Regarding the selection of rotameters
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Discussion on the selection of gas flow meters: The process requirements specify a standard gas flow rate of Q1 = 20 Nm3/h, pressure P1 = 101.325 kPa, and temperature T1 = 273 K. The operating conditions under which the meter will be used are P2 = 4×101.325 kPa and temperature T2 = 273 K. 1. Based on a simple calculation using the ideal gas law, the maximum flow rate of this gas under the given operating conditions is Q2 = 5 m3/h; therefore, the LZB-15 rotameter was selected, with a range of 0.6 to 6 Nm3/h. (Ignoring the effect of gas type on density) Question: Is the above selection of flowmeters appropriate? Please share your opinion? 2. According to relevant information, when a rotameter operates under non-standard conditions, the flow rate actually measured must be corrected before it can represent the true flow rate. Specifically, when there are changes in gas pressure or temperature, the correction formula is as follows: Q1 = Q2 × 1/2. Here, Q1 represents the gas flow rate under standard conditions after correction; Q2 represents the actual measured value of the gas flow rate under the operating conditions. P1 and P2 represent the pressures under standard conditions and under the operating conditions, respectively; T1 and T2 represent the temperatures under standard conditions and under the operating conditions, respectively. If this formula is used to select a flow meter, and Q2 = 40 m3/h, then the LZB-40 rotameter should be chosen, with a range of 4.0–40 Nm3/h. Question: The results obtained from these two selection methods for flowmeters differ significantly. What is the reason for this? Please offer guidance and share your selection approach.
In my opinion, for rotameters with a constant pressure drop at the throttle, the flow rate is primarily determined by the cross-sectional area of the float, the density of the float material, and the conical tube. There is no need to use gas equations for calculation. The corrected formula does not take into account the effect of the gas compressibility factor and changes in fluid density. It is also necessary to understand the relationship between M3/H and NM3/H. When purchasing such a flow meter, it is best for the manufacturer to calculate the range and calibrate it according to the operating conditions.