How cable blowing actually uses air
In cable jetting, the blowing machine pushes the cable into the duct with drive wheels or caterpillar belts, while compressed air enters the duct through a sealed chamber and flows along it at high velocity. The moving air drags along the full length of the cable, so the cable “floats” rather than being pulled from the far end. This distributed drag is what allows long continuous runs in high-density polyethylene (HDPE) ducts and microducts without exceeding the cable’s allowable tensile or compressive load.
Two consequences follow for the air supply:
- Flow matters as much as pressure. The air escapes from the far end of the duct continuously. If the compressor cannot keep up, pressure at the machine sags and the drag force falls, usually just when the cable is deepest into the route and friction is highest.
- Air quality reaches the cable and duct directly. Hot air softens HDPE and can damage the lubricant film; free water and oil aerosol increase friction and leave residue in the duct, and compressor oil can attack some duct lubricants and cable sheaths.
The blowing machine may also have a pneumatic drive motor. Where it does, that drive consumes air in addition to the duct air. Always ask whether the machine supplier’s stated air requirement includes the drive, so the demand is neither double-counted nor missed.
What pressure and flow does fibre cable blowing need?
There is no single figure for every job; the machine and duct supplier’s method governs. As a typical industry pattern, contractors use portable compressors in the 10–14 bar (145–203.1 psig) range, with flow chosen to suit the duct diameter and route length. Small microducts need much less air than 40 mm or 50 mm ducts. The table below is a general planning guide, not a substitute for the machine manufacturer’s data.
| Duct type (typical) | Typical working pressure at machine | Typical compressor class | Notes |
|---|---|---|---|
| Microducts (small bore, micro-cables) | Often 10–14 bar (145–203.1 psig), as per machine | Smaller portable units, often 185–250 cfm class or less | Lower flow, but clean dry air is still essential |
| 32–40 mm HDPE ducts | Typically 10–12 bar (145–174 psig) | 250–400 cfm class | The most common telecom and utility backbone work |
| 40–50 mm HDPE ducts, long runs | Typically 12–14 bar (174–203.1 psig) | 400–600 cfm class | Long routes and tandem (cascade) blowing raise demand |
| Duct testing, cleaning and sponge/foam pig runs | As per duct pressure rating | Same unit as for blowing | Done before the cable goes in; see below |
Two practical rules apply. First, never set pressure above the duct’s rated working pressure or the machine’s limit, whatever the compressor can deliver. Second, compare flows on the same basis: free air delivery (FAD) in cfm or m³/min at the compressor, not “displacement” or a catalogue figure for a different model.
Why the air must be clean, dry and cool
A portable screw compressor discharges hot, saturated air. Oil-injected machines typically discharge at around 75–95 °C at the airend outlet before the package cooler, and even after the internal cooler the air leaving the service valve is warm and carries water and some oil aerosol. On a hot Telangana or Andhra Pradesh afternoon with 40–45 °C ambient, that air can be well above what HDPE ducts and cable lubricants tolerate.
A typical cable-blowing air train is:
- Compressor – portable diesel (or electric where site power exists), sized for the duct and route.
- Aftercooler – brings air temperature down close to ambient so most of the water condenses before the duct, not inside it. See our portable aftercoolers page.
- Moisture separator with drain – removes the condensed water and bulk oil.
- Coalescing filtration – removes fine oil aerosol and particles. Some contractors also specify a dryer or oil-free air where the cable or duct supplier requires it.
- Hose to the blowing machine – correctly rated for the pressure and sized so that pressure drop at full flow stays small.
Where the specification calls for oil-free air, there are two routes: a genuinely oil-free compressor, or an oil-injected machine with aftercooling and staged coalescing filtration. The first removes the oil source; the second depends on the filters being in good condition and drained. Treat the cable and duct supplier’s requirement as the deciding point.
Duct testing and preparation before blowing
Many blowing problems are really duct problems. The same compressor that blows the cable is usually used to prove the duct first, under the contractor’s method statement:
- Pressure integrity test – the duct section is pressurised to the value in the project specification and held to check for leaks at couplers and damage.
- Cleaning and calibration – a sponge or foam pig is blown through to remove dirt and water, followed by a calibration (gauge) pig to confirm there are no crushed or ovalised sections.
- Lubrication – the duct supplier’s lubricant is applied where specified, which is one reason oil carry-over from the compressor is unwelcome.
Duct testing and pigging use stored energy. Keep personnel away from duct ends, secure the receiving end with a pig catcher, and follow the contractor’s procedure for pressure, hold time and release.
How to size the compressor for a cable route
We work from the job, not the brochure. A sound sizing method looks like this:
- Take the blowing machine supplier’s stated pressure and air requirement for the duct and cable combination.
- Add the pneumatic drive consumption if it is not already included.
- Add the hose and fitting pressure drop between compressor and machine, and the drop across the aftercooler, separator and filters at full flow.
- Correct for site conditions: high ambient temperature and altitude reduce what a compressor can deliver against its rating, and dusty sites load intake filters quickly.
- Decide whether you need margin for tandem blowing, where a second machine mid-route is fed from the same or a second compressor.
- Choose the compressor class (185–250, 250–400 or 400–600 cfm) and the pressure rating that covers the result with sensible margin.
Oversizing is not harmful in itself, but a larger unit does not fix a damaged duct, wrong cable stiffness or poor lubrication. Undersizing, on the other hand, shows up as a stall partway along the route.
