JPS5929763B2 - Smoke tube boiler with a smooth tube section in the run-up section - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a smoke tube boiler that uses a spiral smoke tube on the heat transfer surface of the tube group, by limiting a specific distance of the spiral smoke tube on the side where fire or high temperature gas flows into the tube group. By leaving the tube as a smooth tube with no spiral grooves interposed in between, it is possible to prevent an excessive increase in the local heat transfer coefficient in the run-up section due to the flow of fire or high-temperature gas, and to reduce the flow resistance of gas in the tube. It is characterized by:

Conventionally, in smoke tube boilers that use a spiral flue tube group,
The smooth tube section on the side where fire or high-temperature gas flows into the tube group is the length of the smooth tube necessary to insert and expand the smoke tube into the tube sheet, and it is customary to use a length of about 1 to 2 times the inner diameter of the tube. The distance of the smooth pipe from the top of the machine was determined as → without considering excessive heat transfer coefficient, etc.

Therefore, the present invention is intended not only for cleaning work, but also for reducing the increase in tube wall temperature due to the increase in excessive heat transfer coefficient and the flow resistance of gas within the tube group.

In other words, on the fire or high-temperature gas inflow side of the spiral smoke tubes that are stacked in a row in a fire tube boiler, the smoke tubes are inserted and fixed in a tube style with many holes in the flat tube plate, so the smoke tube inlet end is not expanded. Even if the smoke pipe is bell-mouthed, there is a flat plate between the pipes, so the large cross-section becomes suddenly reduced, causing a vortex (Karman
There is a point where the heat transfer coefficient is highest at a point slightly inward from the entrance of the smoke pipe where vortex) occurs, but this is due to the separation of the gas film due to the effects of vortices and swirling currents.In particular, Experiments have shown that the temperature of the pipe wall where the smoke pipe and the tube plate join is much higher than that of the part where the other side is in contact with the boiler water, which causes a localized heat storage or heat insulation effect. has also been revealed.

A more specific explanation will be given based on these four equations and experimental results.

For example, as shown in Figure 3, q/F is the heat transfer surface heat load (
k cal l/m”h input λ is the thermal conductivity of the smoke pipe (kc
al 7m h 'C), Δt is the temperature difference between the inner and outer surfaces of the pipe wall ("Q, 8 is the thickness of the pipe wall in m), then q/F: λΦΔt/s...kcal//W?h'
It is expressed as q Δt=−・S/λ° C. Since S and λ are constant values, there is a relationship of −F with Δt.

In other words, as q/F increases, Δt also increases, and α is expressed as the heat transfer coefficient on the water side (kcal 7m" h
'C), if F is the heat transfer area -1Δtm and the temperature of the water side heat transfer surface surface temperature and the boiler water temperature is ℃, then q -2 α・Δt m ・−・kcal/lt? h, so if F and Δj771 are constant,
If α increases due to an increase in the local heat transfer coefficient, q/F will also increase.

Next, the experimental results will be explained.As mentioned above, the features of the present invention are to prevent excessive increase in the local heat transfer coefficient in the run-up section due to the flow of fire or high-temperature gas, and to reduce the flow resistance of gas in the pipe. There is something to do.

Traditionally, the smoke tubes that make up smoke tube boilers are often smooth tubes with no protrusions on the inner surface or spiral tubes with raised surfaces on the inner surface. It has its own advantages and disadvantages.

The advantages and disadvantages are: (1) In the case of a spiral tube, the heat transfer coefficient is high, but the pressure loss is large.

■In the case of a smooth tube, the pressure loss is small, but the heat transfer coefficient is low.

These are contradictory things.

This is also clear from the experimental results shown in Figures 4-5 conducted by the inventors.

One of the challenges was how to harmonize these strengths and weaknesses.

Therefore, it was thought that by providing a smooth pipe portion at an appropriate location of a spiral smoke pipe, the above advantages and disadvantages could be eliminated, and this is a feature of the present invention.

In that case, simply installing a smooth tube at an appropriate location of the spiral smoke tube will have a satisfactory effect of reducing flow resistance, but it will not be able to sufficiently obtain the effect of preventing an excessive increase in the local heat transfer coefficient. do not have.

That is, as can be seen from FIG. 4, the heat transfer coefficient of the spiral smoke tube is about twice as high as that of the smooth tube.

However, this is the average heat transfer coefficient over the entire length of the smoke pipe, and because there is a flat plate between the pipes at the inlet end of the fire or high-temperature gas, the large cross section becomes rapidly reduced, causing a vortex (Karman vortex) to form around the smoke pipe entrance. ) occurs, and a point where the heat transfer coefficient is highest is created a little further inside than the entrance of the smoke pipe, and the heat transfer coefficient becomes about 2 to 3 times as high.

Therefore, overall, the local heat transfer coefficient in this local area is about 4 to 6 times that of the smooth pipe, and when this happens, the bonding force at the joint between the smoke pipe and the tube plate decreases.
Do joints loosen due to repeated stress caused by thermal expansion of the smoke pipe and expansion and contraction of the pipe due to cooling? The boiler's internal pressure water finally begins to leak.

