Construction Site Air Compressors Solutions for Every Job

Specific power 6.0 vs. 6.8. Difference of 0.8. In percentage terms, 13%.

Two machines, both 10 m³/min. One is 60 kW. The other 68 kW. Purchase price might be about the same. The 68 kW one could even be cheaper. Less efficient machines, some manufacturers discount to move volume. Inventory sitting around is also a cost. Better to sell cheap. Save $500 to $700, a lot of purchasers pick the 68 kW machine.

Over the next ten years, a few thousand extra dollars in electricity every year.

Run the numbers in detail. 68 kW is 8 kW more than 60 kW. Run 10 hours a day. 80 extra kWh. Industrial electricity at $0.10/kWh, $8 a day. 25 working days a month, $200. A year, $2,400. Call it $2,500, some places electricity costs more. Ten years is $25,000 and up. A $500 to $700 price difference traded for $25,000 in extra spending. Nobody does this math at purchase time.

Equipment price is on the quote sheet. Three bids, crystal clear. Operating cost you have to calculate yourself. Calculation is a hassle. You need to know what specific power means. Need to dig out the spec sheet. Find the number. Convert it. Most buyers don't go through this exercise.

Specific power hides in a corner of the spec sheet. Line one is displacement. Line two is discharge pressure. Line three is motor power. These three numbers are big and prominent. Specific power is further down. Small font. Some manufacturers don't even list it. Have to ask. People who don't know won't look. People who know might not have the authority.

Sales won't proactively bring up specific power unless their machine's number is a selling point. Manufacturers with good specific power use it as an argument. Calculate total cost. Calculate electricity savings. Calculate payback period. Manufacturers with poor specific power absolutely won't discuss this topic. Talk about brand history. Talk about service locations. Talk about flexible payment terms. Talk about kickbacks. Topic shifts, buyer's attention is no longer on specific power.

Equipment cost is one-time. Electricity is ongoing. Equipment paid off and done. Electricity bill every month. A screw compressor normally runs ten to fifteen years. Equipment cost gets overtaken by electricity in two or three years. A 60 kW machine, annual electricity around $20,000. Equipment might cost $40,000 to $55,000. Three years of electricity exceeds the equipment cost.

Money saved buying cheap equipment gets eaten by higher electricity bills in three to five years. Save $500 to $700, three to five years spend an extra $10,000. Net loss over $9,000. The further out, the more you lose. By year ten, cumulative extra spending is $25,000 plus.

Three to five years later, who remembers who bought this machine. The buyer may have transferred to another department or another company. Equipment sits there running. Meter keeps spinning. The extra amount on the monthly electricity bill, nobody traces back to that purchasing decision.

Specific power must be compared under the same conditions.

Different discharge pressures, specific power isn't comparable. A 7 bar machine and an 8 bar machine, can't look at their specific power side by side. Higher pressure, higher compression ratio, necessarily more power consumed, specific power number goes up. A machine with 6.0 specific power at 7 bar, switch to 8 bar conditions, specific power might become 6.5 or higher. What pressure was the spec sheet's specific power tested at. Read carefully. Some manufacturers list 7 bar data. Some list 8 bar. Mix them together and the comparison is garbage.

Another confusing point: airend specific power vs. package specific power.

Airend is just the compressor head. Only counts the compression process itself. Package includes the motor, cooling fan, control system, oil circuit. Every power-consuming component counted in. Airend specific power is definitely lower than package. Data looks better. Some manufacturers only show airend specific power on the spec sheet. Don't label it clearly. Buyer takes that number and compares it to another company's package specific power. Thinks this one is more efficient. Buys it. Measures on site and finds it's off.

Spec sheets that aren't clearly labeled, ask. Ask specifically. "Is this specific power for the airend or the package?" "7 bar conditions or 8 bar?" "What ambient temperature for the test?" More specific the questions, more reliable the data. Vague answers, probably something's off.

Most reliable way to verify specific power is on-site measurement.

Have the manufacturer bring the equipment to site. Hook up an energy meter and flow meter. Run it a few hours and look at the data. Energy meter reads power. Flow meter reads air output. Divide and that's specific power. This data can't be faked. On-site measurement. Black and white.

Most purchasing scenarios don't have this option. Can't ask the manufacturer to bring it for testing before buying. The manufacturer may not be willing. Site, power, labor are all costs. Test and don't buy, the manufacturer wasted their time. Unless it's a large order or a long-term customer, you generally can't get this treatment.

Next best, look at third-party certification. Energy efficiency rating certification requires testing specific power. Certification body is third party. Data is relatively credible. Machine with energy efficiency certification, specific power data has at least been verified. Without certification, only the manufacturer's own claims. Hard to tell real from fake.

Can also ask around among existing users. Same model machine, someone else has been running it for years. What's their operating data. More valuable than spec sheets. This kind of information isn't easy to get. Peers may not want to share. Reference customer lists from manufacturers are likely the cooperative ones. Not necessarily objective.

VFD machine specific power varies.

Fixed-speed machine specific power is relatively constant. Motor speed is fixed. Displacement is fixed. Power is fixed. Specific power is one number. VFD is different. Motor speed follows air demand. High demand, spins fast. Low demand, spins slow. Different loads, different power. Different specific power.

At full load, VFD and fixed-speed specific power are about the same. Sometimes VFD is even slightly higher. The VFD itself consumes power. Electrical energy going through the converter has losses. Full load, fixed-speed runs directly at line frequency. No conversion losses. Actually slightly more efficient.

At low load, it flips. VFD specific power is better. Fixed-speed at low load works through load-unload cycling. Low demand, unloads and idles. No air output but still consuming power. Very low efficiency. VFD reduces speed. Output drops. Power drops with it. No idle waste.

Stable full-load operation, VFD may not save. Heavy load fluctuation, VFD savings are obvious. Stable demand, high load factor, a fixed-speed machine with good specific power might save more than a VFD with mediocre specific power.

Some operations have steady air demand all day. Three-shift production. Equipment runs at full load once it's on. Very little fluctuation. This operating condition, pick a fixed-speed machine with good specific power. Highest efficiency. Price is lower than VFD too.

Some operations have big demand swings. Day shift uses a lot of air, night shift uses little. Or different processes have very different air consumption. This condition, VFD. Follows the load. Saves electricity overall.

Then there's the case where equipment was oversized from the start. Worried about not enough, left margin. Result is equipment running long-term at 60% load or even lower. VFD can make up for some of the sizing mistake. Fixed-speed just keeps running in the low-efficiency zone. Bigger waste.

Before sizing, figure out the air demand profile. If possible, measure over a period. What's the peak. What's the valley. What's the average. What's the fluctuation pattern. Take that data and size. VFD or fixed-speed. How much displacement. What pressure level. You'll know. Size by gut feeling, either too small and not enough, or too big and wasting electricity.