Recycling of Waste Heat from Air Compressors in Industrial Enterprises

Air compressor  is a common energy supply power equipment, can provide a large amount of compressed air for industrial production power. Air compressor operation process constantly produces a large amount of heat energy, enterprises typically dispose of this heat by discharging it into the air through the cooling towers  of their cooling water systems, which results in wasted energy and thermal pollution of the air. Our company produces domestic hot water through the Recycling of Waste heat of the air compressor and utilizes it, and calculates and analyzes the rationality of the design, which is of great significance for energy saving, and efficiency and environmental protection.

1. The Significance of Air Compressor Recycling of Waste Heat Recovery and Utilization

Air compressor energy consumption accounts for a large part of the energy consumption of industrial enterprises. In the operation process of air compressor, electrical energy is mainly converted into compressed air internal energy and heat energy. According to relevant statistics, about 50% of the electrical energy is converted into heat and dissipated. If this heat is directly discharged into the air, firstly, it will cause air thermal pollution, secondly, the heat dissipation efficiency is not high, which will make the air compressor in high temperature operation state for a long time, unable to operate normally and shorten the service life. With the progress of waste heat utilization technology of air compressor and the growth of enterprise heating demand, enterprises pay more and more attention to Recycling of Waste heat utilization of air compressor. Waste heat recovery from air compressors can be reasonably used for hot water supply, equipment preheating, etc., which can save investment in equipment, save energy and reduce emissions, improve resource utilization, reduce operating costs and realize sustainable development of enterprises.

2. Working Principle of the Main Equipment

2.1 Oil-free Frequency Conversion Screw Air Compressor

Oil-free frequency conversion screw air compressor generates compressed gas by changing the volume. During the working process, the air pressure in the storage tank needs to be monitored to keep it stable. The air pressure in the storage tank is converted into an electrical signal and sent to the frequency converter and compared with the set pressure value. Based on the comparison results, the working status of the air compressor is controlled, i.e., when the actual pressure in the tank exceeds the maximum setting value, the air compressor suspends its work, and when the air pressure drops to a certain setting value, the air compressor restarts again. In addition, the waste heat utilization rate of oil-free frequency conversion screw air compressor can reach 100%, which is a good practice in heating, and the air it discharges is clean and oil-free, this kind of air compressor is favored by high-end industries.

2.2 Centrifugal Compressor 

When the centrifugal compressor works, the impeller center rotates at high speed to form a vacuum, and the gas continuously enters axially with the impeller rotating at high speed. Under the action of centrifugal force, the gas pressure increases and leaves the impeller at high speed and enters the diffuser guide vane. In the diffuser guide vane, the gas decelerates, and the pressure increases further. The centrifugal compressor can be operated in series with multiple impellers to increase the gas pressure, and the remaining heat recovery effect is also better.

2.3 Zero Air Consumption Compression Heat Regenerative Adsorption Dryer

Zero air consumption compression heat regenerative adsorption dryer is used to dry moisture in compressed air. It utilizes the compression heat generated by the air compressor to dehydrate and regenerate the adsorbent in the drying tower. In this way, there is no need to configure additional heating devices to participate in the regeneration, and the high-temperature compressed air directly heats the adsorbent, maximizing the energy savings for the heating and regeneration process. Compared with other adsorption dryers, the zero air consumption compression heat regenerative adsorption dryer has almost no air consumption, which is an ideal drying equipment.

3. Example of Dormitory Bath Heating in An Intelligent Manufacturing Industrial Park 

3.1 Engineering Background and Conditions 

The Intelligent Manufacturing Industrial Park is equipped with 6 sets of air compressors and 4 sets of dryers for manufacturing according to the actual production needs. The hot water system needs to be designed to supply hot water for bathing in the staff dormitory.

The electrical efficiency of air compressor is not high, and the energy waste is relatively serious. Therefore, it is designed to utilize the waste heat of the air compressor in the production of shower hot water to achieve energy saving, cost reduction and other effects, the design scheme is as follows.

3.2 Design Instructions for Cooling Water Systems 

3.2.1 Operation of Hot Water Systems for Bathing

1. Before the air compressor is turned on for production, fill the hot water tank with tap water and close the float valve when full.
2. One minute before starting the air compressor, start the bath cooling circulating pump and pump out the tap water from the hot water tank to the air compressor. The air compressor works continuously to produce heat, the tap water absorbs the heat of the air compressor and returns to the hot water tank, and then from the outlet pipe of the hot water tank to the bath cooling circulating pump, which is pressurized by the pump and sent to the heat exchanger of the air compressor, and so on, and the temperature of the water in the hot water tank is constantly getting higher. The temperature detection point of the hot water tank is set next to the water outlet of the tank (user side), when the temperature of the hot water tank is lower than 50℃, the bath cooling circulating pump is started, and when the water temperature of the hot water tank rises to 60℃, the bath cooling circulating pump stops working. 
3. The hot water in the tank is sent to the shower room with a hot water pump. When the water level in the hot water tank is designed to drop to 0.4m from the bottom of the tank, tap water is replenished to the hot water tank; when the water level in the hot water tank is higher than 1m from the bottom of the tank, the hot water pump is activated to start a new round of hot water production; when the water level in the hot water tank is 0.2m from the top of the tank, the float valve is closed and tap water stops being fed into the tank.

3.2.2 The Operation of Cooling Tower Circulating Cooling Water

1. When the supply of hot water for bathing is sufficient, the Recycling of Waste heat from the air compressor needs to be treated with a cooling tower.
2. Fill the cooling tower with tap water and start the cooling tower. The water is sent to the air compressor through the secondary cooling circulating pump and returns to the cooling tower after absorbing the heat of the air compressor, and so on, absorbing the waste heat to produce cooling water. 
3. Compression heat regeneration adsorption dryers require both compression heat to participate in the work and cooling circulating water to cool down. 
4. Heat Supply Calculation and Economic Analysis 

The number of enterprise employees showering every day is about 1,000 people, and the amount of hot water per person per day is calculated by 100L.

1. The power of the air compressor is 420kW/unit, the loading ratio is 0.9, the working time is 24h, the electric heat conversion coefficient is 860, the heat recovery ratio is 0.64, and the heat recovered from each air compressor running all day and 24 h is: Q=420×0.9×24×860×0.64=4993229 kcal/d.
2. Waste heat heating, room temperature water 20℃, the daily production of 60℃ hot water for each air compressor is:Qw=4993229/(60-20)=124830kg≈12t.
3. According to the continuous operation of 2 air compressors, the amount of hot water that can be provided at 60℃ for the whole day is：Q2w=125×2=250t.

From the calculation, it is known that The Intelligent Manufacturing Industrial Park can get 250t of hot water at 60℃ per day in summer by utilizing the waste heat from the air compressor, which can satisfy the 100t of hot water for bathing required by the employees per day. If the waste heat of the air compressor can’t meet the bath, it is necessary to design solar energy, air source heat pump and other heating systems. The production of hot water for bathing absorbs the waste heat from the air compressor without consuming electricity. Compared with the heating method of directly heating cold water by heater, the waste heat utilization is more advanced technically and reduces the emission of waste heat and other pollutants from the factory. Relying on the convenience of resources, it saves cost for the enterprise and reduces the total energy consumption, which is very substantial considering the long-term economic benefits.

5. Concluding Remarks

Comprehensive utilization of air compressor Recycling of Waste heat recovery provides a new idea for the heating of industrial enterprises. Through the case study of The Intelligent Manufacturing Industrial Park which uses the waste heat of air compressor to supply employees with bath, the air compressor waste heat recovery system is introduced, and relevant technical and economic analysis is carried out, which is energy-saving and environmentally friendly and avoids the waste of more resources. Timely use of waste heat can also prevent the problem of the air compressor not functioning properly due to the high temperature condition. Reasonable recycling of air compressor waste heat by industrial enterprises has positive benefits for enterprise development and the environment. Read more.