Importance of steady condensate flow in an evaporation system
GLM Hydro Evaporator Problem Solving: – During the course of many falling film and forced circulation evaporator optimization jobs I often see issues of unstable performance of the evaporation systems. There are several issues that can cause an evaporator fluctuating in performance. Sometimes it is a combination of events which all have to be checked and fixed one by one in order to unravel and, more importantly, fix the problem.
In one case it became impossible for the operators to run the falling film evaporator on the limits of its capacity because the system was constantly fluctuating. This resulted in an over- and under-concentration of the product. Temperatures and pressures were all over the place and the unit was basically out of control which created a discouraged team of operators and a very frustrated manager.
I was called to look at a problem which had developed over time. This particular falling film evaporator system is equipped with a Mechanical Vapor Recompression (MVR). It turned out that even the power on the MVR motor was unstable in the same pattern as the entire system. After studying the pattern of the instability and ruling out some of the other process conditions that could have caused the instability in the evaporator it became obvious that the condensate discharge flow was causing the issue.
The condensate collection tank and the suction line to the condensate pump where relatively small. This small size made a proper steady discharge flow control unpredictable. The energy of the condensate discharge is exchanged in a pre-heater where the cold incoming product feed is used to optimize the energy balance of the system. Because of the small size of the tank, the fluctuation in condensate discharge flow caused a fluctuation in preheating temperature of the product feed. As a result, this created a fluctuation of the entire falling film evaporator system, in temperature and pressure. Based on our findings we decided to modify the condensate collection tank so we could control a secure level. By doing so we created a steady discharge flow.
Another challenge in this particular system was that the path the condensate was going was far from optimal. The condensate was either circulating back into the evaporator feed tank or drained next to the evaporator feed tank, both with a relatively low discharge pressure. As soon as the production started and the condensate conductivity was low enough to discharge into the condensate storage tanks, the counter-pressure that the condensate discharge pump was facing became much higher. This changed the characteristics of the control loop as well. We fixed that issue with a simple orifice plate just before the drain and feed tank connection. As a result the differences between the condensate counter pressures would be more consistent.
We renovated this system with some simple modifications. Now the system can be operated on the design conditions without the constant over- and under-concentration. This proves that some relatively simple changes can make the difference between a frustrating production run and a smooth evaporator performance.
Contact us for more information when your installation has these or any other kind of issues.
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