Developing materials for tomorrow’s compressors

Improved compressor technology plays a big part in the development of more energy-efficient refrigerators, air-conditioning units, brake systems and other products. Key components in the development are the compressor valves, where material and design go hand in hand.

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Compressor valves are under constant stress in a never-ending cycle of bending and hitting back as the piston moves back and forth in the compressor. If the valve breaks, the compressor stops working. During its lifetime, a compressor valve should last for 10 billion cycles. This makes the choice of valve material extra important, says Anders Hoel, Senior Research Engineer at Sandvik Materials Technology, who is heading Sandvik’s program of hardened and tempered compressor valve steel.

Reduce energy consumption

“With effective and durable valve steel, the compressor’s energy consumption can be reduced substantially,” he says. “By choosing the right valve steel, you can also reduce noise levels and compressor size, as well as increase compressor efficiency and reliability.”

Hoel says compressor manufacturers are usually on the lookout to increase the compressors’ coefficient of performance (COP), which equates to lower operating costs. Thinner valves are one way to improve compressor efficiency.

“The thinner the compressor valve steel, the more easily it bends,” says Hoel. “This means less force is required to suck the gas in and expel it, making the compressor more efficient. With a thinner valve, you also reduce flow losses at the compressor inlet and exhaust.”

Sandvik manufactures valve steel with a thickness from 1.2 millimeters down to 0.06 millimeters, which is the thinnest compressor valve steel on the market. The challenge is that a thin valve bends more and thereby hits back harder, which places high demands on both the flexural and impact strength of the steel.

A new-generation steel grade

With Sandvik Hiflex, a new generation of high-fatigue-resistance stainless chromium steel alloyed with molybdenum, Sandvik has successfully developed a valve steel with the coveted combination of high strength in terms of both impact and bending. It can withstand a higher number of cycles than any other steel on the market. In addition, the steel can operate in temperatures of 300 degrees Celsius. This is a prerequisite for a long life, as high-efficiency compressors operate at high frequencies and thereby develop heat that otherwise could affect the material properties negatively.

“This particular combination of impact and flexural strength makes a big difference in energy efficiency,” says Hoel. “It also lets you reduce compressor size, since you can maintain the same lift with a smaller-sized valve.”

Material and design must harmonize

But the valve material isn’t everything – it goes hand in hand with valve and compressor design.

“Customers often ask us for even better materials, with improved properties,” Hoel says. “But more important, when you want to get the most out of the compressor, is to make sure the material properties are in harmony with the design. We see the best results when we are included in the design process, and we are continuously searching for collaborations where we hope our materials knowledge could be a helping hand.”

Meanwhile, some industries would benefit more than others from optimizing compressor performance, Hoel says. One example is manufacturers of air-conditioning units for the auto industry, where the ambition level for energy efficiency has not come as far.

“The industry’s sustainability focus has been elsewhere,” says Hoel. “But in my view, each percentage point by which you can reduce energy consumption should be worth the same, regardless of the source. Should air-conditioning manufacturers using our Sandvik 20C compressor valve steel switch to Sandvik Hiflex, for example, energy consumption would be reduced by 1 percent. I would say it’s worth it.”

Higher frequencies call for new materials

Sandvik is also preparing to meet the requirements on future compressor valve steel to enable development of the next-generation compressors.

“Today’s compressors generally work at a frequency of 50-60 Hz (50-60 cycles per second), but we are seeing a trend introducing compressors working at 200 Hz,” says Hoel. “This means we are quickly entering an extremely high number of cycles.

“We want to raise the bar and provide material that meets these higher demands and that can cope with them for more cycles. If we could improve the valve steel properties by 5 to 10 percent in terms of impact and flexural strength, our customers would be able to increase the load on the compressors and further improve efficiency.”

To accomplish this, Hoel and his R&D team are mapping out what is possible with existing materials and what the future needs will be. “We want to learn everything about the processes and the products to be able to bring that knowledge into new applications, new designs and new steel grades,” Hoel says. “I’m convinced that, with our valve steels, we have the potential to influence the future.”

Anders Hoel

Title: Senior Research Engineer at Sandvik Materials Technology
Number of years at Sandvik: Six
Lives in: Gävle, Sweden
Thinks about: “I am generally interested in issues about energy and energy conservation. With our valve steels, there is the potential to influence the future.”

Sandvik compressor steel

Sandvik has three types of steel for compressor valves:

  • Sandvik 20C, a carbon steel
  • Sandvik 7C27Mo2, a martensitic stainless chromium steel alloyed with molybdenum
  • Sandvik Hiflex, a high-fatigue resistance stainless chromium steel, with the best performance of all valve steels on the market

The thickness of the valve steel varies between 0.06 and 1.2 millimeters. The thicker the material, the lower the requirement of impact exhaustion, but a thicker valve steel also results in major flow losses. This is why manufacturers are demanding ever-thinner materials with better properties, in terms of both impact resistance and flexural strength.