If condensation is non-desirable when cooling with an air conditioner or an HVAC system, it is important to keep indoor air above the dewpoint. This will save energy. But does it also make sense and is the constant measurement of temperature and relative humidity in rooms a suitable reference variable?

Depending on the location, season and application, the supply air for rooms, production or storage facilities needs to be humidified or dehumidified in different ways. For example, the ideal value for people is a near as possible constant 40 percent relative humidity in indoor air. This is different in the case of the indispensable antibiotic penicillin. This is most reliably produced at a relative humidity of 60 percent. The same applies to the moisture value in rotary printing in the paper industry.

Technical humidification is carried out isothermally with steam or adiabatically by evaporation, misting and atomisation of water. Dehumidification is achieved by means of adsorption and condensation processes. This almost always requires additional energy input, usually electrical, but occasionally with gas. However, air humidity in rooms is actually rarely the significant reference variable when planning a ventilation or air conditioning system. But would it perhaps be better to work with the operator to determine what the requirements will be and how the air humidity in rooms can be ensured on this basis before designing air conditioning and ventilation systems? It may even make sense to recover humidity. And, if relative humidity is taken into account with foresight and continuously measured, can energy, money and above all CO2 emissions be saved to a significant extent if the air humidity in rooms is introduced as a reference variable for air conditioning and ventilation technology? The report provides answers about the real significance of moisture in the air we consume.

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What is “moist” air, when or why does it become “dry”, how much heat energy does it contain, and what does all this have to do with specific enthalpy? An insight into the correlations between moisture, dryness, thermal energy and enthalpy is provided by physics, more precisely by the laws of thermodynamics. Anyone who has grasped this not entirely simple subject knows, for example, about possible ways of playing with the dewpoint of water in evaporative coolers, can explain the concept of wet bulb temperature without a second thought, and has an impressive grasp of isentropic changes of state – not to mention, being able to sleepwalk through the h-x diagram with complete accuracy, even without an explanatory video as back-up.

These days, however, the design of air conditioning units, HVAC systems or recooling systems is very often carried out by computer programs. The danger of this: Expert knowledge gained by planners or plant engineers about thermodynamic behaviour is increasingly being forgotten. And sometimes there is simply not enough time in day-to-day business to give any thought to an energy-efficient alternative.

However, maybe by thinking about air conditioning technology or recooling units in more depth, one or two kW of mechanically generated cooling energy might be saved or a refrigeration system might even become superfluous? This is especially true when it comes to cooling temperature limit ranges. That is, providing that the possibilities of adiabatic cooling, but also new ones for moisture recovery are properly understood. This is where thermodynamic knowledge helps to deal “correctly” with humid air.

Do you want to read the full technical article? Then register here for the 4-part series.

What is “moist” air, when or why does it become “dry”, how much heat energy does it contain, and what does all this have to do with specific enthalpy? An insight into the correlations between moisture, dryness, thermal energy and enthalpy is provided by physics, more precisely by the laws of thermodynamics.