Discussion on the Energy-saving Nature of Air Compressor Suction Pretreatment for Air-jet Looms

Air Compressor as the main power source of air-jet looms W, has high quality requirements, large air consumption and high energy consumption, so the air pressure station has become the main energy-consuming part of the weaving workshop, which has a direct impact on the production cost of the weaving process. Excessive impurities and moisture in the compressed air will cause impurities along with moisture to adhere to the nozzle, causing nozzle blockage and insufficient power to draw in the weft, which is easy to cause weft shrinkage, it may also produce a lack of weft defects due to the weft blowing the opposite side of the weft yarn, thus affecting the product quality;

Secondly, moisture also causes contamination and rusting of the reed, nozzle and other loom parts, thus affecting the service life. In response to the above problems, many scholars have conducted different energy-saving modifications as well as studies and experiments on reducing the water content of compressed air. Considering the overall energy consumption of the system, we can handle the suction parameters of the centrifugal air compressor by an air conditioning unit to arrive at the optimum range for suction temperature reduction. If using the cooling air of spray room and compressed air to dry the air to reduce the air temperature and moisture content at the suction port of the air compressor, 3.5% of compressed air is consumed as desiccant. Our company analyzes the influenced law of suction temperature and humidity content on the energy consumption of air compressor by testing and using the concept of specific energy of air compressor.

In order to clarify the influence of the temperature and relative humidity of the inhaled air on the energy consumption of the air compressor and the water content of the compressed air, our company, according to the basic principle of the work of the air compressor, as well as the actual state of the gas in the compression and cooling process of the change of the law of the state at all stages, through theoretical calculations to draw the relationship between the state of the inhalation of the air compressor energy consumption, and to find out the main factors influencing the energy consumption of the air compressor and the moisture content of the compressed air.

Taking the screw air compressor station room without oil lubrication over of air-jet loom as an example, on the basis of analyzing the influence of suction parameter on the energy consumption of air compressor and the quality of compressed air, we can adopt the workshop environment air-conditioning room to send air to the air compressor to carry on the transformation of suction pre-treatment, lower the temperature and relative humidity of the inhaled air, reduce the energy consumption of the air compressor during the working process, alleviate the workload of the cold dryer, and decrease the water content of the compressed air in the summer, so as to improve the efficiency of the air-jet weaving machine and the quality of the product.

1. Effect of suction parameters on energy consumption of air compressor

1.1 Theoretical power of compressor compression process

The actual gas compressed by the air compressor is wet air, water vapor in the wet air is in a superheated state, close to the ideal gas. As engineering thermodynamics can be seen, the compression process is a multivariate process, the screw air compressor compresses the mass of wet air within a unit of time G, the compressor needs to consume the theoretical power N as shown in formula (1).

In the above equation: N is the theoretical power to be consumed by the multivariable process compressor (W) ,P1 and P2 are the gas in the suction state pressure and compression to the bottom of the absolute pressure (Mpa), respectively; m is the multivariate index of the compression process,m=1.4; T1 is the suction temperature (K); G is the mass of air compressed per unit time (kg/s); R is the compressed gas constant  [J / (kg*K)], the gas constant of the compressed wet air and the partial pressure of water vapor in the air, it can be calculated by formula (2).

In the above formula: 8314 is the universal gas constant [J/(kg-K)]; B is the local atmospheric pressure (Pa); Pd and Pv are the dry air partial pressure and water vapor partial pressure in air (Pa), respectively, where Pd=B-Pv; Md and Mv are the molecular weights of the dry air and water vapor, respectively, and Md=28.97,Mv=18.02.

From equation (1) and equation (2), it can be seen that under the situation that other conditions are the same, the theoretical power consumed to compress a certain mass of gas increases with the suction temperature, the higher the water vapor content in the air, the higher the partial pressure of water vapor; the higher the gas constant of wet air, the higher the power consumed in the compression process.

