Stairwell Pressurization Studies in Ventus

Reading Time: 11 minutes

Created with software version: 2024-2

1. Overview

This tutorial is intended for Fire Protection Engineers who wish to perform Stairwell Pressurization analysis as part of their Smoke Control projects. It will demonstrate how to model and evaluate a simple fan-based Stairwell Pressurization system to determine if it complies with a given set of criteria. You will learn how to draw zones, create flow elements and flow paths, run the simulation, and evaluate the differential pressures and flows in a stairwell using the tools available in Ventus.

2. Before Starting

Before beginning this tutorial:

Read through the Ventus Basic First Model tutorial to understand the basics of making a model in Ventus.
Download and extract the Stairwell Pressurization model to follow along.

3. Introduction

Most codes and standards regarding Smoke Control systems will provide criteria for stairwell pressurization systems that Fire Protection Engineers must comply with. Typically, these criteria involve a combination of:

Minimum / Maximum Differential Pressures at Doorways
Maximum allowable applied force at Doorways
Minimum Flow Rates when a Stairwell door is open on a Fire Event Level

In this tutorial, we will model and evaluate a building with a similar set of criteria. These criteria are not taken from a specific standard, however they are similar to real standards. When designing your own systems, you should select an appropriate standard for your own use. The criteria we will use are as follows:

Table 1. Design Criteria

Design Criteria
Stairwell Doors Closed
Differential Pressure at Doorways	25-50 Pa
Maximum Applied Force at Doorways	100 N
Stairwell Door Open at Fire Event Floor
Minimum Flow Rate at Open Doorway	1 m/s

Design Criteria

Stairwell Doors Closed

Differential Pressure at Doorways

25-50 Pa

Maximum Applied Force at Doorways

100 N

Stairwell Door Open at Fire Event Floor

Minimum Flow Rate at Open Doorway

1 m/s

4. Geometry Overview

This tutorial will use the geometry from the Ventus Basic First Model tutorial. This geometry features a simple 3 zone floorplan, with two rooms and one stairwell.

Plan View, Room Names — Figure 1. The geometry of the example problem with zone names.

The model we are using specifically is the Summer variant of the model modeled in that tutorial. If you would like to see how this model is created, you can follow along in the Basic First Model tutorial.

Completed Geometric Model — Figure 2. The completed geometric model.

This model as it exists now does not include any forced pressurization system. We will add a fan to the stairwell and size it such that it meets the criteria laid out in Table 1.

The model is also currently in the closed door configuration. We will modify it later to perform the open door analysis. The doors in this model are assumed to have a Cross-sectional area of \(1.85m^2\).

5. The Closed Door Study

5.1. Defining Fans

Fans in Ventus are currently modeled with AHS Zone Points. An AHS Zone Point connects a Zone to a Simple Air Handling System. AHS Zone Points act to add (Supply) or remove (Return) gases from Zones.

In this case, we wish to model the addition of gas (air) to a Zone via a fan, so we will use a Supply AHS Zone Point.

To add the Supply point to the model:

Open the getting started summer.vnts model that you downloaded above.
Right-click on Level 21.0 m in the navigation tree and click Set as Active Level
Click the icon to switch to the Top View.
Click the AHS Zone Point tool .
In the Property Ribbon, give the Zone Point the name fan.
Also in the ribbon, ensure that the Types field is set to Supply, then set the Design Flow Rate field to 1000 scfm.
Place the AHS Zone Point by moving the tool over the central zone in the model, Stair_1_6, and left-clicking.

Drawing of AHS Zone Point — Figure 3. Drawing of the AHS Zone Point

We are now ready to run and analyze the closed door Study.

5.2. Running the Simulation

To run the study:

Click the Run icon . The simulation should take only a few seconds.
When the simulation is complete, click OK in the Run Simulation dialog.

