ASHRAE Journal - August 2011 - 30

“Duct fitting imposed” pressure drop through any fitting is dependent on the velocity. When both ducts of a dual duct system are converted to supply cool airflow, the velocity is reduced. It is not unusual to reduce the original 3,500 fpm to 2,000 fpm (15 to 10 m/s). A VAV box insert is used in place of the dual duct box. This reduction in velocity reduces the pressure drop through the system by the following equation of (V2/V1)2. A system operating at 8 in. of duct static losses at 3,500 fpm (V1) will run at 2.61 in. of static pressure at 2,000 fpm (V2). The velocity reduction results in a pressure drop reduction of (2,000/3,500)2 = 32.6%. The 25,000 cfm (11 800 L/s) dual duct system mentioned previously will see the following savings because of this reduction in pressure. Dual duct fan and reheat savings are the same as the VAV savings previously seen in a constant volume reheat system: $17,794/year. Added savings due to reduced pressures are as follows: [25,000 cfm × (8 in. – 2.61 in.)] (6,356 × 0.7 Eff) = 30 hp reduction 30 hp × 746 kW/hp × 4,368 hours per year × $0.08/kWh = $7,820/year Total dual duct conversion savings = $25,614/year.

Multizone
Exhaust Air Louver Exhaust Air Damper Optional Return Air Fan

Before Retrofit
Return Air System

Return Air Damper Outdoor Air Intake Outdoor Air Damper

Supply Air Fan

Heating Coil

Zone Mixing Dampers

Optional Pre-Heat Coil

Filters Cooling Coil Zone 1

To Remainder of Zones

Exhaust Air Louver

Add New Building Pressurization Sequences

After Retrofit
Optional Return Air Fan With New VFD Return Air System

New Low Leak Exhaust Damper Return Air Damper Outdoor Air Intake New Low Leak Outdoor Air Damper

Supply Air Fan With New VFD

New Larger, Lower Pressure Drop Cooling Coil

New Low Optional Pressure Drop Filters Pre-Heat Coil

Retrofit VAV Box With Reheat Coil

Zone 1

To Remainder of Zones

Figure 3: Multizone systems can have a pressure drop across zone dampers. sure drop. The catalog data for zone damper pressure drops assumes constant velocity across the dampers. In an MZ system, the zone damper section is subdivided into various size sections. The airflow through these sections is not the same and damper segments do not usually have the same airflow per square foot of damper area. The pressure drop across the dampers varies. Even when the engineer specifies certain damper section sizes on the drawings to allow for pressure drop calculations, these damper sections are often ignored by the installing contractors and subdivided into sections that allow easier installations. Because of the different subdivisions, the zone pressure drops vary and the fan speed must be increased to obtain design air to the most restrictive zone. Manual zone balancing dampers are then closed to equalize the total system pressure. The MZ damper section needs to be removed in an RCx process to eliminate this restriction.
ashrae.org August 2011

Multizone Systems
Multizone (MZ) systems consist of an air-handling unit with a heating coil generating 100°F (38°C) air and a cooling coil or economizer generating 55°F (13°C) air. The hot and cold airstreams are mixed with zone dampers on the air handling unit to produce the desired space temperature. Multizone dampers leak air between the hot deck and cold deck in a manner similar to the dual duct systems. This leakage reduces both cooling and heating capabilities of the system. Many multizone systems are typically low pressure systems with duct velocities less than 2,200 fpm (11 m/s). Multizone systems do not have the high static duct losses seen in high pressure systems. Multizone systems have a pressure drop across the zone dampers. This loss is usually ignored in the design process. Engineers typically specify an external static pressure for the duct system using the catalog data for the zone damper pres30 ASHRAE Journal
ASHRAE Journal - August 2011

Table of Contents for the Digital Edition of ASHRAE Journal - August 2011
