Natural ventilation in power generation facilities: roof ventilators or wall louvres for high level extract?

Posted by Graeme Clark on 23/09/14 11:30

Labyrinth natural roof ventilatorsI recently wrote about my view that natural ventilation should always be the first choice for power generation facilities. This leads me to another common discussion point: the choice between wall louvres and roof ventilators for high level extract.

Common concerns for design teams and contractors over roof ventilators include maintenance, planning constraints and rain ingress. However, a high quality roof ventilator will be maintenance free, low profile and 100% watertight.

Why are roof ventilators better than wall louvres for high level extract?

  1. Roof ventilators are less likely to be adversely affected by wind pressures.
  2. Wall louvres are normally installed at least 1m below roof level, which creates an area of warm air pooling above this level.
  3. There is a greater stack effect with roof vents, which reduces the amount of ventilation you require.
  4. The Cv (aerodynamic coefficient) of a roof ventilator tends to be better than a wall louvre.

Performance: let’s look at the facts

Let’s compare performance by using the same project example as my previous blog : we have a typical biomass boiler hall with a 500kW heat load and with building dimensions of 10m wide x 20m long x 20m high:

  • With a roof ventilator: 8.3m2 AvCv of ventilation would be required at high and low level.
  • With a wall louvre: 9.1m2 AvCv of ventilation would be required at high and low level. This is because of the lower stack height.

Furthermore, the Cv of a roof ventilator tends to be better than a wall louvre - 0.4 would be reasonable for a roof ventilator and 0.3 for a wall louvre. Therefore the ventilation requirements would be:

  • Roof ventilator - Low level louvre=27.6m2 (face area), roof ventilator = 20.75m2 (throat area).
  • Wall louvre - Low level louvre=30.33m2 (face area), high level wall louvre = 30.33m2 (face area).

Conclusion: roof ventilators work better and save more 

In this project example, using a roof ventilator would require 9% less equipment at low level and 31.5% less equipment at high level than using a wall louvre. Therefore we are talking about fewer openings in the walls, less steelwork required, lower cost, no warm air pooling at high level, and a reduced effect from wind pressures.

If you want to know more, you can find more information in our other articles on ventilation relating to power generation facilities.

Graeme Clark Graeme Clark is a Senior Consultant for Colt UK and specialises in the design and product application of energy efficient HVAC and smoke control systems.

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Topics: Natural ventilation, Louvre, Power Plants