Why does some steam in a sauna feel harsher than other steam?

[DTQ]

TL;DR Summary

Same temp & pressure but very different feeling.

The steam in some saunas feels much harsher, hotter, burning or 'biting' than in other saunas. The sauna temp is the same at 90°c, pressure is the same or very similar. The amount and temp of water used is the same or similar.

The only obvious difference is that the with the harsher feeling steam some or all of the steam is created from water on steel while in the more comfortable or gentle saunas it is created almost exclusively from water on stones.

I assume the temp of the steam itself is the same, ≈100°c, given the same atmospheric pressure?

Could it be that the steel produces steam much faster than the stones to there's a more sudden or quick burst of steam than with that created by stones?

Other?

Here's a chart of steam creation from stones:

Thanks all,

 

[Baluncore]

Welcome to PF.

Steam is water saturated, but air within the sauna can be less than 100% relative humidity, because it has been heated without the addition of water. Maybe you are sensing the hot, dry air as being harsh because it dries out your body when you breathe it in.

 

[jrmichler]

Steam is invisible. When steam condenses on your skin, the heat release is roughly 1000 BTU per pound of steam plus another 100 BTU per pound to cool it to skin temperature.

When steam condenses in air to form visible fog, that 1000 BTU per pound heat of condensation heats the air to 212 deg F. When that fog touches your skin, the only heat transfer to your skin is the 100 BTU per pound to to cool it to skin temperature.

These BTU numbers are approximate, but close enough to illustrate the concept. Reference to a steam table will get the exact numbers.

The heat transfer coefficient of condensing steam is much higher than the heat transfer coefficient of hot fog or hot air. I had actual numbers for these heat transfer coefficients when I worked in a paper mill back in the 1980's. The numbers are now gone, but I do remember the trends.

 

[Rive]

I assume the temp of the steam itself is the same, ≈100°c, given the same atmospheric pressure?

It's a common misconception to confuse the condensing 'fog' with the steam.
The steam is not visible by default (that's actually why it's so dangerous): but as it meets the ambient air, becomes oversaturated and starts to condense. Only that part is what's visible.

In a sauna the temperature of the heating element may be well over the boiling point of water (especially since this heating element is what provides the heating for the whole cabin), and thus: unless it's over-watered it may produce steam with temperature over 100°C by further heating the output of the boiling water.

With a big pile of rock of low thermal conductivity I expect this to happen less frequently than with an overheated piece of small metal with high thermal conductivity.

 

[DTQ]

Thank you all.

I don't think it's the hot air. I've been in a lot of saunas in my life and a few times every year in saunas that are 120 or higher.

I believe the stones are usually about 200 or higher so well above the boiling point of water. The steam in a sauna is always invisible. The only times we see fog is sometimes when cold air comes in from outside or when the sauna air escapes to outside.

Is it reasonable that the hotter steel could produce noticeably hotter steam by increasing the pressure in the sauna enough? Or would that require a huge increase in pressure?

 

[Janus]

Thank you all.

I don't think it's the hot air. I've been in a lot of saunas in my life and a few times every year in saunas that are 120 or higher.

I believe the stones are usually about 200 or higher so well above the boiling point of water. The steam in a sauna is always invisible. The only times we see fog is sometimes when cold air comes in from outside or when the sauna air escapes to outside.

Is it reasonable that the hotter steel could produce noticeably hotter steam by increasing the pressure in the sauna enough? Or would that require a huge increase in pressure?

Steel has about half the specific heat as stone does. So given equal masses of steel and stone, the stone would have twice the heat content. However, metal has a higher heat conductivity, so it can transfer its heat to the water faster. So, it is possible that what is happening is that metal produces its steam over a shorter period, which is what makes it feel harsher. That being said, most of my sauna experience is with older, wood burning stoves which had metal surfaces which were not what you would called bright and shiny, which likely had an effect on the heat transfer rate, so I don't remember much variation in the steam other than that caused by the amount of water tossed.

 

[Rive]

hotter steam by increasing the pressure in the sauna enough?

In an open system like a sauna pressure and temperature is not connected such a way. You can get very hot steam at ambient pressure - what you can't get is hot (100+°C) and liquid water at ambient pressure.

 

[DTQ]

So, it is possible that what is happening is that metal produces its steam over a shorter period, which is what makes it feel harsher.

This would seem to be at least part of it. I did a quick experiment replacing some of the top stones with steel ingots to see if that made a timing difference but it wasn't very significant (and probably 70% of the water made it to the stones). I may try again with a lot more steel vs stones.

That being said, most of my sauna experience is with older, wood burning stoves which had metal surfaces which were not what you would called bright and shiny, which likely had an effect on the heat transfer rate, so I don't remember much variation in the steam other than that caused by the amount of water tossed.

With a good sauna heater water should interact only with stones and never steel. A criticism of cheaper sauna heaters and DIY heaters is that they often don't have enough stone on top of the fire box to achieve this. Sounds like you had a good heater.

 

[DTQ]

In an open system like a sauna pressure and temperature is not connected such a way. You can get very hot steam at ambient pressure - what you can't get is hot (100+°C) and liquid water at ambient pressure.

I thought that pressure and temp are always connected? If you want higher temp then you must have appropriately higher pressure?

Is it possible that the higher pressure is more localized - only where there is hotter steam? So in this image, if the red steam is for example 105°c then the pressure in that area is appropriately greater (+ ≈80hPa)?

Thank you,

 

[russ_watters]

I thought that pressure and temp are always connected? If you want higher temp then you must have appropriately higher pressure?

They are, it's just that for an open system like a sauna the pressure is always atmospheric and there's nothing you can do to significantly change it.

