A tempering line does not forgive much. A lite comes off the roller hearth, hits the quench, and either it goes on the rack or it goes in the cullet bin. When a plant starts scrapping coated glass for haze or spotting, the first thing anyone blames is the washer. About a third of the time, it is the air.
Glass plants run one of the hardest compressed air duty cycles in manufacturing. The load is continuous, the ambient is hot, the plant is full of fine dust, and the air touches the product. That combination is why compressed air for glass manufacturing gets sized and treated differently from air in a general machine shop.
First, Separate Blower Air From Plant Air
This trips up more glass people than anything else, so get it straight before you price a compressor.
The quench on a tempering furnace is not compressed air. It is huge volume at a few psi, produced by centrifugal or positive displacement blowers. Same with most wide air knives on a washer exit. Those are low pressure, high flow machines, and a rotary screw compressor is the wrong tool and an expensive way to do that job. We do not sell tempering quench blowers, and if someone tries to sell you a 200 hp screw compressor to feed a quench, walk.
What we do supply is everything on the plant air and instrument air side, which in a glass plant is still a very large system:
- Pneumatic cylinders and actuators on the line: transfer arms, stops, tilt tables, lehr gates, cutting bridge heads
- CNC glass machining centers, drills, edgers and waterjets
- Seaming and beveling equipment
- Vacuum cups and venturi generators for handling and load stations
- Blow-off and drying at washer exits where an engineered nozzle beats an open pipe
- IS machine controls and actuation in container glass
- Instrument air for furnace and lehr control valves
Published figures for flat glass put a single ribbon line somewhere around 3,000 to 4,000 cfm at 125 psig just for cylinders and actuation. That is before you add the fabrication side. Even a mid size fabricator running two tempering lines, an IG line, a washer and a couple of CNC centers is usually looking at a 100 to 300 hp compressed air system.
What Your Air Has to Be Clean Enough For
There is no single glass industry air standard the way food has one. What there is instead is a product that shows every defect under a light box, and a coating that costs more than the glass under it.
Low-E and other soft coats are the reason to get serious. Oil carryover that a fabricator would never notice on a piece of steel will show as a bloom or a spot on a coated lite, and it will not wash off. Water is just as bad on the fabrication side: a wet line feeding a blow-off bar at the washer exit puts droplets back on glass you just dried.
Here is a sane target set by application.
| Where the air goes | Target | Why |
|---|---|---|
| General actuation, cylinders, stops | ISO 8573-1 Class 3.4.4 or better, refrigerated dryer | Keeps water out of valves and cylinder seals |
| Blow-off on coated or washed glass | Class 1.2.1, coalescing plus carbon | Air contacts the product surface |
| CNC and machining center air | Class 2.4.2 minimum | Spindle and tool changer air, plus chuck seating |
| Vacuum generators and cup handling | Class 2.4.2, dry and oil free at the point of use | A dropped lite is the most expensive failure in the plant |
| Outdoor or unheated piping runs | Pressure dew point at least 18F below the coldest metal temperature | Prevents freeze-ups in a northern plant |
If you are not sure what those class numbers mean, our breakdown of the ISO 8573-1 air quality classes lays them out in plain terms.
Picking the Compressor
Rotary screw, almost always
Glass runs continuous. A piston compressor sized for that load will beat itself to death, and its duty cycle rating was never meant for 24 hours a day. Lubricated rotary screw compressors are the default for a reason: 100 percent duty, sane maintenance intervals, and enough size range to build a plant system out of two or three machines instead of eight.
Base load plus VSD trim
Glass demand is lumpy. A tempering line cycling, a washer running, and an IG line indexing all land at different moments. The setup that works is one or two fixed speed machines carrying base load and one variable speed machine trimming the swing. That keeps the fixed machines fully loaded (where they are efficient) and stops the whole plant from load/unload cycling all day.
Oil free where it matters
You do not need to go oil free for the whole plant. Most glass plants run lubricated screws with good filtration and then treat the critical legs separately. If you have a coating line where nothing oil bearing can be anywhere near the product, that specific leg is where an oil free machine or a dedicated treatment train earns its money. Buying oil free for the entire plant when 80 percent of the load is cylinders is money you could have spent on storage.
