Design of multistory stoves
Design of multistory stoves.
At present, Russia is experiencing a rise in residential construction. Many of the newly built houses are multistory buildings. Due to unavailability of centralized municipal services such as natural gas lines and hot water lines in rural areas (municipally centralized hydronic heating systems are common in Russia’s large cities), wood-burning stoves are often the only heating source for these houses. Therefore, there’s a demand for efficient multistory stoves. Such stoves have to be of high heating capacity. Plus, it should be made possible for stoves on each story to serve different purposes independently from others.
Prior to creating something new, it is necessary to study what was developed in the field before. This was accomplished with the help of monograph of S. M. Mirkis “Index of Masonry Stove and Fireplace designs published in Russia during the past 100 years”. The monograph was kindly provided by the author. It is being prepared to be published this year or the next year.
At present, two types of multistory stoves are used for this purpose: stoves with a single firebox common to all stories, fig.76; and stoves with a separate firebox on each floor.
A great number of projects of two-story stoves with firebox on each floor have been developed in the past. These projects were mainly for heating and heating-cooking stoves. All of them were designed in accordance with the common diagram, fig.1 comprising:
1- firebox, 2- supply channel, 3-stove convective system (part of the stove where the main heat exchange occurs i.e. system of channels and chambers), 4- chimney. The stove convective system, 3 can be of any type of those shown in figure 45, and can also have multilevel chambers.
Fig.45 Stove convective systems sequential, b) parallel, c) channel-free (bell-type), d) combination, e) with air chamber, single change in direction of the gas flow, 2) double change in direction of the gas flow, 3) multiple change in direction of the gas flow, 4) air chamber
It should be noted that stoves of each story are practically separate stoves provided with their own chimney, built one on top of another. Here, chimney of the lower stove occupies part of the volume of the upper stove. If triple-story stoves are constructed, this volume can be quite considerable.
Multistory heating stoves with a single firebox were designed by L.P.Trigler, V.E. Grum-Grzhimailo and I.S. Podgorodnikov, V. P. Protopopov, I. I. Kovalevsky, N. F. Volkov and other authors. All their designs follow schemes of the multistory stove heating systems from the book:“ Stove Heating of Low-rise Buildings” Moscow, Vysshaya Shkola, 1991. You will find the diagrams with their descriptions below.
Fig. 76 Multistory stove heating systems single channel type, б, в- double channel type, г- triple channel type, firebox, 2. 5 - channels, 3 – stove cores with a system of channels, 4 - chimney Basic design elements of multilevel stoves with a common firebox (fig.76): Firebox 1, located in the basement or on the ground floor of the building; supply channel 2; system of channels 3 and chimney 4. In some cases (г) an additional collecting channel 5 is installed to receive gases effluent from each level. The systems are subdivided into single channel (а), double channel (б, в) and triple channel (г).
Two-story stoves with one firebox can be built with two separate channels outgoing from the firebox. In this case, gases are directed into heat exchanging units of each level separately.
A regulating brick (damper) is installed in the flue above the exit from the stove of the second story to regulate the gas flow volume that comes into the stoves of each story. The drawings of such stove, designed by V. E. Grum – Grzhimailo and I.S. Podgorodnikov are provided in the same book.
What properties such stoves should possess? What are the requirements for the stoves?
A concept of a good stove is given in the article “ The Basics for the Stove Construction”: http://www.stove.ru/new/index.php?lng=0&rs=15 .
A good stove retains and accumulates maximum heat from burning, meets functional requirements, and ensures optimum heat exchange. Design of such stove should be flexible enough to meet client’s and architect’s requirements while maintaining excellent performance. Here is an example of such requirements:
- Heating zone of each floor may have an area or volume of any kind, whereas each floor plan may considerably differ one from another;
- each heating zone may require different thermal and moisture conditions;
- each heating zone may require a functionally different stove or a stove combined with a fireplace;
- a possibility to regulate stove heat transfer (a possibility to adjust thermal conditions on each floor) shall be provided;
- a back-up heating option shall be provided for cases when the house is left unattended for a long period of time.
