Why buying by motor kW or nameplate cfm goes wrong
Two compressors with the same motor rating can deliver noticeably different free air, at different pressures, with different power draw at part load. A nameplate cfm figure may be measured at a different pressure or under different reference conditions from a competing offer. A lower price may exclude the dryer, the receiver, the control system, installation or commissioning.
When those differences are not exposed, the lowest bid often wins and the plant pays for it later through higher electricity use, extra treatment equipment bought separately, or a machine that cannot hold pressure at peak demand. A sound procurement process fixes this in four steps: define what the plant needs, demand comparable evidence, record every deviation, and only then evaluate commercial value over the life of the equipment.
Start with the duty, not the catalogue
The technical specification should be built from measured or well-estimated demand, not from the size of the machine being replaced. Our air demand profiling service logs flow and pressure over representative production periods so the duty is based on evidence.
- Flow: average, peak and minimum demand in m³/min or cfm, stated as free air delivery.
- Pressure: the justified discharge pressure from a pressure-loss review, not an inflated historical setpoint.
- Air quality: the ISO 8573-1 class needed for particles, water and oil at the point of use.
- Load profile: how demand varies by shift, day and season, because this decides fixed-speed versus variable-speed and how many machines.
- Reliability: whether the plant needs N+1 standby, critical-load protection or simply a spare machine.
- Future growth: committed, staged growth only; speculative future load should not oversize today’s machine.
Where you are unsure of the right size, our page on compressor capacity selection and sizing explains the method.
What should a compressor technical specification include?
A good specification is short enough to be read and complete enough that bids can be compared line by line. The table below shows the core content.
| Section | What to state | Why it matters |
|---|---|---|
| Scope of supply | Compressor only, or with dryer, filters, receiver, controls, meters, piping, installation, commissioning, spares and training | Prevents low bids that quietly exclude essentials |
| Battery limits | Air inlet and outlet points, electrical supply point, cooling, drains, control signals, foundations | Stops enabling work falling between contracts |
| Duty points | Required FAD at stated pressure for low, normal and peak demand | Lets suppliers show performance where the plant actually runs |
| Reference conditions | The standard on which FAD is to be stated, with inlet temperature, pressure and humidity | Makes flow figures comparable between bidders |
| Site conditions | Maximum ambient temperature, altitude, dust, humidity, indoor or outdoor installation | Indian summer room temperatures can exceed 45 °C |
| Air quality | ISO 8573-1 class required at the outlet of treatment | Defines dryer and filter selection |
| Electrical | Voltage, frequency, starting method, fault level, VSD harmonic expectations | Avoids starting problems and supply disturbances |
| Cooling | Air-cooled or water-cooled, heat rejection data, water quality where relevant | Determines room ventilation and cooling-water design |
| Controls | Control method, sequencing with existing machines, remote signals, data access | Ensures the new machine works with the station, not against it |
| Testing and acceptance | Commissioning checks, performance verification, documents to be handed over | Gives a clear basis to accept or reject delivery |
Keep the specification technology-neutral where possible. State the performance and function you need rather than naming a compressor type, unless there is an engineering reason, such as a requirement for oil-free air or a high-pressure duty, that genuinely points to one technology.
Comparable performance data to request from every bidder
Suppliers should provide the same data in the same units. Without this, comparison becomes guesswork. Ask for:
- Free air delivery at the specified working pressure, stated to an agreed standard such as ISO 1217, with the test tolerance.
- Package input power at full load, including fan and auxiliaries, not just shaft power.
- Specific power in kW per m³/min at full load and, for variable-speed machines, at several load points across the expected range.
- For fixed-speed machines: unloaded power, unload and blow-down behaviour, and the control method.
- For variable-speed machines: minimum and maximum stable speed, the flow at each, and the power at each.
- Motor efficiency class and rating, starter or drive type, and starting current.
- Maximum ambient temperature for continuous full-load operation.
- Heat rejection to air or water, and required ventilation flow.
- Noise level and the standard used to measure it.
- Recommended service intervals, parts list for the first years of service, and consumable quantities.
Specific power is one of the most useful single comparisons, but it must be read at the pressure and load where the plant runs. A machine that looks efficient at full load may be weaker at the part load where it will spend most of its life. Our specific-power assessment page explains how this figure is measured and compared.
How to evaluate compressor bids step by step
- Check compliance: list every clause of the specification and mark each bid as compliant, deviating or silent.
- Record deviations: for each deviation, note the technical effect and whether it can be accepted, corrected or must be priced in.
- Normalise scope: add the cost of anything one bidder excludes and another includes, such as a dryer, receiver or installation, so all bids cover the same scope.
- Normalise performance: correct flow to the same pressure and reference basis, and check that each offer meets peak demand with the required standby.
- Estimate energy: use the plant load profile and each bid’s power data at those loads to estimate annual kWh.
