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While air compressors are widely used, their energy consumption has become the main issue of increasing concern to all units。In the energy consumption of air compressors, the largest piece is the consumption of electricity, accounting for 77% of the total consumption, followed by maintenance costs, accounting for 18% of the total consumption, and equipment investment only accounts for 5% of the total cost。It can be seen that the power consumption of the air compressor is very amazing。An air compressor, as little as tens of thousands of dollars, more than millions, but the purchase cost is only 5% of the proportion, it can be seen that its power consumption figures are huge。Therefore, to find the reasons for the energy consumption of air compressors, targeted solutions can improve energy efficiency。

1. Common heat recovery methods and applicable places of screw air compressors

Waste heat generally refers to the excess heat generated by the air compressor during the production of high-pressure air。Waste heat recovery is to recover the heat generated in the process of air compression to heat air or water through appropriate means such as heat exchangers, typical uses such as auxiliary heating, process heating and boiler water preheating。Currently commonly used including oil injection screw air compressor waste heat recovery system and oil-free screw air compressor waste heat recovery system。

1.1 Common cooling methods of screw air compressors

At present, there are two common cooling methods for air compressors: air cooling and water cooling。When selecting the cooling method of the air compressor, it should be analyzed and confirmed according to the local meteorological conditions and the water quality of the cooling water source。The details are shown in Table 1。

Figure 1 Selection of cooling mode of screw air compressor

In contrast, due to the evaporative cooling method of water cooling, the cooling water can generally be obtained below the air dry bulb temperature, and the specific heat and density of water are larger than that of air, and the cooling effect of air compressors is good, and it is generally more used。

1.2 Screw type emptyPress waste heat recovery system classification

Today, with the increasing shortage of energy, the application of waste heat recovery system of screw air compressor is more and more。According to the different cooling methods of screw air compressors, the heat recovery system is generally divided into two types: air-cooled air compressor waste heat recovery system and water-cooled air compressor waste heat recovery system。

At present, most of the manufacturers use water cooling system to cool the compressor, coupled with water cooled air compressor waste heat recovery system recovery heat stability, the hot water application site is wide, the compressor cooling effect is better, so the transformation of water cooled air compressor waste heat recovery system is more widely used。

1.3 Advantages of waste heat recovery of screw air compressor

The advantages of air compressor waste heat recovery system are mainly as follows:

a) Green environmental protection, renewable energy, energy saving, no emissions;

b) Improve the welfare level of staff, and have sufficient cost-free hot water to meet the use;

c) Reduce the operating temperature of the air compressor and increase the gas production of the air compressor;

d) Extend the use environment of air compressor oil and reduce the operation failure of air compressor;

e) Saving domestic hot water or heating hot water production funds, reducing the operating costs of production links;

f) Promote national policies to save energy and reduce emissions while reaping benefits。

2, air-cooled air compressor heat recovery system

The heat recovery system of air-cooled air compressor mainly uses the heat of heat exchanger to heat outdoor air and get hot air for hot spot use。

The specific application mode of heat recovery of air-cooled air compressor is shown in Figure 2。

Figure 2 Heat recovery of air-cooled air compressor

In winter, adjust the opening of air valve 1 and air valve 2 as needed, mix the waste hot air from the compressor with outdoor cold air, meet the requirements of heating temperature, and directly send to the office, production workshop and other hot spots to achieve heating hot spots。

When heating is not required in summer, the wind valve 2 is closed and the waste hot gas is directly discharged to the outside。

This system has the advantages of simple structure, low investment and low maintenance cost, which saves part of the cost of winter heating。Suitable for the hot spot distance from the air compressor station into the place。

