Subcooling steam condensate

Meanwhile, the tubes covered with stagnant water would begin to cool. The steam condensate itself around these tubes would cool. This cooled water would be colder than the saturation temperature of the condensing steam. The tubes would then be said to be submerged in subcooled water. 

The vapor is drawn into a steam jet (discussed in Chap. 16). The steam condensate flows into the boot or hot well. The water in the boot is slightly subcooled. This is accomplished by a pair of baffles that create a small zone of condensate backup. The subcooled condensate, cooled to perhaps 10°F below its boiling or bubbling point, is easier to pump. As the pressure in the hot well is subatmospheric, the hot-well pump typically develops a AP of at least 30 to 50 psi. 

Effect of subcooling. When steam condenses at atmospheric pressure, it gives off 1000 Btu per pound of condensing steam. This is called the latent heat of condensation of steam. 

Fig. 1.33 Temperature in a condenser with cooling of superheated steam, condensation and subcooling of the condensate (fluid 1) by cooling waler (fluid 2)...

For shell-and-tube condensation related topics include evaporation (Section 16.11.4.1) and distillation (Section 16.11.4.2). Prefer condensation outside horizontal tubes use vertical tubes when condensing immiscible liquids to subcool the condensate. Assume pressure drop of 0.5 of the pressure drop calculated for the vapor at the inlet conditions. Baffle spacing is 0.2 to 1 times the shell diameter with the baffle window about 25%. Limit pressure drop for steam to 7 kPa on the shell side. U = 0.5 to 0.85 kW/m °C. 

Thermostatic traps are temperature-sensitive traps there are several basic types. They respond to either a temperature difference between the steam and the condensate or directly to a temperature of either steam or condensate. All thermostatic traps are operated and controlled by the temperature in the line upstream of the trap time Is necessary for the operating elements to either absorb heat to cause the valve to close or dissipate heat to cause the valve to open. They usually discharge condensate below the steam temperature and require a collecting leg before the trap to permit some subcooling of condensate. Therefore, sufficient piping length should be provided at the trap inlet to prevent the condensate backup from interfering with the heat transfer surface. 

Steam condensers (after turbo generator sets) are used to maintain the low pressure of turbine exhausts as well as to recover condensate for recycle to the boiler feedwater tank. Here it is necessary not to subcool the condensed hquid, so that the maximum heat recovery can be done (by obtaining condensate at as high a temperature as possible. The configuration of steam inlet nozzle and cooling water inlet/ outlet nozzles etc should be designed accordingly. 

All modules use the 2-fluid model to describe steam-water flows and four non-condensable gases may be transported. The thermal and mechanical non-equilibrium are described. All kinds of two-phase flow patterns are modelled co-current and counter-current flows are modelled with prediction of the counter-current flow limitation. Heat transfer with wall structures and with fuel rods are calculated taking into account all heat transfer processes ( natural and forced convection with liquid, with gas, sub-cooled and saturated nucleate boiling, critical heat flux, film boiling, film condensation). The interfacial heat and mass transfers describe not only the vaporization due to superheated steam and the direct condensation due to sub-cooled liquid, but also the steam condensation or liquid flashing due to meta-stable subcooled steam or superheated liquid. 

Fhix-force-condensation scrubbers combine the effects of flux force (diffiisiophoresis and thermophoresis) and water-vapor condensation. These scrubbers contact hot, humid gas with subcooled liquid, and/or they inject steam into saturated gas, and they have demonstrated that a number of these novel devices can remove fine particulates (see Fig. 25-24). Although limited in terms of commercialization, these systems may find application in many industries. 

To select the proper heat transfer relations to represent the functions, you need to analyze the heat transfer functions that will take place in the unit-tube and/or on the shell side. Some units may have several functions, such as the example in Rubin s recommendations on this subject that is, steam desuperheating and hydrocarbon condensing steam and hydrocarbon condensing, and condensate subcooling. Rubin presents an excellent interpretation of multizone operation for several different sets of conditions. See Figures 10-91Aand 10-91B. 

