Old VAV system - very high air velocities 3

[remp]

I'm replacing the VAV boxes on an old VAV system and retrofitting it to Pressure Independant VAV system.
The ductwork seems very small and original commissioning ducuments verify it ran at approx 12m/s (3pa/m) in the main runs up to the VAV box. After that then it was down to about 5m/s (1pa/m)
Is this high velocity OK. Its circular duct approx 450, 550, 650dia.
If it did it before I assuem it can do it again..what you think,. the original system is about 20 years old at least.
Will this high velvoity affect my pressure independent box control where I maintain a constant pressure at the main air handling unit outlet.
????
Your comments would be greatful
Thanks....

 

[GMcD]

That's a pretty standard VAV system criteria for duct velocities. Those velocities will work just fine for a pressure independent controller. The higher the delta P across the inlet sensor the higher the inlet velocity), the finer it can control. Same as modulating control valves and balancing valves in a hydronic system.

 

[SAK9]

I must say it is a bit on the higher side. If there are no abrupt changes in direction,12 m/s may be allright. Otherwise it will cause turbulence within the duct and resulting in noise. The recommended values are around 8 m/s.It might be worthwhile to check with the building owner if they were happy with the noise levels. High duct velocity does not affect the VAV performance.

As already pointed you need to have around 150 pa pressure drop across the box for stable control. The down of this is higher discharge and radiated noise levels. If you have a silencer on the discharge side, discharge levels can be controlled. As for the radiated sound level, there is no effective way of treatment other than having double walled boxes.

 

[AbbyNormal]

Always seems to be a big debate on where to place the static pressure sensor. The original post sounds like it is at the AHU.

Where is the best location and why. See it argued "2/3 down the run", I would be partial to the last box myself.

What say ye?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[SAK9]

Abby,

With new trends such as static pressure reset,it does not matter where you position the sensor because the set point keeps changing.The BMS is programmed to poll the VAV boxes(digital controlled)at regular intervals.If all the VAV boxes are less than 100% open,the static pressure is reset to a lower value till the system has at least one box 100%open .

 

[remp]

SAK9

Do you go for static pressure reset or supply air temperature reset. You cannot really have both can you?

 

[ChrisConley]

Tell us more about:

does not matter where you position the sensor

I've heard from many manufacuters, and technical people that regardless of the control strategy the sensor should still be located "2/3 down the run" for optimum performance.

 

[AbbyNormal]

Something measuring the pressure at the air handler almost sounds like it was a step up from having a pressure reilef door on the duct in the mechanical room, to prevent the ducts inflating.

Remp, VAV boxes tend to open and closed based on a call for cooling remp, be independent of the temperature of the air coming out.

Boxes start opening up, static drops the VAV ramps up the blower. Boxes close off, pressure will tend to build the VFD ramps the blower down.

The static reset sounds interesting, trying to rationalize it out right now. Is it possible that the farthest zone has the box wide open and there is still not enough coming out, or does the box report "I am wide open and still short of air" ?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[SAK9]

Remp,

You can use both static reset and supply air temperature at the same time as they use different inputs ie damper position and room thermostat reading respectively.

It behaves very much like a variable flow chilled water system with an automatic chiller outlet temperature reset.The system will find a natural balance by itself.

Chris,the sensor position is irrelevant as there is no constant setpoint for the duct static pressure.The system calculates the requred pressure by varying it till there is at least one VAV box 100% open.If you locate it at the AHU out let,this could fall between 400~600 pa,if you locate at 2/3rd it could be between 200~400 but the value has no significance.Any position little away from the AHU outlet in a straight run (far from turbulences) to guarantee accurate and stable measurement will be fine.

Abbey,
Have not seen any systems reporting back box flow measurement back to the controller.The static pressure drop required for the index VAV under partial flow conditions will be considerably low(oversized duct work due to less than design flow).But your lowest set point will be way above that to ensure a certain minimum drop within the box.

 

[AbbyNormal]

SAK, by wide open do you mean at the max CFM for the zone?

A box could be capable of passing more air than what is supposed to be the max for a zone is what I am thinking.

It sounds like we are turning pressure independent boxes back into dependent ones.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[remp]

SAK,

I thought supply air temp reset was based on the box damper position, maintain at least one box 90% open (close to fully open) and reset the temperature up.
How do you do it via room temperature sensor?

 

[SAK9]

Remp,

With static pressure reset control,supply temp is reset upwards based on whether all zone thermostats are satisfied below max airflow (both values available from the box controller).If thermo stats are not achieving set points at max flows,temp is reset down.

 

[simsd]

The static reset sounds interesting. But if you're interested in static tip placement, it's all in the set up. Here's how we do it (let's assume your 2/3's rule but you can put it just about anywhere in the main):

We open all of the VAV boxes to full cooling. We then take a static pressure reading - (We use these sensors

http://veris.com/file_uploads/px series_d.pdf

as they have a digital display so there's no guessing - all others are junk and I will fight anyone to death who disagrees. The reading then becomes our set point.

Then, if you are worried about the box on the furthest main all you have to do is close all of the other boxes to minimum and leave that one fully open. If you lose CFM (LPS?)then bump the static setpoint a little higher until you achieve the desired flow. This new static reading will become your new setpoint. We have tried this method time and time again and it works extremely well.

All too often I see people use the standard 1.5 in. wc. and I see the fan wasting energy for no reason whatsoever.