HOW ADIABATIC WORKS
Adiabatic cooling uses one of the physical properties of air: the characteristic of absorbing and holding a certain amount of water vapour, depending on the temperature.
The ratio between the actual amount of water held at a given time and the maximum amount that the air can actually hold, defines the relative humidity (RH) percentage.
Adiabatic cooling allows the air to hold the remaining amount of water vapour to become saturated or close to saturation.
The water vapour is absorbed by the air by means of the heat contained in the air, which allows a certain amount of water to evaporate and become water vapour.
This evaporation-promoting heat is extracted from the same air, which increases its Relative humidity percentage, whilst simultaneously losing some of its heat.
The drier the air, the more water vapour it can absorb. Therefore, the more humid the air, the less it absorbs.
MOIST AIR
Furthermore, the absolute amount of water vapour that the air can hold depends on the temperature: the hotter it is, the more it absorbs.
This physical air characteristic is expressed in the psychometric chart which indicates the different conditions of moist air.
The conditions that enable the air to collect a greater amount of vapour are those furthest from the saturation zone, i.e. the conditions with a lower RH percentage and higher temperature.
The thermodynamic transformation that the air undergoes during evaporative cooling is comparable to an isenthalpic process, i.e. a transformation in which there are no changes in internal energy or air fluid exchanges. This transformation occurs, by definition, in the absence of a direct exchange of work or a direct exchange of heat with the environment: hence the concept of adiabatic cooling.
MOIST AIR
Our ambient air is made up of a mixture of dry air and water vapour. The weight of the latter is less than 3% of the weight of the ambient air, even in the most humid climates, yet it considerably affects human well-being, in addition to the operation of liquid-cooling equipment.
Relative humidity (RH)
Relative humidity (RH) or hygrometric degree (φ) represents the ratio between the mass of water vapour found in a certain volume of moist air and the mass of vapour that can be contained in saturation conditions at the same temperature and in the same volume of moist air.
Absolute humidity and specific humidity (x)
Absolute humidity is defined as the amount of water vapour (expressed in kilograms or grams) contained in one kilogram of dry air. Specific humidity (x) is also defined as the ratio between the mass of water vapour and mass of dry air contained at the same temperature, in the same volume of moist air (kgv/kga)
THE PSYCHOMETRIC CHART
The psychometric chart is used for the purpose of calculating the transformations undergone by the air and water vapour mixtures and refers to the standard atmospheric pressure.
Moist air is a three-variant thermodynamic fluid, meaning that three variables are required to determine its state. However, if the total pressure of the mixture is fixed, only two state variables are left to be specified and the system’s state can be represented on 2D charts.
THE ADIABATIC SOLUTION FOR LIQUID COOLERS
• There are various types of adiabatic solutions that capitalise on the evaporation effect of water content for temperature reduction purposes.
Let’s try to think in terms of the various transformations: in fact, we are spraying water – which is probably colder than air – towards the intake air at the cells.
• So let’s say, for example, that the air entering the cells is 30°C and the humidity level is 40%.
• And let’s assume that the relative humidity of the air – after being sprayed with the nozzles – is equal to 50%.
• The advantages would be.
