A compressor room approved during a cold Scottish March may behave very differently when every machine is loaded on a warm August afternoon. The same room can develop another set of faults in winter, when snow restricts an intake, condensate freezes in an exposed pipe or an oversized ventilation system pulls cold air across equipment.
The room therefore has to be designed around operating conditions, not floor area alone. The equipment schedule must become a heat-load calculation, a lawful drainage plan, a maintainable arrangement and a set of defined failure responses.
At Design Air, Atlas Copco Premier Distributor in Scotland, we approach this as a compressed air system design problem. The room, equipment, ventilation, drainage, electrical supply and distribution pipework have to work as one installation.
Establish the Design Cases Before Drawing the Layout
Begin with an equipment schedule that identifies what can operate simultaneously. Connected electrical load is not necessarily the same as concurrent heat load. A duty compressor, trim compressor and dryer may all run together during peak production, while a standby machine may start after a duty unit trips.
Record at least the following:
- Compressor type, cooling method, rated electrical input and manufacturer heat-rejection data
- Dryer type, electrical input, purge arrangement and rejected heat
- Receiver volume and maximum operating pressure
- Filtration, condensate drains and treatment equipment
- Ventilation fans, control panels and other heat-producing auxiliaries
- Normal, peak, standby-changeover and future operating cases
- Manufacturer ambient-temperature limits
- Process air demand, pressure and air-quality requirement
- Space allocated for a credible future machine or treatment stage
The selected products should be represented by model-specific data rather than category assumptions. A 55 kW air-cooled compressor, a water-cooled compressor and a desiccant dryer impose different duties on the room.
Use Scottish External Conditions as Design Inputs
The weather file should cover the actual site. An exposed yard in Fife, a sheltered Central Belt factory and a coastal industrial building require different treatment of wind, salt, driven rain and snow.
Set separate summer and winter cases. The summer case determines whether the room can reject heat at the chosen maximum internal temperature. The winter case checks whether ventilation control, drains and exposed pipework can remain operable during frost. Also record contamination sources such as vehicle movements, process exhausts, dust handling and cooling towers.
The result is a design basis that can be checked later. “Suitable for Scotland” is not a specification. An external design temperature, permissible room temperature, calculated airflow and defined weather protection are.
Convert the Equipment Schedule Into a Heat Load
For an air-cooled compressor, almost all the electrical input ultimately appears as heat. If model-specific data is unavailable during concept design, electrical input provides a conservative approximation. Atlas Copco describes the heat released into ventilation air as nearly 100% for air-cooled equipment and approximately 10% for water-cooled compressors in its compressor-room guidance (atlascopco.com).
That difference changes the ventilation calculation, but a water-cooled installation is not heat-free. Motor losses, electrical cabinets, dryers and uninsulated pipework may still release heat into the room. The cooling-water system must also be assessed separately.
Build the schedule without counting the same load twice:
| Heat source | Concept-stage input | Check before detailed design |
|---|---|---|
| Air-cooled compressor | Electrical input if heat-rejection data is unavailable | Manufacturer’s rejected-heat figure at the selected operating condition |
| Water-cooled compressor | Approximately 10% of input into room air | Heat split between room and cooling water |
| Dryers and ancillary equipment | Manufacturer data | Operating mode and maximum concurrent duty |
| Electrical equipment and fans | Declared losses or input where applicable | Whether the heat is released inside or outside the room |
| Building and solar gains | Project heat-gain calculation | Roof, wall, glazing and site exposure assumptions |
Do not add motor power to a compressor’s published total heat-rejection figure if that figure already includes it.
Calculate Airflow From the Permitted Temperature Rise
Once the maximum concurrent room heat load is known, the ventilation airflow can be calculated from:
`qᵥ = Φ ÷ (ρ × cₚ × ΔT)`
Where:
- `qᵥ` is ventilation airflow in cubic metres per second
- `Φ` is heat released into the room in kilowatts
- `ρ` is the density of air at the design condition
- `cₚ` is the specific heat capacity of air
- `ΔT` is the permitted temperature rise between inlet and room or extract air
Use the project’s design conditions and the compressor manufacturer’s maximum permissible ambient temperature. A larger permitted temperature rise reduces calculated airflow, but it also leaves less margin for blocked filters, recirculation, fan deterioration and unusually warm weather.
Check Each Operating Case
Calculate each credible operating case. The highest connected load may not govern if some equipment cannot run together. Conversely, a standby-changeover case can exceed normal duty because two machines may overlap briefly.
Detailed airflow selection belongs in the room’s ventilation design.
Arrange the Air Path Around the Equipment
A fan duty on paper is insufficient if hot discharge air returns to the compressor intake. Position the incoming air so it passes through the occupied equipment zone before reaching the extract. Keep the hot outlet separated from the external intake and check how wind around the building could drive recirculation.
