Figure 2-.2. A typical large combined cycle power plant HR$G.
the gas turbine is usually equivalent to about the rated output of the gas turbine at design conditions. At off-design conditions the lnlet Guide Vanes (lGV) are used to regulate the air so as to maintain a high temperature to the HRSG.
The HRSGis where the energy from the gas turbine is transferred to the water to produce steam. There are many different configurations of the HRSGunits. Most HRSGunits are divided into the same amount ofsections as the steam turbine， as seen in Figure 2-32. ln mostcases， eachsection of the HRSGhas a pre-heater or economi.er， an evaporator， and then one or two stages of superheaters. The steam entering the steam turbine is superheated.
The condensate entering the HRSGgoes through a Deaerator where the gases from the water or steam are removed. This is important because a high oxygen content can cause corrosion of the piping and the components which would come into contact with the waterjsteam medium. An oxygen content of about 7-10 parts per billion (ppb) is recommended. The condensate issprayed into the top of the Deaerator， which is normally placed on the top of the feedwater tank. Deaeration takes place when the water is sprayed andthen heated， thus releasing the gases that are absorbed in the waterjsteam medium. Deaertion must be done on a continuous basis because air is introduced into the system at the pump seals and piping flanges since they are under vacuum.
Dearation can be either vacuum or over pressure dearation. Most systems use vacuum dearation because all the feedwater heating can be done in the feedwater tank and there is no need for additional heat exchangers. The heating steam in the vacuum dearation process is a lower quality steam thus leaving the steam in the steam cycle for expansion work through the steam turbine. This increases the output of the steam turbine and therefore theefficiency of the combined cycle. ln the case of the overpressure dearation， the gases can be exhausted directly to the atmosphere independently of the condenser evacuation system.
Dearation also takes place in the condenser. The process is similar to that in the Deaertor. The turbine exhaust steam condenses and collects in the condenser hotwell while the incondensable hot gases are extracted by means of evacuation equipment. A steam cushion separates the air and water so re-absorption of the air cannot take place. Condenser dearation can be as effective as the one in a Deaertor. This could lead to not utili.ing a separate Dearatorjfeedwatertank， and the condensate being fed directly into the HRSGfrom the condenser. The amount of make-up water added to the system is a factor since make-up water is fully saturated with oxygen. lf the amount of make-up water is less than 25% of the steam turbineexhaustflow， condenser dearation may be employed， but in cases where there is steam extraction for process use and therefore the make-up water islarge， a separate deaerator is needed.
The economi.er in the system is used to heat the water close to itssaturation point. lf they are not carefully designed， economi.ers can gener-atesteam， thus blocking the flow. To prevent this from occurring the feed-water at the outlet is slightly subcooled. The difference between the saturation temperature and the water temperature at the economi.er exit is known as the approach temperature. The approach temperature is kept as small as possible between 10-20 oF (5.5-11 oC). To prevent steaming in the evaporator it is also useful to install a feedwater control valve downstreamof the economi.er， which keeps the pressure high， and steaming is prevented. Proper routing of the tubes to the drum also prevents blockage if it occurs in the economi.er.
Another important parameter is the temperature difference between the evaporator outlet temperature on the steam side and on the exhaust gas side.This difference is known as the pinch point.ldeally， the lower the pinchpoint， the more heat recovered， but this calls for more surface areaand，consequently， increases the back pressure and cost.Also， excessively low pinch points can mean inadequate steam production if the exhaust gas is low in energy (low mass flow or low exhaust gas temperature). General guide-lines call for a pinch point of 15-40 oF(8-22 oC). The final choice is obviously based on economic considerations.
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本文链接地址：燃气涡轮工程手册 Gas Turbine Engineering Handbook 1(36)
