For an offset press, the compressor is sized from the air consumption given in the press installation sheet, added to the other machines in the print shop, with a margin of about 25%. In the worked example below, two sheetfed presses and their peripherals run on a 7.5 kW 4-in-1 screw compressor. After flow, the deciding factor is dry air.
This article looks at the press itself. For the whole print shop, see our page on the compressor for printing and packaging.
Where does the compressed air go on an offset press?
Depending on the model and its options, compressed air on a sheetfed press supplies:
- the cylinders that engage and lift the printing cylinders and rollers;
- plate clamping and, on recent presses, automatic plate changing;
- blanket and roller washing systems;
- the anti set-off powder sprayer at the delivery, which uses air to spray the powder;
- blow guns for cleaning around the machine.
Suction and blowing at the feeder, which separate and carry the sheets, usually come from a dedicated pump supplied with the press, not from the compressed air network. Check the documentation: if your press takes them from the network, consumption is on another scale.
The calculation, step by step
The starting figure is on the press installation sheet: a consumption in L/min or NL/min, and a minimum supply pressure. If it is missing, ask the press manufacturer or installer.
Indicative example: a print shop with two presses and a folding machine. Values marked "assumption" must be replaced with those on your own sheets.
| Consumer | Quantity | Unit consumption | Total |
|---|---|---|---|
| Sheetfed offset press (assumption) | 2 | 250 L/min | 500 L/min |
| Anti set-off powder sprayer (assumption) | 2 | 50 L/min | 100 L/min |
| Folding machine, cylinders and blowing (assumption) | 1 | 60 L/min | 60 L/min |
| Cleaning blow gun | 2 | 150 L/min, counted at 50% | 150 L/min |
| Total | 810 L/min |
- Add up. The presses and powder sprayers use air throughout the run, so they count in full. A blow gun is used now and then: our configurator counts it at 50%, with the receiver absorbing the peaks. Total: 810 L/min.
- Add the margin. 810 × 1.25 = about 1,010 L/min.
- Read the flow at the right pressure. If the press needs 6 bar at its inlet, a compressor set to 8 bar leaves room for network losses. The K 75 delivers 1.1 m³/min at 8 bar, or 1,100 L/min.
If your press sheet gives flow in m³/h or cfm, our flow and pressure converter turns it into L/min.
Which compressor for which print shop?
- One or two presses, no plant room: the K 75. A 7.5 kW 4-in-1 screw compressor with a 420 L receiver, built-in refrigerated dryer and filtration, 66 ± 2 dB(A) according to its data sheet.
- Three presses, or two presses and a finishing line: the K 110. 1.7 m³/min at 8 bar, 340 L receiver, built-in dryer and filtration.
- Receiver and dryer already in place: the S 110. The same flow, as a standalone compressor.
A press alternates between make-ready and production, so consumption rises and falls during the day. Our variable speed screw compressors follow these changes instead of running unloaded, as explained in our article on variable speed.
Traps specific to offset presses
- Water in the powder sprayer. Powder that picks up moisture forms lumps, clogs the nozzles and settles unevenly on the sheets. A refrigerated dryer after the compressor and working drains solve the problem at source: see water in compressed air.
- Oil and particles. Our compressors are lubricated. Where air blows close to the sheets, add fine filters and define the air quality class with our technicians, using our guide to ISO 8573-1.
- Pressure at the press. A long or undersized network makes pressure drop. Fit a gauge at the press inlet and compare it with the minimum pressure on the sheet.
- Make-ready peaks. Automatic plate changing needs a lot of air in a short time. A well-sized receiver absorbs these peaks: our air receiver size calculator gives an order of magnitude.
- Leaks at night. A network left under pressure over the weekend runs the compressor for nothing. Our leak cost calculator puts a figure on the loss.
Send us your press installation sheets with a quote request and we will check flow and pressure with you, or enter your equipment in the configurator.
For your trade: Compressor for printing and packaging.