[chris (OP)]

I was asked on the radio the other day why adding salt to the water fed to a humidifier should cause a significant boost in steam production.

Apart from raising the boiling point of water, I couldn't think of any specific reason why this should be the case.

Can anyone help?

Chris

[Geezer]

This type of humidifier does not actually have a heating element. It has two electrodes immersed in the water that are connected to the live and neutral electricity supply. Without added salt, the water is not very conductive, the current flowing between the electrodes is small, and not much heat is produce to evaporate the water.

Adding salt makes the water much more conductive, so a lot more power is applied to heating the water.

[tommya300]

This type of humidifier does not actually have a heating element. It has two electrodes immersed in the water that are connected to the live and neutral electricity supply. Without added salt, the water is not very conductive, the current flowing between the electrodes is small, and not much heat is produce to evaporate the water.

Adding salt makes the water much more conductive, so a lot more power is applied to heating the water.

I can see a huge safety factor built into this type of design too.

[Geezer]

I can see a huge safety factor built into this type of design too.

It's probably not a bad idea to only use it on a GFI (ground fault interrupter) circuit.

[tommya300]

I can see a huge safety factor built into this type of design too.

It's probably not a bad idea to only use it on a GFI (ground fault interrupter) circuit.

Never can be to too safe!
Also when the water runs out, breaking the circuit, the element no longer conducts.

I always wondered why, after the initial salted water setup, after it is evaporated and then the water is replenished, more salt needs to be added? Isn't there a salt concentrate there already?

[Geezer]

I always wondered why, after the initial salted water setup, after it is evaporated and then the water is replenished, more salt needs to be added? Isn't there a salt concentrate there already?

I supposed most of the salt is driven off along with the water vapour. There isn't that much salt in the first place.

[tommya300]

I always wondered why, after the initial salted water setup, after it is evaporated and then the water is replenished, more salt needs to be added? Isn't there a salt concentrate there already?

I supposed most of the salt is driven off along with the water vapour. There isn't that much salt in the first place.

Precipitating questions!!!!
The separation of salt from water can be done by distillation.
Isn't the evaporation process is part the same?
If the small concentrate was good enough in the initial setup, what happened to the salt?

It always had me perplexed
.
I was thinking .. it is still there, but it had dried to the walls of the reservoir and something physical about the way it is sticking to the reservoir, something is not permitting it to dissolve into the replenished water.

[Geezer]

Precipitating questions!!!!
The separation of salt from water can be done by distillation.
Isn't the evaporation process is part the same?
If the small concentrate was good enough in the initial setup, what happened to the salt?

It always had me perplexed
.
I was thinking .. it is still there, but it had dried to the walls of the reservoir and something physical about the way it is sticking to the reservoir, something is not permitting it to dissolve into the replenished water.

According to
http://www.crscientific.com/electrolysis.html

"Electrolysis of Sodium Chloride Solution

 Cautious electrolysis of NaCl solution with the Brownlee apparatus will produce hydrogen plus aqueous NaOCl if the experiment is carried out in a single, unpartitioned jar with stirring. What happens first is that hydrogen, chlorine, and NaOH are produced, and the aqueous chlorine and NaOH then react to form NaOCl ("bleach").  Higher temperature, however, tends to produce ClO3- (aq.) instead of OCl- (aq.).  Running the electrodes in two separate partitions will produce gaseous chlorine (poisonous!) and hydrogen, plus aqueous NaOH. 
 Some chlorine will undoubtedly escape the hypochlorite cell and go into the air without being consumed, so this whole operation should be done in a fume hood or outdoors.    The odor of chlorine is sharp enough that, if it escapes into the room, you should have sufficient warning to shut off the power supply and get out of the area before there's any injury."


The above refers to DC current flow. I think these humidifiers apply AC across the electrodes, so that will change things somewhat.

[tommya300]

Precipitating questions!!!!
The separation of salt from water can be done by distillation.
Isn't the evaporation process is part the same?
If the small concentrate was good enough in the initial setup, what happened to the salt?

It always had me perplexed
.
I was thinking .. it is still there, but it had dried to the walls of the reservoir and something physical about the way it is sticking to the reservoir, something is not permitting it to dissolve into the replenished water.

According to
http://www.crscientific.com/electrolysis.html

"Electrolysis of Sodium Chloride Solution

 Cautious electrolysis of NaCl solution with the Brownlee apparatus will produce hydrogen plus aqueous NaOCl if the experiment is carried out in a single, unpartitioned jar with stirring. What happens first is that hydrogen, chlorine, and NaOH are produced, and the aqueous chlorine and NaOH then react to form NaOCl ("bleach").  Higher temperature, however, tends to produce ClO3- (aq.) instead of OCl- (aq.).  Running the electrodes in two separate partitions will produce gaseous chlorine (poisonous!) and hydrogen, plus aqueous NaOH. 
 Some chlorine will undoubtedly escape the hypochlorite cell and go into the air without being consumed, so this whole operation should be done in a fume hood or outdoors.    The odor of chlorine is sharp enough that, if it escapes into the room, you should have sufficient warning to shut off the power supply and get out of the area before there's any injury."


The above refers to DC current flow. I think these humidifiers apply AC across the electrodes, so that will change things somewhat.

AC as it cycles the anode cathode is in a shared position.
What you have there explains just what i was wondering for years. I never related the electrolysis just the heating and evaporation

Thanks Geezer

[Diccardo]

Thank you all for the illuminating and ‘salient’ discussion.  This problem has been vexing me for almost a year.  A $14.95 humidifier should not be such a source of bewilderment!  The manufacturer might consider explaining briefly why the salt is sometimes necessary.