Dehumidifiers and the law of Thermodynamics

[barryh]

I've been trying to get my head around the thermodynamics of a stand alone compressor dehumidifier in terms of heat added to the room.

1st law of thermodynamics says that if the unit consumes X watts per hour then X is the minimum heat added to the room.

It seems to me there are two other possibilities:

A compressor dehumidifier is essentially a heat pump even if the COP is low. Does this add additional heat to the room ?

The latent heat of condensation states that 678 watts are released for every litre of water condensed. Is this a real net gain of heat in any circumstances ?

Whether this is a net gain or not must at least depend on whether energy was consumed in creating the humidity in the first place. So if it's steam from cooking there can be no net gain because the two latent heats cancel unless that steam would otherwise have been ventilated outside.

 

[Jetronic]

No what you said first is correct: whatever wattage it extracts from the outlet gets added as heat to the room. The catch is that the power taken isn't constant, sometimes it's just the fan running in defrost mode.

if you putall parts of a heat pump inside a room, it'll heat that room with whatever power it draws from the outlet, no matter if you set it to cool, heat or ventilate. Same thing with a refrigerator.

for a heat pump to work, you need to move a part of it out of the room.

 

[supton]

So the dehumidifier pulls heat from the air so that the moisture will fall out, but mixes the now cold air with the air that the heat was pumped into. That would be net sum zero temperature change, however no process is 100% efficient. The motor used to do the work requires energy, and that waste heat winds up in the room too.

 

[JRay’07]

Users often point these devices at themselves when they are trying to cool off. They are taking advantage of a localized cool body of air even though the net effect in the room remains unchanged.

 

[Jetronic]

Users often point these devices at themselves when they are trying to cool off. They are taking advantage of a localized cool body of air even though the net effect in the room remains unchanged.

my dehumidifier puts out slightly warmer air.... but it would help if you're sweaty I suppose.

 

[Al]

You are adding heat to the room. That heat is the inefficiencies of the closed system.

 

[JRay’07]

You are adding heat to the room. That heat is the inefficiencies of the closed system.

It is a very small amount. Let's say you try to heat your room during the winter by taking advantage of the inefficiencies of your dehumidifier. How well would that work?

 

[Kestas]

Jetronic is correct. You have electricity (energy) coming into the room with no energy going out. That electricity is converted to heat.

I remember this was exactly a question in my thermodynamics class.

 

[Al]

Won't be enough. Just saying it adds heat.

 

[mk378]

As you said, the intended function of a dehumidifier in converting water vapor to liquid releases heat. The heat added to the room is this plus the electricity consumption. The theoretical efficiency of a "perfect" dehumidifier would depend on the dew point in the room; that is how much the air needs to be cooled to condense the water.

 

[SubieRubyRoo]

I don’t see any possible way that 100% of input could be added into the room as heat. There is work being done; you’re blowing air around the room and that is work; simply moving air does not add heat and the cfm moved is a quantity of work. The only pure 100% input = output that I know of in this case would be a resistive heater element with no fan. I’d love to see the equation that shows a dehumidifier would add 100% input to the room as heat.

 

[Rmay635703]

I don’t see any possible way that 100% of input could be added into the room as heat. There is work being done; you’re blowing air around the room and that is work; simply moving air does not add heat and the cfm moved is a quantity of work. The only pure 100% input = output that I know of in this case would be a resistive heater element with no fan. I’d love to see the equation that shows a dehumidifier would add 100% input to the room as heat.

A very efficient dehumidifier in a very wet room makes about 3x more heat than electricity it draws in. Dehumidifier’s are very efficient heaters.

Water vapor releases immense amounts of heat when it condenses

 

[Astro14]

The dehumidifier in my shop (840 square feet, 14 foot ceilings) adds several degrees to the temperature of the entire space when it is running. Argue thermodynamics all you want - the fact is - it adds a LOT of heat.

 

[barryh]

my dehumidifier puts out slightly warmer air.... but it would help if you're sweaty I suppose.

