Natural ventilation in pig buildings

A cheap, popular way of providing cooling and fresh air for everyday operations. Use this information to understand how natural ventilation works, the problems and the solutions.
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There are two mechanisms by which natural ventilation works – wind effects and stack effects.

Wind effects

Wind applies different pressure to various parts of a building, depending on its strength and direction. If inlets and outlets are placed in these different pressure regions, this will drive ventilation.

Stack effects 

This uses the natural buoyancy of warm air to drive air movement. If outlets are positioned high in a building, warm, lighter air will exit at this point and draw outside air in to displace it a low level. This is exactly how a chimney works.

On balance, wind effects can be a much more powerful driver to ventilation than the stack effect, but it is completely dependent on wind pressure so it is very variable.

The trick to achieving good natural ventilation is to have a system that can deliver minimum ventilation rates through the stack effect, but can deliver higher ventilation rates from wind effects.

Fan versus natural ventilation

Characteristics Fan ventilation  Natural ventilation
Running costs Highest Lowest
Accuracy of temperature control Highest  Lowest
Ability to provide high ventilation rates Depend on fan's capacity; independent of external conditions Dependent on size, configuration of inlets and outlets and ambient wind speed, in many cases can actually be higher than with fans
Ability to provide high air speeds in hot conditions Good if designed well Limited by wind
Installation costs High  Low to medium
Fall safe Not intrinsic Intrinsic
Accuracy of minimum ventilation rates (affects running costs of interlocked heating) High Low
Light infiltration control Simple  Can be more difficult

Problems and solutions

The major problem with natural ventilation is that it is largely driven by wind and external influences and is, therefore, unpredictable.

Most wind effect designs will operate effectively at a wind speed over 1m/s and, in the UK, this is available for about 95 per cent of the time. The stack effect can be used to deliver peak ventilation, but ridge openings tend to be enormous, which can sometimes be difficult to engineer. One way around this is to have an opening ridge for extreme conditions and automatic controls for eave-wind-driven openings.

Best applications for natural ventilation are where animals are not oversensitive to changes in temperature. Some simple control of air throughout can be achieved by using manually adjustable air dampers.

Automatically controlled natural ventilation (ACNV)

For thermostatic control, it’s necessary to automate these dampers and introduce automatically controlled natural ventilation (ACNV).

ACNV proposes a system that can cut out the high running costs of fan systems, but deliver good temperature control.

Generally, it does this well, but it has some shortcomings that need to be heeded if applications are to work in most conditions.

Pros and cons of ACNV

Pros Cons
Cheap to run – For a finishing system, it can probably save an average of £0.70 per pig throughput in reduced running costs. Variable performance – Consistency of air distribution from the inlets is low because it depends on wind direction and strength. Therefore, buildings may perform very differently at different times of year – or even different times of the day.
Cheap to install – especially in new structures, because it swaps fans for actuators and flaps. Where structural alterations are required, the cost goes up. Buildings can be ‘leaky’ – even when ‘closed down’, especially at high wind speeds, because they need very large inlet and outlet areas. This can be a problem when low minimum ventilation rates are required or heating is used. Fan systems behave better in these conditions.
It can give higher ventilation rates than a fan ventilation system in some conditions. ACNV can run into trouble in very hot conditions, especially if wind speeds are low. There is no mechanism to artificially raise air speeds to promote cooling. Installers need to consider what contingencies are required in worst-case conditions
ACNV is virtually fail-safe because the system will stop in its current operational position if there is a power failure. Position issues – Close positioning of buildings can hamper air throughput and performance when ambient temperatures are high.

Useful links

Controlled environment for livestock guide Pig buildings Back to Ventilating pig buildings
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