Halo Passes; Operation; Cascade Refrigeration System - Thermo Scientific Harris MBF-700 Installation And Operation Manual

SECTION 300 – THEORY OF OPERATION

HALO PASSES

OPERATION

CASCADE
REFRIGERATION SYSTEM
26 Generic MBF700 Service Manual
The backup unit is powered by 115 VAC, 1 phase, 60 hertz and draws 0.35
amps.
Two copper halo passes are installed on the inner wall around the flange of
the freezer shell. Each of these halo passes is independently and directly
connected to the discharge line of the first stage compressors of "A" and "B"
refrigeration systems. The high discharge gas temperature from the
compressors is thus utilized to raise the flange temperature above the local
dew point to prevent any condensation from forming. When the unit is in
operation, each halo pass can add approximately up to 60 W of heat for
moisture removal in the flange area. The halo heat is only available when the
unit operates.
Ample heat from both halo passes is provided to the flange to prevent frost
accumulation. When the cabinet temperature drops below the auxiliary
temperature control setting only one system ("A" or "B") will operate.
Therefore, the flange is still kept dry even when the cabinet temperature is at
ultra-cold condition.
Principle of a Low Stage in a Cascade Refrigeration System
A two stage cascade refrigeration system typically is comprised of two
separate complete refrigeration circuits. They are normally referred to as
high stage or first stage and low stage or second stage. While the two stages
work separately, the heat transfer duty is carried from the low stage to the
high stage via an interstage heat exchanger or cascade heat exchanger, then
ultimately to the environment by an air-cooled or water-cooled condenser.
In either stage, the basic elements are a compressor, a condenser, an
expansion device, and an evaporator. The evaporator of low stage circuit is
directly for removing heat from payload. In a storage type of freezer, an
evaporator tubing is wrapped around the storage tank where the products
are placed. As the liquid refrigerant boils within the tubing at a low
temperature of about -90 C, it absorbs heat from the products. The
condenser of the low stage is the high-pressure side of the interstage heat
exchanger where low stage refrigerant is condensed to a high-pressure liquid
at about -40 C. The expansion device, which may be a capillary tube or an
expansion valve, regulates the refrigerant flow from the high condensing
pressure to low evaporator pressure, hence producing an ultra lower
temperature in the evaporator. A special refrigerant such as R508b is ideal to
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