Air conditioning

Air conditioning, climate control, air treatment or air control is the regulation of the air in indoor spaces, such as in a building, car or train. Other names are air conditioning, air controller, climate controller, climate control and comfort cooling. The general consumer appliance as used in vehicles is usually referred to as air conditioning or air conditioning; abbreviation A / C. The terms air-conditioning (especially in agriculture), air-conditioning, air-conditioning or air-conditioning are used for equipment or installations for businesses or large buildings.

Air conditioning

With the device or installation, the temperature and air humidity can be kept at a pleasant level, while it is uncomfortably warm (or cold) outside. The air can also be purified by applying a filter system.


In 1902, the young American engineer Willis Carrier devised a system for controlling the temperature in a printing house. In 1924, the first cinema room, the Rivoli in New York, was equipped with air cooling. Cinema turnover increased spectacularly. 5 years later, 300 movie theaters in the United States were air-cooled. Systems had already been devised in the 19th century, but without much success, because too much ice was needed.

Air conditioners

Air conditioning can work in the following ways: with a heat pump, or by (indirect) evaporation of water.
Operation of air conditioning with heat pump
Schematic drawing of a heat pump: 1 is the condenser, 2 is the turbine (possibly the throttle valve), 3 is the evaporator, 4 is the compressor.

A heat pump is a closed circuit of a liquid refrigerant with a low boiling point, such as freon. This refrigerant evaporates in the evaporator and condenses back into liquid in the condenser. The heat is transported net from the evaporator to the condenser, against the existing temperature gradient.

heat pump

There is always a thick and a thin tube going to the evaporator. Gaseous (evaporated) refrigerant flows in the thickest tube; in the thin tube of liquid refrigerant at a temperature approximately equal to the outside temperature. This refrigerant is pumped to the ventilated cooling coil in the indoor unit: the evaporator. Just in front of the evaporator is an expansion system (capillary or expansion valve), which allows the liquid to escape to a lower pressure at evaporating temperature. The liquid starts to evaporate and thereby absorbs heat from the space to be cooled. With an air conditioner, the temperature here is about 0 ° C. A difference of about 20 ° C is taken for an air conditioner, for cold stores it is 7-10 °.

In modern air-conditioning units, the capillary is almost always incorporated in the outdoor unit. This is because a noise is created when the refrigerant is injected from high to low pressure. If the capillary is incorporated in the indoor unit, this sound from injection is uncomfortable and audible. Non-evaporated liquid then flows through the small tube at evaporating temperature.

Because this refrigerant has an evaporating temperature that is lower than the environment, the refrigerant will absorb the heat and evaporate as a result. This heat energy will be absorbed by the refrigerant. In other words: heat from the room is transferred to the refrigerant. The liquid refrigerant evaporates completely and even slightly overheats to prevent the compressor from getting liquid hammer. The compressor of an air conditioner cannot pump liquid, only gas. This does not benefit the efficiency of the system. In order to make a compressor as effective as possible, the harmful space of a compressor is very small, so maximum gas quantity can be pumped. The gaseous refrigerant is compressed here to a higher pressure and temperature and fed to the condenser. The temperature of these gases is in some cases 50 ° above the outside temperature. The temperature at which condensation takes place is called the condensation temperature. A fan is often installed to facilitate heat transport to the outside environment.

Outside the space to be cooled, this gas returns its extracted heat to the condenser and condenses back into liquid. The compressor is the driving force in the entire process, moving the gaseous refrigerant. A special version is the so-called inverter compressor. Due to its speed control, this is a lot more comfortable in controlling the room temperature and moreover more than 30% more energy-efficient than an on-off compressor. The refrigeration cycle is close to the Carnot cycle, with the exception of the evaporator throttle valve.

Operation as an evaporator of water

When water is evaporated, it increases the humidity and the evaporative energy is extracted from the air, causing the temperature to drop. This is called adiabatic cooling. When the moist, cooled air is supplied to the room for use, it is referred to as direct adiabatic cooling.

