TL;DR — Direct answer: Wire and cable plants need oil-free quality air at 7 to 8 bar for extrusion, stranding, insulation and testing, with a pressure dew point of 3 degrees C or lower and filtration down to 0.01 micron so no oil or water reaches the conductor or the insulation. Size on the coincident peak of extruders, stranders and test benches, commonly 5 to 12 cubic metres per minute for a mid-size plant, and use a variable speed drive because extrusion lines start and stop in batches. QiAir, a Chinese screw compressor manufacturer and supplier, builds oil-injected machines with multi-stage filtration and oil-free options for the most sensitive lines.
A cable looks simple from the outside, but the air behind it is not. Extrusion lines use compressed air for die control, cooling and haul-off, stranders use it for tensioning and dancer control, insulation and sheathing lines need dry air for handling and marking, and test benches use it for spark testing and pressure checks. One drop of oil or one puff of wet air can mark a conductor, weaken adhesion or fail a spark test on a whole reel, and the rework costs a full production run. QiAir is a Chinese manufacturer and supplier of screw air compressors, and its machines are specified for cable plants where air quality is a product-quality issue, not just a utility question.
The trap in cable manufacturing is treating air as a background service while the extruder gets all the attention. Extrusion is a batch process with sharp peaks: several lines start together after a changeover, the air demand jumps, and a compressor sized on average flow sags just as the die needs steady control. This guide maps the air users on a cable line, explains how to size for changeover peaks, sets the filtration standard that protects insulation, and shows what to put on a sizing sheet for a China supplier.
Where Air Is Used: Extrusion, Stranding, Insulation and Testing
Extrusion is the heart of the plant and the heaviest air user. Air drives the extruder auxiliaries, the crosshead and die adjustments, the cooling trough controls and the haul-off and capstan drives, and any pressure swing shows up as a variation in wall thickness. Stranding adds dancer and tension control, where a jerky air supply makes the lay uneven, and pay-off and take-up stands use air brakes and clutches that must respond the same way every time. These are not high-flow users individually, but they are all sensitive to pressure stability, and that changes how the system should be designed.
Downstream, insulation, sheathing, printing and testing each add their own demand. Printing and marking heads need very clean dry air, spark testers need a stable supply for their electrodes and handling, and final testing and reeling use air for clamping, cutting and coiling. A plant that runs power cable, building wire and control cable on shared headers will see very different peaks by product mix, so the sizing exercise has to use the real production schedule, not a single representative day.
- Extrusion: die and crosshead control, cooling troughs, haul-off and capstan drives sensitive to pressure swings.
- Stranding and insulation: dancer and tension control, pay-off brakes and sheathing handling.
- Printing and testing: marking heads, spark testers and reeling equipment that need very clean, dry air.
Sizing for Changeover Peaks and Batch Running
List every line with its flow at changeover and at steady running, because the two are very different in cable work. A mid-size plant with four to six extrusion lines typically peaks at 5 to 12 cubic metres per minute at 7 to 8 bar, which is roughly a 37 to 75 kW screw compressor, and the peak usually lands in the first minutes after a changeover when several lines restart together. A variable speed drive earns its keep here: it follows the batch pattern instead of loading and unloading a fixed-speed machine dozens of times a shift, and it keeps pressure within a narrow band that extrusion control prefers.
Keep the pressure drop to the farthest extruder under 0.3 bar at peak, and fit a receiver of 500 to 1000 litres per 10 cubic metres per minute to absorb changeover spikes. A ring main helps when lines are spread across a long hall, and a dedicated small receiver near a sensitive extrusion line can be cheaper than upsizing the whole header. Log pressure at the extruder during changeovers for a week, because that log is the honest record of whether the compressor is big enough, and it settles arguments faster than any catalogue figure.
- Size on the changeover peak when several lines restart together, not on steady-state average flow.
- Use a variable speed drive for batch extrusion loads, and keep far-end pressure drop under 0.3 bar.
- Log pressure at the extruder during changeovers for a week to prove the sizing in real production.