Low-level intake and high-level extract often suit an air-cooled installation because heated air rises. The equipment arrangement may require a different solution where compressor cooling outlets are ducted directly outdoors. Follow the manufacturer’s limits for duct resistance and cooling airflow.
External openings need protection from Scottish weather without imposing an unmeasured pressure loss. Specify weather louvres using certified free-area and pressure-loss data. If a concept calculation uses 60% free area, identify that as a worked assumption. It is not a universal louvre specification.
The intake detail should also address:
- The expected snow line and the possibility of drifting snow
- Driven rain and drainage from the louvre assembly
- Leaves, packaging debris and other windblown material
- Dust or process contamination
- Inspection and replacement of intake filters
- Corrosion resistance appropriate to the site
- Prevention of short-circuiting between intake and exhaust
Control Airflow in Winter
Winter operation normally requires staged or modulating airflow. A system sized for full summer rejection can overcool the room at low load unless its controls respond to temperature and equipment status.
Plan for Ventilation Failure
The design should define what happens when airflow falls below the required value. A spinning fan does not prove that air is moving. A closed damper, blocked filter, broken drive or snow-obstructed louvre can leave the motor running while the room overheats.
Depending on the consequence of failure, the control strategy may use airflow or pressure proving, high-temperature alarms, duty and standby fans, automatic compressor unloading or a controlled shutdown. The response should be agreed with the compressor and ventilation control suppliers so that competing control sequences do not repeatedly stop and restart the plant.
Regulation 6 of the Workplace (Health, Safety and Welfare) Regulations 1992 requires plant used to provide workplace ventilation to include an effective visible or audible failure warning where that is necessary for health or safety. The qualification matters. The risk assessment determines whether the requirement applies and what warning is effective. The statutory wording appears in regulation 6 of the primary legislation (legislation.gov.uk).
Route any required warning somewhere it will be acted upon. An alarm beacon inside an unattended compressor room has little operational value.
Decide the Condensate Route Before Setting Floor Levels
Condensate volume changes with inlet temperature, humidity, airflow and compressor loading. Scottish sites should not treat it as a minor service drain. Atlas Copco reports that a 55 kW rotary screw compressor operating at 24°C and 75% relative humidity can produce 280 litres of condensate per day (atlascopco.com).
The room drainage schedule should include condensate from compressors, aftercoolers, receivers, filters and dryers. Identify peak discharge rates as well as daily volume. Several automatic drains releasing together can impose a higher instantaneous duty than a daily total suggests.
Where winter temperatures can fall below the pressure dew point of downstream air, dryer selection also affects the room and drainage design. Our a beginners guide to desiccant air dryers explains how lower pressure dew points protect exposed distribution systems.
Confirm the Lawful Outlet
Do not discharge compressor condensate to a surface-water drain, yard gully, soakaway or any surface connected to a surface-water system. Current Scottish environmental rules prohibit trade effluent and potentially harmful substances from entering surface-water drainage systems. The operative requirements are set out in the Environmental Authorisations (Scotland) Amendment Regulations 2025.
A connection to the foul sewer is not automatic permission. Scottish Water states (scottishwater.co.uk) that a business with a trade-effluent discharge must obtain consent. That consent sets limits on the volume and nature of the discharge. The appropriate form of consent or agreement should be confirmed for the proposed discharge.
The design sequence is therefore:
- Confirm the destination of every nearby drain from current drainage records and site checks.
- Estimate condensate volume and peak flow for the proposed operating cases.
- Define any collection, separation or treatment equipment.
- Ask Scottish Water or the site’s licensed provider to confirm the required authorisation and discharge conditions.
- Size pipework, collection capacity and inspection points around those written conditions.
- Protect exposed condensate lines and drain devices against frost.
- Include a response for blocked drains, failed treatment equipment and loss of the approved outlet.
A floor drain shown on an architectural drawing is not evidence of a lawful condensate route.
Set the Layout From Equipment-Specific Clearances
Base the room layout on model-specific installation information.
Place each item from its general arrangement drawing. Plot the manufacturer’s installation and service zones, then check them against walls, columns, ducts, cable containment and adjacent equipment. Where overhead lifting is specified by the manufacturer, verify the available height and the supporting arrangement rather than assuming the room volume is sufficient.
Access also has to work after the room is complete. Ducts, acoustic treatment and new pipework are often added after the equipment layout is approved. A coordinated drawing should therefore show the final positions of all services, not separate layouts that occupy the same space.
Reserve Capacity Deliberately
Future expansion should have a stated basis. Reserve a bay for a defined compressor size or treatment stage and carry that allowance through the electrical, ventilation, drainage and pipework calculations.
Leaving an empty rectangle on the floor plan is not enough if the future machine would exceed the extract duty or require a larger incoming electrical supply. Equally, installing the final ventilation duty on day one may produce poor winter control. A staged arrangement can preserve expansion capacity without forcing every fan to run continuously.
