Analysis of the reasons for the failure in the cold oil cooler renovation
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Unit #1 in our plant is equipped with a total of 3 oil coolers, which operate in parallel. All of these coolers are products of Harbin Turbine Factory, and their model is JL—37—6. The technical specifications for the original oil coolers are as follows: cooling area of 37㎡； Cooling water volume: 80 t/h ； The cooling oil flow rate is 600 L/min. The maximum temperature of the circulating water (i.e., the inlet temperature of the oil cooler) is 38°C, with a pressure of 0.15 MPa. The inlet and outlet water pipes of the oil cooler have a pressure of 1.6 MPa and are fitted with 125 mm flanges; the inlet and outlet oil pipes also have a pressure of 1.6 MPa and are equipped with 125 mm flanges. The inlet oil temperature of the oil cooler ranges from 60 to 70°C, with an oil pressure of 0.2 MPa. Due to the poor heat exchange efficiency resulting from years of operation, the original design called for two units in use with one as a backup. Now, during the hot seasons, all three units are operated simultaneously, yet the oil outlet temperature still reaches 48°C. One of the oil coolers was later modified; it still uses copper tubes as the heat exchange elements, and its surface area has been increased to 75 square meters. The number of flow paths remains unchanged at 4, while the tube dimensions are 14x1x1535 mm, with a total of 1120 tubes. Currently, the temperature at the cooling water inlet is 34°C. The temperature at the oil side outlet of the old cooler was 45°C, while after the modification it has risen to 46°C. Although the area of the cooler has doubled, its performance has actually worsened. I was wondering if the experts here could help analyze the reasons for this and suggest some solutions. I would be extremely grateful! I downloaded the trial version of Heat Exchanger Master; the structural parameters cannot be modified, so it’s not possible to see the results. I was wondering if any expert could help me with the calculations. Thank you!
Please provide me with the specific physical properties of the cooling oil, and I’ll help you do the calculations. QQ: 9 5 4 6 9 9 3 5 3
This post was last edited by sus321 on 2015-6-4 at 10:31; the description of the actual operation process is not very clear. Were the original 3 heat exchangers of 37 m2 each operating in parallel, with an oil outlet temperature of 45 degrees? After the modification, 2 heat exchangers of 37 m2 each and 1 heat exchanger of 75 m2 are operating in parallel, resulting in an oil outlet temperature of 46 degrees. The oil outlet temperature increased rather than decreased after the modification; please analyze the reasons for this. In the case mentioned above, I believe the main reason is that the original two 37m2 heat exchangers had a lot of scale buildup on either the oil side or the water side, resulting in high fluid resistance. In contrast, the newly installed 75m2 heat exchanger has a larger heat exchange area and no scale buildup, so the resistance on both the oil and water sides is much lower than that of the other two 37m2 heat exchangers. If the original two 37m2 heat exchangers have a lot of fouling on the oil side, the resistance is high. A large proportion of the oil flows into the 75m2 heat exchanger, which can lead to the aforementioned situation: the oil flow rate in the 35m2 heat exchanger decreases, the heat transfer coefficient drops, and the reduced amount of heat transfer cannot be adequately compensated for by the 75m2 heat exchanger. If the original two 37m2 heat exchangers have excessive scale buildup on the water side, this will result in high resistance. A large proportion of water flows into the 75m2 heat exchanger, which can also lead to the aforementioned situation: the flow rate of water in the 35m2 heat exchangers decreases, the heat transfer coefficient drops, and there is not enough cold water available; as a result, the reduced heat transfer capacity cannot be adequately compensated for by the 75m2 heat exchanger. Solution: Install control valves at both the oil and water inlets of the 75m2 heat exchanger to regulate the balance between the water and oil streams in the two 35m2 heat exchangers and the 75m2 heat exchanger. The heat exchange efficiency after adjustment will definitely be better than that achieved by operating 3 heat exchangers with a capacity of 35 m2 each. (But it won’t help much either; the temperature can only drop by about 2 degrees at most.) To resolve the issue completely: install control valves and thoroughly clean the two 37 m2 heat exchangers
The water supply pressure is low and the temperature is high (generally, the supply temperature in the north during summer should not exceed 32°C; this is our standard – I’m not sure about other units). Low pressure results in a low flow rate; coupled with a small temperature difference between water and oil, the driving force for heat transfer is low, so the heat exchange efficiency is naturally poor. What is the quality of your water? Is there sludge or algae growth, or anything adhering to the pipe walls? (At this temperature, scaling generally does not occur, but such adhesions can increase the thermal resistance of the dirt on the walls and reduce heat exchange efficiency.) 1. Thoroughly clean and maintain the heat exchangers to improve heat exchange efficiency. 2. Continue to upgrade the remaining coolers, replacing them all. 3. It is recommended to use plate heat exchangers, as this will yield better results. 4. No matter how changes are made, it’s always beneficial to have an appropriate amount of margin (of course, considering cost efficiency as well)
What you’re referring to is the oil-siphoning phenomenon that occurs in parallel configurations. By installing a control valve, it’s possible to manually adjust the opening degree of the inlet and outlet valves, but this doesn’t address the root cause of the problem. The factory wants to switch to a series configuration instead
In the old factory, the environmental conditions at the time of design were quite different from those on site; the temperature of the circulating water exceeded 38 degrees Celsius in summer, and it was open-flow water that was quite dirty, so plate-type heat exchangers could not be used.
After changing from parallel to series connection, the heat exchange efficiency will definitely be better； However, the resistance on both sides is at least 18 times greater than that of the original 3 units operating in parallel with 37 units each, and this factor must be taken into account.
After changing to series connection, the resistance increases significantly, but the failure to lower the oil temperature is also a major problem. Could the teacher please offer some feasible suggestions? Perhaps I should leave my contact information so that I can consult you.
I believe a reasonable solution is as follows: 1. As I mentioned above, install control valves and thoroughly clean the two 37 m2 heat exchangers (if cleaning is not possible, replace them with ones of 37 m2 size). Since the original three 37 m2 heat exchangers were sufficient to meet the requirements at the time they were put into use, and those requirements have not changed, the heat exchangers will definitely meet the requirements after being cleaned or replaced. 2. The original 3 units of 37m2 heat exchangers were thoroughly cleaned; they were connected in parallel with the original 3 units of 37m2 units, and then connected in series with the new 75m2 unit (the total resistance on both sides increased by 3 to 4 times), which will certainly yield good results. 3. It cannot be cleaned; provide the manufacturer with accurate information regarding operating conditions and the properties of the oil. Use a new heat exchanger in series or parallel with a 75-square-meter heat exchanger (when used in parallel, be sure to install control valves if the two units have different structures).
Regarding the modifications, there are three units: one old 37-square-meter cooler was removed from each of the three units, and new 75-square-meter coolers were installed in its place; they were then connected in parallel again. All of the equipment had been cleaned after undergoing major repairs.