Ventilation is a simple process of introducing a new flow of air into a restricted space. The new air either mixes with or displaces air already present in the space. The new air flow may be warmer or cooler than the existing air. It could be cleaner, meaning that it does not contain dust and other particulates, or it may be of an entirely different chemical composition, depending on the use of the space or building.
There are three main types of ventilation: mixing, displacement and inter-zonal.
The mixing of existing air with the new air flow is caused by creating turbulence in the space, usually by some mechanical means such as a fan system. The fans may be powered electrically or through the force of the air flow. If the mixing is efficient, the new air flow creates a uniform temperature or particulate concentration within the space.
This type of system can be controlled with a variety of pressure, temperature and chemical sensors that will modify the air flow at specific settings. The disadvantage of this type of system when used for domestic, commercial or public buildings is that it may maintain some proportion of ‘dirty’ or ‘old’ air in the system that could cause eye and throat irritation to occupants.
The displacement ventilation method removes existing air in a space and replaces it with a new air flow in the manner of a piston. This is by far a more comfortable system of ventilation, as the air is new and fresh and pollutants and other particulates can be removed.
However, the problem is that the temperature and pressure distribution of the air within the space, as well as the pollutant concentration, will not be uniform. Pollutant build-up especially may originate from ground-level dust and other contaminants. It’s a common problem in commercial buildings such as office premises and retail outlets.
There could be a build-up of pollution and higher or lower temperatures on one side of the space. This type of system needs a very sensitive control system. In practice, there is always an element of mixing between the old air and the new air flow.
Inter-zonal ventilation is commonly found in kitchens and bathrooms in domestic environments and ‘clean spaces’ such as clean rooms in hospitals and research establishments. Extractor fans are the old-fashioned version of this. Old air is drawn out of the space and is replaced by fresh air. But there is a system of ducts and flow patterns that avoids any mixing or cross-contamination of the two air flows.
One of the greatest problems with any ventilation system is that air extracted from the space at any time will mix outside of it with the incoming flow of fresh air. This is known as short-circuiting and may happen if the air inlet and outlet ducts are installed too close to each other.