单项选择题

In a refrigeration system, foaming of the crankcase oil may cause the （）.

A.expansion valve to overfeed
B.compressor to knock
C.water regulating valve to fail
D.crankcase drains to plug

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Excessive scale formation in a distilling plant may result from （）.

A.poor distillate quality
B.reduced evaporator capacity
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单项选择题

Foaming of the oil in a refrigeration compressor crankcase is caused by （）.

A.refrigerant boiling out of solution from the lube oil
B.liquid refrigerant flooding the compressor and system
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Which of the listed characteristics can be applied to R- 12 vapor?（）

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B.It is odorless
C.It is visible as h blue fog
D.It has a distinctive taste