问答题 6分

Slow speed diesel engines are large, , especially tall, and heavy and operate on the two-stroke cycl...

Slow speed diesel engines are large, , especially tall, and heavy and operate on the two-stroke cycle. These
are the largest diesel engines ever built. Engine powers up to 100,000 kw are available from a single engine.
They are tall to allow for long strokes which improve engine efficiency the large physical allow for long
strokes which improve engine efficiency. The large physical size of the engine and components leads to
slow rotational speed with speeds up to 300 rpm considered to be slow. For equivalent power output, the
two-stroke diesel engine is significantly lighter than its com-parable four-stroke relative. This is most
apparent for large power requirements where the two-stroke engine produces much more power for the
same weight.

Large, slow speed, two-stroke marine diesel engines offer the advantages of:
1. Burning poorer quality, cheaper fuel.
2. Providing large power from a single, less complicated machine comprising fewer individual cylinders and moving parts.
3. Having a low rotational speed allowing them to be directly coupled to the propeller and removing the
need for transmission machinery such as clutches and gearboxes etc.
4. Being reversible and thus eliminating the need for reversing gear or controllable pitch propellers.

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5 问答题 6分
For a certain provision cooling plant, the capacity regulator is generally controlled depending on
pressure, temperature or relative humidity. The pulse emitter is therefore either a pressostat(压力继电器),
thermostat or hygrostat. The switching difference of the control units should be rated as large as possible.
This applies in particular to control by means of the pressure, because change to the load in the
refrigeration circuit results in relatively quick pressure changes compared to temperature, which can in turn
lead to inadmissible oscillation of the regulator.

Under partial load, the suction volume and thus the gas speed can undergo marked changes depending on the
regulation stage. For this reason, the rating and routing of the suction line must be given particular attention.
In order to guarantee sufficient oil return, neither full load nor partial load operation may allow the gas speed
to fall below a minimum level: 4 m/s in horizontal pipe sections; 8 m/s in vertical pipes.

For this reason we recommend that vertical pipes bestaggered into two separate sections (as illustration).
As a result of the falling suction gas speed velocity in partial load operation, the pipe elbow B is
blocked by an accumulation of oil. This increases the gas velocity in pipe section A. In order to guarantee adequate
oil return, both cross sections must be rated so large that the velocity does not fall below the minimum level
in full and partial load operation. In plants with extensive pipe systems it is recommended at least for R22 and NH3
to install an additional oil separator. This reduces the oil circulation period and can have positive effects above
all for cold starts with relatively high oil ejection.