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DANFOSS Energy Efficient Buildings Use Frequency Converter Technology From Danfoss VLT® Drives

Speed control of pumps and fans lowers energy consumption and costs

They usually operate out of sight; but pumps, fans and compressors are vital to many aspects of our everyday environment. We expect them to operate reliably at all times. Without the tasks they perform, such as ventilation and cooling of buildings, our lives would be a good deal more difficult. The pumps, fans and compressors used in these applications must work without a hitch.

For years, energy prices have been rising. Not surprisingly, interest in energy savings for industrial and commercial applications has soared. Overall cost reduction and energy savings have never been more topical, in no small part due to dramatically higher energy costs. And they are still going up.

Owners and operators thus seek energy efficiency measures in production facilities as well as residential and office buildings. Building automation also offers potential for energy savings. Electrical drive technology is a key technology for enhancing energy efficiency and currently the most effective way to reduce energy consumption quickly and significantly.

Speed control of pumps and fans with quadratic load torque

Pumps and fans have particular potential for savings, and they are widely used nowadays in building services. In case of fans, centrifugal pumps and compressors, which have a quadratic load torque characteristic, energy consumption falls proportional to the motor speed to the power of three.

A widely used solution for such equipment is to fit a modern frequency converter so the speed can be perfectly matched to the actual capacity requirement. In most situations, pumps and fans in building services are configured for worst case scenarios. An example is an air conditioning unit dimensioned for the hottest day of the year, when it has to work at full capacity. This naturally means that it operates under partial load the rest of the time. A similar situation is true of pressure boosters in high-rise buildings. This is where frequency converters come in to play.

Speed control as a savings measure

To avoid surprises where speed control for pumps and fans is concerned, operators planning a project should remember that changes in speed alter the operating point and, as a result, affect the efficiency of fans, pumps and compressors. Technical, commercial and logistics issues must therefore be considered before an investment decision is taken, in order to avoid uneconomical and counterproductive actions. To ensure that cost and effectiveness are up to spec with speed-controlled pumps and fans, frequency converters should not be selected on the basis of the lowest price, but instead to yield the most cost-effective and efficient solution throughout the entire life cycle.

Efficiency curves of fans, pumps and compressors

In older systems and even in some new facilities, pumps and fans are often used with dampers, valves or three-way valves. These adjust the pressure or flow in the system to suit the current capacity requirement.

If a centrifugal pump is controlled using a throttling valve, throttling moves the machine's operating point along the pump curve. The required energy is reduced only minimally compared with the pump's set point.

If a pump is speed-controlled, the operating point moves along the system curve. The required energy is reduced by the power of three compared to valve control. At half speed a pump, for example, needs only an eighth of its nominal power. This behaviour is the same for all pumps, fans and compressors with a quadratic torque characteristic.

In addition to pump and system characteristics, the graph (Figure 1) shows a number of efficiency limits. Both valve control and speed control cause the operating point to move out of the optimum efficiency range.

Figure 2 shows the energy consumed by a selected pump operating under speed control. At around 32 Hz, additional losses in the pump start to exceed the savings. The optimum energy efficiency frequency in the system shown is thus 38 Hz. If the pump were not speed-controlled, the energy balance would be significantly worse.

As mentioned earlier, building services systems must be dimensioned to handle the peak load. This unavoidably means that they operate under partial load much of the time. Manufacturers of fans, pumps and compressors take this into account, designing some units so that optimum efficiency is achieved at approximately 70% of the rated delivery volume. When upgrading existing systems or developing new designs, users should consider the optimum efficiency of the pump, fan or compressor and check whether it is suitable for the partial load profile of their system.

Further operating point optimisation – cascading is often a sensible option

Combining a fan, pump or compressor with a frequency converter yields a speed range in which the system saves energy. The machine should operate in this range most of the time. If the difference between the maximum required capacity and average partial load operation is too great, the operator should seek another solution.

In this case the user needs a solution with several units that are nevertheless able to offer the advantages described above. Some examples of such systems are pressure booster or irrigation systems, fans in multi-cell cooling towers, secondary water pump systems typically found in cooling systems, and pump systems for district heating.

There are many reasons for using this type of system architecture. The most important reason is optimal setting of the set point according to the actually required system capacity. For example, efficiency may be as low as 10 to 20% with a wide control range and a single pump dimensioned for worst case operation. In contrast, a multi-pump system can usually achieve an efficiency of over 70.

This makes cascading a sensible option. In addition, investments for the conversion of existing systems often pay for themselves relatively quickly.

When pumps are cascaded, one speed-controlled pump handles the base load. If the load increases, the frequency converter cuts in more pumps one after the other. The pumps accordingly operate at maximum efficiency whenever possible. Pump control ensures that system utilisation is always as energy-efficient as possible. Cascade circuits are a good choice for systems were several pumps share the load. They ensure optimal efficiency, reduce wear and energy costs, and achieve balanced loading by lead pump alternation (Figure 3). The same approach can be used for fan control. Suitable cascade controllers are available as external assemblies.

Ask your specialist

To be sure of obtaining the best solution with maximum energy efficiency, users should always assess the advantages and disadvantages of a particular technology. Here it is important to bear in mind that the quality of a technical solution is usually proportional to its price. As the vast majority of today's users cannot possibly be familiar with every last detail of all technical devices, and as the interplay of components becomes still more complex, it is certainly worthwhile to seek advice from experts where necessary and to discuss all technical advantages and disadvantages with them.

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Arundel Jones Associates Ltd Registered in England and Wales No. 07334149
Hill Farm, Linton Hill, Maidstone, Kent ME 17 4AL
Tel : 01622 745333 email
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