ACHRNEWS

Damper Actuator Savvy

July 23, 2003
It is sometimes called a damper motor, damper operator, and motor pack. The appropriate name, however, is damper actuator, and solid working knowledge of its correct application can be the key to commercial HVAC system operation and troubleshooting.

Incorrectly installed or broken damper actuators can cause ductwork dampers to open only partially or not at all. This, in turn, can lead to comfort complaints, cold/hot rooms, and possible IAQ problems.

That’s why it’s important to specify or install the correct actuators in the first place, or look to them as a possible problem source when troubleshooting commercial systems, according to Mike Wolf, a sales manager for Greenheck, Schofield, Wis. The company makes and markets fans, dampers, and other air-moving products.

Choosing and installing the correct damper in the first place means winning more than half the battle.

Do you know which type of damper is best for your application? Manual, pneumatic, and electric models are appropriate for various applications.

Manual, Pneumatic, Or Electric

  • Manual — According to Wolf, if the damper will be fixed in one position after balancing and other initial adjustments are made, or, if its position only needs to be changed once or twice a year for seasonal changeovers, a manual-locking-quadrant actuator may be all that is required.

    “If the damper is required to operate automatically as part of an HVAC system, an electric or pneumatic actuator should be provided,” he said.

  • Pneumatic — These actuators have operational requirements of their own. They normally need a 20- to 25-psi instrument air supply system to ensure the availability of clean, dry air, Wolf explained. If such a system does not exist, or if it will not be installed as part of a new HVAC system, then a pneumatic damper actuator is not a worthwhile option.

  • Electric — These actuators are widely used for all damper control applications, according to Wolf. Configurations include direct-coupled models. “Since electric actuators are available with supply voltages of 24 VDC and 24, 120, and 240 VAC, it is necessary to designate the voltage when selecting the actuator,” he said. This is critical. If a contractor were to specify too low of a voltage, for example, the unit could burn out when it is hooked up and put into operation.

    There are often several trades involved with the selection and installation of the damper and actuator. For example, the sheet metal contractor may order and install the damper, an electrician or pipe fitter connects power to the actuator, and a temperature control contractor may connect control sequencing to the damper. In this scenario, the power source must be confirmed with the electrician or pipe fitter and the control signal confirmed with the temperature control contractor.

    Electric actuators are not inherently fail-safe, Wolf noted. “For fail-safe operation, spring-return models may be specified.”

    How Will It Function?

    When choosing damper actuators, the temperature controls contractor also needs to consider how the damper will operate in the system, Wolf said. For instance, contractors should ask themselves the following questions:

  • Will the damper be used to adjust airflow to a required design setting, and then be locked permanently in position? This is called a balancing damper, Wolf explained. “It would most likely require only a manual hand quadrant actuator.”

  • Will the damper be required to open fully to allow airflow, and/or to close completely to shut off airflow? This is called two-position control. “If operation is required only one or two times each year, such as for summer/winter changeover, a manual hand quadrant actuator again may be appropriate,” said Wolf. However, if operation is required on a regular basis, or if automatic changeover is desired, he recommends using a two-position actuator.

  • Will the damper need to control temperature, pressure, or airflow by moving to any desired position between open and closed as required by a controller? This operation is called modulating or proportional control, explained Wolf.

  • Finally, will the damper need to return to its open or closed position in case of a power failure or some other emergency? If so, you need a fail-safe actuator. (If the actuator is electric, Wolf added, it will require a spring-return-type actuator.)

    Installation

    “A lot of things can go wrong” in actuator installation, said Wolf. Incorrect location is one potential trouble spot, particularly if the actuator will be installed in the field.

    According to Wolf, external mounting may require that the damper be furnished with a sleeve or side plate, so that the unit can be mounted externally to the ductwork, out of the airstream.

    The advent of direct-coupled actuators has made installation easier in situations where linkage hookups had been used before, Wolf said. “Older models with linkage hookups were often installed incorrectly,” he commented.

    These types of situations could result in dampers that may not open or close fully, he said. In addition, it is possible to connect the actuator so the damper closes and opens properly but the actuator does not rotate through its entire rotation. When this occurs, some actuators fail to engage a stall winding at the end of the stroke, causing the actuator to continue to draw full power and burn out. This would result in damper malfunctioning. And this, naturally, results in comfort problems and tenant discomfort.

    “Most of the time, contractors find out there is a problem with an actuator when somebody isn’t getting enough heat or cooling,” Wolf explained. If the problem is detected during the system repair, or even during regular maintenance or building recommissioning, this is a good time to replace the linkage hookup actuator with a direct-coupled model.

    Torque

    In operation, an actuator must deliver a specific amount of torque to position the damper accurately. This is rated in inch pounds (in.-lb). A damper’s torque requirement is usually expressed in inch pounds per square foot (in.-lb/sq ft).

    “The actual torque required to operate a damper is affected by a number of factors,” Wolf stated, “some of which are under the damper manufacturer’s control — but some are not.” For example:

  • Opposed-blade dampers re-quire slightly less torque than parallel-blade dampers.

  • Low leakage dampers (with blade and jamb seals) require more torque than dampers without low leakage seals.

  • HVAC system pressure and airflow velocities also affect a damper’s torque requirements.

    “How the damper is installed in the ductwork and how the actuator is installed can have a significant affect on torque requirements,” said Wolf. “Dampers installed twisted or out of square may require many times more torque than dampers installed square and plumb.” (Some manufacturers, such as Greenheck, have designs to prevent such problems, as much as is possible.)

    Select a damper actuator with a rated torque that is greater than the damper’s required torque, advised Wolf. If you select an actuator that uses crank arms and linkage rather than the direct-couple connection, the company recommends an additional safety factor of 30 to 50 percent. “When in doubt, the next larger size actuator is always the safest choice,” said Wolf.

    Last Words

    There are additional selection considerations, such as NEMA enclosure ratings and accessories, such as auxiliary switches. Furthermore, due to system and design modifications, and/or missing documentation, you might not have all the information you need to make the best selection.

    “The actuator selection process is not a simple one,” Wolf said. “Often the information necessary to select the appropriate actuator is not available. Unfortunately, this can result in improper actuators being furnished, causing confusion and extra cost.”

    When in doubt, contact the manufacturer to determine the best actuators for the dampers in your project, he concluded.

    For more information, Wolf may be contacted at 715-359-6171.

    Publication date: 07/28/2003