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The test we're going to perform now is a Flow Plate Test, to get system airflow through
our central heating or cooling system. Flow Plate is kind of a generic name for a device
such as this. This is probably the best known, which is the Energy Conservatory true flow
plate. This is the smaller of the two plates. As you might recall, this plate works on the
principle of air moving across this device. There's holes here, there's little pressure
sensing taps on both the front and the back of the plate, and when we hook it up to our
pressure gauge we can convert it into cubic feet per minute or CFM.
There's a couple parts to this test. The first part is to actually come up with what we call
the Normal System Operating Pressure (NSOP). The system, well usually we measure on the
supply side, you can also call this the supply operating pressure, but for that purpose we'll
use our static pressure tap. It has little holes on the side.
We're going to place our static pressure tap up in our supply plenum to get a normal system
operating pressure. This is just with our standard filter in, as though we weren't going
to actually do any additional testing. So we're going to hook up to our pressure tap,
or up to our gauge I should say. Turn it on, conventionally just go on the input side with
this gauge, on the A side I should say. Turn it on.
We have our air going in, and coming out the top, and we have an NSOP of about 57 Pascals.
So now, I'll just take out my system filter. This is a kind of flimsy tin filter cover,
but it's my filter. I'm going to temporarily replace it with my flow plate. Now there is
a front and a back. The front meaning the part where the air will hit first and it's
easy to tell on these devices. The labels are on the front, the little holes have kind
of a depression on the front, and the hoses come out of the front.
Here's going in, we're hitting this side first. Sometimes getting the hoses out without crushing
them is part of the challenge, but you usually find a way. You might replace the actual cover
temporarily with some tape. You just want to make a seal there, so you're not losing
anything airflow wise. Now this plate doesn't need any extra spacers inside, it fills the
whole opening, but it does come with a set of spacers. You can make your own spacers
as well. All you have to do is make a good enough connection to the main plate to get
all the air to go through the plate and not have any go around the edges. If it goes around
the edges, you're not going to be able to measure it and you're going to get an artificially
low flow. Alright. So now, before I connect up to the
plate, I'm going to re-measure my supply static pressure. I didn't have to do anything other
than replace the filter with the plate. I didn't have to move this; it has a magnet
to keep it on. All I do is look back at the number on the gauge. In this case it's oh
around, well it's moving around a little bit, but around 74 ... 74, 75 Pascals. I'll just
choose 74. The abbreviation we use for this is TFSOP, for True Flow System Operating Pressure.
74 Pascals. The purpose of doing this is that, or taking
the two measurements up there, is that the flow plate is a different device from the
filter. It can affect the amount of air the air handler moves. It acts like a different
filter if you want to think of it that way. If we don't keep track of that effect, we're
going to not know for sure what our flow is when we out the filter back in. By taking
this second measurement we can actually make a one-step correction using the tables that
the manufacturer provides. We'll get to that in a moment in detail.
Now, I have to actually measure the pressure drop across the plate. The way I do that is
by hooking up the hoses. The hose order is not important, meaning you can put the green
on the bottom or on the top. This value here will not change, but you'll get a minus sign,
or the minus sign will go away. So what we call the plate pressure looks to be about
oh, 31 Pascals, maybe a little more -- 31.5. We'll call that the plate pressure. Now we're
ready to actually get to a flow and a correction. The plate we put in was a #14 plate.
OK, so we have our measurements. All that's left is to look up what we call the raw flow,
which is what came off from our plate pressure and our plate size, which is #14. Here we
were kind of in between 31 and 32 Pascals of plate pressure, so a flow between about
640 and 650 CFM. We'll just call it 645. And as you recall we had a difference in NSOP
and TFSOP. The manufacturer provides a set of correction tables. Across the top they'll
list the NSOP, and across the side the TFSOP. We were not exactly on these but we were awfully
close and we'll be within a couple of percent by using this value (0.86). If you want to
get very exact you can use this formula, but you've got to have your square root key handy
on your calculator. But our correction factor in this case is a number around .86. That
means we will multiply our correction factor by our raw flow, and that will give us the
corrected system flow. These two numbers together, using the chart,
give us a correction factor of about .86. Our raw flow of these two numbers together
was about 645 CFM, we'll call it our ... these two together give us this raw flow, again
from looking it up. The product of these two values ... ah, the off screen hand off...645
times .86 equals 555 CFM corrected flow. The reason again we have to go through this process
is the true flow device often will create different resistance for the air handler than
the system filter. By taking our normal system operating pressure before we put the plate
in and taking that pressure in the same position with the plate in, we can make one mathematical
correction to our raw flow reading. In practice, every system is different. In
many cases the correction factor is greater than one. Also, in quite a few cases there
is very little difference and the correction is very close to one and is not worth worrying
about. This is especially true with the newer types of variable speed air handlers which
change their RPM to adjust to different types of filters or other obstructions to maintain
very, very consistent flows. So whether you have a filter or a flow plate in there you're
going to see a very similar flow because of how that controller works. On older style
permanent split capacitor fans like we have here; we tend to see correction factors diverging
away from one much more often. So, very important to take both these numbers and use them as
necessary to make corrections to the raw flow reading from the plate.