[0001] The present invention relates to a two-stage rotary compressor for refrigerating
plants, of the type comprising a vane rotor, by means of which it is possible to obtain
high volumetric efficiency and substantial savings in energy while still producing
the same number of frigories.
[0002] In small or medium-capacity refrigerating plants single rotary vane-type compressors
are often used, in order to take advantage of the intrinsic features of this type
of machine. In order to obtain low operating temperatures for a refrigerating plant,
in general it is necessary to have high compression ratios, which are difficult to
obtain using conventional rotary compressors other than those with notably low volumetric
efficiency; it has also been proposed to use in tandem two identical compressors,
each provided with its own control motor and own lubricating circuit, without however
achieving a satisfactory or economic solution to the problem.
[0003] The object of the invention is to provide a two-stage compressor in the form of a
single rotary machine, comprising two functionally and structurally integrated compression
units which use a single control motor and a single pumping means for circulating
the lubricating oil and whose compression ratios are notably reduced according to
the geometric mean between the minimum pressure and maximum pressure of the refrigeration
cycle.
[0004] The above can be achieved by means of a two-stage rotary compressor for refrigerating
plants according to the present invention comprising a first low-pressure vane compression
unit and a second high-pressure vane compression unit, in which the rotors are connected
to a single control motor and in which the lubricating oil circuit comprises an oil
collection chamber positioned between the two compression units, said oil chamber
being maintained at an intermediate pressure and being connected respectively to the
delivery side of the low-pressure compression unit via a first oil separator and to
a feeding pumping means which supplies the lubricating oil to the high-pressure compression
unit, there being provided moreover a connection for supplying the lubricating oil
from the delivery side of the high-pressure compression unit to the low-pressure unit,
comprising an oil separator.
[0005] The two-stage rotary compressor for refrigerating plants according to the present
invention will be illustrated in greater detail hereinbelow, with reference to the
accompanying drawing which shows in schematic form the entire refrigerating plant
with the two-stage compressor according to the invention.
[0006] 10 denotes in its entirety a two-stage rotary compressor of the radial vane type,
comprising a first compression unit 11, hereinbelow also called low-pressure compression
unit BP, and a second compression unit 12, hereinbelow also called high-pressure compression
unit AP, the rotors 13 and 14 of which are mechanically connected together by means
of an intermediate coupling 15 so as to be rotated by a single electric motor 16.
Since the power consumption of the compressor is a direct function of the two compression
ratios, in order to minimise this power, the intermediate pressure of the two-stage
cycle, corresponding to the delivery pressure of the low-pressure compression unit,
is designed so as to be equivalent to the geometric mean between the minimum pressure
of the cycle at the intake side of the low-pressure unit, and the maximum pressure
of the cycle at the delivery side of the high-pressure compressor.
[0007] Therefore, a same compression ratio is obtained for both compressor units.
[0008] 17 denotes moreover a single pumping device for circulating the lubricating oil inside
the two compressor units 11 and 12, making suitable use of the delivery pressures
of both the compression units 11 and 12.
[0009] The two compression units 11 and 12, in a similar manner to conventional vane compressors,
thus comprise a chamber inside which a vane rotor 13, 14 rotates eccentrically, a
refrigerant such as freon or the like being continuously sucked in and compressed
by the two compression units 11, 12, along with the lubricating oil, then separated
and made to circulate inside a refrigerating plant as shown. This plant substantially
comprises a condenser 18 connected to the delivery side of the high-pressure compression
unit 12 via a high-pressure separator 19. From the condenser 18, the refrigerant before
being supplied to the low-pressure evaporator 20 is subcooled in a heat exchanger
21 operating at an intermediate pressure between the minimum pressure of the cycle,
corresponding to the intake pressure of the low-pressure unit 13, and the maximum
pressure, corresponding to the delivery pressure of the high-pressure unit 14. Therefore
the refrigerant leaving the evaporator 20 is returned to the intake side of the low-pressure
compression unit 11, and from here to the intake side of the high-pressure compression
unit 12, via an oil separator 22, having an intermediate pressure, and a mixer 23
forming part of a circuit for subcooling the refrigerant. This circuit comprises a
heat exchanger 21 whose secondary circuit is supplied with the same refrigerant branched
from the line 24, so as to be supplied to the exchanger 21 via a lamination valve
25; 26 denotes a second branched lamination valve for humidification of the mixer
23.
