An air compressor specification should define the air your process needs, the point where it must be delivered, the operating conditions, the equipment included in the quotation and the evidence required for acceptance. Motor power alone is insufficient. A useful specification lets different suppliers answer the same questions and gives your team a clear basis for checking the delivered system.
The difficult part is usually hidden in the quotation details. One offer lists flow at the compressor outlet; another includes a dryer but does not state its effect on delivery. A power figure may describe the motor rather than the complete package. Both proposals can look complete until an engineer asks what the numbers actually represent.
This guide is for factory engineers and buyers preparing a request for quotation, or RFQ. It provides wording you can adapt, a comparison worksheet and acceptance questions. Blank fields are intentional: fill them with measured plant information, machine requirements or confirmed supplier data. No assumed electricity prices, operating hours or site conditions are needed.
Start the air compressor specification at the process
Identify the delivery point
Choose a physical point that both your team and the supplier can identify. It could be the package discharge connection, the treated-air station outlet or a production header. Name that point on the layout drawing and use the same name throughout the RFQ.
The distinction changes the scope. A compressor outlet guarantee does not automatically describe pressure or air quality after an external dryer, filter and distribution pipe. If the supplier is responsible for delivery at the station outlet, the offer should include the equipment and allowances needed to reach that point.
Adaptable wording: “Required flow, pressure and air quality apply at the delivery point marked on drawing [reference]. The supplier shall identify every component between the compressor package outlet and this point, and state which components are included in the offer.”
Describe what happens during production
Give the supplier a description of the load, not just a list of installed machines. Explain which users operate together, which have short demand bursts and which are essential throughout the shift. Identify any shutdown or maintenance arrangement that changes the demand.
For an existing plant, attach a measured demand profile where available. If only machine data are available, label the demand assessment as preliminary and explain its basis. A supplier should be able to distinguish confirmed demand from equipment that has not yet been ordered.
Expansion belongs in a separate field. Describe the proposed additional process and its expected operating relationship with the present plant. This allows the supplier to explain the expansion strategy without quietly building an unspecified allowance into every component.
Assign owners to missing information
A blank field is manageable when someone owns it. Ask production engineering to confirm the machine inlet requirement, facilities to confirm the electrical supply, and the quality team to confirm air cleanliness where the process is sensitive. Procurement can then identify which missing items prevent a comparable quotation.
Do not ask the supplier to infer every requirement from the existing compressor's nameplate. That equipment may have been selected for a different production layout. It can be useful background, but it is not a substitute for the current duty.
The wider screw compressor buying guide covers technology selection. This article concentrates on making the requirements, exclusions and acceptance evidence explicit once the purchasing project has been defined.
Air compressor specification: flow and reference conditions
Specify the basis as well as the unit
A flow entry needs its unit, reference conditions, measurement location and applicable operating pressure. Terms such as free air delivery, actual flow and normalized flow should not be treated as interchangeable labels. Ask the supplier to define the quantity shown on the selected model's data sheet.
Keep the requirement and the offered performance in adjacent columns. This makes a difference in basis visible instead of allowing it to disappear inside a conversion. Where a conversion is required, retain the original value and the assumptions used by the engineer performing it.
Adaptable wording: “State the flow quantity, units and reference conditions used for the offered performance. Identify the test method and discharge measurement point. Any conversion to the purchaser's requested basis shall be shown separately with its assumptions.”
Use the pressure associated with that flow
A catalogue may show several pressure options for one family. The relevant flow is the value associated with the offered configuration and operating condition. Avoid comparing the highest flow in a range with the highest pressure shown elsewhere on the page.
Ask for a matched operating point: model, configuration, pressure, flow and package input power. The supplier should identify whether the information is a catalogue rating, a model-specific test result or a proposed guarantee. Those descriptions belong in the comparison record.
If a quotation says only “up to” a flow or pressure, return that row for clarification. The offer may describe a genuine product range, but the range does not establish the performance of the particular machine being purchased.
Keep temporary peaks separate from continuous duty
An average demand figure can hide the event that causes a production interruption. Describe both the sustained operating requirement and the important short demand events. Include their timing relative to other users where that information is available.
The supplier can then explain whether the proposal addresses the event through generation, storage, controls or a combination. The explanation should include recovery between events. A receiver is not evidence that an indefinitely sustained shortage has been solved.
For background on demand quantities, see our air compressor CFM calculation guide. Use the actual process information in the RFQ rather than applying an unverified percentage to the plant's total connected equipment.
