Evaporative Cooling Towers (part 2)

Water cooling takes place according to three phenomena:

1. CONDUCTION: (disposal of sensible heat1).

The heat flows from a region with a higher temperature to a region with a lower temperature through one or more means that are in direct physical contact, in compliance with the laws of heat conduction (Fourier Transform). Energy is transmitted by direct contact between the molecules, and the potential that governs this phenomenon is the difference of temperature between the two different regions.

2. EVAPORATION: (disposal of latent heat2).

The change of state from liquid to vapor causes the absorption, from the side of the evaporated mass unit, of a quantity of heat called evaporation latent heat, which causes the cooling of the mass unit that remained in the liquid state.

The potential difference that governs this phenomenon is due to the difference of concentration levels, which for the gas phase it's expressed in terms of partial pressures assuming we're using assimilated gases to ideal gases.

    The loss of energy and, consequently, the obtained cooling, with transfer of latent heat, is important:  to evaporate one kilogram of water there ia a need to dispose of approximately 540 kcal or 2257 kJ (evaporization latent heat), i.e.,  100 liters of water get cooled to about 6 ° C ..

3. IRRADIATION

Eelectromagnetic waves propagation, in absence of physical contact, make heat flow from a body that has a greater temperature to a body with a lower temperature. When the radiation emitted by a body meet another body, their energy remains absorbed near the surface.

Thermal exchange by irradiation becomes always more important as the temperature of a body increases, and in the case of temperatures close to the atmospheric ones, the irradiation can be neglected.

We can again highlight the concept that the evaporative towers are essentially based on the use of latent heat by mixing air stream with water flow, by which a small part evaporates by passing through the air current, taking away latent heat from the remaining water.

The heat removed from the water will be dispersed in the environment as water vapor that is contained in the outgoing air stream, so that it will be more humid and warmer with respect to the incoming air.

The water exiting the tower will be colder but in smaller quantities than the incoming water. This is the reason that the tower is replenished with make-up water in quantities (Wo) equal to that lost by evaporation and temperature qo.

Principles

1. Thermal balance (first principle of thermodynamics)

In the following set of equations, we are neglecting the effect of the barometric pressure; although in some cases, for example, plant installations that are 500 meters above sea level, the barometric pressure is important because it helps to vary pressures relative to air.

Cooling system thermal balance

1. Q + (Wo qo ).= L(i2 - i1)
2. Q = W( q2 - q1).+ Wo( q1 - q0).
3. W( q2 - q1).+ Wo( q1 ).= L(i2 – i1)

From the moment the amount of make-up water (Wo) is not thermally significant with respect to the total quantity of water W.

1Sensible heat is the one that transferred/subtracted to/from a body varies the temperature.

2Latent heat is the one that transferred/subtracted to/from a substance causes the physical state to vary.