HVAC Pressure Drop Verification

I am just learning how to use the HVAC and wanted to verify the pressure drop calculation in a duct.

My model is shown below, there are three HVAC nodes and two HVAC ducts. The duct on the left has a fan with a constant flow rate of 1.0 m^3/s. The only loss terms are due to a 0.001 m roughness in the 0.3048 m diameter ducts. The total duct length is 2.0 m. The duct on the right makes is easy to calculate the expected pressure drop for that section. 

hvac problem description

I ran the model and made pressure contour and velocity plots. The pressure contours show low pressure near the inlet and outlet, where the velocity is high, and a high pressure region where the exit flow impinges on the wall.

hvac pressure contours

hvac vel vectors

The calculation for the expected pressure loss in the duct without a fan was done in the Duct Loss Due to Wall Friction Calculator . The duct velocity is calculated to be 13.7 m/s and the pressure loss calculated to be 10.1 Pa. The duct loss calculation is taken from the FDS Technical Reference (to quickly access, on the Help menu of PyroSim, select More Help).

spreadsheet zoom


The figures below show the FDS results for the velocity in the duct and the pressure drop. The values match those calculated in the spreadsheet.

hvac vel in duct with fan

hvac deltaP center to outlet

The pressure difference in the duct with the fan shows the pressure increase needed to maintain the specified fan velocity.

hvac deltaP inlet to center

This problem verifies the flow calculation in the section of the duct without a fan, where the velocity and pressure drop match the expected values.

The expected speed at the vent outlet is 6.25 m/s. The gas velocity measurement device placed at the center of the vent records a speed of 5.8 m/s. Keeping in mind that Smokeview does interpolation, this is fairly close to 6.25 m/s.

Comments or Questions

This post was written by Daniel Swenson. For comments or questions, send email to support@thunderheadeng.com.

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