JPH02126145A - Thermal resistance measuring method - Google Patents

Abstract

PURPOSE:To safely measure the thermal resistance by locally heating the face opposite to the face to which a thermal-resistance material is stuck of a heating wall and comparing the temperature at this time with the temperature which is obtained by giving the same thermal flux at the time of non-sticking of the thermal-resistance material. CONSTITUTION:The outside surface of a heating tube 3 is locally heated by a heating laser 1 until reaching a prescribed temperature. The temperature of the heated surface is measured by a contactless thermometer 2 simultaneously with heating. The surface temperature of the heating tube 3 rises with its own thermal resistance by heating. The temperature rise of the tube surface at the time of sticking of a scale 4 on the internal surface of the tube is greater than that of the absence of the scale 4. That is, the temperature rise is proportional to the extent of thermal resistance. Consequently, the difference between the extent of temperature rise of the surface of the heating tube 3 at the time of applying a certain quantity of thermal flux and that for non-sticking of the scale is obtained to obtain the extent of thermal resistance due to the thermal-resistance material 4 like the scale.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention is directed to the prevention of limescale (scale) that adheres to and grows on heat transfer surfaces.
The present invention relates to a method for measuring thermal resistance (fouling coefficient) caused by such factors. More specifically, the present invention relates to a thermal resistance measuring method for determining thermal resistance in a non-contact manner without destroying a heat transfer surface.

(Prior Art) Thermal resistance measurement is one of the maintenance management techniques common to all heat utilization equipment having a heat exchanger. In particular, heat-resistant substances such as scale and slime adhere to and grow on the heat transfer surface of the heat exchanger as the operating time of the equipment increases. This not only leads to an increase in operating costs due to a decrease in thermal efficiency, but also sometimes causes overheating damage to the heat exchanger, reducing economic efficiency.
It may even become a big problem from a safety standpoint. For this reason, in these heat utilization facilities, measures are taken to regularly remove thermal resistance such as scale and slime.
The basis for this judgment is generally based on experience, and the removal of thermal resistance is rarely judged based on the actual measured value of the thermal resistance itself.

This is because measuring thermal resistance is extremely difficult, and there are currently few techniques that have been put to practical use.

Conventionally proposed methods for measuring heat transfer surface thermal resistance include:
Most of the methods involved embedding a thermocouple in the heat transfer wall and determining the thermal resistance from the time change in the thermocouple reading while the heat exchanger is in operation.

In addition, a method has been proposed in which two thermal pairs are embedded in the heat flow direction of the heat transfer wall and both thermal resistance and heat flow are measured simultaneously (Japanese Patent Application Laid-Open No. 107400/1983), inside the tube wall of a boiler tube. Drill two insertion holes for thermocouples in the hole so that the tips of the insertion holes are spaced apart in the radial direction of the boiler tube, and insert a thermocouple into each hole to measure the temperature at two points. Measurements are taken to determine the basic temperature gradient. That is, the temperature at the temperature detection position of the tube of the operating device is measured, the temperature gradient is determined, and the amount of scale adhesion is detected by comparing the two temperature gradients.

(Problem to be solved by the invention) However, the former method also captures changes in heat flow that occur in the heat exchanger over time, so there is a problem that no correlation can be obtained between the temperature indication value and the thermal resistance value. occurs. Also,
The latter method has been limited to the laboratory level and has not been put to practical use due to the instability of the thermocouple reading or the complexity of processing.

Therefore, the only method of measuring thermal resistance currently in practical use is to cut out the heat transfer wall and determine the thermal resistance of the thermal resistance material in the form of scale thickness and adhesion amount. For example, in thermal power plants that have boilers that are huge heat exchangers, heat transfer tubes are removed during periodic inspections in order to obtain reliable thermal resistance values necessary for managing the heat transfer surface of the boiler. In reality, this extubation inspection is quite expensive, and the reality is that it is difficult to say that a sufficient inspection is carried out on a wide heat transfer surface.

As described above, conventional thermal resistance measurement techniques do not take into account external disturbance factors, simplicity of measurement, or complexity of processing, etc., and do not take into account factors such as the simplicity of measurement or the complexity of processing. Direct methods have problems such as increased testing costs, complicated work, and post-recovery safety.

An object of the present invention is to provide a method for safely, easily, and inexpensively measuring thermal resistance without being affected by disturbances and without destroying the heat transfer wall.

(Means for Solving the Problems) In order to achieve the above object, the thermal resistance measuring method of the present invention locally measures the surface of the heat transfer wall opposite to the surface to which the heat resistance material is attached with a constant heat flux. After heating to a predetermined amount, the steady temperature of the heating surface of the heat transfer wall is measured, and the heat resistance is compared with the steady temperature when the same heat flux is applied when no heat resistance material is attached. The temperature rise due to the thermal resistance of the material is determined, and the thermal resistance and resistance of the other thermally resistant material are measured, which correlates with this.

