Principle of operation of the cooling tower, description

The majority of industrial processes and air conditioning systems generate heat which has to be taken away and disseminated. Usually water is used as the transferring environment for heat elimination from industrial heat-exchange devices, condensers of cooling machines, etc.

In the past it was achieved by continuous water supply from city systems of water supply or from natural sources of water that heated up as the process of heat exchange proceeded and was dumped to the sewerage or back to a superficial source of water.

Now water from the utilities becomes excessively expensive because of constantly growing water consumption and high cost of sewage treatment. Similarly the cooling water from natural sources is rather inaccessible due to violation of the environmental situation of water sources caused by dump of water with the increased temperature.

For the purpose of heat dispersion directly in the atmosphere, air cooling devices can be used but the purchase price and losses of power to drive the fans of these devices are very high.

Beside the listed shortcomings, low efficiency of cooling is relevant - AVO can provide temperature of chilled water that is 11 ∞C higher than the air temperature on the "dry" thermometer. Such temperatures of the cooling water are too high for the vast majority of industrial processes.

Cooling towers enable to overcome the majority of these problems and are widely applied to dispersion of heat from refrigeration units, air conditioning systems and most of the industrial processes. Expenses of reverse systems with coolers constitute only 5% of total circulating water that makes them the cheapest solution  for systems with purchased water supply. Besides,  values of purge figure  for systems with cooling towers are very low so that the impact on environment is considerably reduced. And the key is that the coolers are capable to cool water to value which only by 2-3 ∞C exceeds the air temperature on the "damp" thermometer. Thus, water temperature after coolers can be 20 ∞C lower than at the output from air coolers at comparable overall dimensions.

Such effective cooling is reached due to a combination of the effect of heat transfer and mass transfer. Heated water is pumped in water distribution system and sprayed on the irrigating environment in which the big area (to 150 sq.m of a surface into 1 m3 of a sprinkler) for contact with atmospheric air is put. Air circulation in coolers can be created by fans, convective streams, natural streams or by means of the ejection phenomenon from nozzles. While contacting with air, a part of water will change an aggregate state from liquid to vaporous, which is followed by absorption of heat. Thus, warmth of steam formation is transferred from water in a liquid state to an air stream.

 Fig. 1

In fig. 1 the relation between water and air in process of passing of the counterflow cooler is displayed. Curves show temperature drop of chilled water (from point A to point B) and temperature increase of air on the "damp" thermometer (from point C to point of D) at their contact in the cooler.

The difference between temperature of incoming and outcoming water determines the cooling zone width (temperature difference). For one-planimetric systems, with the set mode and constant hydraulic loading, temperature difference on the cooler corresponds to temperature increase of water in processing equipment.
Accordingly, temperature difference is determined by the water discharge and heat load of the technological and is not related in any way with the size or cooling ability of a cooling tower.   

Difference between temperature of chilled water and the incoming air temperature on the damp thermometer (point B minus point C) in fig. 1 is called the approach to the damp thermometer or cooling depth. Depth of cooling is a function of cooling ability of the cooler. At identical thermal loading, consumption of water and climatic parameters, the cooler with a bigger area of irrigation will provide better (lower) depth of cooling, that is lower water temperature at the output from the cooler.

Thus, the amount of heat disseminated by the cooler in the atmosphere is always equal to the amount of heat generated by processing equipment, and the temperature condition at which heat dispersion occurs is related to the cooling ability of the cooler and air temperature on the "damp" thermometer.

Air temperature on the "damp" thermometer is the most important climatic parameter which influences operation of the cooler. This temperature can be measured by means of a bulb envelopment of the ordinary thermometer by a damp material and the subsequent blowing by its stream of air. Air temperature on the dry thermometer (it is measured by the ordinary thermometer) and relative humidity (it is measured by a hygrometer), given that they are considered separately, have insignificant impact on thermal efficiency of the cooler with compulsory draft. However these parameters have impact on evaporation size in the cooler.

 Fig. 2

In fig. 2 the psychometric analysis of air which passes through the cooler is displayed. Air enters into the cooler under the atmospheric conditions determined by a point A, absorbs heat and weight (humidity) from water, and leaves the cooler under point conditions B in a saturated state - at humidity of 100% (in the conditions of low thermal loading air can be nonsaturated). Amount of heat transferred from water to air is proportionally to a difference of enthalpies of air under conditions on an entrance and an exit from the cooler (hB-hA). As the direction of lines of constant enthalpies coincides with the direction of lines of constant air temperatures on the "damp" thermometer, the difference an enthalpy can be determined by a difference of air temperature of the "damp" thermometer.

Process of heating of air presented by a vector of AB can be divided into two components - a vector AC which shows amount of the dry heat absorbed by air due to a difference of water temperatures and air, and a vector of BC which determines the size of the hidden warmth of phase transition that is formed at evaporation.

If to transfer parameters of air on an entrance to the cooler to a point D (air temperature on the "dry" thermometer at invariable air temperature on the "damp" thermometer increases), the general thermolysis determined by BD vector remains without changes, but strikingly change components of  the dry and hidden warmth. The vector of BD represents process of dry air cooling (that is air doesn't take away dry heat from water, and on the contrary transfers dry heat to water) whereas the vector displays transfer of the hidden warmth from water in the passing air which is significantly higher in comparison with the previous atmospheric conditions. Thus, dry heating of water by air is compensated by the increased share of evaporation of water in the cooler.

The mass transfer (evaporation) is carried out only the componet of  hidden warmth in the course of a heat transfer and is proportional to change of relative humidity. In fig. 2 evaporation discharge in case of AB (wB-wA) is much less in comparison with a case of DB (WB-WD). The ratio of the dry and hidden warmth matters in the analysis of water consumption of the cooler.

As value of the "dry" thermometer or relative humidity on an entrance to the cooler influence a ratio of components of process of a heat transfer, they also influence the evaporation discharge of water in process of its cooling. Evaporation size for usual calculated parameters of air, in the conditions of a temperate climatic zone, makes 1% of the general water flow for each 7 ∞C of temperature difference.

However, the average evaporation discharge for a year is lower, than settlement because of increase in a dry component of process of a heat transfer at decrease in the entering air temperature on the "dry" thermometer.

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