What is the differnce between stroke length adjustment vs. stroke frequency adjustment of reciprocating compressor

As previously mentioned, in addition to adjustment of the metered flow by changing the stroke length, it is possible to control the stroke frequency with a variable frequency drive (VFD) or to combine both alternatives. 

Controlling the metered flow using the stroke frequency with VFD has a higher accuracy than the stroke adjustment by use of an actuator. This is due to the fact that a stroke adjustment has a certain tolerance range and play within which it is at a standstill though being actuated. This ensures judder-free operation of the actuator. In addition, using actuators in multiplex pumps results in high cost and effort since each drive unit has to be equipped with its own actuator and wired individually. Despite these disadvantages, actuators are frequently used in the field since they also offer many advantages in comparison to frequency inverters. 

The actuator is a classic solution that is easier to handle due to its installation in the drive unit as an integral component of the pump. As a result, it is not subject to additional specifications unlike the VFD. In environments with explosive atmospheres, a certified actuator is also significantly easier to use than a frequency inverter. Using actuators, the flow rate can be decreased to zero, which is another advantage. In addition, the design of the motors does not have to be coordinated with the actuator, which means that it is not necessary to select a large motor because of losses arising on the frequency inverter. Due to the constant stroke frequency, actuators are particularly suited for metering processes where precise chemical formulations are to be blended