EP2840331A1 - Method for stagnation detection and stagnation prevention in heat exchangers - Google Patents

Abstract

The invention relates to a method for detecting and avoiding boiling in primary heat exchangers 2 in heaters 1, in particular condensing heaters. One or more of the boiling or micro-boiling characterizing process variables 21, 22, 32, 33 are detected and compared with a threshold value. When the threshold is exceeded, at least one operating parameter is changed, which counteracts boiling.

Description

The invention relates to a method for detecting and avoiding stagnation in heat exchangers, in particular in primary heat exchangers in heaters, in particular in condensing heaters, which transfer the heat of the fuel to the water to be heated and in which a flow is carried out by a plurality of parallel pipes , Stagnation in the context of heat exchangers is understood to mean a local or global boiling of the heat transfer medium. This can lead to partial overheating and damage to the heat exchanger in one or more of the parallel connected tubes. Therefore, it is important to avoid stagnation and, in the event that it occurs, to recognize reliably in order to be able to initiate countermeasures in good time. While it is known that a heat exchanger with a large volume of heat transfer medium is less prone to stagnation, this is in conflict with the desire to make heaters compact and light in weight.

Therefore, it is known to equip devices with an overflow valve, which provides a partial flow directly between the flow and return in the presence of too low a flow rate through the system.

Since high volume flows are associated with a high electrical current consumption for the circulation pump, this is energetically unfavorable. In addition, overflow valves lead to a Admixture of heat transfer medium from the flow in the return and thus reduce the condensing rate in condensing units, resulting in a reduced efficiency.

It is therefore an object of the invention to provide a method for stagnation detection and stagnation avoidance, which does not have these disadvantages.

This object is achieved in accordance with the features of claim 1 in that a characteristic process variable is detected for the occurrence of micro-boiling, which precedes the boiling effecting the stagnation, and is compared with a threshold value. If the threshold is exceeded, a threat of boiling and thus stagnation is detected and at least one operating parameter is changed so that the stagnation is counteracted. This has the advantage that the disadvantages of a higher energy consumption of the pump or a poorer efficiency of the heater only occur when stagnation threatens. In normal operation, however, the heater can be operated efficiently.

In an advantageous embodiment of the method according to the invention, the variance of the pressure of the heat transfer medium to be heated is detected and compared with a threshold value. This process step takes advantage of the fact that boiling is preceded by so-called micro-boiling, in which small gas bubbles are formed in the region of the boundary layer of the flow, and after a short time collapse again in colder regions. This mechanism leads to an increase of the noise value on the pressure signal of the system pressure sensor.

In one development of the method, for example, the variance about the mean value is determined in each case over time segments, for example over 1 s. If the variance exceeds a threshold, for example 3000 mbar ² , microsilking is diagnosed. To avoid a false alarm, for example, in the case of diagnosed microsolvency, a counter may be incremented, for example by 10, the counter for undiagnosed microsilking is decremented by the same or a smaller value, for example by 5. If the counter exceeds a predetermined counter reading, for example 250, then this is an indication that repeated microsyring has occurred over a relatively long period of time.

Alternatively, by detecting the signal dispersion around the mean value, for example with the aid of a high-pass filter, microsilking is detected. In this case, frequencies above 20 Hz or preferably above 100 Hz can be detected. In this case, the signal power or the signal level of the high-frequency component is compared with a threshold value. Alternatively, the ratio to the quasi-static fraction is formed and compared with a threshold value.

Alternatively or additionally, in a further advantageous embodiment of the method according to the invention, the characteristic process variable is the negative gradient of the temperature spread between input and output for the heat transfer medium of the heat carrier to be heated. In order to measure the temperature spread reliably, this is done in quasi-stationary operation. The design makes use of the fact that, when stagnation occurs, the heat transfer to the heat transfer medium is impaired, since no or a significantly reduced circulation occurs in one or more of the pipes connected in parallel. This leads with respect to the total volume flow to the Verteilbzw. Mixing points in the heat exchanger to a lower temperature spread, which is detected according to the invention and compared with a threshold value. Another effect is that under quasi-steady-state operating conditions, ie with a constant burner load and constant water circulation, the stagnation with several pipes connected in parallel leads to an increase in the volume flow of the pipes not affected by the boiling. This means that the temperature difference between the inlet and outlet of the heat exchanger drops in the remaining pipes. If the negative gradient of the temperature spread exceeds the threshold value, either the stagnation is detected and a measure initiated or another operating parameter is used.

