The general purpose of a heat exchanger is to （）.

Heavy wear in a particular area of the inner circumference of a packing ring may be caused by （）.

If the thermal bulb of an expansion valve is incorrectly placed at the center of the coil instead of the end, it will （）.

If the head pressure of a reciprocating refrigeration compressor is excessive, （）.

The usual symptoms of cavitation in a centrifugal pump is （）.

The output volume of a positive fixed displacement pump can be changed only by （）.

One cause of high head pressure occurring in a refrigeration system can be （）.

An internal bypass is provided on some hydraulic system suction strainers to help reduce the possibility of （）.

Low compressor head pressure in a refrigeration system can be caused by（）.

Who is responsible for ensuring that someone is assigned to close the watertight doors in an emergency?（）