The performance adaptation of cooling and air conditioning systems to the actual requirement has always been an important challenge in refrigeration technology. Various control concepts and technologies for capacity control are already in use today. Until now, the focus has rather been on the reliable controllability of systems, and hardly on the efficient part load operation. However, in times of steadily rising energy prices, this is now differently appraised.
Up to now, refrigeration and air conditioning systems have often only been controlled downwards to 30°C or even only to 40°C condensing temperature. Today, modern control concepts are intended to lower the condensing temperature as much as possible to improve the system efficiency.
This, however, sets new requirements for the compressors. Reliable, long-time and more efficient operation at lower pressure differences under part load conditions must be guaranteed. The cooling capacity of the compressors increases by lowering the condensing temperature, so that the effective capacity reduction must be higher. A further enhanced screw compressor series takes this effect into account. This series is not only designed for maximum full load efficiency but also for maximum part load efficiencies. The new mechanical slider control offers another application range with regard to lower condensing temperatures and high efficiencies in part load operations which have so far only been possible in connection with turbo compressors. In many cases the efficiency of such screw compressors exceeds the values of speed-controlled compact screw compressors with the same cooling capacity. This results in reduced investment and operating costs for liquid chillers. This efficient capacity control of compressors and an intelligent control in combination with a refrigerant with low GWP value (e. g. R134a) make it possible to considerably reduce the ecological influences of air-conditioning and refrigeration.
The following article shows the essential design criteria and differences for compact screw compressors with slider control and frequency inverter operation. Since in addition to the technical solution of efficient capacity control, the representation and comparability with other technologies should also be given, the boundary conditions of current certification programs and international standards are also taken into consideration.
2. Design criteria of compact screw compressors with mechanical slider control
Compact screw compressors are mainly used in liquid chillers with average and high capacity. Two separate circuits with direct expansion evaporators are a typical feature of this application. In Europe, mainly air-cooled systems are used which offer a high degree of temperature spread on the high pressure side (spring and summer conditions) in contrast to water-cooled systems or applications with very low ambient temperatures (e. g. Northern Europe). In addition to this version, there are also flooded systems which, however, will be put more and more under the pressure of legal regulations, such as the F-Gas Regulation, in Europe during the next years because of the high refrigerant charges. For medium-sized and larger liquid chillers, reversible heat pumps are frequently used which cool the water in summer and heat up the water for the building heating in winter. The requirements of heat pumps especially with regard to the temperature profile and the load distribution are very different and make general definitions of design criteria very difficult. The draft of the present prEN14825 also gives rise to many variables in the interpretation. However, this means in general that a large pressure difference range from the coldest day of the heating period to the coolest summer day with air conditioning requirement must be realized in contrast to purely air-cooled or water-cooled air-conditioning chillers.
2.1 Use of screw compressors in liquid chillers with low condensing temperature
The application of screw compressors in water-cooled liquid chillers or liquid chillers with low ambient temperature used in many parts of Central and Northern Europe is mainly determined by low condensing temperatures. The thermodynamic efficiency of systems rises, the more the condensing temperature can be lowered. This is also why, in addition to the defined low condensing temperature, the tendency towards increasingly lower temperature differences between the condensing temperature and heat reduction has shown up during the last years. Fig. 1 shows the theoretical coefficient of performance of an R134a system with decreasing condensing temperature. The theoretical coefficient of performance (COP) at an evaporation temperature of to = +3°C and tc = 30°C is 8.95 (toh = 20°C; without subcooling). If the condensing temperature is reduced to tc = 20°C, the COP is 14.91 which represents an increase by 67%. If the ambient temperatures and the compressor characteristics make it possible to use this potential, substantial reductions of the operating costs can be achieved. The importance of part load operation is another aspect which has increasingly been taken into consideration in the past years. Whereas in the past liquid chillers and the compressors used have only been assessed and compared in full load operation under reference conditions (such as the former EUROVENT certification ), the complete capacity range is increasingly assessed today and part load operation becomes more important. Most of the systems are operated at part load more than 95% of their operating time and just a few hours annually (if at all) at the calculated maximum capacity. This, of course, has an effect on the development of the compressors used and their possible adaptation of the offered compressor capacity to the required system capacity. One single compact screw compressor series cannot efficiently fulfil these requirements. This is why two different compact screw compressor versions were developed. On the one hand, the new CSW screw compressor series for applications at low condensing temperatures and on the other hand the CSH screw compressor series for air-cooled liquid chillers and heat pumps. The compressor versions differ by far more than only one Vi adaptation as offered by many manufacturers. The following example of the CSW shows the design elements.
