Factors Affecting the Efficiency of Heating Stoves
Abstract Generated abstract
This paper reports an experimental heat-engineering investigation of a high-heat-capacity channel-and-chamber heating stove with lower heating and partial recirculation of combustion products. Using controlled firings with hard coal, the study measured internal and external temperatures, gas composition, draft, air inflow, material and heat balances, surface heat fluxes, and convective and radiative heat-transfer coefficients over a daily operating cycle. The tests found low outgoing-gas temperature, an excess-air coefficient of about 1.37, and an efficiency of about 73 percent under the longer firing regime, while shortening the firing period increased unburned coke losses and reduced efficiency. The results identify firing duration, charge size, combustion intensity, and recirculation-related design features as important factors governing stove performance.
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Factors Affecting the Efficiency of Heating Stoves
N. N. Norkin
In connection with the questions raised by the research workers of the Moscow Institute of Municipal Construction Engineers, I. F. Livchak and S. N. Shorin (“Increasing the Economy of Heating Stoves,” journal For Fuel Economy, No. 4, 1946), it seems expedient to present the results of a detailed investigation of a high-heat-capacity heating stove with recirculation of part of the combustion products. In the laboratory of the Department of Apparatus of the Tomsk Polytechnic Institute, on assignment from the administration of the Tomsk Railway, an experimental stove of the system of master stove-builder G. I. Budakov was built; it is a modification of the stove of the All-Union Heat Engineering Institute (author Kovalevsky). Since the stove we investigated was intended for mass distribution, the administration of the Tomsk Railway considered it necessary to carry out a detailed heat-engineering investigation; for this purpose an experimental stove was built in the laboratory of the Department of Apparatus, the brief results of the investigation of which are published below.
The investigation was carried out with the participation of fifth-year students: L. Korchinsky, V. Biryukov, and A. Pashin.
I. Method of Measurement
The heating stove investigated is a channel-and-chamber stove with partial recirculation of the combustion products into the firebox, and with lower heating (see figure). The path of the gases in the stove is as follows: the combustion products rise through the combustion chamber up to the crown of the furnace and then, through the upper outlet opening, enter the rear chamber (afterburning chamber). In the afterburning chamber the gases descend. On their way downward, part of the gases returns to the firebox (recirculation), while the greater part of them divides at the bottom into two streams, which enter the side heating channels. In the side heating channels the gases rise upward, and the two streams then join in the upper smoke chamber, from which they are led off into the chimney. The stove was built in a room of about \(400 \, \text{m}^3\) capacity; the room temperature during the test was approximately \(10\text{–}11^\circ\text{C}\). During the experiments the following measurements were made: a) the temperature of the external surfaces of the stove in three zones; the number of points was 12, and the temperature was measured with mercury thermometers; b) the temperature inside the stove: at the bottom of the combustion chamber with a platinum–platinum-rhodium thermocouple, at the top of the combustion chamber with a graphite thermometer, in the afterburning chamber, in the side heating channels, and in the upper smoke chamber with nickel–nickel-chromium thermocouples; before the chimney with an iron-constantan thermocouple. The locations of the points for installing the instruments for temperature measurement are shown in the figure.
Analyses of the combustion products were carried out with a Horpse apparatus from the following sampling points: No. 1—the top of the combustion chamber; No. 2—the afterburning chamber; No. 3—the side heating flue; No. 4—the side heating flue; No. 5—at the outlet into the smoke stack. The locations of the sampling points for gas analysis are shown in the figure. At these same points
Section I–I
Section III–III
Section II–II
Fig. No. 1. General view and sections of the heating stove. Location of control and measuring instruments
Points 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12—mercury thermometers for measuring the temperature of the external surfaces of the stove.
Points A, B, C, D, E, F—thermocouples for measuring the temperature inside the stove.
Points I, II, III, IV, V—gas sampling points for analysis.
the pressure—draft—was measured, as well as in the ash pit. The rate of air inflow through the ash-pit door was determined with an anemometer. The humidity of the surrounding air was determined with a psychrometer. During the stove test, the following firing regime was maintained: wood chips in an amount of 2–2.5 kg were ignited; then, at first after 20–25 minutes, coal was thrown in with a scoop in small portions; and, finally, the last single charge was 7–8 kg. Firing continued for 2 hours 50 minutes. In all, about 16 kg of coal was charged during the firing period. Then the firebox and ash-pit doors were hermetically closed. Twenty-four hours after the cessation of heating, the stove was again fired according to the indicated regime, and so on.
