Refrigeration & Air Conditioning Technology, 8e - 79
Unit 3 Refrigeration and Refrigerants
Follow the sequence below.
1. The refrigerant enters the expansion valve at point A
at 105°F, subcooled 10°F from the 115°F condensing
2. The refrigerant leaves the expansion valve at point B at
220°F (55% vapor and 45% liquid) to be totally evaporated in the evaporator. At point C, the refrigerant leaves
the evaporator as 100% vapor with 10°F of evaporator
superheat and at a temperature of 210°F. The difference
in enthalpy between point B and point C is the net refrigeration effect.
3. The superheated vapor enters the compressor at point D
with 30°F of total superheat and at a temperature of
10°F. The superheated vapor is now compressed to 170°F
along the line to point E.
4. The superheated vapor leaves the compressor at
point E. The refrigerant is now desuperheated from point
E to the saturated vapor line.
5. The now saturated refrigerant is gradually condensed
from the saturated vapor line to the saturated liquid line
at 115°F. This is referred to as rejection of the latent heat
6. The saturated liquid is subcooled 10°F (115°F 2 105°F)
from the saturated liquid line to point A, where it enters the
metering device at 105°F. The process then repeats itself.
Pressure/enthalpy diagrams are useful for establishing the
various conditions around the refrigerant cycle system. They
are partially constructed from properties shown in refrigerant
tables. Figure 3.60 is a page from a typical table for R-22.
Column 1 is the temperature corresponding to the pressure columns for the saturation temperature. Column 5 lists the
specific volume for the saturated vapor refrigerant in cubic feet
per pound. For example, at 60°F, the compressor must pump
0.46523 ft3 of refrigerant to circulate 1 lb of refrigerant in the
system. The specific volume along with the net refrigeration effect
help the engineer determine the compressor's pumping capacity.
The example in Figure 3.48 using R-22 had a net refrigeration
effect of 61 Btu/lb of refrigeration circulated. If a system needed
to circulate enough refrigerant to absorb 36,000 Btu/h (3 tons
of refrigeration), it would need to circulate 590.2 lb of refrigerant per hour (36,000 Btu/h divided by 61 Btu/lb 5 590.2 lb/h).
If the refrigerant entered the compressor at 60°F, the compressor must move 275 ft3 of refrigerant per hour (590.2 lb/h 3
0.46523 ft3/lb 5 275 ft3/h). (There is a slight error in this calculation because the 0.46523 ft3/h is for saturated refrigerant
and the vapor is superheated entering the compressor. Superheat
tables are available but will only complicate this calculation
and the error is quite small.) Many compressors are rated in
cubic feet per minute, so this compressor would need to pump
4.58 ft3/min (275 ft3/h 4 60 min/h 5 4.58 ft3/min).
The density portion of the table tells the engineer
how much a particular volume of liquid refrigerant will
weigh at the rated temperature. For example, 1 ft3 of R-22
weighs 76.773 lb when the liquid temperature is 60°F. This
is important for determining the weight of refrigerant in
components, such as evaporators, condensers, and receivers. The enthalpy portion of the table (total heat) lists the
heat content of the liquid and vapor and the amount of
latent heat required to boil 1 lb of liquid to a vapor. For
example, at 60°F, saturated liquid refrigerant would contain 27.172 Btu/lb compared with 0 Btu/lb at 240°F. It
would require 82.54 Btu/lb to boil 1 lb of 60°F saturated
liquid to a vapor. The saturated vapor would then contain
109.712 Btu/lb total heat (27.172 1 82.54 5 109.712).
The entropy column is of no practical value except on the
pressure/enthalpy chart, where it is used to plot the compressor discharge temperature.
These charts and tables are not normally used in the field
for troubleshooting but are intended for engineers to use
in designing equipment. They do, however, help the technician understand the refrigerants and the refrigerant cycle.
Different refrigerants have different temperature/pressure
relationships and enthalpy relationships. These all must be
considered by the engineer when choosing the correct refrigerant for a particular application. A complete study of each
refrigerant and how it compares to all other refrigerants is
helpful, but you do not need to understand the complete
picture to successfully perform in the field. A complete study
and comparison is beyond the scope of this book.
3.21 PLOTTING THE
REFRIGERANT CYCLE FOR
BLENDS WITH NOTICEABLE
Temperature glide occurs when the refrigerant blend has
many temperatures as it evaporates or condenses at a given
pressure. The pressure/enthalpy diagram for refrigerants with
temperature glide differs from that for refrigerants that do
not have temperature glide. Figure 3.61 illustrates a skeletal
pressure/enthalpy diagram for a refrigerant blend with temperature glide. Notice that the lines of constant temperature
(isotherms) are not horizontal, but are angled downward
as they travel from saturated liquid to saturated vapor. As
you follow the lines of constant pressure (isobars) straight
across from saturated liquid to saturated vapor, more than
one isotherm will be intersected. Thus, for any one pressure
(evaporating or condensing), there will be a range of associated temperatures-a temperature glide. For example, in
Figure 3.61, for a constant pressure of 130 psia, the temperature glide is 10°F (70°F 2 60°F). The isobar of 130 psia
intersects both the 60°F and the 70°F isobars as it travels
from saturated liquid to saturated vapor.