VAV pressures

[Joro--]

Hi all,

I have some questions concerning VAV systems. Differential pressure must be measured at the entrance of each VAV box. The VAV box tupe is in series fan powered (about 2000 - 2500m3/h).

1.I need to specify the scope for the transmitter - how many Pascal should it be able to read by the VAV box???

2.The main fan of the AHU has frequency modulation and the pressure is measured near the AHU. I also need to know the scope of the transmitter there having in mind that the external static pressure of the AHU fan at full airflow is 600Pa.

Any help will be greately appreciated.

[flyrfan]

1.I need to specify the scope for the transmitter - how many Pascal should it be able to read by the VAV box???

The transmitter will be reading velocity pressure, so determine the maximum primary air velocity at maximum flow & that is your upper range. Select a transmitter that has a low range of 45 Pa, which is equivalent to 2.5 m/s velocity or the velocity pressure of your minimum primary airflow rate.

Most manufacturers can help you w/ this.

2.The main fan of the AHU has frequency modulation and the pressure is measured near the AHU. I also need to know the scope of the transmitter there having in mind that the external static pressure of the AHU fan at full airflow is 600Pa.

The static pressure port should not be at the AHU! It should be located about 2/3 down stream of the AHU, in a duct main. Otherwise you are wasting fan energy.

You need to evaluate the minimum system pressure needed at that point in the supply duct to deliver air to the hydraulically most remote VAV terminal in the system.

[Joro--]

Hi flyrfan,

Thanks for responding to my questions. I'm happy to know the opinion of an experienced professional. VAV is not very popular in Bulgaria and I'd appreciate some more clarifications:

What do you mean "a low range of 45 Pa". Does it mean from 0 to 45Pa or over 45Pa.? The company engaged with the delivery provides a range of :

0 to 100Pa
0 to 200Pa
0 to 500Pa
0 to 1000Pa
0 to 2000Pa

The velocity will really be 2.5-3m/s. Does it mean the best choice is 0 to 100Pa? And will it be bad idea to use a wider rage like 0 to 500Pa?

It seems we are not to measure the pressure at 2/3 downstream of the AHU due to other reasons. So we will need to put it near the AHU. The available pressure of the AHU fan is 600Pa and the total is about 900. Is 0 to 1000 Pa range enough. Not sure is it worth considering the situation when the fan works at an airflow less than maximum and then it can provide bigger pressure?

My questions might seem funny to you but I've really got no experience with VAV systems. And just for information, supposedly we could measure the pressure at 2/3 downstream, does it mean we could use a transmitter with lower range?

[dan sw fl]

Hi all,

I have some questions concerning VAV systems. Differential pressure must be measured at the entrance of each VAV box. The VAV box tupe is in series fan powered (about 2000 - 2500m3/h).

1. how many Pascal should it be able to read by the VAV box???

2. The main fan of the AHU has frequency modulation and
the pressure is measured near the AHU.

... having in mind that the external static pressure of the AHU fan at full airflow is 600Pa.

Any help will be greately appreciated.

Use the blower performance curves
Calculate your air flows based on Your layout
and
assure it is balanced.

THEN
Ask Titus
to assist in selecting size and proper noise level

http://www.titus-hvac.com/ecatalog/g...px?fileid=7564

[nik99]

http://www.nailor.com/IOM_Manual/vav...f/IOM_FPTU.pdf

See page 4 of this IOM manual for the pressure ranges of the inlet sensors for Nailor series fan powered terminals.
Basically the 0-200 Pa sensor is the one you need.