Compressor heat recovery and thermal energy

Where compressor heat comes from

An air compressor is, in energy terms, mostly a heater. Almost all the electrical energy taken by the motor ends up as heat: in the oil, in the compressed air, in the motor and in radiation from the package. Only a small share leaves as stored energy in the compressed air itself. In an oil-injected screw compressor, the largest share of that heat is carried by the oil and removed in the oil cooler. Smaller amounts leave through the aftercooler, the motor and the package surfaces.

Makers publish heat-rejection data for their machines, and the recoverable share depends on the design and the recovery method. Use the maker’s figures or measured data for the specific machine. Do not assume that every kilowatt of electrical input becomes a kilowatt of useful heat at the user.

Air-cooled and water-cooled recovery compared

In most of India, space heating is rarely needed, so hot-air recovery has limited use outside drying applications. Water heating is the common opportunity: canteen and washroom hot water, boiler feed-water preheating, process washing in food and dairy plants, and some pharma utilities.

Does my plant have a real use for the heat?

This is the first and most important question. Heat recovery is only worth engineering when three conditions are met together:

1. A real heat sink exists: a user that would otherwise burn fuel, use electric heaters or take steam to heat water or air.
2. Temperature fits: the sink needs heat at a temperature the compressor can supply. A large amount of low-grade warm air may be less useful than a smaller amount of hot water.
3. Timing coincides: the compressor must be loaded when the sink needs heat. Heat from a machine that runs at night cannot heat a tank used only on the day shift unless storage is added.

A simple coincidence check is illustrative. If a compressor runs loaded for 6,000 hours a year but the hot-water user needs heat for only 2,500 of those hours, the useful recovery window is about 2,500 hours, not 6,000. Count only the overlap.

How to estimate recoverable heat

For a water stream, the heat transferred is calculated from the basic heat balance:

Heat (kW) = mass flow (kg/s) × specific heat (kJ/kg·K) × temperature rise (K)

Worked example, for illustration only: 1.5 kg/s of water heated by 8 K, using a specific heat of about 4.18 kJ/kg·K for water, gives 1.5 × 4.18 × 8 ≈ 50 kW of heat. Over 2,500 coincident hours that is about 125,000 kWh of heat delivered.

Three cautions apply to any estimate:

• Use loaded hours from the controller, not calendar hours. An unloaded compressor rejects much less heat.
• Thermal kilowatts (kWth) are not electrical kilowatts (kWe). The value depends on what the heat replaces.
• Meter the result. A heat meter with flow and two temperature sensors on the water side is the only reliable proof.

What is recovered heat actually worth?

The value depends on the heating source it displaces. Heat that replaces electric resistance heating saves roughly one kilowatt-hour of electricity for each kilowatt-hour of heat delivered. Heat that replaces boiler fuel saves fuel in proportion to the boiler’s efficiency, so the saving is measured in litres of diesel, kilograms of LPG or briquettes. Heat that replaces a heat pump saves much less electricity, because the heat pump was already efficient.

Avoid double counting. If the compressor project also reduces pressure or replaces an old machine, the electrical saving from those measures is separate from the heat-recovery saving. Report each on its own basis.

Protecting the compressor while recovering heat

A heat-recovery system must never compromise compressor cooling. Normal oil-injected screw discharge temperature is about 75–95 °C. If the heat user stops taking heat and there is no other way to reject it, oil temperature rises and the compressor trips on high discharge temperature, or worse, runs hot and degrades the oil and airend.

• Keep the original cooler working: the recovery exchanger should sit ahead of the standard cooler, with a thermostatic valve so the compressor’s own cooler takes over when the sink is satisfied.
• Respect fan limits on ducted systems: ducts, bends, dampers and silencers add resistance. Stay within the maker’s allowable external static pressure, or add an assist fan.
• Prevent recirculation: when recovery is not needed, dampers must send hot air outside, not back to the compressor intake. In Indian summers, with ambient temperatures of 40–45 °C, recirculated air quickly causes trips.
• Keep the water side clean: hard or untreated water scales heat exchangers. Plan for water treatment and periodic cleaning.
• Check materials and pressures: exchangers must suit the oil and water pressures and temperatures, and leak paths between circuits must be considered.

