Monday, July 16, 2012

Q&A: How Do You Size Parallel Fan Powered Terminal Units?

Parallel flow (variable volume) fan powered terminals are selected based on their capacity to handle the primary airflow. The same rules which apply to the selection of single duct terminals can be used, except that water coils are not in the primary airstream path, and will not affect sound levels. The pressure drop of the water coils, however, which are on the fan inlet in Titus parallel fan units, must be added to the expected discharge pressure at the fan flow rate when entering the fan curve tables.

The fan is selected based on the minimum airflow requirements for the space or the heating load required. In most cases the fan can be downsized from the cooling flow requirement considerably, reducing both first cost and operating cost. The fan is selected from the fan curves. The downstream static pressure of the secondary air may not be the same as the primary air, however. If the secondary airflow requirements are less than the primary air requirements, the static pressure will be reduced. The following equation can be used to determine the static pressure at reduced airflows. (Do not forget to add water coil pressure drops to the fan requirement).

To select a Titus parallel fan powered terminal, refer to the published fan curves and primary air pressure drop curves, together with the application and sound power data.

 In the parallel flow type of unit, when the primary air is ON, the fan is typically OFF, and vice versa. As shown in the Figure 1, the primary air and the fan discharge air follow parallel paths into a common plenum. Therefore both airflows will encounter the same downstream resistance at a given flow rate.

Since the primary and secondary airflows come from two different sources-and often at two different specified flow rates-the volume vs. pressure relationship in each of these airflows must be checked to ensure adequate flow rates under actual job conditions.

Example: Select a Model DTQP for a maximum of 1400 cfm of primary air with 1.00” wg inlet static pressure. The fan airflow required is 1150 cfm. The downstream resistance offered by the duct and diffusers has been determined to be 0.30” static pressure at 1150 cfm.

Primary Air: From the air inlet pressure table, a size 4 with a 12” inlet will handle 1400 cfm of primary air with a minimum static pressure drop of 0.23” through the primary air section. But since the downstream resistance is 0.30” at 1150 cfm:

The overall primary air static pressure drop is:

0.23”+ 0.44”= 0.67” sp

Since a 1.0” static pressure is available at the inlet, the selection will work. The damper in the primary air section will do some throttling to hold the maximum air flow to 1400 cfm.

Secondary Air (Fan): From the fan curves, a size 4, without coils, terminal will handle 1150 cfm at 0.30” static pressure, with the proper setting of the standard SCR speed control.

Trenton Yarbrough - Director of Engineering

Tuesday, July 10, 2012

Q&A: Why Are Filters for Terminal Units An Option?

Optional filters on fan-powered terminal units are 1” thick disposable filters that are highly recommended if there is any chance that the units will be operating during construction. Construction dust can easily ruin both the motor and blower, if the units are operated without filters in place. Dust tends to deposit unevenly on blower wheels resulting in loss of balance. Any build-up of fine dust or lint on the exterior of a permanently-lubricated fan motor can create a migration path and wick the oil out of the bearings.

 These optional filters should later be removed and discarded during the balancing and commissioning process. Operating units after construction with filters in place is not recommended. Fan-powered units, like all terminal units, are designed for zero maintenance. This is necessary because terminal units, unlike air handlers, are typically located above finished ceilings in tenant spaces. Accessing the units regularly to change filters can be nuisance to tenants, time-consuming for maintenance personnel and can easily result in ceiling damage.

 If filtering of return air is desired, we recommend the installation of filter grilles in the ceiling. This allows the building and the building owner the benefits of less costly standard filter sizes; more filter area for longer service intervals and quicker access for easier replacement.

Randy Zimmerman ~ Chief Engineer