Talking about the waste heat recovery of several typical air compressors
Compressed air is one of the most widely used sources of power in the industry. Because of its advantages of safety, pollution-free, good regulation performance, convenient transportation, etc., it is more and more widely used in modern industrial fields. But to get good quality compressed air requires a lot of energy. In most production companies, compressed air consumes 10% to 35% of total electricity consumption. While continuously improving the efficiency of the compressed air system, the air compressor will generate a large amount of heat of compression during operation. The energy consumed by the compression heat accounts for more than 85% of the operating power of the unit. Usually, this part of the energy is released through the air-cooling or water-cooling system of the unit. In the atmosphere.
Therefore, heat recovery of compressors is a necessary means to continuously reduce air system losses and improve customer productivity. The energy-saving technologies for waste heat recovery are currently studied a lot, but most of them are only for the oil pipeline transformation of the oil-injected screw air compressor. This paper introduces the working principle of several typical air compressors and the characteristics of waste heat recovery system in detail, and more abundantly understands the ways and forms of waste heat recovery of air compressors, which can better recover waste heat and reduce energy costs of enterprises. The purpose of energy saving and environmental protection.
Several typical air compressor waste heat recovery methods and forms are introduced.
1 Analysis of residual heat recovery of air injection screw air compressor
1) Analysis of working principle of oil-injected screw air compressor
The oil-injected screw air compressor is a type of air compressor with a high market share. The working principle is shown in Figure 1.
The oil of the oil-injected screw air compressor has three functions: cooling absorbs heat of compression, sealing and lubrication.
Gas path: The external air enters the machine head through the air filter and is compressed by the screw. The oil and gas mixture is discharged from the exhaust port, passes through the pipeline system and the oil and gas separation system, enters the air cooler, and reduces the high temperature compressed air to an acceptable level. .
Oil circuit: The oil and gas mixture is discharged from the main engine outlet. In the oil separation cylinder, the cooling oil is separated from the compressed air and then enters the oil cooler to remove the heat of the high temperature oil. The cooled oil is re-injected into the main machine through the corresponding oil circuit. Cool, seal and lubricate. Repeatedly.
2) Principle of waste heat recovery of oil injection screw air compressor
The schematic diagram and flow chart of the waste heat recovery of the oil jet screw air compressor are shown in Fig. 2 and Fig. 3.
It can be seen from Fig. 2 and Fig. 3 that the high temperature and high pressure oil and gas mixture formed by the compression of the compressor head is separated in the oil and gas separator, and the high temperature oil is introduced into a heat exchanger by reforming the oil and gas separator oil discharge pipeline. The bypass valve distributes the amount of oil entering the heat exchanger and the bypass pipe in real time, thereby ensuring that the return oil temperature is not lower than the return oil protection temperature of the air compressor, and the cold water on the water side of the heat exchanger exchanges heat with the high temperature oil. The heated hot water can be used for domestic hot water, air conditioning heating, boiler feed water preheating, process hot water, and the like.
3) Primary heat recovery waste heat recovery
There are many forms of waste heat recovery systems for oil-injected screw air compressors. The two most common systems are listed below. Figure 4 is a flow chart of a primary heat exchange system of an oil jet screw air compressor. It can be seen from the figure that the cold water in the heat preservation tank directly exchanges heat with the energy recovery device inside the air compressor through the circulating water pump, and then returns to the heat preservation water tank. The characteristics of such a system are that the equipment is small and the heat exchange efficiency is high, but it must be noted that it is necessary to select a material recovery device with good material quality, and it needs to be cleaned regularly, otherwise it is easy to cause blockage due to high temperature scaling or leakage of the heat exchange device. Pollution application.
4) Secondary heat recovery waste heat recovery
Figure 5 is a flow chart of the secondary heat exchange system of the oil-injected screw air compressor. It can be seen from the figure that the system performs two heat exchanges, and the primary side system that exchanges heat with the energy recovery device is a closed system, and the secondary side system can For open systems, it can also be a closed system. The closed system on the primary side uses pure water or distilled water to reduce damage to the energy recovery unit due to water scaling. In case the heat exchanger is damaged, the heating medium at the application end will not be contaminated.
5) Advantages of adding thermal energy recovery device for oil-injected screw air compressor
After the fuel injection screw air compressor is installed with the heat recovery device, it has the following advantages:
(1) Stop the air compressor's own cooling fan or reduce the running time of the fan. The heat recovery unit uses a circulating water pump. The pump motor consumes a certain amount of electric energy, but the air inlet temperature of the air compressor main unit does not reach 80. At ~95 °C (can be set in this range), the air compressor's own cooling fan is not working, the power of this fan is generally 4 to 6 times larger than the circulating pump power, so the fan stops, the cycle The power consumption of the pump should be 4 to 6 times more energy efficient. In addition, because the oil temperature can be well controlled, the exhaust fan in the machine room can be opened less or not at all, which can save energy.
(2) Converting waste heat to hot water without any additional energy consumption.
(3) Increase the displacement of the air compressor. Since the operating temperature of the air compressor can be effectively controlled by the recovery device in the range of 80 to 95 ° C, the concentration of the oil can be kept good, and the displacement of the air compressor will increase, and the increase is 2% to 6%. This is also equivalent to saving energy. This is especially important for air compressors that operate in the summer, because in the summer, the ambient temperature is higher, the oil temperature can often rise to about 100 °C, the oil becomes thinner, the airtightness becomes worse, and the exhaust volume is reduced. Therefore, the heat recovery device can highlight its advantages in the summer.
2 Oil-free screw air compressor waste heat recovery
1) Analysis of working principle of oil-free screw air compressor
When the air compressor is isothermally compressed, it is the most energy-saving. The consumed electric energy is mainly converted into the compressive potential energy of the air, which can be calculated according to formula (1):
Air mass flow rate of m-air compressor, kg/s
R-gas constant, air is 287J/(kg·K)
T. - inhalation temperature, K
Q-gas volume flow, m3/s
P0 - suction pressure, Pa
P2-exhaust pressure, Pa
P Fu-air compressor heat radiation work, W
P transmission-moving air compressor drive power consumption, W;
ρ---air density kg/m3
Since there is no cooling effect of the fuel injection, the working process of the oil-free screw air compressor is approximately an adiabatic compression process. A two-stage oil-free screw air compressor with interstage cooling is relatively labor-saving. The screw main engine with water jacket or oil jacket has a working process index slightly lower than the isentropic adiabatic index, which is about 1.3. Air compressor power consumption can be calculated according to formula (3):
N-process index
Rg-gas constant, air is 287J/kg·k
T0, T1-first and second inspiratory temperature, k
P0- primary suction pressure, pad
P1, P2-, first and second exhaust pressure, Pa;
P transmission - air compressor thermal radiation power consumption, W
The oil-free screw air compressor has the potential for waste heat recovery compared to the fuel injection air compressor. Since there is no cooling effect of the oil, the compression process is more deviated from the isothermal compression, and most of the power is converted into the compression heat of the compressed air, which is also the cause of the excessive temperature of the oil-free screw air compressor. Recycling this part of the heat energy for the user's industrial water, preheater and bathroom water will greatly reduce the energy consumption of the project, thus achieving low carbon and environmental protection.
Figure 6 shows the energy distribution of the oil-free screw air compressor. It can be seen from the figure that the heat production of the water-cooled oil-free screw air compressor can be recovered 100%.
2) Oil-free screw air compressor waste heat recovery
Figure 7 is a flow chart of the internals of the energy recovery air compressor of the oil-free screw air compressor. As shown in the figure, the cold water is sequentially subjected to heat exchange through an oil cooler, a high pressure compression system, a low pressure compression system intercooler and an aftercooler, wherein the oil cooler must be enlarged.
