One of the most common images of New York’s Big Big Hand is the ‘smoke’ coming out of the sewers, an image that we have engraved on our retina and that we have seen in countless series and movies. The truth is that although it looks like ‘smoke’, what really comes out of the vertical chimneys that are placed in the sewers is water vapor.
And it is that in the subsoil there is an immense network of pipes -169 kilometers- that distribute steam throughout the city, which is used to maintain the heating of the houses, to heat the water, for cleaning tasks, for disinfection …
Although they may seem the same to us, ‘smoke’ and ‘water vapor’ are different. Smoke is made up of small solid particles -with a size ranging between 0.005 and 0.01 microns- found in the air-, and which are formed after incomplete combustion. For its part, water vapor contains particles in a gaseous state that are formed as a result of heating a substance.
Going back to New York ‘smoke’, water vapor is generated when water from rain or snow has seeped into sewers and comes into contact with pipes at very high temperatures.
fogged up glasses
When hot and humid air come into contact with a cold and smooth surface, condensation occurs, which is nothing more than the change of state of matter that is in gaseous form and passes into liquid form.
Water vapor is invisible unless temperature or pressure changes cause it to condense, as happens when it comes into contact with the cold outside.
At that moment it stops being an invisible gas and becomes small particles of liquid water in suspension.
This phenomenon appears in everyday situations, such as opening the dishwasher or oven, uncovering a pot… This explains why glasses fog up in all these situations.
It’s the same chemical reaction that explains why ‘smoke’ comes out of our mouths on winter mornings. The vapor that we give off is nothing more than the vapor from our breath that condenses when it comes into contact with the cold air outside. To understand it, we must bear in mind that our body is at a temperature close to 37ºC and that our breath contains a large amount of water vapor.
From steaming coffee to nuclear power plants
The mist is also responsible for the car windows fogging up during the winter months when we have the vehicle stopped. The answer is again in the temperature difference, in this case between our body, the interior of the car and the exterior. The thermal difference causes the vapor to condense on the crystals and they end up fogging up, to the point that we are not able to see correctly.
The phenomenon of condensation can also be observed in a hot and steaming cup of coffee, in this case the air that is in contact with the surface of the hot liquid raises the temperature and an ascending current of air convention is formed that drags steam from water to cooler upper areas, where it condenses.
To finish, let’s go with another example of “smoke” that, fortunately, is not such. It is the white smoke that emanates from the “chimneys” of nuclear power plants. In this case it is neither smoke nor radioactive, it is simply the water vapor that is produced by cooling the water. The ‘chimneys’ are nothing more than cooling towers, whose objective is to cool the water used in the heat exchangers.