Calculation of flow rate in negative pressure pipelines
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This post was last edited by *aowoniubwb on 2017-2-8 at 11:11. A branch for supplying cooling air is now to be installed on the inlet pipeline of the bag filter, using cold air to cool the bag filter; the control valve on this cooling air branch will be gradually opened once the temperature of the material entering the bag filter reaches a certain value. Given the diameter of the inlet pipeline of the bag filter, the operating conditions and flow rate, as well as the diameter of the cold air pipeline, how can the flow rate of the cold air pipeline be calculated to select the appropriate control valve? Personally, I think this is related to system balance. Under normal conditions, the cold air supply pipeline is not in use; when air supply is needed, the control valve opens to allow air to be supplied from the atmosphere. However, this causes fluctuations in the operating pressure of the inlet pipeline of the bag filter (while the fan behind the filter remains unchanged). Even with a rough calculation, one can only determine the pressures on both sides of the cold air supply pipeline – one end being at atmospheric pressure and the other end connected to the inlet pipeline of the bag filter – but it’s not clear how to determine the flow rate.
There are no flow meters on the existing pipelines, and since a control valve needs to be selected, data such as flow rate is required.
The control valve can only adjust the flow rate, but it is not possible to measure the flow rate
However, the instrumentation field requires flow data when calculating control valves
So when selecting a control valve, it’s sufficient to provide the process parameters
In terms of the matter at hand, it is sufficient to meet the requirements regarding the opening resistance of the control valve. But I think this process design is unscientific and unreasonable. Won’t this require temperature control equipment for the bag filter? Cool the flue gas directly to an appropriate temperature, or mix it with material at an appropriate temperature. Do you think it’s better?
There’s a problem with your diagram: the cooling air for the atmosphere should enter the large bag, while in your diagram it seems to be air that is being discharged into the atmosphere. Flow rate = cross-sectional area of the pipe multiplied by flow velocity. Now that you know the pipe diameter, you can calculate the cross-sectional area. As for the flow velocity, it can be determined based on the pressure difference between the two ends; I’ve forgotten the exact calculation method, so you can search for it online, or simply look up how to calculate flow rate given the known pipe diameter and pressure difference – you should be able to find the information.
There is information on calculating flow rate using known pressure differences and pipe diameters, but I’m not sure if it can be applied to gases. I tried to use the provisions related to vacuum pipelines in HG/T 20570.7-95 \"Calculation of Pipeline Pressure Drop\" for the calculations, but the results were extremely unreasonable; I’m not sure if there was a mistake in the calculations. And the problem ultimately isn’t the flow rate; it’s the control valve on the cooling air duct, and selecting this valve requires flow rate data.
Personally, I think the amount of air to be added can be calculated using the temperature required after mixing