Suction arrangement in a reciprocating hermetic compressor

A hermetically sealed shell (21) contains a reciprocating hermetic compressor that has a suction inlet tube (28) for admitting gas into the shell; a suction orifice (24a) which is provided at the head of a cylinder (22) disposed inside the shell (21) and which is in fluid communication with the suction inlet tube (28). A suction duct (60) has a first end (61) hermetically coupled to the suction inlet tube (28) and a second end (62) hermetically coupled to the compressor suction orifice inlet (24a) and conducts low pressure gas from the suction inlet tube (28) directly to the suction orifice (24a) inside of shell (21), the suction duct (60) providing thermal and acoustic energy insulation to the gas being drawn into the compressor and is dimensioned to produce a load loss reduction in the gas flow from the suction inlet tube (28) to the suction orifice (24a).

FIELD OF THE INVENTION
 The present invention refers to a suction arrangement in a reciprocating
 hermetic compressor of the type provided with direct suction between the
 suction inlet tube and the suction chamber inside its shell.
 BACKGROUND OF THE INVENTION
 Reciprocating hermetic compressors are generally provided with suction
 acoustic dampening systems (acoustic filters), which are disposed inside
 the shell with the function to attenuate the noise generated during the
 suction of the refrigerant fluid. Such components, however, cause losses
 both in the refrigerating capacity and in the efficiency of the
 compressor, resulting from gas overheating and flow restriction. The
 manufacture of said filters from plastic materials have meant a
 significant advance regarding their optimization, although a considerable
 amount of the compressor losses is still due to this component.
 In reciprocating compressors, the movement of the piston and the use of
 suction and discharge valves, which open only during a fraction of the
 total cycle, produce a pulsing gas flow both in the suction and in the
 discharge lines. Such flow is one of the causes of noise, which may be
 transmitted to the environment in two forms: by the excitement of the
 ressonance frequencies of the inner cavity of the compressor, or of other
 component of the mechanical assembly, or by the excitement of the
 ressonance frequencies of the piping of the refrigerant system, i.e.,
 evaporator, condenser and connecting tubes of these components of the
 compressor refrigerating system. In the first case, the noise is
 transmitted to the shell, which irradiates it to the external environment.
 In order to attenuate the noise generated by the pulsing flow, acoustic
 dampening systems (acoustic filters) have been used. These systems may be
 classified as dissipative and reactive systems. The dissipative dampening
 systems absorb sound energy, but create an undesirable pressure loss. On
 the other hand, the reactive mufflers tend to reflect part of the sound
 energy, thereby reducing pressure loss. The dissipative mufflers are more
 used in discharge dampening systems, where the pulsation is high. The
 reactive systems are preferred for the suction, since they present less
 pressure loss. Said pressure loss in the acoustic filters is one of the
 causes that reduce the efficiency of the compressors, mainly in the
 suction case, which is more sensible to the pressure loss effects.
 Other cause that reduces the efficiency of the compressors, when usual
 acoustic mufflers are employed, is the overheating of the suctioned gas.
 During the time interval between the entrance of the gas to the compressor
 and its admission to the compressor cylinder, the gas temperature is
 increased, due to heat transfer from the several hot sources existing
 inside the compressor. The temperature increase causes an increase in the
 specific volume and consequently a reduction in the refrigerant mass flow.
 Since the refrigerating capacity of the compressor is directly
 proportional to the mass flow, reducing said flow results in efficiency
 loss.
 Reducing these negative effects has been achieved with the evolution in the
 acoustic filter designs.
 In prior constructions, the gas coming from the suction line and discharged
 into the shell passes through the main hot sources inside the compressor,
 before reaching the filter and being drawn towards the cylinder inside
 (indirect suction). This gas circulation should promote the cooling of the
 motor. Because of this and because the filters were usually metallic, the
 efficiency of the compressor was impaired due to gas overheating.
 The requirements for more efficient compressors have led to the development
 of acoustic dampening systems with more efficient conceptions. The gas,
 rather than passing through all heated parts inside the compressor, is
 drawn directly to the inside of the suction filter (U.S. Pat. No.
