Masonry heaters of periodic action for Russian steam saunas.
How to choose the correct layout for a Russian steam sauna, what thermal and moisture conditions should be chosen? How to choose the correct layout for a Russian steam sauna, what thermal and moisture conditions should be chosen? Information concerning the Russian steam sauna on our site http://stove.ru.
Two main factors characterize a good steam sauna:
layout that is comfortable and allows the most efficient use of the heat produced by the heater.
a masonry heater that can achieve the selected thermal and moisture conditions in each room, can produce high quality steam and hot water.
In this article we will discuss the questions concerning design of steam sauna heaters of periodic action, i.e. heaters with stone fill heated by hot gases coming directly from the firebox, and passing through the fill (“black” sauna stove).
A great number of such steam sauna heaters have been designed. Let's view some of the designs that are found in publications more often, see fig. 1 and 43. A steam sauna heater (fig. 43) consists of firebox 1, steam chamber 2 and a divider between them, as well as a convective system 7 and chimney 6. a door is installed In the steam chamber from the side of the steam room. Water is thrown onto the stone fill through this door to produce steam. The steam then comes out through the same door into the steam room.
An a ventilation grate or door is installed above the shut-off damper in the chimney flue to allow for ventilation of the steam room. The divider can be made in the form of a dome or arch made of refractory bricks with slots or openings. A coil 4 of hot water supply can be installed into the firebox. A layer of round-shaped stones of volcanic origin is laid on the divider with addition of 20-30% of chunks of cast iron to enhance the heat responsiveness. There are also stoves made of metal and lined inside with refractory bricks that have no convective system (fig. 2). They are designed for heating of steam room and for steam preparation only.
A recommended amount of the stone fill per 1 m3 of the steam room volume varies depending on the author. I. Kovalevsky recommends using 60-62 kg, with a layer thickness of 20-22 cm. V. Litavr and G. Kaidanov, and others recommend 35-45 kg. The stone basalt filling has a volume weight of 2800 kg/ m3 or 2.8 kg/ dm 3. In this case an amount of 12.5-16.1 up to 22.14 dm3 is needed per 1 m3 of the steam room volume. If this amount is laid on 1 m2 of the stove divider the filling thickness will be 1.25-1.61 up to 2. 214 cm per 1m 3 of the steam room volume.
If the standard filling thickness is taken as 22.14 cm (according to I. Kovalevsky-other authors do not indicate the thickness), so for the steam room having an area of 6 m2 and the height of 2.3 m (volume of 13.8 m3) and at the amount of filling of 62kg per 1 m3 of the steam room volume, 1.38 m2 of the firebox ceiling will be required. It is going to be 0.78 m 2 if we are using 35 kg of stone per 1 m3, and 1.0 m2 if we are using 45 kg of stones per 1 m3. In this example equal filling thickness is taken in order to ensure a uniform heating of stoves up to the temperature specified by the authors. As for the design, the firebox ceiling (that is a divider supporting the stone fill) in these examples makes 45-50% of stove square with convective systems and 66 % in stoves having no convective system (fig. 2). This means that the square occupied by the stove for the steam room having the square of 6 m2 will make:
according to Kovalevsky: 1.38/ (0.45-0.50) = 3.07-2.76 m2, according to other recommendations: 1.73-1.56 m2 and 2.2-2 m2 . In other words, the stove can occupy 26-51 % of the steam room square. In order to make the stove size smaller it is necessary to increase thickness of the filling. K. Myakelya recommends to have the filling thickness of 30-50 cm which corresponds to 840-1400 kg/m2 of the stone fill.
According to I.Kovalevsky, the combustion process takes place at temperatures of 800-900 o C for wood and 1000-1200oC for coal, while the temperature of firebox walls is 200oC lower. According to the test results obtained by I. Podgorodnikov, the firebox temperature in the bell-type stoves can reach 975oC. According to the data obtained by Ju. Sosnin and E. Bukharkin ( page 58) the temperature of the firebox wall heating can reach 800-900 o C while burning coal periodically.
It is but natural that if we burn wood the temperature will be about 200oC lower. According to the statements of all the authors (including Ju. Sosnin and E. Bukharkin) the stones get heated up to 1000-1100o C. from the bottom and up to 500-600oC from the top till they are red-hot (until they acquire the colour of raspberries). The basis for making these statements is not clear. In my opinion, this needs to be studied further, as it is not possible to heat the stones to the specified temperature in the wood-burning stove. In order to increase temperature of the stone fill it is necessary to increase the firebox temperature, to heat them from all the sides or to decrease thickness of the fill.
The principle of operation of all the stoves is as follows: the hot gases from the firebox pass through the slots (openings) in the ceiling, go through the stone fill, come to convective system and then to the chimney. The stove’s efficiency, when all the other conditions are equal, depends on thickness of the fill and how well the gases fill the stove’s volume. This means that if the stones fill will have different thickness and individual stones will be of different size, the resistance to incoming gases and heating pattern will be different.
It should be noted that it is practically impossible to obtain high efficiency of steam sauna heaters and to heat the stones quickly to the required temperature. This is to be explained in the following way: When the temperature of the stove’s walls becomes equal to temperature of the gas flow, the accumulation of heat by the stove walls and stones is reduced. Therefore, the temperature of the exhaust gases gets higher. Stoves of this kind are usually built with thick walls, therefore their outer walls are not heated to a great temperature. Such stoves can provide thermal and moisture conditions close to those required for the Russian steam sauna.