The unresolved point is often production growth. If no reliable future demand profile exists, the project team must choose between installed spare capacity and a documented route for later modification. The correct choice depends on shutdown availability and the cost of altering the building.
Resolve PSSR Duties at the Design Stage
The Pressure Systems Safety Regulations 2000 apply to pressure systems used at work, subject to their detailed scope and exceptions. Compressed air at more than 0.5 bar above atmospheric pressure falls within the definition of a relevant fluid, as explained in the HSE PSSR overview (hse.gov.uk).
Establish the pressure-system boundary and list every vessel. Record its internal volume, pressure and protective devices. This avoids discovering late in the project that a receiver, separator vessel or other pressure-containing component changes the examination requirements.
Apply the 250 Bar-Litre Threshold
Schedule 1 provides a partial exception from regulations 5(4), 8 to 10 and 14 where the pressure in bar multiplied by the internal volume in litres is less than 250 bar-litres for each vessel. It is not a blanket exemption from PSSR.
The word “less” is decisive:
- A vessel at 10 bar with an internal volume of 24 litres gives 240 bar-litres and may meet this numerical condition.
- A vessel at 10 bar with an internal volume of 25 litres gives exactly 250 bar-litres and does not meet it.
- Each vessel must be checked. Do not calculate an average across the installation.
The full conditions appear in the primary PSSR text (legislation.gov.uk). A competent person should determine the applicable Written Scheme of Examination requirements. Regulation 4 also requires pressure systems and components to be designed and constructed so that necessary examinations for preventing danger can be carried out.
Coordinate Electrical Safety, Noise and Pipework
Compressor-room design crosses several technical disciplines. The mechanical layout cannot be frozen while electrical, acoustic and distribution requirements remain placeholders.
Electrical Design and Isolation
Confirm the incoming supply against the actual equipment schedule, including starting method, variable speed drives, dryers, fans and future loads. The design review should cover protective devices, earthing, cable routes, control interfaces and safe isolation of electrical and pneumatic energy.
The Electricity at Work Regulations 1989 guidance (hse.gov.uk) applies to electrical systems and equipment used at work. Electrical work, testing and commissioning should sit within the site’s competent electrical design process.
Keep switchgear and isolators away from condensate discharge points and direct hot-air paths. Coordinate emergency controls with the ventilation failure strategy so that a temperature event produces a defined response rather than an uncontrolled loss of the entire air supply.
Noise at the Room Boundary
The room enclosure, doors, louvres and ventilation ducts all affect noise transmission. A quiet compressor package can still create a workplace exposure issue if discharge openings face an occupied area or if several machines operate together.
The Control of Noise at Work Regulations 2005 set the following values:
| Regulatory level | Daily or weekly exposure | Peak sound pressure |
|---|---|---|
| Lower exposure action value | 80 dB(A) | 135 dB(C) |
| Upper exposure action value | 85 dB(A) | 137 dB(C) |
| Exposure limit value | 87 dB(A) | 140 dB(C) |
These are personal exposure values, not permitted compressor-room sound levels. The distinction prevents a common specification error where a machine sound-pressure figure is compared directly with an employee’s daily exposure. The definitions and qualifications are in regulation 4 of the primary legislation (legislation.gov.uk).
Assess occupied areas, access frequency and simultaneous operation. Acoustic louvres and attenuators must then be included in the ventilation pressure-loss calculation.
Distribution Pipework
Size the room header and outgoing main from simultaneous air demand, operating pressure, route length, fittings, permitted pressure loss and future demand. The compressor outlet connection does not determine the correct distribution pipe diameter.
Arrange the header so that equipment can be commissioned in stages and so that the selected control philosophy can measure system pressure at a representative point. Pipe supports, drainage points and isolation arrangements should be coordinated before openings and penetrations are formed.
Our wider blog covers individual equipment and air-treatment decisions. At room-design stage, the important point is that pressure, air quality, pipework and ventilation assumptions all use the same equipment schedule.
Commission the Room Against Its Design Cases
Commissioning should prove the room rather than confirm only that each machine starts. Record the final fan duty, louvre and filter pressure losses, room and intake temperatures, equipment loading, drain operation, alarm transmission and control responses.
Test the credible failure modes included in the design. These may include loss of a duty fan, a restricted intake, a high room temperature, a drain alarm and a standby-compressor start. Where seasonal conditions cannot be reproduced, retain calculations and measured performance that demonstrate the available margin.
Update the operating information after commissioning. The equipment schedule, drain destination, consent conditions, ventilation setpoints, alarm actions and PSSR records should agree with the installed system. Procurement scope, commissioning responsibilities and terms and conditions should describe the same boundaries.
For a compressor-room proposal, send Design Air the site drawings, equipment list, pressure and demand profile, operating hours, external design conditions, drainage information, electrical supply details, access restrictions and expected expansion. Our engineering team can assess the room, specify the compressed air equipment and pipework installation, and provide a quotation for the Scottish site conditions recorded in that design basis.