negative enthalpy???

[ank_gl]

i was just doing some calculations in some numerical when i found out that some enthalpy and entropy values for ammonia and almost all for R744 are negative.
How can the enthalpy be negative?? i m all confused now!!

[US Iceman]

Don't worry about negative values. They simply reflect the reference temperature used for the refrigerant. Look on the tables or PH diagram and find the temperature where the enthalpy goes negative. That's your reference temperature for that set of data.

ASHRAE typically uses -40C (or F). IIR and others usually use different values. Always use the same data for your calculations. If you use one table for one value and another table for a different value you can quickly have an incorrect answer.

Enthalpy difference is hg - hf, so if the values are negative you still get the delta enthalpy.;)

[ank_gl]

i was actually optimizing the heat exchanger temperature in a cascaded system between Ammonia(HT cycle) and CO2(LT cycle) and got those values as negative. now when i calculate the system COP, the negative sign messes everything up.
anyways, can you explain the concept of reference temperature?? i am kinda confused a lil more about it, dont we already use the temperature as a reference, so why use another particular temperature for a reference, or is it a reference for something else that i am missing

[US Iceman]

If you look at a pressure-enthalpy diagram on the "x" axis (enthalpy) for zero enthalpy, then read directly above this to where the enthalpy line crosses the temperature lines you will see the reference temperature used in the data.

For instance, in one Mollier diagram I have here for ammonia it shows the reference temperature is set to -40°F. At that point the enthalpy listed is zero.

Another point to be careful of is what the minus sign may actually indicate. In some textbooks a negative sign is simply a derivation showing heat removed or work extracted from a process. While a positive result shows heating or work input.

[ank_gl]

agreed,
harder i tried, harder it became