Refrigerant entering the compressor of a refrigeration system should be in which of the following conditions?（）

Using a file without a handle may result in （）

If separating effect is not good, which measure should be taken （）

One of the reasons that cause oil flowed from sludge outlet of a self-cleaning separator is（）

The flash point of a residual fuel oil should be used to determine the highest temperature to which oil may be heated（）

A purge recovery system is used in a centrifugal air conditioning or refrigeration system to （）

If two A.C.generators are to be operated in parallel, the load is distributed evenly by（）

（）are used to digest the sewage to produce innocuous sludge in a biological sewage treatment plant.

During the diesel engine running, the water level of F.W expansion tank should normally kept in the （） level of glass tube.

The treatment of the fuel oils involves storage, heating, filtering and（）