Radiation recuperators

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. 

FIG. 27-57 Diagram of a metallic radiation recuperator. (From Goldstick Waste Heat Recovery, Faiimont Press, Atlanta, 1986. )... 

Recuperators are usually designed with very low pressure drop on the flue gas side. In a shell-and-tube recuperator, the flue gas is generally on the shell side, with the air in the tubes, requiring more AP. In a vertical pipe-in-pipe recuperator such as a stack or radiation recuperator, the flue gas goes up the middle pipe (a) to take advantage of the additional stack or natural convection draft, (b) to allow a wider gas... 

At present, waste heat exhausted from the ICE is removed with any efficient radiator system through direct apparent heat exchanging. On the contrary, organic chemical hydrides can recuperate the chemical energy of endothermic reaction heat during exhausted heat removal. Heat transfers accompanying the phase change of evaporation and condensation of aromatic products and unconverted reactants will certainly facilitate the removal of heat from the ICE parts, with adoption of any new radiator system compelled. 

Radiation conductivity, 87—91 Raleigh number, 121 Ramming, 266 Rankinite, 240 Rayon, 162 Raw glaze, 308, 409 Raw materials, ceramic, 245— 246 for enamels, 400 for glass melting, 141 — 142 Reaction sintering, 297, 343, 344—345 Reaumur porcelain, 229 Reboil, 119 Recuperator, 144, 147... 

The overall heat balance should than be calculated. It may be appropriate to determine subsidiary heat balances for the preheating/cooling zones and for any recuperator in the system. The difference between heat input and output is generally ascribed to radiation and convection losses. If a difference is unexpectedly large, additional work may be required to cross-check measurements or to measure losses directly. 

Fig. 5.17. Schematic piping for diiution air for a recuperator. TSBA = temperature sensor for controi of bieed-off air, TSDA = temperature sensor for control of dilution air. Both elbows at the right function as in fig. 5.21 to prevent radiation between recuperator and the furnace load from damaging either. Both eibows aiso assure good mixing between the furnace poc and dilution air, and both eibows prevent the TSDA from being fooled by seeing hotter or colder surfaces in the furnace or recuperator. If a velocity thermocouple at or near the same location, or a wall-mounted sensor, is found to be reading, say, 50° low, the setpoint should be adjusted 50° lower to protect the recuperator.

On the air side of recuperators, heat transfer from the separating wall to the air takes place almost entirely by convection. The radiation absorbing capacity of the small amount of water vapor in the air is practically zero. The coefiicient of heat transfer by convection increases rapidly with the mass velocity (i.e., the product of Velocity x Density) of the air or gases. 

On the flue gas side, however, this rule does not apply. Although an increase in waste gas velocity increases the convective heat transfer, it requires that the gas passages be reduced in cross-sectional area (for a given quantity of gases), and thereby decreases gas radiation from the CO2 and H2O vapor in the poc. The net result may actually decrease the total heat transfer on the gas side of a recuperator. 

Recuperator concerns stem mostly from fouling of the heat transfer surfaces, overheating damage, and leaks. Flame, pic, direct furnace radiation, or condensation should never be allowed to enter any heat recovery equipment. The airflow through any recuperator must never drop below 10% of its maximum design flow until the furnace has cooled several hours after the time when none of its refractory showed even a dull red color. 

When the last two sentences are related to heat transfer within heat recovery devices (instead of within furnaces), the low volume and velocity do present concerns with oxy-fuel firing. Heat recovery equipment with larger flow passage cross sections can benefit more from the triatomic gas radiation with oxy-fuel firing. A good example of this is the double-pipe stack or radiation type recuperator. However, they must have parallel flow at the recuperator s waste gas entrance to prevent overheating there. 

That is, the sensor must not be in a position where it could emit straightline radiation to surfaces that are purposely cool. The dilution air temperature control sensor must not see cold recuperator tubes because that would allow the flue gas temperature to be 100°F to 250°F (55°C to 139°C) above design, reducing recuperator life. Too many recuperators have been burned out on their first day of use. Engineers and operators (who have safely passed the first-day test) should redouble their vigilance from there on. 

Flue gas temperature measurement errors can cause difficulties in heat recovery systems. If a thermocouple can see cold recuperator tubes (i.e., if the T-sensor can radiate heat to cold recuperator tubes), it may read 100°F to 250°F (55°C to 139°C) lower than it actually is, so it will not be able to protect the recuperator tubes. The corrosion reaction rate of steel doubles with every 16°F to 18°F of temperature rise, so an error of 100°F in the flue gas temperature can reduce tube life to about one-third of its intended life. 

Nuclear power propulsion system (NPPS) 1, Recuperator 2, Fuel assembly 3, Control drum (CD) 4, Nuclear safety rod (NSR) 5, CD Drive 6, Radiator 7, Turbogenerator 8, NSR Drive 9, Hydrogen Tank 10, Turbo-pump 11, Hydrogen recuperator 12, Shielding 13, Reflector 14, Nozzle... 

The recuperator is designed to preheat the combustion air to a temperature of about 300 C. The recuperator is a basket type, and the majority of the heat transferred to the tubes from the flue gases is by radiation. 

Figure 3 contains the schematic of one of the two RELAP5-3D input models of the closed Brayton Cycle (CBC) system. The parallel system is identical. Each of these systems contains a turbine, recuperator, a gas cooler, a compressor, and a shaft connecting the compressor, alternator, and turbine. The gas cooler transfers excess etrergy to a heat rejection system (HRS) which contains NaK as its working fltrid and radiates the energy to space. 

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