Become a HVAC Professional - Part 3/3 - Duct Design

Master four duct design methods: equal friction, velocity reduction, constant velocity, and static regain, and learn why the equal friction method remains the most widely used.

The equal friction method, the most universally used method, sizes duct systems using international charts and applies to residential houses, community halls, schools, colleges, and commercial buildings and offices.

Redesign a duct system for a six-room office layout by applying load calculations: six rooms at 1000 cfm and two at 500 cfm, redesigning the main deck and brice deck.

Master the four-step equal friction duct design: placement and route selection, AFM verification, friction loss selection, and size calculation with round-to-rectangular duct charts.

Analyze the friction loss table and velocity charts to size ducts for low, medium, and high velocity HVAC systems using Carrier handbook values.

Begin duct design calculations by locating the air handler unit and planning the duct routing. Use the main branch and supply ducts with elbows and bends, guided by load calculation.

Analyze case two in duct design by placing the unit horizontally to the right, achieving fewer fittings and a symmetrical layout than case one, then choose the best configuration.

Case 1 uses more fittings and elbows, causing greater pressure drop and an asymmetrical layout. Case 2 reduces fittings and length, achieving symmetry and a simpler duct design.

Verify airflow by calculating the total CFM from the load calculation and assign it to the main deck, distributing to diffusers in six office rooms.

Learn how to select and verify friction loss values from a table, compare achieved and recommended duct velocities, and use a friction chart to size ducts in HVAC design.

Use an excel sheet to compute duct friction values and velocities, compare achieved velocity with the recommended velocity, and apply the equal friction method across six duct sections.

Assess the height clearance of 14 inches for duct installation, noting that a 16-inch space with 2 inches reserved above leaves 14 inches of available height before calculations.

Calculate the main duct diameter using the equal friction method, reading incline lines across sections with friction loss and cfm; convert to rectangular if height limits and side clearance require.

Convert circular duct diameters to rectangular cross-sections using a 2009 handbook chart to determine width and height, while enforcing a maximum 4:1 aspect ratio and 14-inch height.

Learn to determine duct sizes with a free calculation tool, switch units, and input height and sections to derive accurate duct dimensions. Consider temperature, humidity, pressure, and friction loss.

Confirm each section's CFM and complete load calculations, comparing manual and software results. Verify flow rates, assess diffuser extraction, and select friction values from the chart for duct design.

Confirm friction loss using the project calculation sheet and brass friction value 0.06 to ensure the achieved velocity stays below the recommended velocity.

calculate branch duct height within a 16 inch clearance by keeping two inches clearance above and below, using a 14 inch available height to yield a 10 inch branch height.

Learn how to calculate branch sizes using the equal friction method for duct design, including subtracting top and bottom gaps and determining remaining height for duct branches.

Adjust duct branch height calculations in hvac design by reconciling upsize results with practical clearances, friction head loss, and overhead accommodations to resolve previous miscalculations.

The universal friction-equal method offers an easy design that reduces air velocity and balances initial cost without numbers for asymmetrical layouts, but long or asymmetric runs cause unbalanced pressure drops.