All Single and Dual Duct VAV Terminal Units

This section describes the catalog selection of single and dual duct VAV terminal units with specific examples. A computer selection program is also available to aid in selection of VAV terminal units, but is not described here.

Selection of single and dual duct VAV terminal units can involve three elements

• Air Valve selection
• Heating coil selection (if required)
• Acoustics

Air Valve Selection

The wide open static requirement pressure versus airflow curve is found in the performance data section of the catalog. To select an air valve, locate the required design cooling airflow for your terminal unit type and find its vertical intersection, with the smallest air valve size that has a pressure drop equal to or lower than the maximum wide open static pressure requirement.

Example:

Conditions

Cooling Only VCCE Terminal Unit

Cooling Airflow, Cfm = 1700

Wide Open Air Pressure

Drop Maximum, Delta P = 0.25 in. wg

Minimum Cfm = 300

From Chart P-1, select a valve size 17 which has a wide open static pressure drop of 0.16 inch wg.

Check the minimum and maximum Cfm desired with the minimum and maximum Cfm allowed in Table G-3. The maximum Cfm of 1700 is within the acceptable range. The desired minimum Cfm of 300 is acceptable for the cooling only box desired. Note that if an electric reheat box was selected, the minimum Cfm would be dependent upon the KW of the electric heater. (See Electric Heat Unit Selection.)

Heating Coil Selection (If required)

First, determine the amount of heat required to meet space and downstream duct heat losses from a load calculation.

Electric Heat

Determine the kw required to meet design heat loss.

KW = MBh / Heat Loss

MBh = Heat Loss

Select nearest available kw with voltage and steps desired from the Electric Heater kw Guideline Table P-8 in the Performance Data section of the catalog.

Example :

Conditions : VCEE,Electric Unit Heat, Size 17(See Air Valve Selection)

Design Heating Airflow,CFM=850

Minimum Air Unit Airflow=300

277/60/1 Voltage

Winter Room Design Temperature,

T=68 F

Design Heat Loss,Q=25 Mbh

KW=Q/3.414

KW=25/3.414

KW=7.3

Selection: Select 7.5 Kw From Table P-8 in voltage and steps desired .The table shows the minimum Cfm allowable for the KW selected.At 7.5Kw the minimum Cfm is 510.The minimum Cfm must be increased to 510 to insure adequate airflow over the 7.5 Kw electric coil.The static prressure requirment is shown in Chart p-4 as 0.17" wg for this example of a design cooling flow of 1700 Cfm.

Check Leaving Air Temperature

LAT=Q/1.085 X Cfm + T

LAT=3414 X 7.5 / !.085 X 85 + 68

LAT=95.8

Decide if leaving air temperature of 95.8 F is satisfactory for your application.