Optimum Cleaning Cycles

The quantity of scale deposited in an evaporator at any time is proportional to the quantity of liquid which has been evaporated and therefore to the heat which has been transferred up to that time. Evaporators normally operate with constant temperature difference. 

In many cases it Is more reasonable to assume that cleaning time is a weak linear function of production cycle. Thus, in general  

Bcc fixed unreducible portion of cleaning time a = constant 

The criterion for the maximum throughput cycle is that R is a maximum. 

The corresponding minimum cost problem can be formulated in a simiiar manner. 

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No one method, however, has all the desired features of simplicity, low cost, and effectiveness. To achieve optimum cleanliness of substrate surfaces, combinations of the various cleaning methods must be used. There is no universal approach to cleaning cycles. There are, however, some general guidelines to follow when establishing cleaning sequences. 

The benefits stated above are due solely to the qualitative early identification of shorts and bad contacts. Beyond this, the HELM tracker will collect quantitative data (primarily current and time), which can be exploited for further benefits. The system can identify the total charge (Amp hours) that has passed through individual cathodes, cells, and banks, to provide an indication of the overall inventory of plated production at any time. By detecting stripping and cleaning cycles, the system can not only identify which cells are next ready for harvesting, but also nominate the optimum time for uniform plate thickness. 

For some processes, though they would not be classified as batch processes, the period of continuous production will be limited by gradual changes in process conditions such as, the deactivation of catalysts or the fouling of heat-exchange surfaces. Production will be lost during the periods when the plant is shut down for catalyst renewal or equipment clean-up, and, as with batch process, there will be an optimum cycle time to give the minimum production cost. 

The regular use of sodium pentachlorophenate has proved successful in many instances, but it is a persistent environmental hazard and cannot be recommended. However, o -benzy 1-p -chlorophenol can often produce goods results, with low bacteria counts and clean systems. But the timing of biocide applications needs to be matched carefully with the production cycles (and process leakage periods) for optimum product effectiveness. 

During the time an evaporator is in operation, solids often deposit on the heat-transfer surfaces, forming a scale. The continuous formation of the scale causes a gradual increase in the resistance to the flow of heat and, consequently, a reduction in the rate of heat transfer and rate of evaporation if the same temperature-difference driving forces are maintained. Under these conditions, the evaporation unit must be shut down and cleaned after an optimum operation time, and the cycle is then repeated. 

The total cost includes (1) fixed charges on the equipment and fixed overhead expenses, (2) steam, materials, and storage costs which are proportional to the amount of feed and evaporation, (3) expenses for direct labor during the actual evaporation operation, and (4) cost of cleaning. Since the size of the equipment and the amounts of feed and evaporation are fixed, the costs included in (1) and (2) are independent of the cycle time. The optimum cycle time, therefore, can be found by minimizing the sum of the costs for cleaning and for direct labor during the evaporation. 

A worked example showing how basic filtration data can be used to assess the optimum cycle time, and hence filter area required to complete a clarification of a v etable oil using precoat filtration on a pressure leaf filter. The filtration is assumed to be constant rate, after having formed a precoat of 2 mm on the pressure leaves prior to clarification. The cleaning and reforming of the precoat takes iproximately 30% of the total cycle time, hence active filtration time is only 70% of any given cycle time. The analysis is based on a 14 h working and 350 days per year. These are easily altered in cells D15 and D16, respectively. 

Various cycle times are used in column A flrom 1 to 8 h, the optimum, i.e. lowest cost per unit volume of filtrate, is 2 h. Alternatively, the lowest total annual cost is given by a 3 hour cycle, but with a significantly lower yield of cleaned oil. 

The term 6cc is a cost-modified value of the cleaning time constant, 6ce< which becomes equal to 0cc when K = F, but is commoniy greater than cc- The optimum throughput for a minimum cost Cycle is such that the tangent from g = (at P = 0) to the curve of P versus 6 touches the curve at (flopt. Popt )- Actual cleaning time is then O c + aPopt - The graphical solution is shown in Figure 12-2. 

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