5.3. Analyzing Results

To analyze the data for our simulated Stairwell, we need to analyze the relevant Flow Paths for doors in that shaft. In our case, this is the D2 Flow Path, and all of its copies on higher floors. To view the results for these Flow Paths:

Click the Path Data button in the Ventus UI to show the Path Data panel.
In the filter box at the top of the panel, type in D2. This will filter the data to only show Flow Paths with D2 in their name. Note that this filter field is case sensitive.

Figure 4. Displaying and Filtering of Path Data in Ventus

The dP column of this table shows the differential pressures across the Flow Path. As can be seen from this data, a 1000 scfm fan only provides a ~4 Pa pressure across the closed door Flow Paths in our stairwell. This is not sufficient to meet the requirements from Table 1.

5.4. Adjusting Fan Size

We will adjust the fan size and run the simulation again to meet our design criteria. To do this:

Click the fan AHS Zone Point in the navigation tree.
Change the Design Flow Rate field to 3500 scfm.
Click the Run icon again to re-run the simulation.

Figure 5. Changing the fan size and re-running the simulation

Note that the Path Data table automatically updates based on the new simulation data. From this new results data, we can see that the fan size of 3500 scfm results in closed door pressures of ~30 Pa.

Figure 6. Path Data results with a `3500 scfm` fan

Repeat the steps above to change the fan size to 4750 scfm.

You can see that the fan size of 4750 scfm results in closed door pressures of ~50 Pa.

Figure 7. Path Data results with a `4750 scfm` fan

With this information we now know that to satisfy our design criteria, we need a single pressurization fan that can provide 3500 scfm - 4750 scfm. We can convert this maximum allowable pressure (50 Pa) and the assumed size of our door (\(1.85 m^2\)) in to applied force to evaluate our compliance with that criteria as well. Using these to values, we evaluate:

\(50 \frac{N}{m^2} * 1.85 m^2 = 92.5 N\)

We know that the maximum force applied to any of our closed doors is 92.5 N, which falls within our design requirements. If we had doors with a larger area in this stairwell, we might have needed to reduce our maximum pressure within the allowable range of 30 - 50 Pa to avoid exceeding our force requirement.

6. The Open Door Study

Now that we know what size fan we will need to satisfy design criteria with all of the shaft doors closed, we will modify the model to evaluate the Open door scenario. To do this, we will need a few new elements in our model. We will need to add:

Table 2. New Flow Elements

Flow Element	Use
Open Door	Modeling an open door in the Stairwell
Open Window	Modeling an open or shattered window on the Fire Event floor

Flow Element

Use

Open Door

Modeling an open door in the Stairwell

Open Window

Modeling an open or shattered window on the Fire Event floor

Once added, we will need to add additional Flow Paths to utilize these Elements, then re-run our study and make any necessary adjustments to the pressurization system.

6.1. Adding New Flow Elements

To add the new Flow Elements to the model:

Use the Model › Edit Flow Elements action to open the Edit Flow Elements dialog.
Click New….
In the New Flow Element dialog, enter the name Door-Open and select the Orifice Area Model.
Click OK to create the Flow Element.
In the Edit Flow Elements dialog for the Door-Open element, enter a Cross-Sectional Area of \(1.85 m^2\).
Click Apply to apply the changes to the flow element.
Repeat the process to create another flow element named Window, with the same Orifice Area model, and a Cross-Sectional Area of \(0.8 m^2\).
Click OK to close the Edit Flow Elements dialog.

Figure 8. Adding new Flow Elements to the model

Now that we have the Flow Elements created, we need to create and modify Flow Paths on the event level to use them.

6.2. Creating and Modifying Flow Paths in the Model

Our Open Door scenario will assume that Level 12.0 m is our event floor. To model the fire scenario on this floor, we will change the existing door Flow Paths on that level to use our new Door-Open Flow Element, and we will add a Flow Path to each room that uses our new Window Flow Element. The Door-Open Flow Paths will represent doors that are opened on the event floor for evacuations, and the Window Flow Paths will represent windows that are either shattered due to heat, or opened for ventilation purposes.

