SUPERHEATERS

Superheated steam is produced by causing saturated steam from a boiler to flow through a heated tube or superheater, thereby increasing the temperature, enthalpy, the specific volume of the steam.

It should be noted that in an actual superheater there will be a decrease in steam pressure due to fluid friction in the superheater tubing.

Maximum work is obtained when a fluid expands at constant entropy, that is, without friction and without heat transfer to the surroundings. By calculations it will be found that the constant-entropy expansion of 1 lb of dry saturated steam at 1000 psia to a final pressure of 1.0 psia will result in the conversion into work of 417 Btu, whereas the expansion of superheated steam at the same initial pressure, 1000 psia but at 1000° F, to the same final pressure of 1.0 psia will result in the conversion into work of 581 Btu, an increase of 39.3 per cent.

In addition to the theoretical gain in output due to the increased temperature of superheated steam as compared to saturated steam, there are additional, advantages to the use of superheated steam in turbines. The first law of thermodynamics states that all the work done by the turbine comes from the energy in the steam flowing through the turbine.

Thus, if steam enters the turbine with an enthalpy of 1300 Btu per lb and the work done in the turbine is equivalent to 300 Btu per lb of steam, the enthalpy of the exhaust steam will be 1300—300 = 1000 Btu per lb, neglecting heat transfer to the surroundings. If sufficient energy is convert­ed into work to reduce the quality of the steam below about 88 per cent, serious blade erosion results because of the sand­blasting effect of the droptets of water on the turbine blades.

Also, each 1 per cent of moisture in the steam reduces the efficiency of that part of the turbine in which the wet steam is expanding by 1 to 0,5 per cent. It is necessary, there­fore, that high-efficiency steam turbines be supplied with superheated steam. The minimum recommended steam temperature at the turbine throttle of condensing turbines far various initial steam pressures is as follows:

Large power plants currently being built in regions of high fuel cost are designed for operation at pressures of more than 1500 psig. At these high pressures, a reduction in the annual fuel cost of 4 to 5 per cent can be made by expanding the steam in the turbine from the initial pressure and 1000 to 1100° F to an intermediate pressure of about 30 per cent of the initial pressure, returning the steam to the steam-generating unit, and passing it through a second superheater, known as a reheater, where it is superheated to 1000 to 1100° F, and then completing the expansion of the steam in the turbine. For initial steam pressures above the critical pressure (3206 psia), a second stage of reheating is employed.

The decreased strength of steel at high temperature makes it necessary to use alloy steels for superheater tubing where steam temperatures exceed 800° F. Alloy steels containing 0.5 per cent of molybdenum and 1 to 5 per cent of chromium are used for the hot end of high-temperature superheaters at steam temperatures up to 1050° F, and austenitic steels such as those containing 18 per cent chromium and 8 per cent nickel are used for higher temperatures.

Superheaters may be classified as convection or radiant superheaters. Convection superheaters are those that receive heat by direct contact with the hot products of combustion which flow around the tubes. Radiant superheaters are located in furnace walls where they "see" the flame and absorb heat by radiation with a minimum of contact with the hot gases.

In a typical superheater of the convection type saturated steam from the boiler is supplied to the upper or inlet header of the superheater by a single pipe or by a group of circulator tubes. Steam flows at high velocity from the inlet to the outlet header through a large number of parallel tubes or elements of small diameter. Nipples are welded to the headers at the factory, and the tube elements are welded to the nipples in the field, thus protecting the headers from temperature stresses due to uneven heating during final welding.

The amount of surface required in the superheater depends upon the final temperature to which the steam is to be superheated, the quantity of hot gas flowing around the superheater, and the temperature of the gas. In order to keep the surface to a minimum and thus reduce the cost of the superheater, it should be located where high-temperature gases will flow around the tubes. On the other hand, the products of combustion must be cooled sufficiently before they enter the superheater tubes so that any ash that may be present has been cooled to a temperature at which it is no longer sticky or plastic and will not adhere to the superheater tubes. In a modern two-drum steam generating unit fired by a continuous-ash-discharge spreader stoker, the superheater is located ahead of the boiler convection surface and at the gas exit from the furnace. In installations burning coal having a high content of low-fusing-temperature ash, it may be necessary to place a few boiler tubes ahead of the superheater.