What Is the Efficiency Rating of a Compressor?

Screw compressor sizing. Specific power is the parameter tied to your electricity bill.

Take a 10 m³/min unit. Specific power 6.0 means 60 kW consumption. Specific power 6.8 means 68 kW. Difference is 13%.

How the Electricity Math Works

In the cost structure of a compressed air system, electricity accounts for over 70%. Equipment is a one-time purchase. Electricity is paid monthly, yearly.

Running 6,000 hours a year, industrial electricity $0.10/kWh. 60 kW machine, annual electricity about $36,000. 68 kW machine about $40,800. About $5,000 difference per year. Ten years is over $50,000. Screw compressors have a design life of 15 years and up. Two machines might differ by only a few thousand in purchase price. The electricity gap is several times the price difference.

Atlas Copco, Ingersoll Rand, these top-tier brands hammer specific power as a core selling point. Among domestic brands, Kaishan and Baosi have been working on efficiency these last few years. Numbers are genuinely improving. Some manufacturers' sales never mention the words "specific power" in an entire conversation. Talk brand history for half an hour. Service locations for twenty minutes. The specific power column on their spec sheet doesn't look good. Naturally they steer the topic elsewhere.

Pitfalls When Comparing Specific Power

Directly comparing specific power from two spec sheets is easy to fall into traps.

Data at different discharge pressures put side by side is meaningless. 7 bar and 8 bar test data produce different specific power numbers. That one's okay. Anyone with basic knowledge won't make this mistake.

Airend specific power and package specific power mixed together. This trap catches a lot of people. Some spec sheets label clearly. Some just show one number. Doesn't say airend or package. Ask the sales rep. Sales may not know either. Or deliberately plays dumb. Package has to include motor transmission losses, cooling fan, control cabinet, everything. Number looks worse than airend. Two manufacturers using different labeling standards, line them up and compare. The lower number doesn't necessarily actually use less electricity. Sullair for a while listed airend and package separately on their spec sheets. That was clear. Later on, seems like not every series did that.

Test conditions are even more uncertain. National standard specifies standard conditions at 20°C inlet temperature. Workshops in the Yangtze River Delta or Pearl River Delta in summer, where's 20°C coming from. 37 to 38°C is the norm. Some manufacturers' test data are from optimal conditions. Get to the site on a hot summer day, that specific power number falls apart. Spec sheet won't note "this data only applies to climate-controlled workshops."

Another situation: the specific power on the spec sheet corresponds to full-load conditions. Machine runs long-term at 70% load. What's the partial-load specific power? Most manufacturers don't provide this data. Ask and you won't get it. VFD machines are better. Fixed-speed machines, partial-load efficiency drops hard. Only looking at full-load specific power during sizing will cost you.

How to Verify Specific Power

Reliable method is measurement. Bring the equipment to site. Hook up an energy meter. Hook up a flow meter. Run three to five days, collect data. 10 m³/min machine. How many kWh did the energy meter log. How many cubic meters did the flow meter accumulate. Divide and you're done.

Most procurement processes can't make this happen. Bidding has deadlines. Equipment is pay-first, ship-later. Supplier won't let you test for a week before deciding to buy. Some large projects where the buyer has leverage can negotiate post-delivery acceptance testing of specific power. Penalty clause if it doesn't meet spec sheet values. Not many contracts include this.

Energy efficiency labels are a ready reference. Domestic compressor energy labels have three tiers. Tier 1 has the highest bar. Stuck right next to the equipment nameplate, visible at a glance. This requires testing by a third-party agency. Not something the manufacturer decides on their own. Data credibility is higher than spec sheets. Gree and Midea entered the compressor market relatively late. Used energy efficiency rating as their door-opener. Tier 1 models deployed broadly. Some established manufacturers' high-volume economy units are actually Tier 2. Cheaper price is right there. Specific power is a bit worse but people still buy.

There's also the old-fashioned method. Ask existing users of that model. Ask peers. A certain model, been running three years, roughly what level are the electricity bills. How much has output declined. A machine with tens of thousands of hours on it tells you more than factory data. Screw wear, seal aging, oil degradation. These accumulate and efficiency drops over time. Some machines are strong for the first three years, then decline fast. Some machines have unremarkable numbers but are steady. Decade after decade. This kind of information isn't on spec sheets. Have to ask around.

