Why the enquiry decides the result
Compressors are often bought by capacity alone: “we need a 500 cfm machine”. Two plants asking for 500 cfm can need very different equipment. One may run three shifts at steady load and suit a fixed-speed screw with a variable-speed trim machine. Another may run one shift with sharp peaks and need a smaller compressor and a larger receiver. A third may need oil-free air at 8 bar for a pharmaceutical line, where air purity matters more than the flow figure.
A clear enquiry avoids three costly mistakes:
- Oversizing, which leaves a machine running unloaded for long periods, wasting power and wearing the inlet valve and contactors.
- Undersizing, which leaves the plant short of pressure at peaks and tempts operators to raise the set point, costing about 7% more power for every extra bar.
- Wrong air quality, which shows up later as water in lines, oil in product or failed validation.
Start with the purpose of the purchase
| Purpose | What we most need to know |
|---|---|
| New plant or new line | List of air users, their consumption, pressure and air-quality needs; planned shifts and future expansion |
| Replacing an existing compressor | Make, model and nameplate of the old machine, how it was loaded, what was wrong with it and any logged data |
| Adding capacity | Existing machines, how they are controlled, where pressure falls short and when |
| Site or project work | Tools or drilling equipment, hose lengths, duration, diesel or electric, and site access |
| Not sure yet | A description of the problem in your own words; we will ask the rest |
If the job is short or uncertain, renting may be the better answer. Our temporary air hire enquiry covers that route.
Pressure: state it where the air is used
The pressure that matters is the minimum needed at the most demanding point of use, not the compressor’s discharge pressure. Between the two sit the dryer, filters, pipework, hoses and regulators, each taking a share of the pressure. When you tell us the pressure:
- Give the unit clearly: bar(g), psig or kPa(g). Gauge and absolute pressure differ by about 1 bar, and mixing them causes real errors. 1 bar is 14.5 psi.
- State the minimum acceptable pressure at the critical user, and any higher peak requirement separately.
- Tell us the present compressor set points if you have an existing system, and the pressure you actually see at the far end of the plant.
Typical bands help as a sanity check: plant air about 6–8 bar (87–116 psig); construction tools about 7 bar (101.5 psig); borewell and DTH drilling about 14–28 bar (203.1–406.1 psig), with special units up to about 35 bar (507.6 psig).
Flow: say how you arrived at the number
Flow is usually given in cfm or m³/min. For a compressor, the meaningful figure is free air delivery (FAD) at stated inlet conditions. When you tell us the demand, also tell us its basis:
| Basis | Reliability | Comment |
|---|---|---|
| Measured with a flow meter over a normal week | Best | Shows average, peak and night load, including leaks |
| Derived from logged compressor load and unload times | Good | Useful for existing plants with known FAD |
| Calculated from tool and machine consumption | Fair | Depends on correct simultaneity and leak allowances |
| Estimated or copied from the old machine | Weak | May repeat an earlier sizing mistake |
Give average and peak demand, how long peaks last, and how many tools or machines run together. A receiver stores air for short peaks but does not supply sustained flow, so do not count receiver volume as capacity. Our compressed air demand calculator explains the method if you are starting from a list of users.
Operating profile: how does demand change?
The same average demand can call for different machines depending on its pattern. Tell us:
- Hours per day, days per week and number of shifts.
- Whether demand is steady or swings widely, and when the peaks occur.
- Whether there are idle periods, weekend loads or night loads (often mostly leaks).
- Whether the compressor must provide standby for another unit.
- Expected growth over the next few years.
A steady three-shift load usually favours efficient fixed-speed base-load machines with a variable-speed or well-controlled trim unit. A widely varying load may favour variable speed and more storage. These are engineering judgements we make from your data, not rules of thumb to apply blindly.
Air quality: what must be kept out of the air?
Air quality is specified by ISO 8573-1, which sets classes for particles, water (pressure dew point) and oil. Tell us what matters to your process:
- Water: is a refrigerated dryer pressure dew point of about +3 °C enough, or do you need a desiccant dryer at about −20 °C or −40 °C for outdoor lines, instruments or sensitive processes?
- Oil: does air touch product, packaging or people? If so, you may need an oil-free compressor, or an oil-injected machine with suitable filtration, depending on the risk.
- Particles: any filtration grade required by your process or your customer.
- Any written specification, such as a pharmaceutical or food-safety requirement.
An oil-free compressor alone does not guarantee a purity class at the point of use. Dryers, filters and clean pipework still decide what arrives. See our oil-free compressors hub for the detail.
