Chiller Minimum Flow Control

1st video in the series: • Chiller Minimum Flow C...
2nd video in the series: • Chiller Evaporator Pre...
3rd video in the series: • Chiller Bypass Control
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[@kevinn2759]

Thanks for the video 👍

[@kidoctane]

Another great YT - always need to cascade the Pump VSD and the by-pass control loops. When I first started as a grad (in Australia's largest and best consultant) back in the 80's we always had the by-pass controlled by "system" pressure - but the sensors where actually across the chiller plant rather than the field Flow Meters are your best friend ..... where have you been all my life !!! However I find where it could get a little wonky is the scenario where there may a field CHW valve close to fully open whereas the other CHWV are near shut (eg: over night cooling for one area). The CHW pumps wind down to a level where they cannot provide the minimum pressure to push the required CHW thro that one outlier CHWV. Do not ask me how I know ☹

[@bryceanderson17]

Mmmm.. This is where the compromise comes in. Traditional comfort control would have picked the most open valve as the process variable (in the pressure set point reset strategy). A comfort biased control strategy would solve this specific problem. However, in energy efficiency land, we would purposely have ignored that 'rough' valve as it wasn't deemed a good representation of the field load, and therefore not a priority in using more pumping power. This is a good example, of where, in that building, if it posed a real problem, that a good BMS engineer would have needed to firstly, spot that problem, and then create a custom solution. Thanks for sharing.

[@mnomanriaz6069]

Best video 👌

[@mabbygoat]

Once again thanks for sharing. Great content. I would also like to listen to your channel on podcast, a lot of downtime on the Monash could be used better.

[@ros879]

Hi Bryce, Thank you for taking the time to share this information. When you mentioned the bypass valve opening to maintain flow to the chiller are you referring to the already chilled water bypassing to back into the return inlet of the chiller evaporator? Thanks in advance.

[@bryceanderson17]

If I understand you correctly, then yes. The bypass valve that is installed between the supply and return pipework, that when opened bypasses say 6°C water back into into the return water (6°C mixing into the return), and then going back into the inlet of the chiller evaporator. It gives the supply water somewhere to go when all the 2-ports are closing off in low load. Note: if you have 3-port valves at the end of some water circuits, which is quite common, then they also provide a return path back to the chiller to assist in maintaining chiller minimum water flow. Technically, if you installed enough 3-ports and you totaled all those individual flow rates and the total was equal to the required chiller minimum flow, then you wouldn't need a main bypass valve. Or you could have a smaller bypass valve as these 3-ports are helping to maintain chiller minimum flow. In my opinion, this is great for reducing the chance of the chiller tripping, but wastes energy as the pumps always need to pump more water as the 3-ports are always bypassing some water.

[@alanjones3875]

Interesting discussion about commissioning. In my view we should just check major water flow to say floors, after that we waste time as 99% of the time the system is below design. I’d say use the BMS for water commissioning, it’s so easy to look at flow meters, energy meters and valve positions to determine if systems are working.

[@bryceanderson17]

That may he a little too far for mechanical designers and consultants. However, if we had networked valves, like the ones IMI and Belimo provide which allow us to monitor the flow rate, well then there would certainly be a case to discuss reduced water balancing to terminal unit control valves. Even with Pressure Independent Control Valves you could stretch that concept. I think the point here is that technology has move on far enough now that we could start challenging the concepts from 50 years ago.

[@barefoottrekker3790]

Thanks for come clean on the subject . This is always in subject where nobody has a point of view. Btw , what is the variable for bypass valves ? Flow sensor at return ? Or based on high differential pressure ?

[@bryceanderson17]

Hi, Not sure if properly understand your question? In Australia we have either flow meters or differential pressure transmitters on each chiller. The bypass modulates open to maintain the flow/pressure of the lowest evaporator flow/pressure of the running chillers. I have an idea that perhaps in the US they have a common flow meter for all chillers and then depending on how many are running, will calculate the minimum flow for the running chillers. I haven't done this so am only assuming from the sort of questions people have been asking me.

[@Peter-dd3nj]

Hi Bryce, agree with you. What do think about before opening the bypass to meet min flow through the chiller keep the field pressure about 10-30 kPa higher to minimise the deltaT trough the chiller so we don't send too cold water back.

[@bryceanderson17]

Hi Peter, I can't fully work out what you are suggesting. Sorry mate.

[@EdwinHongwenGAO]

I think I also can do chiller bypass(in case low flow) +pump speed set to maintain minimum flow, and there is field bypass for overpressure, say 250kpa. How do you think Bryce?

[@vishalrayen6813]

Awesome more informative 😊✌🏻👍🏻. Can you please start a podcast channel in future.

[@bryceanderson17]

Balancing consulting work and the training is just about killing me haha. Definitely no time for a podcast, thanks!

