News | February 28, 2000

Online HVAC Duct Calculations

Online HVAC Duct Calculations
Have you ever wanted to analyze or trouble-shoot quickly a portion of your process ventilation or HVAC system, but did not have the specific computer application or technical handbook to do so? Well a new free online application may be helpful in allowing you to efficiently analyze these problems without tremendous effort and expert input.

This new application, "On-Line Duct Friction Loss and Velocity Pressure Calculation," performs simple pressure drop analysis of single duct lines in ventilation systems. This simple and easy to use tool allows the user to quickly check existing systems, or test modifications to them, using only a web browser.

To use this application the user simply enters in the configuration of the duct on a form accessed via a web page. The user is prompted to enter the following required data:

1. Duct and Flow Conditions (size, flow rate, properties, etc.)
2. Duct Entry Configuration (hoods and duct ends)
3. Elbows (both round and rectangular)
4. Filter(s) / Air Cleaner(s) / Miscellaneous Equipment
5. Duct Exit Configuration (branches, contractions, and enlargements)

Then with a click, the application calculates the pressure drop and velocity pressure, in inches of water, for this section of the system. This pressure drop calculation includes the loss in the straight portion of the duct, plus additional losses associated with the elbows, equipment, and inlet and outlet configurations of the system. Users can run this analysis on either rectangular or round ducts and support for flat oval ducts will be added shortly. The application accommodates most typical duct elbow, fitting, and hood configurations, and also allows for the manual input of pressure losses for equipment like filters. The interactive nature of this web application also allows the user access pictures of the various fittings, elbows, and hoods analyzed in the calculation so that correct choices can be made for analysis.

Although this application can only accommodate one single-sized duct at a time, it can be applied to more complicated systems by dividing them up into sections and analyzing each section individually.

Gases other than air can also be accommodated. Any gas that is operating under conditions that approximate ideality can be analyzed adequately, provided the proper gas properties (molecular weight, viscosity, and specific heat ratio) are entered. This ability to handle non-air systems makes this application particularly useful in low-pressure flow processes like scrubbers and other industrial gas ventilation systems.

Interactive "How To" and "Technical References" links are also available to allow the user to look deeper into how these calculations are done, and get further explanations on how to use the program.

To complement these duct calculations, a link is provided to a page to containing "Fan Law" calculations. Using information generated from the friction loss analysis and the "Fan Laws" page, the user can then access a Fan Data Sheet suitable for printing or direct electronic submittal to vendors for quotation.

Detailed design of facility HVAC and process ventilation systems still requires the skills of an experienced professional. Also, this application does not approximate the functionality and capabilities of industry-proven duct design programs that can easily accommodate multi-line systems. But for the casual user, plant engineer under a deadline, or even the ventilation design professional, the "On-Line Duct Friction Loss and Velocity Pressure Calculation" can be a useful resource to make quick and simple calculations when appropriate.

The "On-Line Duct Friction Loss and Velocity Pressure Calculation," "Fan Laws," and "Fan Data Sheet" applications are all written in JavaScript so that they can be accessed using only a browser. They can be found at by clicking on the "Fans & HVAC" button.

John Kossik is a process engineer for Beacon Engineers Inc., Seattle.

For more information: John Kossik, Beacon Engineers, 18940 Northeast 150th St., Woodinville, WA 98072. Tel: 425-742-9653. Fax: 425-883-2171. Email: