Hot gas automatic defrosting

This is how a commercial dehumidifier becomes a real construction dryer

Commercial dehumidifiers must be extremely tough. Harsh operational environments and frequent site changes require a robust metal housing, sturdy wheels or castors and a solid construction in general. In this respect, most commonly available commercial dehumidifiers do not particularly differ from construction dryers on the outside. But does this mean that every commercial dehumidifier can automatically be used as a construction dryer?

No hot gas automatic defrost system, no use for construction drying

Generally, all commercial dehumidifiers are condenser dryers. They cause the moisture contained in the room air to condense in order to be able to collect and discharge the condensate. This way, moisture is permanently withdrawn from the air, which then becomes drier.

To make the humidity in the sucked-in air condense, its temperature must be reduced to below the dew point. This takes place at the dryer’s evaporator, which is cooled down considerably by a pressure-reduced refrigerant flowing through.

Without technical countermeasures, however, this cooling technique would have a disastrous effect: ice would start to form on the evaporator at a temperature of 16 °C to 17 °C, and the progressing ice formation would not only lead to a steady decrease of performance, but would finally cause a total failure of the entire system.

For this reason, every condenser dryer is equipped with a system which regularly defrosts the evaporator – the automatic defrost system. And this is what makes the crucial difference between a commercial dehumidifier and a construction dryer.

Just because it says construction dryer doesn’t mean it actually is a construction dryer

In case of standard commercial dehumidifiers, 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 defrosting: In defrost mode, the compressor switches off while the fan usually keeps running and circulates the compressor’s waste heat around the evaporator for defrosting. 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 dew point is also significantly lower. This leads to an increased ice formation at the evaporator, which must be defrosted almost permanently in case of air circulation defrosters as they require more time for defrosting. This means that operation in normal dehumidification mode is almost no longer possible with such devices! Bear in mind that on construction sites in Central Europe, temperature usually drop below 15 °C on more than 200 days, at least during night hours.

For this reason, construction dryers are fitted with a completely different defrosting system – a hot gas defrosting system based on the bypass procedure. Here, the hot gas of the refrigerant circuit is used actively for fast and effective defrosting. As soon as ice starts to form, a special valves opens automatically, redirects the hot gas to the evaporator, and closes again after defrosting is completed so that drying operation can be continued via the regular refrigerant circuit.

Unlike air circulation defrosting, hot gas automatic defrosting allows for drastically shorter defrost phases, 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!