HVAC & R Skills Workshop - Module 2 - Pullout - Basics of Duct Design

Oct 13, 2026

HVAC&R Skills Workshop Module 2 Pullout focuses on duct design fundamentals for HVAC and refrigeration systems. It explains what constitutes good duct design by balancing duct size, pressure-loss operating cost, acoustics, and air-balance commissioning cost. The module compares duct sizing approaches, then presents a pragmatic low-pressure sizing guideline attributed to W R (Roy) Ahern, including suggested velocities for different air quantities and a friction-loss gradient target. It covers velocity pressure concepts, total pressure relationships, fitting dynamic losses, common references for K factors, runout and cushion-head considerations, and balancing fundamentals, concluding with key references and related publication context.

LanguageEnglish

CategoryPresentations

FormatPDF

Pages3

Reading Time1 min

AccessFree to download

Basics of Duct Design

Good duct design criteria

Duct sizing methods

Velocity pressure and static pressure friction

Total pressure and fittings

Balancing and references

skills workshop HVAC & R SKILLS WORKSHOP M O D U L E 2 2 P U L L O U T Basics of Duct Design PULLOUT This month we begin the first of HVAC&R Nation’s three-part series on ducts. Two duct design experts, JJW (Bill) Siganto and Murray Mason provide a wealth of information on the topic. What is good duct design? Good duct design optimises: • Duct size (manufacturing costs) • Duct systems pressure losses (operation cost) • Duct systems acoustics (environmental costs), and • Air balance procedures (commissioning costs) Of these, duct sizing is the simplest. In a few minutes, even the most complex systems can be sized with a duct sizing slide rule. The various texts suggest three methods: • Constant friction gradient • Constant velocity, and • Static pressure regain A pragmatic solution to optimise the conflicting elements in duct design, the late W R (Roy) Ahern, offered the following approach(1) for the sizing of low-pressure air conditioning ductwork. • For air quantities greater than 4500 l/s size duct at 10m/s. • For air quantities less than 180 l/s, size duct at 4.5 m/s. • For air quantities greater than 180 l/s, but less than 4500 l/s Then size duct on basis of friction loss of 1.2 Pa/m For these rules, Ahern offered no references other than his own extensive experience backed by his unerring ability to reduce all technical programs to first principles. In the larger size where velocity is the criterion, the limiting factor is noise breakout. His choice if 1.2 Pa/m as the mid-range friction gradient is higher than that used by many designers. For the smaller sizes, the proposed velocity limit often produces a high friction gradient. Consequent dynamic losses are quite low with the resultant velocity pressure around 12Pa. Velocity pressure (or velocity head) The terms velocity pressure and velocity head are interchangeable. For air at comfort temperature the velocity pressure is 0.6 times the square of the duct velocity. Pv = 0.6 x v2, in Pascals (Pa), if v is measured in per second. Static pressure friction Static pressure friction causes static pressure loss. The calculation of the friction gradient “f” (Pa/m) is complex, involving velocity pressure, duct diameter and duct surface roughness. Modern computers can make it routine. However, most designers still rely on duct sizing slide rules. The static pressure loss in a section of duct is: PT = f x L, where L is the duct length in meters. Fitting lengths should be included in the estimation of “L”. Where a fitting involves a change in velocity than fitting should be included in the duct section with the higher velocity. Exposed office ductwork Total pressure Total Pressure is the sum of Static Pressure and Velocity Pressure. PT = PS + PV Since one of the variables of friction gradient is velocity pressure, it follows that total pressure is dependent on the velocity pressure, which for a given duct area varies as the square of the flow. As air flows through a duct system from a fan to terminal the total pressure is always decreasing. Because of changes in velocity, the same cannot be said of velocity pressure and static pressure. Fittings When a fitting changes the direction of airflow or its velocity, then as well as the friction loss referred to above, a dynamic loss is involved. The quantum of dynamic loss is expressed in total pressure. Various fittings are assigned a “fitting loss factor” (K). The dynamics loss through the fittings is then calculated from, PT (Pa) = PV Where the velocity pressures is usually related to the maximum velocity occurring in the fitting. AIRAH(2), ASHARE(3) and SMACNA(4) are the most widely used references for fitting loss factors. Duct designers are often frustrated by the disparity 15 Proudly sponsored by skills workshop between the references for K values for the same fitting, particularly for those involving divided flow. This so concerned Ahern 50 years ago

FAQs

What are the main goals of good duct design covered in the module?

Good duct design optimises duct size (manufacturing costs), duct pressure losses (operation cost), duct acoustics (environmental costs), and air-balance procedures (commissioning costs).

Which duct sizing methods are mentioned, and how does the module position them?

The module lists constant friction gradient, constant velocity, and static pressure regain as common methods, then notes additional approaches such as the balanced pressure drop method and T-Method Optimisation. It emphasizes there is no single method that inherently yields the best design.

How are total pressure and fitting dynamic losses related in the module?

Total Pressure equals Static Pressure plus Velocity Pressure (PT = PS + PV). When a fitting changes airflow direction or velocity, it adds a dynamic loss expressed in total pressure and calculated using a fitting loss factor K and the velocity pressure tied to the fitting’s maximum velocity.

Oct 13, 2026

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