Temporary Air Duty Specification

1 Define the service before requesting a machine

A temporary air duty specification is the technical statement of what a rental compressed-air installation must deliver, where it must deliver it and under which operating conditions. It defines the required flow, pressure, quality, operating profile, duration, continuity and site interfaces. It also identifies the evidence that will demonstrate acceptable performance.

The specification allows a plant team and an equipment supplier to discuss the same requirement. Without it, one quotation may describe compressor outlet capacity while another includes drying, purge losses and delivery through a long hose. Both may appear to offer the same CFM, yet provide different useful service at the plant connection.

A request such as “send a 750 cfm compressor for one month” leaves important questions unanswered. Is that flow required continuously or for a short peak? At what pressure and reference conditions? Does the process need dried or oil-controlled air? Does the rental replace the complete station or supplement a working machine? What happens if the supply trips?

This guide turns those questions into a practical specification. The output is a reviewable service requirement that can be used for enquiries, technical comparison, mobilisation planning and commissioning. Equipment selection follows from that requirement; it should not silently redefine it.

2 Separate the requirement from the proposed solution

Write the required process outcome before naming a compressor model or drive type. A requirement might state that a production header must receive a defined net flow within a pressure range and at a specified quality during two shifts. The supplier can then propose a suitable combination of compressors, treatment and controls.

Some constraints are genuine: electric drive may be required by the site, access may limit machine dimensions, or an existing connection may have a fixed pressure rating. Record these as constraints with their reasons. Other preferences, such as using one large machine rather than two smaller units, may remain open to technical comparison.

Distinguish mandatory requirements from preferences and assumptions. Mandatory items determine whether the proposed service is acceptable. Preferences help compare acceptable options. Assumptions identify information that still needs confirmation. A supplier deviation should reference the exact requirement it changes and explain the effect on production, installation or cost.

Do not use commercial availability as proof of technical fit. An available unit may be useful, but the duty still determines whether it can provide the required pressure, capacity, quality and response. Conversely, a technically suitable configuration is not a confirmed mobilisation plan until equipment and supporting services have been committed for the assignment.

3 Establish the boundary at which performance matters

Name the delivery point precisely: for example, the downstream flange of the temporary treatment manifold at the plant tie-in. Include a location reference, connection identification and a drawing or photograph where helpful. State whether the performance obligation ends there or extends to identified downstream consumers.

Define what lies upstream and downstream of that point. If the supplier provides the compressor, dryer, filter and hose, their effects belong in the supplier's delivery assessment. If plant pipework follows the tie-in, its pressure loss and contamination history remain relevant to the final process service even if the plant owns it.

The flow, pressure and quality boundaries need not be physically identical, but their relationship must be explicit. A flow meter may sit upstream of a dryer while the process duty is specified downstream. In that case, treatment consumption must be reconciled. A quality measurement after a filter does not necessarily establish quality after an old plant branch.

Record maximum permitted pressure separately from minimum operating pressure. A process can require at least 6.2 bar gauge while a connected component imposes an upper limit. Both constraints must be satisfied, including control excursions and credible operating transitions. A request for higher delivery pressure is not permission to exceed the rating of any connected equipment.

4 Build an operating state register

A single maximum-flow number rarely describes a temporary duty adequately. Establish the operating states that the installation will encounter and their expected duration. Include normal production, coincident peaks, low load, idle periods, start-up and any reduced-service condition required during a failure.

Describe how states follow one another. A 30-second peak every five minutes differs from a peak repeated before the receiver can recharge. Start-up can also combine users that do not normally operate together. Use the actual production sequence rather than applying the same diversity factor to every state.

Identify who owns the operating-state information. Production personnel can confirm machine combinations and shutdown plans; maintenance teams can identify service limitations; process specialists can establish the consequences of pressure or quality deviations. The equipment supplier should receive a consolidated duty instead of conflicting estimates from separate departments.

