What is the advantage of multi staging a compressor compared to just a single stage compressor?

The good news about standard one-stage compressors is that they deliver 100-percent output. The bad news is they do it all the time. Most of the time, your home’s cooling load doesn’t require full compressor output, and it’s a waste of energy. Multistage compressors run nearly continuously at approximately 80-percent output in most conditions, automatically escalating to full output only when acute temperatures demand it.

Energy Savings

1. A compressor operating at lower output consumes less electricity. In addition, multistage compressors run much longer “on” cycles than one-stage units, which cycle on and off more frequently. This also contributes to increased efficiency because single-stage compressors draw excess electricity each time they must restart after cycling off. Fewer “off” cycles and restarts means reduced energy consumption.

Performance Gains

1. More continuous operation at lower output results in increased interior comfort. Temperature spikes in living spaces often occur during the “off” cycles of standard one-stage compressors. Because multistage units operate most of the time, these temperature gains are minimized, and climate control is more consistent. Humidity extraction, a critical comfort function of air conditioners, also is optimized by continuous operation at 80-percent output versus the frequent on/off cycles of full output, single-stage compressors.

As noted in the compressor descriptions, most compressor designs are available in single or multiple stage designs.  When air is compressed, its temperature increases.  Compressing in multiple stages allows cooling to occur between the stages, which saves work in the compression process.

Air compressor performance typically approximates isentropic compression, allowing an increase in the air temperature over the course of the compression process. This is approximated in the curve below.  As we reduce volume from V1 to V2, pressure increases from P1 to P2. The blue shaded area represents the amount of work done on the air to change from P1/V1 to P2/V2.

However, in a two-stage compressor, air is partially compressed using isentropic compression, and then cooled in a heat exchanger, called an intercooler.  This brings the compression process closer to isothermal (constant temperature) compression, which is more efficient. The blue shaded area below represents the amount of work done to change the air from P1/V1 to P2/V2 in two stages, with an intercooler. The green shaded area represents the savings achieved by two stage compression versus single stage.

We can see that there is a distinct efficiency advantage to multiple stages when intercoolers are used. Without intercooling, however, the savings would not exist, and the efficiency would match a single stage machine.

Multi-Stage Oil Free and Lubricated Reciprocating Compressors

Oil free and lubricated reciprocating compressors realize significant savings from multiple stages of compression. They have two to three stages of compression for typical plant pressures of 90 to 125 PSIG. Reciprocating compressors may go to 5 or more stages to reach thousands of PSIG operating pressures.

Two Stage Oil Flooded Screw Compressors

Some manufacturers offer oil flooded rotary screws in two-stage arrangements. However, due to oil flooded nature of the machine and the risk of moisture condensation, intercooling is very limited. Every compressor brings in some moisture with the intake air. Higher ambient temperature and relative humidity means more water vapor brought into the compressor.

Air can hold less moisture as pressure increases, but more moisture as temperature increases. For this reason, oil flooded compressors are thermostatically controlled to maintain operating temperatures high enough to limit condensation in the compressed air. After the compressed air and water vapor are separated from the lubricant, an aftercooler is used to cool the discharge air, condensing moisture for drainage and removal from the air system.

If true intercooling (back to ambient temperature) was used in an oil flooded compressor, water would condense in the compressor and mix with the oil, providing reliability problems. Therefore, partial “intercooling” is performed by introducing a spray of slightly cooled oil between the stages. The cooling effect is minimal to prevent condensation.  Since the cooling is minimal, the advantages of the two stage compressor are minimal as well. Efficiency is improved only marginally, and the expense and complexity of the compressor are increased by nearly doubling the number of moving parts.

Continue to Compressor Controls

That’s where multi-stage compressors come in. By increasing the number of cylinder stages, these machines work more effectively and can handle more tools at once.

How Multi-stage Compressors Work

Multi-stage compressors feature a series of cylinders, each of a different diameter. Between each compression stage, the air passes through a heat exchanger, where it is cooled. Cooling the air reduces the amount of work necessary to compress it further.

In a two-stage compressor, air is then forced into an additional chamber where it is pressurized to the required extent. In a three-stage compressor, an additional cycle of compression and cooling occurs before this.

Benefits of Multi-stage Compression

Both single and multi-stage compressors have their benefits. The right one for your application will depend on a number of factors. Specific advantages of a three- or two-stage compressor design include:

• Improved efficiency. Two-stage compressors perform less work to compress air to a given pressure, which means your operating costs are lower.
• Better reliability. The intercooling stage of two-stage compression creates less chance of overheating, which in turn means more uptime and better productivity.
• Less moisture buildup. Cooler air has a lower moisture content. Moisture in compressed air can lead to equipment failure and premature wear. Using a two- or three-stage compressor can potentially save you from having to purchase a separate air dryer.
• Smaller footprint. For heavy-duty applications, multi-stage compressors deliver greater air pressure (PSI) at higher capacities (CFM) than single-stage machines of a comparable size.
• Few maintenance requirements. Thanks to smaller components and cooler temperatures, wearable components don’t wear out as quickly. As a result, recommended service intervals are longer.

Applications

Clearly there are benefits to selecting a multi-stage machine; however, their higher cost means they are not necessarily the right choice for every application. Generally speaking, single-stage compressors are best for light-duty, intermittent use applications, in which pressure requirements don’t exceed 100 PSIG. Two- or three-stage compressors, on the other hand, offer better value and reliability for continuous use applications, such as auto repair and manufacturing.

Multi-stage Machines From Quincy Compressor

Quincy Compressor has been a leading compressor manufacturer for nearly 100 years. We carry a complete range of products, including two- and three-stage air compressors designed for heavy-duty industrial use. Our products incorporate advanced technology to improve efficiency, reduce noise and keep maintenance requirements to a minimum. The best warranty programs in the business back all of our products, and we offer extended coverage on certain components for 10 years or more.

The flagship product, the QR-25, is available in a 5 HP, 2-stage air compressor configuration. Associated Compressor & Equipment also sells rotary screw and three-stage reciprocating machines. To learn more, Contact us.

The easiest way to explain the difference between a single-stage and multi-stage compressor is the number of times that the sir is compressed. In a  single-stage system, the air is compressed once, and in a dual-stage, the sir is compressed twice. So here this article gives the main key difference between single-stage and multi-stage compressors to better understand this topic.

Single-stage compressor:

Multi-stage compressor:

Difference between single-stage and multi-stage compressor:

• In a single-stage compressor only one cylinder for the compression process, While in a multi-stage compressor more than one cylinder is connected in series.
• In a single-stage compressor used in low-pressure ratio application, while in a multi-stage compressor achieve a very high-pressure ratio.
• Volumetric efficiency is low for given pressure ration in a single-stage compressor whereas volumetric efficiency is high for a given pressure ratio.
• The temperature of fluid due to compression is very high. No intercooler, while in Multi-stage compressor temperature is low. Intercooling is more efficient than cooling with a cylinder wall surface. It also reduces thermal stress.
• In a single-stage compressor suitable for the light task, In a multi-stage compressor suitable for the heavy task. It can manage a larger load.
• In a single-stage compressor large size of flywheel required due to high torque fluctuation, while in a multi-stage compressor provide more uniform torque, it needs a light flywheel.
• In a single-stage compressor the size of the cylinder is very large when compared to the cylinders in the multistage compressor, While in In a multi-stage compressor individual cylinders are small when compared to single-cylinder compression.