Superheat and Reheat

The condition of the working fluid in the heat exchanger of the reactor circuit shown in Fig. 6.14 can be illustrated by the temperature-entropy 

For high thermal efficiency, it is desirable that the cycle should approximate to a reversible one. The cycle can be reversible only if the temperature difference between the primary and secondary coolants is everywhere equal to zero, a condition impossible to attain in practice, since there has to be a finite difference between the two fluids in order to promote heat transfer between them. In the diagram, the smallest temperature difference between the primary and secondary fluids occurs between the points B and 3 this gap is called the pinch point. 

In order to approach a reversible cycle, the temperature difference (AT) at the pinch point should be as small as possible. Since the heat transfer rate in the exchanger is also proportional to the surface area of contact between the two fluids, it is possible to compensate for a smll AT by increasing the size of the heat exchanger. There are, of course, obvious limitations to the practical size of the unit, so that with this type of cycle there will always be an appreciable difference between the average temperatures of the two coolants. 

One way of reducing the average temperature difference is by superheating of the saturated steam, as shown by the line CD in the right portion of Fig. 6.21. This not only increases the thermal efficiency of the cycle, but has 

A PWR fuel element is 12 ft long and has a fuel diameter of 0.366 in. Find the total heat generation rate for the element if the heat generation rate per unit volume at the central plane is 6.52 x 10 Btu/h ft.  

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After preliminary shop tests on different mockups (e.g. superheater and reheater headers and steamline pipe sections), since 1996 AEBIL systems have been installed and are at work on several power plant components. In particular, three different components have been monitored throughout 1997 (monitoring still under way at the time being) ... 

An example of a modem, tangentially fired, supercritical, lignite-fuel furnace is shown in Figure 5. This unit, at maximum continuous ratings, supplies 2450 metric tons pet hour superheat steam at 26.6 MPa (3850 psi) and 544°C, and 2160 t/h reheat steam at 5.32 MPa (772 psi) and 541°C. These ate the values at the superheater and reheater oudet, respectively. Supercritical fluid-pressure installations ate, however, only rarely needed. Most power plants operate at subcritical pressures in the range of 12.4—19.3 MPa (1800—2800 psi). 

The oxidant preheater, positioned in the convective section and designed to preheat the oxygen-enriched air for the MHD combustor to 922 K, is located after the finishing superheat and reheat sections. Seed is removed from the stack gas by electrostatic precipitation before the gas is emitted to the atmosphere. The recovered seed is recycled by use of the formate process. Alkali carbonates ate separated from potassium sulfate before conversion of potassium sulfate to potassium formate. Sodium carbonate and potassium carbonate are further separated to avoid buildup of sodium in the system by recycling of seed. The slag and fly-ash removed from the HRSR system is assumed to contain 15—17% of potassium as K2O, dissolved in ash and not recoverable. 

Superheaters and Reheaters A superheater raises the temperature of the steam generated above the saturation level. An important function is to minimize moisture in the last stages of a turbine to avoid blade erosion. With continued increase of evaporation temperatures and pressures, however, a point is reached at which the available superheat temperature is insufficient to prevent excessive moisture from forming in the low-pressure turbine stages. This condition is resolved by removing the vapor for reheat at constant pressure in the... 

The boiler designer must proportion heat-absorbing and heat-recovery surfaces in a way to make the best use of heat released by the fuel. Water walls, superheaters, and reheaters are exposed to convection and radiant heat, whereas convection heat transfer predominates in air preheaters and economizers. The relative amounts of these surfaces vary with the size and operating conditions of the boiler. 

Armitt, J., Holmes, D. R, Manning, M. 1. and Meadowcroft, D. B., The Spalling of Steam Grown Oxides from Superheater and Reheater Tube Steels, EPR1-FP-686,TPS 76-655 Final Report (February 1978). 

Power boilers usually incorporate a reheater and secondary superheater. Superheaters and reheaters may be pendant or horizontal, and some boiler designs may use their roof cooling tubes to provide a support for these various tube bundles. 

Superheaters and reheaters are tube bundles located in either the boiler furnace section or the convection-pass section. They are designed to increase the temperature of generated steam prior to its being passed to a turbine. Many larger power boiler designs incorporate a steam reheater in addition to one or two banks of superheaters. 

Excessive boiler cycling may result in the exfoliation of high temperature oxide scales from superheater and reheater tubes, and from steam outlet piping. The particles and flakes of exfoliated scale may... 

Oxygen in a large operating boiler may corrode steam-water separators and boiler surface components such as the top drum (especially at the waterline) and tubes. Oxygen corrosion also may occur in superheater and reheater tubes, especially in places where moisture can collect, such as in bends and sagging tubes. 

Impingement or erosion-corrosion. Where exfoliated magnetite from high heat-flux areas, such as superheaters and reheaters, occurs, apart from the damage caused in turbine areas or process components, the particles may be swept into the condensate system, ultimately causing further erosion throughout the entire system... 

Boilers with superheaters and reheaters should use only volatile organic chemicals. 

The highest possible quality of water should be used for nondrainable superheaters and reheaters to prevent the risk of solids deposition in the units. 

Fouling is the accumulation of mineral-derived ash on the superheater and reheater tubes in the convective (heat exchanger) section downstream from the furnace. Fouling restricts the flow of exhaust gases and impedes heat transfer through superheater tube walls and thereby reduces the amount of steam generated. 

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