All right so we're going to take a look at the heat rise calculation for airflow. So this is the calculation right up here. Cfm cubic feet per minute equals sensible heat capacity divided by delta t times 1.08. Now the 1.08 is uh brought to us through a separate formula, which is the air density at sea level, which is 0.075 pounds per cubic foot times 60 minutes an hour times.

The specific heat of dry air at sea level, which is 0.24 btus per pound per degree fahrenheit, so it takes .24 btus to heat one pound of air at sea level. One degree fahrenheit. So that's how we get 1.08. So we've calculate our sensible heat capacity.

It's watts times 3.413 btus an hour. There are 3.413 btus in one watt, we're going to calculate our watts. Now we're going to be using electric heat in this particular video, we're on a resistive load to calculate our watts. It is volts times amps times power factor and because we're in a resistive load, it's not an inductive load.

So we have no reactive power here. Our power factor equates to 1 a unity power factor, so we could eliminate it. But in order for us to remain, you know precise in order to calculate watts. That is the formula volts times amps times power factor.

So we're going to take our delta t, which we're going to get from our the difference between our return and our supply. And then our volts and amps and watts, and we're gon na we're gon na go on a goodman air handler we're gon na throw in the heat strips and we're gon na. Do our calculation all right. So here's our unit we've got our supply probe and our return probe.

It's been running for about five ten minutes. Now i'm gon na give it a couple more minutes and then we will make sure that the the delta t is steady and pay attention to where we've got our probe set up. Obviously this is this is a trainer, but we don't want to put it right off of the especially in the elbow like you would never cut a supply run right there in a transition like that regardless, the point is, you want to be measuring the supply air Out of sight out of the line of sight of the coil and the blower or wherever the the source of heat transfer is, which is right off the blower, we want it ideally about five feet away. This turn probably gives us an equivalent leak of that.

We basically we want to make sure that our supply air temperature is as accurate as possible. Okay, so it's been running for about 10 minutes now and we have a pretty steady delta t. Let's take a look at what we got so 77 degree dry bulb return and about a 105 supply. It gives us a delta t of about 28, so we're going to record those measurements and we're going to go ahead and take our volts and amps to get our watts all right.

We're going to be checking the voltage on the load side of the contactor. So at the top so off the load side of the contactor, we have about 243 243 volts and then checking the amperage on the heat strips themselves about 19.5 yeah, sweet so 19.5 and 243.. All right! So we've just inputted all of the data that we were able to collect, and so now we're going gon na. Do the calculations and figure out you know plug it into our grand calculation, our airflow calculation and then solve for airflow.

So our watts was 243 times 19.5 and that came out to 47.39 and obviously that's a a rounding 47.39 and then we multiply this by the btu. How many btus are in each watt multiply 4739 by 3.413. So that's our sensible capacity we're going to plug that into the rest of our calculation. Oh and our delta t, obviously, 70 77 return, air 105 uh degree supply error, and so the delta t was 28.

all right, so 16 174 divided by 1.08 times 28. What was that 30.24 got it, so we plug this in, we get a total of 535 cfm, and so now, what we're going to do is we're going to go back over across and do a traverse over the return grill with a rotating vein anemometer and compare The results all right so in order to use a rotating vein anemometer we've got the testo 410i on the bluetooth, a small vein anemometer. Obviously the larger the the vein, the more accurate because the more area surface area it's able to measure at one time. For the sake of this exercise, i want to kind of demonstrate how you would use this particular method to solve for cfm with this particular type of measurement.

It's not you know airflow times equals sensible. You know capacity times 1.8 times delta, t or divided by 1.8 times delta t this one is velocity times surface area, so we want to measure the free area, the available area of which is from great to great on the grill 15 and a half by 20. That is the surface area that air is going to pass through on this grill. I've already put it into the app the testo app, and so all we have to do at this point is hit the hit the record button and do it a traverse across this grille and it's going to average the uh, the average velocity and then it's going To take into account the surface area that we've got now the free, the free area - that's here, because it's the return! There is no ak factor that we have to worry about with this grill, because we're measuring the air coming in.

If this was a supply, duct or a supply register, we would need to make sure that we look up the manufacturer's ak factor that they've printed for that grill, because we're we're measuring air coming out after the grill, not the case for a return. So i'm going to go ahead and hit record and we want to make sure that we're very careful we maintain a constant pitch or angle and then we're keeping keeping a steady pace. Traversing this this grill we averaged the uh, the total. You know velocity um and then reference that took the average feet per minute plugged that into our velocity times area, and we got an average cubic feet per minute of 550 for airflow.

As pretty close in our other calculation, we got a 535 553 535 fairly close. That's another way you can measure airflow indirectly, of course, because we're not actually measuring actual volume. It's this an indirect method of airflow measurement.