System optimisation
Control options
The most basic form of control is to manage flow by adding friction at the pump or fan outlet. For pumps, this is achieved by a throttle valve. For fans it is done with a damper. It is effective, but inefficient. For example, in situations where maximum flow is not required, and where throttling or damping is used continuously, efficiency can often be improved by reducing the diameter of the impeller. This is called trimming and is best done with the support of your original equipment supplier.
Most industrial systems have pumping requirements with several operating points or variable flow and pressure requirements. Picking the pump (fan) with the optimum efficiency for a specific delivery is only part of the story. The other part is controlling the flow rate to match the process requirements. You can do this in several ways:
- recirculation - continuously runs the fluid round the system through a buffer tank
- throttle control - uses valves or flaps to control the flow rate
- cycle control - turns the pump on and off to control the flow
- VSD or ASD - stands for variable or adjustable speed drive, and controls the pump's speed to control the flow.
The most efficient control option is the one that most closely matches the ideal pump curve, which is shown in the graph below.
To select the appropriate control option you need to balance the capital cost of the control equipment against the savings you will achieve. Although the more efficient control options generally have higher initial set up costs, they can result in large and reoccurring energy savings over the life cycle of the equipment.
Variable speed drive manufacturers often help their customers analyse potential saving that can be achieved through variable speed drives for pump (fan) control. In some situations the savings will quickly pay back1 the cost of the equipment, and from then on the savings go straight to your bottom line.
For fans there is an additional form of control - variable inlet guide vanes. These can control air flow and maintain efficient fan operation, if minimum flow requirements are approximately 85 per cent of maximum flow conditions. For flow requirements below 85 per cent, variable inlet guide vanes can be combined with multi-speed motors to extend their effective range. Variable inlet guide vanes are relatively inexpensive but may not be suited to all situations. For example, they are unsuitable in dirty or corrosive atmospheres.
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