Concept of ventilation

Ventilation is simply the movement of air into a building or a room and the air distribution within that building or room. Ventilation in buildings serves two purposes: it dilutes pollutants and removes them.

There are three elements to it:

  • Ventilation speed — The amount of outdoor air provided to the space and the quality of that outdoor air;
  • Airflow direction — The direction of the airflow through a building’s structure, from clean to dirty areas;
  • Airflow pattern or air distribution — The outside air being delivered to every area promptly. Additionally, the airborne pollutants that have been generated in each space should be removed;

Natural, mechanical, and hybrid ventilation are all possible options for ventilating a building.

Natural ventilation

Nature forces (e.g. Winds and thermal buoyancy forces due to outdoor and indoor air density variations drive outdoor air through purposely-built, building envelope doors.

These openings can be windows, doors, and solar chimneys. The climate, building design, and human behavior all influence natural ventilation in buildings.

The wind will naturally ventilate your home. The direction of the wind can affect how windows are oriented to it. When the wind blows against your house, the air is forced inwardly onto your windows.

However, a natural vacuum effect draws air outwardly (downwind) from windows facing towards the wind.

Mechanical ventilation

Mechanical fans drive mechanical ventilation. Fans can be mounted directly on windows and walls or in air ducts to supply or exhaust air from a room.
The type of ventilation that is used will depend on the climate.

For example, in humid climates, it may be necessary to minimize or prevent infiltration to reduce interstitial condensing (which is when warm, moist air from a building enters a wall or roof and touches a cold surface).

A mechanical ventilation system with positive pressure can be used in these situations. In cold climates, however, it is crucial to prevent exfiltration to reduce interstitial condensation.

This is why negative pressure ventilation is often used. Negative pressure ventilation is used for rooms containing locally-generated pollutants such as toilets, bathrooms, or kitchens.

Positive pressure systems have room for positive pressure. The room air is emitted through leakages of envelopes or other openings. A negative pressure system places the room in negative pressure and compensates by sucking air from the outside.

A balanced mechanical ventilation system is where the air supply and exhausts have been checked and adjusted to meet the design specifications.

You can maintain a room pressure that is slightly positive or negative by using somewhat different exhaust or supply ventilation rates. To minimize interstitial condensation, the room pressure can be slightly negative by exhausting 10% less air than the supply.

An airborne precaution room is used to control infection. The minimum negative pressure in the corridor should be 2.5 Pa ( CDC 2003). Or just simply ask αποφραξεις περιστερι for help.

Hybrid or mixed-mode ventilation

Hybrid ventilation (mixed mode) relies on the natural driving forces to provide the desired flow rate. When the natural ventilation flow rate is too slow, it uses mechanical ventilation (Heiselberg &Bjorn 2002).

Exhaust fans can be used to improve ventilation in areas containing patients with airborne infections. This simple hybrid ventilation, mutually mixed, should be treated with care. The fans should not be placed where the room air can be exhausted to the outdoors through a wall or roof. It is essential to measure and test the exhaust fans before they are installed.

Several problems are associated with exhaust fans, including installation difficulties for large fans, noise from high-power fans, increased or decreased room temperature and the need for continuous electricity supply. Ceiling fans and heating systems may be used if the room is prone to thermal discomfort.

Another option is to install whirlybirds, also known as wind turbines or whirligigs, which do not need electricity but provide a roof exhaust system that increases airflow within a building.

Assessing ventilation performance

These four aspects can be used as a basis for evaluating ventilation performance in buildings.

  • Is there sufficient ventilation?
  • Does the building’s airflow direction change from clean to dirty areas (isolation rooms or areas of containment such as a laboratory)? 
  • How efficiently does the system deliver the outdoor air to each room?
  • How efficiently does the system remove airborne pollutants from each room?

The air exchange efficiency can also be calculated using the air change per hour (Etheridge & Sandberg, 1996). The air exchange efficiency of piston-type ventilation is 100%. Thoroughly mixing ventilation has a 50% efficiency.

Displacement ventilation has an air exchange efficiency of around 50%, while short-circuiting is closer to 50%.

Evaluating ventilation effectiveness can be done by simulation or measurement (Etheridge & Sandberg 1996). The ventilation flow rate is measured by measuring the speed at which the tracer gas is decomposed in a room or by measuring the flow area through the ventilation openings and ducts.

Smoke can be used to visualize the direction of airflow. The airflow direction can be visualized by smoke.

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