What is Enthalpy?

Enthalpy is easy. It's just a state function that depends only on the prevailing equilibrium state identified by the system's internal energy, pressure, and volume. It is an extensive quantity. Simple.

Like most things, the scientific definition is as clear as mud. In HVAC/R, we use enthalpy measurement to come up with the total heat change in a fluid, whether it's refrigerant, water, or air.

That total change in heat content or enthalpy change is called delta H (ΔH), which is just another way of saying “total heat split.” We generally measure it in BTU/lb in the US.

In air, we need to use probes that measure humidity and temperature, like the HUB2 probes shown above or the Testo 605i probes, to calculate the enthalpy of the air. Air has both the energy associated with the temperature of the air and the latent heat stored in the water vapor.

UEI HUB Screenshot

If you want to use the Δto calculate the total heat added or removed from the air, you can use this formula to calculate BTUs of heat added or removed from the air:

Total Heat = (H1-H2) x 4.5 x CFM

In the case above, it would be:

Total Heat = (29.68 – 22.77) x 4.5 x 730 (CFM we measured)

so

29.68 – 22.77 = 6.91 Δ

6.91 x 4.5 x 730 = 22,699.35 BTU/hr

This total air enthalpy change is a required part of calculating total system capacity, and it's a pretty simple thing to understand.

Don't confuse ΔH (total heat change) with ΔT (temperature difference). ΔH includes both latent and sensible heat and is a measure of heat quantity in BTU/lb, while ΔT only calculates temperature difference and isn't converted to BTUs at all.

—Bryan

Related Tech Tips

Liquid Line Solenoid Valves: Long Line Applications
Pump down solenoid valves are commonplace for any refrigeration technician. They are energized with the compressor still running, shutting off flow in the liquid line so that the refrigerant is pumped into the condenser and receiver. The compressor will then shut off once a low-pressure switch opens the circuit when the pressure falls below a […]
Read more
Mistakes in Measuring Ohms
In HVAC and electrical school, one of the first things you learn about electricity is Ohm's law: Volts = Amps x Ohms  Pretty simple, right? Watt's law is just as easy: Watts = Volts x Amps With this newfound knowledge, the student walks confidently into the real world with two equations and some elementary algebra […]
Read more
Non-Bleed TXV & Trapped Nitrogen
The following is based on a true story. No product was harmed in the making of this tech tip, and some facts may have changed to protect the guilty and because I forgot some of them. We got a job installing a new 1-to-1 split refrigeration case with R448a, and it had a typical thermostatic […]
Read more

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from