The Heat of Compression —— Compressed Air Energy Recovery Methods

The heat of compression is essential energy. The operating costs of an air compressor system are usually as follows. Initial investment and equipment maintenance costs account for 25% of the total cost. Energy consumption accounts for 75%. Almost all system waste is reflected in electricity bills. Compressed air leaks and improper use can both reduce system efficiency. As a result, electricity bills continue to increase.

In addition, many experiments have also proved that only about 15% of the electric energy s used to increase the air potential energy. The remaining 85% of electrical energy is almost all converted into heat energy. We usually call it “heat of compression.” The heat energy is discharged into the atmosphere through the air compressor casing and cooler.

What is Heat of Compression?

The air compressor converts electrical energy into mechanical energy and wind energy. When compressing air, the temperature rises rapidly. At the same time, the screw air compressor will also generate frictional heat. This thermal energy mixes with the injected lubricant. So it forms a mixture of high temperature and high-pressure oil and gas. The oil-air mixture transports a lot of heat. They remove the heat of compression from the machine.

In addition, high-temperature lubricating oil must pass through a cooler. After cooling and filtering, it can return to the oil passage for the next cycle. Therefore, the cooling system will dissipate this heat into the environment.

We can see that most of the energy is lost and wasted. Most of them are recyclable. We can use them for heating, cooling, and power generation during production. This helps to reduce the cost of producing this energy. And save energy for the business.

Heat of Compression Recovery And Utilization

There are now many waste heat recovery equipment. They rely on the absorption of waste heat from the air compressor to heat the cold water. There is no energy consumption. Enterprises mainly use them to solve problems such as domestic and industrial hot water for employees. At the smae time, it can also supply heat for workshop.

air compressor

(1) Air Compressor Heat Recovery Applications

The atmosphere extracts heat from the compressed air and lubricating oil. We can use the hot air for the following aspects:

  1. Space heating
  2. Industrial drying
  3. Preheating of intake air for oil burners
  4. Other applications where need warm air

The use of a heat exchanger allows the extraction of waste heat from the compressor oil cooler. Heat exchangers also contribute to the production of hot water. Depending on the design, it can produce either non-potable or potable water. Hot water can be used for the following:

  1. Central heating or boiler systems
  2. Industrial cleaning processes
  3. Electroplating operations
  4. Heat pumps
  5. Laundries
  6. RO water purification

(2) Economic Benefits of Heat of Compression Recovery

Suppose there is a 100 HP (75 kW) air compressor. It works 8 hours a day. And calculate as working 300 days a year.

The oil flow rate is 75 L/min. The oil temperature is 80°C. After cooling, the temperature is 58°C.

The heat generated:75×(80-58)×0.45×0.85×60=37868kcal/hr.

  • If use electric water heater, we need to generate 37,868kcal of heat. Calculated at 0.8 CNY per unit of electricity. The efficiency of the electric water heater is 80%.

1 kWh = 1 kWh = 860.42 kcal.

37,868 kcal ÷ (860.421 kcal/kWh x 0.8) x 0.8 Yuan/kWh = 44 CNY/hr.
Based on 8 hours of work a day, 300 days a year: 44 x 8 x 300 = 105,600 CNY.

  • The calorific value of diesel oil is 1×104 kcal per kilogram. The total efficiency of diesel boiler and heat exchanger is calculated at 0.75. Its effective calorific value is about 7,500kcal per kilogram of diesel. To produce 37868 kcal of heat, there need 5 kg of diesel.

Suppose the boiler works 8 hours a day. There are 300 working days a year. Calculated at 5.5 CNY per kilogram of diesel.
5 kg/hour x 8 hours/day x 300 days/year x 5.5 CNY/kg = 66,000 CNY/year.

In addition, the boiler maintenance also requires a full-time person. Calculated at 20,000 CNY a year. The total cost for one year is 66,000 CNY/year + 20,000 CNY/year = 86,000 CNY/year.

For an air compressor, the annual waste of compression heat is very serious. If the heat can be utilized, it will create considerable benefits. You can see the blow table.

Calorific Value
Thermal Efficiency
Unit Cost
Energy Consumption for 1 Ton of Hot Water
Air Compressor Waste Heat Recovery System
Heat of Compression
Air Energy Heat Pump
Solar Energy
Diesel Boiler
Diesel Oil
Liquefied Gas Boiler
Liquefied Gas
Electric Water Heater

Waste Heat Recovery From Oil Injected Screw Air Compressor

The outside air passes through the air compressor filter. After entering the machine, they are compressed. The temperature rises sharply. At the same time, the high-speed rotation of the screw will also generate a lot of heat due to friction. This heat raises the temperature of the oil-air mixture. Usually reaches 80 ℃ ~ 100 ℃. We call it “heat of compression”.

air compressor filter

Air compressor waste heat recovery machine utilizes the heat energy of high temperature oil and gas. Through heat exchange, the heat energy can be fully utilized. It is not only eco-friendly, but also reduces the energy consumption of the factory.

