HVAC Pressure Drop Verification

Reading Time: 2 minutes

Created with software version: 2020.1

1. Introduction

A HVAC model is shown below in Figure 1, 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 m³/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.

pyro scrn hvac problem description — Figure 1. HVAC Problem Description

2. HVAC Pressure Drop Verification

The contour plot for this simulation is shown in Figure 2 and the velocity plot is shown in Figure 3. 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.

results scrn hvac pressure contours — Figure 2. HVAC Contour Plot

results scrn hvac vel vectors — Figure 3. HVAC Velocity Vector Plot

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 Guide.

excel scrn hvac spreadsheet zoom — Figure 4. Duct Loss Spreadsheet

Figure 5 and Figure 6 below show the FDS results for the velocity in the duct and the pressure drop. The values match those calculated in the spreadsheet.

results scrn hvac duct vel — Figure 5. HVAC Velocity in Duct with Fan

results scrn hvac deltaP c2o — Figure 6. 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.

results scrn hvac deltaP i2c — Figure 7. 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.

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