Ventilation Energy and Environmental Technology
                                     from VEETECH Ltd.              Updated 8th December  2010

Prepare to be Sick in your Airtight Home! 

Tutorial  3    Developing a Flow Network and Introducing Mechanical Ventilation

[See Disclaimer and Important Notes]

CLICK HERE for PHPAIDA The Interactive Ventilation Calculator Based on Opening Area combined with Wind, Stack and Mechanical Ventilation

This tutorial covers:

• Developing a flow network;
• Incorporating Mechanical Ventilation.

Introduction

In Tutorial 1, the various formulations of airflow equations were introduced. This was followed, in Tutorial 2, by the formulation of the wind and stack pressure equations. In this tutorial, the flow network, i.e. the system of openings in the building fabric, through which air exchange between the inside and outside of a building takes place, is described.

The flow equation of Tutorial 1 must be applied to every opening in the building envelope. If any are ignored (e.g. unidentified cracks and gaps in the building fabric) then the calculations will be in error. Typically, the number of leakage openings is underestimated because openings are unwittingly ignored. This results in underestimating the potential air change rate and, almost certainly, incorrectly predicting airflow patterns. The leakier the building, the more likely underestimates will occur. It should also be noted that a building is leaky unless specific measures have been introduced during the construction to ensure that it is not. Usually, only if required by legislation of if specified by the building owner, will the building be anything other than leaky. The only possible exception might be poured concrete structures with few junction elements. 

Incorporating all openings is usually undertaken by defining:

• A network to represent ‘background’ or “air infiltration” paths;
• A network of purpose provided openings such as windows, air vents etc;
• A network of mechanical ventilation fans.

A Network to Represent ‘Background’ Openings

Figure 3.1. Background leakage networks.

Networks representing background leakages are illustrated in Figure 3.1. The basic procedure is as follows:

• Determine the total surface area of exposed building surface (m²);
• Separate into the area of each face and the roof (or ceiling) area;
• Separate each face into a lower half and an upper half (single storey) or upper and lower half for each storey (multi storey building);
• Identify the ‘C’ value of each of each path by multiplying its area by the building porosity (as determined by pressurisation test or as specified in design);
• Check that units are set to a common value (e.g. L/s or m³/s at 1 Pa).

A Network to Represent ‘Purpose Provided’ Openings

  This is determined by identifying for each opening:

• The face of the building in which it is located;
• The height of opening;
• Its dimensions;
• For large openings (e.g. open windows) divide the area into an upper and lower section so that two way flow can be simulated.

  A Network to Represent Mechanical Ventilation Fans

A simple representation of mechanical ventilation can be achieved by establishing a constant flow path for each fan. This is only permissible if the induced flow has no more than a few Pascals influence on pressure drop. The easiest method is to set the flow exponent to the value ‘1’ and the C value to the desired ventilation flow rate (in L/s or m³/s).

The flow network comprises the location and flow characteristics of ALL the above openings.

This completes Tutorial 3

Go To:

• Disclaimer and Important Notes;
  

• Tutorial 1 - Calculating Flow Rate through Openings - The Equations ;
  

• Tutorial 2 - Determining the natural driving forces (wind and temperature) ;
  

• Tutorial 3 - Developing a Flow Network and Introducing Mechanical Ventilation;
  

• Tutorial 4 - Solving the Flow Equations;
  

• Tutorial 5 - Simple Examples
  

• Tutorial 6 - The Online Algorithm PHPAIDA

[Return to Tutorials]