- Estimate maintenance: use each supplier’s stated intervals and parts lists to estimate parts, consumables and labour over the evaluation period.
- Assess risk: consider spare parts availability, local service capability, delivery schedule, and how the machine fits existing controls.
- Compare lifecycle cost: add purchase, installation, energy and maintenance over the evaluation period and rank the bids.
- Clarify and negotiate: send written questions on unclear points and record the answers as part of the contract basis.
For most industrial compressors running long hours, energy over the life of the machine is the largest cost, often well above the purchase price. A small difference in specific power can outweigh a large difference in price. Our page on lifecycle cost and total cost of ownership sets out this calculation.
Common gaps that make a cheap bid expensive
| Gap in the bid | Consequence on site |
|---|---|
| Dryer and filters excluded or undersized | Wet or contaminated air, extra purchase later, higher pressure drop |
| Flow stated at a lower pressure than specified | Real delivery at working pressure is lower than expected |
| No ambient derating at high summer temperatures | High-temperature trips in May and June |
| Installation, piping and cabling “by client” | Unplanned civil, electrical and piping costs |
| Commissioning limited to a start-up | No performance check, no settings record, no baseline |
| No integration with existing machines | Overlapping pressure bands and wasted unloaded running |
| Parts list and intervals not stated | Maintenance cost unknown until the first service |
New, used or rented: framing the purchase decision
Procurement is not always about a new machine. For short projects, uncertain demand or a trial period, renting may be the better option. A well-chosen refurbished machine can suit a secondary duty or a standby role. For long, steady duty, a new energy-efficient machine is usually justified by its lifetime energy use.
The same specification can be used to compare all three routes. Ask for the same performance data, scope and service support whichever route you take, and compare on cost over the period you actually need the air. Our compressor rental pages explain how hire can bridge a procurement gap or cover a project.
Acceptance and handover requirements
The contract should state what will be checked before the machine is accepted. Typical items include correct installation clearances and ventilation, electrical checks, direction of rotation, control and safety-shutdown tests, pressure setting, a run at load with temperatures recorded, dryer dew point at operating flow, and integration with existing compressors. Handover documents should include the operating manual, settings record, parts list, service schedule and any test certificates supplied by the manufacturer. Our page on commissioning, start-up and acceptance covers this stage.
How Reddy Compressors helps
Reddy Compressors has served industry from Tarnaka, Secunderabad since 2002. We sell new and used or refurbished compressors, rent from our own fleet and service machines of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands as an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands. Because we are not tied to one brand, we can help you compare offers on a neutral technical basis.
- Demand and pressure logging to establish the real duty before you specify.
- Help preparing a technical specification and data sheet for suppliers.
- Technical comparison of bids, including scope and performance normalisation.
- Supply of new or used compressors, dryers, filters and receivers to your specification.
- Installation support, commissioning and service across Telangana and Andhra Pradesh, with equipment and spares supplied across India.
- Rental cover while the new installation is being procured or built.
What to send us
- A short description of the application and the users of compressed air.
- Details of existing compressors, dryers and receivers, with nameplate photos.
- Required working pressure and air quality, if known.
- Any demand logs, running hours or electricity data you have.
- Site details: room size, ventilation, electrical supply and any space limits.
- The bids you have received, if you want a technical comparison.
Frequently asked questions
How do I compare two air compressor quotations?
Check that both state free air delivery at the same working pressure and to the same standard, then compare package input power and specific power at the loads your plant actually runs. Next, normalise scope by adding the cost of any dryer, receiver, installation or commissioning one bidder has excluded. Finally, estimate energy and maintenance over the expected life and compare the total, not just the purchase price.
What is specific power and why does it matter when buying a compressor?
Specific power is the electrical input power divided by the free air delivered, usually in kW per m³/min, at a stated pressure. Lower is better. It is one of the most useful ways to compare compressors because energy is usually the largest cost over a machine’s life. Always compare it at the same pressure and at the loads where your plant will actually run, not only at full load.
Should I specify FAD or motor kW when buying a compressor?
Specify the free air delivery you need at a defined working pressure and reference conditions. Motor kW tells you the size of the motor, not how much usable air you will get. Two machines with the same motor can deliver different flows. Ask suppliers to state FAD to an agreed standard such as ISO 1217, together with package input power, so offers can be compared fairly.
What should be included in a compressor tender?
A compressor tender should include the duty points for flow and pressure, reference and site conditions, air-quality class, scope of supply and battery limits, electrical and cooling details, control and integration requirements, data required from bidders, and the acceptance tests and documents expected at handover. A clear scope prevents low bids that exclude essential items such as dryers, installation or commissioning.
Is it better to buy a variable-speed or fixed-speed compressor?
It depends on the load profile. A variable-speed compressor saves energy where demand varies widely and the machine can run within its useful speed range. Where demand is steady and near full load, a fixed-speed machine may be equally efficient and simpler. Many plants use fixed-speed base machines with one variable-speed trim machine. Logging your demand first gives the right answer.
