Latent heat, estimation

TEMPERATURE AND LATENT HEAT ESTIMATION FOR SATURATED STEAM... 

Estimates of the latent heat of vaporization of a pure Hquid at any temperature from a known value at a single temperature are possible by several methods. The known value may be experimental, or it may be estimated by equation 16. Of the methods proposed, the Watson correlation (1), which is both simple and reHable, has found the greatest acceptance ... 

Single-Effect Evaporators The heat requirements of a singleeffect continuous evaporator can be calculated by the usual methods of stoichiometry. If enthalpy data or specific heat and heat-of-solution data are not available, the heat requirement can be estimated as the sum of the heat needed to raise the feed from feed to product temperature and the heat required to evaporate the water. The latent heat of water is taken at the vapor-head pressure instead of at the product temperature in order to compensate partiaUv for any heat of solution. If sufficient vapor-pressure data are available for the solution, methods are available to calculate the true latent heat from the slope of the Diihriugliue [Othmer, Ind. Eng. Chem., 32, 841 (1940)]. 

The solidification speed of salol is about 2.3 mm mim at 10°C. Using eqn. (6.15) estimate the energy barrier q that must be crossed by molecules moving from liquid sites to solid sites. The melting point of salol is 43°C and its latent heat of fusion is 3.2 x 10 ° J molecule F Assume that the molecular diameter is about 1 nm. 

The following expressions can be used to estimate the temperature and enthalpy of steam. The expressions are based upon multiple regression analysis. The equation for temperature is accurate to within 1.5 % at 1,000 psia. The expression for latent heat is accurate to within + 3 % at 1,000 psia. Input data required to use these equations is the steam pressure in psia. The parameters in the equations are defined as t for temperature in F, for latent heat in Btu/lb, and P for pressure in psia. 

Estimation of latent heat becomes umeliable at temperatures approaching or exceeding the critical temperature. The following values can be used as rough approximations ... 

These systems are ealled tempering (i.e., to prevent temperature rise after venting) systems beeause there is suffieient latent heat available to remove the heat of reaetion and to temper the reaetion at the set pressure. The vent requirements for sueh systems are estimated from the Leung s Method [31,32] ... 

Wagle [92] presents an estimate method for the average relative volatility of two components, related to the normal boiling points and the latent heats of vaporization of the two components, in the temperature range of their boiling points ... 

If a compound s latent heat is not known, it can be estimated from the normal boiling points and molecular weight. 

For highly varying vapor loads through the column, pressure drop may be estimated for differences in latent heats from one point in the column to another, the pres-... 

Estimate the molal latent heat of vaporization for pure benzene at its normal boiling point of 80.1°C. 

The heat transfer to the liquid from an engulfment fire has been estimated at around lOOkW/m, and the above formula equates this to the vapor produced from this input as latent heat. The exponential is an area exposure factor, which recognizes that large vessels are less likely to be completely exposed to flames. 

The total furnace heat absorption may be estimated by using the calculated furnace exit gas temperature and analysis to determine the enthalpy (excluding the latent heat of water vapor) and thus deducting the heat rejection rate from the net heat input rate. 

The cooling load of an air-conditioned space comprises estimates of the sensible and latent heat gains, and is (2s + Ql- This rate of heat flow is to he removed hy a cooling medium which may he air, water, hrine or refrigerant, or a comhination of two of these. (See Figure 28.1.)... 

If an experimental value of the latent heat at the boiling point is known, the Watson equation (Watson, 1943), can be used to estimate the latent heat at other temperatures. 

If the degree of superheat is large, it will be necessary to divide the temperature profile into sections and determine the mean temperature difference and heat-transfer coefficient separately for each section. If the tube wall temperature is below the dew point of the vapour, liquid will condense directly from the vapour on to the tubes. In these circumstances it has been found that the heat-transfer coefficient in the superheating section is close to the value for condensation and can be taken as the same. So, where the amount of superheating is not too excessive, say less than 25 per cent of the latent heat load, and the outlet coolant temperature is well below the vapour dew point, the sensible heat load for desuperheating can be lumped with the latent heat load. The total heat-transfer area required can then be calculated using a mean temperature difference based on the saturation temperature (not the superheat temperature) and the estimated condensate film heat-transfer coefficient. 

A reboiler of a distillation column is required to supply 10 kg-s 1 of toluene vapor. The column operating pressure at the bottom of the column is 1.6 bar. At this pressure, the toluene vaporizes at 127°C and can be assumed to be isothermal. Steam at 160°C is to be used for the vaporization. The latent heat of vaporization of toluene is 344,000 J-kg 1, the critical pressure is 40.5 bar and critical temperature is 594 K. Steel tubes with 30 mm outside diameter, 2 mm wall thickness and length 3.95 m are to be used. The film coefficient (including fouling) for the condensing steam can be assumed to be 5700 Wm 2-K 1. Estimate the heat transfer area for ... 

Solution The conditions for the inlet steam are fixed, but the conditions of the outlet steam will depend on the performance of the turbine. First, estimate the steam flowrate from the process heating duty. A good approximation is that the sum of the heat content of the superheat and latent heat is constant from inlet to outlet. At the expander inlet, the heat content of the superheat is higher than that of the outlet, but the latent heat is lower in the inlet than in the outlet. The two trends tend to cancel each other out, with the total heat content of superheat and latent heat being approximately constant across the turbine. From steam properties, the enthalpy of the superheated steam HSup, enthalpy of the saturated steam HSat and enthalpy of the saturated condensate Hi at 100 barg are ... 

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

The terms j32 and I3 have the values given above. Approximately, D3 = D5 = 80 kcal/ mole, and A2 = 88 kcal/mole. The value of D4 is unknown, but 70 kcal/mole is probably a realistic estimate. The Coulombic energy of the ion-pair is q, and assuming an interionic distance of 5 AU, this amounts to 70 kcal/mole. A value of AHSS can be estimated as approximately three times the latent heat of vaporization of methylene dichloride (6.7 kcal/ mole), giving AiTSJ = 20 kcal/mole. Thus... 

In the latest experimental work131 not only the heat conductivities of ice and water, but also the latent heat of fusion were considered, but convection still disregarded. The importance of the refinements of the theory is clear from the comparison of the most recent value for the ysl of the ice - water interface, namely 29 erg/cm2, with the early result129) of 7si = 41 erg/cm2. The probable limits of error were given as 9 erg/cm2 in the early, and as 2 erg/cm2 in the later paper the former estimate appears to be too optimistic. For the interface of solid and liquid lead, 7S) = 76 erg/cm2 was calculated130). 

Taking the density, thermal conductivity and latent heat of fusion of the peas to be 1050 kg m , l.OWm K and 250kjkg respectively and the bulk density within the fluidized bed as 525kgm , the Plank/ Nagaoka model can be used to estimate the time required for the product outlet temperature to reach -20°C. 

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