1. The role of the gas storage tank
1) Gas storage: On the one hand, it can solve the contradiction that the gas consumption may exceed the gas supply in a short period of time in the system, and on the other hand, it can be used temporarily when the air compressor fails or other emergencies (such as power failure). Of course, this depends on how many or how large air receivers are used to store compressed air. 2) Constant current and voltage stabilization: Eliminate or weaken the pulsation of the output airflow of the air compressor, stabilize the pressure of the air source, and ensure the continuous and stable output airflow, that is, the function of constant current and voltage stabilization.
3) Reduce the start-stop frequency of the air compressor: reduce the "start-stop" frequency of the air compressor, and a larger system capacity can extend the "start-stop" or "load-unload" cycle of the air compressor , Reduce the switching frequency of electrical equipment and valves.
4) Removal of pollutants: Use the centrifugation of the air storage tank and the gravity settling of compressed air to separate and remove large particles of water, oil and other pollutants in the compressed air, and further cool the compressed air to reduce other pollution downstream of the pipeline network. Processing equipment workload (can also reduce the investment in purification equipment), so that various gas-consuming equipment can obtain the required quality gas source.
5) Save installation space: For the current popular integrated air compressor, the air storage tank doubles as the compressor body and the installation base for other accessories, which effectively saves installation difficulty and space.
2. Calculation of the volume of the gas storage tank According to the different purposes of setting up the gas storage tank, there are different calculation methods for the volume of the gas storage tank:
1. Actual working condition 1: The pneumatic system selects the volume flow rate of the air compressor according to the average air consumption. During the peak time of air consumption, the air consumption of the system may be greater than the exhaust volume of the air compressor in a short time. To meet the demand for gas consumption during peak hours, the volume V of the gas storage tank can be determined by the following formula:
In the formula:
qmax - the maximum air consumption of the pneumatic system, Nm3/min;
q0--Rated displacement of the air compressor, Nm3/min;
Pa--air compressor suction pressure (absolute), take = 0.1MPa; P1--normal operating pressure of the pneumatic system (absolute), MPa; t--working time of the pneumatic system at the maximum air consumption, s; 2. Actual working condition 2: When the air compressor has a sudden accident or the air suddenly stops due to external reasons (such as power failure), only the air stored in the air storage tank can maintain a safe air supply. In this case, it must be ensured that the air pressure will not drop below the minimum safe pressure for maintaining the normal operation of pneumatic equipment or systems within a certain period of time. The volume of the air storage tank can be calculated as follows:
In the formula:
P2 - the minimum safe working pressure of the pneumatic system (absolute), MPa;
q'--necessary air consumption of the pneumatic system during the power outage, Nm3/min; t--the minimum time for the pneumatic system to maintain normal operation during the power outage, s;
3. Actual working condition 3: The drive motor of the air compressor starts too frequently.
1) It will lead to an increase in the temperature rise of the motor winding and control switch coil, which will aggravate the wear and tear of the mechanical system, especially on medium and large air compressors;
2) The frequent start and stop of the air compressor will increase the energy consumption of the air compressor (the current when the motor starts may be dozens of times the operating power). For this reason, a pressure switch is usually installed on the air storage tank, and the operation of the air compressor is controlled by the pressure difference value (△p) of the speed regulating pressure switch, and the drive motor is "started-up" within a certain pressure difference range. - Shutdown", "full load - no load" or "full load - no load - gap shutdown" and other adjustments. Because of the air storage tank, when the air supply q0 of the air compressor is much larger than the air consumption qk, the driving motor will be in a shutdown or idling state for a long time, and the load startup frequency is not high; when the air supply q0 is close to or less than the consumption When the gas volume is qk, the driving motor will run non-stop for a long time. At this time, the frequency of load startup is not high. The ratio of qk/q0, referred to as the gas consumption ratio, is represented by the coefficient a. At this time, use the concept of gas consumption coefficient to determine the volume of the gas storage tank:
In the formula:
q0--the air supply volume of the air compressor, Nm3/min;
a--air consumption coefficient, as shown in Figure 1, the air consumption coefficient is the largest when the air consumption ratio is 0.5;
f--allowed "loading-unloading" switching frequency, the unit is h-1 (times/hour);
This value is related to the power of the drive motor of the air compressor, the greater the power, the smaller the value of the switching frequency should be.