5 State the airflow basis and data quality

Every specified volumetric flow needs a defined basis. State whether it is free air delivery, flow at explicitly stated reference conditions or actual volume at line conditions. Include reference pressure, temperature and moisture convention. Units such as cfm, m³/min or Nm³/h do not, on their own, resolve all reference-condition differences.

For a simplified ideal-gas conversion involving the same dry-gas quantity, Qref = Qline × (Pline / Pref) × (Tref / Tline), using absolute pressures and Kelvin temperatures. Compressibility and moisture corrections may be required for the intended accuracy and conditions. Do not apply this expression with gauge pressure or Celsius temperature.

For illustration, 1.0 m³/min of dry gas at 8 bar absolute and 313.15 K corresponds to approximately 7.49 m³/min at 1 bar absolute and 293.15 K when ideal-gas behaviour is assumed. This demonstrates why actual compressed volume cannot be compared directly with free-air-equivalent demand. It is not a substitute for the stated measurement or performance-test basis.

Classify each input as measured, derived, manufacturer-provided or assumed. Measured data should include the observation period, meter location, range, verification status and production conditions. Derived data should retain the calculation. An assumed value should have an owner and a plan for confirmation if it materially affects the equipment choice.

6 Convert the consumer register into design demand

List connected users with their operating pressure, air-quality need, consumption basis, cycle and credible simultaneous operation. Separate continuous consumers from intermittent tools and short events. Identify users that remain isolated during the temporary period; replacing a station does not always require recreating its complete historical connected load.

Use simultaneity based on the actual operating arrangement. A mathematical average of tool usage can conceal a short period when several tools operate together. Similarly, summing the rated maximum flow of all connected equipment can overstate the required sustained capacity if those machines cannot run concurrently.

Keep leakage and discretionary air use visible. If avoidable users will be isolated before temporary operation, state that as a condition of the duty. Do not subtract a hoped-for leakage repair from required capacity unless the repair and verification are included in the plan. When pressure changes, assess whether pressure-dependent consumption will change as well.

Separate uncertainty from growth and contingency. A measurement allowance addresses imperfect knowledge; growth covers an identified future load; redundancy addresses an equipment-unavailable state. Combining all three into repeated percentage additions can produce an inflated and poorly explained selection. Show each element once at the boundary where it applies.

7 Specify pressure dynamically

State the minimum acceptable pressure at the delivery point during each required duty state. Add any upper operating limit, permitted short-duration excursion and recovery criterion. “Seven bar compressor” describes equipment capability only loosely; it does not define what pressure the plant must receive while air is flowing.

Develop the upstream requirement from the process back through the active flow route. Include treatment, temporary distribution, couplings, regulators and plant losses where relevant. Distinguish pressure losses at clean initial condition from the maximum condition allowed during the hire. Dirty-filter limits and changes in flow can affect the available margin.

Control behaviour matters. The lower end of the operating band, response delay and peak demand can determine the minimum user pressure. Avoid adding an unexplained pressure margin and assuming that it covers all transients. Request the proposed control philosophy and compare its predicted behaviour with the allowable process response.

Acceptance should record pressures simultaneously at meaningful locations during representative demand. Define the sampling method so it can capture the events of interest. A slow trend may miss a brief but consequential pressure dip, while isolated handheld readings may not explain whether supply and demand events were coincident.

8 Specify quality through measurable requirements

Replace phrases such as “clean dry air” with the particle, moisture, oil and other limits relevant to the application. State the delivery boundary, operating pressure and verification method. Where a recognised classification is required, identify its applicable document and edition in the project specification rather than quoting an incomplete class label.

Pressure dew point must be connected to the operating pressure and lowest relevant downstream temperature. The absence of visible water during a warm commissioning run does not prove that condensation will be avoided during colder operation. Account for outdoor hoses and other sections that may cool below the conditions around the compressor.

Oil-free compression and delivered air quality are related but different questions. Intake contaminants, treatment condition, hoses and existing pipework can influence the final service. Request evidence for the complete air path and the specified boundary. Do not infer sterile, breathing-air or medical suitability from a general industrial oil-free description.

