Analysis
Climaveneta S.p.A


 


 

The advantage of screw compressors for R134a

In the context of various analyses and research programmes, R134a, R407C and R410A proved to be particularly suitable for chillers used in air-conditioning systems, heat pumps and other high temperature applications.
The thermodynamic characteristics of these fluids, however, are considerably different and therefore offer different performance and efficiency levels depending on the type of compressor used.

As regards the theoretical coefficient of performance (COP), R134a certainly comes out very favourably compared with the R407C and R410A refrigerants (fig. 1).
The necessary displacement, however, must be increased according to the greater reduction in the displacement cooling capacity (fig. 2): in screw compressors, this can be obtained fairly simply by using a larger pair of rotors.
The increased size of the rotors offers additional advantages such as reduced leaking and less losses caused by reverse flow during compression. Moreover, the smaller difference between the condensation and evaporation pressures of R134a (about 67% and 45% compared with R407C and R410A respectively) offer an additional advantage:
these conditions are the main reason for the very high and unmatchable isoentropic and COP levels of the screw compressors optimised for R134a.

Fig. 1 - Relative comparison of the theoretical coefficient of performance (COP) The rapid reduction in efficiency of R410A for high condensation temperatures derives from its low critical temperature (73 °C). This disadvantage can only be partly offset by the improved heat exchange efficiency and lower pressure drops that this refrigerant provides.

Fig. 2 - Relative comparison of displacement cooling capacity. R410A clearly shows the highest values with the consequent need for a smaller compressor. At first sight, this seems an advantage; in screw compressors, however, this involves a lower peripheral speed of the rotors with unfavourable effects as regards displacement and isoentropic efficiency. An improvement in the behaviour of R134a is certainly evident when an economiser is used.

 


The new generation of screw compressors optimised for R134a.

Up until now, the additional displacement and the consequent increase in the size of the screws had led to a consequent increase in the geometrical dimensions of the entire compressor. Potential cost reductions were limited to the possibility of using a smaller motor and for this reason were extremely limited.
With a new generation of compact screw compressors optimised for R134a, it has been possible to substantially increase displacement without altering the external geometry of the compressor: in this way cooling capacities identical to those of the respectively larger model in the range designed for R22 and R407C are obtained and at more or less the same cost.


 


Efficiency of screw compressors optimised for R134a

A fundamental characteristic is the truly elevated isoentropic performance, a basic requirement for extremely limited power consumption.
In the above figure, the isoentropic efficiency of the new R134a screw compressors (installed, for example, on FOCS-CA 2652-3152, 4063-4073 and 5414-6314) is compared with the isoenthropic efficiency of a 25HP scroll compressor running on R407C, a model which is generally considered to be superior.
The comparison is made at various evaporation temperatures and at two different condensation temperatures.
In the typical field of application for liquid chillers (SST from 0°C to 5°C) the comparison gives the screw compressor a 5% to 7% advantage over the scroll compressor.
Bearing in mind the difference in the coefficient of performance (COP) of R134A compared with R407C (see fig.2), that is, from 2.5% to 4.5% higher, the advantages in favour of compact screw compressors running on R134A amount to about 10%.
We have not considered the often negative effect of the temperature glide of R407 in the condenser which can lead to higher condensation temperatures, thereby causing a further reduction in the COP.

 


Construction characteristics of the new screw compressors optimised for R134a

The new screw compressors optimised for R134a have an extremely compact structure.

Screw profile: two-rotor design (tooth ratio 5:6). The profiles have been developed using exclusive technology and feature particularly high rigidity and efficiency. On this basis, compared with a traditional screw compressor for R22 or R407C, the profile could be modified so that the increased displacement flow rate required about 10% more overall room for the rotors. It was therefore possible to make the modifications to the housing without affecting the external shape of the compressor.

Roller bearings: due to the favourable torque load and the reduced pressure difference, the same bearings already used in the versions for R22 and R407C now work in even easier conditions, thereby offering a longer lifetime.

Oil separator:
the compressors contain a built-in three-stage oil separator with relative oil management system. These separators, originally developed for the R407C and R22 models, provide extremely high levels of separation in spite of the increases in the displacement flow rate. This derives from the reduced vapour density and discharge temperature of R134a which contribute towards outstanding oil separation.

Electric motor: Given that the power draw is more or less the same, tendentially even lower, than the R134a version, practically the same motors as the R407C / R22 versions featuring the same capacity are used.
Some changes had to be made as regards cooling with suction gas and the higher displacement flow rate which, of course, did not substantially modify the overall dimensions.


Power adjustment with control slide:
the new screw compressors feature a particularly efficient power adjustment system which can be used in the infinite or the step modes, depending on the control logic implemented (and relative switching of the electrovalves shown in fig. 4).
It is generally known and documented by analysis that the dynamic behaviour and consequent efficiency of a cooling system, that is, of a liquid chiller, are mainly determined by the quality of the power adjustment system.
In this regard, the screw compressor, being able to precisely adjust power according to the real load, is superior to any type of adjustment system based on switching individual compressors on and off.
The need to adjust power has led to the development of a control slide contoured to the shape of the profiles, arranged directly between the main and the secondary rotor.
The control slide is hydraulically operated in a sideways direction in order to modify suction volume and therefore adjust power (fig. 4).

Fig. 4 - Power adjustment with a control slide fitted with an integrated economiser channel. Valves CR1…CR4 offer a choice between infinite or 4-step control (100-75-50-25%).

The control slide features unique construction features:

a contribution to high efficiency and operational safety has been made by integrating a part of the discharge channel into the control slide (Fig. 4, see “Contour for Vi adaptation“) which enables a Vi* control at part load conditions. As a result, the internal volume ratio (Vi) practically remains constant down to approximately 70% part load. At even lower load conditions, the Vi is further reduced according to the expected lower system compression ratio.
The economiser channel built into the control slide is another outstanding feature (Fig. 4, see “Sliding ECO-Port“); it enables the operation of the subcooler circuit regardless of the load conditions of the compressor, thereby assuring the highest cooling power at both full load and partial load conditions.

Another feature of the adjustment system is the possibility of automatic no-load starting. This considerably reduces starting torque and, consequently, starting times. The loads on the power supply are reduced as is the stress on the motor and on the mechanical components of the compressor.

* In the screw compressor, the compressed gas is expelled though a discharge port whose size and geometry determine the so-called “internal volume ratio Vi”. This value must have a defined relationship with the mass flow and with the working pressure ratio in order to prevent large losses of efficiency due to over- or under-compression.

 


Additional construction characteristics of the compressor

In the standard version, the compressors are fitted with a sophisticated electronic integrated protection device which:

-controls the temperature of the windings by means of PTC resistors inserted in the motor windings, with consequent interruption of the control current in case of overheating;
-controls the temperate of the oil by means of PTC resistors inserted in the oil sump, with consequent interruption of the control current in case of overheating;
-controls the sense of rotation / phase sequence with immediate interruption of the control current and shut-down if the sense of rotation / phase sequence is incorrect

The standard configuration of the compressor also includes:

-fine mesh (25mm) intake filter for protecting the compressor from damage caused by dirt in the system;
-long-life oil filter with 10 mm mesh;
-oil level sight glass;
-oil heater, fitted in a trap to allow it to be replaced without interfering with the cooling circuit;
-oil intake/drain valve;
-oil level safety switch;
-differential pressure vent;
-check tap and valve on the delivery line. The latter, especially, prevents dangerous reverse flows of gas and consequent triggering of compressor counter-rotation.


Climaveneta S.p.A