Dehumidifiers....?

[morgan]

Could someone please direct me to any websites that give in detail how a dehumidifier works. I don't mean it just sucks the moisture from the atmosphere but an indepth working of it.

Thanks in advance

[Andy Hall]

I don't know of any web sites but the principle is simple enough.

A domestic refrigerator operates by pumping a refrigerant gas around a closed circuit consisting of an evaporator unit (inside the fridge - normally metal plates welded together with a pipe running through) and a condensor (normally a long concertina pipe on the back with fins). A compressor runs and compresses the refrigerant gas. Under compression, the gas condenses to a liquid and releases heat - latent heat of condensation. This heat is released via the condensor. The liquid refrigerant is then passed to the evaporator inside the fridge where it passes from narrower to larger pipework. This causes the pressure to drop and the liquid evaporates. In doing so, heat is absorbed - latent heat of evaporation.

So we have a mechanism for moving heat from A to B. A secondary effect of the cooling inside the fridge compartment is that water vapour will condense on the coolest surface - the condensor. THis is why fridges with ice compartment frost up. In the process, the air inside the fridge is dried.

The dehumidifier works in the same way, except that it is not intended to provide cooling. THe same components are used except that the evaporating component is put in the same box as the condensor. A fan blows air from the room first of all over the evaporator which presents a cool surface to condense the water, then over the condensor which warms the air again before returning it to the room.. Thus, the air temperature of the air going back into the room will be about the same as that taken into the unit. In fact it will be slightly higher because electricity is used to operate the compressor and fan and the energy used ends up as heat. Some dehumidifiers have a supplementary heater to provide additional warmth.

Normally there is a humidistat to control compressor operation so that the air is not made too dry. The evaporator temperature is arranged not to cause the water to freeze. The condensed water drips down and normally into a collecting container in the bottom of the appliance. This then has a float switch to stop the appliance when the container is full. Some dehumidifiers have an optional piped outlet which can be led to a drain etc. for more continuous operation.

Will this do you?

.andy

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[The Natural Philosopher]

It is purely a refigreator with the door open. So the moisture instead of freezing, just drips into a collecting tray. Detail design differs, but the principle is the same...

..in fact a fridge with broken door seal would make a good dehumidifier anyway.

[Andy Dingley]

It's a fridge, without the insulated box. It has some coils out the back, and these get cold (a little below freezing). There's usually a timer to cycle the freezer, and a fan to move air over it.

Then it's just like a bathroom mirror on a cold morning. Find a website on basic meteorology and look into relative humidity. Warm air can "carry" more moisture than cold air. The amount it's carrying at any time, relative to the amount it possibly could at that temperature, is called the "relative humidity". If you take a volume of air, keep the same water content in it and then lower the temperature, the RH increases. If this RH reaches 100%, then you get condensation and water appears.

A dehumidifier doesn't work by being cold (in absolute terms), but by being _colder_ (in relative terms). If the outside air is already cold, then the fridge part can't reach a temperature low enough to extract moisture. For this reason mine is on a timeswitch, as it just isn't effective overnight, although it still uses the same electricity.

-- Die Gotterspammerung - Junkmail of the Gods