What's more efficient, reciprocating compressor or screw ...

Often, when it comes to choosing an air compressor for a service shop, there is a choice between types and sizes of compressors. Do you choose a standard reciprocating compressor or do you go with a more modern lubricated screw? But depending on the way you use your compressed air, and how the compressor is installed, the operating costs may be quite different for the two choices&#;and not in the way you might think.

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Reciprocating compressors use pistons to compress the air to the line pressure you desire. They are simple units, often mounted on a small receiver tank, and they usually run in start/stop mode on a mechanical pressure switch. You can certainly tell when they are running, because the pistons create a lot of noise. The cooling systems of these units are typically just fins across which a pulley mounted fan blows a stream of cooling air. The air produced by these units is usually very hot and oily&#;difficult to clean and dry unless some sort of auxiliary cooling and filtering method is used. If one looks up the efficiency of these units, you may find that at full load they consume more power per unit output than a screw compressor (usually rated in kilowatts per 100 cfm, called specific power). But these units are not designed to run continuously at full load, average loads of over about 60% of the compressor capacity with overheat and damage the internal components, mainly due to the poor cooling method.

Screw compressors, on the other hand, are more complex and expensive units and can often be mounted on the same sized storage tank. These units most often run in a different mode, load/unload which means they run continuously; alternately producing air at full load or running unloaded with zero compressed air output. Screw compressors are much quieter than reciprocating units and have very good cooling systems, similar to automotive radiators across which passes cooling air generated by a fan. These units are designed to run at full load and will produce much cooler and cleaner air, which can be sent directly to an air dryer to remove water vapor. These units usually consume less power per unit output at full load than reciprocating units, making them more efficient in that condition.

But compressors running in a service shop rarely run constantly at full load. Experience has shown that compressors in this type of duty often run at average flows of 10 percent or less. And when one tracks the energy consumption of both types of compressors at this low load quite a surprising picture emerges.

Screw compressors running in load/unload with small storage receivers rarely turn off completely to limit the number of motor starts, which could prematurely burn out the motor; therefore, they will run continuously, consuming power even when they are unloaded and producing no air. This unloaded power consumption typically consumes about 35% of the compressor full load value.

To compare the two types of compressors, the math works like this:

For a 10 cfm load, a 100 cfm 25 hp (21.3 kW at full load) screw compressor consumes 21.3 kW for 10% of the time (2.1 kW) and 7.4 kW for 90% of the time (6.7 kW). This means the total average power consumption is 8.8 kW to produce 10 cfm, a specific power of 88 kW per 100 cfm.

A similar sized 25 hp reciprocating compressor that, say, can produce only 90 cfm at 21 kW, runs 11% of the time to feed the same load (2.3 kW) but consumes no power when it is not producing air. This works to a total average power of 2.3 kW or a specific power of 23 kW per 100 cfm&#;almost 4 times more energy efficient!

Want to get a screw compressor running like a reciprocating compressor? Add much more storage (ten time normal) and widen the load/unload pressure band. And purchase a compressor with auto/dual mode, which will turn off the compressor between cycles. Your compressor supplier can help you with this.

Articles

Welcome back to the Definitive Guide to Industry Products and Services for a Sustainable Future. This penultimate piece focuses on an Atlas Copco compressor that offers average energy savings of 60% &#; launched at the ideal time to help counter rising electricity prices. Check back tomorrow (6 May) to read how mushrooms could make construction more sustainable.

Atlas Copco has launched its most advanced, energy-efficient, sustainable and smart rotary screw compressor. With energy savings of 60% over fixed-speed compressors, the GA VSDS is the most energy-efficient, sustainable compressor on the market today. Thanks to its pioneering smart features, the GA VSDS compressor can adapt and optimise its operation for any application &#; innovative technology now embodied in Atlas Copco&#;s 22-37kW GA oil-injected screw compressor range.

Next-generation VSD offers 60% energy savings and smart, flexible operation. The GA VSDS is the third generation of Atlas Copco variable speed drive (VSD) compressors that significantly reduce energy consumption by adjusting their motor speed to the fluctuations in compressed air demand. 

When Atlas Copco introduced the technology, a first-generation VSD compressor achieved energy savings of up to 35% compared to a fixed-speed compressor. That number increased to 50% with the company&#;s VSD+ range. Now, the new GA VSDS offers energy savings of 60%.

The company is the world’s best compressor open type supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

Sustainability

Together with this unrivalled level of energy efficiency, the GA VSDS also leads the way in sustainability &#; the guiding principle of its innovative design. For example, the new unit not only comprises the minimal number of components but is the first compressor to feature an IE5 ferrite-assisted synchronous reluctance motor that does not rely on rare earth materials in its operation. 

Smart, solid performance

With Free Air Delivery 21% higher than its fixed-speed counterparts, the VSDS introduces a host of smart features that allow it to adapt and optimise performance to the user&#;s applications, requirements and conditions. 

The most important example is the Smart Temperature Control (STC) system, which ensures that the compressor operates with an optimal oil temperature at all times, completely eliminating the risk of condensation while ensuring maximum compression efficiency. The GA VSDS also comes with the all-new Boost Flow Mode, which gives customers the freedom to exceed their compressor&#;s maximum capacity temporarily and safely. 

Small and silent, with sound levels as low as 63dB, the GA 22-37 VSDS does not require a compressor room site. It can be installed on the production floor, occupying the minimum footprint thanks to its vertical, compact design.

Superior connectivity

The GA VSDS concept incorporates industry-leading connectivity features such as the advanced Elektronikon® Touch controller, the EQ2i multiple compressor control (integrated as standard), Smartlink remote monitoring and analysis, and OPC UA integration for connected production environments.

Truly transformative compressor

&#;The new VSDS offers true sustainable innovation,&#; says Stef Lievens, vice-president of marketing in the Atlas Copco Industrial Air Division. &#;As climate change affects us all, Atlas Copco has made sustainable productivity its number-one focus. The GA VSDS set out to meet today&#;s standards but also those of tomorrow. It&#;s a truly transformative compressor for a rapidly changing industry.&#;

A logistics company that trialled the new GA 37 VSDS compressor reports potential reduction of its compressor CO2 emissions by 60 tonnes per year. 

Thanks to the compressor&#;s intelligent optimising features, VSDS customers can save on investment, service and operational cost by installing a lower kW model and still get all the air they need.

Contact Atlas Copco for more information:

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Website: www.atlascopco.com/en-uk/compressors

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