Hey this is the hold on second, that was a little over-enthusiastic. You got a rain in the enthusiasm a little bit even when you're talking about something as exciting is the fact that air has weight and takes up space, which is a very exciting topic when you think about it. Oh, by the way, I'm Brian - and this is the HVAC school podcast - this is a quick episode in the name of this. Quick episode is what I just mentioned, that air has weight and it takes up space, and that has a lot of ramifications for us in the work that we do every day, whether you know it or not, but before we get into that.

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We normally measure CFM right. We talk in terms of CFM, sometimes we talk in terms of static pressure, even which is even more simplistic, but we're generally trying to find out what is the CFM that you're putting out of the air handler? So what do you see if n CFM is cubic feet per minute? That's the amount of space that the air takes out. So we accept that air is something it is matter. It's not just a vacuum.

It actually has some weight to it. There are molecules involved with air, and so when we talk about it, taking up space, we're saying that all right we're moving around these many boxes of air. In fact, when we say cubic feet per minute, CFM you can imagine a cube of one foot by one foot by one foot: cube of air, a cardboard box, a one foot square or cubic cardboard box of air. We're moving around that many per hour and you've probably heard the rule of thumb if you haven't, and you should memorize this 400 cubic feet per ton.

So my time is 12,000 BTUs per hour, 244 thousand BTUs per day and a CFM is a cubic foot per minute. And so you got to remember that it's a cubic foot per minute. It's not a cubic foot per hour or anything like that, and so you're gon na have to do a conversion. If you want to match the two, but what it comes down to is is that we want about 400 CFM for tanta now.

Is that always the case? We always want 400 CFM per ton on all equipment at all times and answer. That is no. We don't in some cases we want a little less than that in some cases, women a little more and there's reasons for that, but where we live here in Central Florida and throughout the Gulf Coast states and even up the eastern seaboard, you get a lot of Really high relative humidities and that's a circumstance where you want to remove more moisture, and so we generally set our blowers to produce 350 cubic feet per ton, which means that per cubic foot of air we're removing more heat per cubic foot, which means we're gon na Colder of a protocol and we're able to pull a little more moisture out, and so that helps us with what we call a sensible heat ratio. And I've talked about this in other podcast, but the point being when you move fewer CFM per ton.

Fewer boxes of air per ton, of cooling capacity or heating capacity that you have then, in the case of cooling capacity, you run a colder of a protocol, and that means that your coil is further below dew point and the air is moving over it more slowly, Which means that you have longer dwell time, which means you pull out more moisture so said, simply lower CFM per ton equals more moisture, which is why we use 350, but for most of the country we talk in terms of 400. Cfm per ton is sort of being our standard, but there's a couple other factors here. So when you're up in the mountains say, you know that the air is thinner. You've heard that before up in the mountains, a thin air well, when you're in thinner air, the air is less dense, and that means that there's fewer pounds of air per cubic foot the density changes.

So in general, when we have 400 cubic feet per minute, that's 400 cubic feet per minute per ton and we see that standard air ways. Oh point: zero: seven, five pounds per cubic foot so for every box of air every cubic foot of air. It wastes point zero five pounds, and so, if you do the math there, you multiply the four hundred cubic feet per minute. Multiply that times, zero point: zero, seven: five pounds per cubic foot: that's equal to 30 pounds per minute per ton of air.

So if we have a three and 50 CFM per ton, then it's gon na be lower than that. But if you're working on the 400 CFM per ton, then it's gon na be 30 pounds per minute per tons. That's a pretty easy thing to remember! So if you have one ton of capacity, that means you need 30 pounds per minute. If you have two tons would be 60 pounds per minute.

You get the point pretty easy math to do, and so that's how many pounds of air we need, and this pounds per minute is what we call mass flow rate. We talked about moving heat in and out of a coil. What we really care about is how much mass there is to the air how much weight there is, and for those of you who we've talked about this previously in the podcast, but really mass is the amount of inertia that an object has weight is affected by Gravity, and so you can kind of say, on earth, you can say mass and weight fairly interchangeably, but it does change with altitude, so mass changes less and so in general it will call it mass flow-rate. We don't call it weight flow rate.

It's the mass of the air, the inertia of the air, and so when you have a box of air you're, assuming that it's going to be point, zero, seven, five pounds per cubic foot standard error conditions, but that doesn't always stay the same. As example. That was recently doing some testing. We were seeing that in where we are in Central Florida inside the building in general, people like to keep it 75 degrees - 50 percent relative humidity.