Fig. 1: Theoretical coefficient of performance of R134a at different evaporation and condensing temperatures
2.2 Requirement profile of compressors for water-cooled liquid chillers
As already explained above, the efficiency of a liquid chiller can be considerably improved by lowering the condensing temperature to the lowest possible value. Fig. 2 shows this characteristic in an example of the new CSW screw compressor application limit (the application limit of compact screw compressors available on the market is marked in red). The coefficient of performance of a CSW8573-90Y compressor at tc = 40°C is 4.70. If the condensing temperature can be reduced to tc = 30°C, the coefficient of performance increases to 6.76 which represents an increase by 44%. If the ambient temperature enables the temperature to be reduced to tc = 23°C, a COP of 8.55, i. e. an increase by 82%, results (operating conditions of this comparison: to = +3°C; ∆toh = 5K; ∆tcu=5K; 50 Hz). Previous compact screw compressors could only be operated up to a condensing temperature according to the red line shown in fig. 2.
The potential for the increase in efficiency is clearly visible if modern system control supports these special characteristics. With these new screw compressors, the condensing temperature in part load applications can be further reduced. When the above-mentioned comparison is carried out at 25% load condition, the following coefficients of performance and increases of COP result:
COP: 25% load condition (tc = 40°C): 2.93
COP: 25% load condition (tc = 30°C): 4.55 ➔ 55% increase
COP: 25% load condition (tc = 20°C): 6.84 ➔ 233% increase
Fig. 2: Application limits of CSW screw compressors
This possible additional reduction of the condensing temperature in part load operation is of particular importance because the ambient temperature in part load operation very often reaches the minimum value as defined in the certification programs according to EUROVENT or ARI 550/590  (see section 4). These new application limits offer considerably improved system performance data and thus favourable classifications according to the EUROVENT or ARI 550/590 certification.
Fig. 3 shows a comparison between the previous compact screw compressors of the CSH series and the new CSW screw compressors according to the EUROVENT conditions for water-cooled liquid chillers. The figure shows the efficiency change at the various load conditions based on the full load values of the CSH8571-110Y.
As can be seen, the CSW compressors are better at all load conditions than the CSH compressors, especially regarding the part load operating points which are weighted more in the seasonal efficiency examinations. In this example, the European annual coefficient of performance (ESEER) could be increased by 23% (according to the EUROVENT calculation). The reduced condensing temperature at the 25% and 50% load conditions has a considerably strong effect on the calculation for the CSW compressors. The reduction of the condensing temperature is based on prior technical modifications which are required to benefit from these advantages. The oil supply in compact screw compressors generally takes place by feeding oil from the oil separator on the high pressure side of the compressor to the bearings and the profile. Insofar, the required pressure difference for a reliable oil supply is a decisive value for the application limit.
Fig. 3: Efficiency change at the various load conditions of CSH and CSW based on 100% of the CSH8571-110Y
2.3 Optimized oil supply
Oil management is a decisive factor for the reliability, use and last but not least the efficiency of screw compressors. During the development, great importance has always been attached to the reliability of screw compressors and the patented unloaded high pressure bearings have provided a decisive edge to the bearings’ service life. For this purpose, the bearing chamber on the high pressure side of all CSH/CSW screw compressors has been sealed and is not subject to high pressure, but the sealing and the vapour recovery towards the closed rotor profile provide pre-unloading of the system. Thus, the refrigerant concentration decreases and the oil viscosity increases which leads to an improved bearing lubrication.
