How to Write a Compressor Enquiry: The Process Data Your Supplier Needs Before They Can Quote

Why Vague Enquiries Produce Useless Quotations

We receive several types of compressor enquiries. The ones that result in fast, accurate technical recommendations and competitive commercial proposals share one characteristic: they contain specific process data. The ones that produce delays, clarification questions, and ultimately a quotation the buyer cannot use tend to follow a pattern: “Please quote for an oxygen compressor, we need it to compress oxygen to 150 bar.”

That enquiry contains two facts — the gas (oxygen) and the outlet pressure (150 bar). It is missing everything else: inlet pressure, flow rate, gas purity, duty cycle, cooling configuration, power supply, applicable standards, and installation environment. Without this data, we can produce a rough order-of-magnitude price range, but we cannot specify a machine, cannot confirm the motor power, cannot confirm the cooling water requirement, and cannot commit to a delivery schedule.

This article is a practical guide to writing a compressor enquiry that gets you a useful answer the first time.

The Eight Essential Data Points

These are the minimum data points required to specify any gas compressor. Missing any one of them either prevents accurate specification or forces us to make an assumption that may be wrong — and a wrong assumption in compressor specification produces a machine that does not perform as required.

1. Gas Type and Composition

State the gas clearly: oxygen, nitrogen, hydrogen, or a mixture. For oxygen: specify whether it is medical grade or industrial grade, and confirm the expected purity range (e.g., 93 ± 3% from PSA, or 99.5% from LOX vaporiser). For nitrogen: confirm the purity and whether it is from PSA, liquid nitrogen, or pipeline. For hydrogen: confirm the purity, the source, and whether any special materials are required.

If trace impurities are present — moisture, CO₂, hydrocarbon vapours from the gas generation process — list them. Some impurities affect material selection or require additional inlet treatment.

For operations that also compress CO₂ as a separate gas stream, the specification process follows the same logic — see our CO₂ compressor resources for application-specific guidance.

2. Inlet Pressure

State the minimum, normal and maximum inlet pressure. These are different values and all three matter:

• Minimum: determines the inlet low-pressure trip set point and confirms the machine will not trip under the worst-case inlet condition
• Normal: used as the design point for stage ratio calculation and inter-stage cooler sizing
• Maximum: confirms the first-stage inlet valve and cylinder design pressure are adequate

For PSA sources, the minimum inlet pressure must account for the lowest point in the PSA cycle, not just the average. Confirm this with your PSA supplier.

3. Outlet Pressure

State the required outlet pressure at the compressor discharge flange. This is not the pressure at the end user — it is the pressure at the compressor outlet, before distribution losses. If the end user requires 14.5 MPa and there is 0.5 MPa distribution loss in your pipework, the compressor outlet must be 15.0 MPa.

Also confirm whether a tolerance applies — some applications require a tightly regulated outlet pressure (e.g., ±0.1 MPa), while others only need the outlet to exceed a minimum value.

4. Flow Rate

This is the most frequently mis-stated parameter. State the flow rate at inlet conditions (not at standard conditions, not at outlet conditions). If you can only provide standard conditions flow, state the reference temperature and pressure used (typically 0°C / 1 atm or 20°C / 1 atm — these give different values).

Also state whether the flow rate given is normal (average), maximum (design) or minimum. We design the compressor for the maximum flow rate at the minimum inlet pressure — this is the most demanding condition. Designing for average flow at normal inlet pressure produces a machine that cannot meet peak demand.

Quick unit conversion: 1 Nm³/h ≈ 0.0167 Nm³/min ≈ 0.0167 m³/min at standard conditions. At 4 bar inlet absolute, 1 Nm³/min at standard conditions ≈ 0.25 m³/min at inlet conditions. Always specify which condition applies.

5. Duty Cycle

State whether the compressor is required to run:

• Continuous (S1 duty): 24 hours/day, 7 days/week — the most demanding duty class. All components must be rated for S1 duty.
• Intermittent: Cycles of run and stop — e.g., cylinder filling applications where the compressor runs until the storage rack is full, then stops. Specify the expected run fraction and cycle duration.
• Seasonal: Extended shutdown periods followed by continuous operation seasons — e.g., aquaculture with seasonal stocking. Wet lay-up procedures will apply during shutdown.

6. Cooling Medium

State whether water cooling or air cooling is preferred. For water cooling: provide the cooling water supply temperature (minimum and maximum), supply pressure, available flow rate, and water quality (hard/soft, treated/untreated, open/closed loop). For air cooling: provide the maximum ambient temperature and whether the compressor will be installed indoors or outdoors.

If cooling water is available, water cooling is preferred for all compressors above approximately 30 kW — water-cooled machines are smaller, quieter, and more efficient than their air-cooled equivalents at the same power.

7. Power Supply

State the available voltage, frequency, and the maximum fault level (short-circuit current) at the supply point. For standard industrial installations: 380V / 3-phase / 50Hz is the norm for all our small and medium machines. For large machines (above approximately 500 kW), 6kV or 10kV may be required.

If the site is in a hazardous area requiring ATEX-classified electrical equipment, state the zone classification (Zone 1 or Zone 2) and the applicable gas group and temperature class.

8. Installation Environment

Provide: indoor or outdoor installation, site altitude above sea level (in metres), minimum and maximum ambient temperature, relative humidity range (important for surface corrosion specifications), and any special environmental conditions (coastal salt air, dust, vibration from adjacent equipment).

Altitude matters because it affects the density of the cooling air (for air-cooled machines) and the motor derating factor. Above 1,000 m altitude, motor power derating applies — a motor that delivers its nameplate rating at sea level may need to be upsized by 5 to 10% at 2,000 m altitude.

Additional Data for EPC and Industrial Projects

For projects being specified by an engineering contractor or involving API 618 procurement, additional data is typically required:

• Applicable codes and standards: API 618 edition, local pressure vessel code, electrical code, piping code
• Documentation requirements: IOM manual, general arrangement drawing, P&ID, material certificates, hydrostatic test certificates, factory acceptance test report, data books
• Inspection requirements: Witnessed FAT, third-party inspection during manufacture, radiographic examination of welds
• Motor specification: Efficiency class, insulation class, IP rating, vibration limits, starting method
• Spare parts requirements: Two-year operating spare parts list, critical spare parts
• Site delivery logistics: Maximum single lift weight (if crane is limited), maximum piece dimensions for access through existing doors or structures, marine export packing if required

A Template Enquiry

Below is a template that covers the essential data points. Copy and complete this when sending an enquiry to us or any other compressor supplier:

What Happens After You Send the Enquiry

When we receive an enquiry with complete process data, our engineering review process is:

• Review inlet and outlet pressures to confirm stage count and frame type
• Calculate required displacement at maximum flow and minimum inlet pressure to select machine size
• ️ Verify inter-stage temperatures are within acceptable limits for the stage ratios selected
• ⚡ Confirm motor power from calculated compression work plus mechanical efficiency
• Confirm cooling water requirement from heat rejection calculation
• Issue technical recommendation with dimensional reference drawing, utility requirements, and commercial proposal

With complete data, we target a response within one business day for standard applications. For non-standard requirements or API 618 projects, we will confirm the timeline after reviewing the data.

If your process data is incomplete, we will tell you specifically what additional information we need — not ask you to re-send a general enquiry. Send your enquiry to our engineering team using the template above, and we will respond with a specific technical recommendation and commercial proposal.