FPB's

[WhatsaCFM]

Hey guys. I'm balancing a project with a bunch of series fan powered boxes. The boxes themselves are existing but they retrofitted all the controls and installed new ECM motors. They want to modulate the fan speed as the primary air VAV goes from max to min. Essentially never pulling in any ceiling air. Never seen FPB's controlled like this and seems like the fan is pretty much pointless in this case. Have you guys ever seen something like this?

[bradluke0]

I have never seen series fan powered boxes controlled like this . I wonder how they are going to modulate the fan and keep the plenum pressure neutral ? Do you have a sequence of operation to share ?

[WAYNE3298]

The only way to do what he wants is to completely block off the FPB air filter. The design minimum primary airflow has to be enough to serve heating needs. If you have water heat with the heating coil located at the filter inlet you won't have any heat.
The design makes no sense whatsoever.
The filter inlet should be neutral with the primary airflow at maximum. At primary airflows less than design the filter pulls in plenum air. That's your first stage of heating.
There's a good chance you misunderstood the design intent.

[bradluke0]

Its a crazy idea for sure , I would love to see a sequence of operation to see if this is really the intent . I really like setting up series fan powered boxes . When the piece of paper falls off the plenum filter , you be calibrated.....lol .

[WAYNE3298]

bradluke0 I balanced a building with series FPB'S and the control contractor lost all the calibration on the boxes. They were trying to prove the balance was wrong and tried to use the velocity grid to neutral the box. Needless to say it didn't work but I watched them try for 2-1/2 hours until the owner got furious. He ask me why they couldn't neutralize the box and I told him the ceiling plenum has return air and they will never neutralize the box because they are picking up return air.
The story is a lot longer but I thought that part would amuse you. I got all that owners work after that circus event because the paper trick made him a believer. Took a few seconds and was undisputable.

[bradluke0]

That's a good story for sure . Would it be interesting if I shared my thoughts on how to properly set up a simple vav system ? I could go through the process and you guys could comment as I go along ?

[WAYNE3298]

Go for it brad. Do a pressure independent single duct system first.

[crazzycajun]

[bradluke0]

And here we go …. I really don't want to do an actual step by step process as I have tried that before and I always forget stuff . I think most everyone can balance a pressure independent single duct system but I think my way offers advantages . First of all , I don't care how much air the ahu is moving , we will deal with that later . I go to the main duct and measure the static near the pressure sensor . If I have 1.25" to 1.50" , I'm good to go ( if they are using "static pressure reset " it should be disabled for balancing ) . Now is a good time to take a quick look at the control system and see if the static pressure sensor is close to being calibrated . While I'm in the control system I make sure the control dudes have entered the correct box sizes and min/max setpoints . After that stuff is ok I go to the closest box and drive to maximum cfm , never wide open , we will get to that later . The rest of the boxes stay at setpoint so they can stay satisfied . Read out the grills , calibrate the box and balance the grilles . The reason I calibrate the box first is as I balance the grilles the box will compensate for the balancing process and always give me the correct airflow ( you are not losing 5 - 10 % while balancing ). Also , when you calibrate the box always get it as close as possible , plus or minus 10% is not good enough here , get it right on or plus 5% . If you have a dual point calibration now calibrate the box on minimum , get it right on or minus 5% . Be sure and write down the calibration factor (s) and damper position of each box at maximum cfm , this will help you identify the worst case box later . When you are calibrating the maximums at 0 to plus 5% and calibrating the minimums at 0 to minus 5% you are giving yourself a bunch of insurance and you will have plenty of air everywhere . After all the boxes are calibrated you can drive all the boxes to maximum . I am assuming there is no diversity on this system which means the box totals add up to the ahu design airflow . Now is where I care about the airflow on the ahu . Why now and not earlier ? The boxes will dictate how much air the ahu has , not the other way around . If you try to setup the ahu first and the boxes are not calibrated you are probably going to have to mess with it again later. I drive all the boxes to maximum , if all of the boxes are controlling at max and none are wide open you know you have enough airflow for sure . You can look at you worst case box and raise or lower the static until it gets 90 - 95% open . I'm tired of typing and have to take the cat to the vet , comments?

[WAYNE3298]

One of the first things I check is what the velocity pressure will be at min flow. If the velocity pressure is going to be below 0.02 inches I set the min to design but don't bother to read the airflow because it will not be controlled. I also check to see if any of the boxes will have a design velocity pressure above 1 inch.
It would be good if you include how to set up constant volume digital and pneumatic controlled boxes.

[bradluke0]

You are correct ( again ). Sometimes the velocity pressure can be so low that the box will not control . Luckily the controls have improved as well as the flow ring and/or pickup tubes . Ill get back on how I use the box positions and static to determine whether to slow down or speed up the ahu . Cheers .

[WAYNE3298]

We are on the same page on almost everything. I was curious as how you approached the final static set point. I use the same method but most balancers just set in high and leave it at that. That wastes energy. I'm not and environment nut but part of our job is efficiency. Glad to see you agree.

[bradluke0]

OK ….The system I am talking about has 10 boxes with a design of 1000 max and 400 minimum each , the ahu is designed for 10,000 cfm . The good part about this system is the ahu will never deliver more than 10,000 cfm ( unless the boxes are driven to wide open instead of maximum cfm ). When you drive all the boxes to maximum and ramp the unit up you could have 1.0" of static or 2.0" or even higher . I like to ramp the static down until the worst case box gets to about 90% open and still has design airflow , that's your setpoint . You did remember to write down the damper positions when you were calibrating , correct? After I ramp the unit down and find my static setpoint I take a look at what frequency the speed drive is at . If its around 50hz or higher I don't recommend a pulley change , if its 45hz or lower I will slow the unit down 15% or so . Why not just leave the pulleys alone and let the speed drive control ? If you slow the unit down and you have a VFD failure you can run the drive on bypass and not have crazy static pressure or noise . Remember when I recommended to set the maximums right at design or 5% high and I said to set the minimums at design or 5% low ? Here's why . If all the boxes are calibrated right at design you would have 10,000 max and 4000 cfm minimum , a 6000 cfm swing . If you are sloppy and the maximums wind up being a bit low , say 9400 cfm and you set the minimums a bit high , say 4300 cfm now you have a difference of 5100 cfm . If you set the maximums 5% high and the minimums 5% low you have a difference of 6900 cfm .That is 1800 cfm more to push around which is 4 1/2 tons . This becomes important a few years down the road when the coil and fan wheel start to get dirty , etc . I also like to drop the supply air to 53 or 54 degrees , this even builds more safety factor into the system . Setting these systems up similar to this ensures a well performing system for a long time , as long as the filters are changed . This isn't the only way to set these up and it probably isn't by the book but it works for me . Cheers guys and thanks for any comments both good and bad .