[@ravikumarkalamraju5044]

Hi Bryce, Great explanation. Could you please explain what happens if primary pump only run but not the chiller and vice versa. I am bit interested to know. Thank You

[@bryceanderson17]

Sorry Ravikumer, I haven’t been on KZbin for a few months. I just uploaded a video and realised there are a bunch of comments in here. You can not run a chiller without the pump first running. The chiller will have a safety interlock to protect the chiller. If you stop the pump while the chiller is running it will fault and need a manual reset at the chiller before it will run again. This is very bad. You can run a pump with no chiller. The pump will push water around the system but the water will not be cold as the chiller is not running. However, in low load conditions, when the chiller is starting and stopping frequently, which is bad for the chiller. You could stop the chiller and let the pump push all the cold water around. Of course the water will warm up as the chiller is not running. But demand on the chilled water system is low, so we are allowing the 'stored' cooth (cold water) to discharge through the chilled water system (like a battery). This is the only time that I run a pump with the chiller off. I am trying to reduce the amount of times the chillers starts and stops per hour.

[@yup735]

Can you do VAV trim and respond strategy in a 2 or 3 part series please?

[@bryceanderson17]

Hello Stan, trim and respond is in the control strategy course 😊 Need to save something for the training course. No more control strategy videos for the rest of the year. Cheers.

[@osamamashal2602]

Great explaination Just one question. If I have only primary pumps (variable speed), with 3-way valves that need to be converted to 2-way system by only closing the bypass section of the valve. We will only open one bypass manually and consider it as our bypass valve. I was suggesting that we will maintain the minimum flow of the chiller by making a minimum limit of pumps speed. Because also we don't have flow meter in the system. Kindly need your suggestions

[@bryceanderson17]

Hi Osama, Ideally the bypass will modulate to maintain the minimum flow though the chiller with a flow meter or differential pressure transmitter across the chiller evaporator. Lets say you don't have a bypass control valve or that it is closed. The pump will not maintain minimum flow through the chiller. If all the 2-ports close, even if you run the pump full speed, the chiller will still trip on low flow. Because the pump can not push the water round. I.e. relying on the pump to maintain minimum flow is not a good idea. If you manually set the bypass open to a certain amount that allows enough water to bypass to maintain the minimum flow through the chiller - this will work. However, this is a waste of energy as you are using more pumping power to push the water through the manually set bypass valve, every day. If you have no chiller differential pressure transmitter and you can not install them, then this may be okay, just not ideal. If you are going to manually fix the bypass open then you could consider this as an alternative solution. If the chiller needs 10 lps of minimum water flow rate, then leave enough 3-port valves in place that totals 10 lps. I.e. if each water coil is 1 lps, then keep ten 3-ports, and convert the rest to 2-port. Try to select these ten 3-ports at the end of the circuits. It may be a good idea to check with the chiller manufacturer to make sure the chiller can operate at a lower flow rate. Not all chillers allow a large difference between the design flow rate and their minimum flow rate. Also, you will need to get a mechanical contractor to adjust the chillers low flow safety flow switch/pressure switch as it will be set too high. Thank you for supporting the channel.

[@osamamashal2602]

@@bryceanderson17 great What about if I have a system with primary and secondary pumps along with bypass. So, we control bypass valve here based on the differential pressure transmitter across it. Why ? To achive what ? Is it related to minimum chiller flow or protectibg system from high pressure

[@bryceanderson17]

@@osamamashal2602 The chiller minimum flow control video only applies to primary only pumping systems. If you have Primary/Secondary then the decoupler protects the chillers min flow. With a Pri/Sec system the chiller can not trip on low flow because the primary pump can always push water around the primary circuit through the decoupler. If you have a bypass valve in the secondary system then it is there to prevent high pressure. So, as the 2-port valves close the system pressure goes up and the secondary pumps slow down to maintain set point. In extreme low load conditions, if the secondary pumps slow down to minimum speed and the secondary system pressure continues to rise then the bypass opens. If this does not make sense then you will need to show me a chilled water schematic so I can understand your system.

[@osamamashal2602]

Thank you very much Well clarified

[@shamsam9605]

Sir it's better if you explain with drawing,

[@bryceanderson17]

I agree, it would be better, it just takes so much longer to do a sketch. Thank you for the feedback, maybe I'll try do more drawings.

[@shamsam9605]

@@bryceanderson17 thanks Sir

[@bilalmohiuddin6518]

However, At times you just need it without sketch to not get lost while attempting to concentrate on what is being said and what is being drawn. You have that captivating talent of Technical story telling that takes the audience to imagination of the system, and your hand gestures are powerful way to get it through our heads easily, which happens very rare while trying to focus on technical subjects. One or two minutes of sketch demonstration should suffice I’d say. Sketch Should not run the size of the video as most other videos do.