In addition, the thermal resistance measurement method of the present invention locally heats the surface of the heat transfer wall opposite to the surface to which the heat resistance material is attached with a constant heat flux, and after heating by a predetermined amount, the heating is stopped and the surface is cooled. Then, measure the temperature and cooling time of the heating surface of the heat transfer wall, and compare it with the temperature change that would occur by applying the same heat flux and cooling under the same conditions when no heat resistance material was attached. Based on the time difference, the thermal resistance of the curved thermal resistance material, which is correlated with this, is measured.

(Function) In the heat transfer wall, the heating surface side and the working fluid side to which the heat resistance substance is attached are partitioned by a tubular or flat metal plate. When this heating surface side is locally heated by a heat source, a temperature rise dT1 occurs in the heat transfer wall due to the thermal resistance of the heat transfer wall itself. Furthermore, if a thermal resistance substance adheres to the working fluid side of the heat transfer wall, a new temperature increase dT2 occurs due to this thermal resistance Rs, and in this case, the temperature on the heating surface side of the heat transfer wall is a state that includes both. That is, dTl + dT2. Therefore, dTl and d
By measuring both Tl and dT2, only dT2 can be easily determined by calculation, and furthermore, by determining dT2, it is possible to determine the amount of thermal resistance. Also,
If the working fluid is cooled after heating, there will be a difference in the cooling time depending on the amount of heat resistance material attached or not, and this difference will also affect the relationship with the temperature. Since it has a correlation with thermal resistance, thermal resistance can be determined using this.

(Example) Hereinafter, the configuration of the present invention will be described in detail based on an example shown in the drawings.

FIG. 1 shows the principle of the thermal resistance measuring method of the present invention.

In national isolation, 1 is a heating source, 2 is a non-contact thermometer, 3 is a heat exchanger tube (heat transfer tube) corresponding to a heat transfer wall, 4 is a heat resistance material such as a scale, 5 is a shutter drive motor, 6 is a shutter that shuts off the heating source and the object to be heated. Note that water at a constant temperature is flowed into the heat exchanger tube 3 at the time of measurement, and is provided to cool the heat exchanger tube 3 to prevent it from being destroyed or deteriorated due to heating.

The heat source 1 has a heat amount sufficient for measurement and does not destroy the tube, for example, 200,000 Kcal/h, or 40 kcal/h in terms of heat flux.
Laser, X
Non-contact heat sources such as infrared radiation or electromagnetic waves are employed.

As the thermometer 2, in order to avoid the influence of a heat source, one that measures radiation temperature using a selected wavelength from a remote location without contact is adopted, for example, a method that uses an antimony element and a color filter in combination.

Such non-contact heating and temperature measurement does not disturb the hot water and is essential for accurate thermal resistance measurements.

In the above configuration, first remove dirt, sludge, etc. from the surface of the heat exchanger tube 3, and then measure the tube thickness in advance using an ultrasonic measuring device.Next, the outer surface of the heat exchanger tube 3 is heated to a predetermined temperature. Heating is performed by the heating laser 1 until the temperature is reached. Heating is carried out in a local area of bin points with a diameter of 5 to 10 taps. Further, it is sufficient that the amount of heating at this time is at least about 200,000 Kcal/h. At the same time as heating, the temperature of the heated surface is measured by the non-contact thermometer 2.

By heating, the surface temperature of the heat exchanger tube 3 increases by one inch due to its own thermal resistance. This temperature rise on the tube surface is greater when the scale 4 is attached to the tube inner surface than when it is not present. That is, the temperature rise is proportional to the thermal resistance 1.

Therefore, if you can find the difference between the amount of temperature rise on the surface of the heat transfer tube 3 when a certain amount of heat flux is applied and the amount of temperature rise when no scale is attached, it is possible to determine the difference between the amount of temperature rise on the surface of the heat transfer tube 3 and the amount of temperature rise when no scale is attached. The amount of thermal resistance can be determined.

Then, stop heating and measure the steady state temperature of the heated part. The heating is stopped instantaneously by rotating the shutter 6 in front of the laser 1 by driving the motor 5. As a result, as shown in FIG. 2, the temperature of the tube surface reaches a steady state and then begins to drop as it is cooled by the water flowing inside the tube.

Therefore, the steady state temperature is measured and the temperature increase dT2 on the tube surface due to the thermal resistance material 4 is determined.

Here, the steady state is a stable state where the temperature is balanced and there is no movement immediately after the heating source is shut off, and exists for a short period of time just before the temperature drops.

The temperature in this steady state is the temperature increase d due to the thermal resistance material 4.
T2 song, temperature rise dT due to thermal resistance of heat exchanger tube 3 itself
1, so dT2 is obtained by subtracting the temperature rise d'T'1 under conditions without scale.

