Mitsubishi Electric City Multi CMH-WM250V-A, Service Handbook

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[1-3 Working with Refrigerant Piping ]
16 -
chapter 1 BS_01_G
1
Che
ck Bef
o
re
Se
rvicing
1-3-3 Vacuum Drying
1. Vacuum pump with a reverse-flow check valve (Photo1)
To prevent the vacuum pump oil from flowing into the refrigerant circuit during power OFF or power failure, use a vacuum
pump with a reverse-flow check valve.
A reverse-flow check valve may also be added to the vacuum pump currently in use.
2. Standard of vacuum degree (Photo 2)
Use a vacuum pump that attains 0.5Torr(65Pa) or lower degree of vacuum after 5 minutes of operation, and connect it directly
to the vacuum gauge. Use a pump well-maintained with an appropriate lubricant. A poorly maintained vacuum pump may not
be able to attain the desired degree of vacuum.
3. Required precision of vacuum gauge
Use a vacuum gauge that registers a vacuum degree of 5Torr(650Pa) and measures at intervals of 1Torr(130Pa). (A recom-
mended vacuum gauge is shown in Photo2.)
Do not use a commonly used gauge manifold because it cannot register a vacuum degree of 5Torr(650Pa).
4. Evacuation time

After the degree of vacuum has reached 5Torr(650Pa), evacuate for an additional 1 hour. (A thorough vacuum drying re-
moves moisture in the pipes.)

Verify that the vacuum degree has not risen by more than 1Torr(130Pa) 1hour after evacuation. A rise by less than
1Torr(130Pa) is acceptable.

If the vacuum is lost by more than 1Torr(130Pa), conduct evacuation, following the instructions in section 6. Special vacuum
drying.
5. Procedures for stopping vacuum pump
To prevent the reverse flow of vacuum pump oil, open the relief valve on the vacuum pump side, or draw in air by loosening
the charge hose, and then stop the operation.
The same procedures should be followed when stopping a vacuum pump with a reverse-flow check valve.
6. Special vacuum drying

When 5Torr(650Pa) or lower degree of vacuum cannot be attained after 3 hours of evacuation, it is likely that water has pen-
etrated the system or that there is a leak.

If water infiltrates the system, break the vacuum with nitrogen. Pressurize the system with nitrogen gas to
0.5kgf/cm
2
G(0.05MPa) and evacuate again. Repeat this cycle of pressurizing and evacuation either until the degree of vac-
uum below 5Torr(650Pa) is attained or until the pressure stops rising.

Only use nitrogen gas for vacuum breaking. (The use of oxygen may result in an explosion.)
(Photo1) 15010H (Photo2) 14010
Recommended vacuum gauge:
ROBINAIR 14010 Thermistor Vacuum Gauge
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