Why cool the water (part 2)
Up to what temperature can air cool water
The refrigerator is the first thing that comes to mind when it comes to cooling. Refrigerating machines require a fair amount of energy to activate the process and transfer the heat from the element you want to cool down to another element. Therefore, we’re thinking of cooling via evaporative air.
During the evaporative cooling, the heat is transferred to air according to a very advantageous physical principle that exploits a principle of air: that it, the ability to absorb moisture and, hence, water and, hence, heat. The more drier the air, the more it absorbs moisture – water – heat. If the air is hot and dry (conditions where the ability to absorb water is very high), it can cool the water even at temperatures lower than that of the air.
Concept of temperature and heat
A hot body is characterized by the amount of energy stored as heat.
Measurement of temperature and heat
The unit of temperature measurement is Celsius degree (° C) or Kelvin degree (° K), where ° K = ° C +273. The unit of energy (heat) measurement is the J (Joule). Since normally heat is related to time, J / s = power = W. Usually the potential of cooling towers is measured in kW thermal power (1,000 W = 1 kW = 860 kcal / h).
Importance of the choice of the cold water temperature according to the process plants that need to be cooled
Our knowledge of the heat it’s usually identified with the temperature. Normally a solid body, a substance, with low temperature has no heat. But, instead there is heat. We realize this if, from that solid body, we want to remove or add more heat.
To begin with, at minimum we must put in contact the body from which we want to add or remove heat in contact with another body with a different temperature; greater if we want to add heat, less if we want to remove heat.
Later, we will also discuss this flow which is “one-way”. Going back to the body that we want to warm up or cool down, we realize that the higher is the difference of temperature between the two bodies, the greater is the sensation of warm-cold.
So, the temperature is a measure of the heat. Or better yet, the difference of temperature multiplied by the weight of the body is the amount of heat of the body itself.
There are different ways to cool the body, one way is explained above, that is, by putting in contact two bodies with different temperatures. But, there are other ways such as hitting the body with a stream of air, or expose the body directly to the sunlight. Well, these phenomena are called:
1) Conduction, if we put the two bodies in contact with each other.
2) Convention, if we hit the body with a stream of air or gas.
3) Irradiation, if we expose the body to the sun.
Each of these phenomena follows different rules that can me mixed. Let us now go back to the flow of heat in one direction. The heat goes always from hot to cold.
There are other possibilities. So if we want to heat a cold body, we can put it in contact with another body, let it be hit by a current of air, expose it to a source, but always warmer to the body we want heat. If you want to cool a body? This seems difficult; we said that the flow of heat is in one direction.
If you want to cool a body, just put in contact with a colder body. Hence, when we say “cool down” a body, it’s not accurate. In reality we remove heat from the body, so it cools down. The problem if ever, is how to get the cooler body. We’re always talking about heat transfer, which we measure with the temperature.
How cool the water in an economical way
As we mentioned above, the problem for cooling a body, a liquid or a substance, is to get a body, a colder liquid. At this point we need to establish how much colder and its cost.
Reference to the operation of refrigerators as “transfer of energy (heat)”
We have learned that cold exists because heat is removed, so it is not produced. The name “refrigerator” from the Latin “to produce cold” is incorrect. But because it’s commonly used, we accept it. Hence, in the refrigerator there is heat transport from bodies that want to be cooled to other bodies or substances.
In practice, in a refrigerator that cools water, the heat is transported toward the outside where it is disposed in an air stream; air-cooled refrigerators or in another stream of water, water cooled refrigerators. If the heat removed from the refrigerator is transferred to the air, air heat ex-changers or radiators are adopted.
The heat can be transferred to the water and, in this case, the process is more efficient for most of the water capacity than the air of heat transport.
At this point we will have hot air that can be dispersed into the atmosphere, there is no environmental damage, we speak of an increase of a few tenth of degrees centigrade that are mixed immediately with the surrounding air.
Or we will have hot water that we discharge in sewers, rivers or other, but we should obtain other water, or recycle it. To recycle the hot water, it must be first cooled down. At this point we shall see how to cool down the water with the least possible cost.