Lesson 8 - Control of Fluid Flow 1
This lesson will consider two main classifications of pumps: centrifugal and positive displacement. These will be introduced together, with aspects of their design and operation. The lesson will also pull together various fluid mechanics concepts from previous lessons, such as the Darcy-Weisbach equation and the use of loss-coefficient to calculate pressure drops in pipe systems. It will use this information to then size pumps for particular pipe systems.
Control of Pumps
As much as we, along with our engineers apply care and consideration in the design of our pipe systems and when selecting appropriate pumps to deliver the required flowrate, in practice, we often need a means of efficiently adjusting the flowrate of a pump to vary the flowrate e.g. during the different phases of lautering or the operation of a whirlpool.
Also, pumps, particularly large ones, are driven by large electric motors, and special consideration needs to be given to starting these motors as the mechanical and electrical ‘stresses’ are considerable.
Starting a pump
The simplest means of starting a pump motor is to connect it to an electrical supply and switch it on, using an appropriate switch, or contactor (a contactor is the electrical engineering term for a switch). Think of it in the same way as you would use your car key to supply power to your car’s starter motor to start your car engine. This is a perfectly acceptable approach for small pump motors typically used in homebrewing or smaller craft breweries. Once the pump motor becomes larger and often provided with a three-phase electrical supply, then switching the motor on in this way creates an initial current surge at the moment the supply voltage is connected to the motor – typically up to six times the full load current. This can require:
-
The costly (over) sizing of the motor installation and associated equipment, such as cabling, contactors, fuses, and circuit breakers, etc.
-
The current surge can reduce the lifetime of these electrical components.
-
The high surge current produces very high starting mechanical forces (specifically torques) within the motor. These can increase the wear on drive train components, such as bearings, driveshafts, and gearboxes thereby reducing their life.
To reduce these negative aspects, we can utilise ‘soft start’ technology to slowly increase the voltage supplied to the motor to limit the surge current. This is done using complex power electronics, which automatically adjusts the supply voltage to the motor.