Mass Flow vs. Volume Flow – Short 103

In this short podcast episode, Bryan discusses the difference between mass flow and volume flow when referring to HVAC equipment processes.

When you are confused as to whether you're dealing with mass or volume flow, think about the units. For example, cubic feet per minute (CFM) is a measure of volume because we're talking about cubic units. We care about the volume when we think about air mixing and velocity, but volume isn't much of an indicator of the actual cooling power.

The mass or weight of the air matters more when we think about cooling a space. There is a lot of variation in how much air weighs, which will impact the performance of HVAC equipment under given conditions. Standard air has a weight of 0.075 pounds per cubic foot, but that can vary depending on humidity, temperature, and pressure conditions.

When you think about volume flow rate, think about moving boxes of matter. As a blower operates, it moves a series of air “boxes,” which is a useful way to look at air velocity.

Compressors have a fixed volume in their compression chambers, unlike blower wheels. (Blower wheels move different volumes of air based on motor staging and other conditions.) However, mass flow is NOT fixed. In a compressor, we can fill those boxes with more weight (higher mass flow). On occasion, too much mass will move at once; a hot pull down is a common scenario where we have too much mass flow. In those cases, we can use crankcase pressure regulators. A system's compression ratio also has a major effect on mass flow rate; the “boxes” might be too light to keep the compressor cool enough to operate efficiently. In the worst-case scenario, the compressor may overheat.

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