△p--the differential pressure setting value of pressure opening and closing, △p=pe-pb, pe is the pressure upper limit value when the air compressor starts to unload, and pb is the pressure lower limit value when the air compressor starts to unload.
4. Actual working condition 4: Based on our practical work experience for many years, in the air compression system, the selection of the capacity of the air storage tank is sometimes determined by empirical values, for example, it is selected as 1/(6～ 8). In the calculation of the above theoretical formula, we did not consider the influence of the difference between the temperature (tk) in the air storage tank and the suction temperature (t0) of the air compressor on the volume of the body. In practice, in a high temperature environment or when there is no rear cooler in front of the air storage tank, the temperature inside the air storage tank is much higher than the suction temperature of the air compressor (i.e. the ambient temperature). Affected by the temperature, the gas volume in the tank At this time, the air storage tank should be selected larger; in high altitude areas, because the pressure of the inhaled air is relatively low, the volume of the air storage tank can be selected correspondingly smaller. At this point, the following formula can be used to correct:
Example: An air compressor with a nameplate parameter displacement of 42Nm3/min and an exhaust pressure of 0.7MPa supplies compressed air with a flow rate of 39Nm3/min (pressure of 0.7MPa) to the air network, and the differential pressure switch setting The value is 0.08MPa, how to calculate the applicable gas storage tank volume?
Assumption: the air compressor intake air temperature is 20°C, the suction pressure is 0.1MPa, and the maximum storage temperature of the air storage tank is 40°C. The air consumption coefficient of the system is 0.5 (a=0.25), the motor power of the corresponding air compressor is 250kW, and f is 3, but it is more appropriate to choose 2. Since the suction temperature of the air compressor is inconsistent with the maximum temperature of the air storage tank, the volume of the air storage tank is calculated according to the following formula:
3. Matters needing attention in the use of gas storage tanks
1. When the compressed air pipe network is very large, the volume of the air storage tank can be slightly reduced or even used, because the volume of the longer pipe itself in the pipe network can store enough air. When the gas consumption peaks of multiple gas-consuming equipment are different, the pipeline network itself is equivalent to a natural public gas storage tank.
2. The gas storage tank is a pressure vessel, so you must choose a product from a regular manufacturer, so as to ensure safety, otherwise it will be a time bomb. It must be equipped with safety accessories, such as safety valves and pressure gauges, and regular safety inspections must be done.
3. The installation position of the gas storage tank should be behind the rear cooler, so as to prevent oil vapor and liquid water from accumulating in the tank and reduce safety hazards as much as possible; the tank body must be equipped with high-quality sewage devices, such as automatic sewage valves, Manual blowdown valve + timed discharge and supervision. If the inner wall of the gas storage tank made of carbon steel or other perishable materials is in a humid environment for a long time, it will easily rust, which will not only affect the service life of the tank, but also cause many adverse effects on the corrosion and peelings that will enter the downstream equipment with the airflow, and more importantly, Increase the safety hazard of the gas storage tank. In order to prevent rust, we usually choose stainless steel gas storage tanks in medical, food, electronic and other gas usage scenarios.
Air storage tanks are very commonly used in our daily work. They are simple in structure and easy to be overlooked. However, in the new era when energy saving in compressed air systems is getting more and more attention and safety supervision is becoming more and more stringent, it is necessary for us to have more knowledge about air storage tanks. more understanding and application. For example, if the compressed air storage system is properly configured, the energy saving rate can be increased by 2 to 10%.
