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Quotes - Engineering - SalesHigh-Pressure Air Dryers for PET Blow Molding
A stretch blow molder runs on two compressed air systems. Plant air at 100 to 125 psig drives the preform feed, the stretch-rod and clamp actuators, the grippers and the conveyors. Blow air at 500 to 725 psig, most often 580 psig (40 bar), comes from a separate multi-stage booster: a pre-blow starts the bubble, then full pressure forces the softened preform against the mold wall in a fraction of a second.
The mold is cooled with 40 to 55°F chilled water so the bottle sets before ejection. Water vapor in the blow air condenses on that cold wall and comes off in the bottle as haze, spots and an orange-peel surface; over months it pits the aluminum cavity and corrodes the blow valve block, and oil aerosol leaves a film and an odor a beverage filler will reject. Both air systems need a refrigerated dryer, and the high-pressure one needs a dryer built for the pressure.
Blow air is dried after the booster aftercooler and the high-pressure receiver, ahead of the blow-air header, with the dryer's flow read from its 500 or 725 psig rating. At 580 psig a given SCFM holds about a fifth of the water it would at 100 psig, and a 38°F pressure dew point there corresponds to roughly -30°F at atmosphere, so a refrigerated high-pressure dryer is enough for bottle quality without a desiccant tower.
Flow follows the bottles. A 500 ml water line at 20,000 bottles an hour draws about 500 to 800 SCFM of blow air without air recovery and 30 to 40 percent less with it; a multi-stage booster delivers roughly 3 SCFM per hp at 580 psig, so that line sits behind a 200 to 300 hp machine. The HHPR Series covers 115 to 2,400 SCFM at 680 psig and the APET Series 45 to 1,000 SCFM at 725 psig.
Request a PET blow-air dryer quote
Most common PET blow pressure (40 bar)At 580 psig each SCFM carries roughly a fifth of the water it would at 100 psig, which lets a refrigerated high-pressure dryer reach bottle quality without desiccant.
What wet or oily blow air does to a bottle
The defects that trace back to the air rather than the preform
- Haze, water spots and orange peel where condensate forms on the chilled mold wall
- Pitting in aluminum cavities and corrosion inside the blow valve block
- Oil film and odor inside the bottle, caught by the filler at incoming inspection
- Sticking pre-blow and exhaust valves, and a recovery loop feeding wet air back to the pre-blow
The specification for a PET blow-air dryer
What to ask for on each side of the booster
- High-pressure side: a dryer rated 680 or 725 psig, capacity read at blow pressure, 38°F pressure dew point
- Low-pressure side: a standard dryer sized to the screw compressor, with a 1 micron prefilter and a 0.01 micron afterfilter
- Condensate drains rated for the pressure on the high-pressure receiver, separator and dryer
- Inlet air at or below 100°F from the booster aftercooler; every 10°F above costs 15 to 20 percent of capacity
Dryers that fit a PET line
High-pressure ratings are at 500 to 725 psig; the plant-air dryers and filters are rated at 100 psig and 100°F inlet. The retired HPET Series cross-references size for size to the HHPR Series.
| SERIES | TYPE | FLOW RANGE | MAX INLET | MAX PRESSURE |
| HHPR Series | High Pressure | 115 to 2,410 SCFM | 120°F | 680 psig |
| APET Series | High Pressure | 45 to 1,000 SCFM | 120°F | 725 psig |
| DXR Series | Non-Cycling | 10 to 4,200 SCFM | 131°F | 203 psig |
| FLXA Series | Cycling | 75 to 2,000 SCFM | 120°F | — |
| GFN Series Filters | Compressed Air Filter | 6 to 1,500 SCFM | — | 232 psig |
| NGF Series Filters | Compressed Air Filter | 20 to 1,500 SCFM | — | 250 psig |
| NETD, NPTD & NHPTD Timed Drains | Condensate Drain | Call for sizing | — | 6,000 psig |
| 530 & 600 Series Electric Drains | Condensate Drain | Call for sizing | — | 1,500 psig |
Specify both sides of the booster before ordering.
