Mitsubishi Electric City Multi CMB-WP108V-G, Service Manual

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VIII
Test Run Mode ]
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[3] Operating Characteristic and Refrigerant Amount
It is important to have a clear understanding of the characteristics of refrigerant and the operating characteristics of air conditioners
before attempting to adjust the refrigerant amount in a given system.
1. Operating characteristic and refrigerant amount
The following table shows items of particular importance.
1) During cooling operation, the amount of refrigerant in the accumulator is the smallest when all indoor units are in operation.
2) During heating operation, the amount of refrigerant in the accumulator is the largest when all indoor units are in operation.
3) General tendency of discharge temperature
Œ
Discharge temperature tends to rise when the system is short on refrigerant.
Œ
Changing the amount of refrigerant in the system while there is refrigerant in the accumulator has little effect on the discharge
temperature.
Œ
The higher the pressure, the more likely it is for the discharge temperature to rise.
Œ
The lower the pressure, the more likely it is for the discharge temperature to rise.
4) When the amount of refrigerant in the system is adequate, the compressor shell temperature is 10 to 60°C [18 to 108°F] higher
than the low pressure saturation temperature (Te).
-> If the temperature difference between the compressor shell temperature and low pressure saturation temperature (Te) is
smaller than 5°C [9°F], an overcharging of refrigerant is suspected.
[4] Adjusting the Refrigerant Amount
1. Symptoms
Overcharging or undercharging of refrigerant can cause the following symptoms:
Before attempting to adjust the amount of refrigerant in the system, thoroughly check the operating conditions of the system.
Then, adjust the refrigerant amount by running the unit in the refrigerant amount adjust mode.
2. Amount of refrigerant
(1) To be checked during operation
Operate all indoor units in either cooling-only or heating-only mode, and check such items as discharge temperature, subcool-
ing, low pressure, suction temperature, and shell bottom temperature to estimate the amount of refrigerant in the system.
The system comes to an abnormal stop, displaying 1500 (overcharged refrigerant) on
the controller. Overcharged refrigerant
The operating frequency does not reach the set frequency, and there is a problem with
performance. Insufficient refrigerant amount
The system comes to an abnormal stop, displaying 1102 (abnormal discharge temper-
ature) on the controller.
Symptoms Conclusion
Discharge temperature is high. (Normal discharge temperature is below 95°C [203°F].) Slightly under-
charged refrigerant
Low pressure is unusually low.
Suction superheat is large. (Normal suction superheat is less than 20°C [36°F].)
Compressor shell bottom temperature is high. (The difference between the compressor shell
bottom temperature and low pressure saturation temperature (Te) is greater than 60°C [108°F].)
Discharge superheat is small. (Normal discharge superheat is greater than 10°C [18°F].) Slightly overcharged
refrigerant
Compressor shell bottom temperature is low. (The difference between the compressor shell bot-
tom temperature and low pressure saturation temperature (Te) is less than 5°C [9°F].)