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General Industrial Ventilation Part 4: Natural Supply/Natural Exhaust

Posted by Gary Leseman, P.E. on Dec 20, 2016 7:54:16 AM

In this last installment in our General Industrial Ventilation series, we discuss an industrial building with a Natural Supply/Natural Exhaust ventilation system.

Natural supply/natural exhaust is the simplest but also the most misunderstood method used to ventilate an industrial building.

natural ventilation.pngEveryone knows that warm air rises and this is the driving principle behind this method: create an opening in the side of the building, and then create an opening in the roof of the building, and then the fresh outside air just naturally flows into the building and the warmer inside air just naturally flows out the top of the building. 

 The openings around the sides of the building can be weather louvers, large overhead doors, or removable building siding. Natural exhaust ventilators are then installed in openings in the roof of the building. These take the form of hooded roof vents, high profile mono-vents that look like a partially opened clamshell, or low profile thermo-vents. The natural exhaust ventilator is designed with a series of internal and external air path change baffles that allows heat to escape the building and keeps rainwater out.

This is method is referred to a natural ventilation or gravity ventilation. It is as old as the hills and works with the natural forces of nature. It is considered to be a “green” method of ventilation because no electrical power is required for it to operate.

If it seems too good to be true, it is for a lot of facilities.

The primary drawback to natural ventilation is that there are a number of external variables required to be present before it works effectively.

External variables required:

  • Tall building height
  • Significant inside to outside temperature differential
  • Very large perimeter openings and geometry of the natural roof exhaust ventilators
  • Constant prevailing outside wind 

The tall building height acts like a chimney. It allows the warm air to rise up high above the floor work area and stay trapped under the roof until it can escape through the roof mounted natural ventilator.

A significant inside to outside temperature differential creates the upward draft of air within the building. For example, a furnace or a forge provides a high internal inside temperature differential in comparison to outside ambient temperatures.

Large amounts of perimeter opening free area to roof free open area of 1.5 to 1.0 is essential to getting adequate outside air into the building so that it can pick up the heat, create a rising force, and ultimately are expelled out the natural roof exhaust ventilator.

The geometry of the perimeter openings and the natural roof exhaust ventilators vary from product to product. Each one has a specific flow coefficient which must be properly applied when designing a system.

A constant prevailing outside wind forces the outside ambient air into the building. It also carries away from the building the naturally exhausted warm air.

If one or more of these external variables is lacking, the natural ventilation system will not work properly. Like a 3-legged stool: if you cut off one leg, the stool will not stand up.

There are excellent computer programs available that use thermodynamics, fluid flow, and actual equipment geometry that allow for the input of variables in order to model the anticipated optimum ventilation rate. However, when one or more of these variables is changed in value, the entire model is changed, and the overall ventilation rate of the building may be reduced.

Advantages:

  • The added weight load on the roof of the building is typically not a factor
  • A high overall building air change rate can be achieved as long as the external variables exist
  • Depending on the air change rate, the temperature at the floor level closest to the intake openings of the building will be as close to the outside ambient temperature as possible.
  • The natural roof exhaust ventilators are designed not to leak rainwater into the building.
  • The overall annual operating cost is practically zero.

Disadvantages:

  • Air change rate cannot be guaranteed under all weather conditions
  • Wintertime down drafting of cold air through the natural roof exhaust may occur
  • Heavy build-up or ice or snow can create some deflection problems and ultimately leaks when the melt comes if the building is not a 24/7 operating process with constant hot air exhaust through the roof unit
  • Build-up of heavy particulate material on the interior of the natural exhaust vent may become a problem adding weight to the roof, “raining” particulate back into the building, or causing unit deflection

With all those perimeter building openings, how do you ventilate in the winter time?

For winter ventilation, Perimeter supply louvers and doors are simply closed. The roof-mounted natural ventilators can be provided with dampers that can be closed (or partially closed) in the winter to maintain heat within the building. The overall air change rate is reduced in the winter to just a few times per hour, which is usually ok.

Can natural supply air be filtered to maintain interior building cleanliness?

No, this is not practical because the perimeter natural supply open area is just too large to cover with filters. A natural supply/natural exhaust ventilation system is typically used on buildings with hot and dirty inside processes such as forge shops, coal fired power plants, and steel mills. Filtering air into these types of process buildings is not required.

Summary

Natural supply/natural exhaust ventilation system is considered to be the “green” way to ventilate an industrial building.

It is an excellent way to ventilate an industrial building when all the external variables are met and when a guaranteed air change rate is not essential.

When properly designed, it prevents the buildup of heat by venting internally generated process hot air the “tried and true/old-fashioned way” using Mother Nature working to maximum advantage.

The annual operating cost is practically zero.

This method is not the solution to every industrial ventilation problem but it does have an important place when called for in the overall arsenal of acceptable industrial ventilation practice.

If you haven’t already, take a look at the other articles in this 4 part discussion of general industrial ventilation methods. I have tried to be conversational without becoming overly technical. This has been a fun exercise and I hope that it has been helpful and informative for you.

Eldridge has been in the business of solving industrial ventilation problems for over 70 years. We welcome an opportunity to be of service to you in this area. Call us for a no obligation consultation and proposal.

Topics: General Ventilation, Industrial Solutions

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