Compressed air cooling load worksheet

Where does the heat from a compressor go?

A compressor converts electrical input into compressed air, but the energy stored in the air is a small fraction of what goes in. The rest appears as heat in the oil cooler, aftercooler, motor, drive and package. Manufacturers commonly state that around 90–95 percent of the input power of an oil-injected screw compressor is available as heat, which is why heat recovery is attractive and why compressor rooms get hot.

For cooling design, the useful question is not “how many kilowatts are installed?” but “where does the heat go during the operating state I am designing for?” The worksheet answers that by separating four paths:

• Heat rejected to room air or a ducted cooling-air stream (air-cooled packages).
• Heat rejected to cooling water (water-cooled packages).
• Heat recovered for a useful service such as process water or space heating.
• Heat that leaves with the compressed air or by other documented paths.

What remains after the deliberate paths are subtracted is the room’s residual load, and that is what ventilation or air-conditioning must handle.

Declare the boundary before entering numbers

Most disagreements about cooling load come from different boundaries. Write down what is inside the boundary and what is not.

The steady-state heat balance

For each compressor at a given operating state:

Q_input = Q_air-side + Q_water-side + Q_recovered + Q_other

The room residual load is then:

Q_room = Q_input − Q_water-side (leaving the room) − Q_recovered (useful) − Q_other (leaving the room) + Q_dryers + Q_extra (other room loads)

All values are in kilowatts of heat (kWth). The worksheet asks for the source of each figure: measured, from the manufacturer’s data sheet, or an engineering estimate. A measured feeder power is better than a motor nameplate, because a motor rarely runs at exactly its rated output.

Sizing cooling air for an air-cooled room

The air flow needed to carry a heat load away with a given temperature rise is:

V = Q / (ρ × cp × ΔT)

where V is volume flow in m³/s, Q is heat in kW, ρ is air density in kg/m³, cp is the specific heat of air (about 1.005 kJ/kg·K) and ΔT is the allowed rise from inlet to outlet in kelvin.

Example: a 100 kW air-cooled screw compressor rejects about 95 kW to its cooling air. With a permitted temperature rise of 8 K and air density of about 1.13 kg/m³ at around 40 °C:

1. Mass flow = 95 ÷ (1.005 × 8) ≈ 11.8 kg/s.
2. Volume flow = 11.8 ÷ 1.13 ≈ 10.5 m³/s.
3. Hourly flow ≈ 37,600 m³/h.

If the allowed rise is cut to 5 K, the flow rises to roughly 60,000 m³/h. Temperature rise is the most sensitive input, and it is easy to choose a figure that looks fine on paper but leaves the compressor breathing its own hot exhaust.

Sizing cooling water for a water-cooled package

The water-side relationship is the same idea with water properties:

m = Q / (cp × ΔT)

with cp for water about 4.19 kJ/kg·K. For 75 kW of heat to water and a 10 K rise, m ≈ 75 ÷ 41.9 ≈ 1.79 kg/s, about 6.4 m³/h. With only a 5 K rise the flow doubles to about 12.9 m³/h.

A warmer outlet does not prove more heat is being removed. If the water flow has fallen, the outlet gets warmer while the heat carried away can actually drop. Measure flow as well as temperature. Water quality matters too: scale and fouling in coolers reduce heat transfer and raise discharge temperature, a common problem with hard borewell water in Telangana and Andhra Pradesh.

Crediting heat recovery honestly

Heat recovery reduces the room’s cooling load only when three things are true at the same time: the recovery circuit is running, there is a real demand for the heat, and the heat leaves the room. If the recovery loop has no load on a given day, the heat goes back to the oil cooler and into the room. The worksheet therefore treats recovered heat as a scenario with its own operating hours, not as a permanent deduction.

Design the room for the condition when recovery is not available, and treat recovery as an energy benefit. The compressor heat recovery calculator handles the energy value separately.

Fleet operation and part load

A compressor house rarely runs every machine at full load. Cooling load should be calculated for realistic combinations. A loaded compressor rejects heat close to its input power. An unloaded screw compressor still draws a significant fraction of its full-load power, and that also becomes heat. A variable-speed machine rejects heat in proportion to its actual input.

Use the peak case for ventilation and cooling-water sizing and the normal case for energy and heat-recovery planning.

Indian conditions: heat, dust and margins

Many compressor rooms in Telangana and Andhra Pradesh face summer outside temperatures of 40–45 °C, and above 45 °C on open sites. That leaves very little room for temperature rise before the compressor inlet reaches its design limit. A normal oil-injected screw compressor discharges at about 75–95 °C; once the room air is too hot, the cooler cannot hold that range and the machine trips on high discharge temperature.

