Airflow

[edit] Types of airflow

Airflow refers to the movement of air from one place to another, within a space or around an object. It can be used to describe air movement in relationship to moving objects, such as cars or aeroplanes as well as fixed objects such as the external form of buildings. It can also be used to describe how air moves within buildings. Finally in a broader sense airflow might describe how air circulates around the globe, forming wind and weather patterns.

[edit] Characteristics

In physics airflow can be determined either by the natural characteristics of that air in relation to heat, pressure, and density, including pressure from external forces such as winds or a fan.

Airflow can impact all aspects of a building from its form in design, its fabric in construction, to occupation and operation. Airflow will often be discussed along with heat flow and moisture flow due to the interrelationships between them, and, in some cases pollution.

[edit] Categories

Inside a building airflow might be simply categorised into just two types:

• Uncontrolled airflow - which includes unwanted airflow that passes through gaps in the building fabric which can impact overall performance in terms of comfort, energy use and detailing. This type of airflow might be described as infiltration, draughts, leaks, breezes, wafts, chills and so on.
• Controlled airflow - might be airflow that is designed into a building, forming part of its servicing strategy, to maintain comfort and performance. This type of controlled airflow normally either forms part of a passive or natural building design ventilation strategy or it is part of a mechanical ventilation strategy, or a combination of the two.

How airflow moves outside or around a building can also impact the internal airflows but is also concerned with external and often public spaces in the built environment and increasingly, air quality and pollution:

• Wind airflow loads on the main structural elements (overall wind loads)
• Wind airflow loads on the facade, openings or roof elements.
• Wind airflow loads on ancillary building items.
• Wind and airflow effects on building design strategies such as natural ventilation.
• Localised wind effects of buildings on external or public spaces.
• Localised wind effects on air quality, particulates and pollution.

At a larger scale, changes in global atmospheric circulation, impacts global weather patterns which can result in changes in localised weather patterns, wind speeds, driving rain and air quality which can directly impact how buildings perform.

[edit] Thermodynamics

Thermodynamics studies the relationships between different properties of a substance such as heat, pressure, density, and temperature as well as moisture. Air as a substance can be effected by these different properties, causing airflows. When air molecules are heated they move quickly and randomly thus requiring more space, so the air is less dense. As hot air has a lower density than cold air it rises, and vice versa, which can result in related airflows, however hot air will not travel towards cold air horizontally unless other pressures such as winds or atmospheric pressures cause it to do so.

For more information see: Heat transfer.

[edit] Weather

In weather patterns when hot air meets cold air, it spreads it out, and is called a warm front. When cold air pushes into warm air, a cold front is created, this pushes the warm air ahead and often creates storm clouds.

In turn when moist air is warmed so too are the water molecules, so warm air filled with these energised water molecules, contains more water molecules than cool air. Moist air is lighter than dry air if both are at the same temperature and pressure.

When the kinetic energy is lost, and air cools, air molecules contract, compress, and lay in closer proximity to each other with a heavier density. Cooler air, which is filled with water molecules can more easily condense and become water.

Fore more information see: Humidity.

[edit] Buildings

In buildings the same principle applies, where, for example warm moist internal air meets a cold surface, as it cools rapidly the water molecules condense and become water. In buildings where this occurs within a wall it might be known as interstitial condensation and can be problematic for the fabric of the building.

Likewise when warm air heated in the building is lost through openings in the fabric, the energy used to to heat the air is also lost. Systems such as Mechanical Ventilation Heat Recovery (MVHR) aim to remove the heat from the air being expelled and use this to preheat the incoming fresh air, thus retaining invested heat energy as well as removing some pollutants.

For more information see: Heat recovery.

Heating, Ventilation, and Air Conditioning (HVAC) systems control the temperature, humidity, and quality of air, using fans to bring fresh air in, exchanging the heat or coolth from the expelled air, as well as heating or cooling the air as required and distributing it to the required locations in a building.

For more information see: Heating, Ventilation, and Air Conditioning.

Both systems rely on airflow to function, the airflow is generally mechanically driven by fans and pressure or naturally driven, or a mixed mode system, which includes the opening of windows or vents.

[edit] Measurement

Airflow both externally (wind speed) and internally is measured using anemometers, of which there are many different types, each focussing on different characteristics, such as air velocity, differential pressure, temperature, humidity or volumetric flow. Other types of measuring devices also use ultrasound to measure energy transfer with passing particles and are capable of calculating air flow along with wet bulb temperature, dew point, and also turbulence.

A more general representation of airflow commonly used within buildings is the rate of air changes per hour (ACH), this measures how often the air in a space is replaced with fresh air. It is calculated by multiplying the volume of air being delivered in one hour (60 minutes) and dividing by the volume of the space. Measures of ACH in houses can vary from 0.2 to 1, whilst passivhaus require less that 0.6 ACH at a certain pressure level (in this case 50 pascals).

Airflow more specifically, for the purpose of mechanical systems is measured by velocity and volume, such as litres per second (l/s), metres cubed per hour (m3/h) or similarly in cubic feet (ft3/min or ft3/h). Occasionally it may also be measured by the mass of air such as kilograms per second (Kg/s)

[edit] CFD

Computational fluid dynamics (CFD) is a technique used to model the behaviour of fluids. In building design it is typically used to model the movement and temperature of air within spaces. This is important as it allows designers to investigate internal conditions before a building is built, allowing them to test options and select the most effective solutions.

For more information see: Computational fluid dynamics.

[edit] Related articles on Designing Buildings

• Air tightness in buildings
• Air.
• Air conditioning.
• Air quality.
• Building services.
• Computational fluid dynamics.
• Domestic Ventilation Systems - a guide to measuring airflow rates (BG 46/2022)
• Domestic ventilation systems performance.
• Draughts in buildings.
• Fresh air.
• Global atmospheric circulation.
• Heat transfer in buildings
• Indoor air velocity.
• Natural ventilation.
• Re-circulation air.
• Ventilation.