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# Modeling a Damper

by Engineered Software, Inc.

A damper is used extensively in the HVAC industry as well as other industries that move air or compressible gases for their processes. A damper controls air flow throughout a duct system in order to evenly distribute air or gas. It operates by changing the size and shape of its flow path, so its flow capacity (or conversely, resistance) changes with position.
This article describes how to model a damper in PIPE-FLO as a Control Valve with an approximated flow coefficient (Cv) profile.  A Fabtech Inc. (www.fabtechinc.com) damper specification datasheet is used as an example in this article.

Some damper manufacturers provide performance curves for their products. An example performance curve graph of pressure vs. air velocity is shown for fully open dampers of 16", 24", and 36".

This performance curve has velocity units in feet per minute and pressure drop in inches of water gauge. Some manufacturer curves may be in other units such as cubic feet per minute (cfm).  PIPE-FLO® needs the flow coefficient (Cv or Kv) at different throttling percentages in able to model the damper as a Control Valve. The first step is converting a set of data points (velocity and pressure drop) for the fully open position to a Cv value.The equation from the Crane Technical Paper TP-410 to be used to obtain the Cv values is shown below.

LaTeX Math Block
alignment center
$C_v = \frac{W}{63.3F_PY\sqrt{xP'_1 \rho_1}}$

The equation can be re-arranged:

LaTeX Math Block
alignment center
$C_v = \frac{W}{63.3F_PY\sqrt{dP \rho_1}}$

The next step is reading data from the performance curve and converting the units to accommodate the equation.
Attached at the bottom of this Knowledge Base is a downloadable spreadsheet that preforms the conversion. A screenshot of the spreadsheet is shown below. Prior to use, verify the accuracy of the equations entered in the spreadsheet.

By entering the damper diameter and reading one value of dP at a select fluid velocity from the performance curve, the Cv can be determined. For example, the 36" damper has a pressure drop of 0.018 in wc at a velocity of 2000 ft/min air velocity. An expansion factor (Y) value of 1.0 is estimated in order to determine the Cv without considering the effect of gas expansion. The piping geometry factor (Fp) is assumed to be 1.0 to evaluate the performance of the damper without attached reducers or other fittings.

At 100% open, the damper has a calculated Cv of 143,649 (which corresponds to a resistance coefficient of about 0.07). Because this is an estimated value based on values taken from a curve, different points on the curve can be used to determine the Cv and an average can be determined.

Once the Cv is determined, the Control Valve Data Estimator in PIPE-FLO can be used to estimate a Cv profile for the damper.  A sample Control Valve Data Estimator dialog box is shown below.

In this example, a Butterfly, Center Shaft body style was used with a trim type of Aligned 60° and equal percentage characteristic curve. Once the Valve Position of 100% and Cv value are entered, PIPE-FLO can estimate the remaining Control Valve data to fit the selected characteristic curve.

It will also enter an FL and xT profile based on typical butterfly valves shown in the IEC and ISA standards for sizing control valves.

It is important to understand that the damper performance is being estimated based on values taken from a log scale on the graph, the profile is assumed by the user, and the FL and xT values are based on typical tested valves. More accurate data should be obtained from the manufacturer or field measurements if possible.

Notes:

1. The above screen shot is from PIPE-FLO Professional version 1216.
2. Always confirm your equation units.

## Attachments

Damper Cv Calculations - FINAL.xlsx