LOUVER AND DAMPER APPLICATION AND SIZING
Intake Louvers: Outdoor air intake louvers must selected on the basis of the following criteria:
a. Size louver on the basis of a maximum face velocity at least 25% below the first point of water penetration rated face velocity, but do exceed 500 fpm face velocity.
b. Select louver to have a pressure loss below 0.2 inches w.g. for most applications. To achieve this, increase the opening size or select a louver model with higher free area, higher first point of water penetration, lower pressure drop, or a combination of these factors. [Note: Louvers for high wind locations must be specified to meet “wind-driven rain” criteria in Section 233300 of the guideline specifications.
d. For outdoor air intakes, provide an indoor plenum at least a 12" deep and with its floor sloped to one or more drains piped to waste. Plenum must include adequate access for damper maintenance.
Discharge Louvers:
1. Louvers for relief or exhaust airflows may be sized for 750 fpm face velocity.
2. For projects in coastal regions, select louvers for high wind conditions as discussed above for intake louvers.
Control Dampers: Dampers used for controlling ventilation airflow rate and/or as part of an airside economizer cycle shall be sized on the basis of ASHRAE Guideline No. 16, as follows:
1. Damper Configuration and Pressure Drop: HVAC dampers are typically available in two configurations, blades configured to open and close parallel to each other or with blades configured to open and close opposed to each other, as illustrated below:
The pressure drop through a wide open damper, parallel or opposed blade configured, can be estimated from the following table (based on Greenheck Model VCD damper with galvanized 3V blade and both blade and jamb seals) :
Face Velocity (fpm) / Pressure Drop (in. wg) /400 / 0.02
600 / 0.05
800 / 0.08
1000 / 0.13
1500 / 0.30
2000 / 0.53
2. Return Fan System (consisting of a supply fan, return fan, and modulating control dampers for airside economizer control):
a. Select the relief air damper first. An opposed blade damper is recommended since it will have lower pressure drop than a smaller parallel blade damper when selected for linear throttling characteristics, thus reducing the cost of operation. The damper selection is based upon the maximum design relief air (design supply airflow rate minus mechanical exhaust minus exfiltration and pressurization air).
First determine the relief air path pressure drop without the relief damper, including ductwork, plenums, and discharge louver. For a linear response opposed blade damper, the ratio between the pressure drop in the relief air path and the pressure drop through the open relief air damper should be 8 to 15. This means that the damper will have a wide-open pressure drop of 7% to 15% of the relief path pressure drop at design flow. The resulting wide-open damper face velocity will frequently be around 1000 to 1500 fpm.
Once thisdamper is sized, the pressure in relief/return plenum relative to ambient can be estimated as the relief air path pressure drop plus the pressure drop across the relief air damper.
b. For constant air volume systems, select a two-position minimum outdoor air damper. This damper may be either parallel or opposed blade. It should be sized for the minimum outdoor airflow rate at the same face velocity as the maximum (or “economizer”) outdoor air damper.
c. For variable air volume systems, the minimum outdoor air damper must be modulating for capacity control. It is should be selected using the procedure similar to that described above for the relief air damper.
d. Select the maximum (or “economizer cycle”) outdoor air damper. The size of this damper is not critical since it does not directly control outdoor airflow. The total flow of outdoor air through maximum and minimum outdoor air dampers is always the difference between the supply airflow and the return airflow, and, therefore, the damper size and type have no effect on the amount of outdoor air. Dampers with a low pressure drop are generally used to minimize the cost of operating the supply fan. Damper face velocities in the range of 800 to 1,500 fpm are typical. Because this damper does not control flow, either opposed blade or parallel blade action can be used.
e. Finally, select the return air damper. This damper selection is based upon the maximum return airflow rate, which is the difference between the design supply air and the minimum outdoor airflow rate. The return air damper should be selected so that the pressure drop across the damper is equal to the differential pressure available between the return air (+ pressure) plenum and the outdoor air (- pressure) plenum.
The pressure in the outdoor air plenum is equal to the pressure drop at the minimum outdoor airflow rate from the outdoors through the outdoor air damper when the damper is at minimum position. The minimum position is up to the designer, but it should be well below full open so that the damper can open further when the VAV supply fan airflow rate is low and the plenum pressure approaches ambient pressure.
A parallel blade damper is usually recommended since it will provide a more linear response.