The temperature increase dT1 under conditions without scale is determined by calculation. If you measure the temperature when not heating, the temperature inside the tube is the same as the temperature outside the tube, so the temperature on the outside surface of the tube itself becomes the fluid temperature (the temperature of the water flowing inside the tube). This aging temperature is constant and uniformly cools the inner wall surface of the tube.

Further, the tube thickness has been measured in advance, and the thermal conductivity of the heat exchanger tube is also known. From this, the temperature dT1 of the outer surface of the tube at a heat flux without scale is q''=l
(■L nie L is determined by Shiyori calculation.

However, 4... Heat flux, k... Thermal conductivity, T1... Tube outer surface temperature, T2... Tube inner surface temperature, and... Pipe thickness. By subtracting the above-mentioned dTl from the tube surface temperature at a constant heat flux given by the heating source 1 described above, the temperature increase dT2 on the tube surface due to the heat resistance material 4 can be determined. From the relationship between the tube inner surface thermal resistance and the tube surface temperature dT2,
The tube inner surface thermal resistance at the corresponding heating heat flux can be determined. For example, made of low-alloy steel, diameter 31°8, pipe thickness 5.5
FIG. 3 shows the experimental results when the open heat exchanger tube 3 was subjected to bin point heating with a diameter of 5.0 mm. As is clear from the graph, for example, when the temperature]-rise dT2 is 0.8°C and the heating heat flux is 20 x 104 Kcal/rrfh,
It can be seen that a thermal resistance of 1.33xfOX10''ih'c/kcal exists on the inner surface of the tube.

When the thermal resistance cannot be determined by the above method, the resistance can be determined by using the difference in cooling time, that is, the difference in the time required to cool down to a certain temperature with and without Sgale. In this case, the difference Δt in cooling time and the temperature are simultaneously measured and their ratio is determined. In other words, there is a certain relationship between the difference Δ in the cooling time to a certain reference temperature and the amount of thermal resistance as shown in FIG. can be found.

For example, in Figure 4, which shows the relationship between the cooling time to the reference temperature and the tube inner surface thermal resistance at a reference temperature of 60゛C1 heating heat flux of 25 x 104 cal/rrfh, when the cooling time is 3301 seconds, the tube inner surface thermal resistance is 20x 105
Cal to rrIlhC/.

(Effects of the Invention) As is clear from the above explanation, the thermal resistance measurement method of the present invention locally heats the surface of the heat transfer wall opposite to the surface to which the heat resistance material is attached with a constant heat flux. , after causing a temperature rise due to the heat resistance material, compare this with the temperature that would occur by giving the same heat flux when no heat resistance material is attached, and calculate the temperature difference at the steady temperature of the heating surface. The thermal resistance of the thermal resistance material on the other side, which has a correlation with this, can be measured by determining the difference in cooling time or by determining the difference in cooling time. Thermal resistance of can be measured.

In addition, according to the method of the present invention, the internal thermal resistance is measured without destroying the heat transfer wall, and there is no need to remove the heat pipes to be measured, so the cost of thermal resistance testing can be reduced to 11 times. However, the accuracy of the measurement is good because it can be measured without disturbing the temperature field on the heat transfer surface due to non-contact.Furthermore, the thermal resistance measurement method of the present invention is capable of locally heating and measuring the tube surface temperature after heating is stopped. Because it only measures , it can be measured in a short time.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the principle of the measurement method of the present invention, Fig. 2 is a graph showing the relationship between cooling rate and tube internal thermal resistance, Fig. 3 is a graph showing the relationship between tube internal thermal resistance and tube surface temperature, Figure 4 shows the cooling time to the reference temperature and the tube inner surface p! ! , is a graph showing the relationship with resistance. DESCRIPTION OF SYMBOLS 1...Heating source, 2...Non-contact type a degree meter, 3...Heat exchange tube (heat exchanger tube), 4...Heat resistance material, 6...Shutter. Heat (warm eaves on the pipe surface by piles)

Claims (2)

[Claims] (1) Locally heat the surface of the heat transfer wall opposite to the surface to which the heat resistance material is attached with a constant heat flux, and after heating by a predetermined amount, measure the steady temperature of the heated surface of the heat transfer wall. , calculate the temperature rise due to the thermal resistance of the thermal resistance material compared to the steady temperature when the same heat flux is applied when no thermal resistance material is attached, and calculate the thermal resistance of the other side that is correlated with this. A thermal resistance measuring method characterized by measuring the thermal resistance of a substance. (2) Locally heat the surface of the heat transfer wall opposite to the surface to which the heat resistance substance is attached, with a constant heat flux, and after heating by a predetermined amount, stop heating and cool the heat transfer wall. Measure the temperature and cooling time of the surface, compare it with the temperature change that would occur by applying the same heat flux and cooling under the same conditions when no heat resistance material was attached, and find a correlation from the difference in cooling time. A thermal resistance measurement method characterized by measuring the thermal resistance of a thermal resistance material on the other side.