3, water cooled air compressor waste heat recovery system

3.1. Transformation and application of heat recovery of oil injection screw air compressor

For the oil screw air compressor，Compressor working process，After outdoor air is filtered into the compressor，Be compressed，The temperature rose sharply: at the same time，The high-speed rotation of the screw of the air compressor will also generate a lot of heat because of friction，This heat causes the oil and gas mixture from the air compressor to rise in temperature，It usually reaches 80℃ ~ 100℃。Ultimately all the enterprise needs is air pressure，In order to ensure the normal operation of the air compressor temperature requirements，Excess heat needs to be cooled quickly by a cooling system，If there is no heat recovery system，Excess heat is released into the atmosphere through the cooling system，It's not just extra power，And it causes waste heat pollution。

Therefore, many old factories have reformed the heat recovery of the original oil screw air compressor to rationally apply this part of the excess heat;In many new plants, the heat recovery system is directly added from the beginning of the design, reducing operating costs, and playing a role in energy saving and emission reduction。

3.1.1 Oil injection screw air compressor recovery system

The main factors affecting the waste heat recovery of oil-injected screw air compressor include: exhaust temperature, oil injection temperature and oil injection amount of screw air compressor [2], and the heat recovered by the waste heat recovery system mainly comes from high-temperature oil。The specific transformation process is to replace the cooling system through the cooling tower with the waste heat recovery system on the premise of not changing the original working state of the air compressor. Through the waste heat recovery system, the high-temperature oil and gas are cooled, and the hot water obtained is used by the hot spot, as shown in Figure 3。

Figure 3 Waste heat recovery system of oil screw air compressor

As shown in Figure 3，Outdoor air passes through the filter，Enter the air compressor for compression，In the compression process，Air is mixed with lubricating oil，A high-temperature mixture of oil and gas is obtained;The oil and gas mixture enters the oil and gas separator，Get high temperature oil and gas，The high-temperature gas is cooled and used by the plant，The high-temperature oil is cooled and then re-entered the compressor for use。When the waste heat recovery system is not used, the high-temperature oil is cooled directly into the cooling system composed of cooling towers.After adding the waste heat recovery system, the high-temperature oil is first cooled by the waste heat recovery system, and in the cooling process, the hot water obtained enters the heat storage tank, which is used when hot water points such as life and production processes are needed。When the waste heat recovery system cannot cool the high-temperature oil to a reasonable temperature, the cooling system composed of the cooling tower cools the high-temperature oil, so that the oil is cooled by two cooling systems, which can ensure that the oil temperature is more reasonable, ensure the efficient and stable operation of the air compressor, and also realize the energy saving of the main engine。

3.1.2 Evaluation of waste heat recovery system of oil injection screw air compressor

Advantages of this system: It can more efficiently cool the high-temperature lubricating oil produced by the screw air compressor，The guarantee of normal working oil temperature when the air compressor is working is more stable，Improve the efficiency of air compressor;In addition，It not only saves energy，Reduced waste heat pollution，Hot water is also produced for use by enterprises，It has brought good economic and social benefits to the enterprise。

The shortcomings of this system: when preparing hot water, the heat exchange temperature difference is large, which makes the heat exchanger easy to scale, affecting the heat exchange efficiency;It is not easy to realize automatic oil temperature control for the whole waste heat recovery system。

3.2. Transformation and application of heat recovery of oil-free screw air compressor

At present, the compressed air obtained by the oil-free screw air compressor mechanism is clean and oil-free and is widely used。For oil-free screw air compressors, the filtered outdoor air enters the compressor for compression, and the high temperature and high pressure compressed air is obtained, and the temperature is between 170 and 190。In order to ensure the normal operation of air compressor equipment，Air compressor lubricants and high temperature compressed air need to be cooled，Generally, the open water cooling system with cooling tower is used to cool the high temperature compressed air and high temperature lubricating oil，Finally, the excess heat is discharged into the atmosphere through the cooling tower，Not only a waste of resources，It also caused waste heat pollution。Therefore, the oil-free screw air compressor has also been transformed to heat recovery, and this part of the heat is applied to life and process water, which not only reduces the daily expenditure for the enterprise, but also reduces the pollution to the environment。