3. Relief Air Fan System (consisting of a supply fan, relief fan, and control dampers for airside economizer control):
a. First, size the relief air damper. This damper is typically interlocked with the relief air fan start/stop control and with relief airflow volume control (variable speed drive and/or fan staging). As such, it should be selected for low pressure drop to minimize fan energy and may be either parallel or opposed blade. Damper face velocities from 400 to 1000 fpm at design flow are typical.
b. Select the minimum outdoor air damper in accordance with the procedure for systems with return air fans.
c. The maximum (or “economizer”) outdoor air damper should be selected for low pressure drop to minimize the cost of operating the supply fan. Damper face velocities from 800 to 1500 fpm at design flow are typical.
d. Finally, select the return damper. Return air damper sizing is based upon the maximum return airflow rate, which is the difference between the design primary supply air and the minimum required outdoor airflow rate. With a relief fan, the pressure in outdoor air plenum must be below the pressure in the relief air plenum. The pressure in the relief air plenum is pressure drop from the space to the plenum through the return air ductwork. This will usually be more negative than outdoor air plenum pressure. Therefore, the pressure drop between the two plenums (including the return damper pressure loss) should be as low as possible to minimize the negative pressure required in the return air plenum and minimize supply fan energy. Damper face velocities from 800 to 1500 fpm at design flow are typical.
e. Where outdoor air and return air dampers are controlled so that one opens when the other closes, both outdoor air and return air dampers should be parallel blade dampers. Tests show that a relatively constant pressure drop across the return air and outdoor air paths throughout the economizer cycle can be achieved by using parallel blade dampers. Where outdoor air and return air dampers are sequenced, either parallel or opposed action may be used.
4. Relief Air Damper System (consisting of a supply fan, gravity relief air damper, and control dampers for airside economizer control): The relief system is not fan-powered and relies on building pressurization to cause excess outdoor air to relieve to atmosphere. Consequently, the relief damper must be located within the conditioned space served, therefore, this concept is applicable only in large spaces (gymnasium, factories, etc.)
a. The relief damper and entire relief path must be sized large enough to prevent over-pressurization of the conditioned space served (a pressure high enough to cause doors to whistle or stand open) when the system is in maximum outdoor air mode. For most buildings, the maximum pressure is in the range of 0.05 to 0.1 in. wg. The relief damper pressure drop includes the pressure required to open the damper when gravity dampers are used.
b. Select the minimum outdoor air damper, maximum outdoor air damper, and return air damper as defined for return air systems.
Fire and Smoke Dampers:
Evaluate/Locate Required Fire or Smoke Dampers on the Drawings: Ideally, any set of design drawings would include the fire and/or smoke ratings for each partition or floor in the building. The HVAC designer, then, must determine which type(s) of dampers are required at each penetration of a rated partition or floor by using standard symbols defining each type of damper at each type of location.
Utilize Dynamic Rated Fire Dampers: Static rated fire dampers are designed and tested to close when the HVAC system fan cycles off during an alarm. Dynamic rated fire dampers are designed and tested to close when the HVAC system fan remains on during an alarm.
When a fan cycles off, it can take up to 20 minutes for air to stop flowing. Specify dynamic rated fire dampers to ensure dampers will close under elevated temperature and airflow conditions. The difference in cost is small compared to the benefit of safe operation.
Select the Appropriate Fire Damper Construction Type: There are basically two types of fire dampers: curtain type and multi-blade type.
A curtain-type fire damper has a temperature-sensitive fusible link that releases an interlocking blade stack to close under gravity or with a spring assist. Sleeve transitions can remove the blade stack from the airstream and provide a 100% free area opening for applications with sensitive static pressure requirements.
Multi-blade fire dampers are similar in construction to control dampers and feature blades, either 3V or airfoil type, located in the airstream. As a result, they typically have a greater restriction to airflow than a curtain-type fire damper for the same size duct. However, multi-blade fire dampers can be used in applications where the system design velocities exceed typical curtain-type fire damper closure ratings and are especially suited for applications that feature larger openings, as multi-blade fire dampers generally have larger UL size listings when compared to curtain-type fire dampers.
Require Class I Leakage Rated Fire and Smoke Dampers: A building’s envelope and compartments must be relatively airtight to maintain proper pressure relationships during a smoke control cycle. Class I dampers are the lowest leaking dampers approved by UL and are available from almost all damper manufacturers who offer a UL-approved product. When comparing cost between Class I and Class II dampers of similar construction type, the cost difference is negligible, yet the amount of smoke leakage allowed for a Class II damper is 2.5 times greater. Low leakage Class I dampers provide the highest level of protection against the migration of smoke, and they ensure that appropriate pressure differentials can be achieved.
HVAC Design Guidelines
LOUVER AND DAMPER SIZING 4