Is it possible that the higher pressure is more localized - only where there is hotter steam? So in this image, if the red steam is for example 105°c then the pressure in that area is appropriately greater (+ ≈80hPa)?

No, there will be no significant difference in pressure throughout the sauna. But there may be a significant difference in humidity in different parts of the sauna or when using different methods to humidify it. Have you measured it?

 

[DTQ]

Thank you. So is the steam in a sauna or banya is then always ≈100°c (at sea level)?

Have you measured it?

The line chart in the first post shows humidity over time as it descends down from the ceiling.

 

[Rive]

If you want higher temp then you must have appropriately higher pressure?

No such thing. In an open system the pressure is ~ constant ambient, not related to the temperature. The minor pressure differences are resolved through airflow (wind).

Just try think this trough properly. What happens if you put a hot air blower (can melt solder: >300°C) above a boiling pot?
Just because of the steam present the output temperature will be reduced to 100°C?
But air always contains steam (water vapor).... It would be impossible to produce air with temperature above 100°C...

 

[russ_watters]

Thank you. So is the steam in a sauna or banya is then always ≈100°c (at sea level)?

No, a sauna doesn't have steam in it except just above the source. The steam quickly disperses and you just have water vapor/humidity at the temperature of the sauna. Note what was said previously: if you can see it, it has already condensed into small water droplets. Then further away you can't see it anymore because it's re'evaporated into vapor and is now just humidity.

The line chart in the first post shows humidity over time as it descends down from the ceiling.

Ok, thanks - do you have one for the steel?

 

[DTQ]

Thank you all.

So… Sauna is 90°c at head height (10cm below ceiling), 72°c at feet and 38°c at the floor. Absolute humidity at all strata is ≈ 25g/m³ (or approx 3-8% RH) - E.G., the brief bit in the line chart above before throwing water on the stones.

I throw a ladle of water on the 200°c stones. The water becomes 'hot water vapor?' that rises from the stones to the ceiling, flows across the ceiling (away from the pressure of the rising plume) to the far walls and then begins to descend on to bathers. Bathers feel an intense increase in heat.

The air temperatures have not changed, they remain constant at all strata. The humidity increases per the line chart above. There is no noticeably visible water vapor anywhere in the sauna (nor will there be in most cases).

The vapor immediately above the stones is steam? Superheated steam? Saturated steam?

What is the invisible water vapor that descends on bathers and causes a feeling of an intense increase in heat (even though ambient air temps have not increased)? And that in some cases can be an unbearable increase in the amount of heat felt.

 

[DTQ]

Ok, thanks - do you have one for the steel?

We did one with a few steel ingots in place of the top ≈8cm of stones but the difference was only minor as most of the water made it by the ingots to the stones below. It looks like it will require replacement of perhaps 40-50cm of stones with ingots and doing a lattice pattern so that most of the steam is produced from the steel. Hopefully sometime in January.

 

[Lnewqban]

The only obvious difference is that the with the harsher feeling steam some or all of the steam is created from water on steel while in the more comfortable or gentle saunas it is created almost exclusively from water on stones.

Compared to the stones, what is the shape and volume of the steel you are referring to in this quote?

Is the feeling difference something like coming from different temperatures or from perceived intensity of heat transfer?

 

[Rive]

The vapor immediately above the stones is steam? Superheated steam? Saturated steam?

What is the invisible water vapor that descends on bathers and causes a feeling of an intense increase in heat (even though ambient air temps have not increased)?

- Yes.
- Superheating is a different phenomenon, just drop it from here. It's just likely >100°C steam-air mixture.
- No, it's not saturated.

The air temperature usually does increase, but what matters most is when hot steam meets the relative low temperature skin: It'll condense and release lot of heat. Check up on 'steam burn'.

 

[DTQ]

The steel ingots I used for testing are half rounds about 4x25cm on the flat side and about 3cm high. I had only 7 of them for the first test. I'm hoping to get a total of 20-30 for a future test.

The steam from steel feels much hotter than that from stones. It also seems to stratify more, sticking closer to the ceiling compared to that from stones that seems to descend and destratify a bit faster.

 

[DTQ]

...

The air temperature usually does increase,

Thank you for all of that.

Here are air temps for the same period as the humidity above:

but what matters most is when hot steam meets the relative low temperature skin: It'll condense and release lot of heat. Check up on 'steam burn'.

Thank you. Yes. Still, why does the steam produced from steel feel so much hotter on my skin than that produced from stones (apologies for being so dense)?

Is it possible that there is more latent energy in it? Thus it releases more heat during condensation?

 

[Lnewqban]

The steam from steel feels much hotter than that from stones. It also seems to stratify more, sticking closer to the ceiling compared to that from stones that seems to descend and destratify a bit faster.

That seems to indicate that there is more thermal energy transferred from the fire (which is a source of the same amount of energy for each case) to the air-steam environment via metal ingots than via stones.

The key for that to happen seems to be the mass of water that is converted to steam in the same period of time, due in part to the differences in specific heat and heat transfer coefficients of both materials.

Please, see:
https://en.wikipedia.org/wiki/Vapor_quality

Even when the environment temperature is about the same for metal and stones, there are more molecules of water-steam interacting with the air and your skin.

The rate of heat transfer induced by those microscopic droplets of very hot water is more intense than the rate at which hot air could achieve.

A film of air sticking to your skin acts like an isolator, while a micro-film of hot water acts like an accelerator of the transfer of energy.

For the very same reason, the air at head level is unable to cool those droplets down as quickly as when the air contains less of them, intensifying the stratification effect.