Set the pressure boundary in your air compressor specification
Separate process pressure from station pressure
List the pressure required at the production equipment and the point where that requirement applies. Then identify who is assessing the treatment and distribution path between the station and the equipment. This prevents a quotation for the compressor alone from being mistaken for a guarantee across the whole factory.
The pressure budget should identify the sections considered and the conditions under which losses are assessed. Filters, dryers, regulators, valves and local connections can affect the available pressure. An unspecified “margin” does not explain how those components were evaluated.
For the contract, name the responsible party for each part of the pressure path. If your own engineer designs the distribution network, provide the station requirement derived from that design and retain the calculation with the project records.
Describe the operating event used for acceptance
A pressure reading with the plant idle cannot establish that the production requirement is satisfied. Identify the representative operating sequence to be used during the site check. Include the important simultaneous users and any agreed standby condition.
State where pressure is measured and how the readings are related in time. A brief disturbance may need recording equipment that can capture it. Let the responsible engineer select the method and interval according to the event, rather than prescribing a convenient generic logging setting.
Adaptable wording: “Site delivery pressure will be checked at [location] during [agreed production sequence]. The procedure shall identify the instruments, recording method, operating configuration and treatment equipment in service.”
Power entries in an air compressor specification
Distinguish the motor from the complete package
Motor nameplate power, shaft power and package electrical input answer different questions. The motor entry helps identify the drive. Shaft power describes a mechanical quantity. Package input describes electricity supplied to the defined compressor package under stated conditions.
The CAGI resource library distinguishes total package input from a motor-only figure and provides compressor data-sheet resources. When reviewing an offer, request the package boundary and the operating condition next to the power entry.
Accessories need attention too. Ask whether fans and controls are inside the declared package input, and whether an external dryer, cooling-water pump or station accessory is excluded. The supplier's boundary should be clear before your team estimates the installed station's consumption.
A documented SEIZE example
SEIZE's SGS-witnessed acceptance report SHES250300470951, dated 2025-04-03, identifies the tested equipment as SVC-75A-II, SV-75A-II and SZ-100TPM. Its nominal motor entry is 75 kW. At the report's full-flow test point, package power input is 90.44 kW and shaft power is 81.92 kW.
These are distinct recorded quantities, not competing answers to the same question. The point is reported at 0.8 MPa discharge pressure, with its own inlet conditions. Testing took place at SEIZE's laboratory with SGS witnessing; the result applies to the tested equipment and conditions, not every current model or customer installation.
For buyers, the useful lesson is to ask which quantity a quotation presents. A nominal motor figure should not be substituted for package consumption in an energy comparison. Request the current offered model's documentation rather than assuming this historical test represents the machine being quoted.
Calculate specific power only from matched data
On a consistently defined basis, package specific power can be expressed as package electrical input divided by delivered flow. If input is in kW and flow in m³/min, retain the resulting unit kW/(m³/min). The pressure and reference conditions belong beside the calculation.
Do not combine a motor rating from one table with a flow maximum from another. Both inputs need to describe the same package and operating point. If the proposal supplies a different efficiency metric, keep its definition and supporting information rather than forcing it into an incompatible ratio.
An engineer should decide whether two results are comparable after checking pressure, flow basis, package boundary and test conditions. A lower printed number without that review is not a sound basis for awarding the contract.
Assess the load profile before estimating energy
The full-load result describes one operating condition. A plant that spends substantial time at different loads needs information about the relevant operating range and control behaviour. Request the available performance information for the anticipated duty and the method used to estimate consumption.
The general relationship is energy = sum of power at each operating condition multiplied by time at that condition. Power in kW and time in hours give energy in kWh. Fill the calculation with measured or justified operating inputs, and show any uncertainty.
No fixed saving percentage is needed in the RFQ. Ask the supplier to describe its comparison basis and exclusions. The eventual energy assessment should reflect the agreed production duty, not an invented annual schedule.
Specify control and availability as an operating arrangement
Ask how the station handles a changing load
For a variable-speed proposal, request the offered model's operating range and intended role in the station. Explain the anticipated minimum demand, normal duty and significant changes. A variable-speed label alone does not describe the behaviour of several machines operating together.
A fixed-speed proposal also needs an explanation of sequencing and load changes. Ask which machine responds to demand and how the proposed controls coordinate with existing equipment. Identify any required interface work in the quotation scope.