Power, site and installation limits
| Item | Why it matters |
|---|---|
| Voltage, phase and supply capacity | Decides motor size, starting method and whether diesel is needed |
| Ambient temperature and dust | Indian summers of 40–45 °C (above 45 °C on open sites) and dusty sites affect cooling, filters and machine rating |
| Room size, ventilation and access | Hot air must leave the room; machines must be reachable for service |
| Cooling water, if any | Water quality and temperature for water-cooled machines |
| Altitude | Thinner air reduces mass flow and engine output |
| Noise limits | Enclosure type and siting |
| Existing pipework and receivers | Connection sizes, pressure drop and storage already available |
Commercial details that shape the offer
- Whether you want new, used/refurbished or rental equipment, or a comparison.
- Brand preferences or brands already on site, which can simplify spares and training.
- Required delivery period and any project milestones.
- Whether installation, commissioning, dryers, receivers, piping or an AMC should be included.
- Delivery location: we supply across India.
A worked example of a good enquiry
Compare two enquiries for the same factory. The first says: “Need 300 cfm screw compressor, send price.” The second says: “Packaging plant, two shifts, six days a week. Pneumatic cylinders and blow-off nozzles need at least 6 bar at the far end of the line; we run our old compressor at 7.5 bar. Logged load hours suggest about 250 cfm average with peaks near 320 cfm during changeovers. Air touches cartons, not food. 415 V three-phase supply, compressor room 5 m by 4 m with one exhaust fan. Water appears in lines in the monsoon.” The second enquiry lets an engineer size the compressor, the dryer and the receiver, question the 1.5 bar gap between generation and use, and spot the ventilation risk, all before a visit.
How Reddy Compressors helps
From our office in Tarnaka, Secunderabad, we have served industry since 2002. We sell new and used/refurbished compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan, Kirloskar and other makes as an independent multi-brand specialist, not an authorised dealer, so our recommendation is based on your duty rather than one brand’s range. We have tie-ups with all major and reputed brands. We also supply gas compressors, centrifugal compressors and blowers, rent from our own fleet of 50–2,000 cfm and 2–30 bar machines, and supply brand-new machines for long-term projects. Over 20 service engineers and technicians support installations across Telangana and Andhra Pradesh, and we supply equipment across India. Where the data is uncertain, we can measure an existing system through an air system audit before you buy.
What to send us
- Your name, company, phone and site location.
- What the air is for and whether this is a new plant, replacement, expansion or project.
- Pressure needed at the critical point of use, with the unit.
- Flow needed (average and peak) and how you arrived at it.
- Shifts, hours and how demand varies.
- Air quality needed: dew point, oil content or ISO 8573-1 class.
- Power supply, site conditions and any existing machines with nameplate photos.
Frequently asked questions
What information do I need to buy an air compressor?
You need the pressure required at the point of use, the flow in cfm or m³/min with its basis, the demand pattern across shifts, the air quality needed, your power supply and site conditions. With these, a supplier can size the compressor, dryer and receiver together. If some values are unknown, say so; guessing figures usually leads to the wrong machine.
How do I know what size compressor I need?
Add the consumption of all air users that run at the same time, include an allowance for leaks and growth, and check peak demand against storage. For an existing plant, measure flow and pressure over a normal week. Then choose a machine whose free air delivery meets the demand at the required pressure, with a sensible control strategy for part load.
Should I give the pressure at the compressor or at the machine?
Give the minimum pressure needed at the most demanding machine or tool, and if you know it, the compressor discharge pressure you run today. The difference is lost in dryers, filters and pipework. Sizing from the point of use lets us keep the generation pressure as low as possible, which matters because each extra bar costs roughly 7% more power.
Do I need an oil-free compressor?
You need oil-free air where oil could contaminate product, packaging, people or sensitive processes, as in pharma, food, beverages, electronics and medical air. Whether that means an oil-free compressor or an oil-injected machine with high-grade filtration depends on the risk and your specification. Tell us the process and any required ISO 8573-1 class, and we will explain the options.
Can I enquire if I do not know my air demand?
Yes. Describe what the air is used for, the tools or machines involved and any existing compressors, and mark unknown values as unknown. Our engineers can estimate demand from equipment lists, review logged data from your current machines or arrange a measurement audit. That is better than giving a guessed figure that could lead to the wrong selection.
Do you supply new and used compressors outside Hyderabad?
Yes. We supply new and used/refurbished compressors, rental machines and spare parts across India, and our service teams cover the whole of Telangana and Andhra Pradesh. We are an independent multi-brand specialist, so we can offer machines from several makes and compare them on your duty, power cost and support needs rather than on one brand’s range.
Related pages
Enquiry
Send your requirement
Share the application, pressure, air demand and any photos or nameplates. We will reply with the questions a service engineer would ask first.