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Approach Temperature-This is defined as the difference between the saturation temperatures of the steam and the inlet water. Lowering the approach temperature can result in increased steamproduction， but at increased cost. Conservatively high-approach temperatures ensure that no steam generation takes place in the economizer.Typically， approach temperatures are in the 10-20 0F (5.5-11 0C) range. Figure 1-40 is the temperature energy diagram for a system and also indicates the approach and pinch points in the system.
Oρρ-Design Perρormance-This is an important consideration for wasteheat recovery boilers. Gas turbine performance is affected byload， ambientconditions， and gas turbine health (fouling， etc.). This can affect the exhaust gas temperature and the air flow rate. Adequate considerations must be given to bow steam flows (low pressure and high pressure) and superheat temperatures vary with changes in the gas turbine operation.
Evaporators-These usually utilize a fin-tube design. Spirally finned tubes of 1.25 in to 2 in outer diameter (0D) with three to six fins per inch arecommon. In the case of unfired designs， carbon steel construction can beused and boilers can run dry. As heavier fuels are used， a smaller number of fins per inch should be utilized to avoid fouling problems.
Forced Circulation System-Using forced circulation in a waste heat recovery system allows the use of smaller tube sizes with inherent increased heat transfer coefficients. Flow stability considerations must be addressed. The recirculating pump is a critical component from a reliability standpointand standby (redundant) pumps must be considered. In anyevent， great care must go into preparing specifications for this pump.

SUPER HEATER
PINCH POINT
APPROACH TEMPERATURE
ENERGY TRANSFER
Figure 1-.0. .ner.y transfer dia.ram in an .RS. of a com.ined cycle power plant.
Back Pressure Considerations (Gas Side)-These are important， as exces-sively high back-pressures create performance drops in gas turbines. Very low-pressure drops would require a very large heat exchanger and more expense. Typical pressure drops are 8-10 inches of water.
Supplementary Firing of .eat .ecovery Systems
There are several reasons for supplementary firing a wasteheat recovery unit. Probably the most common is to enable the system to track demand
(i.e.， produce more steam when the load swings upwards， than the unfired unit can produce). This may enable the gas turbine to be sized to meet the base load demand with supplemental firing taking care of higher load swings. Figure 1-41 shows a schematic of a supplementary fired exhaust gas steam generator.
Raising the inlet temperature at the waste heat boiler allows a significantreduction in the heat transfer area and， consequently， the cost. Typically， as thegas turbine exhaust has ample oxygen， duct burners can be conveniently used.
An advantage of supplemental firing is the increase in heat recovery capability (recovery ratio). A 50% increase in heat input to the systemincreases the output94%， with the recovery ratio increasing by 59%. Some important design guidelines to ensure success include:
.  Special alloys may be needed in the superheater and evaporator to withstand the elevated temperatures.

.  The inlet duct must be of sufficient length to ensure complete combus-tion and avoid direct flame contact on the heat transfer surfaces.

FP0
Figure 1-.1. Supplementary fired exhaust .as steam .enerator.
.If natural circulation isutilized， an adequate number of risers and feeders must be provided as the heat flux at entry is increased.
. Insulation thickness on the duct section must be increased.
Instrumentation and Controls
The advanced gas turbines are all digitally controlled and incorporate on-line condition monitoring. The addition of new on-line monitoring requires new and smart instrumentation. The use of pyrometers to sense blade metal temperatures are being introduced. The blade metal temperatures are the real concern not the exit gas temperature. The use of dynamic pressure transducers for detection of surge and other flow instabilities in the com-pressor and also in the combustion process especially in the new Low N0xCombustors， are being introduced. Accelerometers are being introduced todetect high-frequency excitation of theblades， this prevents major failures in the new highly loaded gas turbines.
　
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