In order to correct this problem, the smooth pipe section was set as described above from the end where the flame or hot gas flows in.

4 and 5 are shown under the same conditions, that is, the inner diameter and length of the tube are made equal, and the smooth tube portion 1d is shown.

10d and 20d.

As is clear from the figures, the desired effect has been achieved by providing the smooth pipe section, but from a practical point of view, the 3D
The range is limited to 10d to 10d as appropriate.

By doing this, a smooth pipe portion exists at the inflow end, and the local heat transfer coefficient remains unchanged from the conventional inflow end.
Also, the average heat transfer coefficient does not lose its characteristics as a spiral tube.

Furthermore, the above is the result of an experiment where the length of the smooth tube part was set to 3d, 10d, and 20d under the condition that the inner diameter and length of the pipe were constant, but when the inner diameter and length of the pipe were changed, the total length of the flue pipe was A similar effect was observed in 4 to 12% of cases.

Hereinafter, illustrated embodiments of the present invention will be described in detail.

FIG. 1 is an enlarged sectional view of the main part of the present invention.
On the flame or high-temperature gas inflow side 2 of the spiral smoke tube 1, a smooth tube distance l of 3 to 10 times the tube inner diameter d is taken, and it is inserted and fixed into the tube plate 4, 4' at the front end 3 of the spiral smoke tube 1, using a roller construction method, etc. By setting the smooth pipe distance l to 4 to 12% of the total length of the helical flue pipe, the local heat transfer coefficient at the pipe inlet side and the prevention of excessively high temperatures at the junction of the flue pipe and tube plate and other parts of the pipe wall are determined. At the same time, it is possible to reduce the flow friction loss due to the flowing gas in the pipe.

FIG. 2 shows an example of a furnace-tube boiler to which the present invention is applied, in which tube sheets 4 and 4' are fixed at both ends of the boiler body 50, and a spiral smoke tube group 1 is inserted so as to pass through both tube sheets. In addition to being inserted and fixed, they are distributed as retaining pipes (not shown) in the spiral smoke pipe group 1.

Further, a furnace cylinder 6 is attached between both tube sheets.

Fig. 3 shows the relationship between heat transfer on the heat transfer surface, and Figs. 4 and 5 show a case consisting of a helical tube and a smooth tube that are entirely spiral, and a case where the helical tube part is made up of a smooth tube. 3d, 10d of the smooth tube part when provided.

The relationship between the gas flow rate, heat transfer coefficient, and pressure drop when the gas flow rate is changed to 20 d is shown in FIG.

Note that the present invention is not limited to furnace-tube boilers, but is applicable to various types of smoke-tube boilers.

As detailed above, the present invention has been used in the past for a long time, and moreover, the problem of boiler internal pressure water leakage from the end of the boiler pipe that occurs periodically has been a problem that has been used, and the present invention has been used to prevent fire or high-temperature gas from being overheated without taking measures. This is extremely useful industrially because it can reduce the excessively high heat transfer coefficient of the run-up section on the inlet side, the excessively high temperature of the tube wall, and the friction loss of the gas flow inside the tube.

Note that the term "excessively high temperature" herein refers to a temperature of around 350°C or higher compared to the steady tube i temperature of 200 to 250°C.

In addition, regarding the change in local heat transfer coefficient and the run-up section,
There are experiments in which A, F, and mills are used when the fluid is air, and the inlet is arranged according to the Nusselt number ratio. Based on the fact that the inlet shape is a combination of a tube plate and a smoke pipe, there is a vortex flow near the inlet.
), and as mentioned above, adjust the inner diameter of the smoke pipe from 3 to 1.
The smooth tube distance is set to be 0 times, and A, F
, not based on Mill's experimental values.

Further, since the combustion gas passage in a fire tube boiler usually has a single circuit to four circuits, the present invention is applied to the inlet of the fire, high temperature gas, or combustion gas of these smoke tube groups.

Further, within the scope of the present invention, the smooth pipe distance and the ratio to the total length of the smoke pipe can be changed depending on the temperature near the inlet of the flame, high temperature gas, or combustion gas, or the number of Ray nozzles.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional side view of the present invention, Fig. 2 is a vertical cross-sectional view showing the → direction of a smoke tube boiler to which the present invention is applied, and Fig. 3 is an explanatory diagram showing the relationship of heat transfer in the heat transfer surface. FIGS. 4 and 5 are explanatory diagrams showing experimental results. 1... Spiral smoke tube, 2... High temperature gas inflow side, 3... Front end of spiral smoke tube, 4... Tube plate, 4/,... Signboard, 5... one boiler body,
6...Furnace tube, d...Bone inner diameter, l...
・Smooth pipe distance, L...Full length of spiral smoke pipe.

Claims (1)

[Claims] 1. In a fire tube boiler equipped with a group of spiral smoke tubes, a length between 3 to 10 times the inner diameter of the tube and 4 to 12% of the total length of the smoke tube between the inlet end of the fire or high temperature gas of each spiral smoke tube and the first spiral groove. 1. A smoke tube boiler comprising a spiral smoke tube made of a smooth tube with the smooth tube portion serving as a run-up section for fire or high-temperature gas.