Define the response to off-specification air. A warning, shutdown, switch to an alternative source or process isolation may be appropriate depending on consequence. Assign responsibility for determining when the supply can return to service. Quality-critical applications require their own process and specialist acceptance requirements; a general rental specification cannot replace them.

9 Reconcile treatment consumption and net delivery

Express process demand as the net flow required at its boundary. Then account for consumption and losses upstream. For an inlet-based purge fraction f, the simplified compressor flow is Qcomp = Qnet / (1 − f), provided the flows use the same reference basis and other losses are absent.

If additional downstream consumption L is included, Qcomp = (Qprocess + L) / (1 − f) for that same simplified arrangement. If the proposed package already declares net treated output, do not deduct purge again. If purge is a fixed flow rather than a percentage, use the applicable fixed-flow relationship instead.

Ask the supplier to state treatment capacity at the actual minimum operating pressure, maximum inlet temperature and required quality. Record pressure loss, purge or regeneration demand, electrical auxiliaries and drain requirements. Confirm whether the stated limits apply continuously or only under a particular test condition.

Treatment performance can constrain the temporary system even when compressor capacity is sufficient. An alternate compressor combination may change dryer inlet temperature or flow distribution. The proposed package should demonstrate compatible performance as a system, including its low-load and equipment-unavailable arrangements.

10 Define continuity in terms of surviving service

Specify what must remain available after an identified failure, how quickly it must be restored and which loads may be shed. “Standby included” is insufficient if the process requires uninterrupted pressure but the spare unit needs manual connection and a lengthy start sequence.

Separate full-duty continuity from essential-duty continuity. A temporary arrangement might support complete production normally and a smaller group of critical consumers during one compressor outage. That can be an acceptable design if explicitly agreed. It must not be presented as full-load redundancy.

Assess shared dependencies. A standby compressor does not address a single failed dryer, blocked common filter, lost electrical feeder or unavailable fuel supply. State which failure scenarios the proposed system covers and which remain outside its scope. The degree of redundancy should reflect the actual consequence of interruption.

Storage may bridge a short response interval, but specify its usable pressure range, connected location and recharge conditions. Define whether control response is automatic or operator-dependent. A contingency based on human intervention requires realistic availability, access and instructions throughout the operating period.

11 Capture site conditions that change performance

The duty specification should list the ambient temperature range, altitude, expected intake condition, cooling arrangement and exposure relevant to equipment selection. Exact site-duty performance should be confirmed using applicable equipment information. Avoid inventing generic percentage derating rules where the proposed machine's data are required.

Record the electrical supply parameters and available capacity if electric drive is considered. Identify whether a generator will be used, and require assessment of starting or drive behaviour and auxiliary loads. For diesel equipment, identify fuel supply responsibility, permitted operating location and refuelling constraints.

State connection type, nominal size, pressure rating and access conditions. Include the approximate route length and whether the supplier provides temporary distribution. Identify location constraints that could affect ventilation, intake contamination, noise, exhaust or maintenance access. The next site-survey guide develops how to verify these physical details.

Separate design constraints from unverified site assumptions. For example, “electrical supply to be confirmed by site electrical team” is more useful than silently treating a nearby connection as available capacity. Unresolved items should be visible to the person making the selection and mobilisation commitment.

12 Worked temporary duty specification

Consider a hypothetical 21-day shutdown replacement serving a production header for 16 hours per day. All example flow quantities are expressed on the same dry-air reference basis of 1.00 bar absolute and 20°C. They are illustrative values, not measured plant data or equipment recommendations.

The net header demand is 18 m³/min during normal production, 24 m³/min for a 30-second peak once every 10 minutes, and 5 m³/min during low-load periods. The header requires at least 6.2 bar gauge during the defined states. Its upper permitted operating pressure is assumed to be 7.2 bar gauge; actual component ratings and protection would be assessed separately.

