ASHRAE Guideline 16 (2010) and Taylor (2014b) state that the RLD should be an opposed blade damper
and sized for a pressure drop equal to 7% to 15% of the relief path total pressure drop to provide a fairly linear control response.
Sound Level Increase in Nc: Vertical Inlet Duct Height: 3 Diameters All Inlet Diameters Case 1 Case 2 Damper: (Sliding No Damper No Damper for Round Inlet, Opposed Blade for Square Inlet) Type Duct Flex Rigid Square 1 0 (Round Inlet) Round 1 0 (Round Inlet) Type Plaque 1 0 Diffuser (Round Inlet) Perforated 1 0 (Round Inlet) Modular Core 0 0 (Square Inlet) Louvered 1 0 (Square Inlet) If a round cross opposed blade damper
is used instead of a sliding damper, subtract 2 dB from the sound increase.
diameter inlet inlet inlet inlet Damper: no no damper damper (sliding damper damper in in for round inlet inlet inlet, opposed blade for square inlet) Type of flex rigid flex rigid duct Square 1.4 1.1 1.7 1.5 (round inlet) Round (round 1.3 1.1 1.6 1.2 inlet) Plaque 1.4 1.1 1.7 1.5 (round inlet) Type of diffuser Perforated 1.3 1.1 1.6 1.2 (round inlet) Modular Core 1.2 1.1 1.4 1.3 (square inlet) Louvered 1.4 1.2 1.8 1.6 (square inlet) If a round cross opposed blade damper is used instead of a sliding damper, subtract 0.3 from the pressure loss ratio.
Damper types used were the round sliding, the round opposed blade, and the square opposed blade dampers. Approach angle is measured from perpendicular to the diffuser side.
The damper used for the close coupling testing was the round opposed blade damper in the full open position.
Sound Level Increase, Field Installations with a 3 Diameter Vertical Duct Height Sound Level Increase in NC: Vertical Inlet Duct Height: 3 Diameters All inlet Case 1 Case 2 Case 3 Case 4 diameters Damper: no no damper in damper in (sliding for damper damper inlet inlet round inlet, opposed blade for square inlet) Duct type flex rigid flex rigid Square 1 0 8 7 (round inlet) Round (round 1 0 10 8 inlet) Diffuser Plaque 1 0 8 7 type (round inlet) Perforated 1 0 5 5 (round inlet) Modular Core 0 0 3 2 (square inlet) Louvered 1 0 7 6 (square inlet) If a round opposed blade damper is used instead of a sliding damper, subtract 2 dB from the noise increase.
Manufacturer C used an opposed blade damper
for both brands of motors.
Sequencing opposed blade dampers
will probably promote better mixing with this sequence, but opposed blade dampers
should not be used if the dampers are modulated simultaneously.