SPEED-CONTROLLED SCREW COMPRESSORS WITH OPTIMIZED PART-LOAD EFFICIENCY

Speed-Controlled Screw Compressors with Optimized Part-Load Efficiency
By: Julian Pfaffl, Manager of Product Performance at BITZER
Introduction
 
Over the years, more and more attention has been paid to the part-load coefficient of performance of liquid chillers used for comfort air conditioning. In addition to existing certification programs for seasonal performance figures (Eurovent ð ESEER, AHRI ð IPLV), legal requirements pertaining to the energy efficiency of liquid chillers will be established in many countries in the future. As part of a regulation mandated by the Ecodesign Directive, the EU will establishing minimal requirements for the Seasonal Energy Efficiency Ratio (SEER) as of 2017, allowing it to continue positioning products on the European market. Similar regulations are in preparation or have already been introduced in other countries such as the US and China. Figure 1 shows the demands on water cooled liquid chillers.
 
 
Figure 1: Requirements on water cooled liquid chillers for comfort air conditioning (as of 10/2014), SEER: Seasonal Energy Efficiency Ratio, IPLV: Integrated Part-load Value. * Tentative SEER requirements the regulation is currently in the formal process.
 
 
 
 
These SEER values are calculated based on national/European standards (Figure 2). When it comes to water cooled liquid chillers, these methods of calculation assume a full load running time of only one to two percent of the annual running time.
 
 
 
Figure 2: Regulations and standards to be applied
 
 
 
 
 
 
 
 
 
Development of screw compressors for water cooled liquid chillers
 
The compressor is the heart of every liquid chiller, so it should come as no surprise that it’s the first to receive attention during optimization measures, particularly in the part-load range. With its CSW compact screw compressors, BITZER offers a series that’s optimized for water cooled liquid chillers, features a slider control and demonstrates especially high efficiencies in the load range between 60 and 100 percent. The new efficiency requirements for part load can, however, only be achieved when using at least two compressors (parallel operation or two separate circuits). For lower load conditions, one compressor is, for example, shut off, so that the base load compressor can be operated in the higher efficiency range.
Speed control enables exceptionally favourable part-load behavior for screw compressors. With the newly developed CSVH series, compressors with integrated frequency inverter were introduced in 2012, designed primarily for use in air cooled liquid chillers.
Another generation of compressors has since been developed for highly efficient use in water cooled liquid chillers, combining the benefits of speed control with the unique characteristics of the CSW models. The standard operating conditions (Figure 3) have been derived from the aforementioned requirements and standards for water cooled liquid chillers in comfort air conditioning systems. These will be used hereafter to evaluate the energy efficiency of frequency controlled compact screw compressors (CSVW series) for low condensing temperatures.
 
 
 
Figure 3: BITZER rating conditions for water cooled liquid chillers, with water inlet temperatures in the condensers in accordance with the EN 14825 standard
 
 
 
 
Capacity control
 
The standards listed in Figure 2 for calculating the SEER require a part-load capacity of down to 21 percent (operating point D) of the full-load capacity. If the chiller cannot achieve this minimal capacity in continuous operation, the coefficient of performance (EER/COPR) is reduced at this operating point using a degradation factor. To make matters worse, the falling pressure ratio causes the volumetric efficiency to rise in comparison with the full-load point A, so that the compressor has to be adjusted to an even lower part load. In addition, the condensing temperature at point D falls in comparison with point A, increasing the usable enthalpy difference and thus the cooling capacity. The compressor should therefore be able to control up to around 16 percent displacement, which corresponds to a control ratio of 6.2:1. The control bandwidth is therefore twice as big as that of conventional slider controls.
Figure 4 compares the efficiency of a slider control with the efficiency of frequency control in a CSVW compressor. While the optimum capacity of the slider control is 100 percent at all times, the optimum capacity of the frequency inverter is achieved in the part-load range. This optimum capacity can be adjusted by adapting the design of the compressor and range of control. Due to the conflicting goals of high full-load EER (COPR) and high part-load efficiency, a compromise must always be made in the design. To optimize the part-load COP by up to 45 percent and achieve a range of control of 6.2:1, a reduction in full-load COP of the CSVW series had to be accepted.
 
