Latent heat of water

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 latent heat of water is about 5 times that for an organic liquid. 

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 water requirement can be calculated from the average latent heat of water over the working range, which is 2425 kJ/kg. The amount of water to be evaporated is 1/2425 = 0.4 X 10 kg/ (s kW). 

Latent heat of water at 294 K Specific heat of ammonium nitrate Specific heat of dry air Specific heat of water vapour... 

Figure 188 shows how much the volume of tank would be reduced by latent heat of water. Although the reduction ratio depends on temperature... 

Thus, if 10% of the solvent is vaporized, this will lead to a yield of 10%, and so on. Energy input is required to vaporize the solvent at 60°C. The latent heat of water is 2350 kl-kg. The product of the mass of evaporation and latent heat gives the energy input ... 

From steam tables, the latent heat of water at a saturated temperature of 230°C is 1812 kJ-kg-1. 

Assuming that radiation losses amount to 20 kJ/kg of dry air used, determine the mass flow of dry air supplied to the dryer and the humidity of the outlet air. The latent heat of water vapour at 295 K = 2449 kJ/kg, the specific heat capacity of dried material = 0.88 kJ/kg K, the specific heat capacity of dry air = 1.00 kJ/kg K, and the specific heat capacity of water vapour = 2.01 kJ/kg K. 

The transition from the sohd state to the hquid state is always accompanied by the absorption of heat. A definite amoimt of heat, which is called the latent heat of fusion, is absorbed when unit mass of a sohd body melts. The latent heat of water, for example, is 80 cal., that is to say, when I gr. of ice melts to form I gr. of water at 0°, the heat absorbed would heat I gr. of water from 0° to 80°. In consequence of the absorption of heat on melting, the temperature of a sohd body cannot be raised above its melting point. The heat which we make the body absorb after it has reached the melting point can no longer raise the temperature, as it is all used up in the process of fusion. Only when the whole of the sohd has melted does a further addition of heat cause the temperature of the hquid to rise. A mixture of sohd and hquid remains permanently at the constant temperature of the melting point. 

The following numerical example shows the variation of the latent heat of water at 100° with the temperature. 

Humid enthalpy H Heat content at a given temperature T of unit mass of dry air and the moisture it contains, relative to a datum temperature To, usually 0°C. As water is liquid at 0°C, the humid enthalpy also contains a term for the latent heat of water. If heat capacity is invariant with temperature, H = (Cpg 4-Cp Y)(T-To) + XqY, where Xo is the latent heat of water at 0°C, 2501 kj/kg (1075 Btu/lb). In practice, for accurate calculations, it is often easier to obtain the vapor enthalpy H from steam tables, when H = H +... 

Feedstock Characteristics. Feedstock characteristics are presented in Table I. Results of the metal analysis will be considered in Phase II of the project. Calorific values were approximated using Dulong s formula and not actually measured. The oat straw had a lower ash content than the stover and a correspondingly higher calorific value. Calorific values, calculated on a dry basis and including the latent heat of water vapour in the products of combustion (high heat value), were 17.9 MJ/kg for the straw and 16.0 MJ/kg for the stover. 

Although natural gas is usually priced in terms of its high heating value (HHV), most users only benefit from its low heating value (LHV), as the latent heat of water vapor condensation is seldom recovered in smaller applications. In Table 4, the heating value calculation for a hypothetical gas composition is illustrated. 

This may be interpreted as the enthalpy of liquid water being 10.519 kcal/mol less than that of water vapor, and as giving the latent heat of water at 1 atm and 25°C. 

Sensible heat is often negligible compared to latent heat. To see why, note that the latent heat of vaporization of water at latm, 212°F is 970.3 btu/lb, where 1 btu is the amount of sensible heat required to raise the temperature of one lb of water 1°F. This means that, for sensible heat to equal latent heat, I need to raise the temperature of water 970°F. A temperature rise of only 30°F, say, represents a sensible heat change of 30/970 or 3% of the latent heat of water. This is an estimate of the relative error made in this estimate of m,. 

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