Cooling, because the plant is already hot
This is the item glass plants get wrong most often. A compressor room downwind of a furnace or a lehr is pulling 105F air all summer. Every compressor is rated at an ambient limit, and running against it means high temperature trips at exactly the worst time. Duct the intake to outside air, filter it well because the plant is dusty, and if the room is tight, look at water cooled packages instead of fighting the air.
Storage, Dryers, and Filtration
Glass loads are pulsed. Transfer arms, tilt tables and IS machine actuation all pull hard for a second and then stop. Without storage, the compressor chases every one of those pulses and your line pressure sags right when the cylinder needs it.
A reasonable starting point for a screw based system is 4 to 6 gallons of receiver per cfm of compressor capacity, more if you have big intermittent hits. Split it: a wet receiver ahead of the dryer to knock out bulk water and let pulsation settle, and a dry receiver downstream to feed demand. Our air receiver tanks cover both spots.
For drying, a cycling refrigerated dryer handles general plant air fine and costs almost nothing to run at part load. Go to desiccant when you have unheated outdoor runs, a coating line, or anywhere you genuinely need -40F pressure dew point. Size the dryer at the actual inlet temperature it will see, not the catalog 100F, because in a glass plant that air is coming in warm. Browse compressed air dryers by type and capacity.
Filtration follows the same order it does anywhere: separator, then particulate, then coalescing, then carbon only where you need it. Do not stack filters you do not need, because every element is a pressure drop and a recurring cost. Full range of compressed air filters and elements here.
The Mistakes Glass Plants Actually Make
Using compressed air for blow-off that a blower should do. A one inch open copper tube at 90 psi eats roughly 30 cfm and never stops. Across a washer exit with six of them, that is a 40 hp compressor running full time for drying. Engineered nozzles cut that a lot, and for wide continuous drying a blower knife is cheaper still.
Sizing on horsepower instead of cfm. Two 75 hp machines from different builders can differ meaningfully in delivered cfm at 125 psig. Compare the CAGI data sheet, not the nameplate.
Ignoring the intake location. Glass plants have fines in the air and heat everywhere. An intake pulling from inside the plant will load the inlet filter fast and push oil carryover up.
Running the whole plant at the highest pressure any one machine needs. If one CNC center wants 110 psi and everything else wants 85, put a booster or a dedicated regulator on that machine instead of lifting system pressure. Every 2 psi of system pressure is roughly 1 percent on the power bill.
Skipping leak repair. In a plant with thousands of fittings and constant vibration, leaks typically account for 20 to 30 percent of total air production. That is the cheapest capacity you will ever buy back.
Frequently Asked Questions
How much compressed air does a glass tempering line need?
The quench itself is blower air, not compressed air, so it does not count against your compressor. The compressed air side of a tempering line is actuation, stops, transfer and blow-off, which typically lands in the tens to low hundreds of cfm depending on line width and automation. Meter it before you buy, because the spread between lines is large.
Do I need an oil free compressor for a glass plant?
Usually not for the whole plant. A lubricated rotary screw with a properly built treatment train gets you to very low oil carryover for general plant air. Where you have direct contact with coated glass or a customer specification that says oil free, put an oil free machine or a dedicated treated leg on that application rather than converting everything.
What dew point should a glass plant run?
Indoor, heated plant air is fine at a refrigerated dryer dew point in the high 30s F. If piping runs outside or through an unheated area, you need a pressure dew point at least 18F below the lowest temperature that pipe will ever see, which in most northern plants means a desiccant dryer at -40F.
Why is there water at my washer blow-off bar?
Three usual causes: the dryer is undersized for the inlet temperature it is actually seeing, there is no drip leg or drop taken from the top of the header at that branch, or a drain has failed and the receiver is holding water. Check the drains first, they are the cheapest thing on the list.
Is a vacuum system separate from the compressed air system?
It can be either. Venturi generators run off plant air and are simple and cheap at low duty, but they consume air continuously while holding. For a load station cycling all shift, a dedicated vacuum pump usually costs less to run. Add up the hold time before you decide.
Where to Start
If you are building or rebuilding a glass plant air system, do it in this order: meter what you actually use, fix leaks, set the air quality target by application, then size the compressor and storage to the measured demand. Getting compressed air for glass manufacturing right is mostly about not paying compressor money for blower work, and not paying oil free money for cylinder air.
