Compressor heat recovery calculator

Where does compressor heat come from?

Almost all the electrical energy that enters an air compressor ends up as heat. Some leaves in the compressed air, a little radiates from the package, and most is carried away by the oil cooler and aftercooler in an oil-injected screw compressor, or by the intercoolers and aftercooler in an oil-free machine. In a normal installation that heat is simply blown out of the compressor room or rejected through a cooling tower.

Heat recovery captures part of it. In an air-cooled package, warm cooling air is ducted to where it is needed. In a water-cooled or oil-circuit recovery package, a plate heat exchanger transfers heat to a water circuit. The heat is real, but whether recovery makes sense depends entirely on whether the site can use it.

The calculator never treats all input power as recoverable. Makers of oil-injected screw recovery packages often quote that a large majority of input can be recovered, but the figure varies by design and operating point. Use the verified figure for the actual package, not a general percentage.

Which recovery method suits which use?

In India, space heating is rarely needed for much of the year, so the strongest cases are usually process heat: boiler feed water preheating, washing and cleaning water in food, dairy and pharma plants, and drying processes. A pharma plant in Genome Valley or a food park near Vizag is more likely to use hot water year-round than to need warm air in winter.

How the calculation works

Thermal potential

Q_recoverable (kW_th) = P_input (kW) × f_recoverable

P_input is the total package electrical input for the operating state being modelled, preferably measured. f_recoverable is the decimal fraction from the maker’s recovery data for that package and operating point. A compressor that spends much of its time unloaded produces less heat, so the input must reflect the real load profile.

Useful heat

Q_useful = Q_recoverable × f_distribution × f_utilisation

f_distribution covers duct, pipe and heat-exchanger losses. f_utilisation is the share of recovered heat the site can absorb while the compressor runs. Keeping the two factors separate stops a seasonal or process mismatch from hiding inside one optimistic number.

Hot-air and hot-water flow

For hot air, the mass flow needed is ṁ = Q ÷ (c_p × ΔT), with c_p for air about 1.005 kJ/(kg·K). Divide by air density to get volume flow.

For hot water, the same equation applies with c_p for water about 4.18 kJ/(kg·K).

Annual energy and displaced fuel

Annual useful heat (kWh_th) = Q_useful × hours of coincident operation per year

Fuel displaced = annual useful heat ÷ efficiency of the heater it replaces. Divide by the fuel’s energy content to express it as litres of diesel or furnace oil, kilograms of LPG or kWh of electricity.

Temperature match

Before any value is counted, the available supply temperature must exceed the temperature the receiving process needs, with enough margin for the heat exchanger. Heat at 60 °C cannot serve a process that needs 80 °C, however much of it there is.

A worked example

An oil-injected screw compressor draws an average of 75 kW over its loaded hours. The maker’s data for its recovery package gives a recoverable fraction of 0.80 at that operating point. The figures are illustrative.

1. Recoverable heat = 75 × 0.80 = 60 kW_th.
2. For hot water with a 30 K rise, water flow = 60 ÷ (4.18 × 30) ≈ 0.48 kg/s, or about 1.7 m³/h.
3. For hot air with a 20 K rise, air mass flow = 60 ÷ (1.005 × 20) ≈ 3.0 kg/s, or about 2.6 m³/s at a density of about 1.16 kg/m³.
4. The site uses hot water for boiler feed preheating. Allow f_distribution = 0.9 and f_utilisation = 0.6, because the boiler does not need all the heat at all times.
5. Useful heat = 60 × 0.9 × 0.6 = 32.4 kW_th.
6. Over 5,000 coincident hours a year, annual useful heat ≈ 162,000 kWh_th.
7. If the boiler runs at 85% efficiency, fuel energy avoided ≈ 162,000 ÷ 0.85 ≈ 190,000 kWh.
8. For diesel at about 10 kWh per litre, that is roughly 19,000 litres a year.

To judge value, multiply the avoided fuel by your own fuel price, subtract any extra pump or fan power the recovery system uses, and compare with the installed cost for a simple payback. The project payback and discounted cash flow tool covers the financial step.

What reduces the real benefit?

