In this video, we're going to show the difference between Acfm or actual cubic feet per minute of air and Scfm standard cubic feet per minute of air, both of which are different means of quantifying airflow as well as an understanding of mass flow rate versus volume flow rate of air. CFM stands for cubic feet per minute. If we think of a cubic foot of air as a box that's one foot long, one foot wide, and one foot tall, then we can think of CFM as the number of those boxes we move by the blower per minute. To get an idea of the air we're moving, we need to understand the temperature and relative humidity of that air.

Here we're showing a box of air at sea level. This particular box of air is 68.3 degrees Fahrenheit and zero percent relative humidity. These are the conditions of standard CFM air or S CFM Air has weight and takes up space. This box of air weighs 0.075 pounds.

As you will see shortly, the weight of an air mass will change in response to changes in temperature, pressure, and humidity. About 78 percent of the air we breathe consists of Nitrogen 21 oxygen and one percent several other tray substances. Since air is a vapor, these molecules are constantly zooming past each other as we add heat to the air. The molecules start to move around more quickly and become more spaced out Since temperature is the measurement of the average molecular velocity or average molecular speed.

As the temperature of the air in the Box increases, the molecules are also getting farther apart because the volume increases as temperature increases according to Charles law. Due to the changes in temperature and volume, hotter air tends to be less dense than cooler air, so this box of air gets lighter as the air heats up. When we remove heat from a given quantity of air, the molecules start to slow down and become more tightly packed together. This increases the weight of the air and causes the volume to shrink.

In general, air with higher humidity will mix with drier air. Up to this point, we've been talking only about dry air, but most air also has a mix of moisture in it that we call humidity. Humid air is lighter than dry air. Yes You heard that right? Humid air is lighter than drier due to the differences between the mass of water molecules and nitrogen and oxygen molecules.

Again, remember, we're talking about water vapor here, not liquid water. When we think about the mass or weight of air, we must consider the molecules within it. Since our air primarily consists of Nitrogen and oxygen, those two molecules will make up most of the weight of air. N2 Or a nitrogen molecule has an atomic mass of 28 units.

O2 Or an oxygen molecule has an atomic mass of 32 units. A molecule of water or H2o has an atomic mass of 18 units, which is fairly obvious because Hydrogen, the lightest molecule there is, makes up the majority of an H2O molecule. Therefore, humid air is lighter than dry air due to water Vapors lower atomic mass than nitrogen and oxygen, which make up most of dry air. We don't just have one box of air though.

HVAC Systems move many boxes of air per minute and these boxes all exert force on each other. Elevation also affects the density of air. The air gets less dense at higher altitudes due to the lower pressures. The surrounding air masses also exert less force on each other at those higher elevations.

As you can see here, the cube of air weighs less in the mountains than it did on the beach due to the lower atmospheric pressure at elevation than the 14.7 PSI present at sea level. In other words, the air weighs less and is less dense at elevation. When working on HVAC systems, we measure air flow in cubic feet per minute or CFM. You may have heard that 400 CFM is a rule of thumb.

Target Airflow CFM Targets will vary by individual circumstances and climate. In actuality, we want a fairly fixed mass flow rate over the evaporator coil or pounds of air per minute rather than CFM for proper system operation. Even in a case where someone is shooting for a CFM Target of 400, the actual cubic feet per minute or Acfm may need to differ slightly because the air may be heavier or lighter than the standard Scfm. Again, Scfm refers to the CFM of a one foot by one foot by one foot box of air at 68.3 degrees Fahrenheit and zero percent relative humidity.

In a dry or arid climate where we have hot dry air, those boxes of air will have a low relative humidity which brings it close to the zero percent RH of Scfm, But they will also often be hotter than the 68.3 degrees. So you really could have a target of more than 400 a CFM. In humid climates, the air will contain a lot more water vapor. Again, because water vapor is lighter than nitrogen and oxygen, these boxes of air will be lighter than in the arid climate and significantly lighter than the Scfm standard.

In many cases, an HVAC system could actually have a target of more like 405 or 410 Acfm to hit the same mass flow rate or pounds of air permitted over the coil, but we also may have a much lower Target than that if we are trying to run a cold evaporator coil on purpose to remove more moisture from the air. You see how complicated this gets in cold dry climates. The air is heavier than an arid. in humid climates.

Because the air is colder and drier, the boxes are denser. These boxes of air could potentially be heavier than those that fit the Scfm standard. In an HVAC system could have a target of less than 400 Acfm. compared to cold dry air, the moist air of cool humid climates is less dense due to the water vapor content.

You can expect the target Acfm to be a bit higher than that of a cold dry climate. Again, this is if the goal is to maintain a consistent pounds per minute of air over the evaporator coil. These same Concepts apply to air at higher altitudes. The boxes of air are less dense due to the lower pressure at high altitudes.

Again, the Acfm will likely need to be higher at those upper altitudes due to the lighter air. All of this is to say that different temperature, humidity, and altitude conditions affect the weight of the air you're moving. While Scfm is a common reference point that we can use to compare the performance of equipment, Acfm gives us a more nuanced view of the air we're actually moving or trying to move in a given environment, and becomes a real factor in how we measure air depending on the instruments that we're using. It also makes a big difference what type of blower motor is in the equipment, how the weight of air impacts The Blower's performance.

In other words, air has weight and takes up space, but that weight per cubic foot or box of air changes and we need to take that into account during system design, commissioning and testing. Thank you.