Danfoss PSH051-4, Инструкции по применению

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Application guidelines
37
FRCC.PC.021.A5.22
System design recommendations
Oil equalisation Suction gas in a hermetic compressor flows via
the oil sump which makes it more difficult to
maintain equal pressure in the sumps of parallel
compressors. Since oil equalisation usually
depends on equal sump pressures this is a
point of special attention. Danfoss Commercial
Compressors has developed specially adapted oil equalisation systems which ensure proper oil
balancing between the manifold compressors
but it is always recommended to carry out
some tests to validate effective operation and
oil equalisation of compressors in parallel
installation at any operating conditions of final
application.
Test conditions
Tests shall be done at three points in final
application envelope:

ARI-based conditions

Low evaporation (SH18°F): low flow rate / pure
oil/ low oil level

High load (SH18°F): high flow rate/ diluted oil/
high oil level
Low
evaporation
ARI
condition
High
load
Test sequences 
Continuous for all compressors: 100% charge
(all compressors continuous running)

Continuous with partial charge: all partial
charges configuration must be tested

On/Off test: after 2 minutes shutdown of any
compressor, the oil level has to retrieve a proper
oil level within 1 minute when the compressor is
switched back on.

Transient 100% load: in transient condition such
as end of defrost with temporary liquid flood
back, check that oil return to normal level.
Oil level criteria 
The oil level of running compressors must
be visible or full in the sight glass of running
compressors at all operating conditions
described before.

The oil level of idle compressors may disappear
in the oil sight glass.

The oil level must retrieve a visible level in all
compressors after the unit is stopped.

Oil level top up might be necessary to retrieve
a visible oil level in the sight glasses. Always
use a Danfoss oil from new can (see section
accessories).

On units working 100% load continuously, a
compressor oil level might decrease. In order
to avoid any loss of oil, regular unit stops might
be needed to re equilibrate oil levels in the
compressors.
above the saturated discharge temperature, this
indicates liquid flood back.
Continuous liquid flood back can occur with
a wrong dimensioning, a wrong setting or
malfunction of the expansion device or in case of
evaporator fan failure or blocked air filters.
A suction accumulator providing additional
protection as explained hereunder can be used
to solve light continuous liquid flood back.
Suction accumulator: a suction accumulator
offers protection against refrigerant flood back
at start-up, during operations or defrosting by
trapping the liquid refrigerant upstream from
the compressor. The suction accumulator also
protects against off-cycle migration by providing
additional internal free volume to the low side of
the system.
A suction accumulator must be carefully
dimensioned, taking into account the refrigerant
charge as well as the gas velocity in the suction
line.
The accumulator should not be sized for less than
50 % of the total system charge. Tests must be
conducted to determine the actual refrigerant
holding capacity needed for the application
.
Depending on the operating conditions it may
happen that the recommended connections of
the accumulator are one size smaller than the
suction line.