Superheated vapor, condensation

If the vapor is superheated at the inlet, the vapor may first be desuperheated by sensible heat transfer from the vapor. This occurs if the surface temperature is above the saturation temperature, and a single-phase heat-transfer correlation is used. If the surface is below the saturation temperature, condensation will occur directly from the superheated vapor, and the effective coefficient is determined from the appropriate condensation correlation, using the saturation temperature in the LMTD. To determine whether or not condensation will occur directly from the superheated vapor, calculate the surface temperature by assuming single-phase heat transfer. 

Cool 7,936 Ib/hr ammonia from 292°F to 105°F and condense at this point. Pressure is 228.9 psia. Reading ammonia superheated vapor tables (or chart) ... 

All gases can be liquefied under suitable pressure and temperature conditions and therefore could be called vapors. The term gas is most generally used when conditions are such that a return to the liquid state, i.e. condensation, would be difficult within the scope of the operations being conducted. However, a gas under such conditions is actually a superheated vapor. 

Water enters the pump at state 1 as a low-pressure saturated liquid to avoid the cavitation problem and exits at state 2 as a high-pressure compressed liquid. The heat supplied in the boiler raises the water from the compressed liquid at state 2 to saturated liquid to saturated vapor and to a much higher temperature superheated vapor at state 3. The superheated vapor at state 3 enters the turbine where it expands to state 4. The superheating moves the isentropic expansion process to the right on the T-s diagram as shown in Fig. 2.5, thus preventing a high moisture content of the steam as it exits the turbine at state 4 as a saturated mixture. The exhaust steam from the turbine enters the condenser at state 4 and is condensed at constant pressure to state 1 as saturated liquid. 

An actual vapor compression refrigeration cycle operates at steady state with R-134a as the working fluid. Saturated vapor enters the compressor at 263 K. Superheated vapor enters the condenser at 311K. Saturated liquid leaves the condenser at 301 K. The mass flow rate of refrigerant is 0.1 kg/sec. Determine... 

R-134a enters the compressor of a steady-flow vapor compression refrigeration cycle as superheated vapor at 0.14 MPa and — 10°C at a rate of 0.04 kg/sec, and it leaves at 0.7 MPa and 50°C. The refrigerant is cooled in the condenser to 24°C and saturated liquid. Determine (a) the compressor power required, (b) the rate of heat absorbed from the refrigerated space, (c) the compressor efficiency, and (d) the COP. 

Draw an isobaric heating process on a T-s diagram for a nonazeotropic mixture from a superheated vapor state to a compressed liquid state. Does temperature remain the same during the condensation ... 

At point 3 the pressure must be such that the steam has a condensation temperature in feedwater heater I of 195 degC, 5 deg higher than the temperature of the feed water to the boiler at point 1. Its saturation pressure, corresponding to 195 degC, from Table F.l, is 1399.0 kPa. The steam at point 3 is superheated vapor at this pressure, and if expansion from P2 to P3 is isentropic,... 

A power plant operating on heat recovered from the exhaust gases of internal-combustion < uses isobutane as the working medium in a modified Rankine cycle in which the upper pressure I is above the critical pressure of isobutane. Thus the isobutane does not undergo a change of p" as it absorbs heat prior to its entry into the turbine. Isobutane vapor is heated at 4,800 kPa to 2 and enters the turbine as a supercritical fluid at these conditions. Isentropic expansion in the turh produces superheated vapor at 450 kPa, which is cooled and condensed at constant pressure, resulting saturated liquid enters the pump for return to the heater. If the power output of the modi Rankine cycle is 1,000 kW, what is the isobutane flow rate, the heat-transfer rates in the heater condenser, and the thermal efficiency of the cycle ... 

In the context of a system containing a single condensable species and noncondensable gases, explain in your own words the terms saturated vapor, superheated vapor, dew point, degrees o f superheat, and relative saturation. Explain the following statement from a weather report in terms a first-year engineering student could understand The temperature is 75 F, barometric pressure is 29.87 inches of mercury and falling, the relative humidity is 50%. and the dew point is 54°E... 

Since only a saturated vapor can condense (why ), to achieve condensation in a system containing a superheated vapor one or more of the variables of Equation 6.3-2 must be changed so that the inequality becomes the equality of Raoult s law, This can be done in several ways, such as by increasing the pressure at constant temperature (the left side increases, while the right side stays constant) or by decreasing the temperature at constant pressure (the left side remains constant, and the right side decreases). 

We can have a similar argument for vapor that enters the condenser as superheated vapor at a temperature 7,. instead of as saturated vapor. In this case the vapor must be cooled first to 3 , before it can condense, and this heat must be transferred to the wall as well. The amount of heat released as a unit mass of superheated vapor at a temperature 7,. is cooled to 75, is simply Cf,r[Ty - 7jjt), where Cp,. is the specific heat of the vapor at the average temperature of (Ty + 7sj,)/2. The modified latent heal of vaporization in this case becomes... 

DisHlIaHon A wide variety of stills are available to produce purified or distilled water. A typical design consists of an evaporator, vapor separator, and compressor. The distilland (raw feed water) is heated in the evaporator to boiling and the vapor produced is separated from entrained distilland in the separator. The vapor then enters a compressor where the temperature of the vapors is raised to 107°C. Superheated vapors are then condensed on the outer surface of the tubes of the evaporator containing cool distilland circulating within. 

Another popular concept has been the idea that the hysteresis is caused at the single pore level by the existence of metastable states analogous to the supercooled liquid and superheated vapor states which can be encoimtered in bulk systems when nucleation of condensation or evaporation is delayed. Hysteresis loops of this type will emerge from any theory of the van der Waals or mean field t3q)e. This idea dates back to... 

One way to increase the temperature of the vapor is to compress it. Fig. 1 shows how adiabatic compression of water vapor (treated as an ideal gas) increases its temperature. Note that the temperature rises faster with increases in pressure than the boiling point increases. Thus adiabatic compression of a saturated vapor nearly always produces a superheated vapor. When heat is removed from this compressed vapor, it will eventually condense at its boiling point for that pressure. 

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