Outlet Temperatures

SH Outlet Temperature

Superheater outlet temperature is the temperature at which the steam leaves the SH. It is usually the highest temperature attained by the steam in a boiler. RH steam is sometimes slightly higher than the SH steam in large utility boilers. Superheater outlet temperature up to —350°C is usually not controlled, being used mainly for process heating. Then SOT reduces as the load reduces. For power application, the SOTs are generally >400°C and are required to be controlled in two ways:

1. By limiting the temperature within a band of ±5°C for the safety of the turbine casing by means of an attemperator

2. By keeping the temperature constant up to usually 70% load by sizing the SH larger (a range of 70-100% is then known as the steam temperature control [STC] range of the boiler)

It is important that the STC is decided judiciously as the SH, the most expensive sur­face of the boiler, becomes unnecessarily larger. Steam temperature control can vary from 50 to 100% MCR, but is mostly limited to 70-100% in power plants. At <70% MCR, the drop in SOT is not all that steep when there is large radiant SH.

Automatic combustion control (ACC) is different from STC. Combustion control is the regula­tion of fuel and air in right proportions to provide an optimum combustion, which is usually performed by ACC logic. This is usually designed to operate in auto mode between —40% MCR and peak load, which is more than adequate for meeting the load variations in most pro­cess and power applications. At lower loads, the boiler has to be operated manually. To make ACC extend to even lower MCR is very expensive as the flow measurements are difficult.

RH Outlet Temperature

Reheater outlet temperature (RHOT) is the temperature at which steam leaves the RH. Reheater inlet temperature is turbine-dependent and it is lower at lower loads. The RHOT is required to be held constant for optimum cycle efficiency. Since RH pressure is one-third of the main steam pressure, the RHOT is usually equal to or preferably higher than the SOT.

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