Existing typical stoves cannot meet these requirements. The majority of the stoves types shown above, have a convective system with forced gas movement or a single bell. These types of stoves have some disadvantages, which I have described in my articles “ Once again about the system” http://stove.ru/new/index.php?Ing=0&rs=16 and “ The Basics for the Stove Construction”: http://www.stove.ru/new/index.php?lng=0&rs=15
It should be pointed out that although standard projects of two-story stoves with a separate fire-box on each floor ПТД-2800/2600,ПТД-3700/3000, ПТД-4400/3500, ПТД –5400/5000 are ranked double-level, channel-free stoves, they cannot be referred to as double bell stoves. They don’t posses “automated damper” feature (more on the feature in the article “Once more… www. stove.ru/ …), and do not correspond to the basic characteristic of bell-type stoves expressed by the following formula (from my patent), “the stove’s lower level and the fire-box are combined to form a single space creating a lower bell” . Besides, these are stoves are heaters only, and not capable of any other functions.
Fig.2 We have acquired a substantial experience in design and construction of multistory stoves with a separate fire-box on each floor. Hundreds of two or three- story stoves operating on one chimney were built. A number of four-story stoves were built as well. The scheme of our stoves is shown in fig.2. The designation is s follows: 5- fireplace. All the rest – as shown in fig.1.
The stoves of each floor have a double bell scheme (except stoves without the bottom bell). The stoves are built in two versions: 1. stacked one over the other; 2. on reinforced concrete slabs with a layout around vertical chimney. Size of the chimney flue is increased when other stoves located above are connected to this flue. It should be noted that in accordance with the Russian Building Codes, each stove, as a rule, has to have a separate chimney. This requirement is valid for stoves of any other systems, because two or three-story stoves of other systems cannot be functioning on one chimney due to a high resistance to outcoming gases. In stoves built according to our system, resistance to gas flow is minimal that allows successful connection of several stoves to one chimney flue.
Stoves of each floor may be of any size and heat output, may serve different purposes (such as heating, cooking, baking alone or in any combination), may have a built-in fireplace; can use electricity as a back-up heat source. All stoves can operate simultaneously or separately in any combination.
The only rule shall be observed: the flue dampers of non-functioning stoves shall be closed. Our stoves possess a number of unique features, which are not inherent to stoves of other systems.
These features are as follows:
1. Bells may have any form and volume.
2. Heat energy is transferred due to a natural power (gravitation).
3. Turbulent gas movement takes place inside a bell.
4. The hottest gases accumulate at the top of a bell.
5. The coldest gases, being the heaviest ones, accumulate at the bottom of a bell.
6. Excessive pressure (overpressure) is being formed inside a bell with the temperature increase.
7. Walls of a bell are evenly heated in each horizontal cross section, and the heat increases in each cross section that is higher.
8. Heat energy source can be located in any place within the lower zone of the bell. Regardless of the location of the source, character of the heating process remains the same.
9. Several heat sources can be used.
10. Vertical placement of consecutive bells (one upon another) guarantees that every horizontal cross-section of the system is heated evenly. The lower bell will always absorb more heat than the upper one.
11. With horizontal placement of consecutive bells, each horizontal cross-section of the system will be heated unevenly. The first bell will accept more heat than the next one.
12. In stoves with a forced gas movement, failure to close a shut-off damper immediately after the burnig cycle is over leads to a significant heat loss. In the similar situation in our stoves the heat loss is insignificant ( automated damper effect).
13. With increase in length of burning cycles the efficiency ratio is not reduced because the heat excess will be taken up by the top bell.
The operation of multistory stoves with one firebox differs in principle from operation of multistory stoves with a firebox on each floor. According to the character of operation, multistory systems with one firebox are very similar to thermosyphon water heating systems, with the only exception, that the heat -carrying agent doesn’t return to the heat generator (firebox). No fuel burning takes place in convective system of multistory stoves with one firebox. The convective system serves to receive the heat of hot gases from the firebox and its useful application.
Such convective system should:
1. possess a minimum resistance to outcoming gases;
2. possess a good heat-accumulating and an optimal heat transfer capability (bottom heating character);
3. allow stoves on different stories to be of different output, size and purpose;
4. possess a possibility to regulate the thermal conditions on each story;
5. possess a possibility of heating the house when leaving it unattended for a long time. (back-up heating)
The three- channel system (fig. 76 г) does not meet “ the requirements for convective system’’ and is not considered.
