Discharge muffler

Abstract

Disclosed is a discharge muffler for a compressor constructed as an inner duct extending inside an outer duct, the inner duct being open at each end and the outer duct being open only at the muffler inlet. The inner duct has a plurality of holes communicating with the outer duct. At least two of the holes are at different distances from the muffler outlet.

Claims

1. A discharge muffler for mounting to take discharge from a compressor, the muffler having a muffler inlet for receiving the discharge and a muffler outlet for delivering discharge, wherein the muffler comprises: an inner duct having an axis and an outer duct having an axis, the inner duct: extending inside the outer duct, being open at each end, and being provided with at least first, second and third holes that communicate with the outer duct, the first and second holes being at different distances from the muffler outlet, and the outer duct: being open at a first end in a region of the muffler inlet, and having a closure at a second end in a region of the muffler outlet, the axis of the inner duct being offset from the axis of the outer duct so that a clearance between the inner duct and the outer duct is different when measured in different radial directions from the inner duct and the first and third holes in the inner duct are therefore at different distances from the outer duct.

2. The muffler according to claim 1, wherein the muffler is configured for use with the compressor and wherein the different distances from the muffler outlet of the first and second holes are determined in relation to wavelengths of discharge pulsations for the compressor in use at different respective speeds.

3. The muffler according to claim 2, wherein the first, second and third holes are at different distances from the muffler outlet, the distances being determined in relation to the wavelengths of discharge pulsations for the compressor in use at least three different respective speeds.

4. The muffler according to claim 2, wherein the different distances of the holes from the muffler outlet are each a quarter wavelength of a discharge pulsation for the compressor in use at different respective speeds.

5. The muffler according to claim 2, wherein the inner duct is provided with a fourth hole, and said first, second and fourth holes are at different distances from the muffler outlet, the distances being determined in relation to the wavelengths of discharge pulsations for the compressor in use at least three different respective speeds.

6. The muffler according to claim 1, wherein the muffler is configured for use with the compressor and wherein the compressor is a screw compressor.

7. The muffler according to claim 6, wherein the discharge is an oil/gas mixture.

8. The muffler according to claim 1, wherein the muffler inlet is provided by open ends of the inner and outer ducts and the muffler outlet is provided by an open end of the inner duct, at the closure of the outer duct.

9. The muffler according to claim 1, wherein the closure of the outer duct at its second end is provided by a transverse wall extending inwardly from the outer duct onto the inner duct.

10. The muffler according to claim 1, fitted between a compressor discharge and an integral oil separator.

11. The muffler according to claim 1, fitted directly to a compressor discharge outlet.

12. The muffler according to claim 1, wherein the first and third holes in the inner duct are at distances from the outer duct which are different by a factor of four.

13. The muffler according to claim 1, wherein a cross sectional area of the inner duct is substantially equal to a cross sectional area between the inner and outer ducts.

14. The muffler according to claim 1, wherein the muffler is configured for use with a compressor and wherein the inner duct is provided with an inlet restriction to reduce a pressure in the inner duct compared with that in the outer duct.

15. The muffler according to claim 1, dimensioned and configured for use with a single screw compressor operating over a range of rotational speeds from at least 50 to 75 Hz.

16. The muffler according to claim 15, wherein the single screw compressor operates at rotational speeds of up to at least 85 Hz.

17. Cooling equipment comprising a muffler according to claim 1, wherein the muffler is configured for use with the compressor and is mounted to take oil/gas discharge from the compressor for delivery to an oil/gas separator.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) A muffler according to an embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

(2) FIG. 1 shows in block diagram a known compressor/separator layout;

(3) FIG. 2 shows a known arrangement of screws in a single screw compressor having two gate rotors;

(4) FIG. 3 shows a tilted axial cross section, indicated by the arrows x-x in FIG. 4, through the centreline of the muffler when fitted between the discharge of a compressor and an integral oil separator;

(5) FIG. 4 shows a vertical transverse cross section of the muffler shown in FIG. 3, viewed in the direction of an outlet to the separator;

(6) FIG. 5 shows schematically the muffler position in a compressor with an integral separator;

(7) FIG. 6 shows schematically the muffler position when fitted on a compressor with a remote oil separator; and

(8) FIG. 7 shows a graph of test results for noise levels with and without the muffler fitted in relation to a compressor.

(9) The drawings are not drawn to scale.

DESCRIPTION OF VARIOUS AND PREFERRED EMBODIMENTS

(10) Referring to FIGS. 3 and 4, the muffler comprises primarily an outer, or surrounding, duct 5 and an inner duct 3. Both ducts 5, 3 are open at a first end to provide the muffler inlet 11, mounted into a discharge aperture in the wall 6 of a compressor. The muffler inlet, in use, receives an oil/gas mixture discharged from the compressor. At its second end, the inner duct 3 is extended, becoming the delivery duct for the gas/oil mixture from the compressor to a gas/oil separator (not shown in FIGS. 3 and 4). The outer duct 5 however is closed at its second end by a transverse end wall 4 which is sealed to the outer surface of the inner duct 3, thus providing a chamber about the inner duct 3. The muffler outlet 12 is thus provided through the inner duct 3 where the end wall 4 closes the outer duct 5.

(11) The discharged gas/oil mixture from the compressor enters both the inner duct 3 and the outer duct 5 at the same time. The frequency content of compressor noise is at least in part determined by the known phenomenon of discharge pulsations. The inner duct 3 has a plurality of holes 8a-8d in its wall and these are positioned to conform to wavelength of the discharge pulsations calculated over the required speed range and operating conditions of the compressor in use, in particular where the speed range is to be extended. The holes are positioned in pairs at distances, first and third holes 8a, 8c at A; second holes 8b at B, and fourth holes 8d at C from the end wall 4 of the outer duct 5 to the center of the respective hole. The cross section of FIG. 4 is taken through the holes at position C and viewed in the direction of the end wall 4 of the muffler.

(12) In addition, the axes 15, 16 of the inner and outer ducts 3, 5, respectively, are radially offset from each other such that at least two of the holes 8a-8d at any one of the positions A, B or C are at a different distance from the wall of the outer duct 5. Gas entering the outer duct 5 is reflected back from the end wall 4 and passes through the holes 8a-8d into the inner duct 3, continuing with the flow in this duct 3 to the muffler outlet and into the oil separator.

(13) The effect of the reflective flow is to introduce a flow into the main flow in the inner duct 3 which is at a wavelength out of time with pulsations in the main flow. This is a recognised noise reduction method which has the effect of damping the pulsations in the main flow. However, it is only achieved for pulsations at one critical frequency. In embodiments of the invention, the effect is extended to cover an increased speed range by the holes positioned at the three distances A, B and C from the end wall 4. These distances can be optimised for different frequencies within the extended speed range, operating together with the radially offset duct arrangement to provide improved noise reduction due to the additional reflection of the wave forms between the two duct sections and the effect on the main reflected wave forms.

(14) Referring to FIG. 3, in an example of the muffler, dimensions may be as follows: inner duct 3 nominal bore: 4 (outer diameter 114.3 mm) outer duct 5 nominal bore: 6 (outer diameter 168.3 mm) uppermost clearance distance marked 1: 32 mm lowermost clearance distance marked 2: 8 mm distance A from end wall 4 to closest holes 8: 70 mm distance B from end wall 4 to middle distance holes 8: 90 mm distance C from end wall 4 to furthest holes 8: 113 mm hole diameter: 25 mm

(15) An example of guidelines in designing a muffler as described above might be as follows:

(16) STEP 1: identify a critical frequency of the compressor discharge. For a single screw compressor, this is the number of rotor flutes times the rotational frequency.

(17) STEP 2: plot the wavelength of this critical frequency in refrigerant at operational condition against a required extended speed range.

(18) STEP 3: select three different wavelengths covering this extended speed range at minimum, maximum and mid speeds. This is achieved by determining the speed of sound in the refrigerant discharge at the temperature and pressure at operational condition and dividing this by the known discharge gas critical frequency. This is calculated for three frequencies over the extended speed range to cover the minimum, maximum and mid speeds of the selected extended range. These give the wavelength dimensions A, B and C in FIG. 3

(19) STEP 4: to establish the length of the muffler, the holes at distance C are positioned at a distance from the muffler inlet which is 0.6 the diameter of the outer duct 5. This dimension is not critical and may be varied to minimise pressure drop.

(20) STEP 5: the radial offset of the axes of the outer and inner ducts is such that the uppermost clearance distance marked 1 is four times the lowermost clearance distance marked 2.

(21) STEP 6: the area of the cross section of the outer duct 5 should be twice the area of the cross section of the inner duct 3, or to put it another way, the area 14 of the cross section between the ducts 3, 5 should equal the area 13 of the cross section of the inner duct 3.

(22) Referring to FIG. 3, in a further embodiment of the invention an additional inlet restriction 7 is added to reduce the pressure in the inner duct 3 compared with the outer duct 5 and thus increase the flow from the outer to inner duct 5, 3, further enhancing the muffler performance.

(23) Referring to FIG. 5, the position of the muffler in an integral compressor arrangement might be inside the separator 2 at the discharge from the compressor 1. Referring to FIG. 6, a typical position for the muffler is again at the discharge from the compressor 1 when the oil separator 2 is mounted separate to the compressor 1. In FIG. 6, it can be seen that the axes of the inner and outer ducts 3, 5 are vertical rather than horizontal. In each of these configurations it can be seen that the muffler can effectively provide the discharge outlet of the compressor.

(24) FIG. 7 shows a graph of test results showing noise levels measured for a compressor with and without a muffler fitted. Maximum noise reduction was achieved at maximum rotational speed of the compressor measured in Hz. Significant noise reduction can be seen.

(25) Several variations may be made in the design of a muffler according to the present invention. For example, the holes 8a-8d and the inner and outer ducts 3, 5 are not necessarily of circular cross section. The radial offset between the axes of the ducts 3, 5 is not necessarily in a vertical plane but might be in a horizontal or other plane or the direction of the offset might be reversed so that the ducts 3, 5 are at their closest point at the top rather than at the bottom of their cross sections. Indeed the muffler does not necessarily run horizontally but may be vertical or otherwise arranged. The outer duct 5 in particular may be replaced by a duct in another component rather than being provided in the form of a tube having a tube wall. The size of the holes in relation to each other or to other dimensions in the muffler may be varied, depending on the design priorities. For example, larger holes may be found to offer a lower pressure drop in the muffler while smaller holes may be found to offer better noise reduction. The inner duct may not have a simple, continuous tube construction but might comprise more than one component along its length, potentially offering for example additional muffling and/or pressure adapting characteristics.