Psychrometrics - SI and Imperial Enthalpies do not Convert?

jpreher

Hello,
I am fairly new to using psychrometrics, so I am hoping that this is a fairly simple solution. I was going back through some old problems and wanted to spot check a few calculations. In the process I converted the units on one of the problems and the enthalpies were not matching up.

I am trying to calculate the enthalpy of a moist air mixture. The air conditions are:

• Tdb = Dry Bulb Temp = 20 C = 68 F
• X = Humidity Ratio = 7.26 grams/kg = 50.82 grains/lb = 0.00726 lb/lb
• P = Baro Pressure = sea level = 760 mmHg = 29.921 inHg

I used both an Imperial and an SI psychrometric chart to do a quick spot check to see if the enthalpies match and got:

• h = 24.4 Btu/lb dry air
• h = 38.5 kJ/kg dry air

I then converted the units and hit a wall, they are not even close to equivalent:

• h = 24.4 * 2.326 = 56.75 kJ/kg
• h = 38.5 * 0.429953 = 16.55 Btu/lb

As another check I did a quick online calculator at http://www.uigi.com/WebPsycH.html, which yielded the same results. As yet ANOTHER check I used Eq 32 from Ch. 1.13, 2009 ASHRAE Handbook:
h = 0.240*Tdb + W(lb/lb)*(1061+0.444*Tdb) = 24.24 Btu/lb

Am I missing some super secret conversion factor here? Can you not convert enthalpies directly? I would think that a thermodynamic property would not change depending on your starting units.

Q_Goest

There is no absolute value for enthalpy, internal energy, entropy and some others like there is for density, pressure and temperature.

You can only use values for enthalpy from a single source. The DIFFERENCES between the enthalpy for two different fluid states will be the same, but the absolute values will very likely be different as you've pointed out.

jpreher

That is my question, I am using them from a single source, just using equivalent SI and imperial units. Correct me if I am wrong, but isn't the enthalpy of the system equivalent to the total heat in the system.

So, if I use completely equivalent SI and Imperial models then the total heat content or thermodynamic potential of the system will be different depending on the units that I start with? I understand that it is not an absolute value and we cannot measure it, but I would think that you should be able to interchange the units.

In my head that would mean that in completely equivalent circumstances that I will somehow have more thermal potential in a room if I sit in it and calculate it in btu/lb than kj/kg.

Q_Goest

If you're only using one source, then what measurement is the source in and how is it you have two different values?

Enthalpy (and internal energy for that matter) is not measured on an absolute scale. It is a measure of the heat in a system but it isn't the 'total heat'. Not sure what that even would be. The difference between two states is equal to some amount of energy per unit mass but there is no total energy associated with that mass. Any zero value of enthalpy is completely arbitrary. In fact, most sources will have negative values of enthalpy, and clearly we can't have negative energy.

Maybe some of these references could help?
http://www.google.com/#hl=en&safe=ac...w=1280&bih=901

jpreher

Thank you for the clarification on the definition of enthalpy, and thank you for your time in helping with this. However, I think we may have two different ideas of what my problem is. Let me throw in an example and a theoretical situation.

Lets say that the source is the air in a single room at sea level. From that single air sample I measure that there is a dry bulb temperature of 20 C and a relative humidity of 50%.

We know that 20 C = 68 F and RH is a percentage.

Now, if I calculate the enthalpy at 20C and 68F (equal temperatures from the same source measurement, just in different units) then I get answers of:

• h(20C,50%RH) = 38.5 kJ/kg = 16.6 Btu/lb
• h(68F,50%RH) = 24.3 Btu/lb = 56.5 kJ/kg

So, where I am confused is that while I took a measurement at one source, the enthalpy is not the same if I convert units.

Where I could see this being extremely bad is in a situation like the following. Two researchers are wanting to know the change in enthalpy of a system. Researcher 1 likes SI units and measures the initial conditions in SI units, calculates the enthalpy in kJ/kg, and records it. Researcher two comes back an hour later to record the final conditions and does it in imperial. He now has the initial enthalpy in SI and the final in imperial.