Summarizing the above said, the following tasks need to be performed in order to get a good stove. We have:
tobe able to make the stove smaller - the filling thickness has to be increased;
to increase heating temperature of the stone fill;
to reduce the resistance to the gas flow;
to decrease the temperature of the exhaust gases, i.e. to increase the stove efficiency;
to absorb maximum amount of heat from burning of the fuel.
How can we do this? This can be done if a steam sauna heater is designed on the principle of « free gas movement». Let's have a look at the diagram shown in fig. 3. This is the diagram of a «double bell» stove. The lower bell consists of a firebox 1 in which stone filling 3 is laid on the slotted slab 2. From both sides of the slab, there are slots 4 not covered with the stones, which connect the firebox with the steam chamber 5. A convective system of the lower bell 6 is arranged close to the firebox, in which a hot water coil 11 is installed. The firebox and the lower bell convective system are connected with each other by a dry joint 7 and a number of openings 8 in the upper zone. A channel 9 is made in the lower bell that goes to the upper bell 10. The diagram shown in fig.3 corresponds to the formula "the stove's lower level and the fire-box are combined to form a single space creating a lower bell". The idea to have a dry a joint and passages not covered with stones going to the upper part is:
To ensure that temperature is increased uniformly in the bell and a uniform heating pattern of the stone fill (from all the sides, including top) irrespective of the filling thickness.
To separate the flows of the gases into cold and hot components inside the bell.
At the same time, resistance to gas flow should not be increased. If an electric heater is used as a source of heat energy, there will be no need to exhaust products of combustion (this is only to better understand the principle). In this case, the heat energy transfer will be done due to natural nature sources, even if the chimney is closed. This system possesses all remarkable features that are given in the article « Once Again about the System»
When there is no dry joint and free passages available, the whole flow goes through the stones with an increased resistance and primary heating of the lower part of the stone fill. The stove operation and the stone heating will depend on thickness of the fill when all the other conditions are equal.
It is known that the channel-free convective system possesses remarkable characteristics and high efficiency. The performance principle of the diagram is as follows: Gases coming through the slots which are not covered with stones (as well as through the stone fill) fill the bell, whereas the hottest gases come in its upper part, and the coldest ones being heavy pass through the dry joint in the lower part of the bell (without coming into it) directly into the channel.
A zone of increased temperature is created in the upper part of the bell, which contributes to combustion reaction (burning the gas component of the fuel), i.e. to an increase of fuel efficiency. The heating pattern of stone fill is uniform that improves stones’ heating, whereas operation of the stove does not depend on the filling thickness. In other words, the resistance to the gas flow is increased insignificantly.
In order to obtain high quality steam it is necessary that the stones should be well-heated. As a rule, heating the stove takes much time, often much more in comparison with an ordinary stove.
It is common knowledge that the longer is the burning cycle the less is the stove efficiency due to the decrease of hot gases heat accumulation by the stove walls and the stones (the gas temperature and that of the walls is being balanced). In this case, care shall be taken to decrease the temperature of the gases coming into the chimney. This can be achieved by 2 ways:
by means of an installation of a hot water coil into the convective system of the lower bell;
by means of arrangement of the second bell over the first one.
The coil and the bell take in the excess heat arising due to the stove burning time increase.
The coil (heater) shall be designed in such a way that the forces arising from the heat extension do not destroy the stove, and it would be easy to change it. This can be achieved if it will have a zigzag form or if an inlet and an outlet pipes be installed at the opposite side of the stove. It shall be designed and installed in such a way that when it is filled with water there will be no air-locks. Otherwise when the water is heated the pressure in the air lens will increase and water hammering can occur.
It is possible to adjust the water heating by changing the speed of water circulation if a valve is fitted on the return pipe or by means of changing gas flow direction. Installation of valve on the direct pipe is prohibited as the pressure arising in the closed circuit can break the pipes.
The following means can be used to regulate water heating:
« the bell inside the bell». The coil is installed in the small bell in the upper part of which a damper is arranged to regulate water heating. The small bell is arranged inside the big one.
«in a sleeve». The coil is arranged inside the «sleeve» , in the bottom of which an adjustment damper is installed. The sleeve is installed in the lower bell.
« in the second bell». The coil is installed across the second bell, in the upper part of the partition between the bells. An adjustment damper is also installed.
About 10-12 years ago we built a similar stove in a municipal bath in Polovinny village, not far away from Ekaterinburg. The stones were heated to such a temperature when they are white-hot. This steam sauna heater heated 1-1.5 m3 of water. The people’s references were very positive. A disadvantage of such a stove is its life expectancy, which is not long. The stones during heating and cooling cycles expand and contract. So, they act like wedges destroying the stove slab (arch) and walls. Therefore it is very important to have stones of a round shape to decrease the forces that can destroy the stove. Radiant heat also acts on the stones, therefore for better heat accumulation stones shall be of black colour. In practice, pouring of water sometimes takes place on the stones that are already cooled. High moisture content contributes to washing of clay based mortar off the joints.
All our bath heaters with stones enclosed in a box (steam generator) made of heat-resistant steel are being made according to the similar diagram. The stones are heated to a lower temperature through the wall made of metal. Due to this, various steam generator solutions are used to get a high quality steam. Even when the temperature of stone fill inside the box is lower it's still possible to get high quality steam, such as superheated steam with the temperature close to critical (374o C). It is possible to create and regulate different thermal and moisture conditions in the steam room using one and the same steam sauna heater. These bath heaters last long.
It should be pointed out that fig.3, shows a generic diagram. In practice, stoves’ design can vary comprising of many different specific solutions. When designing a steam sauna heater, one should follow the recommendations given in the article «The Basics for the Stove Construction»
24/12/2003 © Igor Kuznetsov "Kuznetsov's