Application of Low-pressure Frequency Conversion Technology in the Energy-saving Transformation of Air Compressors
2023-11-19
Technical Term:
- Fixed speed screw air compressor : also called industrial screw air Compressors or industrial frequency screw air compressor.
- Frequency conversion screw air compressor: also called variable speed screw air compressor or variable frequency screw air compressor.
- Permanent magnet frequency conversion screw air compressor: also called PM VSD screw air compressor, permanent magnet variable speed screw air compressor or permanent magnet variable frequency screw air compressor.
Air compressors are indispensable in industrial production, whether using manual, semi-automatic or automated production equipment, air compressors all play a key role，providing a boost to healthy production activities. The common air compressor is the fixed speed screw air compressor (also called industrial screw air compressor)，which has a stable and smooth airflow output, low failure rate, low noise, adapt to the high load for a long time and convenient unattended operation and other advantages, so fixed speed screw air compressor are widely used in the major enterprises. However, against the backdrop of today’s energy constraints and the high cost of electrical energy, fixed speed screw air compressor energy consumption has become a problem for enterprises to reduce operating costs.
We have a metallurgical industry customer, the compressed air used in the annealing equipment is mainly supplied by the fixed speed screw air compressor (also called industrial screw air compressor), and a pressure reducing valve is added at the front end of the equipment to reduce the pressure to 0.1 MPa and then fuel the natural gas. The gas consumption of combustion equipment accounts for more than 80% of the total gas consumption, and the production of equipment according to the adjustment of production capacity, the daily consumption is in a fluctuating state, gas consumption is unstable, the application of low-pressure frequency conversion air compressor (also called variable speed air compressor) can effectively promote the improvement of gas energy saving of combustion equipment. Our company helped the customer to transform the existing air compressor into two systems, i.e. low pressure and high pressure, the low-pressure system uses 0.3 MPa low-pressure frequency conversion air compressor to supply gas for combustion equipment, considering the cost return rate of the high-pressure system renovation, to supply air for pneumatic equipment, we did inverter treatment to the existing 0.8 MPa industrial frequency air compressor(also called fixed speed screw air compressor), and the existing piping is modified according to the calculation, so as to achieve the effect of energy saving, this transformation is expected to achieve energy-saving effect of 40% or more.
1. The operational state of conventional air compressor
The operational state of conventional air compressor is divided into start-up state, loading & unloading state, and no-load state.
Start-up state: The work control of the fixed speed screw air compressor (also called industrial screw air compressor) adopts star-delta starting, loading & unloading control for pumping. Star-delta start instantly, the starting current can be up to 5 to 8 times the normal unit, fluctuations are large, which increases the mechanical failure rate of the equipment, affecting the service life of the equipment, energy waste is also larger.
Loading & unloading state: It is the operating state of the equipment when the gas pressure reaches the minimum or maximum value of the set value. In the loading state, after the pressure reaches the minimum value, the air compressor runs and raises the pressure to the maximum pressure value. Pressurization contains two parts of the energy loss, one is, the pressurization process releases energy to the outside world, with a large energy loss; another is, when the pressure is higher than the maximum pressure value, it is depressurized by a pressure reducing valve, which is also consuming energy.
No-load state: It means that when the pressure reaches the maximum value, the air compressor closes the air inlet valve, so that the motor is in an idle state, and at the same time, the excess compressed air in the separator tank will be emptied through the bleeder valve, which also consumes energy in this part.From the way the traditional air compressor operates, energy is wasted both loaded and unloaded.
Our customer set the upper and lower limits according to the lowest value of the pneumatic pressure of the production equipment end to determine, set the lower limit of the output pressure of 0.6MPa, that is, the beginning of the loading pressure of the air compressor, normal at the lowest value plus about 0.1MPa as the upper limit of the compressor output (take 0.7MPa) that is, the pressure to start unloading. The output working pressure of the air compressor is directly proportional to the power consumption, and its working pressure fluctuates between the upper and lower limits, the higher the operating pressure, the higher the energy consumption, i.e. between a pressure difference of about 0.1 MPa will consume 10% more of the total power.
2. Advantages of low-pressure frequency conversion air compressor
Frequency conversion air compressor use pressure sensors to detect the actual air pressure and air consumption in the system, and realize the instant control of the motor speed (output power) without changing the torque of the air compressor motor (load capacity) through the electrical control and frequency conversion control, by changing the compressor’s rotational speed to change the pressure variation and maintaining a stable pressure (set value), the purpose of outputting high-quality air on demand is achieved. The principle of control is the same as that of adding frequency converters in energy saving of fan motors and pumps, which controls the input voltage frequency according to the load change. When the system air consumption decreases, the compressor provides more air than the system consumption, the inverter-type compressor reduces the speed and decreases the output air volume; conversely, the speed is increased to increase the air volume and maintain a stable system pressure. Technicians in the selection of air compressors according to the maximum required system air consumption and then add a safety factor to select the appropriate model, and the actual use of the number of openers need to change according to the production order, when the operator is in charge of several machines, the production time from the beginning to the start-up of all machines may reach half a day, the selected air volume is higher than the actual usage, therefore, the use of low-pressure frequency conversion air compressor can improve efficiency and greatly save energy loss.
In addition to the advantages of frequency conversion and energy saving, the low-pressure frequency conversion air compressor also has the function of constant pressure, which can stabilize the up and down pressure fluctuation of the pipeline, so that the system can operate at a lower pressure to meet the production requirements, reduce the power loss caused by the upward pressure fluctuation. For certain equipment that requires high stability of the flame, the constant pressure output of the inverter air compressor can ensure flame stability, thus improving product quality.
