In this video, my good friend Jim Bergman from Measure Quick is talking about volume flow rate of Air versus mass flow rate of air. Often we talk about CFM cubic feet per minute, but maybe we should be thinking about the actual pounds of air that it's moving and why we have to kind of calculate some of those differences under different conditions. So it's great. Video Big thanks to a team Adam from HVAC Overtime for sharing this video with me.

Always appreciated when you send in these types of videos that are really just excellent learning opportunities. I Know you're going to enjoy it Jim Bergman My favorite hot air expert Talking about mass flow rate versus volume flow rate of air. This is most technicians don't understand this. almost all air Conditioning if you look at I think it's chapter One Refrigeration Technology Q equals mass times specific heat times delta T You guys have all seen this formula.

Where does it come from? Anyway, No, you've said it multiple times in. Nana No, No, you said it. Um, is one possible Heat equation one BTU is what the amount Mass One BTU is the amount of heat to heat. What? One pound of water? One degree Fahrenheit Okay so this is literally saying Q is one.

BTU equals a mass is is the amount is the amount of air right? One pound of water times specific heat of one right times one degree Fahrenheit That's that's literally what it is. One times one times one and one BT is everything we do in our in our industry is about pounds of air. So when you're looking at 400 standard CFM this is actually 400 times .075 equals 30 pounds per minute per ton, right? So we're talking about a one-ton air conditioner is 30 pounds per minute. We're talking about a five ton air conditioner 150 pounds per minute going across the coil.

All right. So literally we're talking about the mass of air going across the coil. So when all these formulas when you're looking at when you're looking at 400 CFM of airflow, you're actually talking about 400 standard CFM not 400 CFM. So Measure Quick is actually taken into into account the density of the air.

So the air density changes with the air temperature, the relative humidity, and barometric pressure. All right. And what happens is depending on the type of fan you have, you either have a variable mass flow rate at a constant volume. So a fan moves a constant volume of air with a variable mass flow rate.

Meaning that always think of it like a box of air. One foot by one foot and the fan is going I'm going to throw 2 000 boxes across the coil a minute fan doesn't care what's in the box. The Box could be really heavy dense air. The box would be really light.

Dry it. Yeah, Well Well here's let's ask this question before: I Get there. Too much does water in the air? Does it make the air heavier or lighter having Heavy Across the board. What do you think? Ed I Think it gets lighter? It gets lighter? Yeah, why does it get lighter? You don't think it doesn't make sense.

It does it. But what's air? Air is comprised of What two? Um, what's the primary things in the air? Hydrogen? Uh, not hydrogen, Nitrogen, nitrogen, and oxygen Which at a molecular level has more mass than hydrogen which comprise water and oxygen, right? H2O So as we add H2O to the air, the density of the air actually goes down. so dry air is heavier whenever we're riding a fan. What? I was talking about? Now when we talk about this is this is standard air you guys have all heard of.

Standard Air: How much? What? What a standard this .075 is under What conditions? CC level. Okay, so this is F: uh, zero feet, right? Yeah, At what temperature? 68.3 What? Relative humidity? Zero. Zero percent relative humidity? Yeah, wait. I Don't know.

unless maybe you're in Phoenix six days a year. Wow, Yeah, how do you? It's uh, you have to be in a very arid climate. So just a little factoid. in 1924, the Heating and Ventilating Society actually revised their definition of standard error to 70 degrees at 50 percent.

Relatively? Yeah, But this is actually this is considered standard error and the difference, by the way, is less than two percent. Let's get Gary and just on the phone, right? Yeah, All right. So whenever we're talking about this with with Measure Quick, what we're talking about is you're going to see two things. You're going to see Acfm and you're going to see SCS right? So an actual CFM is just a cubic foot, Right? It's a cubic foot of air.

Okay, but it doesn't care about what's in the box. So the the PSC motor we have out there, it moves actual CFOs. So if you have it set for 600 standard CFM So if we look up here on the on the board here right now that you don't have that fan on low and it's moving, that's on medium. Okay, so we're moving 660 actual 663 actual CFM to get an equivalent 638 standard CFM All right so what's going to happen is that fan will always move that 660 AC FM That's always going to be the same number of Cfms.

Never going to change. It's locked in at 660. What we'll change is the standard CFM of air will constantly be changing in there because as the air density goes up and down, the pounds go up and down. So when you're looking at in measure equipment, if you throw out of the bottom you'll see you have a return air density 0.072 right? So if we're to take the the 400, Oh here, let's just do this.

We're moving what 600 and what? 660 standards our actual CFM times What .072 equals how many pounds? 47.52 47. Now if we divide that by .075 to get it to standard here, what's that equal? Six, Six, Thirty, Three Point Six, 633. So right, there's your 660 and 636 Basically the same numbers, right? They're just swapping around changing a little bit as we go. That's literally what we're doing.

So when you're looking at Measure Quick, you're going to see always. You always want to know whether you're moving Scfm or Acfm, because that's that way. You know if you're talking about mass flow rate or volume, right? So that's the difference between the two. All your fans when and this is this one is what threw me for a loop when I got into doing measure Quick was I Was trying to figure out the mass flow calculation for calculating airflow right because we said it before.

