocating compressor are crucial to the longevity of the equipment. Without them in place, the danger of exceeding temperature limitations is much greater if your application surpasses the recommended duty cycle.

Whenever lubricant becomes too hot and the temperature exceeds manufacturer recommendation, the lubricant could break down prematurely. This means having to change the lubrication more often at best, and early failure of components in the compressor and engine at worst.

A double-acting reciprocating compressor has discharge and inlet valves on either end of the cylinder, resulting in two compression cycles with each turn the crankshaft completes.

This design makes double-acting reciprocators incredibly efficient, which is why they are so prominent in the manufacturing industry. It’s rare to find a double-acting compressor under 100 horsepower. However, their power comes with an extremely large footprint that isn’t always practical when space considerations are significant. They also tend to produce a lot of vibration, which requires vibration isolation and making sure the system is set on a solid enough foundation.

A two-stage compressor works very similarly to a single-stage system, but the compressed air does not make its way to a storage tank after its initial compression. Instead, it goes through a second, smaller piston to be compressed again with another stroke. Only after the second compression is doubly-pressurized air put through the cooling system and sent to the storage tank.

A two-stage compressor will produce significantly more air power, which is why they are more commonly found in large industrial applications where continuous operation is necessary. Accordingly, more power equates to the greater up-front cost and continued maintenance needs, so you won’t find them in settings where operations can function on less power.

Single-stage compressors are most commonly found powering handheld pneumatic tools that don’t require more than 100 pounds per square inch (psi) of pressure.

When it comes to your compressor’s speed, you have two options — separable and integral. Beyond the difference in operating speed, a compressor’s function, convenience and cost depend on its type. Whether you have a separable or integral compressor also determines how the machine is designed and how often you can move it.

Separable reciprocating air compressors are high-speed machines, often working between 900 rpm and 1,800 rpm. These compressors get their name due to their design. The compressor and its driver are separate parts, allowing the use of large motors or engines to increase speed. This self-contained modular design makes separable compressors much easier to install and move. While the upfront costs of ownership might seem more manageable, these machines generally need more maintenance than their slower counterparts.

The American Petroleum Institute (API) classifies integral reciprocating air compressors as low-speed. Generally, this type of compressor runs between 200 rpm and 600 rpm. They’re “integral” because the compressor is integrated into a frame that holds anywhere from two to 10 driving cylinders. While they have a higher installation cost and require more upfront work to establish them, integral compressors are highly efficient in their work and have longer life spans than separable units. In stable operations that aren’t constantly relocating, integral units are preferable.

Diaphragm compressors are also called membrane compressors. They use a rotating membrane to pull air into the compression system. A diaphragm reciprocating compressor makes use of two systems — a hydraulic system and an air pressure system with a flexible metal diaphragm acting as a protective barrier between them. The membrane and compressor box are the only components that come in contact with the pumped gas, making these compressors more commonly used to compress toxic or explosive gases rather than just air.

The heat produced during the operation of a compressor has a tangible effect on the durability and longevity of the equipment. A rotary screw compressor uses screws that rotate around each other without touching and therefore produces less friction. Reciprocating compressors, however, produce significant internal friction between the movement of the pistons and other internal components. Operating in this hotter environment is another reason reciprocating compressors have greater maintenance requirements.

Compared to rotary screw compressors, reciprocating compressors hold the advantage in terms of pressure produced. In many cases, a reciprocating compressor can create double the pressure of a rotary screw compressor. Centrifugal compressors can produce up to 1.25 times as much discharge pressure as a reciprocating compressor.

Noise is not always a factor, but in settings where workers need to communicate clearly, the noise of a double-acting reciprocating compressor can be distracting. A rotary screw compressor produces the least noise, while a centrifugal compressor produces the most. A single-acting reciprocating compressor is typically the best choice when weighing power needs against noise and efficiency.

The initial cost of a compressor is a major factor for most buyers. A reciprocating compressor offers more power for less cost upfront compared to rotary and centrifugal compressors. However, the service life expectancy of a reciprocating compressor is lower than either of its competitors. Both rotary and centrifugal compressors have fewer moving parts that suffer from friction, so they cost less in terms of part replacement, downtime, and repairs. However, with regular preventative maintenance, these costs can be somewhat mitigated.