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Environmental Impact of Heat Exchanger Fouling
DOI 10.1615/hedhme.a.000361
3.17 FOULING IN HEAT EXCHANGERS
3.17.5 Environmental impact of heat exchanger fouling
G. T. R. Bott
It is not well appreciated that the problem of heat exchanger fouling has wide environmental repercussions. The effect may sometimes be very apparent but much of the impact is less obvious, and is hidden in general industrial activity.
The essential purpose of a heat exchanger as the name implies, is to transfer heat as effectively as possible. In essence in general terms, heat used in industrial processes is derived from a primary fuel source e.g. fossil fuels, biomass or waste combustion. The heat may be recovered directly for use in the process such as in a pipe still on a petroleum refinery. It is perhaps more common for the combustion heat to be used to raise steam for direct use as a heating medium, or in conjunction with a turbine to produce electricity. Inefficiencies principally due to heat transfer surface fouling, occur in the heat transfer processes, that result in the consumption of additional fuel to make up for the shortfall between what is theoretically possible and what is actually achieved in practice.
In order to reduce energy costs, particularly in large scale processing operations, such as a chemical complex or petroleum refinery, it is essential to recover as much heat as possible. The technology involves the transfer of heat contained in hot product streams, which require cooling, to cold streams that need to be heated to satisfy process requirements. In large-scale operations there is likely to be a complex arrangement of heat exchangers that may be optimized by the application of the concept of process integration or pinch technology Linnoff et al.(1982). The effectiveness of the whole heat recovery process however, is dependent on the sum of efficiencies of each individual heat exchanger in the network.
In broad general terms, it ought to be possible to reuse much of the heat involved in processing, except where the heat is used in endothermic reactions, or where the heat becomes degraded to relatively low temperatures. The opportunity to utilize low grade heat is affected by the overall thermal efficiency. The sum of these individual inefficiencies gives rise again, to a short fall in heat recovery that has to be made up from the energy source, ultimately from the combustion of primary fuel.
The efficiency of an individual heat exchanger for a given set of operating conditions of temperature and flow rate, is largely governed by the extent of the fouling experienced on both sides of the exchanger, as described elsewhere in this chapter. In addition to the resistance to heat transfer, the presence of the fouling deposits restricts flow and, for a given throughput will increase the pressure drop through the exchanger. In order to maintain the flow to satisfy process requirements, this will represent an increase in pumping energy. Many industrial systems will use electrically driven pumps, so that the increased energy requirement will ultimately be manifest in increased combustion of fuel.
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