轮机英语【大管甲类】
考试试题
[问答题]TWO STEP CONTROLLER ACTION is the action of a controller whose output signal changes from
one predetermined value to another when the deviation changes sign. This controller action is mainly on/off
control, for example, a refrigeration unit controlling room temperature, when the temperature rises to a
predetermined value the refrigerating compressor motor automatically starts, then when the temperature
falls to a predetermined value the motor is stopped.
PROPORTIONAL CONTROLLER ACTION is the basic action form of a continuous controller
whose output signal is proportional to the deviation. In the case of a boiler water level control, if the water
level in the boiler, due to increased steam demand, drops by 20 mm, then the automatic controller output to
the feed water inlet valve may change from 0.6 to 0.7 bar. If the level drops by 40mm, then the automatic
controller output would change from 0.6 to 0.8 bar. Offset or droop is an inherent characteristic of
proportional controllers. When a load change occurs in a controlled system a proportional change occurs in
the controller which results in an offset or steady state error to the desired value.
INTEGRAL CONTROLLER ACTION is the action of a controller whose output signal changes at a
rate which is proportional to the deviation. It is used in conjunction with proportional control in order to
remove offset inherent in the proportional controller.
DERIVATIVE CONTROLLER ACTION is the action of a controller whose output signal is
proportional to the rate at which the deviation is changing. The principle object of derivative control is to
give quicker response to system changes.
Sophisticated controllers for rapid response, minimum onsets and reset action would be three term
controllers i.e. proportional plus integral plus derivative.
[问答题]Displacement pumps are those where the volume of the pump chamber is alternately increased to draw
the liquid in from the suction pipe and then decreased to force the liquid out into the delivery pipe.
Centrifugal pumps are those wherein an impeller rotating at high speed throws the liquid by centrifugal
force from the center to the periphery of the impeller where the liquid is discharged through the delivery outlet.
Gear pump consists of two or more meshing gears (spur, single or double helical teeth) enclosed m a
close-fitted housing. These are used extensively for pumping fuel oil, lubricating oil and hydraulic oil.
When gears are unmeshed on the inlet side, a pressure drop occurs that helps the atmospheric pressure
acting on the surface of the oil in the reservoir to force it up the pipe to fill the void, the oil is then
transferred around the periphery of the pump housing. As the gear teeth mesh again on the outlet side, they
form a seal that prevents oil from backing up to the inlet, the oil in the void is then be forced out into the
discharge line.
All vane pumps have a rotor driven within a ring (cam ring) by a drive shaft coupled to a prime mover.
A cylindrical rotor with sliding vanes, generally in radial slots, rotates within the ring. The center of the
rotor is offset from the center of the ring. In a simple vane pump, as the rotor turns, the vanes are forced
outward against the inner surface of the cam ring by centrifugal force. This outward radial movement of the
vanes and turning of the rotor causes the chamber between the vanes to increase as the vanes pass further
away from the inlet port. This increase in volume results in a lowering of pressure until the atmospheric
pressure is sufficient to force oil from the reservoir into the inlet chamber. Oil from the inlet is swept away
by the vanes toward the outlet port through a decreasing series of chambers until it is forced through the
outlet port. A vane pump of balanced design has an elliptical cam ring so that two pumping chambers are
formed. This pump is in hydraulically balance, since the two intake and two outlet ports are diametrically
opposed to each other. Therefore the side loads exerted on the rotor cancel out, thereby increasing bearing
lift and permitting high operating pressure.
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