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What's the pressure needed for VAV box?
- Thread starter lzh007
- Start date
commissioningtech
Mechanical
It would depend on what type of Vav you are using and what controll system it has driving it. Some controll devises have changable pressure ranges and some don't.
I may be able to help further but I'd need more info.
Mark.
I may be able to help further but I'd need more info.
Mark.
- Thread starter
- #5
For an office block, select a pressure down stream of the box of approx 100-150pa. The velocity thro the box inlet max 7-8m/s, keep the duct lenght straight at entry approx 1-1.5m. The big issue is keep the noise down and maintain control with resonable velocity at reduced cooling load with the damper closing off
I'm no VAV expert, but I have always seen VAVs on medium pressure systems with 1.5"-2" of static as a setpoint. And typically the pressure sensor is 2/3 of the way down the duct from the air handler.
I recently ran across a job where duct static was dropping to 0.4" on a design day and we were not getting enough air. We corrected some poor fittings to fix the problem.
So back to your qustion, I think you need a good 1" or better on VAV systems. As I recall on the job I just referenced, the boxes were pressure-independent and each had pressure drops between 0.25" and 0.5". So you have to have enough pressure to get through the box, through the downstream ductwork, and out the diffusers with the corrct velocity.
Unless I'm completely missing something in your question, I would think that there is no way the system will work at low pressure.
I recently ran across a job where duct static was dropping to 0.4" on a design day and we were not getting enough air. We corrected some poor fittings to fix the problem.
So back to your qustion, I think you need a good 1" or better on VAV systems. As I recall on the job I just referenced, the boxes were pressure-independent and each had pressure drops between 0.25" and 0.5". So you have to have enough pressure to get through the box, through the downstream ductwork, and out the diffusers with the corrct velocity.
Unless I'm completely missing something in your question, I would think that there is no way the system will work at low pressure.
With DDC controlled VAV box most of the pressure drop occurs at the reheat coil. Look at Envirotec, Carnes, Trane, Anemostat VAV terminal boxes. I usually allow 1/4" wg but you have to figure in the entire duct run and airhandler components to correctly size your supply fan. Fpr example:
OA intake = 0.10"wg
Filter = 0.40" wg clean, 1.0" wg dirty
preheat coil = 0.15" wg
cooling coil = 1.0" wg
fan discharge & system effect = 0.25"wg
supply duct upstream & downstream of VAV boxes= 1.5' wg
VAV box = 0.25" wg
Total is 3.65" wg with clean filter vs. 4.25" wg with dirty filter. If VAV use 4.65" wg. If CV use 4.0" wg
The VAV box is only a small part of the total pressure drop. I recommend using the full dirty pressure drop only if the system is VAV. For CV use an average say 0.75'wg. This is because the system is balanced with clean filter anyway and the airflow in the CV system will just reduce as the filter is loaded. If the full dirt filter allowance was used, the unit being balanced at the clean filter condition will just either throttle the damper or adjust fan speed down. If damper is throttled, the system will just use up energy and still the aioflow will drop when filters are loaded. If the fan speed is adjusted, you might as well just have used the clean filter pressre drop and still the airflow will drop when filters get dirty. Using the average filter condition puts you in between. Hopefully the balancer set air terminals to more towards the +10% region and you had designed for 10% safety such that the airflow drop when filters get dirty is acceptable.