Heat recovery systems typically consist of two parts. One is the transformation of the internal oil circuit of the air compressor. The other is an external heat exchanger. The heat of compression is recovered through a recovery system. Then the hot water is circulated to the heat preservation water tank through the circulation pump. When need hot water, the hot water is directly taken from the water tank through the heat pump.

Firstly, the lubricating oil is separated in the air-oil separator. Then draw the high temperature lubricant oil out through the pipeline. Afterwards, enter the heat exchange equipment. In the heat exchanger, they exchange heat with water. Finally the hot water is lead out from the pipeline.

The oil-injected screw air compressor mainly bases on double recovery of oil and gas. It is pressurized by cold water first and then passes through the heat exchanger and airend. Lower the oil and gas temperature to about 45°C. We can design to control hot water between 50~80℃. Then transport the hot water into the heat preservation water tank. Finally meet the factory domestic water and process water.

Heat Of Compression Recovery From Oil Free Screw Compressor

Oil-free screw air compressors mainly use compressed air heat recovery. In order to ensure the purity of the compressed air, it recovers the heat of the final-stage compressed air. Combined with the original cooling system, the cold water pressurization first absorbs the heat of the cooling water. Then enter the host after the residual heat. Also reduce the compressed air temperature to about 40 degrees Celsius. Through design, it can control the output hot water at 50~80℃.

Circulating return water——passing through the preheating heat exchanger (temperature 30~40°C)——going through the circulating pump—entering the air waste heat recovery exchanger——adjusting the temperature through the regulating valve (temperature 50~80°C)——sending into waste heat recovery heat exchanger. Heating the external circulating water. So we can use the heat of compression.

Centrifugal Air Compressor Waste Heat Recovery

(1) Single-stage Heat of Compression Recovery

Use the high-temperature air of about 140°C sent by the air compressor. Through the heat exchanger, the heat in the air is converted into high-temperature hot water at 50~80°C. This hot water can be used for process heating or domestic water. After heat exchange, the compressed air temperature drops. Decrease to 60~80℃ and discharge.

Centrifugal air compressor single-stage heat transfer process:

third-stage compressed air——heat exchanger (air-water heat exchange)—inner circulating soft water (transfer)—secondary heat exchanger (water-water heat exchange)—outside circulation pump supplies water.

Item Data
Centrifugal Air Compressor Power 1000kw
Recovery Efficiency 25%
Recovery Power (kw/h) 250kw
Calorific Value Ratio 1 kw*h≈860Kcal
Recoverable Calorific Value 215000Kcal
Hot Water Production (Temperature rise 40°C, water specific heat capacity 1000Kcal/ton*°C) 5.37 tons

For a 1000kw centrifugal air compressor, the third stage of heat recovery: produce 5.37 tons of hot water with a temperature rise 40℃ per hour. This is the value of heat of compression.

centrifugal air compressor

(2) Three-stage Wate Heat Recovery of Centrifugal Air Compressor

Add a set of heat exchangers to the outlet pipes of the original cooling water system. Use high-temperature cooling water to preheat the feed water. Increase the inlet water temperature. The comprehensive recovery efficiency of waste heat is about 60%.

The transmission process is as follows:

the original cooling water cycle —— preheating through the heat exchanger —— the third stage compressed air —— heat exchanger (air-water heat exchange) —— internal circulation soft water (transfer) —— heat exchanger (water-water heat exchange)——outer circulation pumping water.

Item Data
Centrifugal Air Compressor Power 1000kw
Recovery Efficiency 60%
Recovery Power (kw/h) 600kw
Calorific Value Ratio 1 kw*h≈860Kcal
Recoverable Calorific Value 516000Kcal
Hot Water Production (Temperature rise 40°C, water specific heat capacity 1000Kcal/ton*°C) 12.9 ton

It produces 12.9 tons of 40℃ warm water per hour.

Heat of Compression Dryer (HOC Dryer)

Hoc dryer is a kind of desiccant air dryer. And usually we produce the zero air loss HOC dryer. So it is an energy saving product. It fully utilizes the waste heat of the air compressor. The dryer uses the high temperature heat energy to regenerate the desiccant beads. There is no or little regeneration air consumption. At the same time, no need blowers and heaters. So there is no heater and blower power consumption.

The heat of compression dryer is mainly used for oil free and centrifugal air compressor. It requires that the inlet air temperature must be at least 110℃. So how about its energy saving effect? Here we mainly the energy consumptions between heatless air dryer and zero air loss HOC dryer. Assume that there is a 1100KW centrifugal air compressor. It works 365 days each year. Moreover, the machine works 24 hours one day. The electricity price is calculated at 1.0 CNY/kWh. Water cost is 0.2 CNY/ton.

Desiccant Air Dryer Type Energy Consumption Calculation Details Annual Operating Cost
Heatless Regeneration
Air Dryer
All energy consumption is finished compressed air.
It accounts for about 13%
of the air flow.
Zero Air Loss HOC Dryer The adsorption cycle is 4 hours. Cooling water
consumption: 135t/hour.
And compressed air consumption is 0%.
Water consumption: 135t/h×24h×365 days×0.2 CNY/t
=236520 CNY/year.

In summary, the HOC dryer makes full use of the compression heat of the air compressor. So the enery consumption is 81% less than heatless dryer.