With the problem that I am facing, if he just converted the SI enthalpy to imperial to find the delta he would be dead wrong. He would have to convert the SI temperature to F, then recalculate the enthalpy to be correct.

Is there something absolutely obvious that I am just missing here, or is this correct?

Q_Goest

Sorry I'm not as well versed in psychrometrics as I'd like to be so maybe someone else can help. Nevertheless, I looked over your last post and I keep coming back to what I said previously.

I looked at your reference here: http://www.uigi.com/WebPsycH.html

As you say, you can input two exact same states, but just in 2 different systems of measurement, and the outputs are different. I think the problem is still that these are 2 different databases being used. The web site you point to is using 2 different sources, 1 in imperial measurement and 1 in SI units. So even though these inputs are on the same web page, I think they're using different databases, or perhaps there's something else screwed up on that web site.

If you look at your example above, you suggest that researcher 1 measures the initial conditions and researcher 2 measures the final conditions. Problem is, researcher 1 and 2 can't simply measure a single condition and say it's valid because there is no way to measure just 1 condition. You need to produce 2 measurements and determine the difference between them. Only the difference between them is a valid measurement because there's no absolute scale for enthalpy. There's no such thing as measuring the enthalpy of a gas, regardless of it's dew point or relative humidity. You can only measure the difference in enthalpy between two physical states.

jpreher

Hm, perhaps someone else can jump in as well? This is still not clear to me and I do not know if we have reached an answer yet. Pertaining to your questions:

The data on the link is accurate. I have compared it to the 2009 ASHRAE Fundamentals Handbook equations. It also matches psychrometric charts from UIG and Trane.

Let me try to clarify my example. I am not measuring enthalpy. I am measuring the dry bulb temperature and relative humidity at two separate physical states. If you look about half way down the following link you will find a section titled "example".

http://www.powerknot.com/measuring-e...fficiency.html

In the second table you will see that they calculate the enthalpy before and after the evaporator. Now say a plant, for some strange reason, decided to measure the wet and dry bulb temps ahead of the evaporator in F and after the evaporator in C. Using their values, I get:

• Before Evaporator | Twb= 65F | Tdb= 78F | h= 30 Btu/lb
• After Evaporator | Twb= 52F | Tdb= 65F | h= 21.4 Btu/lb
• delta h = 8.6 Btu/lb

My calculated enthalpy matches exactly with what they get.

Now, if I say that I am measuring in C after the air heater then I get:

• Before | Twb= 65F | Tdb= 78F | h=30 Btu/lb
• After | Twb= 11.11C (52F) | Tdb= 18.33C (65F) | h= 31.8 kJ/kg = 13.7 Btu/lb????
• delta h = 16.3 Btu/lb?????

So what is going on here?

sleggett

You are likely not recognizing the difference between lbf and lbm, pounds mass and pounds force. Enthalpy in SI units is kJ/kg but in Imperial it is BTU/lbm. The conversion factor you are missing is g_c.

http://en.wikipedia.org/wiki/Pound_(force)

This problem doesn't occur in SI units, but only Imperial.

Chestermiller

The enthalpy is typically calculated relative to some reference state, in which the enthalpy is taken to be zero. An example would be say 0 degrees C, and 1 atm. pressure. But, different sources may use different reference states. This is what Q_goest was probably driving at. If the reference state for zero enthalpy in one source is different from that in another source, then, irrespective of the units, the tabulated values in the two sources will not agree. However, if you calculate the difference in enthalpy between any two thermodynamic states, the results in the two sets of units should be consistent. Try this out and see what you find.

Jupiter6

I suspect, the psych charts are flaky or the OP might have read them wrong.

OP, try doing these calcs using tables rather than the chart. The deltas should be pretty damn close when converted between SI/English, especially if the tables are from the same textbook.