The evaporation capacity of the double-effect concentrator does not reach the designed value; how to carry out modifications?
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Hello everyone, our company has two sets of automatically controlled double-effect concentrators: one with a rising-film forced circulation system, featuring 1 heater and 1 evaporation chamber in the first effect, while the second effect has two heaters connected in parallel along with 1 evaporation chamber. The designed evaporation rate is 6000 KG/hour. They are located in two different factories; the first set can reach 6000, while the second set can only reach 3500–4000 KG. I can’t find the reason now. Situations where the targets cannot be achieved: 1. The vacuum level in the first stage drops to -0.05 MPa, with the temperature of the liquid medicine at around 70–80 degrees; the vacuum level in the second stage is around -0.08 MPa, and the temperature of the liquid medicine is 45–50 degrees. 2. Increase the amount of steam supplied; the vacuum level in the first stage should be reduced to between -0.03 MPa and -0.04 MPA, with the temperature of the solution at around 90 degrees. The vacuum level in the second stage should be between 0.07 MPa and -0.08 MPA, while the temperature of the solution is 55–60 degrees. 3. When the raw steam flow is increased further, the vacuum level in the first stage drops to near atmospheric pressure, and it is not possible to increase the flow any more. We compared the two sets of concentrators and found that: 1. The area of the heater, as well as the diameter, quantity, and length of the heating tubes, are all the same. 2. The area of the condenser, as well as the diameter, quantity, and length of the tubes, are all the same. 3. The pipes for supplying secondary steam to the two-effect heater are DN200 for the first set and DN150 for the second set. 4. The outer diameter of the liquid medicine pipe in the first set is 38 mm, while that in the second set is 32 mm. 3. The two sets of concentrators were manufactured by different manufacturers; the second set of concentrators comes from another factory, and its control system is different. Planned renovation approach: I believe that the evaporation rate in the second stage has not yet reached an adequate level. It is planned to add a DN50 steam pipe to the secondary steam pipeline, with a stop valve used to control the amount of steam supplied. But here’s the problem: 1. Can a true system-level vacuum level be achieved, especially in the first stage? ； 2. Will it compete with the first-stage heater for steam, as the steam comes from the same main pipeline, and a vacuum exists in the second stage that draws steam away? ； 3. The secondary steam pipeline may not be able to withstand the steam pressure, even with a vacuum present. There are safety hazards. 4. Is it possible to use additional steam for heating in the second stage? Is this approach feasible in principle? I hope experienced friends can offer some good suggestions; we can communicate via phone, WeChat, etc. Thank you!
Check the outlet temperature of the secondary condenser: 1. To increase the vacuum level, it is necessary to ensure that the vapor temperature at the condenser outlet is below the dew point temperature. For the make-up steam in the second stage, it needs to be completely separated from the secondary steam; in that case, the heater in the second stage must be redesigned and modified.
The last edit to this post was made by symc on 2019-3-14 at 08:01. 1. Whether the concentrated products are identical is very important when comparing evaporators from different manufacturers; it is a decisive factor； 2. If it is the same type of product, then please conduct a thorough integrity test on the second set; if there is a significant amount of air leakage, that will cause the differences you described ； 3. If the level of tightness is the same, then please carefully examine the second set of double-effect concentration condensers, and compare them in detail with the design heat load of the first set of double-effect concentrators – obviously, the one with a higher heat load will yield significantly better results ； 4. Check the degree of contamination of the second set of evaporators and condensers; it is advisable to compare it with the contamination level of the first set’s condenser—the fouling coefficient also has a significant impact on the differences you observe ； I’ve just realized there is a difference; based on these parameters, it’s clear that there is an issue with the evaporator design of the second set. The fact that the pipes for the secondary steam are so much thinner results in a greater pressure difference between the two sides, which in turn reduces the actual heat load on the secondary heater (the exact amount of reduction requires further calculation) ； Apart from that, there are the other items mentioned above that need to be checked. Only after all of them have been checked can the cause and the modification plan be determined; otherwise, the modifications will be carried out in a rather blind manner! Furthermore, as explained, the total thermal load on the evaporator is lower than that of the first set, since the heat transfer area is smaller... You can do the calculations yourselves. Heat load is key……
Poster: The diameters of the heating tubes are 32 and 38 respectively; the number and length of the tubes are the same, so how can their areas be the same? .
It’s not the diameter of the heating tube. It are the pipes before and after the external circulation pump. The diameter of the heating tubes is 25 in all cases
It’s not the diameter of the heating tube. It are the pipes before and after the external circulation pump. The diameter of the heating tubes is 25 in all cases
If the diameter of the primary secondary steam pipe is too small. I’m also wondering if a vacuum pump can improve this issue; I’ll install one this afternoon to give it a try.
If the diameter of the secondary heating tube is smaller, it means that the heat load is lower… Vacuum evaporation is used, and the liquid being evaporated needs to absorb heat… However, the amount of heat absorbed does not depend on the vacuum level; rather, it depends on the heat load specified in the design of the equipment… Energy conservation applies here: as much heat is taken away through evaporation, that much heat must be supplied back.
It’s not that the diameter of the heating tube is too small; it’s the secondary steam pipeline that connects the first-effect evaporator to the second-effect heater which is too small. It was originally DN200; now this one is DN150
1. The vacuum level in the first stage drops to -0.05 MPa, with the temperature of the liquid medicine being around 70–80 degrees; the vacuum level in the second stage is approximately -0.08 MPa, and the temperature of the liquid medicine is 45–50 degrees. 2. Increase the amount of steam supplied; the vacuum level in the first stage should be reduced to between -0.03 MPa and -0.04 MPA, with the temperature of the solution at around 90 degrees. The vacuum level in the second stage should be between 0.07 MPa and -0.08 MPA, while the temperature of the solution is 55–60 degrees. 3. When the raw steam flow is increased further, the vacuum level in the first stage drops to near atmospheric pressure, and it is not possible to increase the flow any more. According to 1,2,3, an increase in primary pressure leads to an increase in boiling point, as well as an increase in the temperature of the secondary steam. This indicates that steam can be produced from the first effect, so the heat exchanger in the first effect is likely not the main limiting factor. However, secondary steam cannot effectively remove impurities from the second effect. As the temperature of the secondary steam rises, the temperature of the second effect also increases, which indicates that the solution in the second effect has not reached its boiling point. There are two possible reasons for this: 1. The vacuum level of the solution in the second effect is insufficient, 2. The vacuum level in the secondary steam pipeline is insufficient. It is recommended to check the pumping capacity of the vacuum pump and the diameter of the vacuum tubes.