HVAC systems use limited amounts of outside air to refresh the air inside commercial high-performance buildings. Accurately measuring the amount of outdoor air being pumped into a building is critical for many reasons, including:
- Maintaining the proper pressurization throughout the building
- Avoiding excess costs due to waste of conditioned air
- Preventing overuse of fans which increases maintenance and replacement cost
- Preventing ingress of humidity, dust, debris, and pollutants that can degrade the IAQ
- Meeting environmental regulations for energy efficiency & LEED certification
- ASHRAE Standards such as 62.1 for ventilation rates and 90.1 for high performance DOAS units.
However, accurate outdoor HVAC airflow measurement can be difficult due to weather and local climate conditions, as well as the design of the air inlets.
Climate and Weather Conditions
Outside air pumped into an HVAC system from the rooftop of a building can vary widely in temperature, moisture level, and particulate load. On top of that, the speed and direction of the wind and the barometric pressure can fluctuate from one hour to the next. All these factors can affect the accuracy of an outdoor air flow measurement system.
Duct Design Difficulties
Accurate measurement is made more difficult by the design of some outdoor air inlets, which usually have limited straight runs available to condition the airflow prior to measurement. Some systems require a certain number of upstream straight runs of the duct to reduce flow turbulence before taking measurements.
In addition to the lack of straight duct runs, HVAC inlets are often retrofitted with an economizer inlet to help meet the building’s energy efficiency goals. The addition of an economizer means that the inlet is oversized for maintaining indoor air quality. The result is very low flow velocities at the inlet most of the time, which directly affects measurement accuracy and reliability.
Rooftop packaged units are a worst-case example, as the minimum outside air and the 100% outside air economizer functions are forced into a single inlet. To make matters worse, some manufacturers combine several unit sizes into a single box/frame/control damper/hood size.
Outdoor Airflow Measurement Technologies
The three most often used technologies for measuring outside air are thermal dispersion, Pitot array, and pressure difference across a known resistance. In this post, we will briefly look at each technology to determine the best fit for measuring outdoor air flow.
Thermal dispersion
To get accurate air flow measurements, thermal dispersion systems typically require 2 to 8 diameters of straight duct runs both upstream and downstream – something that’s rarely available in outside air intake duct configurations. Also, thermal sensors do not tolerate any form of moisture in the air and they perform poorly in dynamic and turbulent airflow conditions, making them a poor choice for accurate outdoor hvac airflow measurement.
Pitot array technology
In this type of air flow measurement system, dynamic pressure and static pressure are measured at many points throughout the duct, and the results are averaged. The velocity of airflow is proportional to the difference in pressure, so a differential pressure (DP) transmitter is used to convert the difference into an average airflow velocity. The transmitter then converts average velocity into average airflow volume using the cross-sectional area of the duct.
The performance and price level of airflow measurement systems that use the Pitot measurement principle can vary widely. These systems have two types of components: Pitot probes and the DP transmitter. System accuracy depends on the number of Pitot tubes required, the quality of the transmitter, and to some degree, the local air quality. Air inlets with limited straight runs available to condition the airflow require large numbers of Pitot tubes to achieve accurate flow measurements, which could make them cost-prohibitive.
Pressure difference across a known resistance
Yet a third technology uses small differences in static field pressures created by air moving through a fixed inlet to accurately measure airflow without the need for straight ducts to condition flow. The concept is based on measuring static pressure fields that develop when air flows through any screen or louver with a known velocity vs. pressure drop curve without a significant change in the pressure drop.
These static pressure measurements used in concert with temperature, barometric pressure measurements, and inlet size can be used to calculate the flow rate at any atmospheric condition. This type of system is nearly impervious to dust and moisture, has a high tolerance for dynamic variations in air flow velocity and direction, and requires zero straight runs of duct.
An Accurate Solution for Airflow Measurement in Outside Air Applications
Air Monitor’s OAM II Outdoor Airflow Measurement System uses static pressure field measurements to provide accurate airflow monitoring in this challenging application. The system consists of the OAM II Transmitter with a built-in barometric pressure sensor, one or more uni-sensors or OAM II airflow measuring stations, and one or two temperature sensors. The system has been designed to measure the actual airflow rate to within ±5% by measuring differential pressure across a fixed inlet, such as a louver, screen, or layer of expanded metal.
Precision in outside airflow measurement is essential to ensure that ASHRAE ventilation standards are met, energy consumption calculations are accurate, and proper indoor air quality is maintained for occupant health and safety. The OAM II System allows you to fine-tune the amount of outside air flow to maintain pressurization, meet energy efficiency goals, and keep operating costs low.
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