Let’s view the schemes of multilevel stove heating system shown in fig.76, in fig.3 and 4. Firebox in all solutions is located on the lower floor. There is no convection system at the lower floor and the area is heated by the walls of the firebox itself.
Single channel system (fig 76, a) is a system where gases go straight up the central channel. Convective system of each floor is arranged as a shell around the distribution supply channel. Regulation of the gas flow distribution is achieved by partially closing dampers, which are located in the supply channel above the inlet openings to the convective system of each floor. The system does not meet “ requirements for convective system” number 2,3,5. The adjustment of the thermal conditions by means of partial closing of the supply channel is ineffective. Practically, it is not possible to regulate the heat transfer on all the floors by only changing the size of the exhaust channel in multistory stoves with straight vertical supply channel.
Double channel systems (fig. 76, б, в) and fig 3. In these stoves, gas flow channels on each level are equivalent in size that affects the heat transfer in a positive way. Two forces are influencing the gas flow movement: overpressure in the supply distribution channel, and a chimney draft. The nature of these forces is the same. To compare these designs let’s suppose that the heat energy source is electric heating element. In this case, there are no products of combustion, and therefore, no chimney is needed. In this case, in the diagram fig. 76, в and in fig.3 heat energy transfer will happen not under the influence of external energy (chimney draft) but under the influence of the gravity force, even if the chimney will be closed at the top. On the diagram 76 в, heat energy is brought up to the top zone of the convective system. It is known that liquids and gases shall be heated from the bottom. Therefore the heat energy echange will take place only in the supply distribution channel, and there will be no exchange in the convective system. The whole system will be functioning only due to chimney draft. Comparing the diagrams 76 в and figure 3 in regard to heat accumulation and heat transfer, the diagram in fig.76 в has an advantage over the diagram in fig.3, that has small heat accumulating and heat-release features. Analyzing the above said, it is possible to say that the diagram shown in fig.76 в is the best suitable for application in multistory stoves with single fireboxes. However, this diagram has a significant drawback: the top heating character of each floor. This leads to insufficient heating of the lower part of the room, where a “pit” of cold air is formed.
If bells are created in horizontal channels in the diagram of fig.3 within sections a-б, we will get a diagram shown on fig.4. The stove of each floor in this diagram could be classified as a ” double bell” system, as each of the stoves possesses the “ automated damper” feature. This multistory stove design meets the “requirements for convective system” , a multistory stove of this design:
- Has a minimum resistance to outcoming gases;
- Hasa good heat-accumulating and an optimal heat-release capability (bottom heat character);
- Allows stoves on each story to be of different output, size and purpose;
- Allows stoves on each floor to operate using electricity as a heat source.
By what means can the thermal conditions be regulated? Overpressure is formed in the distribution channel when the temperature rises. Each story is characterized by its own pressure which increases floor to floor. Thus, pressure on the 2-nd floor will be higher than on the 1-st. Its value depends on the height of the channel, gas flow temperature and is proportional to them. As the temperature in the channels increases, the temperature in the chimney flue increases as well. A draft is created in the chimney flue due to the difference in masses of hot and cold air in the chimney volume. The draft’s value depends on the exhaust gases temperature and the chimney height (when all the other conditions remain the same). However, the gas flow parameters are changing differently here if compared with the parameters in the distribution channel. While the hot gases go through the chimney flue, their temperature drops down. The chimney height is also changed (decreased) on each floor. Cumulative force to move the flow on each floor is made up of two constituents: overpressure in distribution channel, and chimney draft. These parameters are changed floor to floor. For example, on the first floor, chimney draft is stronger, but pressure is lower; on the last floor, chimney draft is weaker but the pressure is much higher. The point on each floor where the force is applied is the outlet from the bell in the “single bell” system and the outlet from the second bell in the “double bell” system. If a control valve is installed at this point, the gas flow passing through the system can be redistributed, sending energy in the necessary direction (floor). It is remarkable that the control valve location is natural , optimal and appropriate. The control valve is a damper of each stove on each floor. In diagram 76 б there will be no cumulative force. The points on each floor where the force of overpressure is applied are inlets to convective system of the stoves on each floor. Therefore, there will no temperature increase in the convective system of the stove. Only chimney draft will affect the operation of the system. When the temperature is increased inside the bells, overpressure is formed. Besides the above mentioned, value of this pressure will depend on the size of the outlet channel. The smaller is the opening the higher is the pressure, which will redistribute the gas flow volumes in the direction of smaller resistance.
