Static pressure measurement: Equal friction vs. Static regain

[wayneo]

Is there a critical difference in the location of a s.p. sensor for a zoned system which is designed by the equal friction method vs. static regain??

I've seen w/ equal friction,. the s.p. sensor is located just past the blower outlet, while in larger (typically regain designs) the sensor is located 2/3 down the duct. Is this due to the fact that in a regain system, the s.p. in the duct is uniform throughout the duct length, and with an equal friction, the pressure loss is accumulative??

How specifically will this affect bypass modulation under part load?? (This is assuming constant volume systems)

Thank you!

 

[ChasBean1]

Wayno, two questions:

1. What does the static pressure sensor do? Does it control an HVAC unit equipped with a variable speed drive or volume vanes, or one of those Trane Varitrac type systems with the bypass arrangement?
2. What are the terminal devices, are they variable air volume boxes?

Typically a static pressure sensor can control a central unit from any location so long as it's not too far down the duct in too remote a branch. The most common mistake I see is purchase of sensors in the range of, say, 0 to 5" static and the industry-standard placement of the sensor two thirds down the main where there's 0.3 or 0.4 inches of static, so that the pressure device is too high a range for the application. I've also seen the opposite, where a 0 to 2.5" sensor is placed at a point in the duct that operates at 3" of static.

Also, I've never seen static pressure uniform throughout a duct's length unless there is zero flow, or unless this is some sort of a design application that's really outside the norm.

If you can further clarify I could perhaps be of more help. -Chas.

 

[Vicsidhu]

ChaseBean,

It is very informative answer. Can you please ellaborate it further (or backward ) with following questions:

Static Pressure is used for fan selection:

Why do we not talk about Dynamic Pressure? Is it supposed to be included. I am pretty new at this and would like to know - how do you come to the static pressure for a system. I know this is a very basic question and people would call it even stupid. But I guess if I don't try to get an answer I am stupid.

And do we take system effect factor in consideration when we select a fan. Theoretically I know we should - but how often in field does one do this.

Thanks.