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Fouling Focus
300
250
Fouling Losses Cleaning Cost
Total Cost
200
Cost
150
100
50
0 0 2 4 6 8 10 12 14 16 18 20 22 24 Months
Figure 1 . Optimum cleaning interval .
in a given situation or some might dominate over others . It is important to quantify the major costs to decide what actions to take to minimize the costs . That action could be the use of mitigation techniques ( parts 4 & 5 ), cleaning ( part 6 ), or a redesign of the heat exchanger . All these actions have their associated costs which must be justified based on how much of the losses due to fouling can be recovered . If a heat exchanger fouls it loses its ability to deliver the required heat duty , and that loss of heat duty has many consequences . Table 1 describes the various types of losses and Table 2 lists the costs to consider when applying a mitigation technique .
Benefit vs cost To evaluate the economic benefit of an action against fouling , the first step is to add up all the costs from Table 1 . This can be looked at as the cost of doing nothing , as it represents the situation as it exists . This cost should be projected over some future period , with the current cleaning interval being a good time frame to use . But one year , or six months for severe fouling , can also be reasonably used . Next , evaluate the costs associated with all the relevant items in Table 2 , remembering to include recurring costs . It is especially important to consider all costs when comparing alternative actions for mitigation . A simple indicator to compare two mitigation methods is the ratio of benefit vs cost , evaluated over the same time frame . A more meaningful calculation will consider a life-cycle cost , projected over as many as 25 years .
Optimum cleaning interval If cleaning is the only action to be taken , the time to clean can be optimized by combining the costs and benefits as shown in Fig . 1 . This analysis is relatively simple for a single heat exchanger but requires software when networks are involved , because of the interaction between heat exchangers . Figure 1 shows that the more frequently you clean the higher the cost of cleaning ( blue line ) but lower the losses due to fouling ( red line ). The sum of these two is the total cost ( green line ) which shows a minimum at 6 months , the optimum cleaning time .
Summary There are many types of losses when a heat exchanger fouls , and many types of costs when some mitigation action is taken . All costs should be considered when making decisions about mitigation actions , including cleaning . Such an approach will identify the most cost effective actions and allow for economically better decisions .
Upcoming in Part 9
We will look at design specifications and
practices used to guard against fouling and
provide some guidelines for good designs that
minimize fouling . www . heat-exchanger-world . com Heat Exchanger World June 2024
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