For these reasons, the embodiment described above is also suitable for detecting local boiling.

According to the invention, alternatively or additionally, several measures are used to prevent stagnation after it has been detected. This is on the one hand the increase of the mass flow by the pump speed is increased or by a bypass between the output and input of the heat exchanger is switched. As a result, on the one hand more heat dissipated and on the other hand causes a flushing of the vapor bubbles. Additionally or alternatively, the burner is switched off or the burner power is reduced.

In an advantageous embodiment of the invention, the mass flow of the heat transfer medium to be heated is briefly increased before starting the burner of the heater. As a result, gas bubbles present in the water circuit are optionally expelled from the heat exchanger and divided into smaller bubbles by means of the turbulent flow conditions present in the pump, which have a markedly reduced tendency to stagnate due to the lower buoyancy forces.

The invention will now be explained in detail with reference to FIGS.

FIG. 1 schematically shows a heater for performing the method according to the invention. The heater 1 comprises a burner 3 with a heat exchanger 2, with which the heat obtained from the burner 3 is transferred to a heat transfer medium. The heat transfer medium is usually water, which is circulated in a circuit by a pump 4. The heater 1 is connected to a heat sink 5 provided by the heater 1 is supplied with heat. In the heat exchanger 2, stagnation due to boiling water may occur. In order to recognize and avoid this, a control device 11 is provided, which is connected to temperature sensors 6, 7 and / or a pressure sensor 8. By means of the sensors, the control unit recognizes on the basis of the method according to the invention the entering or announcing stagnation and avoids the occurrence of stagnation by intervention in the rotational speed of the pump 4, in the operation of the burner 3 and / or in the position of the valve. 9 ,

FIG. 2 shows in the course of temperature 20, the time course of the temperatures 21, 22 at the output and input of the heat exchanger 2 from FIG. 1 , which was recorded with the temperature sensors 6 and 7, and in the pressure curve 30, the time course of the with the pressure sensor 8 from FIG. 1 measured system pressure 32, the variance 33 and the measured directly at the heat exchanger 2 pressure 31st

Based on the curves, the occurrence and detection of boiling will be explained below. The course of the pressure 31 at the heat exchanger 2 has at 478 s an onset of high-frequency content. This weakens at 480 s, then rises sharply at 498 s. This is due to the onset of 478 s microsilusion, which then boils at 498 s. The pressure fluctuations are due to the formation and in particular to the collapse of vapor bubbles. The course of the system pressure 32 has these high-frequency components also, but in a lower amplitude. This is due to the fact that the pressure sensor 8 is provided for the system pressure at a certain distance from the heat exchanger. Nevertheless, by determining the variance of the system pressure, the course of which is shown in the curve 33, micro-boiling and boiling can be clearly recognized. By comparison with a threshold value, it is thus possible to detect microsilution and boiling and initiate a measure to prevent stagnation. Advantageously, this threshold can be set so that the micro-boiling is already detected. Here, however, is the risk of a Error identification given. Therefore, optionally or alternatively, the course of the pressure can be used as a further criterion. In any case, when a higher threshold is exceeded, the boiling, as occurs in the range from 498 s, and thus the stagnation certainly recognizable.

While the temperature profile 22 at the entrance of the heat exchanger 2 is almost constant, can be seen from the temperature profile 21 at the output of the heat exchanger microsolutions and boiling. First, the temperature rises in the range between 475 and 477 s. This is due to a heating process and is irrelevant to the detection described here. From about 480 s, however, the temperature drops, which is an indication of the occurrence of boiling due to the mechanisms described above. Thus, the gradient between the temperatures at the output side of the heat exchanger and the input side is negative. This is monitored according to the invention by comparing the negative gradient in the quasi-stationary operation with a threshold value, and used for the detection of stagnation. In addition, it should be noted that the sudden temperature increase at 500 s is due to a vapor bubble formation, through which the heated water is pushed out. In principle, it is also possible to evaluate such curves for detecting the stagnation.

