Appendix; Theory Of Operation - Thermo Scientific XBF40-MD Series Installation, Operation And Service Manual

Appendix

14 Appendix
CAUTION: No user-serviceable components inside unit. All service work
must be performed by a properly trained and equipped
refrigeration/electronic service technician.
14.1 Theory of
°
The XBF40-MD is a -40 C (-40°F) single-stage refrigeration system with a
Operation
Dixell controller. Although it has some unique features due to the blast
freezing application, the basic operation of the unit is similar to any other
low-temperature forced-air freezer.
The compressor is a hermetic reciprocating design with a remote-mounted
run capacitor, start capacitor, and start relay. Waste heat from the
compressor discharge line is used to evaporate condensate water produced
during the defrost cycle.
The condenser fan runs continuously regardless of compressor status in
order to provide condenser cooling during the off cycle and reduce the
compressor starting load.
To ensure rapid load sensing and high heat transfer the evaporator fan runs
near-continuously, only stopping for defrost and door openings. When
running the fan forces cold air from the evaporator coil into the front part of
the cabinet where it is directed downwards over the load before being pulled
under the internal cabinet wall and directed up the rear of the unit to be
cooled and distributed again. If the cabinet contents block this return air
slot the airflow will be reduced and freezing performance will degrade.
The controller has an onboard real-time clock that is used to trigger defrost
cycles. Up to 6 automatic defrosts can be programmed per day. The factory
default is 3 with one occurring every 8 hours. Depending on the ambient
humidity, number of door openings, and type of application more or fewer
defrost cycles may be required to maintain performance. When the defrost
cycle starts the compressor and evaporator fan turn off and electric heaters
on the evaporator and drain tube are energized; melted water will drain from
the evaporator into an evaporating pan in the base of the unit.
The defrost cycle will terminate when the programmed defrost time has
elapsed or when the evaporator temperature probe reaches its programmed
termination temperature. A bimetallic temperature safety switch is installed
in the evaporator enclosure to prevent an overheat condition in the event of
a control failure or programming fault.The evaporator fan and defrost
heaters are powered through a cable that runs up the outside rear of the
cabinet. This cable is protected by a sheet metal guard.
Thermo Scientific XBF40-MD Series Blast Freezers Installation, Operation and Service Manual 29