T-Method Ductwork Simulation Program

The commercially available T-Method simulation program, T-Duct, consists of four major parts: an interactive window-driven preprocessor, a calculation processor, a numerical/graphical post-processor, and a database of fittings. The program calculates the actual airflow distribution throughout a system, adjusts the fan operating point, and checks the brake horsepower of the existing fan. The program output also shows the percent airflow difference between design and actual performance for each duct section, so the user can determine the need for corrective action. [T-Duct Program, 1994]

The program's interactive preprocessor accepts a wide variety of data, particularly for fittings and duct-mounted equipment. It presents only fittings suitable for a particular duct section and can display fittings on the screen. A full range of browse, edit, copy, move, erase, and error-detection techniques saves time for inputting identical sections or branches. The program can automatically convert data among three unit systems (SI, Inch-Pound, and Metric), and it has many help screens.

The processor is an iterative procedure. Iterations allow for matching the fitting and equipment local resistance coefficients with the actual airflows for each section of the duct. The convergence is very efficient, usually requiring few iterations.

The post-processor is partially interactive. It shows the actual operating point on fan and system curves, total and static pressures, and sectional and fitting data, including actual airflows and air velocities. The post-processor allows for correcting cross-sections, closing or opening dampers, and changing fan characteristics. Calculations can be repeated without leaving the program.

The Data Base has a number of libraries containing fittings and duct-mounted equipment from the ASHRAE Fundamentals Handbook [ASHRAE, 1997], the "HVAC Systems - Duct Design" [SMACNA, 1990], and various other sources.

The T-Method has a number of advantages compared to other simulation methods:

• It uses similar techniques for duct optimization and simulation,
• It is appropriate for any duct shape, material, and air density,
• It is appropriate for supply, return, and exhaust systems,
• It recognizes variable C-coefficients,
• It can accommodate any fan characteristic curve, and
• It has an efficient convergence process.

The best way to perform duct calculations for laboratories is by using an appropriate computer program such as T-Duct.