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Before
we start a detailed study of the family of pre-expansion faults,
in this chapter we will examine the greatly misunderstood problem
of instantaneous pre-expansion
in the liquid line,
commonly referred to as 'flash-gas'.
Let's
remind ourselves that the oil used in refrigeration installations
is poorly miscible with refrigerant in the vapour phase. Therefore
the selection and routing of suction side and discharge pipe-work
must be carefully designed to ensure that oil which is
continually leaving the compressor discharge is effectively
returned to the compressor through the suction side of the system
(problems associated with
oil return are considered on page 251).
In contrast,
the oil will mix perfectly happily with liquid refrigerant, and
its passage through the condenser and the liquid line generally
occurs without difficulty, even if the flow rate of the mixture
is slow, or if the pipe-work has low points.
Nevertheless,
in systems where the air-cooled
condenser is distant from the evaporator, even if a poor design
of the liquid line does not pose risks to adequate oil return, it
can still result in a pressure drop that is large enough to produce
the phenomenon of flash-gas... |
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EXERCISE
:
Refrigerant
used: R22 (a 1m column of R22 = 0.12 bar).
DP pipe-work
AB + CD = 0,02 bar/m
DP filter-drier
(BC) = 0,15 bar
DP solenoid
(DE) = 0,21 bar
Total
height difference = 6 m.
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What will be the pressure at the expansion valve inlet
(point E) during operation?
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What must the minimum value of liquid sub-cooling be to remove all risk
of producing flash gas?
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(Solution
: Refrepair Manual pages 87 to 94)
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