How to use the FAQ Centre
Pick the topic closest to your question and read the short answer first. Each answer gives the essential point in plain language, notes any important exception and links to the page that covers the subject in depth. Answers are general: the right choice for your plant depends on measured demand, working pressure, air quality, site conditions and the machines you already have. Where a question needs a calculation, we explain the method and point you to the relevant engineering tool rather than quote a figure that may not apply to you.
| Topic | Typical questions | Go deeper |
|---|---|---|
| Fundamentals | FAD, gauge and absolute pressure, units | Units and symbol dictionary |
| Technology and selection | Screw or piston, oil-free or oil-injected | Compressor type and selection pages |
| Pressure, flow and distribution | Low pressure at the far end, receiver size | Pressure-drop and pipe sizing pages |
| Air quality and treatment | Dryer choice, ISO 8573-1 classes | Dryer and air-quality pages |
| Energy and controls | Leaks, VSD, sequencing | Audit and measurement hub |
| Maintenance and reliability | Service intervals, high-temperature trips | Preventive maintenance and AMC pages |
| Rental and temporary air | When to hire, how to brief us | Temporary air engineering centre |
| Portable and borewell | Tool air demand, DTH pressure and flow | Portable diesel and borewell libraries |
| Spare parts | Identifying the correct part | Spare parts identification enquiry |
If your question is not here, ask us. Real questions from customers are how this page grows.
Fundamentals
What is free air delivery (FAD)?
FAD is the volume of air a compressor actually delivers at its outlet, expressed at the conditions at its inlet. It is the figure to compare between machines, measured to ISO 1217 at a stated working pressure. It is always lower than the swept displacement of a piston machine. Quote it in m³/min, l/s or cfm, and always say at which pressure.
What is the difference between bar, bar(g) and psig?
Gauge pressure, written bar(g) or psig, is pressure above the surrounding atmosphere and is what most compressor gauges show. Absolute pressure adds atmospheric pressure, roughly 1 bar. One bar equals about 14.5 psi, so 7 bar(g) is 101.5 psig and 10 bar(g), 145 psig. Mixing gauge and absolute values is a common cause of sizing errors.
Technology and selection
Should I choose a screw or a piston compressor?
Rotary screw compressors suit continuous or long-running duty with steady demand and give smooth, quiet delivery. Piston compressors suit intermittent duty, smaller demands and higher pressures in compact sizes, and are simple to service. The decision depends on duty cycle, pressure, air quality and running hours, not on capacity alone. Our screw versus reciprocating page compares them in detail.
When do I need an oil-free compressor?
Oil-free compression is chosen where any oil in the air is an unacceptable risk: pharmaceuticals, food and beverage, dairy, electronics, medical and laboratory air, some textile and PET applications. ISO 8573-1 Class 0 is the strictest oil class, set by the user’s specification. Oil-free air still needs drying and filtration, because water and particles come in with the intake air.
Pressure, flow and distribution
Why is the pressure low at the far end of my plant?
Common causes are undersized or long pipework, clogged filters, restrictive quick couplings and hoses, leaks and demand that exceeds compressor output. Raising the compressor set point hides the problem and costs about 7% more power per extra bar. Measuring pressure at several points under load shows where the drop occurs, and the fix is usually in distribution rather than a bigger compressor.
How big should my air receiver be?
A receiver stores air to cover short peaks, reduces load/unload cycling and helps condensate separate. Size depends on compressor output, allowed pressure swing and the demand pattern, so there is no single rule that fits every plant. Fixed-speed machines generally benefit from more storage than variable-speed ones. Every receiver needs a correctly rated safety valve, a drain and periodic inspection.
Air quality and treatment
Which dryer do I need, refrigerated or desiccant?
A refrigerated dryer typically gives a pressure dew point of about +3 °C, enough for most general plant air where pipework stays above that temperature. A desiccant dryer gives typically −20 °C or −40 °C for instrument air, outdoor lines, sensitive processes and pharmaceutical duties. Desiccant dryers use a significant share of the dried air or added heat for regeneration, so specify only the dew point you need.
What does ISO 8573-1 mean on a specification?
ISO 8573-1 classifies compressed air purity for solid particles, water and oil, written as three class numbers such as 1.4.1. Lower numbers mean cleaner air. The specification should come from the needs of the end use, and the treatment train (filters and dryer) is then selected to achieve it. Claims of air quality should be backed by measurement, not assumed from equipment.
Energy and controls
How much energy do compressed air leaks waste?
Leaks often consume a large and unnoticed share of compressor output, running day and night whether the plant is busy or not. The only reliable answer for your plant is a leak survey with ultrasonic detection plus a flow or power measurement. Fixing leaks, lowering pressure where possible and switching off air to idle areas are usually the cheapest energy improvements available.
Is a variable-speed drive compressor always more efficient?
