Why cool the water (part 3)
The ability of the air to cool water has many practical benefits.
As mentioned in the previous post, one way to transfer heat from one element to another is to put them in contact with each other (conduction). This is one of the principles used in cooling water; by putting water in contact with water as much as possible.
Water is dispersed as much as possible in order to break water drops. This action is repeated continuously so as to avoid that the reforming of the drops by the power of attraction of the water. Or, water can be conveyed in rivulets, layers that continuously break down and so on …
In other words, the largest possible water surface is offered to the air, the greater the surface in contact with the air, the greater the flow of heat by contact or conduction.
It’s not just the principle can be exploited to cool the water with air. During the air-water contact another phenomenon occurs. The air has the ability to absorb water up to its saturation. The capacity varies depending on the difference of air temperatures. If the air is cold, even if very dry, it can absorb very little water (a few grams per cubic meter).
If the air is warm, the capacity to absorb water increases dramatically. Therefore: in summer the air has a the capacity to absorb humidity and significant water. If water is transferred from water in order to be cooled by air, the heat contained in that water is also being transferred. And this is what we’re looking for. This way the amount of heat that is transferred from the water to be cooled by air is very significant. In winter we cool the water by conduction, and in summer we cool water by means of a phenomenon called “mass transfer “.
When we talk about conduction we mean only conduction, in the summer, in some air conditions, we will have reverse flow of heat; air is hotter than water and, hence, water heats up by minimum part, but greater heat transfer happens by mass of water.
Another consideration is when we cool water in the winter. Because the output water is warmer than the air inlet, the air itself heats up and thus increases the capacity to absorb water. In spring and autumn season, it is common to cool water using the two phenomena to the benefit of the water temperature in outlet.
Where are the plant used for cooling water?
The cooling of water is the last action of a heat disposal process. Water is used as an element to transport energy (heat), and if at one point heat is absorbed (all machines discard energy), in another point this energy must be disposed always as heat.
Differentiation of plants for cooling water at low temperature required for the process of specific production processes: plastic mold cooling, air conditioning.
Heat disposal is obtained by cooling the element that is used for heat transport (energy): in our case water. The temperature of the cooled water must be chosen in the design phase because it is crucial for the overall performance of the system.
The analysis of the plant allows to know if it’s sufficient to dispose only heat with temperatures from water cooled relatively high at 40 to 50 ° C, as in the case of oil-water heat exchanger where oil can be cooled up to 50 ° C, or the temperature of the water has to be the lowest and obtainable in an economical way, such as refrigerators.