Volumetric Efficiency

Volumetric efficiency is actual output divided by theoretical output.

One revolution of the screw airend can displace a certain amount of air. Based on rotor profile and geometry, you can calculate it. That's the theoretical value. Machine runs, actual output doesn't reach theoretical.

Losses come from several places. Clearance between rotors. Between male and female rotors. Between rotors and housing. High-pressure chamber gas leaks to the low-pressure side. Called internal leakage. GHH airends control internal leakage well. That's one reason German airends are expensive. Air filter used over time, resistance rises. Intake valve response slows, also affects intake volume. Pressure drop in the oil-air tank. Pressure drop across the minimum pressure valve. Pressure drop through post-treatment equipment. Adds up along the way. Compression process releases heat. Gas temperature goes up, density goes down. Insufficient oil injection or poor oil temperature control amplifies this loss.

Stack all these losses. Volumetric efficiency of 92% is respectable. Some domestic economy airends are around 85%.

Machine with low volumetric efficiency. To hit rated output, has to run the speed up. Speed goes up, bearing loads increase. More heat. Mechanical losses go up too. So a machine with poor volumetric efficiency basically can't have good specific power. Reverse is also true. High volumetric efficiency means the same output at lower speed. Power saved. Specific power drops.

Airend manufacturers improving volumetric efficiency. Profile optimization is one avenue. Push internal leakage down. Enlarge intake and exhaust passages. Reduce flow resistance. Oil injection and cooling systems done well. Control gas temperature rise. Get these right and both volumetric efficiency and specific power improve. Domestic airends have made real progress these past few years. Baosi and Hanbell have been iterating their profiles. Genuinely different from ten years ago.

Isothermal Efficiency

Sizing doesn't use this. R&D people and paper writers do.

Isothermal compression is the theoretical limit. Remove all heat during compression. Temperature stays constant. Compression work is minimized. Real screw compressors can't achieve isothermal compression. Oil injection cooling only controls temperature rise within a range. Heat that's going to be generated still gets generated. Gap from the isothermal process remains. Shaft power divided by isothermal compression work gives isothermal efficiency. This number reflects how far the compression process is from ideal.

Airend developers watch this metric. New profile revision, isothermal efficiency up by a fraction of a percent. Gives them something to report in internal reviews. Equipment sizing uses specific power. Efficiency ratings also set specific power thresholds. Industry recognizes this.

Where Should Specific Power Sit in the Purchasing Decision

In a lot of procurement processes, specific power doesn't even make the ranking. Price is number one. A few thousand dollars difference can decide where the order goes. Who's going to calculate ten-year electricity differences. Brand is second. Using Atlas Copco or Kaishan, that's a team to pick. Some buyers specify an approved brand list. Not on the list, your quote doesn't even get received. Payment terms that can be negotiated also count. An extra month of credit puts real pressure on the supplier. After-sales response time, parts availability, local service team. All influence the decision. Specific power? Glance at the spec sheet. Close enough and move on.

Under this procurement logic, efficient machines don't necessarily sell. Getting specific power to 6.0 costs more than getting to 6.5. High-efficiency units are priced higher. In bidding, the price score takes a hit. Some manufacturers just don't push hard on efficiency. Put energy into cutting costs, expanding channels, building relationships. Machine gets sold, that's a win. Whether electricity bills are high later is the user's problem.

Flip the calculation. Add ten-year electricity to purchase price and compare total cost of ownership. High-efficiency units come out ahead. This approach is more common in Europe and North America. High electricity prices naturally drive attention to efficiency. Domestic industrial electricity is relatively cheap. Plus a lot of companies aren't sure if they'll still be using this machine in ten years. Factory lease is only three years. Who cares about ten-year electricity costs.

That said, specific power can't be too far off either. Energy efficiency tiers are there. Below Tier 3 limit, the machine can't be sold anymore. This floor eliminated a batch of products with truly embarrassing efficiency. What's left in the market, specific power can't be pulled apart by huge margins. Just depends on how much premium the buyer is willing to pay for a few percentage points of efficiency difference.