Use of pentaerythritol ester as a lubricant in a compressor

Abstract: An energy efficient hermetic compressor for compressing hydrofluorocarbon refrigerants. The compressor includes a motor compressor unit (20) with a rotating oil pickup tube (30) to which an oil paddle (76) is attached. The oil paddle rotates in a base lubricant of polyol ester mixed with a siloxane ester foaming agent. The combination of the foaming additive with the oil paddle creates a foam layer that floats in the oil sump (74) to reduce the sound level of the compressor.

Description: The present invention relates generally to lubricants for hermetic compressors. An area of interest in the compressor art is how to construct a quieter compressor. One area that has received attention is that of noise generation and suppression within the compressor system. A majority of compressors include suction and discharge valves that open and close thereby creating noise. This noise is irritating and disruptive, particularly in residential applications of compressor systems. It is well known that if oil within the compressor is caused to froth or foam, there is a reduction in the sound transmitted from the compressor. Accordingly, an impeller, generally in the form of paddles, has been placed on the lubricant pickup tube for oil foam generation thus reducing noise of the unit. An example of this is shown in U.S. Patent 4,747,471. For many years the refrigerant of choice in hermetic refrigeration compressors has been dichlorodifluoromethane, CCl 2 F 2 , commonly referred to as Refrigerant R-12. Recently, the scientific community has come to recognize certain environmental hazards associated with the widespread use of chlorofluorocarbons (CFC's) such as R-12, and particularly in regard to its adverse effect on the ozone layer. In recent years, efforts have been made to find a substitute for R-12 that is environmentally acceptable, and that may be utilized in most refrigeration applications without requiring substantial modification of the refrigeration equipment. Tetrafluoroethane refrigerants, such as R134a (1,1,1,2 tetrafluoroethane), have been identified as acceptable substitutes for R-12 in a variety of applications, and widespread use of Refrigerant R134a is expected to increase in the future. The thermodynamic properties of R134a are close to those of R-12, and since R134a is free of chlorine, it is believed to be benign to stratospheric ozone. With the new refrigerant, new types of lubricating oils have been subject to testing. Currently, no one lubricant satisfies the needs of adequate lubrication of the compressor parts and sound reduction within the compressor. US-A-3 792 755 discloses using a siloxane based foaming agent in conjunction with a chlorofluorocarbon (= "CFC") based refrigerant or a hydrochlorofluorocarbon ( = "HCFC") based refrigerant. The combination of CFC's and siloxane does not require the use of an activating agent such as pentaerythritol, to facilitate the foaming effect. Accordingly, no such activant is disclosed or suggested therein. It is the objective of the invention to disclose a lubricant suitable for being used in a compressor, the said lubricant being able to reduce the noise otherwise produced by the compressor dramatically. This problem is solved by the features of claim 1. In the figures and in the following description a compressor is described in which a lubricant as described in claim 1 may be used. Fig. 1 is a longitudinal sectional view of a compressor; Fig. 2 is a enlarged elevational view of the oil pickup tube; Fig. 3 is a front view of the oil pickup tube; Fig. 4 is a top view of an oil paddle; Fig. 5 is elevational view of a test compressor showing the location of microphones; Fig. 6 is a graph of test results showing the sound reduction of the present invention; Fig. 7 is another graph of test results showing the sound reduction of the present invention. Referring now to Fig. 1, there is shown a compressor having a housing generally designated as 10. The housing has a top portion 12 and a lower portion 14 that are hermetically secured together as by welding or brazing. A foot or bracket 16 is welded to the bottom of housing 10 for mounting the compressor. Located inside the hermetically sealed housing 10 is a motor generally designated at 20 having a stator 22 and rotor 24. The stator 22 is provided with windings 26. Stator 22 is secured to the support frame or crankcase 38 by means of screws. The rotor 24 has a central aperture 28 provided therein into which is secured crankshaft 30 by an interference fit. A hermetic terminal 32 is provided on bottom portion 14 of the compressor for connecting motor 20, through cluster block 20a, to a source of electrical power. As shown in