![]() But, the system is moving airflow that is 100F at an elevation of 5,000 feet above sea level. Say the manufacturer’s data requires 1,600 CFM of standard air for a furnace to operate properly under standard air conditions. To determine the amount of air needed under non-standard conditions, divide the amount of standard air needed by the corresponding factor as illustrated here: ![]() Or in simpler terms, there’s only 82% of the weight of air under these conditions as there is at 70 degrees at sea level. To determine the actual amount of air (or, again, the goop in the air) being delivered through your system under non-standard conditions, multiply the volume of air measured by the corresponding factor shown in the air density correction table.įor example: If 1,200 CFM of air was measured at 40F Fahrenheit at an elevation of 7,000 feet above sea level using a non-compensating anemometer by performing a duct traverse, what is the actual amount of air in the duct?ġ,200 CFM x 0.82 (THE FACTOR FROM THE CHART) = 984 CFM of standard air. As altitude or temperature increases, the weight of the air decreases. When testing air at temperatures or altitudes other than at “standard air,” the density of the air changes. Along the top of the table is altitude, and along he left side of the table is the temperature of the air. There are two primary forces that change the density of air that we can plot on the following air density table to make correcting for density quite easy. I have included a table at the end of the article that can be used to correct for air density in our design and air measurements at high altitude and under extreme temperatures. If you read the manufacturer’s data for gas-fired furnaces, each will offer correction factors to apply to BTU output ratings that compensate for air density. Fortunately, this substance is see-through and not very sticky, so we’ve become quite used to living it, moving through it and breathing it. with an average ceiling height of 8 feet, the home holds 16,000 cu.ft. If air wasn’t heavy, it couldn’t absorb the heat and carry it from the heater through the duct and into the rooms in the building.Ī typical home has more than 1,000 pounds of air in it. Air, or, more correctly, the goop in it, passes through a heat exchanger and absorbs the heat. ![]() One good example of the reality of this unseen goop is basic heat transfer from a typical gas fired heat exchanger. Goop? Granted, that's not a very technical term. This is necessary because a cubic foot of air passing through a coil at that elevation has only 75% of the goop in it than a cubic foot of air has at sea level. ![]() While this standard air density is good enough for 90% of us here in the United States that live below 2,000 foot elevation, those living at higher elevations must add an additional step to their engineering and measurement practices to remain accurate.Ī mile above sea level, for example, cooling systems typically need 500 CFM per ton to operate properly. Most efficiency and performance calculations are based on "standard air." Standard air is at sea level, 50% relative humidity, 70F, and weighs. While the basic principal is still true, let’s take a look at airflow from a different angle and see how air density can have a significant effect on how our systems perform. Four hundred CFM per ton has been the gold standard for cooling airflow since Willis Carrier proclaimed it so nearly a century ago. ![]()
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