The SEIZE two-stage variable-frequency screw compressor series is one product route to discuss for an oil-injected application. Its suitability still depends on the selected configuration and the documented plant duty.
Define the unavailable-equipment condition
Availability requirements should state what production must continue when a compressor or treatment component is unavailable. Identify whether the requirement applies during planned maintenance, an equipment fault or both. Separate essential production from demand that may be temporarily curtailed.
Describe the proposed isolation arrangement and how the station operates in that condition. If treatment is part of the supplier's scope, assess whether the same availability requirement applies to the dryer and filters. Compressor redundancy does not establish redundancy of the complete treated-air path.
Ask the supplier to identify the remaining delivery capability and the operating assumptions. This provides a useful basis for assessing the proposal without making an unsupported statement that a particular equipment count guarantees uninterrupted operation.
Air quality requirements in an air compressor specification
Keep cleanliness, drying and delivery location together
Identify the process air-quality requirement and its source. Name the location where it must be satisfied. Where the application involves product contact or another sensitive use, include the process owner's requirements and the applicable verification procedure.
Oil-free generation does not by itself document every aspect of the delivered air. Water, particles and the downstream distribution condition also need appropriate consideration. The complete treatment proposal should explain how the specified delivery quality is achieved and checked.
Adaptable wording: “The required air-quality specification applies at [point]. The supplier shall identify the proposed generation and treatment arrangement, provide evidence relevant to the offered configuration, and state the verification method and exclusions.”
Separate oil-free technology options
SEIZE's local product documentation distinguishes water-lubricated and dry oil-free screw products. These are separate product routes with different model-specific operating and maintenance requirements. Do not combine their data or use one manual's instructions for the other.
For a relevant enquiry, compare the documentation available for the water-lubricated oil-free screw series and the dry oil-free screw series. Ask for the offered model's installation, utilities and service requirements.
If certification is part of the purchase, request the actual certificate and confirm its scope, model coverage and validity. A general product-family statement cannot replace that check. Include the required evidence in the document list before the order is finalized.
Include treatment conditions in the quotation
Give the dryer supplier the relevant inlet conditions and required drying performance. The offered unit should be identified by model and configuration, with any applicable correction or selection basis explained. Ask for the treatment train's effect on the defined delivery condition.
The SEIZE refrigerated dryer product page provides a product starting point. Your RFQ should still state the process requirement and site conditions, rather than choosing a dryer from nominal compressor flow alone.
List consumables, drains and maintenance access in the scope. They are easy to omit when the quotation focuses on the compressor package, yet they affect how the station can be maintained and how the specified air quality is preserved.
Installation fields that change the offered equipment
Cooling and compressor-room conditions
State the available cooling arrangement and the room conditions used for selection. An air-cooled package needs model-specific cooling and heat-rejection information. A water-cooled offer needs the relevant water specification and a clear allocation of responsibility for the cooling system.
SEIZE's historical oil-injected installation manual emphasizes clean, dry surroundings, ventilation and avoiding intake of other equipment's hot discharge. Those principles are useful context, but the current order requires the applicable model's installation sheet. Historical limits should not be generalized across every series.
Include the proposed location, openings or ducts, service clearances and access route in the enquiry. The existing compressor-room ventilation guide explains the room review. In the RFQ, assign responsibility for reviewing the actual room layout before installation.
Electrical supply and integration
Record the confirmed supply voltage, frequency and connection requirements. Identify the party responsible for upstream electrical design and protection. Ask the supplier to provide the model-specific electrical information that the installation engineer needs.
Control integration deserves its own field. Name any existing station controller, required communication interface or remote monitoring requirement. Specify whether the quotation includes configuration, connection and demonstration of those functions.
Do not label an unknown interface as compatible because both systems have a communication port. Obtain the protocol and functionality details, then agree how the integration will be checked. Keep optional monitoring subscriptions or support services visible as separate quotation items.
Access, environment and service support
Provide the route into the room and any restrictions on lifting or placement. Request the equipment dimensions, weight, connection positions and maintenance clearances for the actual offered configuration. Reserve access for removing the components identified in the maintenance instructions.
Describe environmental conditions that materially affect selection, supported by site data. Geography alone does not establish ambient temperature, humidity, dust exposure or elevation. A regional application needs actual design conditions, not a generic claim about its climate.
For overseas projects, request the service arrangement and warranty terms applicable to that order. Identify commissioning, operator instruction, spare parts and the support contact. Domestic response promises should not be assumed to apply to an overseas installation.
