In the above text I have used the terms ‘sensible heat’ of condensate and ‘latent heat’ of evaporation together with ‘enthalpy’ of condensate and the ‘Delta Enthalpy’ of vaporization. This equation I have put into graphical form in the following Excel Chart showing the calculated Condensate Enthalpy ‘h l‘ versus the Reduced Temperature ( click on the graph to enlarge):Įnthalpy of Saturated Condensate versus temperature in degrees centigrade. Thus for the latent heat of evaporation or DeltaHvap we would write ‘H – h l’. As an aside please note I have chosen the notation for the enthalpy of liquid to be in lower case ‘h’ with subscript ‘ l’ and for vapor enthalpy I have chosen to write ‘H’ as a capital letter. The largest error (2 %) is found at the lower temperatures close to the triple point. For the range of 50 to 360 degrees centigrade the average relative percentage error is 0.34 %. It gives rather accurate results over a wide range of temperatures. This corresponding states correlation is valid for temperatures from the triple point up to the critical temperature. Note the above equation can also be written completely in reduced terms either with reduced liquid enthalpy defined as ‘h l/hcrit’ or as reduced term in the form od ‘h l/RT crit’. The symbol ‘T r’ stands for the reduced absolute temperature ‘T / T crit’ and ‘T rt’ is the reduced triple point temperature equal to ‘T rt /T crit’ = 0.4221. ‘T crit’ is the critical absolute Temperature in degrees Kelvin equal to 647.15. The symbol ‘R’ stands for the Universal Gas Constant equal to 8.3145 kJ/kmol/oK. ‘h l’ has a value of zero at the triple point at nearly zero Degrees Celsius or 273.15 degrees Kelvin. In this equation the following symbols have been used ‘h l’ stands for the condensate enthalpy in kJ/kg. This corresponding states correlation reads as follows (note the equation is written in excel format style with as symbols ‘*’ for multiplication and ‘^’ for ‘raising to the power’): The following equation represents a corresponding states correlation of saturated condensate enthalpy versus reduced temperature, I arrived at, that can easily be implemented in one cell of an excel spreadsheet. One short, compact equation that is valid over a wide range of temperatures would be of benefit. At higher temperatures though the specific heat ‘C p’ starts to become a function of temperature and hence the calculated ‘Delta h l’ becomes more like a very rough estimate that duly needs verification against steam table data. As a first approach Enthalpy differences can often be estimated quickly with the following basic equation: ‘Delta h l = C p * Delta T’ as at the lower temperatures the specific heat of condensate is fairly constant. In both cases there is the need to be able calculate the Enthalpy of Steam Condensate as a function of temperature. Which option is most beneficial and economical then depends upon the heating duty requirements and the required temperature levels. The latter has the advantage of generating steam which is much more efficient in heating a process stream ( higher heat transfer coefficient -condensation- than cooling liquid condensate), albeit that the steam is now at a lower temperature level. One option is cooling the condensate and transfer its heat via a heat-exchanger to a process stream, or the other option is to generate flash steam by letting the hot, saturated condensate down to a lower pressure level. When designing, modifying or revamping a steam / condensate system there are two options to further use this heat contained in the hot condensate before returning it to the steam boiler. The sensible heat contained in that hot condensate can be considerable depending on the condensate’s temperature. (Corrected the formula for calculation of the density of saturated Steam Condensate - division ‘/’ slash symbol replaced with multiplication symbol ‘*’ - ) (updated ) Whenever Steam has been consumed for heating purposes hot condensate is being produced.
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