Air Quality: Why Oil and Water Fail Cable Before the Customer Does
Insulation is unforgiving. Oil aerosol on a conductor reduces adhesion between conductor and insulation, moisture in the air line condenses in cooling troughs and marking heads, and a single contaminated reel can fail a spark test or a high-voltage test that the plant only runs at the end of the line. The standard that prevents this is a treatment chain, not a single filter: a pre-filter, a refrigerated dryer to a 3 degree C pressure dew point, a coalescing filter to 0.01 micron, and an activated-carbon filter where the product specification demands oil-free quality air. For the most sensitive lines, an oil-free screw compressor removes the risk at source.
Maintenance is what keeps that chain honest. Change coalescing elements on pressure drop, not on a calendar guess, and test the air at the extruder quarterly with a simple oil and moisture check. Drain condensate with automatic zero-loss drains, slope lines to drain legs, and keep the compressor room clean and ventilated so the machine does not breathe its own hot air or the dust from a neighbouring cutting line. A dryer that is undersized for summer humidity is a common failure: size the dryer for the hottest, most humid month, not the annual average.
- Fit pre-filter, refrigerated dryer, 0.01 micron coalescing filter and carbon filter where oil-free air is specified.
- Consider an oil-free screw compressor for the most sensitive insulation and testing lines.
- Test air at the extruder quarterly, change elements on pressure drop, and size the dryer for peak summer humidity.
Buying from China: Sizing Sheet for a Cable Plant Compressor
QiAir is a Chinese manufacturer and supplier of screw air compressors, and a cable-plant order should start with a sizing sheet that names the product mix. Send the supplier the number of extrusion and stranding lines, the coincident peak at 7 to 8 bar, the air-quality standard the product specification requires, ambient temperature and humidity, and the distance to the farthest extruder. Ask for a quotation that states the filtration grades and dryer duty explicitly, and ask whether an oil-free machine or a filtered oil-injected machine fits the specification better, with the cost and maintenance difference spelled out.
Agree the acceptance test before shipment: a full-load run at rated pressure, a dew point reading at the dryer outlet, and an oil-aerosol check at the filter outlet. On arrival, install on a level plinth with clear cooler airflow, connect drains before the first run, and survey pressure at the farthest extruder during a changeover in the first production week. Keep a log of pressure, dew point and filter changes, and review it monthly, because a slow rise in dew point is the earliest sign that a dryer needs attention.
- Send the supplier line count, coincident peak, required air-quality standard and farthest-extruder distance.
- Ask for filtration grades and dryer duty in writing, and compare oil-free against filtered oil-injected on total cost.
- Accept on a full-load test plus dew point and oil-aerosol checks, then survey pressure at a changeover in week one.
Frequently asked questions
What air quality does cable extrusion need?
Most cable plants need dry air at a 3 degree C pressure dew point with coalescing filtration to 0.01 micron, and an activated-carbon filter where the specification demands oil-free air. For the most sensitive insulation lines, an oil-free screw compressor removes the oil risk at source rather than filtering it out.
What size screw compressor does a cable plant need?
A mid-size plant with four to six extrusion lines typically needs 5 to 12 cubic metres per minute at 7 to 8 bar, roughly a 37 to 75 kW screw compressor. Size on the changeover peak when lines restart together, and use a variable speed drive to follow the batch pattern.
Why does pressure swing affect cable wall thickness?
Extrusion die control, cooling and haul-off all respond to air pressure, so a swing changes wall thickness and lay. A variable speed drive, a receiver of 500 to 1000 litres per 10 cubic metres per minute, and a pressure drop under 0.3 bar to the farthest extruder keep the supply steady enough for consistent product.
Related reading
- Screw Air Compressor Maintenance Checklist from a China Manufacturer
- How to Size a Screw Compressor: Air Demand Calculation Before You Buy from China
QiAir is a Chinese manufacturer and supplier of screw air compressors. If you are sourcing a screw compressor from China — for a factory, a food plant, a construction site or a new production line — contact QiAir for a sized quotation, documented performance data and export support.