HVAC Pressure Loss Calculation

This topic describes how to calculate the total system pressure loss for an indoor ventilation case, how in:Flux contributes to that calculation, and how the result is combined with a fan curve to determine the delivered flow rate.

Sources of Pressure Loss in an HVAC System

Consider the full flow path of the air from where it enters the HVAC system to where it exits. Along that path, the air encounters a series of obstacles, each of which causes a pressure loss:

Ducts, bends, tees, transitions, and fittings
Grilles, louvers, dampers, and filters
Vents at the system's interface with the atmosphere (mushroom vents, goosenecks, hoods, etc.)
The room itself

Pressure loss curves (pressure drop versus flow rate) for each of these component types are typically available from the component manufacturer or from standard references such as SMACNA or the ASHRAE Handbook. Summed together, they account for everything in the system except the room.

What in:Flux Provides

The one component for which a pressure loss curve is typically not available from standard sources is the room being ventilated. Unlike a duct or a vent, the room is filled with equipment and structures specific to the project. The purpose of running an internal ventilation simulation in in:Flux is to produce that missing pressure loss curve - the loss associated with the flow regime inside the specific room being analyzed.

At the end of the analysis you should have:

A pressure loss curve for the components upstream of the room (from manufacturer or reference data)
A pressure loss curve for the room itself (from in:Flux)
A pressure loss curve for the components downstream of the room (from manufacturer or reference data)

Why Not Simulate the Entire System in in:Flux?

Accurately modeling the pressure loss of a simple component such as a mushroom vent requires a very smooth, fine mesh capable of resolving flow physics such as separation at the vent lip. Accurately modeling a room full of equipment requires a mesh that can efficiently represent the full complexity of that equipment. These two requirements are in conflict - a mesh suited to one is not suited to the other. in:Flux uses an immersed boundary approach optimized for complex facility geometry, which is exactly the case where manufacturer loss curves do not exist.

Attempting to compute a mushroom vent's pressure loss in CFD would, at best, reproduce the manufacturer's test data. In that case, using the published manufacturer curve is both faster and more accurate. The general principle is: where good component data exists, use it; use CFD only for the part of the system where it does not.

Building the Room Pressure Loss Curve

As of version 3.2, in:Flux can build the room's pressure loss curve directly from a set of ventilation simulations run at different flow rates. A quadratic curve fit is used, which requires a minimum of three points represented by three different simulations.

The most convenient way to produce three flow rates is to use the HVAC Flow Factor:

Pick a reasonable baseline flow rate and define the HVAC inlets, outlets, and fans with those values.
Create three ventilation cases. For each, open the Advanced options and set the HVAC Flow Factor to a different value - for example "0.5", "1.0", and "1.5". This scales every HVAC inlet, outlet, and fan in the case by the same factor.
Run the three simulations.
Open the Ventilation Analysis Window and select Total Pressure Loss, Pa as the variable.
Click the Loss Curve button. in:Flux plots the three simulation points on axes of pressure loss versus flow rate and fits the quadratic curve through them. The result is the room's pressure loss curve.

The three-simulation minimum is a floor. Additional simulations at other HVAC Flow Factor values can be added to tighten the fit and improve confidence across a wider operating range.

The HVAC Flow Factor

The HVAC Flow Factor, located under the Advanced section of a ventilation case, is a multiplier applied uniformly to every HVAC component in the case. A factor of "1.0" runs the case at the defined flow rates; "0.5" runs all HVAC components at half their defined values; "1.5" at 1.5x. Its primary use is producing multiple simulations at different flow rates without having to redefine each component individually.

Total System Pressure Loss and the Operating Point

Add the room loss curve to the component loss curves upstream and downstream. The sum is the total system pressure loss curve - the pressure drop the fan must overcome at any given flow rate.

Plot the total system loss curve and the fan performance curve on the same axes of pressure versus flow rate. The flow rate at the intersection of the two curves is the expected delivered flow rate of the HVAC system. This is the operating point.

If the delivered flow rate differs from the target design value, either the fan or the system design needs revision. The in:Flux simulations can then be re-run at the revised design flow rate and the operating point re-evaluated.

Air Change Rate (ACH)

The air change rate for a room is:

ACH = HVAC volume flow rate ÷ room volume

This is a hand calculation that does not require CFD. Running a simulation specifically to obtain ACH is unnecessary unless the analysis is evaluating ACH over a sub-volume of the room, in which case a monitor region can be used.

Scope and Limitations

in:Flux provides the pressure loss associated with the room. It does not calculate duct or fitting losses, and it does not evaluate the fan curve. A complete pressure loss calculation therefore requires:

Three in:Flux ventilation simulations (via the HVAC Flow Factor) to build the room loss curve
External calculation or manufacturer data for pressure losses in all other components
The fan performance curve from the manufacturer