Superheated systems

Cp = Speeific heat of the desuperheating water, Btu/lb-°F h, = Enthalpy of superheated system, Btu/lb hj = Enthalpy of saturated steam, Btu/lb Lh = latent heat of vaporization of water at t,j Btu/lb m = Mass of superheated steam into the system, Ib/hr mj = Mass of saturated steam out, Ib/hr n = Mass of desuperheating water used, Ib/hr t[ = Temperature of superheated steam, °F = Temperature of saturated steam, °F t,j, = Temperature of desuperheating water, °F... 

Metastable supercooled or superheated systems can undergo irreversible phase changes at constant pressure, and their entropy changes can be calculated by considering reversible processes with the same initial and final states. 

Classic nucleation theory must be modified for nucleation near a critical point. Observed supercooling and superheating far exceeds that predicted by conventional theory and McGraw and Reiss [36] pointed out that if a usually neglected excluded volume term is retained the free energy of the critical nucleus increases considerably. As noted by Derjaguin [37], a similar problem occurs in the theory of cavitation. In binary systems the composition of the nuclei will differ from that of the bulk... 

Figure A2.5.4. Themiodynamic fimctions (i, n, and C as a fimction of temperature T at eonstant pressure and eomposition x = 1/2) for the two-eomponent system shown in figure A2.5.3. Note the diflferenee between these and those shown for the one-eomponent system shown in figure A2.5.2. The fiinetions shown are dimensionless as in figure A2.5.2. The dashed lines represent metastable extensions (superheating or supereooling) of the one-phase systems.

When dry, or superheated, vapor is used to subcool the liquid, the COP in R12 systems will increase, and decrease the COP in NH3 sys-... 

The system shown in Fig. 11-75 is direct expansion where diy or slightly superheated vapor leaves the evaporator. Such systems are predominantly used in small applications because of their simplicity and light weight. For the systems where efficiency is crucial (large industrial systems), recirculating systems (Fig. 11-77) are more appropriate. 

The maximum or minimum temperature attainable in a vessel can be limited by properly designed jacket heating systems. If steam heating is used, maximum temperatures can be limited by controlling steam pressure. A steam desuperheater may be needed to avoid excessive temperature of superheated steam from a pressure letdown station. 

The method is applicable only to gas-vapor mixtures with the vapor at saturation. However, systems involving superheated mixtures and subcooling can be handled as separate problems and added to the cooler-condenser area requirements to form a complete unit. 

Halocarbon systems are almost invariably controlled by mechanical or electronic thermostatic expansion valves, requiring a superheat signal to operate the control. The superheating of the suction gas into the compressor will cause it to expand, resulting in a lower mass flow for a given swept volume. Reduction of the superheat setting of the expansion valve will therefore result in better use of the compressor. The limit will be reached when there is insufficient signal to work the expansion valve. 

The boiler steam-water circulation system is designed to promote the delivery of steam from the various generating tubes to the boiler steam drum (top drum). From here the steam is separated from the BW and transferred to the steam delivery system for possible superheating and subsequent use in a turbine generator or other downstream process application. 

Nuclear reactor boiler plants consist essentially of a central reactor core providing controlled energy that is transferred via a pressurized cooling system to one or more steam generators. These in turn provide superheated steam for delivery to a turbine. 

To complete the broad picture of what is meant by burn-out, it is useful to consider further the particular burn-out condition which produces tolerable wall temperatures. Such a condition must occur, for example, in any practical, once-through boiler system, where a change from, for example, liquid water at inlet to superheated steam at outlet takes place in a single heated channel. Normal operation of nuclear reactors beyond burn-out has also been contemplated, and Collier et al. (C5) have described successful experiments with irradiated Zircaloy-clad rods operating continuously beyond burn-out... 

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