Compressor heat recovery project economics

Where the heat comes from

Almost all of the electrical energy taken by an air compressor is turned into heat. In an oil-injected screw compressor, most of it is carried away by the oil cooler, a further share by the aftercooler, and the rest is radiated from the package and motor. Manufacturers commonly state that a large majority of the input power can be recovered with a suitable system; the exact share depends on the design and on the temperature at which heat is wanted.

These are typical ranges for planning only. The recoverable temperature and heat for a particular machine must come from its manufacturer or from measurement.

Keep recovered heat separate from compressor efficiency

Heat recovery creates a second useful output from a compressor that is running anyway. It does not reduce the compressor’s electricity use or improve its specific power. Report three numbers separately: compressor electricity, recovered heat, and the extra electricity used by the recovery system’s pumps or fans. This prevents thermal kWh being presented as electrical savings.

Recovered heat also never justifies wasteful air production. Running a higher pressure, unloaded hours or leaks produces more heat, but fixing those problems is still the better first step. Recover heat from the air you genuinely need. See our specific power page for measuring compressor efficiency on its own terms.

Define the heating cost that can be avoided

A heat-recovery project saves money only by replacing an existing heat source. Identify that source exactly: a diesel- or gas-fired boiler, an electric water heater, a steam supply, or a heat pump. Record its fuel, its efficiency and where its heat is delivered. The avoided energy is the recovered useful heat divided by the efficiency of the heater it replaces. A boiler that is 80% efficient needs 1.25 kWh of fuel for each kWh of useful heat, so every recovered kWh avoids more than a kWh of fuel.

In many Indian plants, space heating is rarely needed, so the strongest uses are process hot water, boiler feed-water preheat, washing and cleaning in food, dairy and pharma plants, and hot water for canteens. Warm-air uses such as drying are possible but seasonal.

Match heat supply and heat demand over time

The most common reason heat-recovery projects disappoint is a timing mismatch. A compressor that is loaded only during the day shift cannot supply a night-time hot-water demand without storage. A compressor that unloads heavily at weekends delivers little heat when the canteen is closed anyway.

1. Log compressor loaded hours and power over a representative week, or read them from the controller.
2. List each potential heat user with its temperature requirement, hourly demand and operating hours.
3. Compare supply and demand hour by hour, not as annual totals.
4. Count only heat that can be used at the time it is produced, or stored and used later.
5. Repeat for seasonal changes: hotter summer cooling water and lower process demand can shift the balance.

Worked example: heat matching

A 90 kW oil-injected screw compressor is loaded for an average of 16 hours a day on 300 days a year. The manufacturer’s recovery module can recover, say, 70% of the shaft input as hot water at up to about 70 °C. That gives roughly 0.7 × 90 ≈ 63 kW of heat while loaded.

• Potential heat: 63 kW × 16 h × 300 days ≈ 302,000 kWh a year.
• The plant’s wash-water demand is only during two shifts, and logging shows it can absorb about 70% of the available heat at those times.
• Useful heat: about 212,000 kWh a year.
• Replacing an oil-fired heater at 80% efficiency avoids about 265,000 kWh of fuel energy, or roughly 26,500 litres of light diesel oil at about 10 kWh per litre.

These figures illustrate the method. The real result depends on measured loaded hours, the recovery module’s actual output and the user’s real demand pattern.

Measure useful heat at the delivery boundary

Measure heat where it enters the user’s system, not at the compressor. For water, useful heat equals flow rate × specific heat × temperature rise. Use a heat meter with a flow meter and matched temperature sensors, logged over a representative period. Measurement uncertainty grows quickly when the temperature difference is small, so allow for it in the economics rather than quoting a single precise number.

Deduct the added operating costs

• Pump power for the recovery water circuit, and fan power for ducted air systems.
• Any extra fan or cooler power needed when the heat user does not absorb all the heat.
• Maintenance of heat exchangers, valves and sensors, including cleaning on hard or untreated water.
• Water treatment where scale or corrosion is a risk.

Protect the compressor and the heat user

The compressor must always be able to reject its heat, even when the heat user takes none. A recovery system that blocks normal cooling can cause high discharge temperature trips. A correct design includes:

• A thermostatic valve that keeps the oil within the manufacturer’s temperature range and returns to the standard cooler when recovery demand is low.
• A bypass or backup cooler that can carry the full heat load.
• Correct materials and a double-wall or monitored heat exchanger where any leak could contaminate potable or process water.
• Alarm and trip settings that are not changed to suit the recovery system.