 1,591,239, U.S. Pat. No. 4,242,056). Other technique uses, in the suction
 piping inside the compressor, nozzles or flared tubes (U.S. Pat. No.
 4,486,153), which allow the flow to be directed between the inlet tube and
 the suction filter. Moreover, such filters began to be manufactured with
 plastic materials, which have adequate thermal insulating properties.
 These improvements brought about considerable increases in the efficiency
 of the refrigerating hermetic compressors. Nevertheless, overheating and
 load loss due to the use of the suction filter still represent significant
 amounts in the efficiency losses of the compressors.
 In the reciprocating hermetic compressors known in the art, the gas coming
 from the evaporator enters into the shell and then passes through the
 suction filter, wherefrom it is drawn to the inside of the cylinder
 defined in the cylinder block, where it is compressed up to a pressure
 sufficient to open the discharge valve. Upon being discharged, said gas
 passes through the discharge valve and discharge filter, leaving the
 compressor inside and leading towards the condenser of the refrigerating
 system. In this type of compressor, the discharge filter is always
 hermetic, i.e., the gas is not released into the shell inside, whereas the
 suction filter is in fluid communication with said shell inside.
 The fact that the compressor has low pressure inside the shell brings about
 two negative consequences regarding its efficiency. During great part of
 the compression cycle, the gas inside the cylinder is at a higher pressure
 than that of the gas inside the shell. This pressure difference generates
 a gas leakage from the cylinder towards the shell inside, through the gap
 existing between the piston and the cylinder. This gas is then admitted
 again in the cylinder through the suction filter, in function of the
 pressure balance occurring between the shell inside and the cylinder. Such
 gas is at a higher temperature than that of the gas returning to the
 evaporator, which causes a reduction in the pumped mass explained above.
 This reduction of the pumped mass causes loss of refrigerating capacity and
 of efficiency, as well (loss due to the leakage through the
 piston-cylinder gap).
 The pressure difference between the cylinder inside and the shell inside
 also creates a force at the piston top, which is transmitted, through the
 connecting rod, to the eccentric and bearings. The intensity of this force
 determines the dimensioning of the piston and bearings: the higher said
 force, the larger will be the dimensions of said parts and, consequently,
 the larger will be the dissipation of energy or viscous energy loss in the
 bearings.
 DISCLOSURE OF THE INVENTION
 Thus, it is an object of the present invention to provide a suction
 arrangement in a reciprocating hermetic compressor of the type including a
 hermetic shell comprising a suction inlet tube for admitting gas into the
 shell; a suction orifice, which is provided at the head of a cylinder
 disposed inside the shell and which is in fluid communication with the
 suction inlet tube, said arrangement comprising a suction means having a
 first end hermetically coupled to the suction inlet tube and a second end
 hermetically coupled to the suction orifice, in order to conduct low
 pressure gas from the suction inlet tube directly to the suction orifice,
 hermetically in relation to the shell inside, said suction means providing
 thermal and acoustic energy insulation to the gas being drawn.
 In this solution, the gas flow coming from the evaporator of the
 refrigerating systen is admitted, with no interruption, directly to the
 cylinder inside, before being compressed in the cylinder and discharged to
 the condenser through the discharge filter, which is always hermetic in
 relation to the shell inside.

BEST MODE OF CARRYING OUT THE INVENTION
 According to the illustrations, a refrigerating system of the type used in
 refrigerating appliances usually comprise, connected by adequate piping, a
 condenser 10, which receives high pressure gas at the high pressure side
 of a hermetic compressor 20 of the reciprocating type and which sends high
 pressure gas to a capillar tube 30, where the refrigerant fluid is
 expanded, communicating with an evaporator 40 which sends low pressure gas
 to a low pressure side of the hermetic compressor 20.
 According to FIG. 1 as shown, the hermetic compressor 20 comprises a
 hermetic shell 21, inside which is suspended through springs a
 motor-compressor unit including a cylinder block, which lodges inside a
 cylinder 22 a piston 23 that reciprocates within said cylinder 22, drawing
 and compressing the refrigerant gas when driven by the electric motor.
 Said cylinder 22 has an open end, which is closed by a valve plate 24
 affixed to said cylinder block and provided with suction and discharge
 orifices 24a, 24b. Said cylinder block further carries a head which is
 mounted onto said valve plate 24 and which defines internally therewith a
 suction chamber 25 and a discharge chamber 26, which are maintained in
 selective fluid communication with cylinder 22, through the respective
 suction and discharge orifices 24a, 24b. Said selective communication is
 defined by opening and closing said suction and discharge orifices by the
 respective suction and discharge valves 25a, 26a.
 By suction chamber it is meant only the volume of the cylinder head
 upstream the suction valve 25a.
 The communication between the high pressure side of the hermetic compressor
 20 and the condenser 10 occurs through a discharge tube 27 having an end,
 which is opened to an orifice provided on the surface of shell 21,
 communicating said discharge chamber 26 with condenser 10, and an opposite
 end, which is opened to the discharge chamber 26.
 Shell 21 further carries a suction inlet tube 28, mounted to an admission
 orifice which is provided at shell 21 and opened to the inside of the
 latter, communicating with a suction tube located externally to shell 21
 and coupled to the evaporator 40. In this construction, the gas coming
 from shell 21 is admitted inside a suction acoustic filter 50 mounted in
 front of the suction chamber 25, in order to dampen the noise of the gas
 being drawn into cylinder 22 during the opening of the suction valve 25a.
 This construction has the deficiencies discussed above.
 According to the present invention, as illustrated in FIGS. 3-5, between
 the evaporator 40 and the inside of suction chamber 25 of the hermetic
 compressor 20, there is mounted, interconnecting said parts, a suction
 means 60, which is provided within shell 21 and which comprises, at least
 on a portion of its length, a suction duct, in flexible material for
 instance, having a first end 61 coupled to the suction inlet tube 28 and a
 second end 62 coupled to a gas inlet portion of the suction chamber 25,
 said suction duct 60 being hermetically affixed to both suction inlet tube
 28 and suction chamber 25, so as to conduct, directly and hermetically,
 low pressure gas from the evaporator 40 to said suction chamber 25,
 providing thermal and acoustic energy insulation of the gas being drawn.
 In another constructive option of the present invention, the second end 62
 of the suction duct 60 communicates the gas being drawn directly to
 cylinder 22, for example with said second end 62 being hermetically and
 directly coupled to the suction orifice 24a.
 According to the present invention, the hermetic compressor 20 no longer
 has the suction acoustic filter 50 within shell 21. In a constructive
 option as illustrated in FIG. 4, the suction acoustic filter 50 is mounted
 upstream the suction inlet tube 28. Mounting the filter externally to
 shell 21 allows filters with higher volume and tubes with larger diameters
 to be used, while still providing the same acoustic dampening effect with
 less pressure loss. Since the refrigerating capacity is proportional to
 the suction pressure, the less said loss, the higher will be the
 compressor efficiency. This filter arrangement prevents the gas, while
 passing through the inside of said filter, from being unduly heated as it
 occurs in the prior art construction, although the noise levels generated
 by an assembly mounted as shown in FIG. 3 are very similar to those
 produced by the assemblies mounted according to the prior art.
 According to the present invention, the suction duct 60 is designed so as
 to be produced as a continuous tubular duct, which is constructed, in
 order to avoid interruption of the gas flow being drawn, in an adequate
 material which causes minimum noise and vibration transmission to shell 21
 and which further avoids gas overheating during the admission thereof. In
 order to have these qualities, the present suction duct 60 is obtained
 with a construction that offers high resistance to heat transmission, such
 as for example the constructions using a material with low thermal
 conductivity characteristic (poor thermal conductors) which also have good
 acoustic dampening characteristics.
 Since the gas which is drawn does not have any connection with the shell
 inside, it is impossible that said gas excites the ressonances inside the
 cavity.
 Since the pulsation in the suction is of low energy, there is no
 significant excitement of the external piping to the compressor.
