Air Compressor Selection

Compressor selected too big. This problem is way too common.

Walk into many factory compressor rooms, glance at nameplate output capacity, ask about actual air usage, basically all overcapacity. Some factories 30% overcapacity, some 50%, ridiculous cases even double. Machine idling there every day, can't reach full load, but electric meter spins happily.

Selection

Right-Sizing Your System

Leaving Margin

Reason for oversizing is simple to explain: afraid it won't be enough.

Calculation shows need 350 CFM, selection goes straight for 530 CFM. Ask why, answer is always "what if it's not enough." Like the sky will fall if selected too small.

This mentality is actually understandable. Compressor air supply insufficient, all downstream pneumatic equipment goes down. Production line stopped one day, loss might be more than one compressor costs. So rather go bigger, spend some wasted money for peace of mind.

Problem is, not just one person adding margin.

Equipment department people tally air consumption, each equipment's consumption pushed a bit higher. Process department people set simultaneity factor, conservatively pick a high value. Procurement people see calculated result, figure better leave some more room. Boss making final call, thinking maybe expansion down the road, just go up one size.

There's also a saying that big machines last longer. Small machine running full load every day, hard work. Big machine relaxed, lasts longer. Sounds reasonable, actually the opposite is true. Compressor running at partial load has to frequently load and unload. This repeated impact does more damage to equipment than stable operation.

Electricity Cost Math

Extra money spent buying bigger machine, honestly isn't the most heartbreaking. Equipment is one-time investment, grit your teeth and it's done. What really bleeds continuously is electricity.

A 50 hp screw machine, industrial electricity at $0.10/kWh, running 6,000 hours a year, electricity cost approaches $25,000. Over ten years, electricity is several times what buying the machine cost. Compressors, heart aches once when buying, aches every day when using.

And partial load operation is especially electricity-hungry.

Fixed speed screw machine control logic works like this: system pressure reaches upper limit, inlet valve closes, compressor stops drawing air, but motor still turns. This is called unloaded operation, also called idling. Idling produces no air, electricity still used, about 25% to 30% of full load power. This electricity is pure waste, zero output.

Lower the load rate, higher the proportion of idling time. Machine long-term running only 50% load, nearly half the time idling. Specific power over 20% higher than full load. Same amount of air produced, burning 20% more electricity.

VFD machines are better. VFD machines adjust speed to match air demand. Use what you need, produce what you need. No idling. But VFD machines aren't cure-all either. Speed too low, motor efficiency drops, cooling fan slows down and heat dissipation worsens, oil pump slows and lube oil circulation insufficient. Long-term running at very low speed isn't good for the machine either.

So whether fixed speed or VFD, selecting too big doesn't pay.

How Much Air Do You Actually Need

Rather than agonizing over "whether to leave margin," better to calculate air demand clearly. Calculate clearly, know how much margin to leave, won't add randomly.

First list all air-using equipment, check consumption one by one. Generally on nameplate, can't find it then look at manual, manual doesn't have it then ask supplier. No tricks to this step, just patience, don't miss any.

Pneumatic wrenches, impact wrenches, these tools have big consumption fluctuation. Nameplate shows maximum, averaged out not that much. Cylinders, pneumatic valves, these actuators are relatively stable, calculate by action frequency. Spray guns, blow nozzles, these need separate accounting, related to orifice size, pressure, usage duration.

One easily overlooked area is leakage. Pipe network used long enough, fittings loosen, seals age, leaking everywhere. New pipe network leak rate can be controlled under 5%. Ten-plus year old pipe network, 15%, 20% is normal. Take ultrasonic leak detector and walk along the line once, leak point count often exceeds expectation.

Leakage

Pipe Network Survey

Demand

Flow Recording

After adding up equipment consumption, multiply by simultaneity factor.

Factory can't possibly have all equipment running full load simultaneously. Assembly line equipment has strong linkage, start together when starting, simultaneity factor high, 0.8 or even 0.9 possible. Machine shop equipment each doing their own thing, this one using that one idle, simultaneity factor low, around 0.5, 0.6.

Not sure, install flow meter on main header and record for one or two weeks. See relationship between peak and average.

Most situations taking 0.6 to 0.8 has wide enough coverage.

Calculated number then times margin factor. What is margin for? One is calculation itself has error. Two is production has fluctuation. Three is possible future expansion. Four is pipe network will age and leakage will increase. These added up, 15% to 25% margin is basically enough.

Don't add margin randomly. Add too much goes back to old path, selected machine is another size bigger.