1.2 Theoretical power of wet air compression process with different state parameters

Take for example an oil-free lubricated compressor with standard displacement = 40  Nm³/min, displacement pressure = 0.7 MPa and installed power = 250 kW, the local atmospheric pressure = 98,907 Pa, design of compressor suction volume based on standard conditions ( at 20 ℃, relative humidity at 50% ), the theoretical power of the air compressor compressing the same mass of air under different suction states is calculated by using equations (1) and (2) respectively, as shown in Table 1. The rotational speed of the air compressor is more than 500 r/min, and the multivariate index in the compression process m=1.4.

From Table 1, it can be seen that the calculated theoretical power of the air compressor in compressing the same mass of gas when the relative humidity of the suction gas is the same increases with the increase of the suction temperature. When the suction relative humidity is 50%, the temperature increases from 20 ℃ to 40 ℃, the theoretical power increases by 7.9%, that is, for every 10 ℃ increased, the energy consumption increases by about 3.9% on average; When the suction relative humidity is 80%, the temperature increases from 20 ℃ to 40 ℃, the theoretical power increases by 8.5%, that is, for every 10 ℃ increased, the energy consumption increases by about 4.3% on average. It can also be seen from Table 1 that the calculated theoretical power for compressing the same mass of gas increases with the increase in suction relative humidity when the suction temperature is the same. When the suction temperature is 20 ℃, the relative humidity increases from 50% to 80%, and the calculated theoretical power increases by 0.27%, that is, for every 10 percentage points of increase in relative humidity, the energy consumption increases by about 0.09% on average; When the suction temperature is 40 ℃, the relative humidity increases from 50% to 80%, the calculated theoretical power increases by 0.87%, and the energy consumption increases by about 0.29% on average for every 10 percentage points increase in relative humidity.

From the above analysis, it can be seen that during the process of compression of the same mass of gas, with the temperature of the suction gas increases, the value of the energy consumption of the air compressor increases more, with the relative humidity increases, the value of the energy consumption increases but is not obvious. This is mainly due to the fact that during the theoretical power calculations, the volumetric mass of air is limited due to the small percentage of water vapor content in the air, while wet air compression process are in a superheated state, according to the ideal gas calculation, the specific heat of air in the compression process will be taken as a fixed value, so as not to take into account the impact of water vapor in the compression process caused by the change of state.

2. Status of the air pressure station

Take the example of an air pressure station supplying air to a company’s air jet loom workshop, 240 sets of ZAX-190 air-jet looms with an installed power of 4.77 kW, a single air consumption of 0.9 Nm³/min and a pressure of 0.55 Mpa are installed in the workshop. The water content of compressed air is required to reach the pressure dew point temperature of 4 ℃ ~ 7 ℃. The air pressure station is located in the annex near the weaving machine shop and is equipped with 4 sets of ZR400 water-cooled, oil-free fixed speed screw air compressors (three for use and one for backup), each unit has a single gas production capacity of 62.9Nm³/min, a gas supply pressure of 0.75MPa, and an installed power of 400kW; four SLAD-80NW water-cooled refrigeration dryers with an installed power of 13.5 kW are installed at the back. It is equipped with one ZR315 water-cooled and oil-free frequency conversion screw air compressor with a single production capacity of 50.2 Nm³/min, a set supply pressure of 0.75 MPa and an installed power of 315 kW; one 12.5 kW SLAD-60NW water-cooled refrigeration dryer is installed at the back. Under normal production conditions, the air pressure station is equipped with a separate cooling tower and circulating water system, and the joint operation of 3 sets of fixed speed screw air compressors and 1 set of frequency conversion air compressor can meet the requirements of production gas. The installed power of the whole workshop is 2,799.8kW, and the installed power of the operating equipment of the air pressure station is 1,655kW, and the installed power of the operating equipment of the air pressure station is 59.1% of the total power of the workshop.

