The Magic Stove makes it possible to compare löyly at different temperatures. I have experienced löyly by raising the temperature in a sauna from 50 to over 140 ºC during one bathing. To my surprise, I could throw löyly even at 140 ºC. I also remember when, in my youth, I lost the skin of my back when I got the second place in a löyly contest at a much lower temperature.

Click to see this image bigger... The total heat effect of the löyly depends on the temperature and the humidity of the air. The heat in a sauna of 100 ºC dry air is quite tolerable, but a sauna of 50 ºC air temperature can become a skin burning hell when a lot of water is steamed on the stones.

The heat effect in a sauna can vary from the heat of 50 degrees dry air to the heat of pure steam. In a sauna with a good stove, there is a great variety of different kinds of löyly. In a sauna with an electric heater, there is only a narrow hot end of the variety of löyly as it necessary to heat up the sauna very hot in order to protect the steaming ability of the small amount of stones, in other words the ability to give löyly.

The change in the nature of löyly is best explained with the fact that the ability of air to fuse water increases with the rising temperature. Enclosed is a table of the maximum amount of water that the air can fuse at temperatures from 20 to 100 Celsius degrees, i.e. the water content of air when the relative humidity is 100 percent at different temperatures. Outdoors the water exceeding 100 percent falls down as a rain. In the sauna, the excess water condenses on the shoulders and other surfaces that are colder than the air.

water g/cbm

At the most popular sauna temperatures, 60-90 ºC, the air’s ability to fuse water increases from 130 g per cubic meter to 421 g per cubic meter. In practice, this means that at 90 degrees one should throw over three times more water than at 60 ºC in order to maintain the relative humidity and to get as wet löyly in both temperatures. If the relative humidity is kept constant, the rise of temperature from 60 to 90 degrees is 75 percent (90-20/60-20=70/40) but the increase of the heat effect is much more than 75 percent.

To get an idea about the real increase of heat effect we could make a highly simplified calculation:
Let’s suppose that we have a sauna of 6 square meters and there is 6 cubic meters air above the laude. It takes about 300 kJ energy the heat up 6 cubic meters dry air from 20 ºC to 60 ºC, and 540 kJ to 90º degrees.

In a sauna, most energy is used to heat up the walls and the ceiling. The ventilation takes also a lot of energy. Most of this energy is lost, because a part of it is inside and behind the panelling and a part of is gone with the wind. When taking a sauna bath, one can only take advantage of that part of energy that radiates from the walls and the ceiling, and comes from the laude and the walls as convection energy. It is difficult to say what the heating effect of this energy is. Let us assume that it is as great as the heating effect of air.

We need also the heating effect of löyly, which is in relation to the energy needed to vaporise water and the amount of vaporised water. It takes about 2600 kJ to warm up and vaporise one litre of water, which produces 1700 litres vapour.

Enclosed is simplified drawing about the heat effect of löyly. The horizontal axis is for temperature and the energy needed to heat up dry air in a sauna of 6 cubic meters. As seen in the figure, the energy needed at 60 ºC is 600 kJ ( 300 kJ for air and 300 kJ for radiation and convection from the walls and the ceiling), 1080 kJ at 90 ºC, and 2000 kJ at 150 ºC. We could think that 2000 kJ at 150 ºC is the maximum heat effect in normal use.

The curve A shows that it takes 0.2 litre water to get the relative humidity to 25 percent in a sauna of 60 ºC. It takes 500 kJ to vaporise this 0.2 litre water. In theory, there is momentarily 1100 kJ energy above the laude in sauna, which is equivalent of over 90 degrees dry air. In reality, the maximum momentary amount of energy is less, because a part of the vapour begins condensing as the rest is vaporising.

The curve B shows that at 90º degrees throwing 0.2 litre löyly corresponds 120º degrees dry löyly and that there is only 8% relative humidity. The curve C shows that at 90 ºC degrees it is impossible to keep relative humidity in 25 percent, because the total heat effect corresponds the heat effect of more than 180º degrees dry löyly. At 90º degrees one must throw over 0.6 litre löyly to get the relative humidity to 25 percent.

All this shows the idea: a sufficiently low temperature in the sauna allows you to regulate the humidity and the heat effect. According to my experience this calculation puts too much weight on löyly. In a sauna, there is continuous heat radiation from walls and ceiling but a drop of löyly disappears instantly. The real heat effect of löyly is less than the amount calculated above, but the calculation shows the idea.

There is a curve below that shows the heat effect of a traditional Finnish löyly: In the beginning the temperature is quite low, 60-80 ºC. The curve shows the throwing of löyly. The heat effect is kept at the desired level by throwing almost continuously small amounts of löyly. One can get individually adjusted löyly and heat effect, which is not possible without löyly. Without löyly, bathing is boring and everybody cannot be happy with the constant heat, which can be too much, too little or even just right.

I told above that I threw löyly at 140 ºC. This was possible, because the löyly-coefficient does not rise as much as the temperature. When I threw löyly about as much (or little less) even when temperature rose, the löyly-coefficient did not rise in relation to the temperature. As matter of fact, the relative humidity fell from 30 % to 4 % (let us suppose that the humidity at 50 Celsius was 30 percent.) This means that when starting in a very wet löyly, I finished in a quite dry löyly.

We would appreciate your efforts to give us a better method for calculating and explaining the löyly. Please e-mail your suggestions or comments on anything related to sauna or the vitally important question of löyly. We will publish your comments on our discussion pages. We would also appreciate receiving all kind of thoughts assuming that these thoughts were born in löyly or in the spirits of löyly.
If you have anything to ask about löyly sauna, please ask, we publish your questions and our answers on Questions and Answers Pages.

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