Accurate Outdoor Airflow Measurement

Note: The following article is based on information derived from Air Monitor Corporation in a 2019 webinar “Yes You Can Accurately Measure Outdoor Airflow”. At this article’s conclusion is a link where you may view the webinar in its entirety. 

Introduction

Drawing and conditioning outside air for heating, cooling and ventilation applications remains a standard practice for new constructions, refurbished and older buildings alike.  But in order to ensure occupant safety and comfort, air brought in from the outside must be accurately measured.  Additionally, managing outside airflow is critical in reaching energy savings goals, and regulations associated with today’s high-performance buildings.

Why Measure?

Unlike other utilities, outdoor air is mandated by code requiring a certain amount of outdoor air to be delivered to a space. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) publishes multiple documents and standards to help ensure that ventilation health, regulatory, energy, performance, code and other requirements are met.

ASHRAE regulations include:

ASHRAE 62.1: This is the standard for outdoor air quality and amount of outdoor dilution air brought into a building based on building size, type and occupancy. ASHRAE 62.1 has become the de facto standard for determining the amount of outdoor airflow required for a commercial building. This standard also provides guidance regarding how to balance the need for outdoor air versus the energy cost associated with conditioning that air. While this standard does not directly mandate airflow measurement – measuring airflow directly is the optimum way to ensure compliance.

ASHRAE 189.1: This standard governs the design and operation of high-performance green buildings. This standard incorporated 62.1 and mandates that airflow be measured and accurate at each system level device.  Accuracy is the key; while measuring outdoor air is important, ensuring its accuracy is critical.

ASHRAE 90.1: This is the minimum energy standard defining the minimum energy efficiency requirements for the design and operation of new and existing buildings. While it does not directly mandate outdoor air, this standard relates to the operation and control of ventilation systems as they relate to climate zone, building size, type and so on.

Consequences of Not Measuring Airflow

While there are a variety of measurement technologies available today, most were developed to operate in the confines of duct work. With many of today’s commercial buildings being designed without OA duct (non-ducted), this creates some hurdles for measuring outdoor airflow.  Despite this, failure to accurately measure outdoor airflow is not an option.

The consequences of inaccurate airflow measurement include:

Airflow Measurement Challenges

Measuring outside airflow is not without its challenges; and the primary obstacle is accuracy.  Accurate measurement can be difficult due to the dynamic behavior of air flow itself; and the large variations in air conveying system designs. 

Let’s look at some of the challenges associated with Duct Work, Energy Systems, Actual versus Standard Conditions, and Accuracy versus Repeatability.

Duct Work

Most airflow measurement systems were developed decades ago and were designed to collect measurements within the duct work of a building.  Many ducted installations are unable to meet manufacturers straight run requirements. As a result, compensations for bends, take-offs, elbows and other obstructions must be made. Multiple point airflow measurement is the only way to obtain average airflow accuracy. 

OA Duct-related airflow measurement has created several additional challenges:

Energy Systems

Today’s economizer mode and energy recovery systems impact the amount of outdoor air entering building and how that air is controlled and operated.

Actual vs Standard Conditions

In the HVAC industry, there is generally, no compensation required to account for different airstreams being mixed within a system.  A standard volumetric air flow rate is required when operating above sea level or at temperatures significantly different than 70°. When this is the case, measurements must be compensated due to air density. This change in density affects the measurement and the velocity measure of the air flow. Additionally, compensation for volume differences must be made.

Accuracy vs Repeatability

Measurement accuracy – the ability to measure relative to a known standard – is clearly important; but repeatability is even more so.  Repeatability is the ability to consistently deliver the same measurement under the same conditions.  In the case of outside airflow control, consistency and repeatability is the key. However, when talking about the amount of air being delivered to a space or the energy associated with it, absolute accuracy is critical.

Airflow Measurement Technologies

There is no shortage of capable outdoor air measurement tools and technologies. Often, the problem, however, lies in the application of these systems.  It’s therefore important to select a solution that best matches your unique applications and challenges.

Today there are three methods for measuring outdoor airflow. The most common of these technologies are Differential Pressure and Thermal Dispersion. Let’s examine these measurement technologies along with their pros and cons.

Differential Pressure

Considered the standard of airflow measurement, Differential Pressure (the direct measure of total and static pressure in the duct) has been around for many years and is widely accepted throughout many industries. An advantage of this approach is that it allows for multiple sensing points per probe and meets all ASHRAE requirements regardless of duct size.

Pros and Cons: A distinct advantage of Differential Pressure is that it has many sensing points providing a great deal of accuracy. On the downside, however, the technology was designed to be installed in a duct; and many modern buildings have ductless outside air systems.  This technology may also have accuracy issues when measuring low velocity – as is typical with outside air. The Differential Pressure measurement approach reasonably resistant to dirt, dust, and moisture.

Thermal Dispersion

Another common technology for determining airflow measurement is Thermal Dispersion.  Like Differential Pressure, Thermal Dispersion has been around for some time. This method utilizes two precision-matched thermistors for differential temperature measurement. One thermistor measures airflow temperature while the other is externally heated to a set differential above the airflow temperature. Heat is transferred from the externally heated thermistor to the air stream; as airflow velocity increases, the rate of heat dispersion increases. The relationship between airflow velocity and applied power is defined by an algorithmic function.

Pros and Cons: Because of its ability to accurately measure low velocity air movement, Thermal Dispersion is a great fit for outside air. Like its counterpart, this technology was developed for duct-mounted airflow measurement devices. Similarly, any contaminations such as dirt, dust and moisture will impact the impact the accuracy of the sensors.

Static Differential Pressure

In this method, fixed outdoor air inlets act as a flow conditioner creating a uniform static pressure field on either side of the inlet. Velocity is determined by measuring the static differential pressure across the fixed inlet. The fixed inlet device has a unique mathematical relationship which defines the airflow velocity as a function of pressure drop. This device may be an architectural louver, rain hood with expanded metal or a prefabricated outdoor airflow measuring station.  This technology is ideal for today’s buildings with non-ducted OA applications.

Pros and Cons: With the ability to measure effectively in low or high airflow velocity, Static Differential Pressure is ideal for today’s non-ducted applications.  The technology is also impervious to wind gusts, dirt, debris and moisture. A fixed inlet device combined with a true static differential pressure sensor eliminates the need for straight run duct. Static Differential Pressure technology is also shown to provide excellent performance across a wide temperature range of -40° F to 120° F, and outdoor air conditions of 0% to 100% relative humidity.

Learn More

When it comes to measuring outdoor airflow remember this: if you’re going to measure it; measure it accurately. We’ve just touched on the very basics of outdoor airflow measurement accuracy. 

For a more detailed look at this topic, including installation requirements, the formulas for calculating accuracy and measurements be sure to view the Air Monitor Corporation free on-line webinar: “Yes You Can Accurately Measure Outdoor Airflow”.

View the webinar here: https://www.airmonitor.com/whats-new/

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