Can you help us evaluate bids from other compressor suppliers?
Yes. As an independent multi-brand specialist, we can review your specification and the bids you have received, compare stated performance on a common basis, identify scope gaps and deviations, and estimate energy use from your load profile. We give a technical view to support your decision. The commercial decision, and any negotiation with bidders, remains with your team.
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.
Bid normalisation: Net flow and comparable cost
Bring up to three compressor bids to one flow unit, check net flow against the requirement, then compare specific power, energy and installed cost on a like-for-like basis.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Net flow required (m³/min): 20
- Running hours per year: 6000
- Bid A: offered flow: 21.5
- Bid A: flow unit: m³/min
- Bid A: flow stated at the required pressure?: Yes
- Bid A: internal consumption (m³/min): 1.8
- Bid A: total package input power (kW; 0 if not stated): 124
- Bid B: offered flow: 750
- Bid B: flow unit: cfm (ft³/min)
- Bid B: flow stated at the required pressure?: Yes
- Bid B: internal consumption (m³/min): 1
- Bid B: total package input power (kW; 0 if not stated): 126
- Bid C: offered flow: 22
- Bid C: flow unit: m³/min
- Bid C: flow stated at the required pressure?: No / unclear
- Bid C: internal consumption (m³/min): 0
- Bid C: total package input power (kW; 0 if not stated): 0
Result
- Bid A: offered flow: 21.5 m³/min
- Bid A: net flow: 19.7 m³/min – SHORT by 0.3 m³/min
- Bid A: specific power on net flow: 6.29 kW per m³/min (5.77 on offered flow)
- Bid A: energy per year at the required net flow: 7,55,330 for information only: this bid cannot supply the required net flow. kWh = 6.29 × 20 m³/min × 6,000 h (enter a tariff for the cost). Assumes specific power holds at this flow; use the bidder’s part-load data for a VSD or a part-load profile
- Bid A: comparable installed cost: Enter your own prices above
- Bid B: offered flow: 21.24 m³/min
- Bid B: net flow: 20.24 m³/min – meets, margin 0.24 m³/min
- Bid B: specific power on net flow: 6.23 kW per m³/min (5.93 on offered flow)
- Bid B: energy per year at the required net flow: 7,47,124 kWh = 6.23 × 20 m³/min × 6,000 h (enter a tariff for the cost). Assumes specific power holds at this flow; use the bidder’s part-load data for a VSD or a part-load profile
- Bid B: comparable installed cost: Enter your own prices above
- Bid C: offered flow: 22 m³/min (not at the required pressure)
- Bid C: net flow: Not established clarification needed: ask for data at the required pressure
- Bid C: package input power: Not stated ask the bidder for total package kW at the duty point
- Bid C: comparable installed cost: Enter your own prices above
- Eligible bids by energy at the required net flow: B (7,47,124 kWh) only one eligible bid with stated power
- Bids that pass the net-flow check: B compare cost only between these
Method. Flow is converted to m³/min (1 cfm = 0.0283168 m³/min; 1 L/s = 0.06 m³/min); this is valid only when the bids use the same reference conditions. Net flow = offered flow − internal consumption (dryer purge, drains). A bid whose data is not at the required pressure is not credited: it needs clarification, not arithmetic. Specific power = total package input kW ÷ net flow. Energy per year is compared like-for-like at the required net flow: specific power (on net flow) × required net flow × hours, so a larger machine is not charged for air the plant does not use. This assumes the specific power holds at that flow; for a VSD or a plant that runs mostly at part load, use each bidder’s supported part-load power for each hour band instead. Bids are ranked on energy only if they pass the net-flow check. Comparable installed cost = quoted price + required additions − confirmed discounts. Default: the page’s worked example B (20.0 m³/min net required at 8 bar g).
What to take from it. A low price or a larger gross flow cannot offset a failed mandatory requirement. Compare cost only between bids that pass the net-flow check.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.
Lifecycle cost comparison
Compare two options over their life: purchase, energy and maintenance. The cheapest machine to buy is rarely the cheapest to own.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Option A: input power at working load (kW): 75
- Option B: input power at working load (kW): 66
- Running hours per year: 6000
- Years: 10
- Discount rate (%): 8
Result
- Option A energy per year: 4,50,000 kWh
- Option B energy per year: 3,96,000 kWh
- Lower energy use: Option B by 54,000 kWh per year
- Lifecycle cost: Enter your electricity tariff (cost per kWh) above and the purchase and maintenance amounts
Method. Lifecycle cost (present value) = purchase + Σ (annual energy + annual maintenance) / (1 + r)ⁿ. Energy = input power at working load × hours × tariff. Equivalent annual cost = present value × r / (1 − (1 + r)^−years).
What to take from it. Energy usually outweighs the purchase price over the life of a compressor. Run the comparison with your own running hours.