System curves with recycle lines
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- Опубликовано: 10 июн 2024
- I remember learning about recycle lines in one of my process engineering courses, but it was primarily about mass & component mass balances. I also recall having to iterate to arrive at steady state. When someone asked me about system curves for systems with recycles (pressure drop in recycle lines), I had to do some thinking on how to go about generating one . This was the result.
Link to the sheet where I construct all graphs in the video: docs.google.com/spreadsheets/...
00:00 Different systems and different system curves
01:13 Which flow do we mean?
04:01 How I deal with heat exchangers
05:21 System curves for each individual portion
06:33 Pressures at different points
10:41 Using the Solver add-in for Excel
12:42 Finally, the system curve
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...”
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Brilliant video Pat! It is great to see that you take on suggestions in the comments and make videos to answer those queries as this makes very meaningful content for those that ask, and for everyone else who is curious about the same, so great job! 👏 👍
Wow you are a pure gold for process engineers!
Excellent explanation
For me the system curve can be defined as a function of the recycle flow, the pruduct flow or the pump flow. It just depens on what flow is important in the process.
But maybe the system curve definition is based on the pump flow because the definition is maybe based on a simple example without recycle line.
Thank you Pat !
Great job, I'm recommending your work to all my fellow colleagues
Hi Pat! Thank you very much for the detailed explanation! These videos really helped. Wish you a great day.
Thanks man! I hope it’s useful, or at least thought-provoking! All the best!
Good job on a tricky problem. It seems like you have assumed the recycle valve has a fixed CV through the range of your flow rates, another way is to assume a % of the flow goes through the recycle. in your case, a fixed restriction is like is like using an orifice, which could be a good idea to consider because of cost, but then you would probably need a control valve on the destination to maintain a desired flow to the tank. To keep the problem simple you didn't optimize the size of recycle valve (or the size of the orifice) with the sizing of the pump, or consider using a pressure control valve in that location, but others may want to do so. You made an arbitrary choice on it's CV, but changing that CV could have a significant impact on head and flow of the pump, the long term energy use, the NPSH, and the reliability (how close you're running to the min flow) of the pump.
Probably would be worth you doing a video for a recycle system where the suction and discharge are coming from and going to the same closed tank. That's a fun one to think through. :)
What is the point of having a recycle line here. Are you using a recycle line to control product flow to the tank? Most of us simply use a discharge control valve at the pump to do that. It limits the flow in the system (and pump) to only the flow going to the tank and therefore uses less energy than pumping both product flow PLUS the recycle flow to the same head needed to get your half or so of the total flow actually moving to the tank, the rest (recycle flow) being unnecessarily sent through the pump. Product going to the tank is the only useful flow, so that should be the only real flow we need to pump, right? But your recycle flow is also going through the pump and back to suction and is thus 100% wasted energy.
A recycle line could be used, as you suggest, to control the flow. But, as you say, why not just put a control valve on the discharge which would be more accurate. One reason for a recycle line though is to ensure a minimum flow through the pump at all times. There's a couple of reasons for doing that:
1) Centrifugal pumps require a minimum flow through them. If the pump is shut off on the discharge side, the heat input from the pump impeller losses can begin to overheat the fluid and could cause flashing in the pump which will cause premature seal and bearing failure. I think there's also a possible thrust imbalance problem, but I'd have to look that one up.
2) Another reason might be to have a higher flow rate through the exchanger, this will increase turbulence which will in turn increase heat transfer coefficients, and a high velocity through the exchanger would reduce the RATE of fouling. Having the flow stagnate in a heat exchanger could create a few problems depending on what it's doing (maybe the material would freeze, maybe it would overheat due to turndown and a increased LMTD (that's a tricky one to think about).
3) Another reason would be to very accurately control the discharge pressure on the pump which will improve an downstream flow control devices (which might be manual or automatic).
Overall, Pat's done a good job of getting people to think about pump recycles Every case/need is a bit different, so it's not an automatic plug and play solution.