Natural Ventilation

  • Created by: OctaviaL
  • Created on: 17-01-17 10:27
Why ventilate?
Without ventilation, CO2 concentration rises.
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What is natural ventilation?
Ventilation that relies on natural forces (e.g. wind and thermal buoyancy) to operate. It might use opening windows, holes or large ducts.
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What is mechanical ventilation?
Ventilation that requires on pushing air around the building using a fan.
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What is air conditioning?
Like mechanical ventilation, a fan is used to move air, but cooling can be added to control the temperature of the space.
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Advantages of natural ventilation?
1. Often just needs a window. 2. Occupants not cut off from the outside world.
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Disadvantages of natural ventilation?
1. The amount of ventilation varies due to wind speed and direction and the difference between internal and external temperatures.
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Advantages of mechanical ventilation?
1. You can guarantee a known amount of ventilation as you size and run it.
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Disadvantages of mechanical ventilation?
1. Energy consumed to run fans. 2. Can take up a lot of space. 3. Gives the building a sealed feel.
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Advantages of air conditioning?
1. You can guarantee a known amount of ventilation. 2. Heat can be removed from the building.
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Disadvantages of air conditioning?
1. System uses a huge amount of energy. 2. Very heavy so building might need re-enforcement.
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How can a naturally ventilated building be comfortable if it gives a fluctuating internal environment?
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Name of the temperature at which 90% of people don't feel too hot or too cold?
Neutrality or neutral temperature.
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What is the equation for neutrality temperature?
Tn= (0.31 Te-av + 17.6) +/- 2.5 (C) (where Te-av is the mean external temperature over the last 30 days)
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What are the financial benefits of natural ventilation?
Lower initial, running and maintenance costs.
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What are the health benefits of natural ventilation?
1. Minimises the chance of sick building syndrome. 2. Physchological benefits with feeling connected to the environment (feel of an open window). 3. Potentially more productivity (controlled env. Fluctuations in the environment can produce stimuli)
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What are the environmental benefits of natural ventitlation?
1. Cuts energy use (and bill) by 50-70%. 2. A most effective way to cut GHG emissions. (Wind powered buildings 31x less effective, solar powered 114X; these cost much more with minimal return in next 20-30 yrs).
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When can natural ventilation be used?
In a wide range of building types and circumstances inc. auditoria, modern offices, shopping centres etc.
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Why might people not want natural ventilation to be used?
1. Don't know how to design it. 2. Don't trust it. 3. Don't understand building regs, so play safe. 4. Client has old fashioned 'prestige' image of AC.
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What five things must be considered when designing natural ventilation.
Orientation, zoning, geometry, envelope and control.
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An example of why orientation is important?
Minimise the chance of overheating.
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Two examples of why zoning is important?
1. Maximise the area of passive zone. 2. Realise some areas might need AC.
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What does geometry consist of?
Enough openable windows, good ventilation paths.
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What is an example of poor control?
Big, heavy, high up windows that can't easily be opened.
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What drives natural ventilation?
1. Buoyancy 2. Wind 3. Wind and buoyancy
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What is single sided ventilation?
Both openings/windows are on the same side of the building for any space.
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What is double sided (or cross) ventilation?
Windows/openings are on opposite side of the building and such that air can flow between them?
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What are two alternative names for buoyancy driven ventilation?
Stack or temperature driven ventilation.
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In ventilation, what pressure are we concerned with?
The pressure difference between one location in or around the building and another.
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What is the formula for the difference in hydrostatic pressure compared to ground level?
P = -g
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When does upflow occur in buoyancy driven ventilation? Why?
When Tin > Te. As the pressure gradient inside is less than the pressure gradient outside.
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How does more heating (people, radiators) inside a building affect upflow?
A warmer internal temperature, reduces the inside pressure gradient, so there is a greater pressure difference between inside and outside resulting in more drive and a stronger upflow.
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What is the general rule in terms of gaps for buoyancy upflow?
Bigger holes up high, smaller holes low down.
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What happens to buoyancy driven ventilation if the temperature inside is less than the external temperature?
The interior pressure will be greater than the exterior pressure so the pressure gradient inside > pressure gradient outside, resulting in downflow.
