126
Modern Refrigeration and Air Conditioning
must be kept in insulated-vacuum containers. The con-
tainers have to withstand extremely low temperatures
without losing their strength and be well insulated,
since the temperatures of the fluids inside the contain-
ers are very low. Inside the container, pressure is kept at
a low level corresponding to the fluid’s vapor pressure.
Many commercial food companies freeze food by
using large conveyers to move trays of food through a
cooling chamber. The food is sprayed with the cryo-
genic fluid, which rapidly freezes the product. The
quick freeze action of cryogenics reduces the amount
of ice crystals that form in the food. This process results
in less damage to vegetables and meats during freezing
than traditional refrigeration. In general, cryogenic flu-
ids are expendable. This means that they are only used
once, and then the vapor is vented to the atmosphere.
Caution
Cryogenic Fluid Containerss
Do not attempt to use any of these fl uids in any
container or device that was not designed specifi cally
to hold that fl uid.
Cryogenic Fluid Container
D o not attempt to use any of these fl uids in any
container or device that was not designed specifi cally
to hold that fl uid.
Safety Note
Avoiding Skin Contactt
There are certain precautions that must be
by anyone handling these fl uids. Cryogenic fl uids must
never be allowed to touch the skin. Such contact would
result in immediate freezing of the fl esh. A person han-
dling cryogenic fl uids must have his or her entire body
protected by suitable clothing, helmets, and gloves.g
6.6.3 Expendable Refrigerants
An expendable refrigeration system cools a
substance or absorbs heat from an evaporator and
then releases its refrigerant into the atmosphere. An
expendable refrigeration system uses an expendable
refrigerant, which is used only once in the system. It is
not collected and recondensed for additional refrigera-
tion cycles, as is the case with most compression and
absorption systems. Expendable refrigeration systems
are sometimes referred to as chemical refrigeration sys-
tems or open-cycle refrigeration systems.
The most common expendable refrigerants are:
• Liquid helium (R-704)—boiling temperature at
atmospheric pressure: –452°F (–269°C).
• Liquid nitrogen (R-728)—boiling temperature at
atmospheric pressure: –320°F (–196°C).
• Liquid carbon dioxide (R-744)—boiling temperature
at atmospheric pressure: –109°F (–78°C).
6.7 Refrigeration Lubricants
In a mechanical refrigeration system, moving parts
must be lubricated for long life and efficient perfor-
mance. Refrigeration lubricant is lubricant charged
into a refrigeration system with the refrigerant in order
to lubricate the contact between moving parts. It circu-
lates through the same tubes and components as the
refrigerant, providing lubrication.
Refrigeration lubricants must provide good lubri-
cating qualities under severe conditions. The viscosity
must be correct for the refrigerant and the machine
in which it is used. The lubricant must also be free of
moisture.
6.7.1 Properties of Refrigeration
Lubricants
Refrigeration lubricants must have certain prop-
erties in order to circulate properly through a system
with refrigerant. They must be able to flow in low tem-
peratures. These lubricants also come in direct contact
with hot motor windings in hermetic units. Therefore,
the lubricant must also be able to withstand high
Avoidin g Skin Contac
There are certa in precautions that must be takentaken
by anyone handling these fl uids. C ryogenic fl uids must
n ever be allowed to touch the skin. S uch contact would
result in immediate freezing of the fl esh. A person han-
dling cryogenic fl uids must have his or her entire body
protected by suitable clothin g, helmets, and loves.
Goodheart-Willcox Publisher
Figure 6-17. Note the differences between the boiling points
at atmospheric pressure of some halogenated refrigerants and
the boiling points of fl uids in the cryogenic range.
Boiling Temperature at Atmospheric
Pressure
Fluid
Fahrenheit
(°F)
Celsius
(°C)
Water 212 100
R-134a –15 –26
R-717 (ammonia) –28 –33
R-22 –41 –41
R-404A –51 –46
R-410A –61 –52
R-508B –126 –88
R-744 (carbon dioxide) –109 –78
R-1150 (ethylene) –155 –104
Beginning of the Cryogenic
Range:
–250 –157
R-50 (methane) –259 –161
R-732 (oxygen) –297 –183
R-729 (air) –313 –192
R-728 (nitrogen) –320 –196
R-720 (neon) –411 –246
R-702 (hydrogen)
–423 –253
R-704 (helium) –452 –269
Absolute Zero –460 –273