Refrigeration & Air Conditioning Technology, 8e - 65
Unit 3 Refrigeration and Refrigerants
3.19 RECOVERY, RECYCLING,
OR RECLAIMING OF
It is mandatory for technicians to recover and sometimes recycle
refrigerants during installation and servicing operations to help
reduce emissions of chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs) to the atmosphere.
Examples of recovery equipment are shown in Figure 3.43. Many
larger systems can be fitted with receivers or dump tanks into which
the refrigerant can be pumped and stored while the system is serviced.
However, in smaller-capacity systems it is not often feasible to provide
these components. Recovery units or other storage devices may be
necessary. Most recovery and/or recycling units that have been developed to date vary in technology and capabilities, so manufacturers'
instructions must be followed carefully when using this equipment.
Unit 9 , "Refrigerant and Oil Chemistry and Management-Recovery,
Recycling, Reclaiming, and Retrofitting," includes a detailed description of the recovery, recycling, and reclaiming of refrigerants.
3.20 PLOTTING THE
A graphic picture of the refrigerant cycle may be plotted on
a pressure/enthalpy diagram. Pressure is plotted on the lefthand side of the diagram and enthalpy on the bottom of the
diagram, Figure 3.44. Enthalpy describes how much heat a
substance contains with respect to an accepted reference point.
Quite often, people refer to enthalpy as total heat, but this is
Figure 3.43 Recovery units.
Photo by Eugene Silberstein
Source: National Refrigerants, Inc.
Figure 3.42 Color-coded refrigerant cylinders and drums for some of
the newer refrigerants.
not absolutely accurate because it refers to the heat content
above the selected reference point. Refer to Figure 1.15, the
heat/temperature graph for water. We used 0°F as the starting
point of heat for water, knowing that you can really remove
more heat from the water (ice) and lower the temperature
below 0°F. We described the process as the amount of heat
added starting at 0°F. This heat is called enthalpy. The pressure/
enthalpy diagram is a similar diagram, available for all refrigerants and sometimes referred to as a p-e (pressure/enthalpy)
or p-h (pressure/heat) chart. Since different refrigerants have
different characteristics, properties, and temperature/pressure
relationships, the pressure/enthalpy chart for each refrigerant is different. The pressure/enthalpy chart is used to plot the
complete refrigeration cycle as a continuous loop.
The selected reference point for measuring the heat content in a refrigerant is 240°F. The pressure/enthalpy chart
in Figure 3.45 shows that the enthalpy, or heat content,
in Btu/lb (along the bottom of the chart) has a value of 0 Btu/
lb when the refrigerant is a saturated liquid at 240°F. The
heat content for temperature readings below 240°F saturated liquid are indicated as being negative. As we move
from left to right on the chart, the heat content per pound
of refrigerant increases. As we move from right to left, the
heat content per pound of refrigerant decreases.
As you inspect the pressure/enthalpy chart in Figure 3.44,
you will notice that there is a horseshoe-shaped curve toward
the center of the chart. This curve is called the saturation curve
and contains the same information that is contained on the temperature/pressure chart discussed earlier. The only difference is
that the pressures on the pressure/enthalpy chart are expressed
in absolute pressures, psia, instead of gauge pressures, psig.
Any time a plot falls on, or under, this curve, the refrigerant is saturated and will have a corresponding temperature
and pressure relationship. There are two saturation curves.
The one on the left is the saturated liquid curve. If heat is