• Compressors that spend long periods unloaded or stopped, so less heat is produced
• Heat demand that does not coincide with compressor running hours, such as a day-shift process fed by a night-shift compressor
• Supply temperature too low for the process, or a heat exchanger too small to reach it
• Long or poorly insulated ducts and pipes
• Duct back-pressure that reduces cooling-fan airflow and raises compressor temperatures
• Recovery systems that bypass in summer and were costed as if they ran all year
• Scaling or fouling on the water side of the exchanger when water quality is poor

The compressor’s own cooling always comes first. If the recovery circuit cannot take the heat, the compressor must still reject it through its normal cooler or a bypass. A recovery design that lets discharge temperature rise above the normal range of about 75–95 °C for an oil-injected screw is a reliability risk, not a saving.

Checks before you invest

1. Log compressor power and load hours for a representative period.
2. Identify heat users, their temperature needs and their operating hours.
3. Get the maker’s recovery data for the actual compressor model and operating point.
4. Check the temperature match and the heat-exchanger approach temperature.
5. Estimate distribution and utilisation factors honestly, including the hot season.
6. Confirm the compressor’s cooling and safety shutdowns are protected in every recovery mode.
7. Calculate annual useful heat and displaced fuel, then the payback.

How Reddy Compressors helps

Reddy Compressors, based at Tarnaka in Secunderabad and serving industry since 2002, is an independent multi-brand specialist. We sell new and used or refurbished compressors from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others, rent from our own fleet, provide service and AMC, carry out air audits and supply spare parts. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.

For heat recovery we can log compressor power and load hours, check cooler condition and discharge temperatures, review whether your process can use the heat, and help you compare recovery options or a compressor change. Our teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment and spares across India.

What to send us

• Compressor make, model, rated power and cooling type (air or water)
• Running and loaded hours, or a power log
• Where you could use the heat, the temperature needed and when
• The fuel or electricity the heat would replace and the heater’s efficiency
• Compressor room layout and distance to the heat user
• Water quality details if a water circuit is proposed

Frequently asked questions

How much heat can be recovered from an air compressor?

Almost all of a compressor’s electrical input becomes heat, and makers of recovery packages for oil-injected screw compressors often quote that a large majority of it can be captured. The real figure depends on the package design, operating load and recovery method. What matters most is how much your site can use at the right temperature and time, which is usually less than what is recoverable.

What temperature of hot water can I get from a screw compressor?

It depends on the compressor and the recovery package. Oil-injected screw compressors recover heat from the oil circuit, so water temperature is limited by the oil temperature the compressor needs for reliable operation. Oil-free machines can sometimes offer higher temperatures from their coolers. Always use the maker’s data for the specific model and check it against the temperature your process needs.

Is compressor heat recovery worth it in India?

It can be, but usually for process heat rather than space heating. Boiler feed water preheating, washing water in food, dairy and pharma plants, and drying processes can use heat year-round. The case is strongest when the compressor runs long loaded hours and the heat demand coincides with them. A calculation using measured hours and real utilisation will show whether it pays.

Does heat recovery affect compressor reliability?

It should not, if designed correctly. The compressor’s own cooling must always have priority, with a bypass or standard cooler taking over when the recovery circuit cannot absorb the heat. Ducting that restricts cooling airflow, or a water circuit that lets oil temperature rise, can cause high discharge temperature trips and faster oil degradation. Safety shutdowns must stay active in every mode.

How do I calculate the hot water flow from compressor heat recovery?

Divide the useful heat in kW by the specific heat of water, about 4.18 kJ/(kg·K), multiplied by the temperature rise you want. For 60 kW and a 30 K rise, that is about 0.48 kg/s, or roughly 1.7 m³/h. A smaller temperature rise gives more water at a lower temperature; a larger rise gives less water at a higher temperature.

Can you assess heat recovery on our existing compressors?

Yes. We can log power and load hours, check cooler condition and operating temperatures, and review your heat users and their temperatures. From that we estimate useful heat, the fuel it could displace and the practical installation options, including whether a compressor change would give better results. We work with compressors of all major brands across Telangana and Andhra Pradesh.

Related pages

• Compressed-air heat recovery and thermal-energy engineering
• Compressor heat recovery project economics
• Heat-recovery rotary screw systems
• Compressor room ventilation worksheet
• Project payback and discounted cash flow tool
• Compressed air knowledge and calculation centre