Danfoss VZH052 Manual page 29

Manage superheat
Test, criteria and solutions
Test N° Purpose
Steady-state
Liquid flood
back test
Transient
Check liquid
floodback
Defrost test
during defrost
cycle
*Suction accumulator offers protection by trapping the liquid refrigerant upstream from the compressor. The accumulator should be sized at least 50 % of the total system
charge. Suction accumulator dimensions can impact oil return (gas velocity, oil return hole size...), therefore oil return has to be checked according to section "Manage oil in the
circuit".
Test condition
Liquid flood back testing must be
carried out under expansion valve
threshold operating conditions:
•Lowest foreseeable evaporation,
and highest foreseeable
condensation.
•Minimum speed running.
For reversible system, perform test
in both heating and cooling mode
A
Tests must be carried out with most
unfavorable conditions :
• fan staging,
• compressor staging
• ...
Defrost test must be carried
out in the most unfavorable
condition at 0°C (32°F) evaporating
temperature).
(reference at 20°C / 68°F ambient temperature)
-22
18
16
14
12
10
8
6
4
2
0
-30
Oil temperature sensor must be placed between
oil sight glass and compressor baseplate. Some
thermal paste shall be used to improve the
conductivity. The sensor must also be correctly
thermally insulated from the ambiance.
The Oil superheat is defined as:
(Oil temperature - Evaporating temperature)
Pass criteria
Suction superheat >5K
Oil superheat shall not be more than
30 sec below the safe limit defined in
the Dilution Chart. (see graph above)
Oil superheat shall not be more than
30 sec below the safe limit defined in
the Dilution Chart. (see graph ablve)
Dilution chart
Evaporating Temperature °F
-13 -4 5 14 23 0 32 50
Acceptable
Unacceptable
-25 -20 -15 -10 -5 0 5 10
Evaporating Temperature °C
Solutions
1. Check expansion valve selection
and setting.
-For Thermostatic expansion valve
(TXV) check bulb position...
-For Electronic expansion valve
(EXV) check measurement chain and
PID....
2. Add a suction accumulator*.
1.Check defrost logic.
In reversible systems, the defrost
logic can be worked out to limit
liquid floodback effect. (for more
details see "Control Logic").
2. Add a suction accumulator*.
59 68 77 86
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29
25
22
18
14
11
7
4
0
15 20 25 30
AB237586440343en-000601
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