The rotary screw air compressor and the main unit of the cold dryer are cooled by circulating water, and the cooling effect is better in summer. However, the ventilation of the air pressure station room is used to open the air inlet under the outer wall, and the axial flow ventilator is set above to exhaust the air, and the cooling effect of ventilation is not good. In the operation process, due to the direct use of outdoor fresh air ventilation cooling, and in recent years in central China in the summer for a long time in the high temperature and high humidity, the inlet and exhaust air inlets are close to each other at the top and bottom, and the temperature difference is small, coupled with the heat dissipation of the motor in the air pressure station and the high temperature radiation of the enclosure, which result in high air temperatures and high relative humidity in the air pressure station. In summer, the temperature of the air pressure station can reach up to 45 ℃ or more, and the relative humidity reaches 70% in the high humidity season. As a result, the actual energy consumption of the air compressor increases in the high temperature and high humidity season, even the exhaust temperature is too high, which will touch the high temperature protection shutdown, and affect the normal production of the workshop.

As the site where the project is carried out is located near the Yellow River beachland, the surrounding environment is influenced by the Yellow River wetland factors, and the relative humidity in the air is high during the high temperature and high humidity seasons in summer for a long period of time. The cold dryer is often in overloaded working condition, resulting in the compressed air dehumidification effect is not good. The storage tanks are located outside the air pressure station, where the ambient temperature is high, the cooling effect is limited, and the water content in the compressed air is high, it is easy to have the phenomenon of water accumulation at the end of the compressed air dry pipe in the weaving machine workshop, and the dampness and rust at the weft-inducing nozzle and the attachment of flecks will affect the stability of the weft-inducing air pressure and the effect of the weft-inducing effect, and produce defects, which is not conducive to the normal production of the workshop. High water content in compressed air is also a major cause of damage to the regulator solenoid and auxiliary nozzle. The period from June to September is also the time of the year when the most damage and replacements are made to solenoid valves, auxiliary nozzles, and other weft-inducing specialties.

3. Factors affecting the energy consumption of air compressor and compressed air moisture content

3.1 Suction temperature

From the operation observation, it can be found that the operating power of the fixed speed operation air compressor during the summer with high temperature does not increase a lot more than that with the normal temperature, but the operating power of the frequency conversion air compressor increases more. This is mainly due to the fact that as the suction temperature rises, the volume and mass of the inhaled air decreases, and the actual suction volume of the compressor operating at fixed speed frequency decreases. In the subsequent cooling down process, reflected in the air compressor air supply is reduced, then it is necessary to increase the frequency of operation of frequency conversion air compressor, to supplement the fixed speed screw air compressor to reduce the supply of air. This leads to the following conclusion:

producing the same mass of compressed air, the actual energy consumption of the compressor rises as the suction temperature rises. It can also be understood that as the suction temperature rises, the air compressor’s gas production drops, then you need to frequency converter to increase the frequency to supplement the gas supply, so the frequency converter’s operating power increases. From this phenomenon can be derived from the change rule of energy consumption of air compressor when the temperature of air intake air increases.

3.2 Suction water content: Air Compressor

During the compression process of an air compressor, as the air is compressed and the pressure rises, the amount of water molecules that can be held in the air decreases, but as the temperature of the air rises during compression, the amount of water vapor that can be held in the air increases. Therefore, the moisture is not condensed and precipitated during the compression process. During the high temperature and high humidity season, the air compressor inhales the high temperature and high humidity air after compression, the volume of the gas becomes smaller and the water content per unit volume increases. As the compressed air is cooled, a large amount of moisture in the air condenses and precipitates out, and in the case of poor aftercooler cooling, the temperature of compressed air entering the cold dryer rises and the water content increases, which will cause the cold dryer to be overloaded, the dehumidification effect decreases, and the water content in the compressed air increases.

