Safety Devices; Temperature Control - Thermo Scientific Harris MBF-700 Installation And Operation Manual

SECTION 300 – THEORY OF OPERATION

SAFETY DEVICES

TEMPERATURE CONTROL

24 Generic MBF700 Service Manual
(see Figure 7, notes 2,3 & 4)
Both first and second stage compressors are protected by an internal pressure
relief valve that will rapidly equalize the high and low sides of a given stage
in the event of a pressure imbalance greater than ~425 psi. Additionally, all
compressors are equipped with internal Klixon-type electrical overloads that
will break the motor circuit in the event of an overcurrent or
overtemperature condition. These two devices form a conservative
application envelope that the compressors should operate in. Under normal
conditions, the trip points are far from actual operating pressures. In the
event of a trip, both devices will automatically reset to their normal
operating mode once the triggering condition has been resolved.
A Reverse Acting Low-Pressure Control (RALPC) is mechanically
connected to the suction side of the first stage. It is however electrically
connected to the control side of the contactor for the second stage
compressor. When the first stage operates properly to trip at the setting of
the control ( 7 PSIG), the switch turns on the second stage compressor.
Therefore, it protects compressors in both stages from starting up
simultaneously. When the suction pressure rises to 21 psig, the 2
de-energized to prevent flood back of liquid refrigerant to the 2nd stage
compressor.
High/Low Temperature Alarm (see Figure 9)
Through the main temperature control, both high and low process variable
alarm settings can be made. Should the freezer temperature deviate beyond
these settings, a relay will activate the master alarm circuitry and the control
panel will display an alarm message.
In addition to the two refrigeration systems, redundancy is also incorporated
into the control system. Two temperature control systems are included – the
main and the back-up controls. The main control is an electronic Dixell
controller that provides the primary refrigeration demand signal, as well as
the high and low alarm setpoints. If the Primary control loses
communication with its sensor probe, it defaults to a 100% run condition.
The back-up control is an electromechanical thermostat. The factory has set
this thermostat 3-5 degrees warmer than the desired cabinet operating
temperature. When this safety thermostat functions, it indicates that the
second cascade system has been required to maintain proper temperature.
This can be the result of a significant heat load added to the freezer or, it can
mean that the system operating as primary at the time it occurred has a
problem. If a failure occurs, the failed system may be switched off by
nd
stage is
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