The fuel used was Anzherka hard coal; its proximate analysis was: \(W = 1.67\%\); \(V^{r}\) (volatile matter)—11.5%, and \(A^{p}\) (ash)—9.6%. The ultimate composition
coal, converted to working fuel: \(C^p=80.9\%\); \(H^p=3.82\%\); \(N^p=1.63\%\), \(O^p=1.95\%\); \(S^p=0.45\%\); \(W^p=1.67\%\) and \(A^p=9.58\%\). The calorific value of the coal is \(Q_n^p=7580\) kcal/kg.
II. Material and Heat Balances of Combustion. Combustion Intensity
As a result of threefold tests, the following material and heat balances of combustion were established:
A. Material Balance
| Input | Amount | Output | Amount |
|---|---|---|---|
| 1. Wood chips | 2.5 kg | 1. Combustion products | 209.5 kg |
| 2. Coal | 16 kg | 2. Unburned coke | 2.485 kg |
| 3. Air | 196 kg | 3. Ash and slag | 2.232 kg |
| Total | 214.5 kg | Total | 214.217 kg |
B. Heat Balance
| Received | Amount |
|---|---|
| 1. Heat of combustion of wood chips | 8250 kcal |
| 2. Heat of combustion of coal | 121200 kcal |
| Total | 129450 kcal |
| Heat delivered / losses | Amount | Percent |
|---|---|---|
| 1. Heat loss with the outgoing gases at their temperature of \(130^\circ\mathrm{C}\) | 6530 kcal | 5.05% |
| 2. Losses from chemical incomplete combustion | 8060 kcal | 6.23% |
| 3. Losses in coke | 17900 kcal | 13.82% |
| 4. Losses in slag and ash | 3350 kcal | 2.58% |
| 5. Heat accumulated by the stove (by difference) | 93610 kcal | 72.32% |
| Total | 129450 kcal | 100% |
Considering the heat accumulated by the stove as usefully utilized, and the heat of the unburned coke as lost, the efficiency of the stove is expressed by the value 73.32%. However, part of the unburned coke can be used during the subsequent firing of the stove, especially since the coke obtained is in large lumps, as a result of which the efficiency increases. Of the heat not utilized in the stove, the greater share is due to loss from chemical incomplete combustion, which is caused by the considerable CO content in the exhaust gases; in compiling the heat balance the average composition of the exhaust gases was taken as: \(\mathrm{CO}_2=13.3\%\); \(\mathrm{O}_2=4.0\%\) and \(\mathrm{CO}=1.8\%\), and the excess-air coefficient \(\alpha=1.37\).
The main dimensions of the firebox are \(27\ \mathrm{cm}\times 41\ \mathrm{cm}\times 165\ \mathrm{cm}\). The volume of the firebox (together with the combustion chamber) is \(0.182\ \mathrm{m}^3\). The grate is \(0.27\cdot 0.29=0.0783\ \mathrm{m}^2\). The grate intensity, with a coal consumption of 16 kg and a firing duration of 2 h 50 min, was \(62\ \frac{\mathrm{kg}}{\mathrm{m}^2\cdot\mathrm{h}}\).
The thermal intensity of the volume of the firebox with the combustion chamber is \(250{,}000\ \frac{\mathrm{kcal}}{\mathrm{m}^3\cdot\mathrm{h}}\).
It should be noted that when the firing duration was reduced to 1 h 40 min (instead of 2 h 50 min), with the same fuel charge, the residue of unburned coke amounted to 3.78 kg (instead of 2.485 kg), as a result of which the heat losses in coke were 21% (instead of 13.82%); the efficiency of the stove consequently decreased to 63.2% instead of 73.32%.
III. Analysis of combustion products, temperature distribution in the stove, and draft
During the firing period of the stove (the first 2.5–3 hours), gas analysis, carried out after 30 minutes, showed the following results, the average data of which are given in Table 1.
Table 1. Gas analysis during the firing period of the stove
| Name | Top of combustion chamber No. 1 | Afterburning chamber No. 2 | Right side channel No. 3 | Left side channel No. 4 | Outlet to chimney No. 5 |
|---|---|---|---|---|---|
| CO₂ | 10.09 | 12.30 | 9.75 | 9.50 | 11.20 |
| O₂ | 4.74 | 2.90 | 3.20 | 3.4 | 5.00 |
| CO | 4.0 | 4.40 | 3.70 | 3.8 | 1.8 |
Table 2. Gas analysis 7 hours after the cessation of firing
| Name | Top of combustion chamber No. 1 | Afterburning chamber No. 2 | Right side channel No. 3 | Left side channel No. 4 | Outlet to chimney |
|---|---|---|---|---|---|
| CO₂ | 0.8 | 0.2 | 0.2 | 0.3 | 0.2 |
| O₂ | 2.0 | 1.16 | 8.0 | 8.5 | 19.3 |
| CO | 9.2 | 4.7 | 4.7 | 4.6 | 0.1 |
The temperature inside the stove during the firing period is distributed as follows.
