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The advantage of screw compressors for R134a The new generation of screw compressors optimised for R134a Performance of screw compressors optimised for R134a Construction characteristics of the new screw compressors optimised for R134a Additional construction characteristics of the compressor
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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. As regards the theoretical
coefficient of performance (COP), R134a certainly comes out very favourably
compared with the R407C and R410A refrigerants (fig. 1). 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.
Efficiency of screw compressors optimised for R134a A fundamental characteristic is the truly
elevated isoentropic performance, a basic requirement for extremely limited
power consumption.
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. 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. 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. 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; 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; Climaveneta
S.p.A
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