Problems with heat exchangers and thermocouples
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I encountered a problem today: we have a heat exchanger on site, and when no water (the cooling medium) flows through it, the inlet temperature of the heat exchanger is 54 degrees Celsius, while the outlet temperature is actually 38 degrees Celsius. The flow rate of the hot medium is 5.0 cubic meters per hour； The external ambient temperature is minus 5 degrees Celsius ； We found the instrumentation technician, who, after making some checks, confirmed that both the inlet and outlet temperatures were accurate. They also used a mercury thermometer inserted into the hole of the outlet thermocouple on site, and the temperature measured in this way differed from the temperature shown by the thermocouple by only about 1 degree; therefore, the technician concluded that the temperature displayed by the thermocouple must be correct. Moreover, our leadership also believes it is a problem with the manufacturing process, saying it has nothing to do with the instruments; please help analyze what exactly is going on. This post was last edited by zhaoming4865 on 2007-12-6 11:39.]
But can air have such a strong cooling effect?
It should be air cooling, as the temperature of air is very low; even without a heat exchanger, the temperature drops significantly in the straight pipe section. It is recommended that you upload the material parameters of the heat exchanger to facilitate a detailed analysis
I think that’s the case too; it’s related to the type of valve used, or perhaps the valve is damaged. For example, even though a disc valve is closed, the flow rate can still be quite high! ! ! Of course, air cooling also plays a very important role!
It’s probably due to the cold air; you can do the calculations! Did the temperature change during that journey? What type of heat exchanger do you have? Use the heat exchanger calculation formula to figure it out, and you should know the answer!
After reading the original poster’s post, I wanted to ask: was the temperature measurement taken when the heating medium was first introduced, or after everything had stabilized? The outdoor temperature seems quite low; if the measurement was taken at the beginning, it’s likely that the large temperature difference between the heating medium and the condenser caused a sharp drop in temperature. If the measurement was taken after stabilization, then there are two possible explanations: either the room temperature is too low, leading to air cooling – and it should be noted that air cooling can also be quite effective; or there might be a leak in the cooling water valve. As the other posters mentioned.
The poster didn’t clarify what is meant by the heat exchanger not having water flowing through it! Is it that the inlet and outlet valves are fully closed, keeping the water inside, or has the water been drained? If there is water inside, then heat transfer certainly occurs; if the water has been drained, the heat exchanger itself acts as a good conductor, and it will still provide cooling effects even at such low ambient temperatures! Even with a section of pipe that isn’t insulated, there will still be a temperature drop. Therefore, the situation you described is possible – and it’s not strange at all!
This heat exchanger acts as an air cooler!
Yes, I also think this heat exchanger has become an air cooler. With an ambient temperature of -5 degrees, it is entirely possible to cool a hot fluid at 54 degrees down to 38 degrees
It’s unlikely that it’s entirely due to a leaky recirculation water valve. Because with water cooling the temperature is around 20 degrees, while in reality the temperature dropped from 54 to 38 degrees, which suggests that if the heat exchanger is a good conductor, it will still be able to provide cooling even at such low ambient temperatures. The combination of these two factors is likely the answer; moreover, it’s best to use a thermometer for further measurement – it might be a problem with the thermocouple
1. What is the size of your heat exchangers? What kind of insulation is it? 2. The flow rate of the high-temperature medium is 5 m3/h, which is not a very high flow rate, and the temperature difference between the inlet and outlet is only 7 degrees Celsius.
Air is used as the cooling medium, with the entire heat exchanger functioning as an air-cooled unit. The heat exchange area can no longer be calculated using the traditional values; the interface of the hot substance serves as the heat exchange surface, while the interface of the non-hot substance functions as an auxiliary surface for heat dissipation, acting like cooling fins. So the heat exchange capacity is greatly improved.