LIST OF REFERENCE NUMBERS

1

heater

2

Heat exchanger

3

burner

4

pump

5

heat sink

6

Temperature sensor at the output

7

Temperature sensor at the entrance

8th

pressure sensor

9

Valve

10

bypass

11

control unit

20

temperature curve

21

Temperature at the outlet of the heat exchanger

22

Temperature at the entrance of the heat exchanger

30

pressure curve

31

Pressure at the heat exchanger

32

system pressure

33

Variance of system pressure

Claims (15)

Method for detecting and avoiding boiling in primary heat exchangers (2) in heaters (1), in particular condensing heaters, characterized in that the boiling preceding the boiling is detected, wherein one or more Mikrosieden characterizing process variables (21, 22, 32, 33) are detected and compared with a threshold value and that, when the threshold value is exceeded, at least one operating parameter which counteracts boiling is changed. The method of claim 1, wherein a characterizing process variable is the variance (33) of the pressure (32, 31) of the heat transfer medium to be heated. Method according to claim 2, wherein the variance (33) of the pressure (32, 31) is measured over short periods of time, a counter is increased by one value when the threshold value is exceeded, and a counter is lowered by a second value when the threshold value is undershot, the second value is preferably smaller than the first value, and wherein at least one operating parameter is changed as soon as the counter reading has exceeded a second threshold value. The method of claim 1, wherein a characterizing process variable is the signal component of the pressure (32, 31) of the heat transfer medium to be heated in a high frequency range or the ratio of the signal component of the pressure in a high frequency range to the steady state signal of pressure (32, 31) is heating heat transfer medium. The method of claim 4, wherein the high-frequency frequency range is an area above 20 Hz, preferably above 100 Hz. Method according to one of the preceding claims, wherein a characterizing process variable is the negative gradient of the temperature spread between input and output (22, 21) for the heat transfer medium of the heat exchanger (1) to be heated in quasi-stationary operation. Method for detecting and avoiding boiling in one or more tubes of a primary heat exchanger (2) comprising a plurality of pipes connected in parallel in heating appliances (1), in particular condensing heating appliances, characterized in that a process variable (21, 22) characterizing the boiling is detected and are compared with a threshold value and that, when the threshold value is exceeded, at least one operating parameter which counteracts boiling is changed, and the characterizing process variable is the negative gradient of the temperature spread between input and output (22, 21) for the heat transfer medium of the heat exchanger to be heated (1) is in quasi-stationary operation. A method according to claim 7, wherein the threshold for the negative gradient of the temperature spread is 0.05 K / s, preferably 0.1 K / s. The method of claim 7, wherein the threshold for the negative gradient of the temperature spread less than, preferably 63% of the temperature spread in the stationary Case divided by the number of parallel tubes of the primary heat exchanger (2). Method according to one of claims 7 to 9, wherein the gradient over a period of at least 5 seconds, preferably of at least 12 seconds is averaged. The method of any one of the preceding claims, wherein changing an operating parameter is decreasing the ratio of supplied and discharged heat. A method according to claim 11, wherein the reduction of the ratio between supplied and discharged heat quantity is achieved by switching off the burner (3) or reducing the burner output. Method according to one of claims 1 to 11, wherein the change of an operating parameter is achieved in that the mass flow of the heat transfer medium to be heated is increased, in particular by increasing the pump speed of the heat transfer medium to be heated pump (4). Method according to one of claims 1 to 10, wherein the change of the operating parameter is achieved in that a bypass line (10) between the output and input for the heat transfer medium to be heated of the heat exchanger is opened. Method according to one of the preceding claims, wherein when starting the burner (3) of the heater (1), the mass flow of the heat transfer medium to be heated is briefly increased, in particular by increasing the pump speed of the heat transfer medium to be heated pump (4).

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