Latent heat of steam

At the low absolute pressure of the flash chamber, the entering water partially evaporates and in so doing absorbs heat from the bulk of the water in the compartment. The latent heat of steam (greater than 1,000 Btu/lb) at the evaporator pressure is removed and the water in the compartment is cooled an equivalent amount. Figure 11-8 indicates the conditions for one system. 

This is the figure obtained by dividing the total heat of steam at working condition by the latent heat of steam at atmospheric condition (i.e. 2256kJ/kg). Then... 

These are designed to use the latent heat of steam at the heat emitter. Control of heat output is generally by variation of the steam saturation pressure within the emitter. For heating applications with emitters in occupied areas, low absolute pressures may be necessary in order to reduce the saturation temperature to safe levels. 

Clausius (1850), in considering Regnault s data for the latent heat of steam, introduced a new specific heat, applicable to either phase of a saturated complex of two phases, viz., the amount of heat absorbed in raising the temperature of unit mass of a saturated phase by 1°, the pressure being at the same time varied so as to preserve the substance in a saturated state. In the case of a vapour, this is called the specific heat of saturated vapour (a). 

Taking the coefficients of heat transfer on the water side as 4.0, and on the steam side as 8.5 kW/m2 K, caleulaic the outlet water temperature and the total mass of steam condensed per second. The latent heat of steam at 372 K is 2250 kJ/kg. The density of water is 1000 kg/nr. 

Assuming the heat content of the superheat and latent heat of steam to be constant, the flowrate of steam can be estimated to be ... 

Joseph Black, upon completion of his study on magnesia alba, received his medical degree. He published very little after this study. Black taught at universities in Glasgow and Edinburgh, and continued to do solid research, which he presented in his lectures. One area in which he did important work was heat and the latent heat of steam. His work in this area inspired one of his students James Watt to apply Black s ideas in making improvements to the steam engine. 

A condenser consists of 30 rows of parallel pipes of outer diameter 230 mm and thickness 1.3 mm with 40 pipes, each 2 m long in each row. Water, at an inlet temperature of 283 K, flows through the pipes at 1 m/s and steam at 372 K condenses on the outside of the pipes. There is a layer of scale 0.25 mm thick of thermal conductivity 2.1 W/m K on the inside of the pipes. Taking the coefficients of heat transfer on the water side as 4.0 and on the steam side as 8.5 kW/m2 K, calculate the water outlet temperature and the total mass flow of steam condensed. The latent heat of steam at 372 K is 2250 kJ/kg. The density of water is 1000 kg/m3. 

The latent heat of vaporisation of water at 100° C. is 539 calories u°. Sometimes the value is given for water at 0° C., m which case the amount of heat required to raise the water from 0° to 100° C. must be added to the above quantity. The following are the most noteworthy attempts to determine the latent heat of steam, undoubtedly the most accurate results being those of Richards and of Mathews. It is remarkable that the values obtained by Black and by Watt should approximate so closely to that accepted at the present day. 

A similar explanation may be offered for the high values observed for the latent heat of steam. Thus, the thermal dissociation of dihydrol molecules is accompanied by an absorption of 189x18x2 calories, or, expressed as an equation ... 

In a long note describing his experiments on the variation of the latent heat of steam with temperature, Watt acknowledged that Mr. Southern is inclined to conclude, from the experiments on the latent heat of steam at high temperature [presented in the Appendix]... that the latent heat is a constant quantity, instead of the sum of the latent and sensible heats being so .55 This seems tantamount to an abandonment of what is known as Watt s Taw - that the sum of the latent and sensible heats is a constant. Watt had used this idea not only in his development of expansive working of steam engines but also, as we will see, it was important to his ideas about the chemical transformation of water into air.56 So, to abandon Watt s Law was to jettison a key part of Watt s chemical material theory of heat. We will see shortly, however, that a place was retained for it in a clever fashion. 

The Appendix referred to contains Southern s letter to Watt on the steam experiments that he and William Creighton conducted in 1803. From measurements of the elasticity and density of steam, Southern stated that These results appear to support the conclusion that the density of steam is nearly if not accurately, proportional to its elasticity. And subsequently he states The opinion which I entertain from these experiments as to the latent heat of steam is,... 

William Irvine was a native of Glasgow, matriculating at the university at the age of thirteen in 1756. Black soon noticed his interest in chemistry and his disposition to apply mathematics in his studies. Like Black, Irvine s main lines of communication for his work were through his students and the occasional excursion before the local Philosophical Society. Working as Black s assistant, Irvine helped in his professor s determination of the latent heat of steam and contributed values of that quantity for melting tin, zinc and spermaceti and beeswax. He also worked with Black on establishing experimentally the specific heats of various substances. However, Irvine became very much his own man. Andrew Kent puts it colourfully ... 

Black 1 about 1764 determined an average value of the latent heat of steam by. a crude method as 450 g.cal./g. Irvine found 430, and Watt, in 1781, found 533, which is a good value. Other experiments were made by Lavoisier and Laplace,3 Ure, Despretz, Brix and Andrews. ... 

As a result of inexact experiments, Creighton and Southern (1803) supposed that the latent heat of steam, 4, was independent of temperature, whilst Watt supposed that the total heat, was independent of temperature. Experiments of Clement and Desormes seemed to confirm Watt s assumption, which was generally adopted by engineers and called JVatt s law. Both laws were shown to be incorrect by the careful experiments of Regnault, who found... 

The Joly steam calorimeter ( 6.VIIE) has been used to determine the latent heat of steam 9 by measuring the amounts of steam condensed on the metal bulb first empty and then full of water, from which 4 can be calculated. 

L denotes latent heat of steam entering steam chest, ti denotes temperature of boiling liquid, and t denotes temperature of liquor entering evaporator. 

Local temperature excursions arise where the latent heat of steam condensation is released to the environment through the cooling water return streams from steam condensers. 

V = vapor rate from reboiler Aj = latent heat of steam X = molal latent heat of mixture... 

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