The dehumidifier in my shop (840 square feet, 14 foot ceilings) adds several degrees to the temperature of the entire space when it is running. Argue thermodynamics all you want - the fact is - it adds a LOT of heat.

I'm in no doubt that my dehumidifier adds heat to the room. The air is chilled by the evaporator to condense out the water and then reheated again on it's way out by passing through the condenser. I've measured the exit air temperature and it's something like 5 deg C warmer than the room temperature. It's what prompted my question about thermodynamics in the first place because subjectively it seemed to add more heat to the room than could be accounted for by the few hundred watts consumed by the device. This subjective additional heat must come for the latent heat of condensation and whether that is a net gain to the room will depend on how the humidity was created in the first place.

 

[Al]

It puts out cold and puts out heat and water.

Imagine a room of a specified humitity. You pur 16 oz of water and boil it with an electeric heater that itself adds no heat. AQgter the water is boiled the room will be boiled and the humitity will j raise up to 100% if there is enough water. Assuming the room does not get to 100% there will be very little condensation.

If you magically collect that water again perfectly the room will go back to where it was (cooler).

If you look at the big picture it turns out very little heat will be added by the humidifier. If entropy remains constant in the whole room there will be no temp. increase. Of course that is not possible. But only a small heat will be added in the real world. IMHO

 

[RDY4WAR]

We have a slightly oversized dehumidifer. I've run down to 45% humidity and the temp in the room hardly changed, only increasing by 1°F, but the room felt much cooler since the humidity was lower. We've been able to use the HVAC less since we can keep the house at 72°F and feels like it did when we kept it at 68°F with higher RH. The electric cost ends up being a wash.

 

[KrisZ]

It all probably depends on the sizing of the evaporator and condenser and probably also depends on the conditions and air humidity.
Technically speaking the net temp gain should be zero as both should cancel each other out, but we all know it would be impossible to perfectly balance them out like that.

So from thermodynamics perspective, the only heat gain should be from the friction of the running compressor and the fans. The gas phase change that actually does the dehumidifying should be a net zero conversion.

 

[Al]

It all probably depends on the sizing of the evaporator and condenser and probably also depends on the conditions and air humidity.
Technically speaking the net temp gain should be zero as both should cancel each other out, but we all know it would be impossible to perfectly balance them out like that.

So from thermodynamics perspective, the only heat gain should be from the friction of the running compressor and the fans. The gas phase change that actually does the dehumidifying should be a net zero conversion.

yep

 

[Rmay635703]

It all probably depends on the sizing of the evaporator and condenser and probably also depends on the conditions and air humidity.
Technically speaking the net temp gain should be zero as both should cancel each other out, but we all know it would be impossible to perfectly balance them out like that.

So from thermodynamics perspective, the only heat gain should be from the friction of the running compressor and the fans. The gas phase change that actually does the dehumidifying should be a net zero conversion.

Water releases .627 kwhrs of heat per kg when it condenses.

If you get 3kg of water in an hour that is the same as running an 1887 watt electric heater for an hour.

When it’s wet I get 5 gallons a day of water out of the dehumidifier that is 12,300 wattshrs of heat

 

[KrisZ]

Water releases .627 kwhrs of heat per kg when it condenses.

If you get 3kg of water in an hour that is the same as running an 1887 watt electric heater for an hour.

When it’s wet I get 5 gallons a day of water out of the dehumidifier that is 12,300 wattshrs of heat

Except, the air is then passed through the condenser coil to change the gas inside the coil from gas back to liquid, so that the phase change cycle can continue inside the humidifier.

But the water doesn't actually release much heat by condensing on the evaporator. It's the gas inside the evaporator that absorbs the heat from the air passing thorough and condenses, making the coil cold. Water simply condenses on the cold coil.

Edit:
Here is a graph with specific heat of air with different humidity levels. It only starts to make a difference at high temperatures. Hence steam is used for generating power, it's really energy dense, but at very high temperatures.

Your number above is likely for 100% RH and close to 100C from the graph below. That's not what the dehumidifier or AC systems have to deal with.