Why “send a 750 cfm compressor” is not a specification
Most rental enquiries begin with a machine size. That is understandable, but it leaves the important questions open. Is 750 cfm needed continuously or only for a short peak? At what pressure, and measured where? Does the process need dry or oil-controlled air? Is the rental replacing the whole station or topping up a working machine? What happens if it stops?
Two quotations for “750 cfm” can deliver very different useful air. One may be a compressor alone; another may include an aftercooler, dryer and filtration whose pressure drop and, for some desiccant dryers, purge air reduce what reaches the plant. A specification puts both offers on the same footing: the air delivered at your connection.
Separate the requirement from the solution
Write the process outcome before naming a model, brand or drive type. For example: “the production header must receive a defined net flow between 6.5 and 7.5 bar (94–109 psig), at a pressure dew point of about +3 °C, during two shifts a day for six weeks.” The supplier can then propose a suitable mix of compressors, treatment, hoses and controls.
Some constraints are real and belong in the specification with their reason: electric drive because diesel exhaust is not permitted in a clean area, a footprint limit at the placement point, or a fixed pressure rating on an existing connection. Other items are preferences, such as a favourite brand. Label them as preferences so they do not quietly exclude a better solution.
The core parameters of a duty specification
| Parameter | What to state | Common mistake |
|---|---|---|
| Flow | Net delivered flow at the receiving point, average and peak, in cfm or m³/min (1 m³/min ≈ 35.3 cfm) | Quoting nameplate FAD of an existing machine without knowing actual demand |
| Pressure | Minimum and maximum at the receiving point, in bar and psig | Specifying compressor discharge pressure and ignoring losses |
| Air quality | Dew point, oil and particle needs, ISO 8573-1 classes where applicable | Assuming “dry air” means the same to everyone |
| Operating profile | Hours per day, shifts, peaks, start-up events | Sizing for an average that the plant rarely sees |
| Duration | Expected start and end, and likely extensions | Short-hire assumptions for what becomes a long project |
| Continuity | Longest acceptable interruption, need for standby | No plan for a trip or refuelling stop |
| Site interfaces | Connection point and size, power or fuel, placement, access | Discovering the cable or hose route on delivery day |
| Ambient conditions | Maximum temperature, dust, altitude, indoor or outdoor | Selecting for 35 °C when the site sees 40–45 °C, and above 45 °C on open sites, in summer |
How to work out the flow you actually need
Flow is the parameter most often guessed. A few methods, from best to roughest:
- Measured data: logged flow or compressor load data from the existing system, ideally over a representative week.
- Existing compressor behaviour: if a 500 cfm machine runs loaded most of the time and pressure still sags, demand is above 500 cfm; if it unloads often, demand is lower.
- User inventory: list the tools and process users with their air consumption and a realistic simultaneity factor. Breakers and rock drills typically use 85–145 cfm each at about 7 bar (101.5 psig); blasting nozzles vary by size, for example a No. 6 nozzle typically needs 280–300 cfm at 7 bar.
- Nameplate of the failed machine: a reasonable starting point for breakdown cover, provided it was correctly sized in the first place.
Add allowances for leaks, hose losses and treatment losses. If a heatless desiccant dryer is used, it consumes a significant share of the dried air as purge, so the compressor must be sized above the plant demand. Stating flow at the receiving point makes that the supplier’s job to account for.
Pressure: specify it where the air is used
State the pressure the process needs at its connection, not the compressor set point. The supplier then adds the expected losses through hoses, couplings, aftercooler, dryer and filters. On short runs those may be a few tenths of a bar; on long hose routes they can be much more.
Avoid adding generous margins “to be safe”. Every extra bar of discharge pressure costs about 7% more power or fuel, and some equipment has a lower maximum pressure than the plant expects. For high-pressure duties such as pipeline testing or deep drilling, specify the pressure clearly in both bar and psig, for example 21 bar (304.6 psig), so there is no conversion error.
Air quality, without ambiguity
“Clean dry air” means different things to a construction crew and a pharmaceutical plant. Specify:
- Pressure dew point: around +3 °C typical for refrigerated drying; −20 °C or −40 °C typical for desiccant drying.
- Oil content: whether oil-free compressors are required, or whether oil-injected compressors with suitable coalescing and carbon filtration are acceptable.
- Particles: filtration grade or ISO 8573-1 class.
- Any validation or sampling the plant requires before use.
Where the process already has an air-quality specification for its permanent system, copy it. The temporary air should not quietly be of lower quality than the system it replaces.
Continuity and standby
Ask what a stop costs. For a construction site, a short stop may be an inconvenience. For instrument air or a continuous process, it can mean a plant trip. The specification should state the longest acceptable interruption and whether standby capacity is required, for example N+1 compressors, a second dryer or a receiver sized to ride through a short changeover.
Include planned stops too. Diesel units need refuelling and every unit needs servicing; on our hire jobs that is every 300 running hours. If those stops cannot interrupt the air, the arrangement must allow one machine to be serviced while another carries the load.
Site interfaces and conditions
Describe the site honestly. Useful details include:
- Placement area: size, ground condition, access for delivery vehicles and lifting.
- Connection point: pipe size, valve type, pressure rating and distance from placement.
- Power: available voltage and supply capacity if electric units are preferred.
- Fuel: refuelling access and storage rules for diesel units.
- Environment: maximum ambient temperature, dust, rain exposure, noise limits and working hours.
