A shop laying up a 42 foot hull pulls the bag down at 4 p.m., checks the gauge, and walks away happy. At 7 the next morning the bag is soft in two places and there is a milky haze on the inside of the film. Nobody touched it. The vacuum pump never shut off. What changed overnight was the shop temperature, and the water that was already sitting in the compressed air lines feeding the pneumatic hold-down clamps and the control valves came out of solution and went where it should not have gone.
That is the whole problem in one sentence. Compressed air for composites manufacturing is not a big-horsepower problem. It is a cleanliness and consistency problem wearing a horsepower costume. Shops that get burned here almost always bought enough CFM and never bought enough treatment.
What Makes a Composites Shop Different
Walk through a metal fab shop and the air system is doing one job: feeding tools. Walk through a composites shop and the air is doing four unrelated jobs at once, each with a different tolerance for dirt.
- Tool air. Pneumatic sanders, die grinders, trim routers, drills, rivet guns. High volume, forgiving about quality, brutal on total CFM.
- Process air. Chopper guns, gel coat guns, resin dispensing pumps, catalyst metering. This air touches the part. Oil carryover here causes fisheyes and bond failures.
- Control and clamp air. Mold clamps, press cylinders, autoclave door seals, valve actuators on infusion manifolds. Low volume, zero tolerance for water, because a stuck clamp in the middle of a cure is a scrapped part.
- Vacuum generation. Some shops run venturi generators off shop air instead of electric pumps. That is a legitimate choice for small bags and a terrible one for a 40 foot infusion.
One compressor feeds all four. The treatment train has to satisfy the fussiest one.
Start With Sanding, Because Sanding Sets the Number
Most composites shops are surprised by how much air their finishing bay eats. Everything else is rounding error by comparison.
| Tool or process | Typical demand at 90 psi | Notes |
|---|---|---|
| 6 inch random orbital sander | 6 to 9 CFM intermittent, 8 to 12 CFM continuous | Finishers run these nearly all shift |
| Inline or dual action board sander | 10 to 17 CFM | The single biggest draw in most layup shops |
| Die grinder, 1/4 inch | 4 to 6 CFM | Trimming and port grinding |
| Pneumatic trim router | 12 to 20 CFM | Short bursts, high peak |
| Gel coat or chopper gun | 10 to 18 CFM | Must be oil free and dry |
| Venturi vacuum generator | 4 to 6 CFM each, continuous | Cheap to buy, expensive to run |
| Pneumatic clamp or cylinder | Under 1 CFM average | Spiky, but tiny |
Count the sanders that genuinely run at the same time, not the ones hanging on the wall. Three finishers on board sanders is a 35 to 50 CFM load by itself, all day, no breaks. That is where your compressor size comes from. Add process guns and clamps on top and apply a diversity factor rather than stacking every nameplate, because nobody in a real shop triggers everything simultaneously.
A reasonable starting point for a mid size shop running three to four finishers plus a spray booth is 50 to 75 CFM, which is a 15 to 20 HP rotary screw. Our air compressors category covers that whole range, and if you want to work the math from scratch, read air compressor diversity factor before you buy.
Why a Rotary Screw and Not a Piston
A finishing bay is a continuous load, and that is what kills pistons. Most two stage pistons are honest for 50 to 75 percent duty. Sanders will hold you above that for six hours. The compressor runs hot, carries more oil past the rings, and dumps more water into the tank because the discharge temperature stays high. In a shop where oil carryover causes a cosmetic defect, this is not a reliability question, it is a scrap rate question.
Dew Point Is the Spec Nobody Writes Down
There is no composites-specific air quality regulation. No auditor is going to hand you a citation for a 50 degree dew point. The standard everyone in compressed air works from is ISO 8573-1, which grades air in three independent numbers: particles, water, and oil. You pick the class. Nobody assigns it to you.
Here is a practical target that holds up in a real shop:
| Where the air goes | Sensible target | What that means in practice |
|---|---|---|
| Sanders, grinders, general tool drops | Roughly ISO Class 1.4.2 | Refrigerated dryer plus particulate and coalescing filtration |
| Gel coat, chopper, and paint guns | Roughly ISO Class 1.4.1 | Add activated carbon at the booth drop |
| Prepreg freezer rooms and cold layup areas | Class 2 water or better, around minus 40 F | Desiccant dryer, because the room is cold |
| Lines that run outdoors or through an unheated bay | Pressure dew point below the lowest pipe temperature | Desiccant, no exceptions |
Note that ISO 8573-1 dew point classes are referenced to 68 F and 102 psig. If your system runs at a different pressure, the number on your hygrometer is not directly comparable to the class table until you convert it. That trips up more people than it should.
The rule that actually matters: your pressure dew point has to be below the coldest temperature any part of your piping ever sees. A 38 F refrigerated dryer is perfect for a heated building. Run that same air through 60 feet of pipe across an unheated shipping bay in February and you have built a condenser. Shop air dryers against the coldest pipe in the building, not the average.
Oil Is the One That Scraps Parts
Water makes a mess. Oil makes a warranty claim. A trace of lubricant carried through the line and onto a prepped bonding surface gives you a fisheye in the gel coat or, worse, a bond that passes visual inspection and fails in service. Secondary bonding surfaces are unforgiving.
You have two honest paths. Run a lubricated rotary screw with proper coalescing filtration and a carbon stage feeding the spray and bonding drops, which is what most shops do and it works. Or buy an oil-free compressor for the process air and keep the lubricated machine for tools. The second option costs more up front and removes an entire category of defect from your life. Which one is right depends on whether you are building fairings for a tier one aerospace customer or bass boats.