[0010] According to the present invention, lubrication of the two compression units 11 and
12 of the two-stage compressor 10, unlike conventional compressors, is obtained by
means of an integrated circuit which makes use of the intermediate pressure of the
cycle at the delivery side of the low-pressure compression unit. For this purpose,
between the low-pressure compression unit 11 and the high-pressure compression unit
12, there is provided a chamber 27 for collecting the lubricating oil 28, inside which
the intermediate pressure of the cycle is maintained. This chamber 27 for the lubricating
oil forms a functionally and structurally integral part of the two-stage compressor
according to the invention. The chamber 27 is therefore connected to the delivery
side of the low-pressure unit 11 via the conduit 22', the oil separator 22 and conduit
22''; at the same time, the bottom of the chamber 27 is connected to the oil circulating
pump 17 which supplies the oil to the delivery side of the high-pressure compression
unit 12. Finally, the low-pressure compression unit 11 is further supplied via the
conduit 17'' with lubricating oil from the high-pressure oil separator 19 via the
lines 19' and 19''. In this way, the various differences in pressure of the cycle,
and in particular the intermediate pressure inside the chamber 27, are used to supply
the lubricating oil to the intake side of the feeding pump 17. It is therefore possible
to use normal commercially available pumping devices even if they have a relatively
low pressure differential insufficient for compressors with high-compression ratios,
thus reducing considerably the costs of the compressor.
[0011] As previously mentioned, the intermediate-pressure oil chamber 27 is located between
the low-pressure unit 11 and the high-pressure unit 12, defining a chamber for collecting
the lubricating oil. Therefore, advantageously this oil chamber 27 surrounds the coupling
15 connecting the shafts of the rotors, thus making it possible to achieve a two-stage
compressor with an integrated structure, which occupies a limited amount of space
and is able to use conventionally designed compressors.
[0012] According to a further characteristic feature, in order to simplify further the problems
associated with the seals, the ends of the two compression units 11 and 12 opposite
to the intake ends of the compressor are directed towards the intermediate-pressure
oil chamber 27. Therefore, on this side of the compression units, it is not necessary
to provide any special seal since any leaks or seepages of gas are prevented by the
said oil chamber 27.
[0013] From the above description and the illustration of the accompanying drawing it will
thus be understood that a two-stage rotary compressor had been provided, comprising
two blade compression units which are structurally and functionally integrated so
as to form a single machine with separate rotors, lubrication and cooling of which
is effected by making suitable use of the different pressures of the cycle, thus resulting
in a simplified compressor design and reduced operating costs.
1. Two-stage rotary compressor for refrigerating plants, characterized by comprising
a first low-pressure compression unit (11) with a vane-type rotor, and a second high-pressure
compression unit (12) with a vane-type rotor, the rotors (13, 14) of said compression
units (11,12) being connected together and to a common control motor (16), and in
that the lubricating-oil circuit of the compressor (10) comprises an oil chamber (27)
which is maintained at an intermediate pressure and is positioned between said low-pressure
compression unit (11) and said high-pressure compression unit (12).
2. Two-stage compressor according to Claim 1, characterized in that the oil chamber (27)
is connected respectively to the delivery side of the low-pressure compression unit
(11) via a feeding line (22',22'') comprising an oil separator (22) and to the oil
input of the high-pressure compression unit (12), via a line (17',17'') comprising
pumping means (17) and in that there is provided a connection (19',19'') supplying
the lubricating oil from the delivery side of the high-pressure compression unit (12)
to the oil input of the low-pressure compression unit (11).
3. Two-stage compressor according to the preceding claims, characterized in that said
oil chamber (27) surrounds the coupling (15) connecting the shafts of the rotors (13,14)
of the two low-pressure and high-pressure units (11,12).
4. Two-stage compressor according to Claim 1, characterized in that the ends of the low-pressure
compression unit (11) and high-pressure compression unit (12) which are opposite to
the refrigerant intake ends are protruding into said intermediate oil chamber (27).
