HVAC Grille and Register Testing in the Airflow Sciences Lab
By Dr. Kevin Linfield, P.E. and Kelly Hile
The Airflow Sciences flow laboratory conducts testing on HVAC components to quantify their aerodynamic performance. Every piece of an HVAC system plays a role in the system’s performance. Any barrier in the duct system contributes to static pressure loss, including air filters, elbows in the ductwork, and grilles or registers. The system designer must ensure that the fan or blower in the HVAC system will be powerful enough to overcome the pressure losses of each component while also providing enough air supply to each room.
The international standard ANSI/ASHRAE 70-2006 details the method of testing the performance of air outlets and air inlets. It is used to define laboratory methods to test ducted and unducted systems for distribution and return of building air. It includes specifications for test instruments, facilities, installations, and procedures and methods of calculation for determining aerodynamic performance. Typical air inlets and outlets consist of grilles, registers, diffusers, vents, and slots.
Measuring Static Pressure Loss
As air flows through a grille or air supply register, a loss in pressure occurs because of the flow restriction caused by the grille geometry. A very open geometry will cause a smaller pressure loss than a more restricted design. A grille face design undergoes testing so that a designer can factor these losses into the overall design of the HVAC system. The higher the flow rate through the grille, the greater the pressure loss will be, so grilles are tested over a range of flow rates. If the register contains adjustable dampers, these can also be tested at multiple orientations.
Pressure loss will vary based on the size and geometry of the grille
A good test system will feature a plenum with internal flow control devices to ensure smooth, even flow to the grille. The discharge velocity or core velocity is measured by a vane anemometer placed near the center of the register face, and the pressure loss is defined by the plenum pressure. The system flow rate is measured with an AMCA/ASHRAE flow tunnel.
Measuring Throw
Another factor in the design of an HVAC system is the effective delivery of air to room occupants. The supply register or vent must be sufficiently sized to project air into the main occupied zone of a room. This parameter is measured in feet and is called “throw.” Throw is defined as the distance it takes for the air stream to slow down to a particular air velocity (called terminal velocity). When choosing a properly sized register for a given room, designers can compare the throw distance at various flow rates with the overall room size.
Supply air slows down as it discharges from the register into a room
The throw depends on both the flow rate and the core velocity of the air stream as it discharges from the register. For example, if the core velocity at the register discharge is 300 ft/min, at some distance away from the register the air stream velocity will slow down to 150 ft/min. This distance is measured in feet and is the throw for 150 ft/min. Throw is also determined for slower terminal velocities of 100 ft/min and 50 ft/min. Throw values are measured along a center line normal to the grille face using a hot mandrel probe to determine the free air stream velocity. This process is repeated for a range of flow rates/core velocities.
Throw is the distance where the terminal velocity is reached
Performance Data
Results of these flow tests are often supplied in the form of data tables that grille manufacturers can include in their product specification documents. For product lines with many variations in sizes and shapes, test data from a representative subset of grilles can be used to determine performance data for full range of product sizes. Performance data includes static pressure, flow rate, and throw for a number of face velocities.
Sample performance data table
In addition to laboratory testing, Airflow Sciences also utilizes computer modeling and CFD simulations to assess flow through registers, duct systems, and flow patterns within rooms. CFD can also be used to determine the relative noise generation between two different register designs, which can be helpful during product development.
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