Outside air control on VAV system with common return air fan

[Waramanga]

Hi all,

the title of this post says it all (nearly!). What is the best way of achieving good outside air control in this case? I am thinking that the AHUs will have the return and intake dampers modulate so that the mixing plenum pressure (when not in full economy mode) is some negative pressure, pressure set by the minimum OA damper full flow damper pressure drop. The return fan would run on a plenum pressure with a spill damper???? I cant find a description of such an arrangment in any of my books, what do you guys think?

thank you

 

[imok2]

This may be of some use

The following article was published in ASHRAE Journal, October 1999

VAV Systems And Outdoor Air

HVAC system outdoor airflow rates are usually measured and set
at the air-handling unit. The effectiveness of an HVAC system at delivering the outdoor air to the occupied spaces of a building is
varied and often disputed. To investigate the delivery of outdoor air to the
occupied spaces of a variable-air-volume system, ASHRAE Technical Committee
9.1, Large Building Air Conditioning Systems, funded Project 687-
TRP, Minimum Ventilation Airflow Rates with VAV Systems. This article
presents some of the information collected during that project.

 

[Waramanga]

thanks Imok, do you know of any books that deal with practical control issues also? I wonder how much the Ashrae journal costs to subscribe to....hmmm I will check it out!

 

[imok2]

When I google "Minimum Ventilation Airflow Rates with VAV Systems" I get the complete paper

 

[imok2]

A typical Economizer Cycle Control

Maintain the return air fan for volume tracking of the supply fan

Operate the 3 economizer dampers in sequence. The % open for the OSA damper shall be equal to the % open for the exhaust damper. Maintain a fixed minimum position for the OSA to assure at least a minimum OSA intake at all times. If the OSA temperature is greater than 70*F (adjustable) the position of the economizer dampers are at a minimum position. If the OSA is less than 70*F (adjustable) and greater than the discharge air set point, the positions of the economizer dampers are at 0% return air, 100% exhaust air and 100% OSA intake. If the OSA temperature is less than 70*F (adjustable) and Less than the discharge air set point (very cold condition) then modulate the economizer dampers to maintain the mixed air set point.