If we know cubicles, mass times, specific heat, we can figure out and it's cubicles, mass times, specific heat times the Delta T. So in this case, if we're to say 12 000 BTUs equals what Mass would be 30 pounds per minute per ton, right? times specific heat of 0.24 times delta T Well let's just solve this for a minute. So we take 12 000. divided by 30 times 0.24 27.5 27.5 So if all if all the energy went to converting the water vapor to water, excuse me if all the energy went to cooling the air.

12 12 000 BTUs cooled down to 30 pounds of air, right? It has specific heat of 0.24 BTUs per pound, then it would change the air temperature to 27.5 degrees right? But typically what's the typical late sensible split on a machine? How much if we design it? What's the typical Lane What do you think? Ed 0.75 So if we're to multiply this times 0.75 that's where the magic 20 degrees delta T comes from. I Still stole this off your website six or seven years ago and you have it line by line. Yep, so this literally comes down to if if you know. So if you figure this out backwards, if I know what this is going to be and I know what this is I can calculate how many pounds of air going across Like Well now here's a weird one for you.

Anybody know what the inverse of the specific density is. So you've all seen this. If you've looked at a psychometric chart, so you got psychometric chart going up, you got your lines. You got lines going this way.

Anybody know what? Those lines are relatively. no relative humidity or curved like this. and these are these. these lines going vertical or dry ball.

These lines come down at an angle, not weapon. Well, there are wet balls, but they're more of us. These are really steep angle wet bulbs. More like this.

Um, so is. uh uh. you're close specific. uh.

volume? Yeah. So a lot of times you'll see like this point Here you'll see this is 14.7 right? Or in this case here if we're to say it at zero degrees relative humidity at 68 degrees right? 68 degrees Fahrenheit this is our 68 degree line. This line of specific volume would come across here. This would be 13.33 So this this spot here would be 13.33 feet cubed per pound.

Why is that ringing a bell? What's the inverse of it? Well, this is over one, right? Okay, so now take the inverse of it. 1 divided by 13.33 equals what. Ah, pounds per foot cubed. So if I if I take if I take the If I can figure out the mass going across the coil.

Or if you figure out the specific volume, I can calculate the mass or massive specific volume. They're interchangeable. Just got to take the reciprocal of it. So literally all the stuff we're doing.

This is why it's so important to understand mass flow and and or understand at least what measure puts measuring. because you're going to have and what's I Heard of a CFM What's an Acfm or Scfm? Well, Acfm or actual CFM Scfmr standard. A standard. Meaning it's got associated with a weight.

An actual. it's just a cubic foot. A standard means of weight. Okay, so that's why I can say that that a certain amount of actual CFM are equal to this many standard.

CFM That's all we're doing. So now this this thing gets more interesting because what's the latest motor technology? We came out with Ecms. Now, Ecms work off of two things They work off RPM and anybody know: static pressure, torque, Torque Very good. That's exactly what I was looking for RPM and torque.

So now, as the air density changes as the air gets lighter, what happens? The amount of torque that's required to actually push the air. It goes down. As the air gets heavier, the torque goes up right. So an ECM motor? it's It's chucking pounds of air across the coil and it's saying, oh, this air is getting a little lighter, so guess what it does.

It speeds up to actually move more actual CFM across the airflow to get the standard CFM the same. So in ECM when you set it for 400, it's 400 standard CFM So this is where it gets tricky. Sometimes you're going to hold your capture Hood up or a measurement device up and it's going to read like, let's say 440. you're like, oh, the airflow is a little high up, it's 10 over.

Maybe not. It might just be your air density's oil, right? So again, this is why we're doing all this stuff in Measure Quick. There's a lot of complicated formulas of measure quick, but we do it because the computer can do it, and because the computer she can do it. Why, Why, why not do it right? So we don't use any Standard air formulas? A matter of quick, this is our what's called Standard Air formulas that uses those in manual Manual D Manual J Manual s are all Standard Air formulas.

We use the ashtray fundamentals psychometric calculations which are moist air equations they're I'm in Wexler psychometric calculations but it just just an important concept to understand is that we're cooling the mass of the air, not the volume of it. And and when we look at if you were to ever order an evaporator coil separately, they say how many pounds of air you're moving across the coil and now you know how to tell them. You just take your required air flow times .075 for Standard air and you can tell the pounds. Now if you had really hot air, you go to Psychometric chart and you could you could.

You could find out the temperature and the relative humidity of the air temperature, relative humidity. or you can find out the dew point and temperature. You can find out the wet bulb and temperature. You can find out the wet bulb relative humidity.

Get plot on a chart. Then you look at the lines of specific volume and you take the inverse of the specific volume and you get the density. So they're just interchangeable just by just by taking the inverse of it. Big thanks to Jim Big thanks to Adam Big thanks to all of you who share I Hope you enjoyed this video as much as I did.

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