We will start by modifying the existing door Flow Paths. To modify these:

Right click Level 12.0 m in the navigation view and click Set as Active Level
Click the icon to switch to the Top View.
Select the two red Flow Paths visible.
In the Properties Ribbon, switch the Element field to use the Door-Open flow element.

Figure 9. Switching door Flow Paths to use the `Door-Open` Flow Element

Now we will add two Flow Paths to represent the windows. To do this:

Double-click the icon to select the Two Point Flow Path tool in pinned mode.
In the Tool Properties ribbon, select the Window Flow Element in the Element field.
Draw two lines across the outer walls of each Unit in the model to add the two new Flow Paths.

Figure 10. Adding new Flow Paths with the `Window` element

We are now ready to run the Open Door scenario.

6.3. Running the Simulation

To run the study:

Click the Run icon . The simulation should take only a few seconds.
When the simulation is complete, click OK in the Run Simulation dialog.

6.4. Analyzing Results

Like the Closed Door study, analyze the results of the Open Door study by clicking Path Data to open the Path Data Panel, then filter down to the D2 elements.

Figure 11. Path Data results of the Open Door study with a `4750 scfm` fan

As can be seen in Figure 11, the differential pressures of our closed shaft doors drop well below our design criteria because we opened the doors and windows on the fire event level. However, the flow across the open door boundary, shown by the Flow0 column for the D2_1_3 Flow Path, increases significantly. To convert this Mass Flow value to a velocity at the boundary, we will assume the standard density of air at sea level to be \(1.225 \frac{kg}{m^3}\) and use the following equation:

\(\frac{flow \frac{kg}{s}}{1.225 \frac{kg}{m^3} * 1.85 m^2} = velocity \frac{m}{s}\)

Plugging in our flow value:

\(\frac{2.282441 \frac{kg}{s}}{1.225 \frac{kg}{m^3} * 1.85 m^2} = 1.007 \frac{m}{s}\)

As can be seen, this flow velocity meets our design criteria requirement of \(1 \frac{m}{s}\). The flow value is sufficient, however we need to make adjustments to satisfy our differential pressure requirements at the closed stairwell doors.

6.5. Adjusting Fan Size

There are several adjustments that we could make in a real Pressurization system to achieve the design criteria such as adding dampers, adding an additional fan at the bottom of the shaft, and using a Performance curve model for our fan, however for this tutorial we will simply increase the Design Flow Rate of our constant flow fan.

We will adjust the fan size similar to how we did during the closed door study. To adjust the fan size, select the fan and enter a new value of 25000 scfm, then click the Run icon to run the simulation again.

Figure 12. Path Data results of the Open Door study with a `25000 scfm` fan

As can be seen from the results in Figure 12, our closed door differential pressures now meet our design criteria. Plugging the new Flow0 value for D2_1_3 in to the equation above yields:

\(\frac{12.077871 \frac{kg}{s}}{1.225 \frac{kg}{m^3} * 1.85 m^2} = 5.329 \frac{m}{s}\)

We now meet our design criteria in the Open door configuration.

6.6. Final Results

To summarize our analysis, we determined that the fan size needed for our Pressurization system to meet the design criteria laid out in Table 1 is as follows:

Table 3. Final fan size requirements of our pressurization system

Scenario Fan Size (scfm)

Doors Closed 3500 - 4750

Fire Level Doors Open 25000 minimum

Scenario	Fan Size (scfm)
Doors Closed	`3500 - 4750`
Fire Level Doors Open	`25000` minimum

For this simple system, we would need to source a fan that could ramp from 3500 scfm to 25000 scfm, or add additional components that would give us more control over the pressurization and flow rate of our stairwell.

7. Conclusion

You should now be familiar with how to use Ventus to perform a Stairwell Pressurization study. Ventus accelerates this type of study by providing tools that makes processes of modeling and analysis much faster.

To download the most recent version of Ventus, please visit the Ventus Download page. Please contact support@thunderheadeng.com with any questions or feedback regarding our products or documentation.