Saturation temperature definition

In order that hot condenser water may be re-used in a plant, it is normally cooled by contact with an air stream. The equipment usually takes the form of a tower in which the hot water is run in at the top and allowed to flow downwards over a packing against a countercurrent flow of air which enters at the bottom of the cooling tower. The design of such towers forms an important part of the present chapter, though at the outset it is necessary to consider basic definitions of the various quantities involved in humidification, in particular wet-bulb and adiabatic saturation temperatures, and the way in which humidity data are presented on charts and graphs. While the present discussion is devoted to the very important air-water system, which is in some ways unique, the same principles may be applied to other liquids and gases, and this topic is covered in a final section. 

As long as the wall temperature stays below that required for the formation of vapour bubbles, heat will be transferred by single-phase, forced flow. If the wall is adequately superheated, vapour bubbles can form even though the core liquid is still subcooled. This is a region of subcooled boiling. In this area, the wall temperature is virtually constant and lies a few Kelvin above the saturation temperature. The transition to nucleate boiling, is, by definition, at the point where the liquid reaches the saturation temperature at its centre, and with that the thermodynamic quality is r h = 0. In reality, as Fig. 4.53 indicates, the liquid at the core is still subcooled due to the radial temperature profile, whilst at the same time vapour bubbles form at the wall, so that the mean enthalpy is the same as that of the saturated liquid. As explained in the previous section, the... 

When heating with steam, a selection of the proper steam pressure level must be made when designing the evaporator. No definite rules for the selection can be established because of changing plant steam balances and availability. However, it is generally more economical to select the lowest available steam pressure level which offers a saturation temperature above the process temperature required. Some evaporator types require relatively low temperature differences. Some products may require low temperature in order to reduce fouling or product degradation. 

This definition is also applicable for pure substances and for mixtures. In the case of mixtures, the local composition of the boiling liquid gives rise to the saturation temperature at the local pressure. 

Vapour pressure. If a liquid is admitted into a closed vacuous space, it will evaporate or give off vapour until the latter attains a definite [)ressure, which depends only upon the temperature. The vapour is then said to be saturated. Experiment shows that at a given temperature... 

A saturated aqueous solution in contact with an excess of a definite solid phase at a given temperature will maintain constant humidity in an enclosed space. Table 11.4 gives a number of salts suitable for this purpose. The aqueous tension (vapor pressure, in millimeters of Hg) of a solution at a given temperature is found by multiplying the decimal fraction of the humidity by the aqueous tension at 100 percent humidity for the specific temperature. For example, the aqueous tension of a saturated solution of NaCl at 20°C is 0.757 X 17.54 = 13.28 mmHg and at 80°C it is 0.764 X 355.1 = 271.3 mmHg. 

The temperature at which water condenses from natural gas is called its dew point. If the gas is saturated with water vapor, it is by definition at its dew point. The amount of water vapor saturated in the gas is given... 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

We shall now assume that it is possible to have a system in equilibrium composed of the various phases at a specified temperature and total pressure. This will be characterised by certain definite relations between the compositions of the phases (for example, a solid salt, saturated solution, vapour of the solvent). Let 77, T = total pressure, and temperature, of the system. n = number of components (cf. 84). r = phases ... 

To estimate the stage, and the condenser and reboiler temperatures, procedures are required for calculating dew and bubble points. By definition, a saturated liquid is at its bubble point (any rise in temperature will cause a bubble of vapour to form), and a saturated vapour is at its dew point (any drop in temperature will cause a drop of liquid to form). 

It is worth noting that one of the great advantages of the matrix technique is that trace impurities are rarely a problem because there is little or no diffusion in the matrix. Recently Bonneau and Kelly Q ) have obtained definitive results on this solution system, partly by reference to data obtained in solid matrices (21), which suggest that saturated perfluorocarbons should interact with Cr(C0)5 less than other practicable room temperature solvents. Thus, Bonneau and Kelly have investigated the laser flash photolysis of Cr(C0>5 in perfluoromethylcyclohexane (C7F14) at room-temperature. A transient species 620 nm) formed... 

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