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Types of Ventilation Systems - Coggle Diagram
Types of Ventilation Systems
Local Exhaust Ventilation (LEV)
capturing the contaminant at source before it spreads into the room air
the most effective method, especially where localized sources of hazardous contaminants are present
the rapid increase in concentration occurs close to the extract hood
relative ventilation efficiency (E) > 2
Piston Ventilation
represents a unidirectional flow of air in which outdoor air propels the contaminated room air ahead of it like a front
employed in the clean/sterile room
the air turbulence must be reduced to a minimum so that contaminant dispersion is minimized
the room air is continuously swept by outdoor air and little spread of contamination generated within the room takes place before this is carried by the outdoor air to the extract duct
supplying the air through laminar flow panels containing HEPA filters or ULPA filters placed on the ceiling (down flow) or on a wall (cross flow) at a velocity between 0.4 and 0.5 m/s [ULPA more efficient and more small particle handled than HEPA]
relative ventilation efficiency (E) = 2
the driving force is mainly the momentum of supply air
Displacement Ventilation
As in piston ventilation, this method also relies on the displacement of room air by a fresh supply of outdoor air but in a less discreet way than in piston ventilation
Used in
: large occupancy and large internal heat gain
The momentum is small, but buoyancy is the dominant force in creation of room air movement
Type of displacement
: upward and downward
more widely used than the downward system because it produces better indoor air quality for occupants
outdoor air and recirculated air is delivered close to the floor of the room at a low velocity which rises through the room by a combination of momentum and buoyancy forces, the latter caused by the presence of people or warm surfaces such as IT equipment in offices
Disadvantage
: it can only be used for cooling and is not suitable for winter heating
relative ventilation efficiency (E) < 2
Mixing Ventilation
This method requires that the supply air and room air are mixed well by the actions of the supply jet momentum and buoyancy
it can cope with much larger room loads than in displacement ventilation
The supply air is used to dilute the concentration of contaminants in the room
the outdoor air is usually supplied at high level (i e above the occupied zone) the concentration levels above the occupied zone are generally lower than those existing within the zone, which results in an overall relative ventilation efficiency and mean air change effectiveness of less than one
relative ventilation efficiency (E) < 1
Impinging Jet Ventilation
As the jet impinges on the floor it spreads over a large area
Unlike the displacement system which floods the floor with supply air, the resulting flow from an IJV is a very thin layer of air over the floor
Advantage
: both the mixing and displacement ventilation systems and has lower momentum than mixing and higher momentum than wall displacement ventilation (when the air terminal unit is wall mounted at low level)
Although higher momentum than wall DV, IJV produces a similar flow field
Personalized Ventilation
In certain types of occupancy it may be sufficient to supply the fresh air directly to the occupants
provide fresh air directly to the breathing zone of every occupant in the space
Disadvantage
: it restricts occupants movement and sometimes causes excessive draft on the occupants faces
Demand Control Ventilation
demand for ventilation using sensors and supplies the outside air as needed
As employees arrive to a building in the morning for work, a DCV system will increase the number of air changes in occupied rooms (related to the CO2 concentration). The DCV system will decrease demand for air changes when employees leave at the end of the day
Benefits
Reduce energy cost
Improve IAQ
Increase occupant comfort
Important considerations
Consider the outdoor ventilation rate for exhaust system in certain room. not to reduce the outdoor air flow rate so low that it results in unwanted building pressurization
Consider the placement of sensor
Sensor needs to be maintained routinely. It also requires calibration over time