Suppose the assessed pressure loss from compressor discharge through the temporary treatment and distribution to the header is 0.6 bar at the peak duty. A preliminary upstream requirement is 6.8 bar gauge before verifying the actual control band and transient behaviour. The final arrangement must preserve the minimum header pressure and avoid exceeding the permitted limits at lower flow when losses decrease.

Assume, only for this example, that treatment purge consumes 12% of its inlet flow. Sustained production therefore requires approximately 18 / 0.88 = 20.45 m³/min before treatment. Directly supplying the complete peak requires 24 / 0.88 = 27.27 m³/min before treatment. The supplier can assess direct peak capacity or a combined source-and-storage solution, provided the same net service is demonstrated.

If a source maintains 18 m³/min net throughout the event, the peak deficit is 6 m³/min for 0.5 minutes, requiring 3 m³ of reference air. For an ideal isothermal receiver at 20°C operating between 8.0 and 7.5 bar absolute, V = 3 × 1.0 / 0.5 = 6 m³. This is a simplified inventory calculation, not a selected vessel size. Dynamic discharge, connection losses, protection, operating limits and site requirements remain to be assessed.

Most importantly, a source supplying exactly 18 m³/min has no spare flow to recharge storage while normal demand remains 18 m³/min. Between peaks, the 3 m³ deficit must be restored within the available 9.5 minutes. That requires at least 0.316 m³/min additional net flow on average over that interval in the simplified cycle, before other allowances and response limits. Storage cannot solve a repeating deficit without recharge capacity.

For continuity, assume the plant permits reduction to an agreed 8 m³/min essential load during one compressor outage. The specification would state that reduced duty, its minimum pressure and quality, the permitted recovery time and the authorised load-shedding arrangement. It would not simply request an unspecified spare compressor.

13 Reusable specification schedule

Use the following schedule as the enquiry and technical-response framework. Enter actual values and evidence references for the project. An unconfirmed item should remain explicitly unconfirmed, with responsibility for resolving it; it should not be replaced by a convenient supplier assumption without agreement.

Attach the duty-state register, available measured trends, relevant process requirements and the connection information. Keep this technical schedule distinct from a commercial price summary so qualifications are visible even when prices are compared separately.

14 Write clauses that can be checked

A usable clause names the required outcome, location, condition and evidence. For example: “Provide the agreed net treated flow at the named plant tie-in during the specified normal and peak states, on the stated reference basis. Declare all treatment consumption upstream of that boundary and demonstrate the agreed minimum pressure during the representative acceptance run.” Project values must replace the references before issue.

A pressure clause should distinguish continuous limits from permitted transient behaviour. A quality clause should identify the contaminants or classification, sample location and test method. A continuity clause should identify the failed component, surviving duty and restoration time. A schedule clause should distinguish delivery, installation readiness and accepted service.

Avoid blanket wording such as “suitable for all plant requirements” or “complete in every respect” as a substitute for technical definition. Such wording leaves different parties free to imagine different boundaries. Resolve interfaces through named tasks, equipment and responsibility rather than relying on broad descriptions.

Allow alternatives where they improve the solution. Ask for a compliant base proposal and clearly separated alternatives, each with a deviation statement. This preserves comparable quotations while permitting a supplier to explain a better control, treatment or capacity arrangement.

15 Compare technical responses before comparing rates

Review net delivered capacity at the specified pressure and site conditions, not nominal machine flow alone. Confirm treatment performance, minimum-load behaviour, peak response and the defined outage case. Examine whether the response includes the entire delivery route or leaves essential equipment to the customer.

Classify each requirement as compliant, compliant subject to a named condition, proposed deviation or unanswered. A condition needs a responsible person and a resolution date appropriate to its consequence. “To be confirmed” is not a performance commitment, even if the quotation otherwise appears complete.

Compare energy and operating costs on the same duty profile. Ask for the basis of fuel or electrical estimates and identify auxiliary consumption. Keep transport, treatment, hoses, connection work, operation and demobilisation within the same comparison boundary. Different scope should be reconciled before declaring one price lower.

