Ventilation

Question 1

Explain the advantages and disadvantages of natural ventilation.

Answer 1

Natural ventilation is based on natural forces as wind, pressure differences and sun (temperature) to provide the ventilation that we need to our building instead of expensive mechanicsal ventilation. Natural ventilation is operated by pressure varieties between one section of a building and another, or between the outside and inside, it naturally driver and remove air from an interior space without assistance of a fan or other mechanical device and provide cooling by using external airflow.

Advantages:
Energy effective
Low maintenance
Sustainable
No fans or system noise
Suitable for buildings in mild or moderate climates
Occupants control the ventilation after their desires, often popular in mild climates
Cheap
No HVAC-room neede
Disadvantages
Based on climate situations and maybe can be difficult for some location with cold, unstable and very windy climate. Hard to control for climate with different seasons.
Hard to control ventilation rate and it leads to indoor air quality issues
Hard to deliver fresh air to large, deep plan, and multi-roomed buildings.
Pollutants and noisy locations
No particular filter
Only operates when natural power is high and provided
Does not operate as anticipated
Hard to cool in hot climates

Question 2

Explain the mechanism of stack ventilation.

Answer 2

Stack ventilation is a natural ventilation, that relies on the principle of buoyancy (principen om flytkraft). In other words on temperature differences, that makes a difference in the air density. So warm air will rise and create an airflow within the building.

Temperature difference: warm air rises, often by solar radiation.
Inlets and outlets: a room often has strategic placed inlets and outlets, outlet high up and inlet near the floor.
Natural convection: Warm air rises.
Replacement air: As the warm air rises and exits the building it creates a low pressure inside → air from outside is drawn in.

Question 3

List three common ventilation modes and explain their advantages and disadvantages.

Answer 3

CAV - constant air volume, the simplest system, without demand control and constant air flow
Advantages:
- Cheap and easy to install
- Can work good for building with same room size and use (they all will need same ventilation rate)
- Work well for buildings/zones with low solar radiation, underground

Disadvantages:
- Do not fit larger buildings and offices
- Expensive operational cost
- Low efficiency
- Same volume rate in all rooms

VAV - Varied Air volume. The system varies the air flow at a constant temperature, adapted according to the indoor temp and quality.
Advantages:
- Low operational costs
- Adjustable air volume
- High efficiency
- Works well for bigger buildings and offices

Disadvantages:
- Expensive investment costs

DCV - Demand Controlled Variables. More a mode to control a VAV-system with different kinds of sensors. Often controlled by CO2.
Advantages:
- Adjusts air flows, temperature depending on need and presence in the zones.
- High energy efficiency
- Can control by occupancy
- Save a lot of energy

Disadvantages:
- Expensive to install
- No occupant controllability