Air vents are most effective when they are fitted at the end of a length of 300 mm or 450 mm of uninsulated pipe that can act as a collecting/cooling leg. Air is an excellent insulating material, having a thermal conductivity about 2200 times less than that of iron. The last place where it can be allowed to collect is in the steam space of heat exchangers. Further, as it contains oxygen or carbon dioxide, which dissolve readily in any subcooled condensate that may be present, the presence of air initiates corrosion of the plant and the condensate return system. 

Note that the method described assumes that the high-pressure condensate has not been sub-cooled. If any subcooling has taken place, then the figure taken for the enthalpy of water at the higher pressure is reduced by the amount of sub-cooling. The chart or table can still be used if the upstream pressure is taken as that corresponding to saturated steam at the same temperature as the sub-cooled condensate. 

Bubbles later eject into the flow for subcooling below 60°C (108°F). Bubbles become elongated as they slide on the wall and condense while sliding along the wall. These bubbles are shaped like inverted pears, with the steam touching the wall just prior to ejection. 

The temperature driving force will be taken as the difference between the temperature of the condensing steam and that of the evaporating water as the preheating of the solution and subcooling of the condensate represent but a small proportion of the total heat load, that is AT = (394 - 325) = 69 degK. 

Mechanics of subcooling. As the condensed steam flow out of the radiator is restricted, the surface area of the radiator, available to cool the hot water, increases. Hence, the water temperature leaving the radiator decreases. To summarize, the effect of restricting the condensate flow from a radiator or condenser is to... 

In the aforementioned process, the heat for the reboiler is usually available as waste heat from the steam cracker, for example, and is essentially cost-free. If this heat is not available, a heat pump can be used. The heat pump can upgrade the heat, at an exergetic cost, to the desired temperature level. If the separation is viewed in isolation, this means that the heat rejected by the condenser at relatively low temperature, can be upgraded to be the higher temperature heat input for the reboiler. A schematic of the heat pump process is given in Figure 10.2. The overhead vapors are heated slightly in the reflux subcooler, which enables these vapors to be compressed and cooled in the condenser-reboiler. 

Barolo et al. (1998) developed a mathematical model of a pilot-plant MVC column. The model was validated using experimental data on a highly non-ideal mixture (ethanol-water). The pilot plant and some of the operating constraints are described in Table 4.13. The column is equipped with a steam-heated thermosiphon reboiler, and a water-cooled total condenser (with subcooling of the condensate). Electropneumatic valves are installed in the process and steam lines. All flows are measured on a volumetric basis the steam flow measurement is pressure- and temperature-compensated, so that a mass flow measurement is available indirectly. Temperature measurements from several trays along the column are also available. The plant is interfaced to a personal computer, which performs data acquisition and logging, control routine calculation, and direct valve manipulation. 

The Jakob number is basically a measure of the importance of subcooling expressing, as it does, the change in the sensible heat per unit mass of condensed liquid in the film relative to the enthalpy associated with the phase change. The Jakob number is small for many problems, i.e the sensible heat change across the liquid film is small compared to the latent heat release. For example, for cases involving the condensation of steam, Ja, is typically of the order of 0.01. 

Consider the situation described in Problem 11.15 in which stagnant saturated steam at 80°C condenses on a 0.4-m high vertical plate with a surface temperature of 75°C. Calculate the heat transfer rate per meter width of plate with and without taking subcooling effects into account. Do subcooling effects have a significant influence on the condensation rate in this situation ... 

The conditions of steam generation in the boiler are the same as in Example 8.1 8,600 kPa and 500°C. The exhaust pressure of the turbine, lOkPa, is also the same. The saturation temperature of the exhaust steam is therefore 45.83°C. Allowing for slight subcooling of the condensate, we fix the temperature of the liquid water from the condenser at 45°C. The feedwater pump, which operates under exactly the conditions of the pump in Example 7.10, causes a temperature rise of about 1°C, making the temperature of the feedwater entering the series of heaters equal to 46°C. 

Note that the calculation has not included various operating conditions that could alter the condensation jnocess such as fouling effects, presence of noncondensibles in the steam, subcooling of the condensate, etc. 

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