In high Indian summer ambient conditions, where cooling margins are already small, this protection matters even more. See thermal performance and overheating.

Calculating payback and lifetime value

Simple payback is the installed cost divided by the yearly net saving in avoided fuel or electricity, less added running and maintenance costs. It is useful for screening but ignores what happens after payback. A fuller comparison uses discounted cash flow over the expected life of the system and shows how the result changes if loaded hours fall, fuel prices change or the heat user’s demand drops.

Always show a sensitivity check: the break-even number of useful heat hours per year. If that figure is close to what the plant actually achieves today, the project carries significant risk. Installed cost must include heat exchangers, pipework, insulation, valves, controls, metering, any thermal store, installation and commissioning.

Verify performance after commissioning

• Log useful heat, compressor loaded hours and pump power over a representative period.
• Check that oil and discharge temperatures stay within limits during low heat demand.
• Confirm the existing heater’s fuel use has fallen in line with the recovered heat.
• Repeat in a hotter month, when cooling margins and demand differ.

How Reddy Compressors helps

Reddy Compressors, based at Tarnaka in Secunderabad, has worked in compressed air since 2002. As an independent multi-brand specialist for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others, not an authorised dealer but with tie-ups with all major and reputed brands, we:

• Read controller data and log loaded hours and power to establish available heat.
• Help you list heat users, their temperatures and their demand pattern.
• Check that a proposed recovery arrangement keeps the compressor within its temperature limits.
• Supply new compressors with recovery options, or advise on retrofitting existing machines where the manufacturer offers it.
• Service oil coolers and heat exchangers, which keeps both cooling and recovery working.

Our 20+ service engineers and technicians cover all of Telangana and Andhra Pradesh, and we supply equipment and spares across India.

What to send us

• Compressor make, model, rating and nameplate photo, with loaded and running hours.
• Cooling type: air-cooled or water-cooled.
• Details of the heat users: purpose, temperature needed, flow and hours.
• The heat source you want to replace and its fuel.
• A layout showing the distance from the compressor room to the heat user.

Frequently asked questions

How much heat can be recovered from an air compressor?

Almost all the electrical energy a compressor uses becomes heat, and manufacturers commonly state that a large majority of it can be recovered with a suitable system on an oil-injected screw compressor. How much is useful depends on the temperature you need, the compressor’s loaded hours and whether a heat user is available at the same time. Measure loaded hours and user demand before estimating.

What temperature of hot water can a screw compressor produce?

Oil-injected screw compressors with a plate heat exchanger on the oil circuit commonly produce water up to about 70 °C. Oil-free compressors can often provide higher temperatures from their stage coolers, depending on design. The available temperature must be confirmed with the manufacturer, and the recovery system must not raise oil temperatures beyond the compressor’s limits.

Is compressor heat recovery worth it in India?

It can be, where there is a year-round need for hot water or process heat, such as boiler feed preheat, washing in food, dairy and pharma plants, or canteen hot water. Space heating is rarely a strong use in most of India. The project works best when compressors are loaded for long hours and the heat user operates at the same time.

Does heat recovery make my compressor more efficient?

No. Heat recovery reuses heat that the compressor would otherwise reject, but the compressor uses the same electricity to make the same air. The benefit is a reduction in fuel or electricity bought for heating elsewhere. Keep compressor specific power and recovered heat as separate figures so that each can be judged fairly.

Can heat recovery cause compressor overheating?

It can if it is badly designed. If the heat user stops taking heat and there is no automatic return to the standard cooler, oil and discharge temperatures rise and the machine trips. A correct design uses a thermostatic valve and a cooler that can carry the full heat load at any time, with the compressor’s own protection settings left unchanged.

How do I calculate payback on compressor heat recovery?

Estimate useful heat from matched supply and demand, divide by the efficiency of the heater it replaces to find avoided fuel or electricity, subtract added pump and maintenance costs, and divide the installed cost by the net yearly saving. Then test how the result changes if loaded hours or heat demand fall. Verify after commissioning with a heat meter.

Related pages

• Compressed-air energy optimisation hub
• Compressed-air heat recovery and thermal-energy engineering
• Heat-recovery rotary screw systems
• Specific power and delivered-air performance
• Compressor cooler and heat-exchanger performance