No. A VSD compressor saves energy where demand varies over a wide range, because it matches speed to demand instead of running unloaded. At steady full load, a well-matched fixed-speed machine can be equally or more efficient. The right choice comes from a demand profile logged over representative days, and in multi-compressor plants from sensible sequencing.
Maintenance and reliability
How often should a screw compressor be serviced?
Follow the maker’s intervals for your model, adjusted for site conditions. Dust, heat and continuous running shorten filter and oil life, so Indian sites often need more frequent checks than a temperate schedule suggests. On our hire jobs we change oil and service every 300 running hours. A preventive plan or AMC sets the schedule against actual running hours and condition.
Why does my compressor trip on high temperature in summer?
Typical causes are blocked cooler fins, poor room ventilation with hot air recirculating, low oil level, degraded oil, a faulty thermostatic valve or a blocked oil filter. Normal discharge temperature for an oil-injected screw is about 75–95 °C. Clean the cooler, check ventilation and oil, and call an engineer if trips continue, because repeated overheating shortens oil and airend life.
Rental and temporary air
When does it make sense to hire a compressor?
Hire suits breakdown cover, planned shutdowns, short projects, seasonal peaks, trials before purchase and sites where capital is better spent elsewhere. Buying usually wins for long, predictable duty. We rent from our own fleet of 50–2,000 cfm machines at 2–30 bar, diesel and electric, and supply brand-new machines for long-term projects where that suits the customer better.
What information do you need to quote a compressor rental?
We need the air demand in cfm or m³/min, working pressure in bar or psig, air quality required, diesel or electric preference, site location, duration, working hours per day and the tools or process being supplied. Tell us about hoses, dryers, aftercoolers and operators if needed. The more precise the brief, the closer the first proposal will be.
Portable and borewell compressors
What size portable compressor do I need for breakers and rock drills?
Typical hand-held breakers and rock drills each use about 85–145 cfm at around 7 bar (101.5 psig), and rock drills may need 7–10 bar (101.5–145 psig). Add the demand of each tool that runs at the same time, allow for hose losses and wear, and choose the class: 185–250 cfm for one or two tools, 250–400 cfm for two to four.
What pressure and flow does a borewell compressor need?
Borewell and DTH drilling typically uses 600–1,500 cfm at 200–400 psig, about 14–28 bar, with special units up to about 35 bar (around 500 psig). The right combination depends on hole diameter, hammer size, depth, formation and water. We supply all types, brands and models of borewell compressors, new and used, for hire, with service and spare parts.
Spare parts and enquiries
How do you identify the right spare part for my compressor?
We start from the machine: make, model, serial number, a photo of the nameplate and, ideally, the old part or its number. From those we confirm the correct part and its fit before quoting, because similar-looking machines often use different parts. We supply genuine and quality aftermarket parts for many makes and explain the difference so you can choose.
How Reddy Compressors helps
Reddy Compressors, Tarnaka, Secunderabad, has served industry since 2002. As an independent multi-brand specialist we sell new and used compressors, rent from our own fleet, and provide service, AMC, overhaul, air audits and spare parts for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and many more. Our 20+ service engineers and technicians answer the questions behind these FAQs every day, on site across Telangana and Andhra Pradesh, and we supply equipment and spares across India.
What to send us
- Your question and the decision it relates to.
- Make, model and nameplate photo of the machine, if any.
- Working pressure, air demand and air quality required.
- Site location and duty: continuous, intermittent or temporary.
- Name and phone or WhatsApp number.
Frequently asked questions
Are the answers in the FAQ Centre checked by engineers?
Yes. Answers are written and reviewed by people who work on compressors and air systems, and they are updated when practice or our understanding changes. They are deliberately general, because the right answer for a particular plant depends on measured conditions. Each answer links to a fuller article, and our engineers can apply the principle to your installation.
Can I ask a compressed air question that is not listed?
Yes. Send it by phone, WhatsApp or the enquiry form with as much detail as you have: machine, pressure, demand, symptom and site. Our engineers will reply directly. Questions that many people ask are added to the FAQ Centre so that the next reader finds the answer straight away.
Why don’t the FAQ answers give prices?
Prices depend on make, model, specification, condition of used machines, hire duration, site location and the market at the time, so a published figure would mislead more often than it helped. We give a clear quotation once we understand your duty. FAQ answers focus on how to decide and what to specify, which is what drives cost.
Do the FAQs apply to all compressor brands?
The engineering principles apply across makes, because screw, piston, oil-free and centrifugal machines work in the same basic way whoever builds them. Settings, service intervals and parts differ between brands and models, and the maker’s manual governs those. We work across many brands as an independent specialist, not an authorised distributor. We have tie-ups with all major and reputed brands.
Where can I learn compressed air in a structured order?
Our learning paths arrange the technical articles into ordered routes for operators, maintenance engineers, buyers and plant owners. Each route starts with fundamentals and moves to selection, air quality, energy and reliability. The FAQ Centre is better for quick answers; the learning paths are better when you want to build understanding step by step.
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.