 
Figure 4: Comparison of the control characteristics of conventional screw compressors with slider control/frequency inverter (CSVW) at operating point A with refrigerant R134a
 
 
 
 
 
 
Vi control
 
The Vi of a screw compressor is the ratio of the geometric displacement following completion of the suction process to the geometric displacement when the discharge gas mass flow begins to be discharged. The Vi is relevant in all positive displacement compressors without a working valve, and is used to define the design point of the compressor. To assess Vi control, we have to consider isentropic efficiency, which, in a compressor with integrated frequency inverter, is defined as the ratio of the theoretically required power to the power actually consumed, including frequency inverter losses. If you observe the isentropic efficiency behavior of a compressor with a constant Vi (Figure 5), you will see that the operating points A, B and C are very close to optimum, while operating point D shows lower efficiency.
 
 
Figure 5: Isentropic efficiency with a constant Vi
 
 
 
 
 
 
 
 
 
 
 
 
 
If you integrate a control slider into the compressor to change the Vi ratio to a second value, and always switch to the optimum depending on the system conditions, the isentropic efficiency can be improved by 11 percent in part-load point D. 
Figure 6 also reveals that another Vi ratio or seamless Vi control can only slightly increase isentropic efficiency in the typical operating conditions of liquid chillers.
 
 
 
Figure 6: Isentropic efficiency with two alternating Vi ratios
 
 
 
 
 
 
 
 
In CSVW compressors, the control module of the frequency inverter takes control of the Vi. Via two integrated pressure sensors the suction and discharge pressures are measured and, using integrated functions, the optimum Vi for the current operating point is calculated.
Permanent magnet motor
The asynchronous motors integrated into the compact screws work with efficiencies of up to 95 percent in full-load and around 90 percent in part-load. If you want to further increase efficiency here, asynchronous machine losses have to be reduced. Figure 7 shows the losses of a conventional 200 kW motor, according to where they occur. The losses were determined theoretically via the equivalent circuit diagram of the asynchronous machine.
 
 
 
 
Figure 7: Losses of a 200 kW asynchronous motor
 
 
 
 
 
 
 
 
 
Because a permanent magnet motor doesn’t induce voltage in the rotor, it doesn’t allow current to flow. The aluminium losses in the rotor that occur in the asynchronous motor are therefore not an issue and efficiency can be increased at part-load point D by around 2.5 percent. Figure 8 shows a direct comparison of an asynchronous motor and a permanent magnet (PM) motor, measured on a motor brake. The PM motor delivers efficiency that’s 2.5 to 3.5 percent higher throughout the entire application range. Because the rotor passes on less heat to the refrigerant (suction gas cooling), the suction gas maintains a lower temperature, further increasing volumetric and isentropic efficiency.
 
 
Figure 8: Comparison of a permanent magnet motor and asynchronous motor on the motor brake at 2,000 min–1
 
 
 
 
 
 
 
 
Due to the already very high efficiency of the asynchronous motors used in the compact screw compressors, the improvements resulting from the use of PM motors don’t appear to be significant at first glance. But it’s important to remember that the compressor in water cooled liquid chillers has much more of an influence on power consumption and thus the annual performance factor than the compressor in air cooled units. Due to the relatively high energy requirement of systems like these, the added costs associated with PM motors are offset in a very short period of time.
Summary
Using a frequency inverter in screw compressors can approximately double the control range compared with conventional slider control.
A second Vi that is adjusted with a control slider can increase part-load efficiency by 11 percent. Seamless Vi control doesn’t provide any benefits to water cooled liquid chillers.
Using a permanent magnet motor eliminates aluminium losses in the rotor, which makes it possible to further increase efficiency throughout all areas of operation.

Compact screw compressors with integrated frequency inverter offer exceptional energy efficiency and an impressive control range to meet the legal requirements of the future. 

The CSVW series (Figure 9) can be used to achieve very high SEER values for direct-evaporative water cooled liquid chillers, due to the latest system technologies.

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