Although the storage tanks can separate out condensed moisture, they are ineffective at eliminating gaseous moisture, and when combined with the high temperatures in which the storage tanks work in the summer, the moisture that can be held in the compressed air is relatively high. This compressed air with a certain water content enters the workshop, and after entering the compressed air pipeline below the loom, the temperature decreases, and some of the water vapor molecules in the air condense, accumulating water at the end of the dry pipe in the workshop. When compressed air containing water is sprayed from the loom nozzle, the water precipitates at the nozzle, causing condensation and humidity at the nozzle, attaching dust and rust, and affecting the working effect of the nozzle.

The moisture content state of the air intake air of the air compressor has a great influence on the moisture content of the compressed air, take the air compressor with a gas production capacity of 62.9 Nm3/min and a supply air pressure of 0.75 Mpa as an example, and assume that its aftercooler exhaust temperature is 40 ℃, and the cold dryer cools it down to a pressure dew point of 4 ℃. Calculate the displacement of water in each stage of compression and cooling at state 1 (20°C of intake air, 60% relative humidity) and state 2 (45°C of intake air, 65% relative humidity), respectively. According to the equation of state of the air compression process, the formula for calculating the water content of the air compressor’s suction and discharge air are shown in formula (3) and formula (4), respectively, and the formula for calculating the drainage volume is shown in formula (5).

In the above formulas: V1, V2, d1, d2 are the volume of process air (m³) and air water content (g/m³) before and after compression or cooling, respectively; Φ1、Φ2、ρsb1 and ρsb2 are the relative humidity (%) and the saturated water vapor volumetric mass (g/m³) of the air before and after compression or cooling, respectively; and G is the volume of drainage (g).

Combined with the saturated wet air state parameter table, calculate different suction state compressor compression process and post-cooling and drying of each stage of the water required to be discharged, the results are shown in table 2:

From the above table, it can be seen that in the compression cooling process, the aftercooler bears the main task of cooling the high temperature air after compression. Compressing the same volume of gas, due to the high suction temperature and relative humidity, the water content of the suction air is large, and the aftercooler needs to separate out a large amount of water in the cooling process. Suction state 2 requires 9.13 times more water to be separated than suction state 1, greatly increasing the aftercooler’s workload. Coupled with the fact that the water vapor in the after-cooler from the gas state condensed into a liquid state but also to release a large amount of latent heat of vaporization, it will make the after-cooler cooling effect decreases, the exhaust temperature increases, the dew point of the compressed air rises, the water content increases.

This increases the workload of the cold dryer and reduces the dehumidification effect of the cold dryer. Taking the inspiratory state 2 as an example, if the exhaust temperature of the aftercooler is increased to 55°C, which corresponds to an air saturated water vapor volume mass of 82.77 g/m3, the water content of the air in the aftercooler is 322.33 g/m3, and the aftercooler unit gas drainage is 322.33 – 82.77 = 239.55 (g/m3), with a reduction in drainage of (286.79 – 239.55)/286.79 = 16.5 (%).The unit gas drainage of the cold dryer is 82.77 – 6.35 = 76.42 (g/m3), and the unit gas drainage increases by (76.42 – 51.34)/76.42=32.8 (%).This makes the workload of the cold dryer much larger and the dehumidification effect is reduced.

A large amount of condensate If it cannot be discharged in time, it will flow into the pre-cooler of the cold dryer along with the compressed air, so that the water content of the compressed air increases and the actual pressure dew point rises. This is the main reason for the high temperature and high humidity season in summer, resulting in high water content of compressed air. Through the above analysis and calculation, it can be seen that the increase in the suction temperature of the air compressor is the main reason for the increase in energy consumption; An increase in suction water content will mainly lead to an increase in compressed air water content, in addition to a small increase in energy consumption.

