D. JEFF BURTON, MS, PE, FAIHA (former CIH and CSP, VS), is a past president of AIHA and an industrial hygiene engineer with broad experience in ventilation used for emission and exposure control. He is also an adjunct faculty member at the Rocky Mountain Center for Occupational and Environmental Health at the University of Utah in Salt Lake City. Send feedback to The Synergist.
Common Properties of Air
Air can be friend or foe, depending on its conditions and our management of those conditions. Because my Synergist articles for 2022 will cover ventilation topics, it will be helpful for readers to review some properties and behaviors of indoor air. Please keep this article handy as you read my upcoming articles.
CHARACTERIZING AIR Volume. Earth’s atmosphere weighs about 5.5 quadrillion tons, which is still only about one millionth of the Earth’s total mass. The atmosphere has an average useful depth of about 7.5 miles above the earth’s sea level (about the highest altitude jet passenger airplanes fly), but most people are comfortable only in the first 2 miles. Air inside a jet plane is usually pressurized to an equivalent height of 6,000–8,000 feet.
Composition. Uncontaminated dry air mainly consists of oxygen (about 20.9 percent), nitrogen (78 percent), argon (0.9 percent), and carbon dioxide. (The current average of CO2 in the atmosphere is approaching 420 ppm, up from about 325 ppm in 1971.) At near-standard conditions, air can also contain water vapor (up to 2.5 percent), particles, aerosols, and other gases and vapors. When these other airborne gases, vapors, particles, and aerosols exceed certain concentrations, odors and adverse health effects can occur. As OEHS professionals, we need to understand how these chemicals and concentrations relate to the comfort, health, and well-being of those who are exposed.
Physical conditions. Air has a standard condition—known as STP, or standard temperature and pressure—at which its temperature (T) is 70 F and its barometric pressure (BP) is 29.92 inches of mercury (inHg). At STP, air contains little or no water vapor (that is, its relative humidity, or RH, is essentially 0 percent), weighs 0.075 pounds per cubic foot (lbs/cu ft), and has an air density correction factor (df) of 1.0. The df parameter can be used to correct the density of air and other factors at non-STP conditions.
Under normal circumstances, water vapor in the air varies between 0 and 2.5 percent and averages about 1 percent at sea level. At 70 F and 100 percent RH, about 2.5 percent of the air volume is water vapor. Because water vapor displaces other gases and vapors, the oxygen concentration in this example is reduced from 20.9 percent to about 20.4 percent. (The vast majority of people don’t notice this small reduction in the percentage of oxygen.)
Mass/weight. At STP, air weighs about 0.075 lbs/cu ft and varies with changes in temperature, altitude, and pressure. The air density correction factor (df) can be used to estimate the actual weight of air. (See examples below.)
Movement and mixing. Air molecules at standard conditions are separated from each other by about 12 molecular diameters and are moving at high speeds in random directions. This constant mixing of the air causes currents that move at about 20 feet per minute (fpm) in “quiet” indoor environments such as offices and classrooms. While we rarely notice air moving at this velocity, we do notice that an odor can travel quickly through a room, which proves that the air is mixing. Near air supply registers, the mixing velocities are typically higher. People can feel air movement on their hands at about 100 fpm and on their neck and ankles at about 50 fpm. (That is one reason we wear shirts or blouses with collars and socks that go above our ankles.)
Constant mixing of the air causes currents that move at about 20 feet per minute in offices and classrooms.
Pressures. Air exerts pressure in all directions due to gravity. The normal air pressure is 14.7 pounds per square inch (psi) at STP conditions, or 29.92 inHg in a mercury barometer, or 407 inches of water in a water barometer. (In other words, a column of water 407 inches high exerts a pressure of about 14.7 psi.) Air movement can also create force or pressure. The properties of air include velocity pressure, or VP, which is the pressure exerted by the air moving in one direction; static pressure, or SP, which is the pressure of the air relative to the barometric pressure; and total pressure, or TP, which is the sum of SP and VP.
Air can also be described by its “partial pressures.” The partial pressures of air gases (nitrogen, oxygen, and so on) are simply the pressures they would exert if they were isolated in the same volume of space at the same temperature. Since atmospheric pressure averages about 760 mmHg at sea level, the 78 percent of air molecules consisting of nitrogen exerts about 590 mmHg of pressure while the 20.9 percent contributed by oxygen exerts about 160 mmHg of pressure.
When air moves to higher altitudes, it loses some of its density and pressures at sea level. For example, the density correction factor (df) at Salt Lake City, Utah, at 70 F and at an elevation of about 4,250 feet above sea level, is about 0.85. This means the density of air (and of its oxygen) is 15 percent less than at sea level and that the partial pressures are also reduced 15 percent. The average barometric pressure is reduced to 645 mmHg and the partial pressure of oxygen is reduced to 135 mmHg.
Psychrometrics. This term is related to the properties of air and water vapor mixtures. Seven properties of atmospheric air are commonly included in psychrometrics: dry bulb temperature, wet bulb temperature, dew point, humidity ratio, relative humidity, specific volume, and specific enthalpy. I will discuss the use of psychrometrics in future articles.
BASIC ESTIMATION METHODS The formulas discussed in this section all use U.S. units of measurement.
A common formula for airflow rates is given by the equation Q = V x A, where Q is the airflow rate in cubic feet per minute (cfm), V is the average velocity of the airflow rate in feet per minute (fpm), and A is the cross-sectional area in square feet (ft2) the air is flowing through. For example, suppose air is flowing through a classroom window two feet high and 3 feet wide at an average velocity of 400 fpm, as measured using a velometer:
As mentioned earlier, a density correction factor (df) can be used to correct the density of air and other air factors. The common formula is
where BP is the local barometric pressure, F is the local temperature in Fahrenheit, and 530 is the absolute temperature in degrees Rankine at these standard conditions. This formula can be used, for example, to estimate actual velocities and airflow rates at non-STP conditions. Suppose the air in the classroom has a temperature of 80 F and the barometric pressure is 27.5 inHg. Therefore:
The air at this location is about 10 percent less dense than at STP. This difference can impact other measured parameters, such as the air velocities and flow rates in the space. The density correction factor can be used, for example, to help estimate air velocity in ductwork. Suppose the air moving in the classroom ductwork has a velocity pressure (VP) of 0.53 inches of water. The duct cross-sectional area is 0.65 sq. ft. What is the airflow rate in the duct? To use the equation Q = V x A, we need to estimate the area and velocity of the air in a duct. To estimate velocity, we often use an equation first described by the 18th-century Swiss mathematician Daniel Bernoulli:
Now that we know the velocity, we can estimate the airflow rate (rounded for significant figures):
In some cases, the units of Q (cfm) are further designated as “airflow at actual barometric conditions” (acfm) or “airflow at STP” (scfm). Because we corrected for air density in this example using df, we can designate the airflow units as “acfm.”
MORE TO COME More detailed information on ventilation practices, standards, and estimating techniques will appear in future articles. My next article will summarize the new ANSI/ASSP Z9.5 standard on laboratory ventilation and its applications.