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 out the volume of air flowing beyond a point in the duct per unit of time. Usually, volume flow is measured in cfm (cubic feet per minute) or (cmm) cubic meters per minute.
Considering Pressures
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 pressure and motion to the air by either a paddle wheel action or screw propeller. 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 consumes to move air via the duct. When static pressure’s high, usually the air velocity is 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 three types of pressure can be measured using pitot tubes, manometers, and anemometers.
Pitot Static Tubes and Manometers
A pitot tube can find out the volume of air that is being delivered to a conditioned space by having a slender inserted, hollow tube that contains 2 holes on it into the air stream over a tiny hole in the duct. As air overflows into the tube’s end, 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 the air speed’s accurate measurement. of the air speed. The pitot static tube’s a blend of a static tube for measurement of static pressure in the flow and a pitot tube for measurement of total pressure, 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).
Anemometers
Anemometers are one of the most effective instruments for measuring air flow and volume and are available in two technologies – vane and hot-wire anemometers.
Vane anemometers can be used to average air velocities at filter banks, walk-in ducts, and supply openings. They function when the airflow gets in contact with, and rotating the vane blades. An optical or magnetic sensor is converting the signal to a velocity measurement in fpm (feet per minute) ranging from 50 to 6000 fpm.
Wind speed is measured with Hot wire anemometers based on the rate of loss of heat to air that’s flowing by a sensor by utilizing a really fine wire (micrometers) that has a 0 to 10,000 fpm measurement range. The wire’s heated up electrically to some temperature that’s beyond the ambient temperature by passing a current via an electrical resistance. Then, the energy’s converted to heat. Air that’s flowing past the wire has the effect of cooling on the wire. As the electrical resistance of many metals relies on the metals’ temperature, a relationship can be gained between the flow speed and the resistance of the wire.
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 made and operated to not only cool and heat the air, but also to circulate and draw in outdoor air. If they re 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