Common problems on site and what usually causes them
| Symptom | Likely air-side cause | What to check |
|---|---|---|
| Cable stops advancing after a few hundred metres | Flow too low; pressure sags at the machine | Gauge at the machine inlet under load, hose size, compressor unloading |
| Water spitting from the duct end | No aftercooler, separator not draining, or saturated air in heat | Aftercooler fan and core, separator drain, filter bowls |
| Oily residue on cable or duct | Oil carry-over from the compressor | Separator element condition, oil level, coalescing filters |
| Duct softening or deforming near the machine | Air too hot | Air temperature after the aftercooler; shade and cooling airflow |
| Compressor shuts down in the afternoon | High discharge temperature in high ambient | Cooler cleanliness, oil level, cooler and radiator airflow, dust load |
| Pressure fluctuates widely | Control setting or unloader fault | Regulator and unloader settings, air demand vs capacity |
For deeper fault-finding on portable units, see diesel portable compressor troubleshooting.
Hire, buy or used: what suits a cable contractor
Telecom and utility contractors across Telangana, Andhra Pradesh and other states often work in campaigns: weeks of intense blowing, then a gap. Hire suits short or uncertain work, because the compressor, aftercooler and separator arrive as a matched package and servicing stays with us. Buying suits contractors with steady, year-round route work who can maintain the machine themselves or under an AMC. A used or refurbished portable can be a sensible middle path, provided the airend, cooler condition and engine are checked before purchase.
Whichever route you take, plan for maintenance. On our own hire jobs, portable compressors are serviced every 300 running hours, with an oil change and filter service. On cable work, intake filter and cooler cleanliness need extra attention because roadside duct routes are dusty.
How Reddy Compressors helps
Reddy Compressors, based at Tarnaka in Secunderabad and serving industry since 2002, has supplied compressors for fibre-optic cable blowing, both for sale and on hire. We are an independent multi-brand specialist, supplying, hiring and servicing machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and others; we are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.
- Rental from our own fleet – diesel and electric compressors from 50 to 2,000 cfm and 2 to 30 bar, supplied with hoses, aftercoolers, moisture separators and dryers where the job needs them.
- Sales – new and used/refurbished portable compressors, and brand-new machines for long-term route projects.
- Air treatment matched to the specification – aftercooling, separation and filtration set up for the cable and duct supplier’s requirement, including oil-free options.
- Service and spares – our team of more than 20 service engineers and technicians covers all of Telangana and Andhra Pradesh, and we supply equipment and spare parts across India, with project-based site support elsewhere where feasible.
We do not set the blowing method, duct test pressure or cable force; those stay with the cable, duct and machine suppliers and your engineer. We make sure the air arrives at the machine at the agreed pressure, flow and quality.
What to send us
- Duct size and type (microduct or HDPE, inner and outer diameter) and its rated pressure
- Cable type and diameter, and route length per blowing section
- Blowing machine make and model, and whether it has a pneumatic drive
- Required pressure and air quality (dry, oil-free, temperature limit) from your specification
- Site location(s), duration of work, and whether site power is available
- Whether you need duct testing and pigging support as well as blowing
Frequently asked questions
What size compressor do I need for fibre-optic cable blowing?
It depends on the duct diameter, route length and blowing machine. As a typical pattern, microduct work uses smaller portable units, 32–40 mm ducts often use the 250–400 cfm class, and long 40–50 mm runs use the 400–600 cfm class, generally at 10–14 bar. Start from the machine supplier’s stated requirement, add any pneumatic drive consumption and hose losses, then choose the class with sensible margin.
Why does cable blowing need an aftercooler?
Compressor discharge air is hot and saturated. Without an aftercooler, it can soften HDPE duct near the machine, and its water condenses inside the duct as the air cools, increasing friction and leaving the duct wet. An aftercooler brings the air close to ambient so that most of the water condenses where a separator can remove it, before the air reaches the duct.
Do I need oil-free air for cable blowing?
Many cable and duct suppliers ask for oil-free or well-filtered air because oil residue raises friction and can interact with duct lubricants and sheaths. You can meet this with an oil-free compressor, or with an oil-injected portable fitted with aftercooling, a separator and coalescing filters that are properly maintained. Follow the requirement written in your cable and duct specification.
Why does the cable stop moving halfway through the duct?
Often the cause is mechanical: a crushed duct, a poor coupler, missing lubricant or a cable unsuited to the duct. On the air side, the usual cause is insufficient flow, so pressure at the machine sags as friction increases along the route. Check the pressure at the machine inlet under load, the hose size, and that the compressor is not unloading or tripping on temperature.
Can I hire a compressor for cable blowing in Hyderabad or across Telangana?
Yes. We hire portable diesel and electric compressors from our own fleet, with hoses, aftercoolers, moisture separators and dryers where required. Our service team covers all of Telangana and Andhra Pradesh from our base at Tarnaka, Secunderabad, and we supply equipment for project work across India. Send us the duct, machine and route details so we can match the package.
How is an HDPE duct tested before cable blowing?
Under the contractor’s method statement, the duct section is usually pressurised and held to check for leaks, then a sponge or foam pig is blown through to clean it, followed by a calibration pig to confirm there are no crushed sections. The test pressure and hold time come from the project specification and the duct rating, not from the compressor’s maximum pressure.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.