• Check the manufacturer’s maximum ambient for each package and design the room around it.
• Duct hot exhaust air outside so that it cannot be recirculated to the intake.
• Provide filtered intake air; dust clogs coolers and reduces their capacity.
• Add a stated margin for cooler fouling and future equipment, and record why it was chosen.

Reading the worksheet result

The worksheet reports cooling duty by path and by scenario, with the source of every figure. Read it in this order:

1. Check that the input power for each machine is measured or taken from the package data sheet, not guessed from the motor rating.
2. Confirm that the heat paths add up to the input power; a large unexplained remainder means a boundary or data error.
3. Look at the peak scenario first, because ventilation fans, ducts, cooling towers and pumps must handle it.
4. Compare the calculated air or water flow with what the installed fans or pumps actually deliver, preferably measured.
5. Review the sensitivity to temperature rise; if a small change in allowed rise doubles the flow, the design has little margin.

The result is a design and diagnosis aid. It does not replace the compressor manufacturer’s installation requirements, which remain the governing limits for each package.

How Reddy Compressors helps

Overheating is one of the most frequent service calls we attend. Since 2002 our team, now over 20 service engineers and technicians, has worked on compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and many other makes across Telangana and Andhra Pradesh, from our office in Tarnaka, Secunderabad.

• We review compressor room layouts, ventilation and ducting for new and existing installations.
• We clean and service oil coolers, aftercoolers and heat exchangers, and check thermostatic valves, fans and oil condition.
• We measure discharge temperature, room temperature and power during air audits.
• We supply new and used compressors and can propose packages suited to high-ambient rooms.
• We provide rental compressors when a machine has to come out of service for cooler work.

What to send us

• Make, model and nameplate photo of each compressor and dryer.
• Whether each machine is air-cooled or water-cooled.
• A sketch or photos of the room, with openings, fans and ducts.
• Measured room and discharge temperatures, especially on hot afternoons.
• Any heat-recovery equipment and how often it is used.

Frequently asked questions

How much heat does an air compressor produce?

Nearly all of the electrical power an air compressor draws is turned into heat. Manufacturers commonly state that around 90–95 percent of an oil-injected screw compressor’s input is available as recoverable heat, with the rest leaving in the compressed air and as radiated losses. A compressor drawing 100 kW therefore puts close to 100 kW of heat into its cooling air or water.

How do I calculate ventilation air for a compressor room?

Divide the heat that must be removed from the room by air density, the specific heat of air and the allowed temperature rise. For 95 kW and an 8 K rise at about 40 °C, that is roughly 10.5 m³/s, about 37,600 m³/h. A smaller allowed rise needs much more air. Always check that hot exhaust cannot recirculate to the compressor intake.

Why does my compressor trip on high temperature in summer?

Usually because the cooling air reaching the compressor is too hot, or the cooler cannot transfer heat. Common causes are hot exhaust being recirculated, poor room ventilation, dust-clogged coolers, low oil level, a faulty thermostatic valve or a failed cooling fan. In Indian summers rooms often exceed the package’s design ambient. Measure room and discharge temperatures before replacing parts.

How much cooling water does a water-cooled compressor need?

Divide the heat rejected to water by the specific heat of water, about 4.19 kJ/kg·K, and the allowed water temperature rise. For 75 kW and a 10 K rise, that is about 1.8 kg/s, or roughly 6.4 m³/h. Halving the allowed rise doubles the flow. Use the manufacturer’s data for heat to water and maximum inlet temperature.

Does heat recovery reduce compressor room cooling load?

Only while the recovery system is running and the heat is actually being used outside the room. If there is no demand for the heat, it returns to the normal cooler and enters the room. Design ventilation for the case without recovery, and count recovered heat as an energy saving with its own operating hours rather than a permanent reduction.

Should a compressor room be air-conditioned?

Generally not for the compressors themselves. Air-conditioning a room to remove heat that could be ducted outside wastes energy. Good practice is to supply filtered outside air and duct the hot cooling air out, keeping the compressor inlet within its permitted temperature. Air-conditioning may make sense for sensitive electronics or dryers in a separate enclosure.

Related pages

• Compressor room ventilation worksheet
• Compressor heat recovery calculator
• Compressor thermal performance and overheating
• Cooler and heat-exchanger service
• Compressed-air heat recovery and thermal energy
• Compressed air knowledge and calculation centre