 In doing calculations for the vapor and dry air calculations, what we want to be able to do is have an approximation for the enthalpy and what we'll do now is take a look at that. So this is an approximation and we are able to do it because we're looking at temperatures in the range like I mentioned before between minus 10 degrees C and 50 degrees C. So what we'll begin with is look at the dry air and if we recall we said that enthalpy can be calculated using this equation here. Sometimes CCP changes if you have variable specific heats but what we want to do is we want to integrate that and so what we'll do is we'll integrate between zero degrees Celsius and up to some arbitrary temperature not yet determined that will be part of our calculation and I'm going to pull these specific heat out because we're dealing with relatively small temperature changes so we can take an average value for the specific heat and we get that equation there if we integrate now notice I'm dealing with degrees C here not Kelvin this you got to be a little careful when you're dealing with H back it's different than what we looked at before and the other thing we're going to say is we're going to assume zero degrees C as being the reference state and what that implies is the enthalpy at zero degrees C will say is equal to zero and so when we do that we can simplify this equation notably this term is going to go away and what we end up with is the enthalpy of dry air now notice the units degrees C kilojoules per kilogram degrees C multiplied by temperature also in degrees C so that is something a little different from what we've seen before and consequently we can then evaluate the change of an enthalpy of dry air as 1.005 delta t and the units of that would then be kilojoules per kilogram so that's an approximation that we will be making for dry air let's proceed on and take a look at how we can deal with the water vapor so for water vapor we don't want to have to go into the steam tables every time we want to determine the enthalpy of water vapor so we make an approximation here as well and what that approximation is is for a given temperature we will assume that the enthalpy is equivalent to the saturated vapor enthalpy value at that particular temperature and if you look in in your book it would vary from book to book but the value of hg can be approximated with this equation here and this is again in degrees C and the units of this are kilojoules per kilogram now just be a little careful your book may use a slightly different value than this first term here but it probably won't change by a significant amount but but just prefer to the value that might have in your book because it might be slightly different than what I have here but these are two approximate equations that we can then use to calculate enthalpy of the water vapor and the one previously this was enthalpy of our dry air and it's kind of a quick way of doing the calculations but that's what we do a lot of with the HVAC so that is how to treat enthalpy as an approximation