Mitsubishi Electric CITY MULTI PUHY-400YMF-C Service Handbook page 62

(9) Judgement and control of refrigerant amount
• Judge refrigerant amount by detecting refrigerant liquid surface accumulator.
1) Judgement of accumulator liquid level
• Return refrigerant from accumulator liquid level detecting circuit to compressor inlet pipe, detect piping tempera-
ture, and judge liquid level.
When heated with heater, liquid refrigerant temperature is almost equal to low pressure saturation temperature,
and gas refrigerant temperature is a little higher than low pressure saturation temperature. By comparing these
temperatures in accumulator inlet portion, refrigerant liquid level can be judged.
Accumulator liquid level is judged in 3 steps as shown in the figure, from temperature A and liquid level detecting
temperatures (TH3, TH4). After deciding refrigerant status (Liquid: TH3 and TH4 are TH2 + 9˚C or less, Gas: TH3
and TH4 are TH2 + 9˚C or more), judge liquid level by comparing TH3 and TH4.
Balance pressure pipe
Inlet pipe
TH2
2) Control of liquid level detection
1 Prohibition of liquid level detection
Liquid level is detected in normal conditions except for the following;
(Cooling)
• For 6 minutes after starting unit, and during unit stopping.
(Heating)
• For 6 minutes after starting unit, and during unit stopping.
• During defrosting.
• For 10 minutes after refrigerant recovery.
(Note that liquid level determination is being performed even when liquid level detection is being disregarded.)
2 In case AL = 2 is detected for 3 consecutive minutes during liquid level detection (control at excessive refrigerant
replenishment and trouble mode)
• Changed to intermittent fault check mode preceded by 3 minutes restart prevention. But it is not abnormal when
the discharge SH is high. Error stop is observed when trouble is detected again in the same intermittent fault
check mode (for 30 minutes after unit stops for intermittent fault check).
• When turning on liquid level trouble ignore switch (SW2-4), error stop is not observed, and 3 minutes restart
prevention by intermittent fault check mode is repeated. However, LED displays overflow.
(Turning SW2-4 on makes the error of TH6 < outdoor air sensor > ineffective.)
3 When operation mode shows "Stop," excessive or insufficient refrigerant display and excessive or insufficient
refrigerant ignore display are extinguished.
Dividing plate
TH4
TH3
-59-
*Temperature A: low pressure saturation temperature
(TH2).
AL=2
AL=1
AL=0
• Judgement by the AL is at best only a
rough guideline.
Outlet
Please do not add refrigerant based
pipe
on the AL reading alone.