Use an air compressor specification worksheet
Fill the purchaser's column before requesting a price
| Field | Purchaser input | Supplier response |
|---|---|---|
| Delivery point | Drawing reference and marked location | Confirmed boundary and included path |
| Flow | Required quantity, unit, basis and duty | Matched model value and test basis |
| Pressure | Required condition at the defined point | Offered operating point and allowances |
| Air quality | Process requirement and source | Generation, treatment and verification |
| Power | Required comparison boundary | Package input and included accessories |
| Load pattern | Profile or preliminary demand basis | Control strategy and performance evidence |
| Availability | Production required during an outage | Remaining capability and isolation strategy |
| Site conditions | Confirmed utilities and environment | Model limits and installation requirements |
| Acceptance | Required evidence and site sequence | Proposed procedure and responsibilities |
Attach references rather than writing vague entries such as “standard industrial conditions.” When information remains unresolved, name its owner and the date needed for confirmation. This keeps the quotation process moving while making uncertainty visible.
Use a separate deviation schedule
Ask the supplier to list every difference from the requested duty or scope in a deviation schedule. The schedule should reference the corresponding RFQ field and explain the offered alternative. An empty schedule should have a clear agreed meaning, rather than relying on silence.
Distinguish a technical deviation from a commercial option. A different delivery boundary affects performance comparison; an optional spare-parts kit affects scope and price. Mixing those into a single total makes it harder to judge either.
Adaptable wording: “The supplier shall provide a field-by-field response. Deviations and exclusions must be listed explicitly. Alternative configurations may be offered separately and shall not replace the compliant base offer without identification.”
Compare each quotation against the air compressor specification
Resolve technical differences before ranking price
| Quotation observation | Why comparison is incomplete | Clarification to request |
|---|---|---|
| Motor kW only | Complete package input is unidentified | Package power, boundary and operating point |
| Flow without a basis | The quantity cannot be compared consistently | Units, reference conditions and test location |
| Maximum pressure only | Required-flow performance is unresolved | Matched flow at the offered duty |
| Dryer included without selection details | Delivered quality and pressure are unresolved | Model, inlet conditions and treatment duty |
| Saving percentage without a baseline | Energy benefit cannot be evaluated | Comparison equipment, duty and calculation |
| Certificate without offered-model scope | Compliance coverage is not demonstrated | Original certificate and applicable scope |
| Commissioning described as optional | Site acceptance responsibility is unclear | Included tasks, personnel and procedure |
This table is a clarification tool, not a claim that every incomplete entry makes an offer unacceptable. Mark each item as resolved, unresolved or an accepted deviation. Your technical reviewer can then state why a proposal is ready for commercial comparison.
Keep optional technologies within the same RFQ
If the duty justifies reviewing different technologies, issue the same delivery requirements to each proposal. The SEIZE gear centrifugal compressor series can be considered as another product route where the application fits. The selection still needs a model-specific assessment.
Do not assume that technology names establish energy performance or maintenance cost. Request the relevant operating data, utilities, service scope and availability arrangement. Identify any difference in treatment or cooling infrastructure in the comparison.
The benefit of a shared RFQ is visibility. Buyers can see whether an alternative meets the same duty and what additional equipment it requires. A lower package price then remains a commercial fact rather than being mistaken for a lower installed-system cost.
Write down the award decision
Record the reasons for selecting the preferred proposal. Reference the accepted operating point, delivery boundary, resolved deviations and included scope. Name any condition that must be closed before manufacture or shipment.
A short decision record is more useful than a generic statement that one supplier is best. It lets project staff check that the ordered configuration matches the technical assessment and prevents an optional item from being lost between quotation and purchase order.
If a supplier revises its offer after selection, compare the revision against that record. Check model changes, included treatment and utility requirements before accepting the new document. Price continuity does not establish technical continuity.
Acceptance evidence for the air compressor specification
Separate documentation, factory evidence and site operation
The acceptance plan should identify which requirements are checked through documents, which through factory performance evidence and which through site operation. These methods answer different questions. A factory report cannot establish the pressure at the end of a distribution network that was not part of that test.
Agree the procedure before the purchase is finalized. Record the applicable test method, instruments, operating conditions, tolerances and responsibilities. Use the offered equipment's requirements and the contract, rather than inserting an unverified generic tolerance.
