Recuperators configuration

The baseline recuperator effectiveness has been increased from 0.90 to 0.92 and the thermal load has been increased from 514 IdA/ to 761 kW relative to the Hamilton Sundstrand recuperator configuration in Reference 9- 54. 

The simplest configuration for a recuperative heat exchanger is the metallic radiation recuperator (Fig. 27-57). The inner tube carries the hot exhaust gases and the outer tube carries the combustion air. The bulk of the heat transfer from the hot gases to the surface of the inner tube is by radiation, whereas that from the inner tube to the cold combustion air is predominantly by convection. 

Figure 8.3 plots the efficiency versus pressure ratio at three turbine inlet temperatures (1100, 1200 and 1300 K) for the recuperated case, without intercooling, and the simple cycle. Also plotted are the power curves that apply to both cycle configurations. 

In this examination, the cathode recycle case will have no recuperator and the recuperator case will have no cathode recycle. However, it is quite possible that a hybrid system could use both. The anode exhaust is combusted at the turbine inlet in all cases. This could be done in other locations, such as the recycle loop, which would reduce the recycle required. It is apparent that a number of other configurations could be imagined (such as intercooling) however, each will likely be a modification of one of the base configurations given here. 

Due to the absence of cokes (and CO), no latent heat is lost from the cokeless furnace system. Full heat recuperation from the flue-gas occurs in the shaft. In duplex configurations (for example in conjunction with an induction furnace), efficiencies in the range of 40 to 60 % may be obtained. Thermal efficiencies for coke fired cupolas vary between 25 % (cold blast) and 45 % (hot blast, long campaign). 

Fig. 5.20. Recuperator flow types, shown schematically. All but types 1 and 2 have many, many tubes. Cross-flow recuperators (types 3, 4) often have the configuration of a square shell-and-tube heat exchanger. For the same heat exchanging area, temperature levels, and type, the average heat flux rates (see glossary) of parallel flow, cross-flow, and counterflow are about proportional to 1.00 to 1.40 to 1.55, respectively.

Heat exchangers required for the gas turbine are a recuperator, a preheater and an mtercooler, and their high efficiency performances and compact designs are demanded. Table 4 lists specifications of these heat exchangers and Fig.7 shows configurations of the heat exchanger elements. 

Figure 7.7 Simplified process flow diagram of superheated steam dryer configuration to recover energy from dryer exhaust a) excess steam with recuperation by high-pressure steam b) excess steam recompressed to superheat c) excess steam with recuperating by electricity.

Of the above features, the combination of heat input to the cycle with exother-nuc reactions in reheaters and recuperators was seen as producing substantial cycle efficiency iuCTeases and useful chenucals. This would involve a variety of heat exchanger reactors and similarly-configured compact/intensified reactors - the effect of a reheater on efficiency, for example, is given in Table 8.2. The compact reformer/combustion unit, the intensified reactor, was attractive for emissions reduction and efficiency improvement. 

As for the recuperator, the precooler is designed using the techniques and extended heat transfer surface configurations found in Kays and London. The extended surface performance detail presented in the Kays and London appendix may be somewhat dated. More recent surfaces are most likely being used by manufacturers, but the design detail necessary to size the precooler as well as the recuperator is also most likely proprietary. For the scope of this concept, the extended surface detail given in the appendix of Kays and London is sufficient. 

Figure 5 Valve Configuration for a Share Recuperator and Cooler...

The bypass flow concept utilizes a fractional cooling flow ( 1% of total gas flow) in parallel with the hot leg gas. The bypass flow is supplied from the recuperator outlet and is discharged into the turbine outlet. The bypass flow concept was evaluated with and without internal insulation. The un-insulated option was not viable due to excessive cold gas temperature increase, as illustrated by the results of the counter flow thermal analysis presented above. In the un-insulated bypass flow configuration, this will cause the cold gas temperature to exceed the temperature limit of the piping material. In order to maintain an outer wall temperature of 900K in the insulated bypass flow concept, the insulation thickness must approach that of the internally insulated concept. The addition of insulation will significantly increase pressure drop due to a reduction in the area available for gas flow and will also increase manufacturing complexity. 

Configuration 2 using a recuperator decreases the amount of natural gas necessary to heat the air, thus increasing the electrical efficiency to 40% or higher. However, it is a more expensive and complicated system, and there will be a slight loss in maximum power due to the pressure losses in the recuperator. 

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