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What happens to the downflow if there is more cooling inside (more thermal mass, more chilled ceilings)?
A cooler internal temperature results in a larger internal pressure, increasing the pressure gradient inside, so there is a larger difference in pressure between inside and outside, this creates more drive for a stronger downflow.
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What are the two basic forms of buoyancy driven ventilation?
1. Mixing. 2. Displacement.
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How can the effectiveness of buoyancy driven ventilation be increased?
Controlling solar gains to create higher pressure where you want air to be drawn to.
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How does height affect wind speed?
Increase in height = increase in wind speed.
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What does the wind power profile law describe?
How wind speed changes with height.
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Why is the wind power profile law very approximate in built up areas?
Due to the presence of surrounding buildings?
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What is the equation for the wind power profile law?
Uh = Ur(h/r) ^0.143 U=wind speed, h=height above the surrounding ground of the opening, r= reference height.
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How will wind affect pressure across a building?
Pressure will be greater on the side of the prevailing wind.
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How is flow affected for buoyancy-dominated, wind-assisted ventilation?
More flow.
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How is flow affected for buoyancy-dominated, wind-opposed ventilation?
Less flow.
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How is flow affected for wind-dominated, buoyancy opposed ventilation?
More flow and ventilation regime changes.
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What might happen if enough people leave the room to result in a change of heat flux?
The flow direction can be reversed.
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How can we take advantage of wind and minimise its detrimental impact?
Design and locate inlets and outlets so that wind and buoyancy work together.
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Why is sizing easy for mechanical ventilation?
You set the levels for the maximum CO2 or cooling needed and size to give this.
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Why is sizing much harder for natural venitlation?
Wind speed and pressure and buoyancy keep changing.
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Why do we size ventilation?
1. Provide enough air for good air quality. 2. Cool the building in summer.
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What is good air quality defined by?
Building regulations, e.g. in schools 3 litres/second/person.
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What is the equation for heat removed via ventilation?
Heat removed (W) = ventilation rate (m^3/s) x density of air (kg/m^3) x specific heat capacity of air (kJ/kg/K) x (internal temp - external temp)
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What heat gains do we need to control?
1. Solar gains. 2. Gains from lights. 3. IT gains.
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Is wind or buoyancy driven ventilation more effective in reducing summertime overheating?
Wind, but it needs openings on two sides.
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What affect does increasing the distance between openings have?
Greater stack effect (hence you need to be able to create the height difference.
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What does Cd (the discharge coefficient express?
How easy it is for air to pass through the hole because of air on the other side getting in the way.
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Who gives guidance equations for sizing?
The Chartered Institute of Building Services Engineers.
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What is the equation for openings on opposite sides, wind only?
Qw =
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What is the equation for buoyancy driven ventilation?
Qb =
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What are the equations for wind and temperature driven ventilation?
Qt =
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How can you estimate the internal/external temperature difference when sizing?
See how often certain external temperatures are typically reached at a location.
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What appropriate features might a north face have? (BDP Studio, Manchester)
1. Aluminium curtain walling system. 2. Glazing to provide good levels of daylight. 3. Opening lights. 4. 'Hopper' vents (concealed behind timber louvre cassettes)
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How could you shield a south face from traffic noise and direct sunlight but still have openings for cross ventilation? (BDP Studio, Manchester)
1. Wrap in stainless steel skin. 2. Slot windows and flush-faced ventilation louvres.
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What is biomimicry?
'Studies nature's best ideas and then imitates these designs and processes to solve human problems.
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What example of biomimicry could we use in ventilation?
Termite mounds include flumes which vent through the top and sides. As the wind blows, hot air from the main chambers below ground is drawn out of the structure.
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Other cards in this set

Card 2


What is natural ventilation?


Ventilation that relies on natural forces (e.g. wind and thermal buoyancy) to operate. It might use opening windows, holes or large ducts.

Card 3


What is mechanical ventilation?


Preview of the front of card 3

Card 4


What is air conditioning?


Preview of the front of card 4

Card 5


Advantages of natural ventilation?


Preview of the front of card 5
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