T-Method Capabilities

Another example that illustrates the T-Method's capability is a simulation of the example in the Fundamentals Handbook Chapter, "Duct Design" [ASHRAE, 2001]. The example was analyzed to determine the airflow rates when system dampers are fully opened. Drastic differences were discovered where airflows changed from 23% to 37% compared to the original system design [Tsal, et al., 1990]. System analysis can also be done for situations such as partially occupied buildings, so engineers and contractors can predict flow distribution, locate dampers, and save energy by balancing the system, repositioning dampers and adjusting fan speed. The results of the simulation calculations are flow rate and pressure resistance of each section of duct, pressure at each node, and an operating point on the fan performance curve. The T-Method for duct simulation has also been used to analyze systems with: (1) fans working in parallel when one fan is shut down, and (2) a VAV system working in the minimum and maximum flow regimes.

Duct system simulation is needed in several situations for HVAC designs, including:

• Analyzing air-flow redistribution in a multiple-fan system when one or more fans shut down,
• Analyzing pressure differences between adjacent confined spaces in a nuclear facility when a DBA occurs,
• Analyzing air-flow distribution in a VAV system when there is terminal box flow diversity,
• Analyzing air flow redistribution resulting from modifications to the HVAC system,
• Analyzing system air flow for partially occupied laboratories,
• Determining the need for fan or motor replacement during retrofitting of an air distribution system,
• Analyzing smoke control system performance during a fire when some fire/smoke dampers close and others remain open,
• Analyzing pneumatic conveying systems and manifolds,
• Analyzing the influence of a change in damper blade angle on air flow at existing terminal outlets,
• Finding the operating point on the fan performance curve when duct size or damper blades angles are changed,
• Connecting additional terminal outlets to an existing system,
• Analyzing the possibility for damper-generated noise,
• Determining the maximum static pressure that can be experienced by dampers when all system dampers are closed except one.