Why compressor packages differ
A compressor “package” can mean anything from a bare airend and motor on a frame to a complete compressed-air station with receiver, dryer, filters, drains, controls and installation. Catalogue phrases such as “ready to run” or “complete package” are not definitions. One supplier may include an integrated dryer and a star-delta starter; another may quote the compressor alone and leave the dryer, receiver, cabling and commissioning to the buyer.
The gap between two offers is usually filled later, at the buyer’s cost and often in a hurry. A careful comparison before award shows the real scope, the real performance and who is responsible for each interface.
What to compare: a scope checklist
The table below lists the items that most often differ between offers. Mark each one as included, excluded or optional for every quotation.
| Item | Why it matters | Question to ask |
|---|---|---|
| Compressor technology and pressure variant | Sets flow, efficiency and air quality | Oil-injected or oil-free? Fixed or variable speed? Which pressure variant? |
| Integrated or separate dryer | Affects dew point, footprint and power | Refrigerated or desiccant? Rated at which inlet and ambient temperature? |
| Filters | Decides air-quality class | Which grades, how many, with differential gauges? |
| Air receiver | Stability, control and storage | Volume, design pressure, safety valve, drain, inspection documents? |
| Condensate drains and treatment | Air loss and environmental compliance | Timer or level-sensing drains? Oil-water separator? |
| Starter and electrical panel | Starting current, protection, integration | DOL, star-delta, soft starter or VSD? Panel IP rating? |
| Power cables and earthing | Often excluded and expensive | Who supplies and lays cables from the main panel? |
| Isolation valves, pipework and hoses | Connection to the plant | Up to which flange or point is supply included? |
| Cooling and ducting | Plant-room temperature, efficiency | Air or water cooled? Hot-air duct included? |
| Controls and remote monitoring | Multi-machine control, alarms | Sequencer? Remote signals? Which protocol? |
| Foundation, transport and unloading | Site cost and time | Who handles delivery, lifting and placing? |
| Installation and commissioning | Correct start-up and settings | Who installs, commissions and trains operators? |
| Documentation | Service, parts and inspection | Manuals, parts lists, test reports, vessel documents? |
| Warranty and service | Life-cycle support | What is covered, for how long, and what is the first-year service scope? |
Compare performance on the same basis
Flow and power figures are only comparable if they are stated the same way. Ask every supplier for:
- Free air delivery to ISO 1217, at the same working pressure, stated in m³/min or cfm (1 m³/min ≈ 35.3 cfm).
- Total package input power in kW at that pressure, including fan motors and integrated dryer, not just the motor nameplate rating.
- Specific power: package input kW divided by FAD in m³/min, at the same pressure. A lower figure means less energy per unit of air.
- Part-load performance, especially for variable-speed machines: power at the flows you actually expect to run, not only at full load.
- Dryer performance at realistic inlet and ambient temperature. A refrigerated dryer typically gives a pressure dew point around +3 °C at its rated conditions; in a hot Indian summer, real capacity at that dew point is often lower.
- Pressure drop across integrated dryers and filters, since it raises the pressure the compressor must make.
Remember that each extra bar of pressure costs about 7% more power. An offer that looks cheaper because it quotes flow at a lower pressure than your plant needs is not a like-for-like offer.
New, used or rented: comparing different routes
Sometimes the real comparison is not between two new packages but between buying new, buying a used or refurbished machine, or renting. A used compressor may cost less to buy, but its scope must be checked even more carefully: which treatment equipment comes with it, the condition of the airend and motor, and whether parts are still supported. Rental moves the scope question onto the hire contract: hoses, dryers, delivery, servicing and operators may or may not be included. Use the same checklist for every route so that the decision rests on complete scope and running cost, not on headline price.
Electrical and control interfaces
The electrical boundary is where most surprises appear. A direct-on-line starter on a large motor can cause a heavy starting current that the site’s supply or transformer may not accept. A star-delta starter reduces this; a soft starter or variable-speed drive reduces it further and, for a VSD, gives part-load efficiency. Ask who designs and supplies the incoming cable, the isolator, the earthing and any harmonic filtering a VSD may need. For multi-compressor stations, check whether the controller can be sequenced with existing machines and what signals are available for a plant control or building management system.
Installation, ventilation and service access
An air-cooled compressor rejects almost all its input power as heat. Ask whether the offer includes ducting to remove hot air, and whether the supplier has checked the room’s ventilation. In Telangana and Andhra Pradesh summers, a machine installed without ducting can run hot and trip. Check also that the layout leaves room to open panels, change separators and filters, and remove the motor or airend when needed. A package that cannot be serviced easily costs more over its life.
How to compare offers step by step
- Write your own duty first: flow demand pattern, pressure at the point of use, air-quality class under ISO 8573-1, ambient conditions and running hours.
- Send every supplier the same specification and ask them to state exclusions clearly.
- Build a comparison sheet with the checklist above, marking each item included, excluded or optional.
- Cost the gaps: add the price of every excluded item you will need, to put all offers on a complete-scope basis.
- Normalise performance: compare FAD and specific power at the same pressure, and part-load power at your expected flows.
- Estimate running cost: annual energy in kWh from specific power, flow and running hours, plus expected service and parts.
- Check support: availability of spares and service engineers in your region, and how parts are identified for the machine.
- Clarify and confirm in writing before award, with a scope-of-supply list attached to the order.
Typical package scopes compared
The three scopes below are common in Indian industrial purchases. None is right or wrong; the point is to know which one each offer represents.
| Scope level | Usually included | Usually left to the buyer |
|---|---|---|
| Compressor only | Compressor with canopy, controller and starter | Receiver, dryer, filters, drains, cables, pipework, ducting, installation |
| Compressor with treatment | Compressor, integrated or separate dryer, main filters, drains | Receiver, cables, pipework, ducting, oil-water separator, installation |
| Complete station | Compressor, receiver, dryer, filters, drains, piping in the room, ducting, commissioning | Incoming power cable, civil work, distribution main beyond the room |
When one supplier quotes a complete station and another quotes the compressor only, their prices cannot be compared until the missing items are added to the second offer. Equally, a complete station may include items you already have, such as a working receiver, and those can be removed to sharpen the comparison.
Red flags in compressor quotations
- Flow quoted without a pressure, or at a pressure lower than you need.
- Motor kW given instead of package input power.
- “Package” with no list of included items.
- Dryer rated only at mild reference conditions.
- No mention of cables, ducting, commissioning or documents.
- Warranty conditions that depend on parts or fluids you cannot easily source.
How Reddy Compressors helps
Reddy Compressors, at Tarnaka in Secunderabad and serving industry since 2002, sells new and used compressors of many makes, including ELGi, Atlas Copco, Ingersoll Rand, KAESER and Chicago Pneumatic, 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 set out a clear specification and compare offers on the same basis, including our own.
We state what is included in our scope of supply, carry out installation and commissioning where agreed, and support the machine afterwards with service, AMC and spare parts. Our engineers cover Telangana and Andhra Pradesh, and we supply compressors across India; for long-term projects we can supply brand-new machines to site. If a new package will take time to arrive, a rental compressor from our own fleet can cover the gap.
What to send us
- Your air demand: flow, pressure at the point of use and running pattern.
- Required air quality or the processes the air serves.
- Copies or summaries of the quotations you are comparing.
- Plant-room layout, available power supply and existing compressors.
- Your planned timeline.
Frequently asked questions
How do I compare two air compressor quotations?
List everything each quotation includes and excludes, such as dryer, filters, receiver, starter, cables, ducting and commissioning, and add the cost of missing items. Then compare free air delivery and total package input power at the same pressure, and calculate specific power in kW per m³/min. The cheapest offer is often not the cheapest once scope and running cost are equal.
What is specific power and why does it matter?
Specific power is the total package input power divided by delivered free air, usually in kW per m³/min, at a stated pressure. It shows how much electricity the compressor uses to make each unit of air. Because energy is usually the largest share of a compressor’s life-cycle cost, a small difference in specific power can matter more than a difference in purchase price.
Should I buy a compressor with an integrated dryer?
An integrated dryer saves space and simplifies installation, and suits many smaller and medium plants. A separate dryer can be easier to service, can be sized independently and can be bypassed during maintenance. Either way, check the dryer’s rating at your real inlet and ambient temperatures and the pressure drop it adds, as hot Indian summers reduce dryer capacity.
Is a variable-speed compressor always better than fixed speed?
Not always. A variable-speed machine saves energy where demand varies widely, because it reduces power at part load instead of running unloaded. Where demand is steady and close to full load, a fixed-speed machine can be as efficient and costs less. Compare part-load power at the flows you actually expect, and check the electrical supply for harmonics.
What should be included in compressor installation and commissioning?
Installation normally covers placing the machine, connecting pipework, drains and electrical supply, and fitting ducting. Commissioning covers checks of rotation, oil level, settings, safety devices and alarms, a running test, and operator training. Ask each supplier to list exactly what they include and which tasks fall to the buyer or the electrical contractor.
Why does the motor kW not match the compressor’s power consumption?
The motor rating is its output at the shaft. The package also powers cooling fans, an integrated dryer if fitted, and controls, and the motor itself has losses. Total input power is therefore higher than the motor nameplate figure. Ask suppliers for package input power at your working pressure so that offers can be compared fairly.
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 17 October 2026. How our engineering content is reviewed.
Specific power comparison
Compare two compressors on what matters over life: the electrical power needed per unit of air delivered, and what that costs a year.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Machine A: input power (kW): 75
- Machine A: delivered flow (m³/min): 11.5
- Machine B: input power (kW): 80
- Machine B: delivered flow (m³/min): 12.6
- Running hours per year: 6000
- Air actually needed (m³/min): 11
Result
- Machine A specific power: 6.52 kW per m³/min
- Machine B specific power: 6.35 kW per m³/min
- Energy to deliver the air needed: A: 4,30,435 kWh per year
- Energy to deliver the air needed: B: 4,19,048 kWh per year
- Energy difference: 11,387 kWh per year in favour of B
- Cost difference: Enter your electricity tariff (cost per kWh) above
Method. Specific power = input kW ÷ delivered flow (m³/min) at the same pressure and reference conditions. Annual cost = kW × hours × tariff. Use the full-load input power on the same test basis for both machines.
What to take from it. A small gap in specific power is multiplied by thousands of running hours. Compare on the same pressure and reference conditions.