Airflow considerations when designing HVAC ductwork
Ductwork used to channel or direct air to spaces throughout a building can impact the efficiency of the structure’s HVAC system. That’s why it’s important to properly design and install ductwork and its components, including the dampers used to regulate or stop airflow.
As they select dampers that will best suit their ductwork design, HVAC construction engineers have the option of specifying either airfoil, triple-V groove or modified flat blades. Airfoil blades offer several advantages, beginning with blade strength.
The strength of a blade is determined primarily by its shape. Modified single-skin blades are difficult to keep flat and true when compared with a hollow, airfoil-shaped blade. When side linkage is used to keep linkage out of the airstream, a hollow blade will not twist from side to side.
An airfoil blade provides higher static-pressure design. This becomes important when dampers are designed to close rapidly, as is the case with fire dampers with fusible or bimetal links. Any instantaneous or sudden, momentary static pressure that results will far exceed the design static pressure due to the mass of air flowing through the duct, downstream of the damper at closure. The negative static pressure can actually collapse a duct or destroy the damper, making the higher static-pressure design of an airfoil blade critical.
Lower static-pressure loss
The potential for static-pressure loss should also be noted as an engineer considers blade design. The static-pressure drop on an airfoil blade is less than that of a single-skin blade, as indicated in testing performed to Air Movement and Control Association Standard No. 500 (see Figure 1).
Because many systems use multiple dampers in a series, static-pressure drop becomes especially important when the velocity drop exceeds 1,500 feet per minute.
Airfoil blades offer three additional advantages. First, they provide excellent blade-to-blade sealing action because the blade’s hollow shape does not twist when force is exerted from one edge.
Single-skin blades depend on offsets or bends in the blade to make it strong. However, these bends do not support the blade in a twisting action. Unlike single-skin blades, the airfoil blade moves throughout its length when twisted on one end, maintaining the seal.
Additionally, the hollow airfoil blade can withstand the most rigorous fire endurance testing because the blade shape actually provides heat resistance not found in a single-skin blade design. Even in a four-hour British standard fire test performed under positive pressure, giving a hotter test than an Underwriters Laboratories’ No. 555 fire test, the temperature on the exposed side of an airfoil blade damper was 212°F cooler throughout the test than a damper with V-groove blades.
In addition to blade shape, engineers must consider the shape of the damper when designing a duct system. True round dampers offer better pressure drop and airflow performance than their square or rectangular counterparts. This includes round control, fire or combination fire/smoke dampers. Round fire and combination fire/smoke dampers are cost-effective and easy to install.
As engineers design ductwork with fire dampers, the ductwork downstream from the fire damper must be specifically designed to handle negative pressure. Negative pressure is created by the air mass that continues to flow through the ductwork when the damper closes very fast. Figure 2 shows a negative 8-inch water gauge static pressure that built downstream after the fast closing of the curtain damper in the ductwork.
To prevent the ductwork from collapsing under this high negative pressure, engineers can specify an access door to relieve the pressure. A high-pressure relief access door installed downstream from the fire damper automatically relieves the negative pressure generated by fast-closing dampers, no matter what the damper type — dynamic curtain fire damper with springs or multiblade fire damper with fusible link. Combination fire/smoke dampers use an electronic fuse link device and spring-return actuator, which provides controlled closure. Standard access doors can be installed downstream of the damper for inspection purposes only.
The placement, shape and blade design of dampers can impact HVAC system installation costs and performance. Specifying the right damper for the job can go a long way toward ensuring satisfied customers who are confident their system is delivering conditioned air safely and efficiently.