Step-by-step heat-recovery assessment

1. List compressors, their cooling type and loaded hours from the controllers.
2. Obtain heat-rejection data from the maker for each model.
3. Identify candidate heat users, their temperature needs and when they need heat.
4. Calculate coincident hours and the realistic heat that can be delivered.
5. Identify the displaced energy source and its efficiency.
6. Check compressor cooling safety, space for exchangers and pipe routes.
7. Decide on metering so the result can be verified.
8. Commission, record baselines and review after a full season of operation.

Heat recovery and the rest of the air system

Heat recovery is one part of compressed-air energy work, not a substitute for the basics. Leaks, excessive pressure and unloaded running waste electricity before any heat is generated. Fix those first, then recover heat from the compressors that remain loaded for long hours. For new machines, many makers offer factory-fitted heat-recovery options, which are often simpler than retrofits.

Oil-free compressors and heat recovery

Water-cooled oil-free compressors reject heat at higher temperatures in their intercoolers and aftercoolers, and makers often offer recovery packages for them. The same rules apply: a real user, matching temperature, coincident hours and a cooling path that protects the machine when the user stops taking heat.

How Reddy Compressors helps

Reddy Compressors, at Tarnaka in Secunderabad, has worked with compressed-air users since 2002 as an independent multi-brand specialist. We sell, rent and service ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands, and we are not an authorised dealer of any one maker. We have tie-ups with all major and reputed brands.

• We read controller hours and log power and pressure to find which compressors run loaded for long enough to justify recovery.
• We collect nameplate details so heat-rejection data can be obtained for the exact model.
• We check cooler condition, oil temperature control and room ventilation before any recovery equipment is added.
• We supply new compressors, including models with heat-recovery options from their makers, and used machines where suitable.
• We service coolers, thermostatic valves and oil circuits under service and AMC arrangements.

Our service teams cover all of Telangana and Andhra Pradesh, and we supply equipment and spares across India.

What to send us

• Nameplate photos of each compressor and its cooling type.
• Running and loaded hours from each controller.
• The heat users you have in mind, their temperature needs and operating hours.
• What currently heats that water or air: boiler, electric heater, steam or other.
• A sketch or photos of the compressor room and the distance to the heat user.

Frequently asked questions

How much heat can I recover from an air compressor?

Almost all the electrical input to a compressor becomes heat, and with a well-designed oil-circuit exchanger a large share of it can be captured. The exact figure depends on the model and method, so use the maker’s heat-rejection data. What you can actually use is limited by your loaded hours, your heat user’s demand and the hours when both coincide.

Is compressor heat recovery worth it in India?

It can be where a plant heats water for long hours, for example for boiler feed, process washing, canteens or food and dairy cleaning. Space heating is rarely needed in most of India, so hot-air recovery is limited mainly to drying uses. The case depends on coincident hours and on the fuel or electricity that the recovered heat displaces.

What temperature of hot water can a screw compressor provide?

Oil-injected screw compressors run with oil discharge around 75–95 °C, so an oil-circuit exchanger can heat water to a useful temperature below that level. The achievable water temperature depends on the exchanger design, flow rate and the maker’s recovery package. For a specific project, confirm the outlet temperature and heat output with the maker’s data for that model.

Can heat recovery cause my compressor to overheat?

It can if it is designed badly. If the heat user stops taking heat and there is no alternative cooling path, oil temperature rises and the compressor trips or degrades its oil. A correct design keeps the original cooler in circuit with a thermostatic valve, respects fan pressure limits on ducts and prevents hot air from returning to the intake.

How do I measure the savings from heat recovery?

Fit a heat meter on the water side that measures flow and the inlet and outlet temperatures, and record delivered heat in kWh over time. Then convert it into the fuel or electricity saved, using the efficiency of the heating source it replaces. Keep it separate from other energy measures so that savings are not counted twice.

Should I recover heat or fix leaks first?

Fix leaks, reduce excess pressure and cut unloaded running first. Those measures reduce electricity use directly, while heat recovery only makes better use of energy already spent. Once the air system is efficient, recover heat from the compressors that still run loaded for long hours and have a heat user close by.

Related pages

• Compressor heat-recovery project economics
• Heat-recovery rotary screw systems
• Compressor heat-recovery calculator
• Compressor thermal performance and overheating
• Compressed-air energy and efficiency