Face Velocities

Harvey Ramer

Just curious. What face velocities are you guys using for return air grills and for round ceiling diffusers?

I just completed a system design and on the return air filter grills, I used slightly below 300. Reason being, to allow a 1" merv8 pleated filter to be used without exceeding a .17" pressure drop.

On the supply i'm using RZ16 Round Ceiling Diffuser H&C. There I stayed at 425 face velocity to try to reduce noise and again, reduce the pressure drop. It cut back on the throw a bit though.

Wondering how everyone else is doing it?

NJ, Designer

Harvey,

For the filter grill returns, the common H&C 659, @ 300 FPM make more noise than I would like, if you dear get near 400 FPM Some of them start to whistle, So yes definantly don't go over 300 FPM I'm trying to get my hands on the 673 to see if they would be quieter. However .17" drop across a filter, not sure what type of duct work your attaching and who's blower and coil your using but you are pushing the limits on most residential equipment, what's your total static pressure?

For the supply's I attached the Specs from H&C for the Model I think you are talking about, I would definitely try get a higher velocity if possible, up to 600-700 FPM. Depending on the size I would go up to .03" which is ACCA's default....

So for the supply's speed them up if you can for better air mixing and circulation, and for the return you doing fine, maybe go a little slower......

Harvey Ramer

That is the supply diffuser I want to go with. I think i'm missing something though. How come does the pressure drop increase as the neck size increases. I'm not completely understanding that one.

This is what I used to reference filter pressure drop data. Courtesy of some university field test's. I can't seem to find manufacturer data on filters.


For the returns I am using the H&C 673

I will wait to state my esp until I get clarified on the supply diffusers. Of course I want to stay as low as possible. .3 to .4 I would be happy with.

NJ, Designer

The reason why the pressure drop goes up with the collar size is a little complicated but It has to do with this particular diffuser that is a one size fits all and with the smaller collar the velocity pressure is dropping before it hits the diffuser, ( the 8" and 10" collar is there),

If someone can explain it better I'm listening.

Any way I don't like those diffusers unless the job is a retrofit and there is no money on the job, the throw is not very good, So I attached the engineering data for the metal round ceiling diffuser check it out, (and the price of course (and don't forget the labor that goes with them))

I also attached a chart from the wonderful H&C on face velocity's so you don't have to trust me.......

What brand and size filter are you trying to use I might have the data for you (in my office, Not there now..) Or I love fetching that kind of stuff.... If your trying to stay so low on the external static pressure you definitely want to go with a 2" filter..... The purpose of thicker filters are, 1 to lower the Pressure drop and 2 to last longer (collect more dust).

Nathan

Harvey Ramer

Last night while I was trying to sleep, I had a revelation which revealed with great clarity, the mystery of the pressure drop in the diffuser when using the larger neck size.

For an example lets use a 6" duct and an 8" duct, both moving 100cfm. Obviously the velocity in the 8" duct will be lower than in the 6" duct. Now we come to the diffuser neck which is 6" in this case. With the 6" duct connected, the air will maintain velocity through the neck and start slowing down as it enters the diffuser. Very minimal pressure drop. With the 8" duct, the air will have to increase velocity in order to pass 100cfm through the 6" neck. With any gas or liquid, an increase in velocity requires energy to overcome the inertia of the mass and increase it's speed. It also requires additional energy to overcome the friction created by the increase in velocity.

That energy required for the 8" duct comes in the form of a pressure drop.

The same scenario would happen in trunks without reductions or oversized, incorrectly built junction boxes.


I revised the math and design on this system and my ESP stands at .41 even with the 1" Merv8 filters. I expect the ESP to rise slightly once I balance the system to design terminal air flows.

NJ, Designer

Well said...

What's the PD across the coil?
How many Square feet is the area being conditioned?
How many returns?

Harvey Ramer

NJ, Designer said:

Well said...

What's the PD across the coil?
How many Square feet is the area being conditioned?
How many returns?

It's an airhandler. Cfm and ESP ratings already have a dry coil included.

The conditioned space is 1398 sf.

There will be three returns. 1 20x20, 1 16x16 and 1 12x12

2Ton system. 800 Cfm in cooling and 830 Cfm in heating.

NJ, Designer

Nice!

Rich_49

Nj Designer , Take a look at this and start at 18:30 of video . The products from there till the end should interest you . Am getting answers to a couple questions about these things on Monday and will call and share that with you . We were just discussing this type of stuff .

http://forum.heatinghelp.com/discussion/153576/htp-whats-new#latest