• switch off lights if they are not needed, for example, outdoor lighting during daytime;
• clean lamps regularly;
• ensure that walls and floors are light in color;
• install motion sensors in rooms that are lit and not used continuously;
• install time switches for simple switch-off functions.
226 | 7 Energy Efficient Unit Operations and Processes Heat exchangers
• improve control concept to reduce utility consumption;
• avoid sub-cooling or over-heating if not required;
• use lowest grade of possible heating and cooling media, for example, cooling water instead of chilled water;
• monitor fouling and clean surfaces frequently to ensure best possible heat
Equipment that consumes a large amount of power is pumps and blowers. The energy efficiency of these operations can be improved with the use of more efficient equipment, better matching of equipment capacity to throughput, and more energy efficient flow control such as variable-speed drives. Energy efficient equipment is discussed in detail in Chapter 8
Pumps account for a large part of industrial electricity consumption. In addition, the energy costs account for a significant part of the lifecycle costs of a pump. Therefore, optimization of the design is also essential. Since energy losses can be found in each part of a pump system consisting of pipeline, pump, gear, motor and frequency converter as illustrated in Figure 7.1, the whole system needs to be considered to identify improvement measures.
A large part of the energy consumption of a centrifugal pump can be saved by optimization of the control concept. In older plants we usually observe throttle control with fixed-speed motors where the flow is adjusted by a control valve in the pipe. This concept often causes a high energy loss mainly because a part of the hydraulic energy of the pump is converted into heat in the control valve. The transfer.
Frequency Motor Gear Converter
Frequency Motor Gear Converter
Figure 7.1 Schematic energy losses in a pump system.
throttle control is only recommended for smaller pumps or if there are minor deviations from the operating point or it is necessary for process reasons to have defined pressure conditions downstream of the control valve. The pressure drop at the control valve has to be minimal.
Bypass control is used primarily with displacement pumps where throttling of the volumetric flow is not possible. A further application is the control of minimum quantities with high-pressure centrifugal pumps. An advantage of bypass control is that the pump is always operated at its design point - but this is also the main disadvantage if it is operated for longer periods in bypass mode, since during this period it will require 100% energy without effective work. Bypass control with centrifugal pumps is in most cases less favorable in terms of energy than throttle control. An exception are pumps that have a power consumption which increases with a reduction in the volumetric flow rate, for instance axial pumps. In this case bypass control is more favorable in energy terms than throttle control. This type of control is only used in approximately 5% of pumps to be controlled, for instance small metering pumps.
Speed control influences the hydraulic performance of pumps and therefore the pump characteristic curve. With centrifugal pumps a variation of speed enables control of pressure and volumetric flow. The speed n of a centrifugal pump influences the volumetric flow Q, the pump head H and the power consumption P in the manner shown in the following equations:
Based on these relationships, it can be seen that if the flow rate is reduced by 50% through a 50% reduction in speed, the power consumption will be reduced to 12.5% of the value at full speed. On the other hand, it is only possible to reach 25% of the pump head H.
The effect of reducing the speed of a centrifugal pump is comparable to the effect of reducing the impeller wheel diameter. Energy savings become more pronounced for pumps with steep characteristic curves (e.g., axial and semi-axial pumps) then for pumps with flat characteristic curves (e.g., radial pumps).
Frequency control drives offer considerable potential savings for load profiles that fluctuate substantially. The costs of a variable-speed drive will almost always be recouped if the load profile exhibits substantial differences over the operating period. A detailed analysis is not worthwhile for pumps with a power consumption less than approx. 15 kW or if the operation time is less than 2000 h/a.
Figure 7.2 shows a comparison of an old pump system design with a fixed-speed motor and throttle control with a new pump system design with variable speed control and a high efficiency motor. Considering all elements a difference in the system efficiency of approx. 40% is possible.
228 I 7 Energy Efficient Unit Operations and Processes Old design: Fixed-speed machine, flow adjusted by control valve, system efficiency: app. 30 %
New design: Variable speed control with high efficiency motor, system efficiency: app. 70 %
Important improvement measures for pump systems are:
• Consider if flows can be reduced.
Often low temperature difference in circulation systems for heating or cooling media can be observed due to high circulation flows; reduction of circulation flow saves electrical energy.
• Switch off when not required.
• Adjust impeller wheel diameter.
A variation in the diameter of the impeller wheel has an effect similar to that of a change in speed. This is a suitable measure for pumps with a constant output. After adjusting the impeller wheel diameter the pressure drop at the control valve should be minimal.
• Reduce pressure drop in piping.
• Consider replacement of undersized lines and valves.
• Optimize motor drive.
• Consider replacement of old motor with low efficiency by new motor with higher efficiency.
• Avoid oversized pumps and motors.
Pump and motor should have maximal degree of efficiency at required operating conditions; over-sizing increases energy consumption.
• Implement predictive maintenance.
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