Indian summer conditions deserve special mention. At summer ambients of 40–45 °C, and above 45 °C on open sites, compressors and dryers lose margin, and dust loads coolers and filters quickly. A specification that states these conditions gets equipment selected for them.
Turning the specification into acceptance criteria
A good specification already contains the commissioning test. If it says “net flow at the header between 6.5 and 7.5 bar during peak demand, dew point around +3 °C”, the acceptance test simply measures those values at the header under peak demand. Add who will witness the test and what evidence is needed. This links the enquiry directly to how the installation will be proven.
A short duty specification template
- Purpose of the hire and what it replaces or supports.
- Net flow at the receiving point, average and peak.
- Pressure band at the receiving point, in bar and psig.
- Air quality requirements.
- Operating hours and duration.
- Continuity: acceptable interruption and standby needs.
- Site interfaces: placement, connection, power or fuel, access.
- Ambient and environmental conditions.
- Constraints with reasons, and preferences labelled as such.
- Acceptance test and who signs it.
How Reddy Compressors helps
Reddy Compressors rents, sells and services compressors as an independent multi-brand specialist working with ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands; we are not an authorised dealer of any of them. We have tie-ups with all major and reputed brands. Our own rental fleet covers 50–2,000 cfm and 2–30 bar (29–435.1 psig), diesel and electric, oil-injected and oil-free, with hoses, aftercoolers, separators and dryers.
If you do not have a complete specification, send what you know. Our engineers will help fill in the gaps, check flow and pressure against your users, and propose a package against the duty rather than just a machine size. For long-term projects we can also supply brand-new compressors to site. With over 20 service engineers and technicians, we support sites across Telangana and Andhra Pradesh from Tarnaka, Secunderabad, and supply across India. See also temporary air site survey and diesel or electric rental air.
What to send us
- What the hire is for and how long you expect it to last.
- Flow and pressure needed, or the nameplate of the machine being replaced.
- Air-quality needs: dry, oil-free or filtered.
- Working hours and how critical continuity is.
- Power available or preference for diesel.
- Site location, photos of placement area and connection point.
Frequently asked questions
What size rental compressor do I need?
Start from the air your process needs at its connection, not from a machine size. Add up the users with a realistic simultaneity factor, or use measured data from your existing system, then allow for leaks, hose losses and treatment losses. If you are covering a breakdown, the nameplate of the failed compressor is a reasonable starting point. A supplier can then propose one or more machines to meet that delivered flow and pressure.
What information do I need to give when hiring an air compressor?
Give the flow and pressure needed at the point of use, air-quality needs, working hours, duration, how critical continuity is, and site details such as power, fuel access, placement and connection size. Photos of the site help a lot. With that information, quotations can be compared on the air they actually deliver rather than on compressor nameplates that may include or exclude treatment and hoses.
Should I specify pressure at the compressor or at the point of use?
At the point of use, or at the plant header that feeds it. Hoses, couplings, dryers and filters all cause pressure drop, so the compressor must run higher than the process needs. Stating the receiving-point pressure makes the supplier responsible for those losses. Avoid adding large safety margins, because each extra bar of discharge pressure costs about 7% more power or fuel.
Do I need a dryer with a rental compressor?
That depends on what the air is used for. Tools on a construction site often need only an aftercooler and moisture separator. Production lines, instrument air, painting and coating usually need a refrigerated dryer, typically giving a pressure dew point around +3 °C, or a desiccant dryer for lower dew points. If your permanent system has a dryer, the rental air should match its quality.
How do I compare two compressor rental quotations fairly?
Compare them against the same duty specification: delivered flow and pressure at your connection, air quality, operating hours, continuity, and what is included, such as hoses, treatment, delivery, installation, servicing and operators. Check whether a desiccant dryer’s purge air or treatment pressure drop has been allowed for. Two quotes with the same cfm can deliver different useful air at your plant.
Can a rental compressor run in 45 °C Indian summer heat?
Yes, if it is selected and placed for it. High ambient temperatures reduce cooling margin and raise discharge temperature, and dust blocks coolers and filters faster. State your maximum site temperature and dust conditions in the specification so the supplier can choose suitable equipment, allow for shade and clear airflow, and plan cooler cleaning during the hire.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Compressed air demand estimate
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Unit for the air each tool uses: L/s
- Group A: number of tools: 6
- Group A: free air each (in the unit above): 4
- Group A: duty (%): 40
- Group B: number of tools: 2
- Group B: free air each (in the unit above): 15
- Group B: duty (%): 60
- Group C: number of tools: 1
- Group C: free air each (in the unit above): 40
- Group C: duty (%): 80
- Simultaneity of all tools (%): 65
- Leakage allowance (%): 15
- Growth allowance (%): 10
Result
- Connected demand: 94 L/s = 199 cfm
- Average demand (duty-weighted): 59.6 L/s = 126 cfm
- Simultaneous peak: 61.1 L/s = 129 cfm
- Design basis (higher of average and peak): 61.1 L/s = 129 cfm
- Design capacity with allowances: 77.29 L/s = 4.64 m³/min = 163.8 cfm
Method. Average demand = Σ(number × air per tool × duty). Peak = Σ(number × air per tool) × simultaneity. The peak can never be lower than the average, so the design basis is the higher of the two. Design capacity = design basis × (1 + leakage) × (1 + growth).
What to take from it. Leakage and growth allowances move the answer as much as any single tool. Where you can, replace catalogue figures with measured ones.
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.