Performance of Industrial Natural Roof Ventilators

A COMPARISON ON THE PERFORMANCE OF STATIC & ROTATING TYPE VENTIALTION EQUIPMENT

Introduction

Natural ventilation itself is not a new concept. The beginnings of Natural ventilation design can perhaps be considered as a time when the enclosures started to become purpose ventilated. Early designs were primarily empirical and evolved from experience. Modern buildings are very demanding in which all standards of health and comfort have to be met, provided that they satisfy low energy consumption and sustainability. In big industries, factories & storage facilities, the best practical and economical solution is using natural ventilation systems. Traditionally Static integrated-building ventilation systems were used which are bulky, ineffective and vulnerable to dust accumulation, birds and vermin infestation.

The best and the most cost effective way for ventilating a building is using roof mounted ventilators which comes in different forms and size. But, there is a misconception in the market about these roof mounted ventilators which are basically rotating type equipment. The working principle behind these equipment is not known to all and this creates a misbelief that the rotation causes suction of hot air, which is absolutely false.

How Natural Ventilation works?

In order to understand how a Natural ventilation system or an equipment works, we first need to understand the characteristics of air. We all know that hot air rises and cooler air moves into its place due to the low pressure created by the rising hot air, since air moves from high pressure zones to low pressure zones. There are two principles related to air which affects Natural ventilation, Buoyancy effect and wind siphoning. What are these?

  • Buoyancy is an upward force exerted by a fluid on some other object immersed in the fluid, it occurs due to differences in densities.
  • Wind siphoning creates a low-pressure wake region in the downwind side of any object placed in wind.

During hot months, solar heat load on a building creates heat buildup inside the enclosed space. As a result, the air density reduces and it rises and accumulates near the roof attic, this phenomenon is known as Buoyancy effect. Heat, fumes and other VOCs from machining and other processes also gets entrapped inside the building. Under such circumstances, roof mounted natural ventilation equipment are used which works on the principle of wind siphoning pressure to exhaust the hot air, fumes and other VOCs.

An object when placed in a fluid stream, creates a low-pressure wake region in the downwind side. The natural ventilation equipment work on similar principle, the exhaust gases are removed through these low-pressure zones. More the wake, more will be the pressure difference and better will be the flow. The geometry of the ventilation equipment decides the amount of wake region that it will create.

Ventilation Equipment and Performance

Natural ventilation equipment has got only one standard in the world, that is Australian/New Zealand Standard AS/NZ 4740:2000. As per this standard, ventilation equipment are classified into four types. Among these the most common types are the rotating and open stub or static type ventilation equipment. Both these types of equipment work as per the above-mentioned principles.

The performances of such roof mounted ventilation equipment are dependent on many nominal parameters like air density, height of the building, gravity, temperature gradient, wind speed and also on ventilation equipment’s geometry, aerodynamic area, coefficient of flow, coefficient of discharge etc.

Testing of a ventilation equipment

The natural ventilation equipment is very difficult to test since their performance depends of unsteady parameter which changes with time, like temperature difference, wind velocity etc. As per the standard, tests on such ventilators are carried out either by stimulating wind by axial fans or inside a wind tunnel. Both these methods do not give accurate results because, the axial fans do not actually resemble an actual wind and gives erroneous results and second, testing such big ventilators inside a wind tunnel is practically not possible.

The best way to test this equipment is in situ. These tests give the most accurate and actual results when compared to any other methods. There are two tests from which we can determine the performance of these ventilators, they are static pressure test to determine the discharge coefficient and flow coefficient test.

Flow through natural vents

The air flow through these vents are basically of two types, first due to the temperature difference and this is enhanced by wind induced flow. Once the ventilation system starts working, the temperature difference reduces due to air circulation and Wind induced effect takes over and forms the major part of the flow.

The tests as mentioned above help us to compare the performances of different types of ventilators to determine the parameters that affect wind induced flow. Similar tests were carried out on commercially available Round-rotating units and Hurricane®Plus Static-type ventilation equipment.

The following graphs, show a direct comparison between Static type ventilation equipment and a rotating type unit. The flow coefficient values plotted against the anemometer probe distance inside the throat of the ventilation equipment gives a direct comparison of flow through different spots inside the ventilator. As we can figure out from the graphs, the flow coefficient of static vent is much larger than a rotating ventilation equipment.

The reason behind this being the high static resistance inside the rotating equipment. Whereas, an open stub type or a Static ventilation equipment offers a very less static resistance due to it geometry. Thereby making it a better and a high performing one under any working conditions.

What does a flow coefficient mean, or what effect it has on ventilation?

Flow coefficient is a dimensionless number which is the ratio of velocity of exhaust air inside the vent’s throat and the wind velocity at that particular moment. It drives the wind siphoning flow rate of any ventilation equipment.

It has got direct impact on the performance of the ventilation equipment during both high wind and low wind conditions. More the flow coefficient better will be the performance.

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