Why is there a ball on the roof? So many people don't know

Audit expert: Luo Huiqian

Associate Researcher, Institute of Physics, Chinese Academy of Sciences

Are there any friends who, like me, are always attracted by the metal balls spinning on the rooftops when looking at the scenery? Are you also curious about these big metal balls that frequently appear on rooftops or factory roofs? What are these round guys used for, and why do they always appear on the rooftops?

Today, let me take you to learn about the past and present of these little things.

Non-powered hood

To be honest, I have always called these things "silver rolling balls" because they always "roll" on the roof. I just learned their real name from the workers a few days ago. It turns out that these guys are called "unpowered hoods", also known as spherical ventilators. The reason why they are called "unpowered" is not because they don't need any power, but because they don't need additional electricity, and natural wind is enough to drive them.

Non-powered hood installed on the roof of the house Source | Sohu website

So, what are they used for? Simply put, a non-powered hood is a device that accelerates the air flow inside and outside the house by rotating itself.

Structure of non-powered hood

Next, let us understand what kind of internal structure is hidden under their round appearance.

First of all, the non-powered hood is mainly composed of four parts: the blade, the neck, the internal bracket, and the bearing. The circle of arc-shaped blades we see is the result of careful scientific calculations based on the principles of aerodynamics. It not only prevents the hood from being invaded by rainwater when working, but also greatly improves the exhaust efficiency. It can be said that the blade is the most important part of the entire hood.

Source of the internal structure of the hood | Zhihu @Unpowered hood

There is a large space inside the round exterior of the hood, which contains two brackets. The upper bracket is an aluminum bracket, and the lower bracket is a nylon bracket. They mainly play a supporting role, which greatly improves the overall mechanical strength and flexibility of the hood.

The stainless steel material used to make the hood means we no longer have to worry about the hood being corroded. At the same time, the stainless steel material also greatly improves the mechanical strength of the hood, ensuring that the hood can work continuously and stably.

In short, the hood, which has been precisely designed by experts, is not only waterproof, fireproof, explosion-proof, and wind-proof, but also can effectively prevent foreign objects from falling into the device, which greatly improves the service life of the hood.

How does the non-powered hood work?

After understanding the internal structure, another question arises before us: How does this seemingly simple hood work?

I presume that everyone has seen chimneys, and perhaps range hoods in the kitchen. In fact, the principles of non-powered hoods, range hoods and chimneys are similar. They essentially achieve the exchange of internal and external gases through the pressure difference between indoor and outdoor gases.

Source of the range hood in the kitchen | Zhihu @ What is worth buying for straight men

If there is natural wind outside, the hood can rotate under the force of the wind. The centrifugal force during the rotation can reduce the air pressure around it, forming a negative pressure area, that is, the air pressure inside the hood is lower than the indoor air pressure. Due to the pressure difference, part of the air flowing in the parallel direction of the room will be transformed into air flowing vertically from bottom to top, thereby pumping the stale air in the house to the outside.

As for the rate of exhaust, it mainly depends on the size of the natural wind. In this case, the working principle of the hood is similar to that of a range hood, the difference being that the range hood consumes electrical energy to drive the internal fan to rotate, thereby forming a negative pressure area.

Source of the towering chimney | Baidu Encyclopedia

If there is no horizontal wind outdoors, the thermal updraft formed by the higher indoor air temperature than the outdoor air temperature (similar to the principle of a chimney) can also drive the hood to rotate. At this time, the centrifugal force generated by the rotation will form a negative pressure area, further accelerating the flow rate of the updraft, thereby accelerating the indoor and outdoor air circulation.

Small hood, many benefits

From an environmental perspective, the hood does not produce any pollution during operation and makes very little noise, making it an environmentally friendly and practical product. It can also improve the lighting environment of the user's home. The hood installed on the roof can let in light through the gaps between the fan blades. A bright home environment can also improve our mood for the day.

From an economic point of view, it does not require electricity or energy and can automatically remove excess heat from the room, improving the temperature and sanitary conditions in the room. It requires no maintenance after one installation, thus reducing installation costs and electricity bills.

From a safety perspective, the hood can also protect the user's personal safety in special circumstances. If a fire occurs in the corridor or public area, this ventilation device can speed up the discharge of internal smoke and dust, thereby providing more escape space for those in distress and reducing the losses caused by the accident.

Environmentally friendly and economical non-powered hood sources | Zhihu @宅宅装修驿站

In short, as a ventilation facility, the non-powered hood is one of the best choices for smoke exhaust, rain protection, and convection air in building flues. In particular, the inherent high cost performance of the hood makes it stand out from other products. At present, more and more non-powered hoods have been applied to the construction of residential houses, and perhaps it will become the choice of more houses and buildings in the future.