3 Centrifugal air compressor waste heat recovery
1) Analysis of working principle of centrifugal air compressor
The centrifugal air compressor is driven by the impeller to rotate the gas at a high speed to generate centrifugal force. Due to the diffusing flow of the gas in the impeller, the flow rate and pressure of the gas after passing through the impeller are improved, and compressed air is continuously produced. The centrifugal air compressor is mainly composed of two parts, a rotor and a stator, and the rotor includes an impeller and a shaft. There are blades on the impeller, in addition to a balance plate and a part of the shaft seal, the main body of the stator is a casing (cylinder), and the diffuser, the curve, the return, the intake pipe, the exhaust pipe and the partial shaft are arranged on the stator. Sealed. The working principle of the centrifugal compressor is that when the impeller rotates at a high speed, the gas rotates. Under the action of centrifugal force, the gas is sucked into the diffuser at the rear, and a vacuum zone is formed at the impeller, at which time the fresh air from the outside enters. impeller. The impeller keeps rotating and the gas is continuously sucked in and out, thus maintaining a continuous flow of gas. Centrifugal air compressors rely on changes in kinetic energy to increase the pressure of the gas. When the bladed rotor, that is, the working wheel, rotates, the blade drives the gas to rotate, transferring the work to the gas, so that the gas obtains kinetic energy. After entering the stator part, the energy head is converted into the required pressure due to the expansion of the stator, the speed is reduced, the pressure is increased, and the guiding action of the stator part is used to enter the lower stage impeller to continue to increase, and finally discharged by the volute . For each compressor, in order to meet the design requirements pressure, each compressor is provided with a different number of stages and segments, and even several cylinders.
2) Centrifugal air compressor waste heat recovery process
The centrifuge generally undergoes three-stage compression. The first- and second-stage compressed air is not suitable for waste heat recovery due to the influence of the outlet temperature and pressure. Generally, the third-stage compressed air is recovered by waste heat. It is necessary to add an air aftercooler, as shown in Figure 8. It shows that when the hot end does not need to use heat, the compressed air is cooled without affecting the operation of the system.
4 Another type of waste heat recovery method for water-cooled air compressor
For water-cooled oil-injected screw machines, oil-free screw machines, centrifuges and other air compressors, in addition to the waste heat recovery of its internal structural transformation, it is also possible to directly transform the cooling water pipeline to realize waste heat without changing its body structure. Recycling.
1) Schematic diagram of waste heat recovery of water-cooled air compressors.
Fig. 9 is a schematic diagram of waste heat recovery of an air compressor using a water source heat pump for energy boosting. It can be seen from the figure that the cooling water is introduced into the water source heat pump main unit by installing a secondary pump on the cooling water outlet pipe of the air compressor, and the temperature sensor of the main evaporator inlet adjusts the electric three-way regulating valve in real time to control the inlet temperature of the evaporator. At a certain set value, 50-55 °C hot water can be prepared for life and production process through the water source heat pump unit.
If there is no high-temperature hot water demand, a plate heat exchanger can be connected in series with the air compressor circulating cooling water circuit. The high-temperature cooling water exchanges heat with the soft water from the soft water tank, which reduces the internal water temperature and increases the external water temperature. The heated water is stored in the hot water storage tank and then transported to the heat network for use at a low temperature heat source. As shown in Figure 10, when the heat of the cooling water is utilized, the load of the water cooler is greatly reduced, or even A cooling tower is needed again. However, in order to ensure the safe and reliable operation of the system, the cooling tower will still be retained, but the control valve in the normally closed state is installed on the inlet and outlet pipes of the cooling tower. The control valve adopts temperature control. Once the waste heat recovery system fails, the internal water temperature rises to a limited temperature, the valve opens, and the original cooling tower cooling system is put into use, ensuring reliable cooling of the air compressor.
2) Analysis of waste heat recovery of water-cooled air compressor
It is simple and convenient to directly transform the cooling water pipeline of the air compressor, and it does not modify the structure of the air compressor body, which can reduce the risk of reconstruction and increase the reliability of the operation of the waste heat recovery system.