4. Suction pre-treatment measures: Air Compressor

The main reason for the high energy consumption of the air compressor and the high moisture content of the compressed air that occurs in the summer is the high temperature and high moisture content of the inhaled air. Therefore, reducing the inhalation air temperature, is conducive to the reduction of energy consumption of the compressor and the improvement of the working condition; reducing the moisture content of the inhalation air can reduce the compressed air cooling equipment workload, drying effect to improve the efficiency of the weaving machine, reduce peccadilloes. For the actual situation of the project’s air compressor room, taking into account the air pressure station above the loom workshop preparation process air conditioning air supply system, summer workshop air conditioning air supply dew point parameters:

temperature 22 ℃, relative humidity 60%. Adopting air-conditioning air supply to treat the suction air of the air compressor to reduce the suction air temperature and humidity content can reduce the energy consumption of the air compressor and the workload of the cold dryer. In order to reduce the investment, our company directly organizes the process: The air conditioner sends filtered air into the air compressor room, which uses galvanized steel ducts to bring in the air directly. The wind then passes through the spray room treated low-temperature air to the vicinity of the suction port of the air compressor.

A state of 22 °C suction temperature and 60% relative humidity was formed, and the volume of air introduced was 15 000 m³/h. Due to the inconvenience of compressed air flow measurement, it is used to measure and compare the energy consumption of the air compressor and the pressure dew point of the compressed air under normal production conditions with the same workshop production and pipeline network pressure parameters. The energy consumption and cold dryer operation before and after testing the air compressor using suction pretreatment in the hot summer season are shown in table 3.

From the above table, it can be seen that after the use of suction pretreatment, the actual air supply of the fixed speed screw operating air compressor increases, and the energy consumption of the operating air compressor changes slightly. However, inverter compressors have a large reduction in energy consumption due to the reduction in actual air supply. The energy consumption of the cold dryer is slightly reduced and the total energy consumption of the air pressure station is lowered. Under normal production conditions, the total energy consumption of the air pressure station before and after suction pretreatment can be reduced by 89.55 kW, minus the energy consumption of air treatment and blowers, which is about 8.6 kW, the actual energy saving is 80.95 kW. Comprehensive energy-saving effect: (1,231.3-1,141.75-8.6)/1,231.3 = 6.6 (%). That is, the equivalent suction temperature of every 10 ℃ lower, the integrated energy saving 2.9%. The dew point temperature of compressed air reaches 4 ℃, which improves the quality of compressed air and slows down the contamination and rusting of the weft-inducing nozzles of air-jet looms.

Comparison of different outdoor meteorological parameters and the results of regular testing of the total energy consumption of the operation of the air pressure station shows that the use of suction pretreatment technology in the summer high temperature season (May 20-July 20) can reduce the suction temperature by 15 ℃ on average, realizing the comprehensive power saving of the air pressure station by 4.35%, and the energy-saving effect of the air compressor is obvious; In the high humidity season (July 20 – September 20) using suction air pretreatment technology can reduce the suction air temperature by 12 ℃ on average, realizing a reduction of 3.48% in the comprehensive energy consumption of the air pressure station, but the water content of compressed air is greatly reduced, and the failure rate of the workshop is reduced. When the average outdoor temperature is lower than 20 ℃, the suction pre-treatment equipment is stopped, and the annual operation season is calculated on the basis of 120 days, an average energy saving of 3.92% can be realized, and the air pressure station can realize a power saving effect of 139,000 kW-h, and the index of the compressed air water content has been improved greatly.

5. Conclusion

The compressed air used in the air-jet loom workshop has high consumption and high quality requirements, and the power of the equipment running the air pressure station accounts for 59.1% of the total power of the loom workshop, so the air pressure station consumes a lot of energy. In the summer high temperature and high humidity season, the air compressor suction temperature has a great impact on the energy consumption of the air compressor, and the suction moisture content has a great impact on the water content of the compressed air.

Cooling and dehumidifying the air intake air of the air compressor can effectively reduce the energy consumption of the air compressor and the water content of the compressed air, and improve the productivity of the workshop. Taking the conditions of the tested area as an example, with the suction pretreatment technology, the energy consumption of the compressor is reduced by an average of 2.9% for every 10 °C reduction in the equivalent suction temperature, taking into account the cost of pretreatment. The use of suction pretreatment measures can reduce the operating energy consumption of air compressors and improve compressed air quality. Read more.