Calculation of air compressor flow rate
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With 3 air compressors of 60 m3/min, the air is transported through pipes with a diameter of DN250 and a pressure of 0.8 MPa; differential pressure flow meters are used in this setup. How should the maximum flow rate be calculated?
The maximum flow rate within the range is about 1.5 times the actual flow rate.
Select the flow meter based on a flow rate that is 1.5 to 2 times the actual designed flow rate. Do not calculate based on the air compressor, as two or three compressors may be running simultaneously; in cases of low demand, variable frequency operation is used. For DN250 air ducts, venturi meters or orifice plates are generally used
It is recommended not to consider options related to the air compressor side; either variable frequency or a backup unit can be used as appropriate. It is best to ask the process design team for the maximum flow rate required, and then use that value along with a certain coefficient to select an orifice flow meter for use in the straight pipe section.
Some of the process parameters for the instrument are provided by the process engineering team. The design pressure is set at 0.8 Mpa (trying to match the actual operating conditions; for example, if a buffer tank is installed at the outlet of the air compressor, and the flow meter is usually placed on the main pipe at the outlet of that buffer tank, then there may be some pressure loss, resulting in a pressure of around 0.76 Mpa). The design temperature is 25°C. The measurement range is generally indicated in standard units. If you install the instrument on the main pipes at the outlets of 3 air compressors, with 2 compressors in operation and 1 as a backup, then the flow rate should be calculated based on the operation of 2 compressors at full capacity: 2 × 60 × 60 × 8.5 = 61,200 Nm3/h. You can choose the appropriate flow range according to the process requirements; it’s important to understand how the equipment operates, including whether it has variable frequency control Whether it should operate at 1 frequency for normal operation and 1 frequency for variable speed operation, or at 2 frequencies for normal operation, etc.: to ensure accuracy, you need to be as precise as possible. Otherwise, if the measurement range is too large, the readings will not be accurate. Modern differential pressure flowmeters have relatively large measurement ranges, so you can set a range that is as small as possible. If it becomes necessary to increase the range in the future, you can calculate it based on the linear relationship, or ask the manufacturer to do the calculations again