Last step is selecting machine. Total demand times margin factor, get a number, find model with capacity close to it.

One trap to watch here. Capacity in manufacturer catalog is measured at certain discharge pressure, like 115 psi. If use pressure is 100 psi, actual capacity will be a bit bigger than rated. If use pressure is 145 psi, actual capacity will be noticeably smaller. Selection must verify pressure matches. Don't be fooled by numbers.

High altitude areas, air is thin. Same machine, capacity needs discount. Denver at 5,280 feet, capacity about 15% lower than sea level. High ambient temperature same thing. 105°F has less output than 70°F. These factors need to be corrected in.

Let's Calculate an Example

A shop has 20 pneumatic equipment, each averaging 18 CFM consumption, working pressure 100 psi.

If following "leave margin" old thinking, directly select 530 CFM, output capacity exceeds demand by nearly 80%. This machine installed, long-term idling below 60% load. Fixed speed machine loading and unloading every day wasting electricity. VFD machine also can't run in optimal efficiency zone.

Not Just Output Capacity

Pressure rating needs to match. Common industrial air use is 100, 115 psi mostly. Most screw machines can cover. Some special processes need high pressure. PET blow molding needs 435 psi or even 580 psi. That's another category called high pressure compressors. Price and configuration are different.

Selection, discharge pressure should be slightly higher than end demand, leave room for pipe network pressure loss. Pipe small, distance far, many elbows, pressure loss is big. Pressure loss big and end pressure isn't enough, some people's solution is turn up compressor discharge pressure. This is drinking poison to quench thirst. Each 15 psi discharge pressure increase, energy consumption increases about 7%. Correct approach is optimize pipe network, reduce pressure loss.

Air quality needs to be looked at by industry. Food plants, pharma plants, electronics plants have strict requirements on compressed air oil content, dust content, dew point. ISO 8573 has specific grading standards. Configuring aftertreatment equipment must check against grade requirements. What spec refrigerated dryer, desiccant dryer, filters to use, can't be sloppy. Common machine shop isn't that particular. Standard refrigerated dryer plus two stages of filtration is enough.

Station room environment also needs consideration. Poor ventilation makes machine cooling difficult. Summer easily gets high temp alarm. Dusty environment, air filter clogs fast, maintenance interval needs shortening. Noise sensitive locations, like near office area, select low noise models or do sound insulation treatment.

Sizing

Capacity Planning

Configuration

Multi-Unit Setup

Multiple Small Machines or One Big Machine

When air demand is large, two configuration approaches. One is buy one big machine, done in one shot. Other is buy two or three small machines combined.

Latter has better flexibility.

Day shift air demand high, three machines all on. Night shift air demand low, shut off one or two, remaining run in high efficiency zone. One big machine can't do this kind of adjustment. Day shift night shift it alone carries, night shift load is low same waste.

Redundancy is also an advantage. One machine breaks, send for repair, other two cover, production doesn't stop. If only one big machine, breaks and whole plant loses air. Waiting for parts, waiting for repair, loss can't be calculated.

Phased investment is also convenient. Current capacity, two is enough, buy two first. Expand in two years, add third. Don't need to dump all the money in at the start.

Multi-machine combination premise is having coordinated control system. Coordinated control system automatically dispatches each machine's start-stop based on header pressure. Pressure low, start machine. Pressure sufficient, stop machine. No one needs to watch. Without coordinated control system, manual start-stop responds slow, easy to make mistakes. Might as well have one big machine for less hassle.

VFD and fixed speed pairing is also common approach. VFD machine responds fast, handles fluctuation portion. Fixed speed machine is efficient, carries base load. Two types of machines working together, efficiency and cost can both be balanced.

Air receiver capacity sized by total output capacity, leave buffer space for unit start-stop. Tank too small, pressure fluctuation big, machines frequently starting and stopping, actually wastes electricity and hurts machines.

Is Energy Efficiency Rating Worth It

Compressors on market have energy efficiency ratings. First tier efficiency saves most electricity. Third tier is entry threshold. First tier machines are expensive, but running electricity cost is low. Third tier machines are cheap, but electricity cost is high.

How to choose depends on runtime. Running over 6,000 hours a year, first tier efficiency can earn back price difference through electricity savings in few years. After that it's all profit. Running only 2,000 hours a year, payback period stretches to seven or eight years. Equipment might be at end of life by then.

Do a total cost of ownership calculation: purchase cost plus ten years electricity plus maintenance cost. Whichever option has smaller total, pick that one. Only looking at purchase price is shortsighted. Compressors, small money when buying, big money when using.