Cooling Unit; Description; Heat Absorption; Leveled Operation - Norcold 1200 series Service Manual

COOLING UNIT

Description

The cooling unit is a self-contained gravity flow
absorption refrigeration system. The refrigerant charge
is a solution of water, ammonia, sodium hydroxide,
and sodium chromate (a corrosion inhibitor).
Refrigeration is accomplished by applying heat energy
and venting the heat extracted from the freezer and
fresh food cabinet while maintaining the unit leveled.

Heat Absorption

The refrigerant transfers the heat from the freezer and
the food cabinet to the absorber coils. At the absorber
coils the metal surface absorbs the heat and air flow
over the external surfaces of the coils carries away the
heat. A thermal air current is created by the rising hot
air and flows out of the enclosure through the roof
exhaust vent or sidewall exhaust vent (depending on
installation). The flowing air mass passes over the
surface of the condenser fins where it absorbs heat
transferred from the ammonia vapors flowing through
the condenser.
The thermal airflow process creates a "chimney
effect." The fresh air drafted by the chimney effect
through the intake vent removes rejected heat,
supports combustion, and expels the exhaust gases
produced by the combustion process. Any obstruction,
restriction, or modification to vents or the enclosure
will affect the cooling 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 screening covering vents.
Plastic sheeting covering vents.
Items stored in the enclosure.
Modifications to vents or enclosure.
No roof or sidewall exhaust vent opening.
Information pertaining to ventilation and enclosure
requirements appears on pages 19-23.

Leveled Operation

The circulation of the refrigerant through the cooling
unit is accomplished by gravity flow; therefore, the
refrigerator must be operated leveled. Off-level
operation will effect the flow 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 are not affected
when the vehicle is in motion.
1200XX/120X-IMXX Models

Gradual Decrease in Cooling Efficiency

A gradual decrease in cooling efficiency can be caused
by the following factors:
Ventilation obstruction.
Heat source failure.
Off-level operation.
Lack of service or maintenance.
Inadequate repairs.
Unauthorized field modifications.
Cooling unit failure.
Note that a gradual decrease in cooling efficiency is
not always an indication of cooling unit failure. If any of
these factors exist and are not corrected, a
replacement cooling unit will also perform inefficiently
or fail.
Step-by-step troubleshooting is the best approach
when dealing with a gradual decrease in cooling. It is
important to consider that the cooling unit is working,
though not efficiently. A Cooling System Diagnostic
Flowchart can be found on pages 34 and 35. The
cooling unit must reach peak efficiency before
troubleshooting can take place. It takes an average of
four hours for the refrigerant to reach peak efficiency.
This time frame depends on ambient air temperature.
Additionally, ambient air temperature plays a
significant role if the unit was not installed correctly.
Incorrect installation leads to poor ventilation, which in
turn relates to poor cooling performance.
The unit's service and maintenance history should also
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
the cause and resolution of a cooling problem.

Cooling Performance Monitoring Control

Units with Serial Numbers 832171 and Higher Only

The Cooling Unit Monitoring Control monitors the fin
assembly temperature through the thermistor. This
control activates when the refrigerator is turned on or if
the fin temperature rises above 40° F. When the
temperature is above 40° F and a change in cooling is
not detected in a specific amount of time, the
refrigeration heat sources are turned off. The Cooling
Unit Monitoring Control will not activate if:
The fin assembly temperature stays below 40° F.
If the thermistor is disconnected, open, or shorted.
If the fresh food compartment door is sensed open
(resampling starts when door closes).
The AC heater is open.
AC voltage is low.
Frost management is active.
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