A heat exchanger is a device that exchanges heat between two fluids of different temperatures. Heat exchangers are widely used as heat transfer equipment in fields where energy consumption is large. With the rapid development of energy-saving technologies, there are more and more types of heat exchangers. It is suitable for heat exchangers with different media, different working conditions, different temperatures and different pressures, and the structure type is also different.
At present, most of the heating systems use wall-heat exchangers, and plate heat exchangers fall into this category. After the plate heat exchanger is operated for a period of time, the heat transfer efficiency will decrease. This paper analyzes the operation failures and causes of water-water plate heat exchangers (hereinafter referred to as heat exchangers).
In the heat exchanger, the cold and hot fluids respectively flow on both sides of the solid wall, and the heat of the hot fluid is mainly transmitted to the wall surface by convection, and is transferred to the cold fluid through the wall surface. In order to enhance the heat transfer effect, the hot and cold fluids often adopt a countercurrent heat transfer method. The heat exchanger usually works in steady state. The heat transfer equation is shown in the literature [1~3]. The temperature change of the hot and cold fluid in the heat exchanger is shown in Figure 1.
Under normal circumstances, the cold fluid outlet temperature approaches or exceeds the hot fluid outlet temperature, but in actual operation, the temperature of the hot and cold fluid is often abnormal due to heat exchanger failure. The main failure of the heat exchanger is as follows: Case 1, the hot fluid outlet temperature reaches 1.1 times of the cold fluid outlet temperature, which does not meet the reverse flow heat transfer law; Case 2, the hot and cold fluid outlet temperature is close to and low.
2 Cause analysis and troubleshooting1 Situation 1
When the hot fluid outlet temperature reaches 1.1 times of the cold fluid outlet temperature, it indicates that the heat exchanger has been fouled, and the fouling layer constitutes an additional heat conduction thermal resistance, which affects the heat transfer effect.
The heating area of ​​a district government thermal station is 4 & TImes; 10 m, the heating capacity is 2.8 MW, and the heat exchanger has been used for 10 a. In the winter of 2005, the heat exchanger was found to have problems in case 1. The operating parameters were: primary side inlet temperature of 70 ° C, outlet temperature of 65 o C; secondary side inlet temperature of 42 ° C and outlet temperature of 45 ° C. At this point, the primary side valve of the heat exchanger is fully open and cannot be adjusted normally. As the outdoor temperature decreases, the thermal station is no longer able to meet the heating demand. The heat exchanger operating parameters of another thermal station of the same scale are: the primary side inlet temperature is 70 ° C, the outlet temperature is 44 o C; the secondary side inlet temperature is 41 ° C, and the outlet temperature is 48 ° C. At this time, the primary side valve of the heat exchanger is not fully opened, and there is room for adjustment. The heat exchanger was disassembled and found to be fouled on the surface of the plate, and the surface was muddy (with piping during construction), and the scale layer was about 1 mm thick. After descaling and cleaning, the heat transfer effect is obviously improved. The specific operating parameters are: primary side inlet temperature is 70 °C, outlet temperature is 45 oC; secondary side inlet temperature is 41 °C, and outlet temperature is 49 °C. At this time, the primary side valve
The door is in a half-open state and resumes normal adjustment.
2 Situation 2
In the case of Case 2, it can be determined that the flow rate of the hot fluid is too small, and the cold fluid cannot be sufficiently heated, and the temperature of the cold fluid cannot be continuously increased. The reason for this is that the heat exchanger is clogged and there is a gas blockage on the primary side inlet pipe of the heat exchanger.
The operating parameters of the heat exchanger of a local tax bureau heat station are: the inlet temperature of the primary side is 70 °C, the outlet temperature is 45 °C, the inlet temperature of the secondary side is 41 °C, and the outlet temperature is 49 °C. The operating parameters when the anomaly occurred were: the primary side inlet was 70 ° C, the outlet temperature was 32 ° C, the secondary side inlet temperature was 28 ° C, and the outlet temperature was 33 ° C. The heat exchanger was disassembled and it was found that most of the primary side passages of the heat exchanger were blocked by debris, and the flow area was already small. After the heat exchanger is backwashed and the decontamination device is cleaned, the operating parameters return to normal, which is a condition in which the heat exchanger is clogged. The failure phenomenon of the heat exchanger of Guangchuan Thermal Station is the same as that of the above-mentioned local tax bureau, but after cleaning the heat exchanger and the decontamination unit, the effect is not obvious. Observing the on-site pipeline layout found that after a water inlet station, the pipeline rises and then descends into the heat exchanger. The pipeline has a 1r-bend in the station, and there is only one pressure gauge and no venting valve. Remove the pressure gauge, open the plug valve on the bend of the watch to deflate, and find a lot of air in the pipe. After venting for about 15 minutes, the water gradually enters the heat exchanger, and the temperature of the primary side gradually increases, the temperature of the secondary side increases, and the heating returns to normal. This is a case where the primary side inlet pipe is blocked.
Based on the above situation, we have taken the following measures: Strengthen construction management to prevent debris from entering the pipeline during construction; install a bleed valve at the highest point of the primary inlet pipe; often deflate during operation.
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