Pressure drop (and system curves) in parallel
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- Опубликовано: 14 май 2024
- Why do electrical engineers have nice equations for resistors in parallel, and we process engineers don't?! We have a look at why this is the case, and how it doesn't matter because we can use the same principle to analyse fluid flow through parallel branches.
00:00 Analogies between fluid flow & electrical current
02:29 Ohm's law makes things easy
03:48 The equation for resistors in parallel
06:14 Applying the principle to fluid flow
08:40 Worked example
Link to sheet with the example I work through:
docs.google.com/spreadsheets/...
Process with Pat is the place to come for perspective and to ask stupid questions. I want you to leave more knowledgeable, confident, motivated, and most importantly, curious. I also want to invigorate a field that seems tired and uninspiring, at least if you get your perspective from internet forums. These are not lectures. This is a place for you to leave thinking “Oh! That’s why...”
This channel is not only for chemical engineers - anyone who works with processes should be able to find something of value here.
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This is the best process engineering channel!
We need some more videos, these are great!
Nice video Pat. Very thought provoking.
Pretty nice vid. I remember embarrassing myself in front of our client several years ago for not fully understanding this concept. It can also be very difficult and complex to incorporate parallel branches with hydraulics into a process simulation - I find you have to adjust the flow rates to each branch manually (or with an adjust block) so the downstream pressures at the merge point equalize. But it can get really complicated really quickly! Nice intro to the concept!
The analogy goes deeper than is discussed here. If you go further into EE, you find that there are many examples of non-linear resistance. For example, the analysis of total current through two transistors in parallel can be done with a similar graphical analysis as done here.
1:41 1 bar pressure drop with the valve on the end, doesn’t mean you get equal pressure drop when the valve is on the beginning. The pressure drop depends on the flow rate inside the pipe, and it’s different in both situations. When the opening of the valve is the same and the pipe is equal, you get different results when you switch the position of the valve. Not always, but noticeable when you get into higher flow rates.
Wow! Never think about that the sum of regressions of exponential function. Very cool.
Great analogy analysis between electricity and pressure flows, very interesting to see how the nature of the results change depending on the system! Great video as always 👍
Just great! Thank you
Thank you very much!
Hi Pat, great video. Just a question. How is the pressure drop for the valve calculated? It doesn't seem to correspond to the Cv equation where Cv = Q X [(SG/DeltaP)^0.5]. What is the 27.3 value in your spreadsheet?
Thank you very much for the great videos. As a young chemical engineer they have really changed my way of thinking in pressure drop.
I have a quick question that relates a bit to this video but also on your first video about what happens when two pipes meet in the pressure.
Question: we have a feed stream of known pressure that mixes with a recycle stream. The mixture goes into a pump. After the pump you have equipment that causes pressure drop and at the end some of the product is the recycle and some you take as product.
I am struggling to understand what is the pressure before the pump (pump suction pressure). Before watching your video I thought it was the minimum pressure but now I am not sure.
From your video I should look at what happens downstream, but because I have the recycle that is complicated, especially because the pump works at constant pressure output, meaning that my flow through the pump will change based on the differential pressure.
In essence you have a single loop system
Just so you know I’m thinking about this one. When I get a chance I’ll get to it!
Edit: as promised
System curves with recycle lines
ruclips.net/video/VEgu41wUwCM/видео.html
Very nice video! I would like to ask you how did you come up with this fitting equation?
Hey! What do you mean? I just used the fitting function that's in all spreadsheets.
Very nice explanation, Pat! I belive that when it is an open-loop system pumping to different heads it gets more trick. Could you approach that kind of problem?
Absolutely! It’s a good point and I’ll put something together.
As promised: ruclips.net/video/yFjxJoX6We0/видео.html
Thank you so much, Pat! Really appreciate your clarity!
What happens in branches which don't join back up at the same point? Do you define pressure drop for each branch and then calculate flow based on this? If a valve to one of these branches was closed and the suddenly open, would there be a difference in pressure at the tee of points for the branch?
Great question! Rather than reply in a comment I’ll post a video and go through an example of this. Shouldn’t take too long if you can hang tight.
As promised: ruclips.net/video/yFjxJoX6We0/видео.html
@@ProcesswithPat that's awesome thank you
Hi Pat,
I hope you are doing well.
I have a problem with my client over the pressure drop of a control valve which is considered to control the flow in the parallel lines. I wonder if you could give me your email address so that I can ask my question. I am sure it won't take more than 3 minutes for you to answer.