Heat-recovery rotary screw compressor systems

Where does compressor heat come from?

Compressing air takes energy, and almost all of that energy leaves the compressor as heat. In an oil-injected rotary screw compressor, oil is injected into the airend to seal the rotor clearances and carry away the heat of compression. That hot oil then passes through the oil cooler before returning to the airend. The rest of the heat leaves through the aftercooler, the motor and radiation from the package.

The main heat sources in a typical oil-injected screw package are:

• The oil circuit. This carries the largest share of the heat and at the most useful temperature. Discharge temperature of an oil-injected screw is typically about 75–95 °C.
• The aftercooler. The compressed air is cooled before it leaves the package; this heat is at a lower temperature and smaller in quantity.
• The motor and enclosure. Heat from the motor and package surfaces warms the cooling air leaving the canopy.

Oil-free two-stage screw compressors have no injected oil. Their heat leaves through the intercooler, aftercooler and, in water-cooled designs, the cooling water, often at higher air temperatures between stages. Recovery is possible but the hardware is different.

What are the ways to recover the heat?

The water temperature you can reach depends on the oil temperature, the heat exchanger and the water flow. It will always be lower than the oil temperature. Read the achievable temperature from the manufacturer’s heat-recovery data for your model, not a general rule.

When does heat recovery make sense?

A project is worth pursuing when all of the following line up:

1. Enough heat. The compressor is large enough and loaded enough to give useful heat. A small machine that runs unloaded half the time gives little.
2. The right temperature. The recovered temperature is high enough for the intended use. Pre-heating boiler feed water is often a good match; producing steam is not.
3. Matching hours. The compressor runs when the heat is needed. A compressor that runs on day shift cannot heat a hostel’s water at night without storage.
4. A steady heat user. There is a real, continuing demand: boiler make-up water, process washing, CIP pre-heat, canteens, hostels or dyeing.
5. Practical integration. The heat user is close enough, the pipework is simple, and the compressor can still cool itself when no heat is wanted.
6. Sound economics. The fuel or electricity saved over the expected running hours justifies the equipment, installation and maintenance.

If any one of these is missing, the project may not pay. That is why we start with measured compressor data and the site’s heat demand, not with a percentage from a brochure.

How to estimate the heat you can recover

A first estimate uses simple, measurable figures:

1. Record the compressor’s actual input power over a typical week, loaded and unloaded, from a power meter or controller data.
2. Note the loaded hours and the hours when the heat user actually needs heat.
3. Take the share of input power that the manufacturer states is recoverable for the model and the recovery method.
4. Multiply the recoverable power by the overlapping hours to get recoverable kWh of heat.
5. Compare that with the heat the user needs, in kWh, worked out from water flow and temperature rise. Heating 1,000 litres of water by 1 °C needs about 1.16 kWh.
6. Express the saving in the fuel it replaces: litres of diesel or furnace oil, kilograms of LPG or kWh of electricity.

Worked example (illustrative only): a compressor recovers 40 kW of heat for 16 hours a day, giving 640 kWh of heat. A canteen needs 5,000 litres of water a day heated by 30 °C, which is about 174 kWh. The heat is more than enough, so the useful recovery is set by the canteen’s demand, not by the compressor. The size of the user, not the size of the compressor, decides the saving.

Protecting the compressor

A heat-recovery circuit must never put the compressor at risk. The main rules are:

• The compressor’s own cooler must stay in service so the machine can cool itself when the heat user needs no heat or the water flow stops.
• Oil temperature must stay in the range the manufacturer specifies. If the oil runs too cool, water condenses in the oil, which damages bearings and shortens oil life. If it runs too hot, the oil breaks down and the machine trips on high discharge temperature.
• A thermostatic valve or control must divert oil correctly between the recovery exchanger and the cooler.
• Water-side quality matters. Hard water scales a plate exchanger and cuts heat transfer, so treat or soften the water and plan cleaning.
• Consider the water-side pressure so that, if the exchanger leaks, oil cannot contaminate a potable or process water system. Use double-wall or intermediate-loop designs where contamination would be serious.

