A rack of zinc-plated brackets comes out of the dry-off oven with fisheyes in the chromate. The plater adjusts the chemistry. Two weeks and three bath additions later, somebody finally pulls the element out of the filter feeding the blow-off station and finds it soaked in oil. That was the whole problem, and it cost a month of rework.
We get this call a lot. A plating line is not a machine shop with tanks in it. Your air touches the product, and in some shops it touches the chemistry. When it carries oil or water, it does not just slow the line down, it scraps parts and contaminates baths you paid good money to build. So compressed air for metal finishing deserves more thought than picking a compressor big enough to keep the gauge up.
Start with the thing most shops get wrong
Tank agitation should not come off your air compressor. Not in a zinc line, not in an anodize line, not in an acid copper line for boards.
There are two reasons, and the second one is the expensive one.
First, oil. Even a well-maintained oil-flooded screw compressor puts some oil carryover into the air stream, and a plating bath is an excellent oil trap. Once it is in, you are filtering, carbon treating, or dumping. Platers have been saying this for decades and they are right.
Second, money. Agitation needs a lot of air at almost no pressure. Common practice is around 1 psi for every 18 inches of solution depth, which on a 4-foot deep tank is roughly 3 psi. Your compressor makes 100 to 125 psi and then you throttle it back to 3. That is like heating your shop by running the truck in the parking lot.
The numbers make it obvious:
| Air source | Typical delivery | Useful pressure | Oil risk to the bath |
|---|---|---|---|
| Regenerative blower | Roughly 28 CFM per hp or better | 2 to 7 psi | None (oil-free by design) |
| Oil-flooded rotary screw | Roughly 4 CFM per hp | 100 to 125 psi, throttled to 3 | Real, and cumulative |
Seven times the air per horsepower, and no oil. That is why the answer is a low-pressure blower.
Here is the honest part: we do not sell regenerative blowers. Your chemistry supplier or a local fluid handling house will set you up. What we do sell is everything else on this list, and the everything else is usually where the shop is losing money anyway.
If you already run agitation on shop air
Some older lines do, and ripping it out tomorrow is not always practical. If you are stuck with it for now, at minimum put a coalescing filter and a carbon filter in that leg and change the elements on a schedule, not on a hunch. Size the agitation at about 0.5 CFM per square foot of tank surface for normal work and up to about 1.5 CFM per square foot for hard anodize, where the air is doing cooling duty and not just mixing. Then plan the blower purchase, because that load is eating your compressor alive.
What compressed air actually does on a finishing line
Strip out agitation and a plating shop still has a serious air bill. The loads that matter:
- Air-operated double diaphragm pumps. Chemical transfer, rinse water transfer, sump pumping, filter feed. In most shops this is the single largest air consumer. They are self-priming, they dead-head safely, and they do not care if you pump abrasive sludge through them, which is exactly why platers love them.
- Hoists and rack shakers. Air hoists on a hand line, pneumatic shakers on barrels and racks, air cylinders on automated transfer.
- Blow-off drying. Between rinse and chromate, and again before the oven. This is the leg where oil kills you.
- Spray application. Chromate and sealer spray booths, touch-up guns, and in some shops airless or air-assist paint after the plate.
- Pneumatic valves and actuators. Rectifier interlocks, tank fill valves, scrubber dampers, filter press cylinders.
- Waste treatment. Plate and frame filter press, chemical metering pumps, sludge transfer.
The air quality target, by leg
You do not need the same class of air everywhere, and paying for the best air on every drop is a waste. Split the system and treat each leg for what it does. The classes below refer to ISO 8573-1, which we break down in ISO 8573-1 Air Quality Classes, Decoded.
| Where the air goes | Practical target | What it takes |
|---|---|---|
| Blow-off before chromate or paint | ISO 8573-1 Class 1.4.1 or better | Coalescing filter plus carbon, dryer to 38 degrees F pressure dew point or lower |
| Spray application | Class 1.4.1 or better | Same train, plus a point-of-use filter at the booth |
| Anything entering a bath | Oil-free source, ideally a blower | See above. Do not solve this with filters alone. |
| AODD pumps, hoists, cylinders | Class 2 to 3 particulate, dry enough not to rust | Particulate filter, refrigerated dryer, and a lubricator where the pump calls for one |
| Instrument and valve air | Class 2.3.2 | Refrigerated dryer is usually enough indoors, desiccant if lines run outside |
A refrigerated dryer plus a good coalescing element handles most of a finishing shop. Shops that run lines outdoors, or that keep a nitrogen or clean-dry leg for instruments, step up to desiccant. If you are not sure which way to go, read Refrigerated vs Desiccant Air Dryer.
Sizing the compressor
AODD pumps drive the number, and they are rated on a curve, not on a single figure. A 1-inch pump moving around 15 gpm at full line pressure often lands near 12 SCFM. Heavier duty pumps and higher heads climb from there. Pull the curve for the actual pump, at the actual head and flow you run, and do not take the catalog headline number.
Then build the list honestly:
- Add up every AODD pump that runs at the same time, at its real duty point.
