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8
Refrigerator Service Manual
NX6 / NX8 / NXA Models
THEORY OF OPERATION
The NX6X/NX8X MODEL gas absorption refrigerators are
comprised of two separate systems that together allow the
refrigerator to cool. When performing service it is important
to have a basic understanding of each system and their
interaction with one another and how this interaction provides
for cooling/refrigeration.
These two “systems” are the:
• Cooling
Unit
• Refrigerator Controls (Electronic Controls)
Overview
Cooling Unit
The cooling unit is a self-contained gravity
fl
ow absorption
refrigeration system. The refrigerant charge is a solution of
water, ammonia, sodium hydroxide, and sodium chromate.
In order to produce cooling a precise heat must be applied to
the boiler area which in turn initiates a chemical reaction that
extracts heat from the freezer and fresh food compartments,
thus providing "cooling."
This precise heat is supplied to the cooling unit via heaters
and/or a LP gas burner which are controlled by the
refrigerators electronic controls. The refrigerant transfers the
heat from the freezer and fresh food cabinets to the absorber
coils. At the absorber coils the metal surface absorbs the heat
and air
fl
ow over the external surfaces of the coils carries
the heat away. A more detailed description of the absorption
process can be found under the heading “Cooling Unit
Detailed Description" on the following pages.
A thermal air current created by the rising hot air
fl
ows out of
the enclosure through either a roof exhaust vent or a sidewall
exhaust vent (depending on installation). The
fl
owing air
mass passes over the surface of the condenser
fi
ns where it
absorbs heat transferred from the ammonia vapors
fl
owing
through the condenser. The thermal air
fl
ow process creates
a "chimney effect" that creates a continuous draft of cooling
and combustion air. The fresh air drafted by the chimney effect
removes rejected heat, supports combustion, and expels
the exhaust gases produced by the combustion process.
Obstructions, restrictions, or modi
fi
cations to vents or the
enclosure will affect the heat absorption cycle. Poor cooling
unit performance may be due to:
• Loose insulation interfering with the ventilation process
• Construction material or debris left in the enclosure
• Insect screen covering vents
• Plastic sheeting covering vents
• Items stored in the enclosure
• Modi
fi
cations to vents or enclosure
• No roof or sidewall vent openings
Leveled Operation
The circulation of the refrigerant through the cooling unit is
accomplished by gravity
fl
ow; therefore, the refrigerator must
be operated leveled. Off-level operation affects the
fl
ow of the
refrigerant through the cooling system. The maximum off-level
operation limits are:
•
3° (Degrees) from side-to-side
•
6° (Degrees) from front-to-back
Exceeding the maximum off-level limits can permanently
damage the cooling unit. The cooling unit or its performance is
not affected when the vehicle is in motion.
Gradual Decrease in Cooling Effi ciency
A gradual decrease in cooling ef
fi
ciency is not a clear
indication of cooling system failure. Other factors that affect
cooling ef
fi
ciency include ventilation, the heat input, off-level
operation, lack of service and maintenance, inadequate
repairs, or unauthorized
fi
eld modi
fi
cations. If any of these
factors exist and are not corrected, a replacement cooling
unit will also perform inef
fi
ciently or fail. Step-by-step
troubleshooting is the best approach when dealing with
a gradual decrease in cooling. It is important to consider
that, though not ef
fi
ciently, the cooling unit is working.
Troubleshooting should always begin by checking ventilation,
then thoroughly checking the cooling unit, and heat sources.
In the majority of reported cases, the problem is related to
the installation, which in turn hinders cooling unit ventilation.
Additionally, ambient air temperature plays a signi
fi
cant role
if the unit is not installed correctly. Incorrect installation can
lead to poor ventilation, which in turn relates to poor cooling
performance.
The unit's service and maintenance history should be
considered when checking a cooling unit for poor cooling
performance. The service history and the scope of service
work performed may lead directly to cause and resolution
of a cooling problem. The cooling unit has to reach normal
operating temperatures before troubleshooting can take place.
It takes an average of four hours for the refrigerant to reach
normal operating temperatures. The time frame to reach
operating temperatures depends on ambient air temperature.
