The importance of water in the cooling tower industry – Water (part 4)
UP TO WHAT TEMPERATURE CAN WATER BE COOLED
The capacity of air to cool water, according to latent heat. Practical benefits.
The physical states are well known: solid, liquid, gaseous.
When an element goes from one physical state to another, it frees energy but, at the same time, it requires energy.
If a solid element changes from solid to liquid, it requires energy (e.g., a solid metal requires heat/energy to become liquid/molten);
In the reverse proceedings, the molten metal cools down (losing energy-heat) and becomes solid.
Other example: a liquid that is heated to the right temperature, e.g., in the case of water it’s 100 ° C, it evaporates and passes to the gaseous state. This is called “evaporation temperature“. This process required heat, hence, energy.
Let us take a closer look:
we have employed heat-energy to heat up water to its evaporation temperature, which is 100 ° C.
By the definition of the unit of heat measurement, the amount of heat is determined in an exact way. In fact, 1 (one) calorie of energy is required to increase the temperature if 1 ° C in a liter of distilled water.
The increase in temperature, with respect to ambient temperature, is called “sensible heat“. This is because it is perceived by one of our senses: touching.
If you put a finger in a pot of water that is heating up on top of a fire, we soon realize, or rather we feel, the increase in temperature. This is the sensible heat.
Let us now calculate how much sensible heat is required to bring a liter of normal tap water to the evaporation temperature. Normally, tap water comes out at a temperature of 15 ° C.
We mentioned that:
the water must reach a temperature of 100 ° C to evaporate;
1 calorie of energy is required to raise the temperature in a liter of water by 1 ° C.
Hence, the 1 liter of tap water must be brought from 15 ° C to 100 ° C. The operation is simple arithmetic: 100 – 15 = 85 calories. So that the sensible heat of the water is 85 calories, and we have the same water at 100 ° C and, hence, in the evaporative state. If we continue to heat the water temperature will always remain at 100 ° C, but the water will evaporate until it is exhausted.
For the latter operation 539 calories are required, which are called latent heat of vaporization. It’s called latent because it’s not detected, since the water temperature will always be 100 ° C.
This principle is also used in the kitchen to cook foods “in a water bath”, that is, the food is cooked in a container immersed in another container containing water that is brought to the boiling point. With this method the food will not exceed a cooking temperature of 100 ° C, with respect to fried foods or foods that are in direct contact with fire.
So we have seen that to evaporate one liter of tap water at 15 ° C, there is a need of 85 calories of heat sensitive and 539 calories of vaporization latent heat: a total of 624 Cal. This is a fixed datum.
To evaporate a liter of water it takes 624 calories.
Now let’s try to do another experiment:
We have a liter of water and can make it evaporate with another stratagem. We will describe it in another paragraph, and it’s not the pot on the stove. The new system will evaporate a liter of water by removing 539 calories, that is, the quantity of vaporization latent heat.
Remember, removing heat means cooling!
But let’s see what is this thing that is well known since ancient times. First, we must use another element present in nature: air.
The air, with the exception when it is raining, is not saturated with moisture. The air has the possibility to always absorb water up to its saturation.
The air that we find in the environment, hence, has this important feature, which is to absorb water.
Absorb …., hence, make evaporate.
By now the concept should be clear!
If we can “transfer” a liter of water to the air, we have transferred the 539 calories and the heat was removed with the new system (and not with the pot on the stove).
Air absorbs water, hence, latent heat from water. This is the important phenomenon that we exploit to cool water in cooling towers.
Relative humidity is the percentage of water vapor that the air holds under certain conditions, in relation to the amount of water vapor contained from saturated air in the same conditions.
If the relative humidity is 80% and the temperature is 20 ° C, since the saturated air contains 17.7 g / m3, the ambient air taken under consideration will contain 80% of the water, or better 0.8 x 17 7 = 14.6 grams of water per m3.