Flow rate is the quantity of material that is moved per unit of time. Generally, the quantity of material can be expressed as a mass or a volume. For example, mass flow rates are in units of pounds per minute or kilograms per hour. Volumetric flow rates are stated in cubic feet per minute or liters per hour. The trick begins when volumetric flow rates are used for a compressible gas in different altitudes.
From the history of air compressors, they could calculate the volume of air being drawn into the air compressor by the size of the cylinder. With the volume of the compression chamber and the rotations per minute of the motor, RPM, they could calculate the volumetric air flows. As conditions change like air density, temperatures, and relative humidity; the values of the volumetric flowrate changes.
Since we are looking at the intake flow rates of an air compressor, what happens when they run at different altitudes? I remember that when I was in Denver, I got easily winded. Now, this could be that I was out of shape, but it was also because the air is less dense. That means for a volume of air, the mass of air was less. This is called the specific volume. Air compressors work the same way. So, let’s look at the Ideal Gas Law; Equation 1.
P * v = R * T
v – Specific Volume
R – Universal Gas Constant
T – Absolute Temperature
P – Absolute Pressure
In a comparative relationship, we can show the changes that can occur with an air compressor at different altitudes. Since we are looking at altitude, the air density and pressure will change at different elevations above sea level. If we keep the temperature the same, we can derive a formula from Equation 1.
P1 * v1 = P2 * v2
P1 – Absolute Pressure at Sea Level
P2 – Absolute Pressure at elevation
v1 – Specific Volume of air at P1
v2 – Specific Volume of air at P2
Specific volume is the inverse of density, so it has the units of ft3/lb or M3/Kg. If we use an example of a 40 CFM air compressor at sea level, it will produce 40 cubic feet per minute. We can calculate the flow rate of air that it can produce at 5,000 feet of elevation. The absolute air pressure at sea level is 14.7 PSIA, and at 5,000 feet, the air pressure is at 12.2 PSIA. So, if we look at Equation 2, we can rearrange the values to find the change in specific volume from sea level (position 1) to 5,000 feet (position 2):
v2 / v1 = P2 / P1 = 12.2 PSIA / 14.7 PSIA = 0.83
With the 40 CFM air compressor, it will now only produce 40 * 0.83 = 33.2 CFM of compressed air at 5,000 feet.
When sizing an air compressor, it is important to know the conditions. In this blog, I discussed the effects of altitude as it applies to the intake of an air compressor. But, no matter the size, elevation, or type of air compressor, EXAIR blow-off products like Super Air Knives, Super Air Nozzles, and Safety Air Guns will help you to save energy and increase safety. You can speak to an Application Engineer to see how.