3.2.1 Application of primary heat transfer in oil-free screw air compressor

The main factors affecting the waste heat recovery of oil-free screw air compressor are: oil free screw air compressor exhaust temperature, lubricating oil temperature and exhaust volume, etc. The heat recovered by the waste heat recovery system mainly comes from high temperature compressed air。The specific transformation process is to replace the cooling system through the cooling tower with the waste heat recovery system on the premise of not changing the original working state of the air compressor. Through the waste heat recovery system, the high-temperature oil and gas are cooled, and the hot water obtained is used by the hot spot, as shown in Figure 4。

FIG. 4 Primary heat transfer waste heat recovery system

As shown in Figure 4, the outdoor air is filtered and compressed into the compressor to obtain high temperature compressed air。In the non-residual heat recovery system, the high temperature compressed air is exchanged with the cooling water after cooling by the cooling tower, and the compressed air with the required temperature is obtained for the use of the plant。After adding the residual heat recovery system，According to specific needs，The high temperature compressed air is first exchanged with the heat exchanger in the waste heat recovery system，To meet the requirements of hot hot water used in waste heat recovery system;When the hot water used in the hot spot is not enough to carry away the heat lost by the high temperature compressed air to reach the required temperature，Cooling system operation with open cooling tower，The excess heat is lost to the atmosphere。In this way, the high temperature compressed air is cooled by two cooling systems, which can ensure that the high temperature compressed air is at a more appropriate temperature, ensure the stability of the high temperature compressed air outlet temperature, and realize the energy saving of the main engine。

3.2.2 Evaluation of waste heat recovery system for primary heat exchange of oil-free screw air compressor

The advantages of this system: it can better ensure the output temperature of high temperature compressed air, reduce the waste heat pollution, and make hot water for the enterprise. Compared with the waste heat recovery system of oil injection screw air compressor, it can get higher temperature hot water, and the application range of hot water is wider, which has brought good economic and social benefits for the enterprise。

Disadvantages of this system: the heat transfer temperature difference of the heat exchanger in the waste heat recovery system is large, and the compressed air temperature is too high, which makes the scale of the heat exchanger serious, which greatly affects the heat exchange efficiency, and requires frequent cleaning, increasing the maintenance cost。

3.3.3 Application of secondary heat transfer in oil-free screw air compressor

Because the structure of the waste heat recovery system of the oil-free screw air compressor is very serious, the waste heat recovery system of the oil-free screw air compressor is more of a secondary heat exchange waste heat recovery system, as shown in Figure 5。

Figure 5 Waste heat recovery system of secondary heat exchange of oil-free screw air compressor

As shown in Figure 5, the secondary heat exchange and waste heat recovery system of oil-free screw air compressor is also added to the softened water circulation system on the basis of the primary heat exchange and waste heat recovery system of oil-free screw air compressor. Therefore, the heat exchange of high temperature compressed air and softened water will not cause serious scaling during operation when the temperature difference of heat exchange is large。This can be worth a higher temperature of softened hot water, can improve the overall heat recovery heat transfer efficiency。

As shown in Figure 5, like the secondary heat exchange waste heat recovery system of the oil injection screw air compressor, the softened water system and the open cooling tower cooling system should not run at the same time。Of course, it is also possible to add a new heat exchanger and high temperature air to the softened water circulation system for heat exchange as shown in Figure 3, and the two can run at the same time, but the scaling phenomenon is not obvious when the softened water and high temperature air heat exchange, and the addition of a new heat exchanger will bring about an increase in initial investment, which is not very economical。Specific needs to consider the actual situation of the enterprise to determine。

3.3.4 Evaluation of waste heat recovery system for secondary heat exchange of oil-free screw air compressor

The advantages of this system: in the heat exchange with high temperature and high pressure air, the heat exchanger is not easy to scale, better ensure the heat exchange efficiency of the heat exchanger, so that the air compressor can work more stable and efficient。