Compressed air is the most expensive utility on most thermoforming sites, and the most commonly oversold. A compressor that is too small starves the line; one that is too large wastes energy every hour it runs. This guide sets out the calculation in a form you can apply to your own machine list.
What Actually Consumes Air on a Thermoforming Line
Before choosing a compressor, it helps to know where the air goes. On a typical forming cell the biggest consumer is the vacuum system, followed by the forming press cylinders, the trim station, and continuous users such as blow-off nozzles and part conveying. Downstream of the former, a take-out robot adds its own intermittent demand, and a counting and packing machine takes air for its pneumatic actuators.
Each of these behaves differently. Cylinders draw in bursts, vacuum generators draw heavily but only during forming, and blow-off runs continuously. A demand schedule that mixes the three is what makes the sizing calculation meaningful.
Free Air Delivery Is Not Usable Air
Compressor data sheets quote free air delivery, measured at the discharge flange under reference conditions. By the time that air has passed through a dryer, two or three filter stages and a pipe network, some of it has been lost to pressure drop and some to the treatment itself. The usable figure is what remains at the point of use, and it is typically five to twenty per cent lower.
This is why our own tables publish both numbers. A 20HP unit delivering 2.54 m³/min is quoted with 2 m³/min of usable air, and it is the second figure that should be compared against your demand schedule.
Step 1 — Build a Demand Schedule
List every consumer, the volume it uses per cycle and the number of cycles per minute, then add the continuous loads on top. Where a machine’s consumption is not documented, measure it or ask the supplier rather than guessing. The total, not the largest single consumer, is what the compressor has to supply.
If two machines can run at once, add their demands. If they never overlap, the larger of the two sets the requirement. Getting this wrong by assuming everything runs together produces an oversized machine; assuming nothing overlaps produces a line that stalls.
Step 2 — Add the Treatment Allowance
Once the demand is known, add the allowance for drying, filtration and distribution. A practical approach is to take the calculated demand, add the pressure-drop allowance for your pipe run, then divide by a treatment efficiency figure of about 0.9 to arrive at the free air delivery the compressor must produce.
If that result falls between two sizes, the right move is usually to step up rather than to shave the margin, because air demand tends to grow as a plant adds machines.
Step 3 — Size the Receiver
The receiver performs two jobs: it damps the pressure drop that occurs when a large cylinder fires, and it lets the compressor unload or slow down without the header pressure collapsing. A common rule of thumb is one litre of storage for every litre per minute of free air delivery, but forming cycles are often sharper than that rule assumes.
Where the cycle is short and the drawdown is deep, size at the top of the recommended band. Our own recommendations run from 500 litres on the smallest frame to 2000 or 3000 litres on the largest, and the reason for the range is precisely this variation in demand shape.
Step 4 — Choose the Dryer and Filtration
A refrigerated dryer is sufficient for general workshop air and is what we recommend for forming cells. Its rated capacity should exceed the compressor’s delivery by a comfortable margin, because a dryer that is matched exactly becomes the bottleneck on a hot afternoon. Three-stage filtration then removes particulate and oil aerosol before the air reaches the process.
Note that in a permanent-magnet variable-speed installation the dryer is normally sized for the compressor’s maximum output, not its average, since the machine can run flat out when demand requires it.
Step 5 — Fixed Speed or Permanent-Magnet Variable Speed
A fixed-speed compressor produces a fixed volume and unloads when the receiver is full. That is efficient only when demand is close to its rated output. As soon as demand falls, the machine continues to consume a substantial part of its rated power doing very little.
A permanent-magnet variable-speed machine varies its rotational speed to match demand, so the motor produces only the air that is actually needed. Direct drive removes belt losses as well, and soft starting keeps the electrical inrush modest. On a forming line where demand swings through the shift, the difference shows up in the electricity bill rather than in the specification sheet.
The VihoX PM VSD Range at a Glance
Every frame below shares the same design basis: 0.8 MPa working pressure, permanent-magnet direct drive, variable-frequency soft start, air cooling and three-stage filtration. The differences are output, connection size and the ancillary equipment that should accompany them.
| Model | Motor | Free Air Delivery | Usable Air | Outlet | Receiver | Dryer | Typical Duty |
|---|---|---|---|---|---|---|---|
| VihoX-10PM | 7.5 kW | 1.13 m³/min | 0.9 m³/min | G3/4 | 500 L | ≥1.5 m³/min | One small pneumatic cell |
| VihoX-20PM | 15 kW | 2.54 m³/min | 2 m³/min | G1 | 500-1000 L | ≥3.0 m³/min | Medium forming equipment |
| VihoX-30PM | 22 kW | 3.64 m³/min | 2.9 m³/min | G1-G1¼ | 1000 L | ≥5.0 m³/min | Plant drawing 2.0-2.8 m³/min |
| VihoX-50PM | 37 kW | 6.31 m³/min | 5 m³/min | G1½ | 1000-1500 L | ≥8.0 m³/min | One complete cup line |
| VihoX-100PM | 75 kW | 14.05 m³/min | 11 m³/min | G2 | 2000-3000 L | ≥16.0 m³/min | Two to three forming machines |
Mistakes That Show Up Months Later
The most common error is sizing on peak demand alone and then discovering the treatment equipment cannot keep up, so the line runs wet. The second is buying the smallest machine that meets today’s figure and having no room to add a second machine. The third is neglecting the header: undersized pipework raises velocity, increases pressure loss and forces the compressor to run at a higher setting than the process needs.
All three are avoidable at the specification stage, and all three are expensive to correct once the plant is running.
Frequently Asked Questions
Why is usable air lower than free air delivery?
Free air delivery is measured at the compressor outlet. Drying, filtration and the pipe network all remove pressure and volume before the air reaches a machine, so the usable figure is the one you can actually plan production around.
How do I estimate what a thermoforming machine consumes?
Add the swept volume of every cylinder, multiply by the strokes per minute, then add continuous consumers such as blow-off, conveying and any vacuum generator. Vacuum forming is usually the largest single draw on a line.
What receiver volume should I choose?
Enough to absorb the difference between the compressor’s output and peak demand for the duration of the peak. Short, sharp forming cycles need a larger vessel than steady processes, which is why suppliers quote a range rather than a single size.
Is PM VSD worth the extra cost over a fixed-speed machine?
Where demand varies during the shift, yes. A permanent-magnet variable-speed drive trims motor power to match demand instead of running at full output and unloading, and that saving accumulates over years of operation.
What dew point do I actually need?
It depends on where the air is used. General workshop air is usually satisfied by a refrigerated dryer; outdoor lines, instruments and any application exposed to freezing conditions need a desiccant or membrane dryer.
How often do filters and separators need attention?
Follow the manufacturer’s interval, but watch the pressure differential. A rising drop across a filter stage means the element is loading up, and that costs energy every hour it continues.
Can one compressor serve several machines?
Yes, and it is a common arrangement. Size the header and the treatment for the combined demand plus margin, and consider whether the plant can tolerate a single point of failure before deciding against a second unit.
What pressure should the system run at?
Set it as low as the highest-pressure consumer allows. Every extra bar costs energy continuously, so it is worth checking whether the machine that demands the highest pressure really needs it.
Have Your Air Demand Checked
List the air consumers on your line and tell us which of them run simultaneously. From that we will calculate the free air delivery, receiver volume and dryer capacity your plant actually needs. The PM VSD screw compressor range covers duties from a single forming cell up to a hall running several lines. Reach the factory on WhatsApp: +86 147 7547 6896.