The importance of air storage tanks and the calculations for selecting such tanks in compressed air systems

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Air receivers are a crucial component in compressed air systems. Under different circumstances, what kind of air receivers are required? Let’s take a look below: First, we need to clarify one question – what is the role of the air compressor storage tank in an air compression system? 1. Store air to meet the sudden increase in gas consumption by gas-using equipment ； 2. Store air to eliminate pulsations in the airflow within the pipelines ； 3. Preliminarily cool the air to allow some of the liquid water to precipitate out. Therefore, the air storage tank needs to be drained regularly ； 4. It ensures the automatic shutdown of the air compressor. When the air tank is fully charged at the set pressure, the air compressor will automatically shut off, preventing it from running continuously and wasting electrical energy. Theoretically, the larger the air storage tank, the better, as it can reduce the frequency of frequent startups of the air compressor. Star-delta starting is a type of power-frequency starting; at the moment the motor starts, the current is about 7 times the rated current. Frequent starting causes significant damage to the electrical components of the air compressor, reducing their lifespan. It also creates a large impact on the power grid, affecting the proper operation of other electrical devices. Variable-frequency air compressors feature soft starting, with the starting current being around 2 times the rated current, so the impact is not significant. So the question arises: can’t we use the air storage tank of the screw air compressor instead? The air storage tank located at the outlet of the air compressor not only helps to stabilize the output pressure and provide buffering, but it also serves another important function that most users don’t notice. That function is to prevent compressed air in the pipes from returning to liquid form for some reason during the compressor’s shutdown, thereby avoiding damage to the compressor. If this air storage tank is removed, an inverted U-shaped bend must be used in the pipeline design to achieve the same purpose. Furthermore, the function of the air storage tank is to maintain stable pressure in the piping network of the compressed air system, preventing large fluctuations. Since the air consumption at the end of the compressed air system cannot remain stable at all times, air storage tanks are used to maintain stable system pressure and reduce the frequent starting and stopping of the air compressor. Another function is to cool the compressed air after it has been dried, in order to reduce the moisture content in that air. For individual compressed air systems where the capacity of the air compressor is very high (such as centrifugal air compressors), the volume of the system piping network is also large (with large diameters and long lengths), and air consumption is relatively steady, it may not be necessary to install an air storage tank. But typical compressed air systems require an air storage tank. There is no space inside the air compressor for storing compressed air; once it is generated, it must be used immediately. This working method is not ideal. With a gas storage tank, compressed air can first be stored in the tank until it reaches a certain pressure level, and then it can be used as long as the pressure drops to a certain extent before the compressor starts up again. This approach is more efficient both in terms of energy utilization and the quality of the compressed air. So a gas storage tank is necessary. However, for devices that require a small amount of compressed air and are used infrequently, an air compressor can do without a gas storage tank. However, considering the stable performance of the air compressor and the constant pressure and quality of the compressed air, it is advisable to use an air storage tank. Systematically speaking, equipping a air compressor with an appropriate air storage tank offers the following advantages: The operation of the air compressor – Since the amplitude of movement of the air compressor is relatively large, frequent opening and closing of the pressure vessel during operation increases wear on the valves; it also leads to instability in the oil and gas circuits, thereby causing instability in the machine. However, if an air storage tank is used, this problem can be reduced to some extent. Energy-saving effect: The frequent starting and stopping of air compressors results in high electricity consumption by the motors. Therefore, the compressor is often in an idle state during use; if this state persists for too long, it leads to a significant waste of energy. However, in an air system equipped with a gas storage tank, the stability of the air compressor is maintained, ensuring very stable operation and reducing unnecessary waste of energy due to idling. Provide a stable air supply. Why must an air compressor be connected to an air tank? In an air compressor system, the presence of an air storage tank provides a buffer for the air output by the compressor, allowing the air pressure to be maintained at a set level. This ensures that the systems that use air receive a constant pressure, which is essential for modern factories. The air quality is good. The compressed air produced by the air compressor is first sent to an air receiver tank, where it remains for a certain period of time. This allows impurities and moisture in the air to settle out. The temperature of the compressed air also decreases. Even without a refrigerated dryer, relatively high-quality air can still be delivered. In fact, the function of storage is similar to that of a reservoir; the air compressor serves as the air source, while the air tank functions as the reservoir. The air receiver can maintain the air pressure within a set range, thereby buffering pressure fluctuations and reducing the number of times the air compressor needs to start up. Additionally, the air receiver serves to precipitate impurities and moisture present in the air. Basic requirements for selection: 1. Gas storage tanks should be selected from manufacturers that strictly comply with the GB150-2011 standard for \"Steel Pressure Vessels\" ； 2. The volume of the air receiver tank should be between 10% and 20% of the compressor’s discharge volume; generally, 15% is chosen. When a large amount of gas is required, the volume of the gas storage tank should be increased appropriately ； If the gas consumption on-site is low, it can be below 15%, but it is best not to be below 10% ； 3. The dryer is installed after the air storage tank, which allows it to fulfill its functions more effectively – providing buffering, cooling, and waste removal – thereby reducing the load on the dryer and ensuring a more uniform air supply in the system. By installing the dryer before the air storage tank, the system can provide a higher peak regulation capacity, and it is often used in applications with large fluctuations in air demand. Selection calculation: The volume of the air receiver tank can be determined using the following formula: V = N × Q ÷ (P + 1). V: Volume of the air receiver tank; Q: Volumetric flow rate of the air compressor; P: Discharge pressure (gauge pressure), in bar; N: Parameter. It is recommended to set N to 1 when the air consumption remains relatively stable. For situations where air consumption fluctuates frequently but the variation range is small, N should be set to 3. In cases where air consumption fluctuates frequently and the variation range is large, N should be set to 4 or higher. Practical example: A company is adding a new two-stage reciprocating compressor operating at power frequency (discharge pressure: 0.8 MPa, capacity flow rate: 12 m³/min). What size air receiver tank is required? 1. For standard selection, the formula V=N×Q÷(P+1) can be used; applying this formula with N=1, Q=12, and P=8 gives V≈1.3 cubic units. An approximate value of V=1.5 cubic units can be adopted. 2. In cases with special requirements, such as when the air compressor or the external piping system suddenly stops supplying air, but the pneumatic equipment still needs to operate for a certain period of time, the formula for calculating the volume of the air storage tank is: V≥Pa×Q×t/（P1-P2). Here, Pa represents atmospheric pressure and is taken as 0.1 MPa; Qmax refers to the maximum air consumption rate of the pneumatic equipment, in cubic units per minute; t is the time during which the pneumatic equipment must continue to function after the supply of air is stopped, in minutes; P1 is the pressure at the moment the air supply stops, in absolute terms, in MPa; P2 is the minimum allowable pressure for the pneumatic equipment, also in absolute terms, in MPa. Assuming t=10, Qmax=2, P1=0.9, and P2=0.5, then V=Pa×Q×t/（P1-P2）=5 cubic units