OA intake = 0.10"wg
Filter = 0.40" wg clean, 1.0" wg dirty
preheat coil = 0.15" wg
cooling coil = 1.0" wg
fan discharge & system effect = 0.25"wg
supply duct upstream & downstream of VAV boxes= 1.5' wg
VAV box = 0.25" wg
Total is 3.65" wg with clean filter vs. 4.25" wg with dirty filter. If VAV use 4.65" wg. If CV use 4.0" wg
The VAV box is only a small part of the total pressure drop. I recommend using the full dirty pressure drop only if the system is VAV. For CV use an average say 0.75'wg. This is because the system is balanced with clean filter anyway and the airflow in the CV system will just reduce as the filter is loaded. If the full dirt filter allowance was used, the unit being balanced at the clean filter condition will just either throttle the damper or adjust fan speed down. If damper is throttled, the system will just use up energy and still the aioflow will drop when filters are loaded. If the fan speed is adjusted, you might as well just have used the clean filter pressre drop and still the airflow will drop when filters get dirty. Using the average filter condition puts you in between. Hopefully the balancer set air terminals to more towards the +10% region and you had designed for 10% safety such that the airflow drop when filters get dirty is acceptable.
The reason I said .1 at the box was because I thought you wanted to know the minimum to get a good reading. And again I'm trying to remember from another job from two years ago how we did it. Why I say this is because we couldn't get a decent CFM reading out of the boxes and couldn't figure out why. When we were looking at the DP at the DDC VAV we noticed that it was extremely low. We then discovered that 1.5" in. wc. duct static was not high enough and that we needed to pump it up to 2.5" (it was a nutty design - inlet guide vanes spec'd!). I believe that as we were bumping the static up, it was when we got to .1 in. wc.
I'm doing a new system this week. Let me see what I can find out through testing and I'll get back to you.
I'm doing a new system this week. Let me see what I can find out through testing and I'll get back to you.
One other thing I forgot to mention. While 1.5" is the norm for duct static design, we do work with many lower static pressures.
Let me explain. Generally as the designer, you design everything around 1.5" - generally. What we do however is once everything is ready is we open all of the VAV boxes to their full max required CFM. Then we set the static to control the fan at 1.0". We see what happens from here. We then bump it up or down accordingly in order to achieve the desired CFM at all of the boxes (this also assumes all VAV's are to be at MAX on the warmest day of the year). Once we find the correct static, that becomes our setpoint (and it's never 1.5").
The reason we do this is because I noticed on many systems we had the fan blasting 1.5" on the days when we didn't need it and just wasting energy. We've done it like this ever since and have NEVER had a problem.
Let me explain. Generally as the designer, you design everything around 1.5" - generally. What we do however is once everything is ready is we open all of the VAV boxes to their full max required CFM. Then we set the static to control the fan at 1.0". We see what happens from here. We then bump it up or down accordingly in order to achieve the desired CFM at all of the boxes (this also assumes all VAV's are to be at MAX on the warmest day of the year). Once we find the correct static, that becomes our setpoint (and it's never 1.5").
The reason we do this is because I noticed on many systems we had the fan blasting 1.5" on the days when we didn't need it and just wasting energy. We've done it like this ever since and have NEVER had a problem.
I agree with simsd. On my example, 1.5" was the design setpoint. Even after our correction of the poor ductwork, we could only achieve 1.3" with all the boxes wide open, but we did achieve the design airflow (which was the ultimate goal). So we changed the setpoint to 1.3" instead of the original 1.5" There is no reason to run any higher since 1.3" was enough at max airflow. So it was certainly enough at other conditions.
One note of caution, this job had a host of design issues, including some flaws with the air handler itself. The mixing box section was too short, and that caused the freeze stat to trip out on a regular basis. This unit is in Chicago. To correct the problem, we had to add baffles, perforated plate, etc. to help mix the air. This resulted in a higher pressure drop when running in economizer mode as opposed to a design day where OA would be minimum. This is not a normal situation, but we had to consider it in our static setting.
One note of caution, this job had a host of design issues, including some flaws with the air handler itself. The mixing box section was too short, and that caused the freeze stat to trip out on a regular basis. This unit is in Chicago. To correct the problem, we had to add baffles, perforated plate, etc. to help mix the air. This resulted in a higher pressure drop when running in economizer mode as opposed to a design day where OA would be minimum. This is not a normal situation, but we had to consider it in our static setting.
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