Drive and drive unit components for reciprocating compressor

The main function of the drive and drive unit components of reciprocating positive displacement pumps is to provide a reciprocating linear movement for driving the plunger or the diaphragm of a pump head. Figure 1shows an overview of various drive unit designs with and without stroke adjustment.

Figure 1: Drives and drive units for metering pumps

Drives and motors

The term "drive" refers to the group of all functional elements that take primary energy and convert it into the linear, reciprocating form of kinetic energy necessary for operating the pump. 

The primary energy can take various forms, such as electric, pneumatic, hydraulic or chemical. 

In addition to the motors working as the actual energy converters between electrical and mechanical energy, a power train may include frequency inverters and external gears where appropriate. 

The secondary energy form is always mechanical energy. Depending on the type of kinetic energy delivered by the drive, it can either be applied directly, as in the case of linear drives, or has to pass through one more conversion step in the drive unit, as in case of rotating motors.

Linear drive – Direct drives for reciprocating pumps

The simplest solution for driving a reciprocating positive displacement pump is to directly convert electric, hydraulic or pneumatic energy using electromagnets, a linear motor or a stroke cylinder that is operated pneumatically or hydraulically. Figure 2 shows a schematic example of a linear drive using an electric solenoid.

Figure 2: Electric solenoid with stroke adjustment

In principle, the electromagnetic or pneumatic linear drive is the ideal drive for metering pumps. It delivers the kinetic energy directly in the desired linear and reciprocating form. No additional mechanical transformation is necessary as it would be for rotating electric motors or other rotating drives. 

This eliminates the gear and the crank mechanism, and adjustment of the stroke length is also simple. 

The advantages of a solenoid actuator include both the elimination of the mechanical overload protection, since the metering pump simply comes to a standstill in the event of overload, and the easy adjustment of the stroke frequency by electrical means. Figure 3 provides an example of an electromagnetic metering pump.

Figure 3: Example of an electromagnetic metering pump

As a matter of principle, pneumatic drives offer better explosion protection than electric solutions. Hydraulic drives allow large forces and make it relatively easy to control the stroke frequency. 

However, they require a relatively large effort in terms of providing the necessary hydraulic energy and control. Both the electric and the pneumatic stroke drive are usually equipped with stroke adjustment.

 A threaded spindle can be used to adjust a stop axially in order to limit the return stroke of the magnet or the pneumatic cylinder. That is why these drive units have a constant dead center (see the dashed line in figure 4).

Figure 4: Stroke function for linear drives

Primary advantages: 

– Straightforward mechanical structure 

– Compact, cost-effective design 

– Few moving parts resulting in a lower number of wear parts 

– Simplified explosion protection for pneumatic cylinders 

– Overload protection

– Easy stroke frequency adjustment (solenoid) 

– Easy stroke length adjustment 

– Constant front dead center when adjusting the stroke

Disadvantages of the electromagnet drives are usually the impact-type motion and a corresponding pulsation in the hydraulic and fluid part of the pump. Furthermore, only relatively low axial forces can be implemented using electromagnets. 

In pneumatic drives, poor efficiency and high effort for noise protection get in the way of increasing the capacity. Thus, the use of linear drives is mostly limited to the low performance range.

Primary disadvantages:

 – Impact motion sequence 

– Relatively low axial forces 

– Only suitable for small flow rates and pressures 

– Higher sound level for pneumatic drive 

– Poor efficiency for pneumatic drive 

– High complexity for hydraulic drive 

Areas of application: 

– Gas odorization, chemical process engineering 

– Micro flow metering 

– Laboratories and pilot plants