 Though not illustrated, other constructions for the suction duct are
 possible, such as a duct formed by suction duct portions connected to each
 other in a sealing condition. In any one of the solutions, the suction
 conducting means should be located so as to operate with an extension of
 the suction piping, connecting the shell 21 to the evaporator 40, allowing
 a fluid communication, without interruption between the suction inlet tube
 28 and the cylinder 22 of the present compressor.
 The requirement of suction piping flexibility is due to the relative
 movement existing between the mechanical assembly and the shell 21, since
 the mounting between said parts is made through flexible springs. The
 flexibility will prevent said piping from being broken during the normal
 operation of the compressor or during transportation and handling.
 The suction duct 60 is further dimensioned in order to minimize the noise
 generated by the pulsing flow resulting from the excitement of both the
 suction line piping and the evaporator.
 Another characteristic of the dimensioning of the suction duct 60 is its
 larger diameter in relation to the diameter of the piping upstream the
 suction inlet tube 28. The diameter of the suction duct 60 is determined
 to cause a load loss reduction in the gas flow coming from the suction
 inlet tube 28 and, consequently is led to the suction chamber 25 or also
 directly to the suction orifice 24a.
 Due to the characteristics of the gas flow, smaller length and larger
 diameter of the suction duct 60, there will be less pressure loss in the
 filter, if used, in relation to the pressure loss existing in the suction
 filter of the art.
 Using the suction duct 60 causes a reduction of the path made by the gas
 inside the shell, previously to being admitted into the cylinder. By
 reducing the path, the overheating effect of the gas being drawn is
 smaller, which increases the refrigerating capacity and efficiency.
 In a constructive option of the present invention for the suction means 60,
 as illustrated in FIG. 5, said means is in the form of a loop tube, which
 is "U" shaped with rounded sides and internally provided with or
 incorporating (for example by material injection) at least one spring
 element 63 which constantly mantains said tube in a condition of
 structural stability, in order to prevent it from collapsing when
 submitted to pressure differences, such as during the compressor
 operation.
 Due to the suction tightness, the pressure inside shell 21 is higher than
 the suction pressure and results from the gas leakage through the gap
 existing between the piston 23 and the cylinder 22. This leakage increases
 the pressure inside the shell 21 to a pressure value intermediate between
 the suction and discharge pressures, usually close to a medium pressure
 value between the compression start pressure and compression end pressure.
 The pressure increase inside the shell allows the compressor to start each
 new operation, working with less load and therefore requiring a low torque
 from the motor during the operation thereof. During the suction and the
 compression start, the inside of shell 21 is at a pressure which is higher
 than that of the inside of cylinder 22, which makes the gas leak into the
 latter. From the moment in which the compression pressure in cylinder 22
 is higher than that inside the shell 21, which occurs till the end of the
 discharge, the gas leakage inverts its direction, traveling from the
 inside of cylinder 22 to the inside of the shell 21. Due to the
 characteristics of the phenomenum, the leakage towards the shell inside
 exceeds the other leakage direction, till reaching a medium balance
 pressure inside the shell 21. In this situation, the leakage is null, if
 integrated in time, which consequently causes a reduction in the losses
 due to leakage between the piston 23 and cylinder 22.
 With the solution of the present invention, since the pressure inside the
 shell 21 is intermediate between the compression start pressure and the
 compression end pressure, the pressure difference actuating over the head
 of the piston 23 is lower than that observed in the prior art compressors.
 Since the force transmitted to the bearings is smaller than that observed
 in the constructions of the prior art compressors, there is a condition of
 less loading for the operation of the bearings, which increases their
 reliability. Another advantage that comes from less force transmitted is
 the reduction of the mechanical losses caused by viscous attrition of the
 bearings. Another important advantage caused by the smaller difference
 over the piston is the lower deformation of the mechanism throughout the
 cycle. This lower deformation results in a reduction of dead volume and
 consequently higher refrigerating capacity, due to less wear reduction of
 the parts of this mechanism and cost reduction of the components, since
 their rigidity may be reduced to the same levels of the actual
 deformations, making possible to use less noble materials.