VAV system Pressure Control - OPEN LOOP CONTROL

[remp]

Im constructing a pressure independent VAV system in a high rise building. The air handling unit fans ramp up and down on a vatriable speed drive (freq. inverter) to maintain a constant pressure in the supply duct through a pressure sensor about 2/3 the way down the duct.
At present I have supply air temperature reset based on the zone demanding the most cooling, so if all VAV boxes are say 60% closed the supply air temperature will reset above the design temperature. This will eventually cause the boxes to open more and the fan to ramp up. This method has lower chilled water gennerating costs.

The consultant on the job wants to delete the pressure sensor from the system because its a pressure independent system. ok he has a point here. But he wants to control the supply fan speed based on the box that requires the most cooling by using "OPEN LOOP CONTROL", if the box requiring the most cooling is just off its max position slow down the fan a little. Again after another 5 minutes watch if and VAV box is not preforming then ramp up the fan a little.

He want to do this for enegry saving. With this method there will be no supply temperature reset so less energy savings on the chiller. The consultant reckons there will be more savings on fan power.

What are you thoughts on this OPEN LOOP CONTROL?

Im in Australia, someone from Trane here told me this is how its done in US. Is this true?

 

[ChasBean1]

Re-read your first four lines and delete everything below it. Billy, this is deja vu, didn't we talk about wagging the dog - cycling central plant components to maintain positions of throttling devices? Start playing with temperature resets based on VAV damper positions and you have major humidification issues in the summer. If you want, reset it based on OA temperature only. Better yet, based on OA dew point.

The open loop control also wags the dog - what if the limiting VAV thermostat is adjusted? Is it really a VAV or a CAV? Do the boxes go through a periodic auto-calibration? What if that box fails, or the damper slips so the command indication fails?

If any of this is done here in the U.S. it means I haven't been involved with that job. I would like to see the central fan sheave adjusted per NEBB (accounting for diversity), steady 42°F CHW temperature, and VFD control via static pressure sensor with the setpoint optimized so that the most remote VAVs are nearly 100% open during cooling design conditions.

 

[remp]

Thanks for the advise Chas. Can you tell me how you would reset the pressure sensor set point with out using open loop control. I would love to know how you can practically do this without upsetting the entire system. I like to keep the pressure sensor set point constant.

Also, I cannot believe you dont reset your supply air temperature, Here in Australia we do it all the time based on zone high temperature select or zone average temperature select. If all VAV boxes can satisify conditions with a supply temp of a few degrees higher than design, why not adjust the off coil temperautre, it will reduce CHW plant power consumption. I do agree you have to watch the space humidity and set some limits here as well.

 

[quark]

If sensible heat is the only concern with your process, you can fiddle with chilled water temperature. Seasonal change of the set temperture will give you more headaches.

At low load conditions you will definitely save energy by controlling the fan speed. Moreover as the load on chilled water is less, the system quickly comes down to the set temperature and your chiller stops. You do save energy here.

I don't think CB is suggesting static pressure reset. He is hinting at operating constant static pressure to maintain sufficient flow(correct me, if I am wrong CB!). Increased static pressure means low flow required and fan speed reduces. So static pressure is maintained and vice versa.

Note: I don't have knowledge about systems in US.

Regards,

 

[ChasBean1]

Quark/Billy - yeah I'm not sure about what was meant by changing the pressure. I didn't suggest it.

Billy, two thoughts.

1) You said "At present I have supply air temperature reset based on the zone demanding the most cooling." How many zones are there in your high rise? You plan to have the system alter the supply air temperature if one guy's office on the 11th floor needs more cooling? What about the people on the shady side of the building if he's on the sun side?

2) Also you noted that "if all VAV boxes are say, 60% closed the supply air temperature will reset above the design temperature." If all are 60% closed then probably the static pressure setpoint is too high, and will remain too high after the air warms and VAVs open. Some might normally control at 20-30% open due to duct sizing/design, some might be at the end of runs causing them to control at 90% open.

My point here is not to say it would be catastrophic to control the way you suggest, but it would be a practice that I wouldn't employ due to the nature of the variables providing input. If somebody puts a refrigerator in front of a thermostat, there goes the AHU setpoint. If it's a hot/humid but cloudy day, solar load goes away which might pinch back VAV boxes, but do you really want do raise your discharge setpoint with the ambient humidity?

Hopefully this helps clarify my previous post. Please let me know if I misinterpreted your intent.

 

[remp]

I have one AHU serving 12 floors of North zone perimeter, one AHU serving 12 flors of south zone perimeter and so on...Seperate AHUs serve centre zones.
Hot water coils in the perimeter AHUs only for early morning warm up. No reheats on any of the VAV boxes unless a tenancy fitout warrnets it....by the way this is in Sydney Australia.

If a zone on level 12 is looking for 80% cooling (VAV box 80% open)and the rest are less than 60% open ( 60% cooling), Id like to reset the supply temp up from design a little so the the box that was 80% open is now 100% open. This will cause the boxes that were doing 60% to maybe do 80% say. You see I have a minimum setting on the VAV boxes 50% and no heaters so if the load goes too low I dont have heaters to bring the temp back up...thats way I like to reset the temp up...

 

[SAK9]

Billyq,

There is no doubt about the fact that chilled water reset brings in more energy savings than AHU fan speed control even after accounting for the increased power drawn by the variable speed secondary chilled water pumps due to the greater flow requirements.I have used chilled water resets based on the position of two way valves at the AHUs.It is pretty much similar to the concept you have used for the VAV boxes.I would look at the AHU fan speed control more from a comfort control point of view rather than any significant energy savings.I had put in external dry bulb and wetbulb temperaure monitors in the BMS to keep track of the out side air conditions.Therefore the chilled water reset is done taking into account the two way valve positions as well as outside air conditions.For eg, a shopping complex which uses chilled water at 7C in the day may reset it to 8C in the night.