Where urgent operation requires a provisional choice, record the limited service it is intended to provide and the evidence still required. A temporary reduced-duty arrangement may be valuable, but it must not silently become acceptance of a different full-production requirement.

16 Define acceptance before mobilisation

Agree the measurements, locations, operating states, observation periods and pass criteria before equipment arrives. The method should demonstrate the required service without asking a commissioning team to infer the intended duty from the delivered machines. Retain measurement uncertainty and instrument suitability in the assessment.

Capacity may be supported through an agreed combination of measured flow, documented equipment performance and representative operating evidence. State what each method proves. A successful pressure hold at low consumption does not establish peak capacity, and a brief run cannot prove performance for all seasonal conditions.

If the full production load is unavailable, identify which requirements were demonstrated and which remain deferred. Agree an acceptable interim operating scope and the later verification condition. Do not convert a partial test into a complete-performance claim simply because the equipment has begun operating.

Define how results will be recorded and who accepts them. Include equipment identity, configuration, settings, relevant ambient conditions, process state and any deviations. These records also establish the reference for subsequent changes during hire.

17 Manage revisions and responsibilities

Give the duty specification an issue date, revision and named owner. Record changes to flow, pressure, quality, duration and continuity requirements, together with the reason and affected supplier response. Keep earlier issued versions so a change in scope is traceable.

An added production line can reopen capacity and treatment sizing. A moved tie-in can reopen pressure loss and hose selection. A longer hire period can change servicing and fuel logistics. These are engineering changes even when the rental equipment itself remains the same.

Assign the plant's responsibilities for process requirements, site access, isolation and utilities, and the supplier's responsibilities for the agreed equipment and performance evidence. Identify shared commissioning tasks explicitly. Competent personnel should perform the specialist work appropriate to the site and equipment; the specification describes the required interfaces rather than replacing their working procedures.

After the temporary period, retain the accepted duty and actual operating observations. They provide a stronger basis for the next shutdown or contingency than an old quotation containing only a compressor capacity and monthly rate.

18 Frequently asked questions

Is the existing compressor size enough to specify replacement rental air

It is a starting reference, not a complete duty. Existing equipment may be oversized, undersized or serving loads that will be isolated during the shutdown. Establish net demand, pressure, quality, operating states and the required contingency before final selection.

Should I specify compressor flow or net treated flow

Specify the process service at the agreed delivery boundary. Ask the supplier to declare upstream compressor output and treatment consumption as supporting information. This makes different treatment arrangements comparable and avoids deducting purge twice.

How should short peaks be included

State peak flow, duration, frequency, pressure tolerance and the operating state between events. The proposed solution must supply the event and recover before it repeats. Average daily flow alone does not establish that capability.

Can a pressure range replace a single setpoint

Yes. A defined acceptable range and transient requirement often describe the process more usefully than one requested machine setpoint. The supplier should show how its controls and losses maintain that range across the required duty states.

What if no flow meter is available

Use a documented consumer register and available operating evidence to create a provisional duty. State assumptions and uncertainty. Where the selection is sensitive to those estimates, obtain targeted measurements or another agreed verification method before making an unsupported performance commitment.

Does a standby machine prove redundancy

No. Define which outage must be survived and what net flow, pressure and quality remain available. Include start or changeover time and shared dependencies such as treatment, power and connections.

19 Submit the duty for an engineering review

Send the completed duty schedule with available process data, operating trends and tie-in information. Include the required service date, expected duration and any restrictions that could affect equipment selection or installation. Clearly identify essential loads if reduced-service operation may be needed.

Reddy Compressors can use this information to assess the proposed temporary-air requirement and coordinate a project-specific response. Equipment configuration, specialist capability, availability and mobilisation timing remain subject to confirmation. Enquiries from Telangana and Andhra Pradesh can include the exact site location; support for other locations should be discussed against the particular project.