Cold evaporator of a compression refrigeration dryer
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  4. Practical knowledge concerning dehumidifiers
  5. Overview of dehumidification methods
  6. Condenser dryers with compressor technology

Condenser dryers with compressor technology

Technical differences and functional principle of compression refrigeration dryers

Since most dehumidification tasks at home are carried out at temperatures between 15 and 25 °C, and thanks to an excellent ratio of price, performance and efficiency, this device type is among the most frequently used dehumidifiers in the private sector and building industry.

Compressor-operated condenser dryers work on the same principle as a refrigerator. The device houses a compression refrigeration system conveying a refrigerant through two heat exchangers – a condenser and an evaporator.

Made possible by abrupt cold shock

By means of a compressor and an expansion valve, the refrigerant is exposed to changing pressures within this closed circuit, which results in the gas heating up on the condenser side during compression, and abruptly cooling down far below the room temperature on the evaporator side during decompression.

At the evaporator, the temperature is literally brought to a full stop – the air is abruptly cooled to below its dew point temperature, the moisture bound in the air condenses and turns into drops of water dripping into a collection container. The cold, dry air is now guided through the hot condenser, absorbs heat and then flows back into the room as dry, warm air before taking up moisture again.

Ice doesn’t stand a chance

Depending on the ambient temperature and humidity level, the evaporator can be extremely cold and there might be icing at its surface at room temperatures below 15 °C.

An increasing formation of ice virtually “clogs” the fins (icing) and reduces the device’s dehumidification capacity.

This is why all compressor-operated condensation dehumidifiers are equipped with mechanisms for regular evaporator defrosting – usually by means of air circulation or hot gas, as described in the section “Types of defrosting” below.

If there was no defrosting by air circulation or hot gas, the evaporator (cooling element) would freeze completely over time until a full-fledged “ice wall” would make any kind of air flow impossible.

The functional principle of compression refrigeration dryers
A refrigerant compression dryer opened for servicing shows the heat exchanger integrated in the upper part of the device with the evaporator (1) at the front, on the cold surface of which the air condenses, and the channel (2) behind it serving to discharge the accumulated condensate into the water collection container. The compressor (3) for the refrigerant is integrated in the bottom part of the device.
Opened refrigerant compression dryer
View of heat exchanger and condensate collection tray

Types of defrosting in compression refrigeration dryers:

Air circulation defrosting

With this method, defrosting usually takes place electronically in a time- or sensor-controlled manner by means of air circulation, which is why it is often referred to as electronic or electric defrosting:

if an increasing amount of ice forms at the evaporator, the compressor switches off and starts the defrosting process while the fan usually keeps running and circulates warm room air around the evaporator to remove the ice.

This technique has proven itself and works quite well in heated surroundings above approx. 15 °C.

If, however, such dryers are used in cooler environments – below 15 °C –, the surface temperature of the evaporator is below 0 °C. This leads to an intense ice formation at its surface, which must be defrosted almost permanently in case of air circulation defrosters as they require much more time for defrosting.

In cold surroundings like these, regular dehumidification becomes nearly impossible with dehumidifiers equipped with an air circulation defrost system as the devices are almost constantly busy defrosting themselves!

This is why – from an economic point of view – refrigerant dryers with an air circulation defrost system are almost always a good solution for all operating conditions in warm rooms with moderate air temperatures above 15 °C.

Hot gas defrosting

Dehumidifiers that are designed to be used in cooler rooms are equipped with a hot gas defrost system based on the bypass procedure instead of an air circulation defrost system.

Here, the hot refrigerant gas of the compression circuit is used actively for fast and effective defrosting. As soon as ice starts to form, a special solenoid valves opens automatically and redirects the hot gas from the compressor directly to the evaporator (instead of to the condenser) via a bypass. The valve closes again after defrosting is completed so that dehumidification operation can be continued via the regular refrigerant circuit.

Unlike air circulation defrosting, hot gas automatic defrosting allows for drastically shorter defrost phases of only a few minutes, which is an essential requirement for effective dehumidification in low-temperature surroundings such as unheated rooms. After all, the actual dehumidification process only takes place when defrosting is not in progress!

For the dehumidification of unheated rooms at temperatures below 15 °C, thus, dehumidifiers with hot gas automatic are always better suited and more efficient than devices with air circulation defrost function despite identical compressor performance.

At temperatures of more than 15 °C, however, the capacity of these two dehumidifier types is increasingly assimilating until being virtually identical at temperatures above approx. 18 °C.

Conclusion: Refrigerant dryers with a hot gas defrost system are versatile all-rounders since their defrost system enables operation at ambient temperatures ranging from 5 °C to 35 °C. This is why the devices can be used variably in warm and cold rooms – both in the summer and in the winter. For process-related reasons, the use of a device with an air circulation defrost system, on the other hand, is only reasonable at temperatures between 15 °C and 35 °C from an economic and energetic point of view.


Dehumidifier equipment technology – practical knowledge from Trotec

Practical knowledge concerning dehumidifiers – summary of all chapters

Chapter 1: Basic knowledge regarding humidity – relative is everything
Chapter 2: Overview of dehumidification methods – condensation and desiccation
Chapter 2.1: Condenser dryers with compressor technology
Chapter 2.2: Condenser dryers with Peltier technology
Chapter 2.3: Desiccant dehumidifiers
Chapter 3: Which dehumidification method for which purpose?