Refrigeration & Air Conditioning Technology, 8e - 82

82

Section 1 Theory of Heat

Figure  3.62 illustrates an actual pressure/enthalpy diagram for refrigerant R-407C, a zeotropic refrigerant blend
with very similar properties to those of R-22 in air-conditioning equipment. It is a blend of R-32, R-125, and R-134a
with a fairly large temperature glide (10°F) in the ranges
of air-conditioning applications. Notice the isotherms
angled downward from left to right, showing temperature
glide.
The terms near-azeotropic and zeotropic refrigerant
blends are used in the HVACR industry because they describe
temperature glide and fractionation. Azeotropic blends
have negligible franctionation and temperature glide, nearazeotropic blends have some fractionation and temperature

glide, while zeotropic blends have the most fractionation
and temperature glide. Refer to Unit  9, "Refrigerant and
Oil Chemistry and Management-Recovery, Recycling,
Reclaiming, and Retrofitting," for more in-depth information on refrigerants, refrigerant blends, temperature glide,
and fractionation. Charging methods for refrigerant blends
will be covered in Unit  10, "System Charging." ✔✔SAFETY
PRECAUTION: All refrigerants that have been discussed in
this text are stored in pressurized containers and should be
handled with care. Consult your instructor or supervisor about
the use and handling of these refrigerants. Goggles and gloves
should be worn while transferring the refrigerants from the
container to the system.●✔ ✔

SUMMARY
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Bacterial growth that causes food spoilage slows at low
temperatures.
Refrigeration of products at temperatures above 45°F
and below room temperature is considered hightemperature refrigeration.
Refrigeration of products at temperatures between 35°F
and 45°F is considered medium-temperature refrigeration.
Refrigeration of products at temperatures from 0°F to
210°F is considered low-temperature refrigeration.
Refrigeration is the process of removing heat from a place
where it is not wanted and transferring it to a place where
it makes little or no difference.
One ton of refrigeration is the amount of heat necessary to
melt 1 ton of ice in a 24-hour period. It takes 288,000 Btu
to melt 1 ton of ice in a 24-hour period, or 12,000 Btu in
1 h, or 200 Btu in 1 min.
The relationship of the vapor pressure to the boiling
point temperature is called the temperature/pressure
relationship.

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A compressor can be considered a vapor pump. It lowers the pressure in the evaporator to the desired temperature and increases the pressure in the condenser to a level
where the vapor may be condensed to a liquid.
Liquid refrigerant moves from the condenser to the metering device, where it again enters the evaporator.
Refrigerants have a definite chemical makeup and are
usually designated with an "R" and a number for field
identification.
A refrigerant must be safe, detectable, environmentally
friendly, have a low boiling point, and have good pumping
characteristics.
Refrigerant cylinders are color-coded to indicate the type
of refrigerant they contain.
Refrigerants should be recovered or stored while a refrigeration system is being serviced, then recycled, if appropriate, or sent to a manufacturer to be reclaimed.
Pressure/enthalpy diagrams may be used to plot refrigeration cycles.

REVIEW QUESTIONS
1. Name three reasons why ice melts in an icebox:
_________, _________, _________.
2. What are the approximate temperature ranges for
low-, medium-, and high-temperature refrigeration
applications?
3. One ton of refrigeration is
A. 1200 Btu.
B. 12,000 Btu/h.
C. 120,000 Btu.
D. 120,000 Btu/h.
4. Describe briefly the basic refrigeration cycle.
5. What is the relationship between pressure and the boiling point of liquids?

6. What is the function of the evaporator in a refrigeration
or air-conditioning system?
7. What does the compressor do in the refrigeration
system?
8. Define a superheated vapor.
9. The evaporating pressure is 76  psig for R-22 and the
evaporator outlet temperature is 58°F. What is the evaporator superheat for this system?
A. 13°F
B. 74°F
C. 18°F
D. 17°F



Refrigeration & Air Conditioning Technology, 8e

Table of Contents for the Digital Edition of Refrigeration & Air Conditioning Technology, 8e

Contents
Refrigeration & Air Conditioning Technology, 8e - Cover1
Refrigeration & Air Conditioning Technology, 8e - Cover2
Refrigeration & Air Conditioning Technology, 8e - i
Refrigeration & Air Conditioning Technology, 8e - ii
Refrigeration & Air Conditioning Technology, 8e - iii
Refrigeration & Air Conditioning Technology, 8e - Contents
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