Negative enthalpy on psychrometric chart

[nuuvox000]

I was looking at a low-temp psychrometric chart and I saw that the enthalpy (BTU per pound of dry air) is negative at low temperatures. Could someone explain this to me please? Seems like there would be some energy in a pound of air even if it's tiny so I'm confused.

 

[BronYrAur]

It is just the way the scale is defined, no different than a temperature of 0 degrees F does not mean you can't get any colder. In other words, it is not an absolute scale. 0 deg F dry bulb air with 0 moisture is the condition that defines 0 enthalpy.

 

[nuuvox000]

But isn't a BTU a defined amount of energy? You can't say that a lb of dry air that has 1 BTU now has 5 BTU's of energy because you use a different scale right?

 

[EnergyProfessional]

The energy is relative to the surrounding. A 20C block of steel in a 20C environment has zero thermal energy stored. the same 20C block in 0C environment has thermal energy (mass*heat capacity*dT).

When the chart was defined, they selected a practical 0-point that is good to understand for humans. Same way we don't use the Kelvin scale in daily life (even when I think we should!).

Note there are charts and tables with different 0-points!

 

[nuuvox000]

Thanks, I think it's making more sense now; I guess I never realized energy was defined by it's environment. Do you happen to know what the BTU is defined to for HVAC equipment? When they say 24,000 BTUs in one hour, is that in reference to the expected return air temperature? Or it seems like it would have to match the same reference as the psychrometric chart or else they wouldn't work together?

 

[EnergyProfessional]

BTU is defined as amount of energy required to heat an amount of water by a certain dT. Same way metric energy units are defined.

What is important in the psychrometric chart is the delta. If you bring air from state to another and it requires 10 BTU/lb, it doesn't matter if you went from 0 to 10, or from 20 to 30. The delta is 10 in both cases and determines the energy required.

The way you size cooling equipment for air treatment is to determine the 2 states of air (entering and leaving), the delta in enthalpy, and the flowrate. From that you can derive how many Btu/h. Look up some psychrometric chart tutorials and/or thermodynamics text books. The chart is just a handy tool instead of a table or a software. Also look up psychometrics. Unlike just heating of air (no mass transfer), cooling is more complex since we also condense water vapor. So thee is not a simple equation like for heating air, hence the complex chart.

Best thermodynamics book ever:
[URL unfurl="true"]https://www.amazon.com/Thermodynamics-Sanford-Klein/dp/0521195705[/url]