ASHRAE Journal - August 2011 - 28

Reheat is reduced by the following amount: 25,000 – 17,500 = 7,500 cfm [(7500 cfm × (72 – 55)°F × 1.08) × 4,368 hours per year × $10/MMBtu]/0.8 Eff = $7,518/year Subtotal $17,794/year. The following formula is used to determine fan horsepower: Fan hp = (cfm × Total Pressure) (6,356 × Fan Eff) Eliminating the 1 in. (249 Pa) constant volume box pressure drop produces the following savings: [(25,000 cfm × (11 in. – 10 in.)] (6,356 × 0.7 Eff) = 5.6 hp 5.6 hp × 0.746 kW per hp × 4,368 hours per year × $0.08/kWh = $1,464/year Total savings by implementing the above changes = $19,258/year.

Dual Duct
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

Filters Optional Pre-Heat Coil Cooling Coil

Supply Air System (Hot)

Mixing Boxes

Supply Air System (Cold)

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

Supply Air Fan With New VFD

New Larger, Lower Pressure Drop Cooling Coil

New Low Leak New Low Pressure Optional Outdoor Air Pre-Heat Coil Drop Filters Damper

Dual Duct Systems

Enlarge Duct to Lower Pressure Drop

Dual duct systems consist of an airPressure Drop handling unit delivering hot air through one duct and cold air through the other duct. A terminal mixing box mixes the hot and cold air to produce the proper Figure 2: Dual duct systems can have issues with damper leakage. space temperature. These constant volume dual duct systems have the same There are temperature control sequences that can reduce eninefficiencies as the constant volume reheat systems outlined ergy use by minimizing hot deck and cold deck temperatures above. Dual duct boxes also have the added inefficiency of and limiting damper leakage by shutting a hot deck or cold damper leakage between the hot deck and cold deck. deck discharge damper to equalize hot deck and cold deck The damper leakage of these systems increases as the flow duct pressures. Before a design professional decides on implebecomes more predominant through either the hot deck or menting these sequence changes, they should evaluate the cost the cold deck. For example, during the summer, most of the and cost savings of converting the air-handling system into a air flows through the cold duct. With a small volume of air modified VAV system. The conversion is done by installing flowing through the hot deck, the pressure of the hot deck variable frequency drives (VFDs) on the fan drives and using will rise and approximate the discharge pressure of the fan. both the hot deck and cold deck duct work to supply cool air This higher pressure differential at the box damper increases to VAV terminal reheat coils. the leakage of the hot air through the hot deck damper. The It was common for dual duct systems to operate under the air leakage from the hot deck into the cold airstream causes SMACNA High Pressure Duct Construction classification. the 55°F (13°C) cold duct air to increase in temperature so Duct system static pressures of 8 in. to 10 in. (1992 to 2490 that the actual discharge air into the space is warmer than Pa) were common. The restrictions caused by duct air veloci55°F (13°C). The airflow must be increased because of the ties operating in the 3,000 fpm to 3,500 fpm (15 to 18 m/s) higher temperature air. range caused these high pressures.
28 ASHRAE Journal ashrae.org August 2011

Cap Unused Duct Retrofit VAV Boxes With Reheat Coils

Enlarge Duct to Lower

ASHRAE Journal - August 2011

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