Heat Transfer

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Together with combustion, heat transfer is a complementary and critical process for energy efficient operation. Combustion is just the heat-releasing phase and its good performance just guarantees maximum availability of this energy. Good heat transfer is the other side of the process, guaranteeing maximum absorption of liberated heat. In a simple thermodynamic approach, to achieve maximum yield in a heat machine, it is necessary to minimize heat escape. If a refinery is treated like a huge heat machine, avoiding heat losses by containment and recovery is paramount for energy efficiency, and it is obtained by controlling heat transfer.

Heat is transferred by three ways: radiation, convection and conduction, and all are dependent on temperature from bodies exchanging energy. For convection and conduction, physical contact between bodies is essential, but not for radiation. So in a heat exchanger like a shell and tube or tube bundling, heat is transferred by conduction and convection, while a fired heater is in fact a heat exchanger in which the major heat amount is transferred by radiation. Conduction transfer depends mainly on the constituent material of the equipment, and convection, on the physical characteristics of fluids exchanging heat and equipment design. For radiation, like in process fired heaters or in boilers, the interaction between combustion flame, flue gases and absorbing heat tubes must be addressed at the design stage, to achieve high heat transfer rates by radiation and convection, the last for economic purposes, to increase overall energy efficiency of the equipment.

During operation either in furnaces or heat exchangers to guarantee maximum heat transfer rates, avoiding fouling is a big energy efficiency opportunity. Fouling occurs due to film deposits, such as oil films or scaling due to corrosion. It can be avoided in fired heaters by proper combustion, as mentioned in the previous section, avoiding low air conditions with soot formation and averting flame incidence over heat absorbing tubes. This can be accomplished partially by the same procedures for good combustion, with monitoring of tube skin temperature inside the furnace. Keeping heat absorbing surfaces clean is one step for quality heat transfer in furnaces. Heat containment is another, obtained by proper refractory and insulation. Refractory is any material that can withstand high temperatures and abrasive or corrosive conditions, can tolerate sudden temperatures changes, and possesses low thermal conductivity and a low thermal expansion coefficient. Also it has a reasonable emissivity, defined as the ability to both absorb and radiate heat. These properties together give more chance for heat absorption while pre venting it from being wasted to the atmosphere. Monitoring the condition of the refractory is another relevant operational task.

One improvement opportunity related to this area is preheating the combustion air. Recovering heat from stack flue gas to preheat air entering the furnace is a form of heat recovery. And once heat is recycled to the furnace, energy losses are reduced and reagents mixing and reaction improves. However, higher air temperature for combustion increases nitrogen oxide formation, that is a regulated pollutant, so finding out what should be its limit and tracking it, is another significant operational procedure.

Heat recovery from all fluids processed or produced heat transfer is the most important operation for energy efficiency in a refinery. This is achieved by proper design and sequencing of heat exchangers. Fouling is taken into account usually by appropriate flow velocity design inside the heat exchangers and some extra heat transfer area known as ' fouling factor' - This factor should not be exaggerated otherwise an oversized exchanger may signify lower velocities and ends up increasing fouling. The optimal sequence of exchangers is also a design task where methodologies like ' pinch' apply. For operations, when running a train of heat exchangers, it is important to maintain flow rates and track temperatures of fluids passing through the exchangers, to guarantee the expected heat transfer exchanges to occur. If they don't comply, fuel demand on fired heaters or steam demand on exchangers along the process rises, revealing a low energy efficiency point. Recording these facts indicates a maintenance intervention spot for next turnaround.

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