- Above the fuel layer . . . . . . . . . . 1000°C
- At the bottom of the combustion chamber . . . . . . 725°C
- At the top of the combustion chamber . . . . . . 456°C
- In the afterburning chamber . . . . . . . . 392°C
- Temperature at the entrance to the chimney . . 130°C
Draft in the stove according to measurement results:
- Ash pit . . . . . . . . . . 3.74 mm water column
- Combustion chamber (firebox) . . 1.27 mm water column
- Afterburning chamber . . . . . 2.76 mm water column
- Side channels . . . . . . . 2.25 mm water column
- At the entrance to the chimney . . 1.95 mm water column
From the results of measuring temperature and draft, it was additionally calculated: the total resistance of the stove is 5.82 mm water column; the intrinsic hydrostatic potential is 2.1261 mm water column; consequently, the remaining resistance of 3.6939 mm water column is covered by the draft created by the chimney.
IV. Temperature and heat transfer of the external surfaces of the stove walls
For the stove under investigation, during the test period a daily regime was established, i.e., the stove was fired every other day. During the day, temperature measurements were made every hour in 3 belts on each of the 4 surfaces of the stove. Table 3 gives the results of the measurements.
Table 3. Measurement of the temperature of the stove’s exterior surfaces
| Time | Air temp. in the room | Front wall, °C | Rear wall, °C | Side walls, °C |
|---|---|---|---|---|
| 6—00 | 12.38 | 40.0 | 46.2 | 35.8 |
| 7—00 | 12.55 | 43.25 | 47.0 | 41.4 |
| 8—00 | 12.88 | 44.5 | 48.4 | 42.0 |
| 9—00 | 12.90 | 48.75 | 59.0 | 46.0 |
| 10—00 | 12.90 | 48.6 | 66.5 | 50.3 |
| 11—00 | 12.8 | 52.0 | 77.7 | 55.0 |
| 12—00 | 12.80 | 55.5 | 80.7 | 57.6 |
| 1—00 | 12.8 | 58.15 | 82.7 | 60.00 |
| 2—00 | 12.8 | 60.50 | 83.0 | 61.4 |
| 3—00 | 12.8 | 61.35 | 82.00 | 62.0 |
| 4—00 | 12.8 | 62.00 | 80.07 | 62.30 |
| 5—00 | 12.90 | 62.05 | 78.5 | 61.70 |
| 6—00 | 12.8 | 61.75 | 75.8 | 61.5 |
| 7—00 | 12.50 | 60.75 | 74.3 | 60.0 |
| 8—00 | 12.45 | 60.1 | 71.0 | 58.5 |
| 9—00 | 12.50 | 58.15 | 67.8 | 55.5 |
| 10—00 | 12.45 | 56.85 | 65.5 | 55.4 |
| 11—00 | 11.70 | 54.75 | 63.2 | 53.3 |
| 12—00 | 12.50 | 53.3 | 61.0 | 51.8 |
| 1—00 | 12.5 | 52.25 | 59.4 | 50.7 |
| 2—00 | 12.5 | 50.4 | 56.5 | 48.7 |
| 3—00 | 12.5 | 48.7 | 54.75 | 47.4 |
| 4—00 | 12.5 | 47.35 | 52.8 | 45.8 |
| 5—00 | 12.5 | 45.9 | 51.1 | 44.5 |
| 6—00 | 12.5 | 44.4 | 49.2 | 43.0 |
| average | 12.6 | 53.0 | 65.5 | 52.5 |
Note: 1) the air temperature in the heated room was measured at 4 points at different distances from the stove; 2) the average temperature of the walls was determined as the arithmetic mean of the temperatures in 3 zones.
Analyzing the tabulated data, we find that the greatest heating occurs in the rear wall of the stove (the exterior surface of the rear flue channel): namely, the average temperature of the rear-wall surface is 65.5°C, with the maximum temperature reaching 83°C 6 hours after firing has ceased. The front wall (the exterior surface of the firebox) is heated on average to 53°C, with a maximum temperature of 62°C 8 hours after firing has ceased. The side walls (the exterior surfaces of the last side flue channels) are heated on average to 52.5°C, with a maximum temperature of 62.3°C 7 hours after firing has ceased.
