Why heat recovery needs proper evidence
Almost all the electrical energy taken by an air compressor ends up as heat: in the oil, in the compressed air, in the motor and in the cooling air or water. That is why heat recovery is attractive. It is also why claims about it are easy to exaggerate. A brochure can show how much heat is available; only measurements show how much was used.
Three questions decide whether a heat recovery claim is real:
- Did the heat cross into a process that actually needed it, at a temperature it could use?
- Did the compressor and the heat user run at the same time, and for how many hours?
- Was the heat measured at the receiving side, or calculated from ratings?
A record that cannot answer all three is a design estimate, which is useful, but it is not a verified project.
Where the heat comes from in different compressors
| Compressor type | Main recoverable heat source | Typical recovery route | Points to protect |
|---|---|---|---|
| Oil-injected rotary screw | Hot lubricating oil, plus cooling air | Oil-to-water plate heat exchanger, or ducting warm cooling air to a space or dryer | Minimum oil temperature, oil viscosity, thermostatic valve function, cooler bypass |
| Oil-free screw | Intercooler and aftercooler, oil for gears and bearings | Water-cooled coolers piped into a recovery circuit | Stage temperatures, cooling water quality, the maker’s design limits |
| Reciprocating | Cylinder jackets, intercoolers, aftercoolers | Water jacket and cooler circuits | Jacket temperature limits and valve life |
| Centrifugal | Intercoolers and aftercooler | Cooling water circuit at a lower temperature level | Surge margin and stage cooling performance |
For an oil-injected screw, the normal discharge temperature is about 75–95 °C before the aftercooler, and recovery packages typically produce hot water in the region of 50–70 °C, depending on design and load. Recovery must never override the compressor’s own cooling: if the heat user stops drawing heat, the standard cooler has to take over automatically.
What makes heat “useful”
Heat is only useful if a process wants it, at that temperature, at that time. In Indian plants, good matches include boiler feedwater preheating, process wash water, canteen and hostel hot water, and pre-heating for certain process tanks. Space heating, which drives many European examples, is rarely relevant in Telangana or Andhra Pradesh. That changes the economics and is a common reason imported case studies mislead.
- Temperature match: a process that needs water at 80 °C may only be pre-heated by a recovery circuit, with the boiler providing the rest.
- Time match: a compressor running three shifts paired with a canteen water heater used for two hours a day recovers far less than the nameplate suggests.
- Load match: a compressor that runs unloaded much of the time produces less heat than at full load.
- Displacement: the record should state what the recovered heat replaced, such as boiler fuel or electric heating, and how that was established.
A simple test before any design work is to plot two profiles over a typical week: the compressor’s loaded hours and the heat user’s demand. Where the two overlap for long periods, recovery is worth studying in detail. Where they rarely overlap, a storage tank may help, but it adds cost, space and heat loss. In many plants the honest conclusion is that only one compressor in the room is worth connecting, usually the base-load machine that runs loaded for the longest hours.
How useful heat is measured
The standard method is to measure on the receiving side of the heat exchanger. For a water circuit, the delivered heat rate is flow multiplied by the specific heat of water and the temperature rise:
Heat rate (kW) = water flow (kg/s) × 4.19 (kJ/kg·K) × temperature rise (K)
As an illustration only, a recovery circuit carrying 1.0 litre per second of water with a 20 K temperature rise delivers about 1.0 × 4.19 × 20 ≈ 84 kW of heat while it runs. If the compressor and heat user ran together for 4,000 hours in a year at that rate, delivered heat would be about 335,000 kWh. Real figures vary with load and must be logged, not assumed.
A sound measurement set includes:
- Water flow, from a calibrated flowmeter or a heat meter.
- Water inlet and outlet temperatures at the exchanger.
- Compressor oil or discharge temperatures, to confirm the compressor stayed within limits.
- Compressor power and loaded hours, so heat can be related to the compressor’s actual duty.
- Heat user operating hours, so only simultaneous hours are counted.
- Pump power and any added fan power, which reduce the net benefit.
Our instrument calibration and measurement evidence page explains how instruments and readings are recorded.
Common reasons claimed savings do not appear
| Problem | What happens | How the record shows it |
|---|---|---|
| No heat demand when the compressor runs | Heat is dumped to the standard cooler | Low temperature rise logged during those hours |
| Compressor mostly unloaded | Less heat produced than the rating suggests | Loaded hours and power logs |
| Fouled plate heat exchanger | Lower heat transfer, higher oil temperature | Approach temperatures rising over time |
| Thermostatic valve fault | Oil too cold or too hot, risk of condensation in oil or overheating | Oil temperature trend |
| Recovery measured from ratings | Paper result with no field confirmation | No receiving-side measurements in the file |
Protecting the compressor
A heat recovery circuit becomes part of the compressor’s cooling system, so it must not harm the machine. In an oil-injected screw, oil that runs too cool lets moisture condense in the oil, which shortens oil and bearing life; oil that runs too hot causes high discharge temperature trips and faster oil breakdown. The design must keep the thermostatic valve working, provide a bypass to the standard cooler, and stay within the maker’s limits. Where the compressor is under a maker’s warranty, check the maker’s conditions before connecting recovery equipment.
In the hot, dusty Indian summer, compressor coolers already work hard. A recovery exchanger can relieve them when the heat user is drawing heat, but the standard cooler and ventilation must still be sized for full load when it is not.
