Evaporative Humidifier Questions

[Axelrod12]

We picked up a relatively inexpensive evaporative console humidifier off amazon recently to try to bump up the humidity in our apartment for the winter. We've been hovering around 40-45% typically. I wanted to test out what the humidifier could do so I put the hygrometer about 10ft away and ran it at max humidity and max fan speed for about 24 hours. It brought the humidity up to about 57%, a 15% increase from where it started, in about an hour and then maintained that the rest of the time. At this humidity setting the fan didn't shut off once and the unit emptied around 3.5gallons of water in a day. It is a 9gpd system running in a ~1200 sqft apartment.

So the 15% increase is definitely nice, I appreciate it already, but I was hoping for a bit higher and from some of the reviews I was reading I got the impression it should be able to do more in an apartment this size. Does anyone have any ideas how I could make this unit more efficient so it can bump the humidity up a bit more?

Two thoughts I had and no idea if either would do a thing. The first was to run another small fan near the humidifier, hoping more air circulation will allow more air to get through the unit faster.

Second idea was when I was looking inside the unit. It draws water up from the base of the unit and pours it into a sort of trough over the top edge of the filter/wick pad. The trough has 4 small holes in it allowing water down into the wick/filter which the air passes through. Watching the unit while operating it looks like a lot of the water is just sort of spilling over the end of the trough. If I drilled a couple additional holes and allowed more water into the wick would that do anything to help?

Oh and the apartment is mostly an open design so shutting doors to lightly used rooms won't work, they already stay shut most of the time.

 

[Av8tor1]

The rate of increase in RH with this type of humidifier will decrease as RH increases. So you will find the next 15% much more difficult to achieve than the first 15%.
Kind of like the second law of Thermodynamics, but in RH ;-)

But this is why you don't see them used much in our hobby

 

[Axelrod12]

I figured that was the case. In some probably flawed way that sort of led me to idea 1 up there. I figured if I placed a couple fans around strategically i could circulate some of the humid air in the immediate area away and get some less humid air into the unit.

 

[David F]

Is that why vented enclosures are so hard to get above 70 ish percent humidity? I'm aware that about 70 is constant with condensation and evaporation, it must be some kind of saturation benchmark?

 

[Av8tor1]

David,

Not sure I can provide a good answer to your question... not really my field of expertise.
I would think that condensation and evaporation points would related back to dewpoints, and any change in state requires additional energy transfer.

Typically, the greater the difference between two items be it temperature or humidity, the greater the rate of change. As the difference decreases so does the rate of change given the same amount of "effort".

Maybe Zu or one of our other physic's gurus can chime in and give a better explanation.

or if you just love death by powerpoint :http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCYQFjAB&url=http%3A%2F%2Fwww.jsg.utexas.edu%2Ffu%2Ffiles%2FGEO387-LectureW7-1.pptx&ei=uEmIVO_FMpGQyATTlYCoBA&usg=AFQjCNHVFCFYFmsk3qXMybJtkN0NlAdqbg

(powerpoint lecture on moist air thermodynamics)

 

[Steve Booth]

Hi Greg
As AV8 suggests the amount of moisture given off by these units is dependent on air inlet temperature, humidity, airflow from the fan and amount of water available at the evaporation point, as well as the size of the area, air tightness and permeability of surfaces etc.

Without knowing more of the parameters it is difficult to answer your questions but putting another fan in the area wont increase the output of the unit itself, but may well distribute the air better throughout the space. However if the humidifier unit is at its limit due to lack of airflow or ineffective wicking then the only way to increase its output, as you suggest, is to introduce a faster evaporation rate i.e. more water (you hole idea may assist in this) and a higher airflow. You no doubt will have noticed the drop in temperature too, if your room is at say 20C d.b. and 40% RH when you start and you put the unit on, then without addition of external heat, if the room RH rises to 60% the room temperature will have dropped to 16.5C d.b.

Working against you is, again as you rightly alluded to, is the fact that it is a recirculatory unit, so as the onto the wick humidity increases, due to its effect on the room air already imparted by the unit, unless you can increase the temperature of the air onto the wick the quantity of water absorbed reduces leading to lower off wick moisture content.

If you can get a psychrometric chart on line that will give you the information you need. Plot your room condition on the chart, the evaporative humidifier (cooler) when switched on will then follow a constant wet bulb line, the amount of moisture imparted to the air in kg being the difference between the on and off medium moisture content in kg multiplied by the mass flow of the air from the fan (volume x 1.2) in metres cubed per second.

Sorry it sounds complicated, but warmer rooms, more water and more air is what you need, but with a small re-circulatory unit, relatively low temperatures and rising room humidity levels, you wont get there I’m afraid. Just do as you suggested and that may well improve things slightly.

Cheers
Steve

 

[RL7836]

Be careful what you wish for. You don't list your location but if you're in a cold area, your windows will act as a dehumidifier, condensing the moisture you're putting into the air (& potentially creating some ugliness on the sills).

 

[Av8tor1]

Be careful what you wish for. You don't list your location but if you're in a cold area, your windows will act as a dehumidifier, condensing the moisture you're putting into the air (& potentially creating some ugliness on the sills).

also one of the issues with using frozen 2-liter bottles in cp enclosures,
(dehumidifier)

 

[Axelrod12]

I'll have to keep an eye on the windows, they are mostly covered in the winter so I don't really see if there is condensation or not. I'm probably gonna see what drilling those holes does first. When running it definitely seems like the wick can have more moisture in it.

Maybe we can turn the heat up a couple degrees too or would that act as a dehumidifier as well, we have gas baseboard heat. And if it does will it outweigh the benefit of higher heat with the unit?