When the PM VSD air compressor (also called PM variable speed air compressor) is selected, the energy-saving effect is more obvious, which has the advantages of high efficiency, high power factor and high starting torque, and there is no rotor winding, so there is no need to excite the rotor, which reduces the consumption of energy. From the point of view of maintenance cost, permanent magnet motor has no bearing, no need to add grease, reducing the bearing failure point, greatly saving the maintenance cost.
The efficiency output curve from no-load to full load of a PM VSD air compressor selected by a company is shown in Figure 1.
3. Application and modification of low-pressure frequency conversion air compressor
The compressed air used by the customer’s annealing equipment is supplied through an industrial screw air compressor (0.7 MPa), and a pressure reducing valve is added at the front of the equipment to reduce the pressure to 0.1 MPa for natural gas combustion, according to the data collection, the gas consumption of the combustion equipment in a factory area reaches 49m3/min, and the actual gas consumption of the pneumatic part, when it is fully loaded, only requires 1 set of 10.2 m3 /h, 55 kW screw air compressor (0.7 MPa) to meet the usage demand. When the pneumatic section is fully loaded, only one “10.2m3 /h and 55 kW” screw compressor (0.7 MPa) is required to meet the actual air consumption. The second and fourth floors of the customer’s company are equipped with air compressor stations, and all of them are industrial screw compressors (0.7 MPa), most of which have been in use for about 10 years, the parts began to enter the aging cycle, in addition to the routine maintenance costs, the annual maintenance costs by 5% to 10% upward incremental cost of $1500 to 3000 per year, moreover, the air compressors happened to be in the overhaul period, and the cost of each overhaul was about $15,000 (including rotor overhaul, oil change, and aging components). The idea of the renovation proposed by our company is to place the air compressor station on the second floor, in which three new low-pressure frequency conversion air compressors are added to supply gas for combustion equipment, and the pneumatic part is supplied by the existing 10.2m3/h screw air compressor. The air compressor station is unified and centrally arranged for easy management and maintenance, and the downtime is shorter, which reduces the impact on the production.
4. The Do’s and Don’ts of Pipe Remodeling
For modification of piping, it is necessary to consider whether the old piping meets the pressure requirements. Normal 0.3MPa low-pressure piping requires a larger pipe diameter than 0.7MPa piping. Pressure loss at the equipment end is mainly reflected in the piping, elbows, tees, valves, etc. Resistance loss causes pressure drop, if the piping diameter is not selected properly, it will cause pressure fluctuations and end pressure insufficiency, affecting the stability of the whole system.
The fourth floor of this renovation uses 900m3/h of gas, so calculate the inner diameter of this section of pipe as:
Where: d is the inner diameter of the pipe, mm; qv is the gas volumetric flow rate, m3/h; u is the average flow rate of the gas in the pipe, m/s, and the flow rate of the air under the pressure of 0.3-0.6 MPa is taken to be 10-20 m/s.
The pressure loss is calculated by the formula:
Where: △p is the pressure drop, bar; q is the volumetric flow rate, L/s; d is the inner diameter of the tube, mm; L is the length of the tube, m; p is the exhaust pressure (absolute pressure), bar (a). The calculation result meets the requirement of supply pipe pressure drop <0.03 bar.
At the outlet of the air compressor, due to the compressor work, the temperature at the outlet of the air compressor is high, and water will be generated in the exhaust pipe, so the pipeline at the outlet end of the air compressor is designed in a way to prevent backflow, and a straight section is reserved at the bottom of the pipeline, and the bottom of the straight section is increased with an automatic drain valve so as to automatically drain the water in the air pipe. When the pipe diameter is selected on the small side, the pressure fluctuation of the end pipe is large, and the pressure will be slightly reduced. According to the calculation, the original need to use DN125, in order not to affect the production and reduce the amount of transformation, retained the end of the fourth floor of the pipeline DN100 as a low-pressure pipe, add DN50 as a high-pressure pipe, large pressure fluctuations at the end and a slight decrease in pressure occurred during operation. Utilizing the gas storage tank at the end of the original fourth floor, the end pipeline is connected to the gas storage tank to solve the pressure fluctuation problem.
5. Comparison of energy saving values
Comparison of the actual measured values of power consumption between the industrial frequency screw air compressor and the PM low-pressure frequency conversion air compressor, taking the comparison of the values of power consumption in the interval in which the equipment maintains the same amount of air compressors on. Permanent magnet low pressure frequency conversion (21.5m3/min, 63kW, 0.3MPa), 14:30 to 15:30 power consumption accumulated as a dotted line, the instantaneous power as a solid line, as shown in Figure 2.
The operating power of the industrial air compressor (10.2m3/min, 55kW) is the loading power and standby power, and the pressure of the output air is the upper limit of 0.7MPa and the lower limit of 0.6MPa, as shown in Figure 3.
A comparison of power consumption of industrial frequency air compressor and frequency conversion air compressor is shown in Table 1.
At 7600h per year, with gas consumption calculated based on existing usage, the annual saving in electricity costs would be about $80,000 (at $0.35 per kWh).
6. Concluding remarks
Frequency conversion technology has been widely used in a variety of occasions, in the field of energy saving, low-pressure frequency conversion air compressor also occupies a certain market share, especially in the combustion equipment and pneumatic use of the occasions, the air compressor station is divided into two air compressor stations of high and low pressure, energy saving effect is obvious. Our company has conducted an in-depth discussion on the application of low-voltage frequency conversion in the energy-saving transformation of air compressors, combined with the characteristics of the customer’s existing compressed air use, pilot renovation was carried out on the air pressure station of a certain factory, and the energy saving situation was analyzed. The actual energy saving data measured verified the energy saving effect and payback cycle of the low-pressure frequency conversion air compressor, which has now been promoted in the air compressor stations of other factories. Read more.