What properties the firebox ( the heat generator) shall possess? If it is not necessary to heat the room where it is located, it should:
- Have a heat rating sufficient to cover total heat losses of the building;
- Extract maximum possible heat from the fuel;
- Direct the emitted energy in maximum volume to stove’s convective systems on the floors.
A “single bell” well isolated (by thermal resistant mineral isolation) metal stove possesses these characteristics. It also fits the formula “lower deck and the fire-box are connected to form a single space making a lower bell” . In such version, the combustion reaction takes place at the optimal thermal conditions with a high efficiency. The stove’s metal walls absorb heat quickly. The walls’ temperature approaches the temperature inside the firebox and no significant heat transfer as well as heat accumulation taking place, due to a small heat storing capacity of the stove. In common masonry stoves, the burning cycle is usually 1 to 3 hours depending on the type and mass of the stove. With longer burning cycles, the difference between temperature of exhaust gases and temperature of firebox decreases, the heat exchange slows down, and subsequently, the stove efficiency drops down. Therefore, the output of the firebox in multistory stoves with one firebox has to be designed to produce the necessary amount of heat within time of a normal burning cycle.
It is possible for a multistory stove with one firebox to have the firebox built into the stove on the lower floor. A triple-story stove with a single firebox being a part of the stove on the lower floor was designed by professor V.I. Grum-Grzhimailo and professor Podgorodnik (I.S. Podgorodnikov). There are two outlets for exhaust gases from the firebox. The exhaust gases are directed separately into the convective system of the stove on the lower floor and into stoves on the second and third floors. An adjustment brick (damper) is installed above the exit to the stoves of the second and third floors to regulate the volume of the gas flow coming into the stoves.
The stove design is similar to the design of double –story stoves of the same authors described at the beginning of the article and in the book of A. E. Shkolnik ” Stove heating of low-story houses” Moscow, Vysshaya Shkola, 1991. The stoves of the second and the third floors are designed according to the diagram ”single bell”. The drawback of this type of stove is overheated top and a significantly colder stove’s bottom. In order to get rid of this drawback, the stoves of the second and third floors should be designed in accordance with the diagram “double bell”, fig.5. The firebox shall have an output sufficient to cover total heat loss of the building. The firebox is provided with 2 outlets with gas flow adjusting dampers.
One issue shall be pointed out. When the stove is located on the floor within a multideck stove provided with a single fire-box, heating- and cooking stove( stove for the bath or other type requiring a power thermal impact) its cooking properties are reduced. The temperature of stove heating is reduced due to the absence of direct impact of radiant energy of the burning fuel.
Viewing the operation of Russian stoves called Teplushki designed by Podgorodnikov, as well as considering their operation in practice the following shall be pointed out:
The stoves Teplushki 2,4,15 have the exhaust gas outlet directly into the cooking chamber. The fire-box and the cooking chamber are combined into single space. The cooking chamber is heated quite well as a certain impact of radiant energy is revealed;
In teplushka stoves 9,10 the exhaust gases from the fire-box go to the cooking chamber through the channel. There’s no impact of the radiant energy in the cooking chamber, and its heating is not sufficient.
Besides, all these stoves are not provided with the “ gas damper” effect, that also reduces the chamber’s temperature.
Summarizing up the above said we can make the following conclusion: The stove decks requiring a powerful temperature impact shall have an additional fire-box, providing a possibility to bring the functional requirement to the necessary parameters. The stoves shall also correspond to the formula ”the lower part of the stove and the firebox are combined into a single space comprising the lower bell”.
I. V Kuznetsov
24/12/2003 © Igor Kuznetsov "Kuznetsov's stoves"