Heat recovery in Indian plants

In much of India, space heating is rarely needed, so ducted warm air is usually less useful than water heating. The strongest cases in Telangana and Andhra Pradesh are often:

• Boiler feed-water pre-heating in pharmaceutical, food, dairy and textile plants.
• Hot water for washing, CIP pre-heat and bottle or crate washing.
• Hot water for canteens, hostels and staff facilities at large sites.
• Process water for dyeing, electroplating rinse tanks and similar duties.

High summer ambient temperature also matters in the other direction. In April and May, a compressor room that is already hot needs its cooling to work well. Ducting warm air away from the compressor and into a useful space, or out of the building, protects the machine whatever happens to the heat.

Common problems with heat-recovery systems

How Reddy Compressors helps

Reddy Compressors, an independent multi-brand specialist at Tarnaka, Secunderabad, helps plants decide whether heat recovery is worth doing on their rotary screw compressors and keeps those systems running. We are not an authorised distributor of any manufacturer. We have tie-ups with all major and reputed brands.

• Site visit and air audit to measure compressor load, hours and power, and to list possible heat users.
• Neutral advice on heat-recovery options from the compressor manufacturer, for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands.
• Sales of new and used/refurbished compressors where a new machine with factory heat recovery is the better answer.
• Service, AMC and breakdown repair covering coolers, thermostatic valves and oil condition, with teams across Telangana and Andhra Pradesh, and spares supplied across India.

What to send us

• Compressor make, model, rated power and nameplate photo.
• Typical loaded and unloaded hours per day, or controller running-hour data.
• The heat user you have in mind, with water flow, start and target temperatures, and hours of use.
• Fuel currently used for heating and the approximate daily consumption.
• Layout sketch showing the distance between the compressor room and the heat user.

Frequently asked questions

How much heat can I recover from a screw compressor?

Almost all of the electrical input becomes heat, and much of it passes through the oil circuit, but the recoverable amount depends on the model, the recovery method and how loaded the machine is. Use the manufacturer’s heat-recovery data for your model, then check it against your actual loaded hours. A heavily loaded machine on long shifts gives far more useful heat than a lightly loaded one.

What temperature water can I get from compressor heat recovery?

The water temperature is always lower than the oil temperature and depends on the heat exchanger and water flow. Typical oil-injected screw discharge temperature is about 75–95 °C, so recovered water is suitable for pre-heating, washing and similar duties rather than steam. Take the achievable water temperature from the manufacturer’s data for your model and recovery kit.

Is compressor heat recovery worth it in India?

It can be, where a plant has a steady hot-water demand that overlaps with compressor running hours. Boiler feed-water pre-heating, washing and canteen or hostel hot water are common good matches. Space heating is rarely useful in most of India. Without a real heat user running at the same time as the compressor, the equipment will not pay for itself.

Can heat recovery damage my compressor?

A badly designed system can. If the recovery circuit removes too much heat, water condenses in the oil; if it blocks the cooler path, the machine overheats and trips. A correct design keeps the compressor’s own cooler in service and controls oil temperature with a thermostatic valve, so the compressor stays protected when the heat user needs no heat.

Can I add heat recovery to an existing compressor?

Often yes. Many manufacturers offer retrofit heat-recovery kits for oil-injected screw compressors, and plate exchangers can be added to the oil circuit with the right controls. The work must follow the manufacturer’s guidance for oil temperature and pressure. We check the model, its condition and your heat demand before recommending a retrofit.

Does a VSD compressor give less recoverable heat?

A VSD compressor uses less energy at part load, so it produces less heat at those times. That is a good result for energy, but it means the heat available rises and falls with air demand. Size the heat-recovery case on measured running data, and use hot-water storage if you need a steadier supply than the compressor gives.

Related pages

• Compressed-air heat recovery and thermal energy
• Rotary screw compressors
• Water-cooled rotary screw compressors
• Compressor cooler and heat-exchanger performance
• Compressed-air energy and efficiency
• Compressed-air energy optimisation