The moist cooled air can also be used to cool a second, separate air stream. This technique is called indirect adiabatic cooling. The temperature of the second air flow is hereby lowered by means of the first air flow. The second air stream is supplied to the user space. The absolute humidity of the second air flow is lower than with direct adiabatic cooling, which gives a higher comfort.

This air conditioner has the great advantage that only energy is needed for the fan that pumps the air to the consumption room, which is often done anyway (mechanical ventilation). The supplied air is also fresh air (with heat pump air conditioning, often only the room air is cooled and dehumidified, and pumped around but not refreshed). Furthermore, some water is needed to evaporate. The disadvantage of this air conditioner is that the achievable temperature reduction depends on the humidity outside and that it cannot be used as a dehumidifier.


To dry

High temperatures are less tolerated when the humidity is also high. Reducing the humidity already contributes to being able to tolerate high temperatures. This can be done, for example, by means of desiccation, or the use of drying agents.


An air conditioner equipped with a four-way valve can heat, in addition to cooling, by reversing the process described above. Many air conditioners are also equipped with additional functions: cooling (and automatic dehumidification); heating (if the device is equipped with a four-way valve); only dehumidify; only ventilate. Heating is possible even in moderate frost.

In many warmer regions of the globe, the air conditioning enables a Northern European daily rhythm. Without these systems there would be e.g. in the Gulf states in the Middle East not much activity is possible in the summer.

Type of air conditioners

There are various types of air conditioners. The most common air conditioning is the split-unit air conditioning. This has a separate unit on the outside of the facade where the coolant cools down. Furthermore, mobile air conditioning is increasingly common. This combines all parts in a movable unit. The moisture is discharged via a thinner hose and collected in a condensation tray and / or warm moist air is discharged via a thicker air hose that can be hung through the window or semi-permanently via a fixed opening in a wooden outer wall or wall. A new type that is emerging is the standalone air conditioning. This is a closed system in which both separate air conditioning units are combined into an indoor unit. Some types of air conditioners are: ceiling model, window model, mobile air conditioner and cassette model.


Cooling with air conditioners also has drawbacks. The process uses a lot of energy, and although that energy is needed when the sun provides the most energy, that solar energy is hardly used. Furthermore, the air-conditioning system requires that buildings be kept tightly closed, otherwise the resulting coolness will be lost by mixing with the warm outside air. Comparable losses occur with a portable air conditioner that dissipates the heat through an air hose to the outside; inevitably, (warm) outside air is drawn in from elsewhere.

An air conditioner (especially when it is poorly maintained by never or rarely cleaning the air filter, for example) can give rise to all kinds of diseases and allergies (the so-called sick building syndrome).

Our body cannot absorb a difference of more than four to six degrees. It is therefore advisable to keep the indoor temperature no more than 4-6 ° C lower than the outdoor temperature. Inside it will still feel fresher when you go in from outside, all the more because you will not be bothered by the direct heating of the sun and the ‘oppressive’ or ‘brooding’ feeling that moist, warm outside air gives. Too low a humidity (lower than the ideal 45-60%) results in a reduced resistance, which gives bacteria and viruses a chance to multiply.



Trains with air conditioning are on average (measured over the whole year) more energy efficient than trains without air conditioning. Because the windows can no longer open, the train surface is smoother and more streamlined. The air resistance is therefore smaller and less energy is needed for the drive. By using windows that cannot be opened, the train is also better insulated, so that less energy is needed to heat the train in winter. In addition, the turbine efficiency can be increased by using a generator to convert the entropy loss of the air conditioning into kinetic energy that is then used to power the train. The Dutch VIRM trainsets (new generation of double deck trains) and the modernized ICRm carriages and ICMm trainsets of the NS are air-conditioned or in GTWs of Arriva. At the Belgian NMBS, the I11 and M6 coaches (new generation of double-decker coaches) have air conditioning. The AM96, the MW41 and some I10 and I6 coaches are also equipped with air conditioning.