Be clear about what oil free does not mean: an oil-free compressor still makes water, still pulls in whatever hydrocarbons are in your shop air, and still needs filtration. The intake sits in a building full of styrene and solvent vapor. Oil free is not a substitute for a treatment train. Our air filters category has the coalescing and carbon stages either path needs.
Autoclaves, Ovens, and Where Shop Air Stops
If you cure in an autoclave, do not assume shop air pressurizes it. Most autoclaves above roughly 100 psi operating pressure are fed by a dedicated booster compressor or by nitrogen, not by your plant air header. Many aerospace specifications require an inert pressurizing medium rather than air because of the fire risk at elevated pressure and temperature. Your plant compressor typically feeds the booster suction, the door seals, and the control valves, not the vessel itself.
Be honest with yourself about what that means for sizing. The booster has a suction CFM requirement that goes on your load list. The vessel volume does not.
Oven cure and out of autoclave processes are simpler. There your shop air runs the clamps, the bag connections, and sometimes the venturi generators, and the sizing is ordinary.
Vacuum: Venturi Generators Versus a Real Pump
This is the decision that quietly costs shops the most money. A venturi generator is a block with no moving parts that converts about 4.5 CFM of 90 psi shop air into roughly 1.3 CFM of vacuum, pulling as deep as 29 inches of mercury at sea level. For a small repair bag you hold for two hours, it is excellent. You buy it for a couple hundred dollars and it never needs service.
For a 40 hour infusion cure, it is a bad trade. That 4.5 CFM of compressed air costs you roughly a horsepower of compressor energy, continuously, for 40 hours, to do work a fractional horsepower electric vacuum pump would do for a fraction of the power. Run four bags at once and you have dedicated several horsepower of your compressor to vacuum generation around the clock. If that is your pattern, buy pumps. If your pattern is short bags in a repair cell, venturis are the right answer. Our venturi vacuum generator versus vacuum pump piece runs the numbers properly.
Storage, Because Composites Shops Are Spiky
Trim routers, rivet guns and clamp banks all fire in short, violent pulls. A compressor cannot respond to those. Storage can. Put a receiver downstream of the dryer, sized at a minimum of 2 to 4 gallons per CFM of compressor output, and the spikes come out of the tank instead of out of your system pressure. Browse air compressor tanks and size for the hits, not the average.
Dry storage downstream of treatment is what buys you ride-through. Wet storage ahead of the dryer protects the dryer from slugs. Most shops want both.
The Mistakes Composites Shops Make
- Sizing on the compressor in the brochure instead of the sanders on the floor. Finishing is the load. Everything else is garnish.
- One filter for the whole plant, set to the strictest requirement. You pay a pressure drop penalty on every CFM to protect the 10 percent that goes to the spray booth. Filter the plant to a sane general class, then add point of use treatment at the booth and bonding drops.
- Buying a refrigerated dryer for a building with an unheated bay. The dew point follows the coldest pipe. Always.
- Leaving venturi generators on long cures. Check what you are actually spending in compressor horsepower before you call it free vacuum.
- Ignoring the compressor room. A 20 HP machine rejects roughly 60,000 BTU per hour. In a shop already fighting exotherm and cure temperature control, dumping that into the building in July is not free.
- Letting styrene-laden shop air into the compressor intake. Duct the intake to clean outside air if your layup area is in the same building. Solvent vapor degrades compressor oil and shortens separator life.
Frequently Asked Questions
What size air compressor does a composites shop need?
Count the sanders and grinders that genuinely run at the same time, since finishing dominates the load. Three finishers on board sanders is 35 to 50 CFM continuous on its own. A mid size shop with a spray booth usually lands between 50 and 75 CFM, which is a 15 to 20 HP rotary screw. Apply a diversity factor instead of adding up every nameplate on the wall.
Do I need an oil-free compressor for composites?
Not necessarily. A lubricated rotary screw with correct coalescing filtration plus an activated carbon stage at the spray and bonding drops produces air that is clean enough for most marine, automotive and industrial composites work. Oil free makes sense when your customer specification calls for it, or when a single bonding failure costs more than the price difference.
What dew point do I need for prepreg and infusion?
For normal shop temperatures, a refrigerated dryer at roughly a 38 F pressure dew point is adequate for tool and clamp air. If any line passes through an unheated space, or if you feed a cold prepreg storage room, you need a desiccant dryer in the minus 40 F range. The governing rule is that the pressure dew point must sit below the coldest temperature your piping ever reaches.
Can I run my vacuum bags off shop air?
For small bags and short holds, yes. A venturi generator uses about 4.5 CFM at 90 psi to produce roughly 1.3 CFM of vacuum and can pull 29 inches of mercury. For long infusions or multiple simultaneous bags, an electric vacuum pump costs far less to run, because a venturi left open for 40 hours is burning roughly a horsepower of compressor energy the whole time.
Does my autoclave run on plant air?
Usually not directly. Most autoclaves above about 100 psi are pressurized by a dedicated booster or by nitrogen, and many aerospace specifications require an inert gas rather than air because of the fire risk at elevated pressure and temperature. Your plant compressor normally feeds the booster suction, the door seal, and the control air, so size for those loads rather than for the vessel volume.
Where to Start
Measure before you spend. Put a flow meter on the header for a week and find out what your finishing bay actually draws, then watch your dryer outlet dew point on a humid afternoon with everything running. Most composites shops find the same two things: the compressor is closer to right than they feared, and the treatment is one stage short of where it needs to be. Getting compressed air for composites manufacturing right is mostly about putting the money in the dryer and the filters rather than in more horsepower you will never use.
