A reciprocating compressor, also known as a piston compressor, is a positive displacement machine that uses a piston-driven crankshaft located inside a cylinder to pressurize air. This style of compressor has been widely used for over 200 years! Air compression makes materials (natural gas, hydrogen, oxygen, and other gasses) easier to store, transport and use. Reciprocating compressors are used where high compression ratios (rate of discharge to suction pressures) are required without high flow rates and the process fluid is relatively dry. They are better than other compressors (centrifugal, rotary) when it comes to handling wide capacity swings and generating a high discharge pressure.
Reciprocating compressors are used in the chemical, oil, and gas industries for reliably moving compressible fluids. You’ll find them in refineries, gas transmission pipelines, petrochemical plants and many other applications. Their popularity is in part due to their ability to attain high-pressure levels at a range of crankshaft speeds.
Different Designs of Reciprocating Compressors
Single-stage
With a single-stage, the air is drawn into the cylinder and compressed in a single stroke. There is one compression cycle for each time the crankshaft turns. The compressed liquid is then transferred to a storage tank. Single-stage reciprocating compressors are an affordable style of compressor.
Two-stage
A two-stage design employs an extra piston and is capable of higher pressure levels. There are two compression cycles for each turn the crankshaft completes, making this style of compressor extremely efficient. However, two-stage reciprocating compressors produce a great deal of vibration, requiring vibration isolation and a solid foundation.
Separable
Separable reciprocating compressors are high-speed (between 900 rpm and 1,800 rpm). The compressor and the driver are separate components, enabling the use of large motors/engines to increase speed. They’re easy to install and move, when necessary. However, they often require more maintenance than slower compressors.
Integral
Integral reciprocating compressors are slow speed in comparison (they run between 200 rpm and 600 rpm). The compressor is integrated into a frame, holding from two to ten cylinders. They have a high installation cost, require a great deal of upfront effort to establish but are highly efficient and have a longer lifespan than separable units. This style of compressor is useful for stable operations when relocating is not needed.
Diaphragm Reciprocating Compressors
Also known as membrane compressors, these compressors use a rotating membrane to pull air into the system. They utilize two techniques (hydraulic and air pressure) with a flexible metal diaphragm that acts as a protective barrier. This type of compressor is often used for toxic and/or explosive gases.
There are a number of styles/models of reciprocating compressors, all of which increase density while decreasing volume. However, these compressors are sensitive to dirt, requiring chemical cleaning.
How Does a Reciprocating Compressor Work?
Reciprocating compressors generally draw power from a diesel engine through a direct drive or a belt drive system. Sometimes a governor (the device that automatically maintains the rotary speed of an engine within close limits regardless of the load) is mounted directly to the compressor where it works to enforce specific intake and cut-out pressure. There are a number of stages to the workings of a reciprocating compressor.
- Intake cycle: In the first stage of compression, a vacuum is created by the movement of the piston causing air to enter the cylinder through a suction/inlet valve, which controls flow into and out of each cylinder. The piston then changes direction, the inlet valve closes and the air is compressed. A closed discharge valve serves to block air from moving back into the low-pressure part of the cylinder after it has been pressurized. When the interior pressure in the cylinder exceeds the pressure in the discharge pipes, valves open allowing air out of the cylinder. Each cylinder of the compressor will discharge the same volume, at the same discharge pressure each time it completes the discharge stroke. Air exiting the discharge port cycles through an air receiver tank to smooth out the low-frequency pulsation it acquires during compression. As the rotation speed of the compressor changes, so does the discharge volume.
- Unloading process: Once the air in the receiving tank has reached the high-pressure cut-out threshold, it signals the compressor to unload, which may be full or partial depending on the design of the compressor. As the compressed air is used, the pressure level of the tank gradually reduces. When it reaches the predetermined load point, the control device signals the compressor to restart and the compression cycle builds the tank pressure again.
- The duty cycle is determined by comparing the time the compressor spends loaded compared to the time the equipment runs while completely unloaded/turned off. Reciprocating compressors are designed for a 20 to 30% load time, ensuring they are unloaded for 70 to 80% of the time. To maximize the life of your compressor, ensure it’s operating within duty cycle limits. Never use an undersized compressor or artificially increase the load as it pushes the system beyond its capabilities and can result in premature wear of components.
- Cooling: Lubricant is the primary cooling source for the compression system. An engine oil cooler chills the lubricant which is then passed through the compressor. Cooling is essential to the longevity of a reciprocating compressor, reducing the danger of exceeding temperature limitations.
There are various types/models of reciprocating compressors suitable for different needs and purposes but all have similar stages of operation.
The Advantages of Reciprocating Compressors
Reciprocating compressors have many advantages over other models/styles.
- They’re more tolerant of changes in gas composition
- Their machinery is less complicated
- They’re cheaper to manufacture
- Their technology is well-known and understood
- Finding maintenance support is easy
- Spare parts are abundant and affordable
- A wide range of sizes are available for different applications
- They have a relatively high maximum discharge pressure
There are numerous applications for which a reciprocating compressor is the ideal choice.
What is the Clearance Volume of a Reciprocating Compressor?
Clearance volume is the space remaining in the compressor cylinder at the end of the stroke. It’s made up of spaces in valve recesses and the space between the piston and cylinder. Clearance volume accommodates the valves in the head of the compressor, gives mechanical freedom to the moving parts, reduces noise/vibration and prevents damage to the moving components. Depending upon the type of compressor, clearance volume varies from 1.5% to 35%. In a reciprocating compressor, the clearance volume is generally between 4% and 10%. To determine the clearance volume of a reciprocating compressor use the following formula:
Clearance ratio equals: Clearance volume / Swept volume
Things to Consider When Purchasing a Reciprocating Compressor
Before purchasing a reciprocating compressor consider the following issues:
- Do you need to produce clean air, dry air or oil-free air?
How many hours will you be running the compressor?
Will the compressor be running constantly or will there be shifts?
Does your demand fluctuate throughout the day?
Will you be expanding/will requirements change in the near future?
- What are the operating conditions (present and future)?
- What skills are available for maintenance?
- How familiar is your staff with the machine?
- Do you require automation and/or digital controls?
- Are parts readily available?
- How long is the delivery time for a unit?
- What is the price and the life-cycle cost?
The most widely used style of compressor, reciprocating compressors come in a variety of models and provide long-term, effective compression on demand. They’re a crucial part of many processes. As technology and materials improve, reciprocating compressor design is constantly being refined.
Interested in a reciprocating compressor unit for your facility? Contact Gaspro Compression Corp. We have a focused team that takes pride in their work, striving to provide dependable products and prompt service to all clients. We’re continually improving and innovating our products to meet changing industry demands. Gaspro is a leading provider of compressor packages in Canada. Contact us to request a proposal. Call 403-443-8886 or email us at clint.praski@gaspro.ca