What a published record contains
- Site type and process that receives the heat, anonymised unless the site permits naming.
- Compressor type, rating and duty profile.
- Recovery arrangement: exchanger type, circuit, controls and bypass.
- Measurement period, instruments and their calibration status.
- Delivered heat in kWh over the period, and simultaneous operating hours.
- What the recovered heat replaced and how that was established.
- Limitations, such as seasonal variation not covered, or estimated rather than measured values.
- Written permission from the site to publish.
Where only an estimate exists, we call it an estimate. The compressor heat recovery calculator can help with a first estimate before any measurement.
How Reddy Compressors helps
Reddy Compressors, at Tarnaka, Secunderabad, has served industry since 2002 as an independent multi-brand compressed-air business. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands. For heat recovery we:
- Review your compressors, from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic or other makers, and identify which heat sources are practical.
- Log compressor power, loading and temperatures as part of an air audit, to show how much heat is really produced.
- Help you find a heat user whose demand matches the compressor’s operating hours and temperature level.
- Service oil coolers, thermostatic valves and heat exchangers to keep the compressor protected.
- Measure delivered heat on the receiving side and report it in kWh, including what was not measured.
Our team of over 20 service engineers and technicians covers Telangana and Andhra Pradesh, and we supply equipment and spares across India. Read more on our heat recovery and thermal energy engineering page.
What to send us
- Compressor make, model, motor rating and nameplate photos.
- Typical running hours per day and how often the compressor is loaded.
- The process you want to heat, its temperature need and its hours of use.
- Current heating method, such as boiler, electric heater or none.
- Any existing power, temperature or flow readings.
Frequently asked questions
How much heat can I recover from my air compressor?
Most of a compressor’s input power becomes heat, but the useful amount depends on how many hours the compressor is loaded, the temperature your process needs and whether the heat user runs at the same time. A rated figure is only an upper limit. Logging compressor power and the heat user’s demand gives a realistic estimate before you invest.
What temperature of hot water can a screw compressor provide?
An oil-injected screw compressor runs with discharge temperatures of about 75–95 °C, and recovery packages typically provide hot water in the region of 50–70 °C, depending on design and load. That suits preheating boiler feedwater, wash water and canteen hot water. Processes needing higher temperatures usually use recovery as a preheat stage.
Can heat recovery damage my compressor?
It can if it is badly designed. Oil that is cooled too much lets moisture condense in it, while oil that runs too hot causes trips and faster oil breakdown. A correct design keeps the thermostatic valve working, provides an automatic bypass to the standard cooler and respects the maker’s limits. Check warranty conditions before connecting anything.
Is compressor heat recovery worth it in India?
It can be, where there is a steady demand for warm water or process heat during the compressor’s running hours. Space heating is rarely relevant in Telangana or Andhra Pradesh, so imported case studies can overstate the benefit. Boiler feedwater preheating and process washing are usually the best matches. Measure first, then decide.
How do you verify heat recovery savings?
We measure water flow and temperatures on the receiving side of the exchanger, count only the hours when the compressor and heat user ran together, and report delivered heat in kWh. We also record pump power and what the heat replaced. Estimates based on ratings are clearly labelled as estimates and never presented as verified results.
Can heat be recovered from an air-cooled compressor?
Yes. Warm cooling air can be ducted to a space, a drying process or outside, depending on the season and need. For hot water, an oil-to-water exchanger is fitted in the oil circuit. Ducting must not restrict the compressor’s cooling airflow, or it will run hot and trip, especially during an Indian summer.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Water-side heat duty of a water-cooled compressor
Compare two runs of the same cooling-water circuit. A larger water temperature rise does not mean more heat is being removed: heat removed depends on flow multiplied by rise.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Reference run: water flow (m³/h): 6
- Reference run: water in (°C): 30
- Reference run: water out (°C): 36
- Later run: water flow (m³/h): 3
- Later run: water in (°C): 30
- Later run: water out (°C): 39
Result
- Reference run: water rise: 6 K
- Reference run: heat removed: 41.8 kW
- Later run: water rise: 9 K
- Later run: heat removed: 31.4 kW
- Change in water flow: -50 %
- Change in water rise: +50 %
- Change in heat removed: -25 %
Method. Q (kW) = density × flow × specific heat × (water out − water in) ÷ 3,600, with flow in m³/h, density 1,000 kg/m³ and specific heat 4.18 kJ/(kg·K) for clean water.
What to take from it. If the rise goes up while the heat removed goes down, the flow has fallen. Check strainers, valves, pump and scaling before the compressor.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Compressor heat recovery
Estimate the heat from a compressor that could be put to use, and what it is worth.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Compressor input power (kW): 90
- Recoverable fraction (%): 70
- Hours when the heat is used: 4000
- Displaced heater efficiency (%): 85
Result
- Recoverable heat: 63 kW
- Heat recovered per year: 2,52,000 kWh
- Fuel energy displaced per year: 2,96,471 kWh of fuel
- Value per year: Enter the cost of the fuel you would otherwise burn (per kWh) above
Method. Recoverable heat = input power × recoverable fraction. Value = heat × hours × cost of the fuel replaced ÷ efficiency of the displaced boiler or heater. Recoverable fraction depends on the machine and the heat sink; use a measured or supplier figure.
What to take from it. Heat is only worth recovering when there is a use for it for enough hours. Confirm the sink before the equipment.