With this construction, only a partial quantity of oil (and not the full oil flow) is fed to the bearings. This means, that the bearings are not “floating” in oil and offer improved rolling characteristics as oil mist is sufficient for the lubrication of the roller bearings.
The floating of the rollers or balls leads to undefined rolling characteristics of the bearings, similar to those caused by aquaplaning when driving on a rainsoaked road, and, apart from the loss in reliability, also to an increased power consumption of the compressors. Many screw compressor manufacturers pass the full oil quantity and full high pressure through the bearings and then to the main injection. This results not only in a higher load of the bearings but also in a imitation of the minimum high pressure. Very often, monitoring of the application limits via a pressure difference switch is then required to guarantee the oil supply. The required pressure difference for a reliable oil supply of CSW compressors is then only 2.2 bar. This is the prerequisite for extending the application limits as described above.
A second decisive aspect in the development was the optimization of the compressors with regard to the typically high percentage of the part load operating time in relation to the total operating time. The compact screw compressor series described here is the first series which has been optimized with regard to these special conditions and operating conditions. The special adaption of the application limit to applications at low condensing temperatures offers an additional three options of increasing the efficiency of compressors and systems: Firstly, the oil mass flow which is normally designed for the cooling of the compression process at high condensing temperatures can be reduced considerably. Secondly, the absolute loads on the bearings are reduced. This reduced load and the patented unloaded high pressure bearings make it also possible to change the oil selection.
Thirdly, the measures described above lead to a 10 –15% reduction of the oil carry over from the oil separator section handled by the CSH compressors. The CSH series is already fitted with an efficient and integrated oil separator characterized by a low pressure drop. This integrated triple oil separator is not only efficient within the application limit, but also in all capacity steps with oil carry over rates below 0.5%. Secondary oil separators are available for flooded systems. Thus, the CSW can be universally used in direct-evaporating and flooded applications.
2.4 Vi selection
The Vi selection for full load only plays a minor role for these compressors as the adaptation range is relatively small and primarily the part load conditions have to be taken into consideration in addition to the full load conditions. As shown in section 4, the operating hours of a water chiller at full capacity represent only a small percentage (1– 3%). The 75% and 50% operating points are more important for the calculation of seasonal efficiencies (e. g. according to EUROVENT or ARI 550/590).
This must be taken into consideration for the screw compressor development and especially for the Vi design. With screw compressors with slider control, it is possible to work with two outlet windows. The radial window sits on the slider and the axial window is integrated in the discharge flange. In full load position, the outlet window sitting on the slider is decisive for the Vi. When moving the slider (forwards according to fig. 4) in part load operation, the outlet window in the discharge flange is decisive for the Vi.
The volume ratio can thus be optimally matched in part load operation according to the ESEER or IPLV curves. Decreasing pressure differences (decreasing condensing temperatures a constant or increasing evaporation temperature) result in smaller volume ratios which are reproduced by means of this slider geometry. This is a special advantage compared to the previously available compact screw compressors with frequency inverter which only need one Vi for all load conditions.
This Vi can only be a compromise between full load and part load conditions. The Vi of the previously available compressors with frequency inverter was designed for full load operation in order not to suffer further disadvantages regarding the full load efficiency in addition to the inverter losses.
Fig. 4: Axial and radial pressure outlet windows in a screw compressor with capacity control slider
2.5 Motor selection
The design of the motor has also been matched to the new application conditions. The motors are generously sized in order to cover the full application range. However, the design criteria for optimal efficiency of the motors have largely been shifted towards the equired torques for part load operation. In this case, optimum efficiencies ranging from 0.93 to 0.96 are also achieved in the application ranges with the highest expected operating time. In addition to the increase in efficiency, a reduced starting and operating current is also achieved which enables the use of smaller and cheaper contactors, cables and fuses.
