Backup Temperature Control System - Thermo Scientific Harris MBF-700 Installation And Operation Manual

SECTION 300 – THEORY OF OPERATION
BACKUP TEMPERATURE
CONTROL SYSTEM
(See Figure 9, Note 2)
The electromechanical backup control system will take over control of the
refrigeration systems if the main control system should fail to maintain the
desired temperature.
In this mode of operation, the sequencing relay will respond to a demand
signal from the secondary control by energizing the side of the system that is
not currently being controlled by the primary controller. As these two
demand signals are independent, it is possible for both systems to be
running simultaneously with one driven by the primary controller and the
other by the secondary controller. Control operation is transferred to the
backup system simply as a result of a main control system failing to
maintain a temperature colder than the backup control setpoint. (typically
3-5°C warmer than the desired operating temperature) Under normal
operation the freezer will not warm up to the temperature at which the
backup system will engage. However, if the freezer lid is left open for
extended periods or a large product load is placed inside the freezer, the
backup control system may energize both refrigeration systems. Also note
that when the freezer is first turned on at room temperature, the backup
control system will be engaged. As the freezer cools beyond the setting of the
backup control, operation of the freezer will transfer to the main control.
While the "On" cycles are fairly consistent, the off cycles will be very
responsive to the density of product loading inside the cabinet and the ease
of cabinet air circulation as well as ambient temperature. Off cycles can vary
from 4 to 30 minutes. The system is protected against overloading through
the action of the RALPC in the electrical circuit of the second stage.
However; prolonged overloaded operation will shorten the life of all
refrigeration compressors; overloading should be avoided by loading
pre-frozen product after the cabinet interior is at operating temperature.
When the secondary controller is engaged, the red "Backup Control" alarm
light on the lower status indicator panel will be lit. When both systems
operating have cooled the cabinet below the Backup Control's setpoint, the
Backup Control allows the Sequencing Relay to shut down one of the
refrigeration systems and the Backup Control red warning light will be
extinguished. The second system will continue to run until the cabinet
temperature has lowered to the temperature control set point. At that time,
the operating system will cycle off. Upon cabinet warm-up to the primary
control setpoint, the "lead" system (as determined by the position of the
sequencing relay contacts and indicated by the LED on top of the relay) will
start. When the cabinet control setpoint is once again achieved, the
sequencing relay will de-energize and will exchange the lead systems. The
Thermo Fisher Scientific Generic MBF700 Service Manual 29