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We have now detected that the compressor, which is usually still driven by the combustion engine, transports refrigerant under pressure through the condenser in a circuit. Then it becomes liquid. Then it goes through a pressure valve, causing the pressure to drop enormously. In the subsequent evaporator, it then becomes gaseous while absorbing heat.

If the pressure valve consists simply of a spring-loaded valve disk, the refrigerant does not completely change to the gaseous state. However, since the compressor would be destroyed by the liquid components, there is also a collection container that reliably keeps such components away.

In case a so-called expansion valve is used instead of the simple pressure valve, such liquid components do not arise in the first place. The expansion valve includes the inflow to the evaporator and controls the inflow in such a way that all the refrigerant evaporates there. The collection container mentioned is not necessary here.

However, there must always be a container in such a circuit that has a certain reservoir of refrigerant ready, otherwise even the smallest amounts lost would cause major functional losses. This so-called liquid tank is installed in systems with an expansion valve in the high-pressure circuit, usually near the condenser.

It is also called a 'dryer' because of another function. Since water or moisture hardly dissolves in the liquid refrigerant, it is either carried along as droplets or its vaporous part mixes with the vapor of the refrigerant. Moisture can also combine with the oil portion in the circuit.

Just as we know ice formation with every major pressure drop, e.g. on oxygen or air bottles with large extraction rates, the water components in the refrigerant (medium) circuit can form ice on the expansion valve or the throttle. The function of the air conditioning is then disrupted.

The dryer removes this moisture. In addition, it prevents the formation of vapor bubbles on the way to the expansion valve. It calms and filters the refrigerant and also reduces the number of gas bubbles. Its dryer lasts a long time if it is always properly treated, although it can actually only hold a few grams (observe the manufacturer's information).

It is best if only liquid flows through the expansion valve. Therefore, in principle, it continues with a line below from the liquid area of the dryer. This is then obtained in the space above either directly or from gas bubbles and freed from moisture by small, highly hygroscopic solids in the appropriate container.

The dryers used to be combined with a sight glass. There you could see from a certain formation of gas bubbles and discoloration that something was wrong. The former could be caused by insufficient refrigerant and/or a lack of passage through the dryer. For example, if the refrigerant takes on a green-yellowish colour, it contains too much moisture.

Both the liquid to fill up any shortages and the vapor space are important. Even under the most unfavorable conditions, this must not be replaced by liquid. Then each additional drop would theoretically increase the pressure to infinity. The fill factor is just over 1 kg/litre.

Defects in the dryer bottle can be detected by measuring the temperature while the system is running. Since the temperatures here can be well above body temperature, measuring by hand is not advisable. A remote thermometer (laser pointer), for example, would be better here, it should not show a temperature difference of more than 4°C between the input and output.

The detection of leaks is made easier by a special liquid that is matched to the refrigerant and brought into the circuit. Even less than 5 grams can be tracked down with an electronic search device.

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