Reset pressure setpoint method and primary pump variable speed 1

[saeedplc]

Dear Experts,

Could you please let me know how is "reset pressure setpoint " method implemented in chillers secondary loop?(in aspect of control algorithm)
And another question is that: How should the primary pump that supply chiller evaporator be controlled by VFD drive?(it should be control
according to secondary loop valve flow or position or something else?


Regards,

 

[saeedplc]

Your reply will be highly appreciated

No one has experience my friends?

 

[BronYrAur]

The questions are too vague.

Many secondary loops are controlled by a pressure differential measured out in the system. The VFD adjusts to maintain a predetermined differential as 2-ways valves on the coils modulate.

primary pumps should ideally be controlled to match secondary flow (or to be slightly more then secondary). However, other factors need to be considered. Maintaining the minimum evaporator flow is chief among the concerns. Cycling chillers on/off will cause abrupt changes in primary flow which can knock other chillers off line or temporarily disrupt your ability to maintain setpoint leaving the plant. when a chiller first comes online, it is not making cold water, so you lose setpoint. Control valves out on the secondary will see this, but there will be a delay. Valves will open, causing the secondary pumps to ramp up. The secondary flow may get so high that the another chiller is required just so primary flow can keep up (even though you don't need additional cooling).

There are many factory to consider, and each system has its own unique quirks.

 

[saeedplc]

Thank you for the reply

Sorry if i couldnt convey my mean
As far as i know except putting a diffrential pressure transmitter at end of line or on critical unit to adjust the vfd of secondary pump there is another way that pump speed is controlled by position value of control valves of critical units( demand higher flow compare to others).
I am going to know ths second method but nothing have found in the websites. How the second method is implemented according to the valves positions and it's algorithm?
Or How to find critical coils or units?

Thank you so much

 

[GT-EGR]

Secondary loop differential pressure reset algorithm generally takes feedback of some or all of the critical chilled water consumers in the loop, decides if a high quantity or all of the valves are in need of that much flow/pressure, and then resets the DP setpoint up or down in response to that.

Simple example is you have 10 CHW fan coil units, connected and all valves are between 40-80% open. This tells you that you don’t need that much pressure and can reduce your DP. Some people then set the algorithm to keep reducing pressure until one or maybe 2 valves are at least 90% open depending on how critical every zone is, (or set priority to some valves over others), and then when one or 2 valves are more that 95% open you start ramping the DP setpoint back up.

 

[EnergyProfessional]

What GT-EGR said. You want to run the system at open throttle. A valve is basically a throttle or brake. Whenever you close it, you are wasting energy. Reduce pump speed as much as you can to have them as close to 100% open as possible. Once the most open valve drops to 95% or under, your pump is too fast (hence the valve "applying the brake"). the valve is at 100% (not meeting DAT setpoint for example), you need to increase pump speed. It's more tricky for valves that don't have a direct output feedback and are digital (i.e. hydronic unit heaters).

 

[saeedplc]

Thank you so much for your replies

Could you please tell how to determine the critical valves?
Could you please let me have a sample performed project or flowchart of reset setpoint conttol?.
To be honest I have always executed the method based on placing DPT at most remote unit with constant pressure setpoint and this is my first time for reset setpoint pressure method.

Thanks a lot.

 

[saeedplc]

Your reply will be highly appreciated

 

[EnergyProfessional]

I think I already said that in your other thread. there isn't a single critical valve in a variable flow system. The controls will adjust pump speed to the most critical (most open) valve being 100% or lose to it open. Basically eliminate throttle losses as much as possible.

That critical valve could be AHU 1 in the morning, AHU 2 at noon, and AHU 3 the next day. There isn't "the one" critical zone like in a CV system. The upper and lower pump pressure limits are more fail saves and will be determined during commissioning.

 

[GT-EGR]

It’s very similar to the traditional way where you picked a DP reading and target setpoint somewhere remote in the system.

Major install difference is you need feedback on the valve positions to your controller.

Otherwise pick and design to a starting DP setpoint as you always would have, but during operation some computer programming is now going to look at all the connected valve positions and incrementally reduce or increase this setpoint to provide as much pressure/flow as the valves need. It renders you initial DP setpoint somewhat moot over time.

 

[EnergyProfessional]

You don't need valve position feedback for SP reset to work. You can just use the command signal for the control.

What feedback does is tell you if there is a problem with the actuator. So yes, it is useful, but for other reasons. We usually set u alarms if the command/actual signal differ by 5% or so.
Also note, if the linkage or connection to the actual valve is broken, the actuator may still move and from BAS side it will look like the actuator works (and it does, it just doesn't operate a valve). With modern ball valves this is less problem. But globe valves can rip the actuator off the valve body and the actuator still will work, just not the valve.

Newer PICV have a flowmeter built in. That IMHO is a much better feedback. Wth any feedback, evaluate what it is you see. Don't jump to conclusions. With actuator feedback people see the actuator actual position. But they think they see the actual valve position - that can be a huge difference. Like if you put your car on a lift with the wheels fre-spinning. run the engine and the speedometer will tell you are driving fast, even if you are stationary. Know what is fact and what is imagination.