2.6 Economizer operation
The use of economizers has now proven to be efficient for liquid chillers with R134a. Even in the case of moderate pressure differences which occur in air-conditioning applications, the economizer generates an increase in cooling capacity of approx. 20% (with to = 0°C and tc = 50°C) at moderate cost increases. Thus, in many cases it was possible to compensate the competitive disadvantage regarding the specific cooling capacity of the R134a compared with the R22.
The extension of the CSH series was the first extension which has been launched to the market many years ago in combination with the advantages of the economizer especially for the R134a. This feature has clearly confirmed the expected efficiency advantages. The economizer connection (fig. 5) integrated in the slider made it also possible to use subcooling efficiently in part load operation and to maintain a uniformly subcooled liquid in the mechanical expansion valves.
Fig. 5: Schematic diagram of the integrated economizer in the capacity control slider
In the case of air-cooled systems, the advantage is now obvious. The temperature stroke is relatively large, but is the subcooling useful for water-cooled systems with low temperature stroke? Taking only the compressor into account, the increase in capacity of 10 –13% and increase in efficiency of 6% at full load (to = +3°C; tc = +38°C; toh = 5K; tcu with economizer; 50 Hz) are not extraordinary. But the economizer may be important for the entire condensing range because the increase in cooling capacity of the compressor at reduced condensing temperature is lower. Fig. 6 shows the cooling capacity of various compressors and refrigerants compared to the system requirement. The required cooling capacity of systems will decrease or at maximum remain constant at reduced condensing temperature. “System type A” in fig. 6 represents a process cooling with constant cooling capacity requirement and “system type B” shows a comfort air-conditioning system with decreasing cooling capacity requirement at reduced ambient temperatures. With all compressor technologies, the cooling capacity generated by the compressor increases with decreasing condensing temperature. It can be clearly seen that screw compressors with economizer show the lowest relative increase in cooling capacity with decreasing condensing temperature compared with the selected layout point (to = +3°C, tc = 50°C). This is an advantage for the system control because almost all systems have the same constant or reduced cooling requirement with decreasing condensing temperature, as shown by the lines for the system types A (process cooling) and B (comfort climate) in fig. 6. R134a screw compressor systems without economizer and scroll compressors with R410A show more disadvantages due to their considerably higher compressor performance increase.
Fig. 6: Relative compressor cooling capacity increase and system requirement variation with decreasing condensing temperatures
This means for screw compressors with economizer that single compressors can be operated in higher part load slider positions in order to obtain the same reduction in percent of the system performance. This results in improved part load efficiencies of the system. Fig. 7 clearly shows this by means of the slider positions of a water-cooled comfort air-conditioning system according to the EUROVENT conditions. Two CSW8583-110Y were selected for the comparison. First of all, you will see that the compressor with economizer (system B) generates 10% more cooling capacity under these operating conditions than without economizer (system A).
Fig. 7: Slider positions for different part load conditions for systems with (system B) and without (system A) economizer
Operation under the individual part load conditions is then realized with deactivated economizer. The individual part load conditions refer to the corresponding full load capacities as defined in the EUROVENT certification. The 75% operating point requires a slider position of 69% system A or 76% system B due to the reduced condensing temperature. This tendency remains for all part load conditions. In ON/OFF operation at minimum system performance, the cyclic operation for system B is shorter than for system A without economizer. Although the cooling capacity of both compressors is the same, the compressor in system B must cycle less because the full load capacity in system B is 10% higher than without economizer. For systems with economizer, this means not only an efficiency advantage for full load operation but also for all part load conditions. This leads to an overall increase of the ESEER.
The economizer has therefore been integrated in the CSW screw compressor housing and is thus optimally suitable for the full load condition. It can then be deactivated for part load operation to achieve a lower residual capacity of the corresponding part load condition.
3 – Compact screw compressors with integrated or external frequency inverter
Compact screw compressors of conventional construction can generally also be operated with external frequency inverter for performance adaptation. In this case, the slider control is used for start unloading. During normal operation, the compressors are controlled via the frequency inverter in full load position. The control range depends on certain boundary conditions which are explained in the following. The same restrictions also apply to compact screw compressors with integrated frequency inverter.
3.1 Motor design
The motors of compact screw compressors available on the market are normally designed for 400 V/3/50 Hz or 460 V/3/60 Hz. Many special voltages are also possible. In Central Europe, the supply voltage 400 V/3/50 Hz is generally offered. The selection of the correct motor voltage and frequency inverter is possible based on these parameters. Due to the investment costs for the frequency inverter, it is the aim to increase the compressor speed to obtain a more favourable price/kW cooling capacity ratio. When using a 400 V/3/50 Hz part winding motor, under-voltage is supplied to the motor if the output frequency of the frequency inverter exceeds 50 Hz, because the output voltage of the frequency inverter is, in general, not higher than the input voltage. If the motor disposes of sufficient capacity reserve, it may be operated up to 60 Hz (20% under-voltage). In an air-conditioning application, it is rather unusual for motors to dispose of such a reserve at the maximum operating point. This is why compressors with special motors (such as 230 V/3/50 Hz) are required. These motors could be operated up to 87 Hz without under-voltage in a 400 V/3/50 Hz mains. This selection presents the disadvantage that the compressor with standard part winding motor cannot be connected directly to the mains and that the operating current and thus the frequency inverter value increases by the factor root 3 due to the lower motor voltage at constant power consumption. Furthermore, the cooling capacity potential of up to 87 Hz cannot be used completely as shown by the following points.
3.2 Pressure drops
Conventional compact screw compressors are designed for a frequency range from 50 Hz to 60 Hz. The internal flow areas and shut-off valve geometries are optimized with regard to the volume flows in this range. Now, if the speed is increased to 70 Hz or 80 Hz, the required volume flow increases by 40 – 60% (based on 50 Hz). The quadratic volume flow is used for the pressure drop calculation and is therefore a decisive factor. Efficient use above a speed of 60 Hz is therefore not possible only due to the inverter losses but also due to the increasing pressure drops.
3.3 Oil separator efficiency
Like the pressure losses, the oil separator efficiency also depends on the volume flow. Above a certain value which normally lies outside the application limit of 50 – 60 Hz, the separator efficiency decreases overproportionally. At a value of 70 or 80 Hz, the oil separation efficiency decreases drastically to values below 97%. This leads to increased concentration of oil in the system and may also cause oil migration with strongly variable power requirement. A downstream secondary oil separator could be a remedy in this case, but would also further increase the investment costs.
3.4 Speed range
The speed range is determined by several variables. The today’s construction of compact screw compressors with integrated or external frequency inverter generally only enables efficient use within the range from 20 (25 Hz) to 60 Hz. Any use above this limit range causes excessive pressure losses and oil carry over. Any use below approx. 20 Hz causes excessive leakages in the profile and excessive inverter/motor losses. The losses caused by the frequency inverter are not taken into consideration in most of the compressor documents. This is also not possible when using external frequency inverters, as the assignment between the compressor and the frequency inverter is normally not done by the compressor manufacturer and the selection may have a considerable effect on the motor power. Even companies offering compressors with integrated frequency inverter do not document the inverter losses which leads to unrealistic efficiency values in the documentation.
Fig. 8: Efficiency change at the various load conditions of CSH and CSW with and without frequency inverter and based on 100% of the CSH8571-110Y
3.5 Application limit
The above-described possibilities for extending the application limit in part load operation cannot be easily realized with conventional compact screw compressors with frequency inverter. For capacity control with frequency inverter, the compressor is always operated in the “full load” slider position. Thus, the increases in efficiency realized by decreasing the condensing temperature during part load operation cannot be entirely realized for compact screw compressors with frequency inverter. This results in lower ESEER or IPLV values as shown in the comparison in fig. 8 of the various load conditions for a water-cooled condensing unit according to EUROVENT. Under 100% load condition, a CSH with frequency inverter generates inverter and pressure losses which make the EER turn out lower. In the 75% load condition, the efficiencies of the compressors with frequency inverter or with slider control are very similar and with 50% and 25%, the efficiencies of normal compact screw compressors with conventional slider control drop considerably. The CSW compressor is more efficient under all load conditions than the CSH with frequency inverter which results in an ESEER improved by 10 –15% compared with CSH screw compressors with frequency inverter. Conventional compact screw compressors with internal or external frequency inverter can therefore achieve a better ESEER compared with the previous CSH series with slider control, but they cannot reach the capacity characteristic of the new CSW.
