In a water-cooled central air-conditioning system, the chiller dissipates heat from the condenser to the outside, and the condensation heat is carried by the cooling water to the cooling tower. After the heat is dissipated by the cooling tower, it is reduced from 37°C to 32°C, and then returns to the condenser of the chiller, so that the heat is circulated by the cooling water system.

The heat dissipation of cooling water in the cooling tower depends on three temperatures: the water temperature of the cooling water entering the tower, the dry bulb temperature and the wet bulb temperature of the outdoor atmosphere, which is divided into contact heat transfer and evaporative heat dissipation.

1. Contact heat dissipation

Contact heat dissipation is based on the difference between the cooling water temperature and the outdoor air temperature (dry bulb temperature).

When the cooling water temperature is higher than the outdoor air temperature, the direct contact between water and air will cause sensible heat transfer based on the temperature difference, and convective heat transfer will also occur between the water surface and the air flow, thereby reducing the temperature of the cooling water.

Therefore, the cooling tower is designed with a spray system and a packing layer, and the water flow is dispersed into fine water droplets or thin films, thereby greatly increasing the surface area for heat exchange between water and air, so that the heat in the water can be transferred to the air more quickly.

Subsequently, these heat-carrying air are continuously extracted and discharged into the atmosphere through natural wind or forced fan, so as to achieve continuous and effective removal of cooling water heat.
2. Evaporative heat dissipation

Evaporative heat dissipation transfers heat based on the difference between the cooling water temperature and the wet bulb temperature of the outdoor air.

When the dry bulb temperature of the outdoor air is higher than the wet bulb temperature, it means that the outdoor air is “unsaturated”. If the cooling water temperature is also higher than the wet bulb temperature at this time, part of the cooling water will evaporate, carrying a large amount of evaporative heat into the air, thereby reducing the temperature of the cooling water.

As evaporation proceeds, the water vapor content in the air increases, tends to be saturated and is taken away by the fan, thereby continuously removing heat and cooling the cooling water.

As the cooling water temperature drops and gradually approaches the wet bulb temperature of the outdoor air, the evaporative heat dissipation will gradually decrease. Until the cooling water temperature drops to the same level as the wet bulb temperature of the outdoor air, the evaporative heat dissipation is zero and fails.

3. Heat dissipation analysis

Under normal circumstances, the ambient air is “unsaturated” and its dry bulb temperature is higher than the wet bulb temperature. Based on the relationship between them and the water temperature, the heat dissipation of the cooling tower can be divided into three situations:

When the cooling water temperature is higher than the outdoor air temperature, it must also be higher than its wet bulb temperature. The cooling tower is in the best heat dissipation state, and has contact heat dissipation and evaporative heat dissipation to the surrounding air.

When the cooling water temperature is equal to the outdoor air temperature but higher than the wet bulb temperature, there is no sensible heat temperature difference between the cooling water and the air, and the contact heat dissipation is zero and invalid, and there is only evaporative heat dissipation based on the latent heat temperature difference.

When the cooling water temperature is lower than the outdoor air temperature but higher than the wet bulb temperature, the contact heat transfer is reverse, from the air to the cooling water, while the evaporative heat dissipation is still positive, from the cooling water to the air.

At this time, if the outward evaporative heat dissipation is greater than the inward contact heat transfer, the cooling tower still shows outward heat dissipation as a whole. Therefore, when the cooling water temperature is lower than the outdoor air temperature, the cooling water may still dissipate heat outward and be cooled.

Usually, the evaporative latent heat is greater than the temperature difference sensible heat, until the cooling water temperature drops to the wet bulb temperature of the outdoor air, the evaporative heat dissipation is also invalid, and the cooling tower completely loses the ability to dissipate heat to the surrounding air.

In summary, the heat dissipation effect of the cooling tower depends on the difference between the cooling water temperature, the dry bulb temperature of the outdoor atmosphere and the wet bulb temperature. In particular, the wet bulb temperature represents the lowest temperature at which water can be cooled under the current temperature conditions, and is also the limit value of the cooling tower outlet water temperature.