Knowing How to Measure Air Movement Ensures Proper Operation & Uncovers Defects
Supplying the appropriate amount of air to a building to ensure the comfort and health of its occupants requires a thorough knowledge of pitot tubes, manometers and anemometers – instruments used to measure air volume and velocity in air ducts.
The primary measurement of air velocity (speed) and air flow (volume) in an HVAC system is the differential pressure output, and in some cases, the difference between the duct pressure and atmospheric pressure. To understand how air movement is measured, it’s helpful to understand how the instruments work.
Air velocity is the measurement of the rate of displacement of air or gas at a specific location and is a statement of distance traveled in a unit of time, typically measured in feet per minute (fpm) or meters per second (mps). By multiplying air velocity by the cross section area of a duct, you can determine the air volume flowing past a point in the duct per unit of time. Volume flow is usually measured in cubic feet per minute (cfm) or cubic meters per minute (cmm).
In an HVAC system there are three types of pressures present, all measured in inches of water column (wc) or Pascal (Pa). The first is when air gets distributed by fans or blowers that add motion and pressure to the air by either a screw propeller or paddle wheel action. The wind produced from the fan blades causes the air to accelerate, creating a force in its direction of motion called Velocity Pressure.
The second type of pressure, Static is the amount of resistance produced when air is moved through the duct. The higher the static pressure or resistance, the more energy it takes to move air through the duct. When static pressure is high, air velocity is usually low. This is a good place to start a diagnostic evaluation if you suspect a defect in the system. The third pressure is Total pressure, the combination of static and velocity pressure. All of three types of pressure can be measured using pitot tubes, manometers, and anemometers.
Pitot Static Tubes and Manometers
A pitot tube can determine the volume of air being delivered to a conditioned space by inserting a slender, hollow tube that has two holes on it into the air stream through a small hole in the duct. As air floods into the end of the tube it creates pressure. By comparing the pressure inside the tube with the natural pressure of the air around the tube (the static pressure), you get an accurate measure of the air speed. The pitot static tube is a combination of a pitot tube for measuring total pressure and a static tube for measuring static pressure in the flow, allowing the velocity to be determined at the point of measurement. Imagine a tube within a tube where the air space between the tubes allows the transfer of pressure from the two sensing holes to a manometer through connecting tubing. Where the pitot tube detects total and static pressures, the manometer measures the dynamic velocity pressure – the difference between total and static pressures. Used together with a pitot tube, manometers can measure standard air velocities (air density of 0.075 lbs per cubic foot or 70° F and a barometric pressure of 29.92 inches).
Vane anemometers can average air velocities at supply openings, walk-in ducts an filter banks. They work when the airflow makes contact with, and rotates the vane blades. A magnetic or optical sensor converts the signal to a velocity measurement in feet per minute (fpm) ranging from 50 to 6000 fpm.
Hot wire anemometers measure wind speed based on the rate of heat loss to air flowing by a sensor using a very fine wire (micrometers) that has a measurement range spanning from 0 to 10,000 fpm. The wire is electrically heated up to some temperature above the ambient temperature by passing a current through an electrical resistance. The energy is then converted to heat. Air flowing past the wire has a cooling effect on the wire. As the electrical resistance of most metals is dependent upon the temperature of the metal, a relationship can be obtained between the resistance of the wire and the flow speed.
Importance of Air Motion
Outdoor air quality and how it’s delivered indoors, including the breathing zones of occupants, are critical to maintaining good indoor air quality. Mechanical ventilation systems in large office buildings are designed and operated not only to heat and cool the air, but also to draw in and circulate outdoor air. If they are poorly designed or operated causing inadequate air motion inside the building or if vents are blocked or ineffectively placed, air can’t reach the breathing zone of the occupants, resulting in poor ventilation. It’s important for the air to change on a consistent basis to provide temperate air movement as well as continuous distribution of contaminants. ASHRAE recommends that the average air movement in the occupied zone for the winter season not exceed 0.15 m/s and summer air movement should not exceed 0.25 m/s. It’s important to rebalance the HVAC system if you add office equipment like printers, copiers and computers – devices that might increase the building temperature to ensure proper air movement is taking place.
Assessing indoor air quality is important, not only for keeping occupants comfortable, but keeping them healthy as well. Properly measuring air movement should be a routine part of all HVAC performance and preventative maintenance programs.
Source by: Dan Martin, Territory Sales Manager – REED Instruments