3.6 Sound power
Modern drive units consisting of motor and rotors are designed for a speed range from 50 Hz to 60 Hz. Resonances and vibrations may be caused within an extended speed range. Generally, the corresponding frequencies are then not taken into consideration in the frequency range. However, if the compressor is operated above 60 Hz, sound emission increases overproportionally so that special measures for sound insulation are required. All in all, the compact screw compressors with integrated or external frequency inverter which are available today are a compromise based on the situation that no efficient slider control has been available so far. The SEER values (Seasonal Energy Efficiency Ratio) of liquid chillers with screw compressors could be increased by means of frequency inverters. The investment costs for a frequency inverter are, however, quite high which makes the amortisation compared with conventional screw compressor systems difficult. The frequency inverter is often as expensive as the compressor. The comparison between the available frequency inverter solutions and the new CSW systems does not show any amortization potential regarding the additional investment costs. For special projects in which the temperature criterion or a very low starting current is very important, compact screw compressors with frequency inverter may be useful. In this case, the solution of a semi-hermetic screw compressor with frequency inverter and external oil separator in the control range from 20 to 90 Hz would be the more economical, efficient and flexible solution .
4 – Certification programs and international standards with the required availability of the compressor documentation
Different standards and requirements for the design and documentation of liquid chillers and heat pumps exist worldwide. Over the past years, different certification programs have been developed on the basis of these standards, which should contribute to a better transparency and comparability of the manufacturer’s specifications. The EUROVENT certifications ARI 550/590 and ASHRAE 90.1  (both USA) and GB19577-2004 in China have been established as the essential standards and certification programs for liquid chillers. In addition to that, there are further national certification programs which, in many cases, are based on the former ones (e. g. in India the ECBC [Energy Conservation Building Code]). Certification programs and standards for heat pumps are in development. The standards and certifications formerly defined only the operating points and classification for the full load operation. As the operating hours under full load represent only a small percentage, efficiency comparisons on the basis of these conditions are not very meaningful. This is why meanwhile part load conditions have been integrated in the capacity appraisal and certification. The load conditions are assigned and appraised in the corresponding programs depending on the ambient temperature. Fig. 9 shows the frequency distribution of the part load values depending on the ambient temperature for the ESEER (European Seasonal Energy Efficiency Ratio according to EUROVENT) and IPLV (Integrated Part Load Values according to ARI 550/590) calculations.
Fig. 9: Frequency distribution of the load conditions according to EUROVENT and ARI 550/590
This classification allows a better appraisal of liquid chillers in comfort air-conditioning systems. The allocation of load conditions and ambient temperature defined as linear function represents a rough average only and in particular does not consider the part load operation at normal and high ambient temperatures. The statistical evaluation of the operating times of liquid chillers in Europe (for ESEER) and USA (for IPLV) is the basis of the weighting of the SEER values. It refers to the average temperatures in both climate zones. Both calculation procedures are based on a time distribution of the corresponding load conditions. See the ESEER example in fig. 10. However, it has to be considered for this appraisal that it is not the compressor coefficient of performance (COP) which is decisive for the system’s energy efficiency but the actual power consumption above the defined load profile. The power consumption at full load is four times higher than with 25% load condition and should actually be weighted more in the SEER with the given time distribution.
Fig. 10: Time distribution of the load conditions and weighting according to EUROVENT
This may cause shifts of the actual energy efficiency, as shown in fig. 11 by means of an example of two systems (A and B) with identical cooling capacity in all operating points.
