Chiller Bypass Control

[@jnlraboin]

Very good advice on chiller bypass control! Had problems when I was in the field adjusting the setpoint to meet the flow requirements. The most of the time the chiller would trip on low flow because the valve could not respond. 80% of the jobs I worked on were butterfly valves because they were cheap! Thanks for your help in this matter! Jeff Raboin- retired control tech

[@bryceanderson17]

Hello Jeff. When I do new construction witnessing. I ask the BMS engineer to override all the cooling valve from, 100, 90, 80, 70, 60, 50, 40, 30 (over about 30 minutes)... and every time the chiller will trip on low flow as this scenario was never tested (tuned). Thanks for the support, mate. Have a good Christmas 🎄

[@randomguy5965]

You are simply a legend. Thank you so much!

[@bryceanderson17]

Thanks, man 😊

[@xhajjar]

Thanks for sharing your experience. Usually controlling bypass valve through an actual flow meter. And when adding a chiller to the load you just multiplying the minimum flow set point by the number of running chillers if they're equal.

[@bryceanderson17]

Yes, this has been mentioned to me, thanks! I usually measure the flow at each chiller and pick the lowest to control the bypass. But, I guess you could have 1 large common flow meter and do as you suggest. I wonder if 1 large flow meter would be cheaper than 3 smaller ones? Also, when u have individual flow meters, you can calculate kWr for each running chiller, which is what we use for capacity staging. Thanks for sharing.

[@xhajjar]

@@bryceanderson17 you're welcome. I suggest a current transmitters for each chiller for capacity staging (cheap solution)

[@yup735]

keep them coming

[@bradgeary3467]

awesome

[@travisoutten6641]

Bryce, great content as always. Any chance you can do a video on SCU compressors staging on too quickly, essentially doing the same thing you are talking about here, creating demand spikes and quickly staging off? The ask being, (from a non controls person) how would you control the speed of the staging? I imagine through implementing some delay language but was curious to your thoughts. Thank you

[@bryceanderson17]

You may not be a controls person, but I am not a mechanical person 😆 I'm not a specialist in the equipment you are talking about. I'd be guessing.

[@markbell6864]

If only they got it right to begin with 😂

[@PawlikC]

hi Bryce , just found your channel and LinkedIn profile recently - great source of hands-on knowledge for a mech engineer! just wondering on the "equal percentage" there for chiller bypass valve ( kzbin.info/www/bejne/pJLUkoWEnb6niJI ) - wouldn't it be more convenient to use linear characteristic for this application (I mean you don't have a coil in that "circuit" you would normally want to flatten for the capacity(stroke) function and perhaps linear flow(stroke) response would be more beneficial here?)

[@bryceanderson17]

Hello Pawel, I have not watched that video for a long time, lol. The chiller bypass control valve is maintaining the minimum flow through the chiller evaporator, and the evaporator is a coil. So, if I undersatnd you comment correctly, yes you do still need an equal percentage charateristic control valve. If you have a primary/secondary system with the bypass on the secondary system or a bypass control valve on the cooling towers, then yes, they probably could be linear.

[@RG-pi7ij]

@@bryceanderson17 I had a similar thought as PawlikC when watching this video. First, I should say that I am new to variable chilled water flow design and really appreciate your videos. I have been having difficulty finding information on the best valve type to use both for minimum chiller flow control and for isolating chillers during staging. For controlling heat transfer capacity through a coil, a valve with equal percentage inherent characteristic makes sense to me. This is because we are trying to achieve a linear valve position vs heat transfer capacity characteristic for good control. The valve position vs flowrate characteristic of a properly selected equal % valve when combined with the (opposite behaving) heat transfer capacity vs. flowrate characteristic of the coil, ideally leads to a near linear "installed characteristic" for valve position vs. heat transfer capacity for the system. The linear installed characteristic is ideal for good control since the valve position % should match closely to whatever heat transfer % capacity that we are trying to achieve. In the case of a minimum flow bypass control valve, we are not controlling for chiller heat transfer capacity, we are controlling to maintain minimum flow. So we would want as close to a linear valve position vs flowrate characteristic that we can get (for good controllability). The only thing we are relying on to achieve this linear installed characteristic in the bypass line would be the valve itself. The bypass line is a short pipe with relatively little head loss and relatively constant differential pressure across it so it can't do much to affect this. Based on this reasoning, I would think a valve with linear characteristic, such as a globe valve with linear trim, would be best. Again, I have no experience with this and would really appreciate hearing more about what you've seen work for this application.