- High-pressure dryer rated 680 or 725 psig, sized at blow pressure
- Standard dryer and filter pair on the 100 to 125 psig plant air
- Drains on the high-pressure side rated for the full blow pressure
Pressure works for the dryer, temperature works against it
Compressing air to 580 psig squeezes most of its water out in the booster's intercoolers and aftercooler before the dryer sees it, which is why a high-pressure dryer handles more SCFM in a smaller frame than a plant-air unit. What it cannot shrug off is hot inlet air: a booster aftercooler fouled by a dusty compressor room sends 120°F air to the dryer and takes a third of its capacity away, and the first sign is haze on the afternoon shift's bottles. Read the dryer inlet temperature on a summer afternoon before assuming the dryer is undersized.
- Read the dryer capacity at blow pressure, then correct for inlet and ambient temperature
- Keep the booster aftercooler and the dryer condenser clean; both live in the same dusty room
- Dry downstream of the air recovery return, or dry the recovered stream separately
Where the two dryers go
Low-pressure side: screw compressor, aftercooler, receiver, 1 micron prefilter, refrigerated dryer, 0.01 micron afterfilter, then the plant header that feeds the molder's actuators and preform handling. High-pressure side: the booster discharge runs through its aftercooler, a high-pressure receiver, the high-pressure dryer and on to the blow-air header. Feeding the booster with dried plant air lightens the load on the high-pressure dryer but does not replace it; the booster's own cooling condenses more water and the receiver keeps the air saturated.
Sizing at blow pressure
The sizing figure is the booster delivery in SCFM at 580 psig, corrected for inlet and ambient temperature like any refrigerated dryer: 110°F inlet air costs about 20 percent, a 110°F compressor room about 12 percent. A booster delivering 600 SCFM into a 105°F room wants the 750 SCFM model, not the 600. The operating pressure page covers the correction factors.
Air recovery systems
Most new blow molders recover part of the exhaust from each blow and reuse it for the pre-blow or return it to the booster inlet. That air has been through a chilled mold and back, so the recovery loop is the wettest part of the machine and the usual reason a dryer that looks large enough on paper still lets haze through. Keep the high-pressure dryer downstream of the point where recovered air rejoins the supply, or dry the recovered stream separately.
Non-cycling on both sides, usually
A bottle line runs three shifts at a rate that changes only at a size changeover, so the high-pressure dryers are non-cycling machines and the plant-air side is the same steady-load case: a DXR Series non-cycling dryer costs least and holds its dew point. A FLXA Series cycling dryer on the plant air only pays back where the molder is one of several lines that come and go through the week. A lubricated booster needs oil removal downstream of the high-pressure dryer in housings rated for blow pressure; an oil-free final stage, which most beverage lines specify, avoids the question.
Drying 580 psig blow air for bottle quality
Downstream of it, unless the recovered stream gets a dryer of its own. Recovered exhaust has just been through a chilled mold and the blow valves, so it is the wettest air in the machine, and rejoining it to the supply after the dryer sends that water straight back into the pre-blow. Many haze problems blamed on an undersized dryer turn out to be a recovery loop piped in on the wrong side of it, which no amount of extra capacity fixes.
It lightens the load but does not let you drop a size. Compression to 580 psig multiplies the partial pressure of the remaining vapor about fivefold, so feed air dried to 38°F leaves the booster with a dew point not far below its aftercooler outlet temperature, and cooling in the high-pressure receiver or piping condenses it again. The dryer is still sized on the booster's delivery at blow pressure, corrected for inlet and room temperature. Dry feed air does buy less booster drain cycles and longer valve life.
Coalescing oil-removal filters in housings rated for blow pressure, placed downstream of the high-pressure dryer and served by high-pressure drains. A plant-air filter housing rated for a few hundred psig has no place on a 580 psig line. Most beverage lines avoid the question with an oil-free final booster stage, because oil film and odor inside a bottle are what a filler rejects at incoming inspection, and filtration only manages a source that oil-free compression removes.
About a third of its capacity once the dryer inlet reaches 120°F, and a dusty compressor room gets there quickly. Every 10°F above a 100°F inlet takes away 15 to 20 percent, so a dryer correctly sized in spring lets haze through by July. Read the dryer inlet temperature on a hot afternoon, clean the aftercooler and the dryer condenser together, and only then judge whether the dryer is genuinely too small for the booster.
More on high-pressure blow air
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The correction factors from 60 to 725 psig, and why they favor the high-pressure dryer.
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