Fig. 11: Comparison of the coefficient of performance and power consumption of two systems according to the EUROVENT conditions
System A has a particularly good part load efficiency for low load conditions and system B has a significantly better full load efficiency. The weighted power consumption in time over the load conditions is identical in both cases, the ESEER for system A, however, is by more than 5% better as compared to system B. The reference conditions and weightings in this example refer to the ESEER calculation according to EUROVENT for water-cooled systems. Operating conditions for the part load conditions that deviate stronger (air-cooled systems) can result in essentially larger differences. The conclusion from this is that a high ESEER (IPLV) value does not inevitably allow the conclusion that minimum energy is needed. The actual temperature and load behaviour cannot be represented sufficiently either, as the application conditions and external temperatures on the installation site can differ considerably. These rigidly defined specifications are of minor effect when liquid chillers with the same condensing technology are compared but show shifts in the actual efficiency in case of different condensing technologies and depending on the operation. A system with multiple liquid chillers in one application cannot be evaluated by means of the above mentioned methods.
In spite of this, evaluations which take the part load conditions into consideration are always better than a pure full load comparison. The draft of the standard prEN14825 defines new conditions for the testing and power measurement of liquid chillers and heat pumps based on a predefined temperature and load profiles.
This is a further step in the direction of more precise representations of the actual efficiency of liquid chillers and heat pumps. IPLV and ESEER comparisons can definitely be used for a first overview, but for an exact cost-benefit analysis the calculation on the basis of the actual boundary conditions with the software of the different manufacturers of liquid chillers should be used.
New requirements for the choice and design of compressors derive from standards, certification programs and real operating conditions. Previous software packages allow only the choice of compressors for full load conditions. Some programs also allow the calculation of part load conditions.
The BITZER Software offers as of version 5.1.3 more – the choice of compressor technology and compressor types according to ESEER, IPLV or individual specifications for four different load conditions. In accordance with the ESEER or IPLV specifications, evaporation and ambient or water temperatures for four load conditions (presetting: 100%, 75%, 50% and 25%) and the belonging time weighting can be specified. Furthermore, the number of circuits and compressors can be varied. The compressor capacity characteristic as a function of the load conditions can also be defined as an additional input parameter. The software determines the optimal compressor configuration and shows all the data necessary to select the components. This simplifies the simulation and configuration of liquid chillers or heat pumps considerably.
5 – Summary
The application of compressors in refrigeration and air-conditioning has been subject to continuous changes in the past years. The energy costs rise and the direct and indirect greenhouse potential of the systems becomes more and more the centre of interest in the social discussion. In addition to the choice of refrigerants with the lowest possible GWP (e. g. the currently technically usable refrigerants with minimum GWP: R134a for air-conditioning and normal refrigeration and CO2 in low temperature application with cascade), the system efficiency during the year is of crucial importance to the reduction of the CO2 emissions originating from refrigeration systems. In this case, the part load efficiencies of the compressors and the system are of particular importance as the systems are operated at part load most of the year. New standards and certification programs will increasingly take this aspect into account and the simple full load assessment still used in many applications today will play a minor role.
The new compressor generations will therefore be optimized with regard to the increase in efficiency at part load operation, whereas the full load operating point will not be neglected but will only be the second priority. BITZER presented a new CSW compact screw compressor series. This series could increase the ESEER according to the EUROVENT conditions for water-cooled condensing units by more than 23%. The efficiency of essential part load operating points was increased by more than 50% based on the new application limits. This enables the realization of modern flexible technology featuring low operating and maintenance costs and minimized CO2 emissions at the same investment costs.
 BITZER, 2008, A-600-2: Competence in capacity control
 Website EUROVENT certification: www.eurovent.com
 ARI 50/590: Performance Rating of Water-Chilling Packages Using the Vapor Compression Cycle
 BITZER, 2009, SV-0801: A new generation of frequency controlled screw compressors for liquid chillers and heat pumps
 ASHRAE 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings