HVAC Ductwork Systems Flashcards

What is the significance of duct pressure classification (e.g., 1/2", 1", 2", 3", 4", 6", 10" WG)?

Answer: It determines the construction standards including metal gauge, reinforcement spacing, and sealing requirements

Duct pressure class determines all construction requirements including minimum metal gauge, joint types, reinforcement methods and spacing, and sealing class. Higher pressures require heavier construction.

What is a 'mixed air plenum' in an air handling unit?

Answer: A chamber where return air and outdoor air are blended together before passing through filters and coils

The mixed air plenum is located upstream of the filters and coils where return air from the building is blended with fresh outdoor air. Dampers control the ratio of return to outdoor air.

What is the purpose of a 'flexible connector' between the air handling unit and the ductwork?

Answer: To isolate fan vibration from the duct system and accommodate thermal movement

Flexible connectors (typically fabric or rubber) isolate mechanical vibration from the AHU fans, preventing it from transmitting through the rigid ductwork. They also accommodate thermal expansion.

What duct material is required for a fume exhaust system handling corrosive chemicals?

Answer: Stainless steel, PVC, or fiberglass reinforced plastic depending on the chemical

Corrosive chemical fume exhaust requires materials resistant to the specific chemicals involved. Stainless steel, PVC, polypropylene, or fiberglass reinforced plastic are selected based on chemical compatibility.

What is the purpose of a 'test and balance' (TAB) procedure after duct installation?

Answer: To measure and adjust airflows, temperatures, and pressures throughout the system to ensure design conditions are met

Test and balance is a systematic procedure to measure actual airflows at all supply and return outlets, adjust dampers, and verify that the system delivers the designed air quantities, temperatures, and pressures.

What is the maximum permitted duct leakage rate for a SMACNA Seal Class A system at 2" WG?

Answer: Approximately 1% of system airflow or less, as specified by CL = 6 formula

SMACNA Seal Class A at 2" WG permits minimal leakage calculated by the formula CL x P^0.65 where CL=6 for Class A. This typically works out to approximately 1% or less of system airflow.