The Pressure - Enthalpy Chart
By Dave Demma, Senior Application Engineer - Supermarket Refrigeration
How often have you heard the statement “it isn’t cooling?”
Well it may seem a bit picky, but it is not entirely accurate to
say the refrigeration system “cools”. If the system is operating
properly, the refrigerated space should be “cooler” than its surroundings,but it is the result of a heat transfer process. Heat is
transferred from the refrigerated space to the refrigerant, and
ultimately from the refrigerant to the ambient (at the condenser). A lower temperature in the refrigerated space is the
byproduct of this heat transfer process. Perhaps this is a minor
shift in thinking, but in viewing the refrigeration system for
what it is…a heat transferprocess…a more fundamental
approach for diagnosis may be obtained.
In an effort to gain a better understanding of the various heat
transfer processes occurring in a refrigeration system, the pressure-enthalpy chart can be of great use. Additionally, once
understood, “the chart” can be a tremendous benefit in analyzing the relative health of a refrigeration system. Let’s follow
therefrigerant on a quick journey through a refrigeration system to see what it experiences, and plot it on “the chart” as we
go. Before we start, a few technical definitions are in order:
Refrigeration - The achievement of a temperature below that
of the immediate surroundings.
Latent Heat of Fusion - The quantity of heat (Btu/lb) required
to change 1 lb. of material from the solid phase into theliquid
Latent Heat of Vaporization - The quantity of heat (Btu/lb)
required to change 1 lb. of material from the liquid phase into
the vapor phase.
Sensible Heat - Heat that is absorbed/rejected by a material,
resulting in a change of temperature.
Latent Heat - Heat that is absorbed/rejected by a material
resulting in a change of physical state (occurring at constant
temperature).Saturation Temperature - That temperature at which a liquid
starts to boil (or vapor starts to condense). The saturation temperature (boiling temperature) is constant at a given pressure,*
and increases as the pressure increases. A liquid cannot be
raised above its saturation temperature. Whenever the refrigerant is present in two states (liquid and vapor) the refrigerant
mixture will beat the saturation temperature.
Superheat - At a given pressure, the difference between a
vapor’s temperature and its saturation temperature.
Subcooling - At a given pressure, the difference between a liquid’s temperature and its saturation temperature.
* Except for zeotrope refrigerants
Ton of Refrigeration - The amount of cooling required to
change (freeze) 1 ton of water at 32ºF intoice at 32ºF, in a 24
Btu - British Thermal Unit: The amount of heat required to
raise 1 lb. of water 1ºF.
1 Ton - 12,000 Btu/hr
Fig. 1 illustrates some of these definitions, using water as the
medium experiencing a heat transfer process. This graph plots
the water temperature vs. the enthalpy of the water (heat content in Btu/lb). We all know that water boils at 212ºF(atmospheric pressure at sea level). By definition, water at atmospheric pressure, at a temperature lower than 212ºF, is subcooled.
So, we start with subcooled water at 42ºF, and begin transferring heat to it. Assuming we are working with 1 lb. of water,
for every Btu added, a corresponding temperature increase of
1ºF will be achieved (the definition for one Btu). It we continue to add heat,eventually the water’s temperature will increase
to 212ºF (the saturation temperature at atmospheric pressure).
At this point, the water begins to change states from a liquid to
a vapor (boil). As noted on the graph, the water will experience
no further temperature increase…for a given pressure, the
saturation (boiling) temperature is the highest temperature
a liquid can ever achieve. Increasing...