- Add blow-off. If you are still blowing parts off with open pipe or a standard blow gun, assume it is eating more than you think. A 1/4-inch open tube at 80 psi can pull well over 30 CFM.
- Add spray guns from the gun data sheet, not from memory.
- Add hoists, shakers and cylinders at a realistic duty cycle. These are intermittent, so do not sum them at 100 percent.
- Add 15 to 25 percent for leaks if you have never done a leak survey. If you have, use the real number.
- Size storage so the compressor is not chasing every pump stroke. A finishing line is pulsing air all shift, and a receiver flattens that out.
For most plating and anodizing shops that lands on a rotary screw in the 25 to 100 hp range. Continuous AODD duty is exactly the load a screw compressor is built for, and exactly the load that chews up a piston machine. If your shop has a wide swing between first shift and off shift, look hard at a variable speed unit before you buy a fixed-speed machine plus a smaller trim unit. Our rotary screw sizing guide walks through the math.
Regulations that actually touch your air system
OSHA chromium (VI), 29 CFR 1910.1026. The permissible exposure limit is 5 micrograms per cubic meter as an 8-hour time-weighted average, with an action level of 2.5 micrograms that triggers periodic monitoring and medical surveillance. Blowing off a freshly chromated part with compressed air is an excellent way to put chromium into the breathing zone. Air knives inside an enclosure with the scrubber pulling on it beat a hand-held gun in open air every time, and it is not close.
OSHA 1910.242(b). Compressed air used for cleaning must be reduced to less than 30 psi dead-ended, with effective chip guarding and PPE. That is dead-end pressure at the nozzle, not the pressure on your regulator, so a safety nozzle that relieves when blocked is what keeps you compliant. A plain open tube is not.
Your pretreatment permit. Compressor condensate from an oil-flooded machine is not clean water. It carries lubricant, and in most jurisdictions it cannot go down the floor drain untreated. An oil water separator is cheap compared to a violation. We walk through how this lands on a categorical metal finishing permit, and how to size the separator for a hot plating room, in Oil Water Separators for Plating Shops. The general rules are in Compressed Air Condensate Disposal.
Three mistakes that show up over and over
Chasing bath contamination for weeks before checking the air. If a bath goes bad and nothing changed in the chemistry, pull a filter element and look at it. Oil in the air is a quiet failure that nobody suspects until the third week.
Running blow-off with open pipe. It is loud, it violates 1910.242(b) dead-ended, and it is usually the biggest single waste point in the building. Engineered nozzles and air knives cut the air and cut the noise. We ran the numbers in Engineered Air Nozzles.
Treating the whole plant to spray-booth quality. You do not need carbon filtration on the drop feeding a sump pump. Split the header, treat the clean legs properly, and stop paying pressure drop on every element in the building. The order you build that train in matters, and we laid it out in Compressed Air Treatment.
Frequently Asked Questions
Can I use my shop air compressor for plating tank agitation if I filter it well enough?
You can, and shops do it, but it is the expensive answer on both counts. Filters slow oil carryover, they do not eliminate it, and a plating bath accumulates whatever gets through over months of operation. Compressed air also costs roughly seven times as much per CFM as blower air at agitation pressures. Use a regenerative blower for agitation and put your compressor on the loads that actually need pressure.
How much air does a plating shop need per square foot of tank?
For agitation, figure about 0.5 CFM per square foot of tank surface for normal plating and rinse work, and up to about 1.5 CFM per square foot for hard anodizing where the air is also removing heat from the parts. Pressure runs about 1 psi per 18 inches of solution depth. Those are blower numbers, not compressor numbers.
What dew point do I need for blow-off before chromating?
A refrigerated dryer holding a 38 degrees F pressure dew point is enough for indoor blow-off in most shops, as long as the piping does not run through a cold area where the air can condense again. If any of that header runs outdoors or through an unheated bay, you need desiccant drying on that leg or you will get water at the nozzle no matter what the dryer says.
Do I need an oil-free compressor for a plating shop?
Usually no. An oil-flooded screw with a properly built filtration train handles blow-off and spray fine, because that air contacts a part you are about to coat, not a food product or a sterile process. Oil-free becomes worth the money when you are feeding air directly into a bath, and even then a blower is the better tool for that job.
What kills compressors in finishing shops?
Acid mist in the intake air. If the compressor room shares air with the plating room, you are pulling acid fume across the inlet filter, into the airend and into the oil. Duct the intake from clean outside air, keep the compressor room under slight positive pressure, and watch your oil analysis for acid number climbing. It is the single biggest difference between a machine that runs 60,000 hours and one that dies at 20,000.
Where to start
If you are building a finishing line from scratch, buy the blower for agitation, buy a rotary screw sized on your AODD load, and split the header so the blow-off and spray legs get real treatment. If you are fixing an existing shop, start by pulling the elements on your clean legs and looking at them. In our experience that answers the question about half the time before you spend a dollar.
Good compressed air for metal finishing is not exotic. It is a correctly sized screw compressor, a dryer that actually holds dew point, filtration only where it earns its pressure drop, and agitation kept off the compressor entirely.