I don't know why I said in Central Florida: that's pretty universal. Seventy-Five degrees, Fahrenheit and 50 % relative humidity, the air does not weigh point zero, seven, five zero: it weighs point zero, seven, three one, which means that the amount of density that you get the mass flow rate isn't exactly the same as it would be if you're Using the standard error equation, the old 400 CFM per ton figuring that you have 30 pounds per minute per ton. So if what we really care about is the pounds of air that we're moving, then why do we talk in terms of CFM? That's an interesting question. One that I don't necessarily have the answer to, but the reason probably is that, when we're dealing with fans, a fan doesn't depend on the weight of the air.

So a fan is going to move the same volume of air, regardless of its density and when we measure airflow with a rotating vane, anemometer you've seen them have a little fan that spins you hold it up that measures the volume of air. So the boxes of error, independent of its density, and so when we have a blower, let's say it's a blower and it's running at 70 degrees Fahrenheit. It's going to move the exact same CFM when it's 80 degrees, Fahrenheit and regardless of the density, regardless of the relative humidity. And so that's probably why we generally talk in terms of CFM, because that's what the blower affects.

But when we think about a coil. For example, how much air is going over a coil? The coil doesn't really care so much about the volume of air. It cares about how many molecules what is the overall mass of the air, and so that coil cares about the mass flow rate, but your blower can only produce a fixed volume flow rate, and so what do we have to do? Well, what it comes down to is: we have to make adjustments for our volume flow rate in order to achieve a fixed mass flow rate, and this is important in a couple different situations - it's important when you're to get to altitude. So when you have air that is thinner air, we're kind of looping back to that when air is thinner, that means that it's less dense, I mean we know that getting an altitude, it's less dense, and so that means you need more of it to achieve the Same mass flow rate, you have to correct for the change in altitude, so you just give an example.

So 70 degrees at sea level would be kind of a standard error equation. But if you go up to that same situation, so you have 70 degree air and you go up to 5,000 feet. Well, now you have to correct using a point, eight three correction factor at 5,000 feet of altitude, which essentially means that you have less mass for the same amount of volume, but your blower is not going to adjust your blowers. Gon na keep they're moving the same CFM and if you measure the CFM coming out of your blower you're gon na measure, the same thing that you measured before you're, not gon na see a difference there and your vein.

Anemometer is gon na measure the same, but the fact is that you're not going to be producing the same number of molecules. You have fewer molecules per cubic foot again. I keep restating the same thing because so you can get a picture of this. You have the box, which is the volume and you have how much stuff is in the box and that's the density which turns into mass right mass flow rate and so that blower keeps moving the same amount of boxes of air.

But what's in the Box decreases when you go up into altitude, and that makes a big difference with the way that the equipment operates. Rubber meets the road we could have a system in Florida where we have relatively high air density because we're at sea level - and it could work great at 350 CFM per ton. You take that same unit at 350, CFM per ton and put it up in the mountains of Colorado and you'll notice that the system starts to freeze. Why is that? Well? Because you need more molecules that it's moving the same amount of boxes but there's not as much stuff in the box and when you're measuring using different methods.

So, if you're measuring with say a PDO tube or if you're measuring static pressure, for example, those are density. Dependent measurements, meaning that those measurements change as the density changes when you measure with something like a vane anemometer, for example, one of those spinning things is you hold up to the vent? Those do not change, meaning they're, going to measure volume, they're gon na measure. The number of boxes and not what's in the box, whereas static pressure probe in a manometer is going to measure. What's in the box, a hot wire is gon na measure what's in the box, because it actually is contingent on the number of molecules.

So, there's a lot of different little things like this. When you talk about density, dependent versus density, independent measurements, and it really just all comes down to what are you measuring? Are you measuring the number of boxes or you're, measuring how much stuff is in the box? What are you moving? Are you moving boxes, or are you moving the stuff? That's in the boxes for all all key things to think about, and the same concept goes into when you talk about compressors, for example, in a compressor and older beginning compressor, you imagine that piston going up to its top point. It has a fixed volume in there, and so it's going to move a fixed volume of refrigerant, regardless of the density regardless of the mass, and that has a lot of ramifications and how the refrigerant circuit operates, which we're not going to go into now. But the main thing to know is: is that air takes up space, that's the volume and it has weight.

That's the mass affected by gravity, that's how we get weight. So hopefully, that's helpful to you, and especially if you're, in a circumstance where you wonder why you see a lot more under charge systems freeze up in Florida, whereas in Colorado you're much more likely to see a system freeze out because of low air flow. Both things apply, but it does vary from place to place because up there you got to get your CFM over 400 CFM per ton in order to move the same amount of stuff in the box. Hopefully that's helpful, alright.

Well, thanks for listening and as always, you can find all of our podcasts. By going to blue-collar roots comm, you could find our daily tech tips by going to HVAC our school comm. I appreciate you and we'll talk next time on the HVAC school podcast.