Fig. 1, a motor relay 34 is connected over terminal cluster 32 and a terminal shield 36 covers both terminal cluster 32 and motor relay 34. Within housing 10 is mounted a support frame or crankcase 38 resiliently suspended within housing 10 by suitable spring mounts such as compression springs 40 connected to crankcase 38 and bottom portion 14 of lower housing 14. Although only one spring is shown, it is to be understood that a number of springs are provided at proper positions to support crankcase 38 within lower housing 14. Crankcase 38 has a horizontally extending cylinder bore 42 which is sealed off at the end adjacent to housing 10 by cylinder head 44 including a suction valve 46, discharge valve 48, suction plenum 50 and discharge plenum 52. Connected to cylinder head 44 is a suction muffler 54. A suction return tube 55 permits refrigerant to enter compressor from a refrigeration system (not shown). Discharge plenum 52 is in communication with a discharge muffler, shockloop and a discharge tube (not shown) leading out of housing portion 14. From the center of crankcase 38 extending upwardly is a bearing hub 56 defining a vertical bearing bore 58. Bearing hub 56 has an end face 60 facing upwardly on bearing hub 56. Crankshaft 30 is journalled for rotation within vertical bearing bore 58. A ball thrust bearing device 62 extends radially from crankshaft 30 separated from end face 60 by a thrust washer 63 and, on the other end, from rotor 24 by another thrust washer 63. Bearing 62 rides upon thrust washers 63. A connecting rod 64 is attached to the end of crankshaft 30 that extends through bearing bore 58. Connecting rod 64 is attached to piston pin 66 that fits within cylinder bore 42, and causes piston 67 to reciprocate within cylinder bore 42 as crankshaft 30 rotates. The reciprocating compressor described herein provides a lubrication system for lubricating the components of the compressor including the crankshaft 30 and bearing 62. An offset oil pickup tube 68 is disposed within crankshaft 30 and is in communication with spiral groove 70 extending around the outer surface 72 of crankshaft 30. Oil pickup tube 68 has an opening 72 that is partially immersed in an oil sump 74. During operation, oil pickup tube rotates within the oil in sump 74 pulling lubricant up into tube 68 and urging it through groove 70. The compressor includes an oil paddle 76 attached to oil pickup tube 68. Paddle 76 is preferably a strip paddle made from a shaped piece of metal and attached to tube 68 by either welding or brazing. As shown in Figs. 2-4, paddle 76 includes a flat portion 78 that, when rotated with oil pickup tube 76 within the lubricant of sump 74, causes foam to be produced. Without oil paddle 76, the rotating oil pickup tube 68 does not cause foam. Paddle 76 includes an arcuate portion 80 with inner arcuate surface 81 and an outer surface 83, in the inner surface 81 shaped to conform to tube 68. Paddle 76 is brazed to tube 68 at an attachment point 82. As shown in Fig. 4, attachment point 82 may include an edge to help locate paddle 76 in a notch (not shown) on oil pickup tube 68. The invention provides a base lubricant with a controlled quantity of a foaming additive. The environmental hazards associated with the use of chlorofluorocarbons (CFC's) have been widely reported, and there has been an ongoing search for environmentally-acceptable substitutes. Of particular concern has been the search for an acceptable substitute for Refrigerant R-12 (dichlorodifluoromethane), widely used as a refrigerant in hermetic compressors. R134a, among others, has been identified as a promising substitute for R-12. The physical characteristics of R134a (1,1,1,2 tetrafluoroethane) enable it to be used in place of R-12 in refrigeration compressors, which substitution may be accomplished without requiring substantial changes to be made to the compressor. Mineral oils have been widely used as lubricants with Refrigerant R-12 in refrigeration compressors. These mineral oils provided sufficient lubrication to the moving parts of the compressor. In addition, the mineral oils were generally fully miscible with R-12 throughout the operating temperature range of the compressor. Mineral oils, however, are almost completely immiscible with R134a. No matter what proportion of oil and refrigerant one employs, the refrigerant and the oil remain as two distinct layers, regardless of temperature. This immiscibility increases the likelihood that the compressor will not be adequately lubricated if a mineral oil lubricant is utilized with R134a, and that the lubricant will not return to the compressor from the evaporator in the refrigeration loop, thereby resulting in the eventual premature failing of the compressor. Instead of mineral oil lubricants, polyol ester type lubricants along with other types of synthetic oils have been found to work with R134a in a compressor environment. The polyol esters of choice are the fatty acid esters of pentaerythritol. Although the particular fatty acids used may be straight-chain, branched or a mixture of these, polyol esters of branched chain fatty acids having between about five and about nine carbon atoms are preferred. If acids having more than about nine carbon atoms are used in predominant amounts, the viscosity of the lubricant becomes greater than desired. If the acids have less than about five carbon atoms in the chain in predominant amounts, the viscosity may not be sufficient to properly lubricate the moving parts of the compressor. The lubricant has a greater miscibility in the refrigerant when a branched chain fatty acid is used; however straight chain lubricants, and mixtures of straight and branched chain lubricants may generally also be employed. It is highly desirable, however, that the lubricant be fully miscible in the refrigerant throughout the operating temperature range of the compressor. Therefore, the use of a branched chain fatty acid is preferred. A particular preferred polyol ester is sold by the Henkel Corp, Emery Group, under the commercial designation of 2927A. Other readily available polyol esters may be substituted for 2927A, but it is preferred that the fatty acid component is within the general guideline of chain length range discussed above. It is preferred that the viscosity of the polyol ester be between about 29.5 centistokes and 36.0 centistokes at 100°F. Although commercial pentaerythritol esters often contain small amounts of dipentaerythritol esters, this small amount of the dipentaerythritol esters is normally acceptable in the lubricant composition. It is conventional to nonetheless refer to the esters as pentaerythritol esters, notwithstanding the presence of the dipentaerythritol esters. Other polyol esters which may be used as lubricants with R134a are based on trifunctional alcohols such as trimethylol propane (TMP), or even on difunctional alcohols such as neopentyl glycol. A particular siloxane ester favored by the inventor is sold by Dow Corning, under the commercial designation of silicone additive, DC 57. This particular ester in actually a copolymer containing the ingredients of dimethyl, methyl, and polyethylene oxide acetate capped siloxane copolymer. The oil foaming additive creates an oil foam while the compressor is operating but also serves as a wetting agent, increasing the lubricity of the lubricant and thereby reducing friction within the compressor during startup. The present oil and foaming agent is not compatible with R-12 and mineral oil compressor systems. In operation, compressor 10 begins functioning as electrical power is supplied to motor 20 causing piston 67 to reciprocate within bore 42. Motor 20 also rotates oil pickup tube 68 attached to crankshaft 30. Oil paddle 76 rotates with pickup tube 68 causing flat portion 78 to move through the lubricant in sump 74. This rapid rotating movement of paddle 76 causes the lubricant in combination with the siloxane ester additive to foam. Foam generation can be controlled by varying the speed at which the compressor is operated or by varying the quantity of the oil additive within the lubricant. Normally the operating speed of the compressor motor is not varied. It has been found that a quantity of siloxane foaming additive of between approximately .10% and 1.0% by volume relative to the lubricant creates an adequate supply of sound insulative foam for use in compressors. Concentrations of siloxane ester outside of the above limits tend to decrease the performance of the compressor. Low concentrations do not cause enough foam to be produced or increase the oils lubricity. High concentrations cause too much foaming, increasing the entrained lubricant in the R134a refrigerant. A foam blanket thickness of between 6,3 - 50,8 mm (.25 and 2 inches) is desired. A preferred quantity of siloxane ester is between approximately .25% and .50% by volume relative to the lubricant, which makes a superior thickness of foam to dampen sound levels within the compressor without negatively impacting compressor performance. As discussed below and as shown in Figs. 6 and 7, the single best concentration of siloxane foaming agent is approximately .50% by volume relative to the lubricant. This concentration, with the lubricant and oil paddle disclosed above, creates the largest sound reduction potential over a broad spectrum of sound frequencies. TEST RESULTS In order to objectively determine the overall sound reduction inside the compressor with the addition of the siloxane additive, five acoustic microphones (#2-#6) were mounted inside a compressor (Fig. 5). Microphone placement was determined from earlier testing showing the major acoustic resonant modes inside the housing. Note that #2 microphone is under the oil. Since the sound power levels of the four microphones above the oil were so similar, they were averaged to reduce the amount of date presented. In addition, 1/3 octave bands, A-weighted and linear levels were also measured. The insertion losses db (attenuation) from 50 Hz to 20 KHz are shown in Figs. 6 and 7. The attenuation was derived from the baseline readings taken without additive and the calculated differences among seven different additive concentrations. Figs. 6 and 7 plot this acoustic data reduction. The loudest tones from the microphones above the oil are at 400, 800-1250 and 4000 Hz. The foaming agent works to reduce the sound pressure levels at all frequencies, from 400 Hz through 20 KHz. Minimum concentration tested (.05%) does nearly as well as the maximum (5.0%) above 2500 Hz. Nominal concentration of 0.5% is as good as 5% for all frequencies except 1600 Hz. Above 2500 Hz, no clear pattern is apparent, with all concentrations producing up to 4db of attenuation. Below 2150 Hz, more specifically the 400-500 Hz and 1600-2500 Hz ranges, higher concentrations produce higher attenuation. Little is gained by increasing concentrations of additive from 0.5% to 1.0%. The loud frequencies measured by microphone #2 under the oil are 315-400 Hz, 800-1250 Hz, 1600-3150 Hz and 6000 Hz. The tones from 1600-3150 Hz and above 5KHz are the loudest recorded for any of the microphones. Higher concentrations of foaming agent lowers the level above 4 KHz, but sound levels were very unsteady. There was little effect below 1000 Hz except at the 400-500 Hz range. The sound pressure levels below the oil are much higher in the important frequency range above 2500 Hz. Sound transmission through the oil is probably the most important path for major noise problems. The foam layer, created by the foaming agent, causes overall noise reduction and reduced sound transmission when used in conjunction with the oil paddle, polyol ester oil and refrigerant. Without the foaming agent additive, the ester oil will not foam. The geometry of oil pickup tube 68 and oil paddle 76 do not create a harmful pressure gradient or cause sloshing sounds within the compressor. Because there are no oil sloshing sounds created, no oil baffle is needed within sump 74. During operation, the refrigerant vapor bubble concentration is not high enough to reduce the amount of oil flow to the bearing, therefore all bearings receive adequate lubrication.

Claims: The use of pentaerythritol ester as a base lubricant in a compressor for compressing a hydrofluorocarbon refrigerant, said compressor comprising: a hermetic housing (12, 14) containing a motor compression unit (20) including a rotatable shaft (30), said housing including an oil sump (74); a supply of a base lubricant of pentaerythritol ester located in said oil sump; a foaming agent of siloxane ester mixed with said base lubricant; and an oil paddle (76) attached to said shaft, said paddle rotatable in said lubricant, whereby an oil foam layer is created during compressor operation to reduce compressor noise transmitted through said housing. The use as per Claim 1 characterized in that said siloxane ester foaming agent is a mixture of dimethyl, methyl and polyethylene oxide acetate capped siloxane copolymer. The use as per Claim 1 characterized in that the volume concentration of said foaming agent relative said lubricant is 0.05% to 3.0%. The use as per Claim 1 characterized in that the proportion of said foaming agent relative to said lubricant is 0.25% to 0.50%. The use as per Claim 1 characterized in that said oil foam layer has a thickness of between 6,35 - 50,8 mm (.25 inches and 2.0 inches). The use as per Claim 1 characterized in that the volume concentration of said foaming agent relative to said lubricant is 0.10% to 1.0%. The use as per Claim 1 characterized in that the volume concentration of said foaming agent relative to said lubricant is approximately 0.50%. The use as per Claim 1 characterized in that said hydrofluorocarbon refrigerant is R-134a (1,1,1,2 Tetrafluoroethane). The use as per Claim 1 characterized in that said hydrofluorocarbon is one of R-32, R-125, R-134a or R-143a. The use as per Claim 1 characterized in that said hydrofluorocarbon is a blend consisting of two or more of R-32, R-125, R-134a or R-143a.

Inventors: SCHMITZ III RAYMOND J, SCHMITZ III, RAYMOND J.
Applicants: TECUMSEH PRODUCTS CO, TECUMSEH PRODUCTS COMPANY
Application year: 1993
Main classification: F04B39
IPC codes: F04B39, F25B1, C10M169, C10M171