Under the regular daily operating regime of the stove, as a result of the established heat flow, the total amount of heat given off by the stove to the room is, from the heat balance, equal to 93,610 kcal. The exterior heat-emitting surface of the stove walls is 11.3 m²; then the magnitude of the heat flow according to the heat balance is expressed as:
\[ \frac{93610}{11.3 \cdot 24}=346 \ \frac{\text{kcal}}{\text{m}^2\text{hour}} \]
The temperature-measurement data given in Table 3 made it possible to calculate the magnitudes of the heat flows over the entire period of the daily regime.
Without presenting here the tabulated data for all the thermal calculations, we shall indicate only the minimum and maximum values.
At the beginning of firing the stove and at the end of cooling (the first and last hours of the daily cycle), the heat output is minimal and amounts to:
\[ \begin{array}{lll} \text{for the front wall} & Q \text{ minimum} & 266\ \dfrac{\text{kcal}}{\text{m}^2\text{h}},\\ \text{for the rear wall} & \text{” ”} & 338\ \text{”},\\ \text{for the side walls} & \text{” ”} & 273\ \text{”}. \end{array} \]
The maximum heat output occurs 9 hours after firing for the rear wall, 11 hours later for the front wall, and 10 hours later for the side walls, and is characterized by the following values:
\[ \begin{array}{lll} \text{for the front wall} & Q \text{ maximum} & 577\ \dfrac{\text{kcal}}{\text{m}^2\text{h}},\\ \text{for the rear wall} & \text{”} & 846\ \text{”},\\ \text{for the side walls} & \text{”} & 660\ \text{”}. \end{array} \]
The coefficients of heat transfer by convection and by radiation were also calculated for each hour of the daily cycle.
Table 4 gives the minimum, maximum, and mean values of the heat-transfer coefficients.
Table 4
Heat-transfer coefficients from the outer surface of the stove walls
| Name | Front wall | Rear wall | Side walls |
|---|---|---|---|
| Heat-transfer coefficient by convection $\alpha \dfrac{\text{kcal}}{\text{m}^2\text{h deg}}$ — minimum | 5.05 | 5.30 | 4.34 |
| Heat-transfer coefficient by convection $\alpha \dfrac{\text{kcal}}{\text{m}^2\text{h deg}}$ — maximum | 5.82 | 6.37 | 5.35 |
| Heat-transfer coefficient by convection $\alpha \dfrac{\text{kcal}}{\text{m}^2\text{h deg}}$ — mean of 24 determinations | 5.58 | 5.98 | 5.10 |
| Heat-transfer coefficient by radiation $\tilde{\alpha}_{l}\dfrac{\text{kcal}}{\text{m}^2\text{h deg}}$ — minimum | 4.60 | 4.70 | 4.34 |
| Heat-transfer coefficient by radiation $\tilde{\alpha}_{l}\dfrac{\text{kcal}}{\text{m}^2\text{h deg}}$ — maximum | 5.46 | 5.70 | 5.45 |
| Heat-transfer coefficient by radiation $\tilde{\alpha}_{l}\dfrac{\text{kcal}}{\text{m}^2\text{h deg}}$ — mean of 24 determinations | 5.00 | 5.07 | 4.93 |
Conclusions
The investigation carried out made it possible to establish:
-
The design features of the stove, such as bottom heating and partial recirculation, provide good performance indicators for the stove: a relatively small amount of chemical incompleteness of combustion, a low temperature of the outgoing gases, and a comparatively high efficiency of the stove.
-
Observations during the investigation of the stove showed: a) the combustion of hard coal can be carried out with an amount of air corresponding to an excess-air coefficient not exceeding 1.4; b) each opening of the firebox door to load a new portion of coal entails a decrease of the temperature in the firebox by 200–250° C; however, this has to be tolerated in the initial period after firing; the latter
a single charge may be allowed in the amount of 7–8 kg. An increase in the weight of the last single charge entails an increase in the chemical incompleteness of combustion.
- The measurements and the calculations performed on their basis revealed: a) the optimal firing time of the stove, affecting the coke residue; b) the heating-up and cooling-down time of the stove; c) the optimal values of the weight and thermal intensity of combustion; d) the magnitudes of the heat fluxes; e) the calculated heat-transfer coefficients.