Where witnessing or third-party evidence is required, specify the actual scope. Do not describe a witnessed test as independent laboratory testing unless that description is supported. Keep model identification consistent between the quotation, test records and delivered nameplate.
Prepare a site acceptance record
| Acceptance item | Evidence to retain | Responsible party |
|---|---|---|
| Equipment identity | Model, configuration, serial identification and order reference | Buyer and supplier |
| Installation readiness | Utilities, connections and access checked against instructions | Installation engineer |
| Delivery condition | Readings at the agreed point during the agreed duty | Commissioning team |
| Air quality | Required verification and treatment configuration | Process or quality owner |
| Controls | Demonstrated operating sequence and approved settings | Controls specialist |
| Availability | Agreed unavailable-equipment sequence and result | Plant operations |
| Handover | Manuals, maintenance requirements and support details | Supplier and maintenance team |
Keep unresolved items in a separate punch list. Name the required action and the owner, and distinguish an accepted temporary arrangement from final acceptance. A signature should refer to the identified configuration and evidence, not simply confirm that a machine was delivered.
Protect the baseline after handover
Save the accepted configuration, settings and operating records with the maintenance documents. If the plant later adds a machine or changes the treatment train, the baseline helps the engineer identify which original assumptions have changed.
Include a practical process for evaluating modifications. Staff should know where the installation information is kept and which team reviews changes affecting delivery or air quality. This is especially useful when different contractors handle electrical work, piping and production equipment.
The purchase specification then remains useful beyond procurement. It connects the original duty to the installed equipment and gives maintenance staff a clear description of what the station was intended to deliver.
Send an RFQ that a supplier can answer
Attach the right supporting files
Send a marked layout, the completed requirement worksheet and the available demand information. Include the machine requirements behind the requested pressure and air quality. Provide confirmed utility information and identify any remaining site-data gaps.
Ask for a model-specific response, an itemized scope and a deviation schedule. Request the documentation needed for selection, installation and the agreed acceptance plan. Separate optional items so that the base offer can be compared consistently.
There is no need to distribute confidential production details that are irrelevant to the enquiry. Use the information needed to explain the duty, and manage any sensitive drawings through your company's approved process. The supplier should be able to state what additional information is genuinely necessary.
Make the commercial boundary match the technical boundary
List who supplies treatment equipment, receivers, distribution connections, cooling infrastructure and electrical installation. Include commissioning and operator instruction where required. If an item is outside the supplier's scope, identify who provides it.
For an overseas order, state the agreed delivery and commercial requirements in the purchasing documents. Clarify local installation responsibilities, required documentation language and the applicable support arrangement. Do not infer these terms from another customer's project or a domestic service description.
Request a revised scope if the technical assessment changes the selected configuration. The commercial comparison should describe the system that will actually be purchased, with exclusions visible alongside the quoted price.
Questions buyers ask about an air compressor specification
Can I copy the old compressor's kW rating into the RFQ?
You can include it as background, but define the present delivery requirement separately. Production may have changed, and motor power does not establish delivered flow, treatment scope or package consumption. Request a matched operating point for the offered equipment.
Must every quotation use the same compressor technology?
Alternative technologies can be useful if each proposal answers the same process requirements. Keep a comparable base scope and require the supplier to explain configuration, utilities, control behaviour and deviations. Technology differences should remain visible in the assessment.
What if demand has not been measured?
Use available machine requirements to prepare a clearly labelled preliminary duty and identify its owner. Ask what further assessment is required before final selection. Do not convert an uncertain estimate into an unconditional performance requirement without review.
Is a standard data sheet enough for acceptance?
It can support equipment comparison, but the contract should identify the evidence and procedure used for acceptance. Model coverage, test conditions and delivery boundary matter. Site operation requires its own agreed scope when downstream equipment or actual production is part of the requirement.
Should I demand a fixed energy-saving percentage?
A meaningful energy comparison needs a defined baseline, duty and measurement boundary. Request those first. A percentage without them can conceal different pressures, operating hours or equipment scope and does not establish a benefit for your plant.
Finish with a comparable, traceable enquiry
A complete air compressor specification connects process requirements, equipment boundaries and acceptance evidence. It makes uncertainties visible before they become purchasing assumptions. The result is an enquiry that suppliers can answer and a record your team can use through installation and operation.
Send SEIZE AIR your completed specification worksheet, marked delivery-point drawing and available demand information. Ask for a model-specific quotation with a defined scope, documented deviations and the installation information needed for your project.