US2620968A

US2620968A - Machine of the screw-compressor type

Info

Publication number: US2620968A
Application number: US707084A
Authority: US
Inventors: Nilsson Hans Robert
Original assignee: JARVIS C MARBLE; LESLIE M MERRILL; PERCY H BATTEN
Current assignee: JARVIS C MARBLE; LESLIE M MERRILL; PERCY H BATTEN
Priority date: 1945-11-03
Filing date: 1946-11-01
Publication date: 1952-12-09
Anticipated expiration: 1969-12-09

Description

Dec. 9, 11952 H. R. NILSSON 2,620,953

MACHINE OF THE SCREW-COMPRESSOR TYPE Filed Nov. 1, 1946 8 Sheets-Sheet l Dec. 9, 1952 H. R. NILSSON 2,620,958

MACHINE OF THE SCREW-COMPRESSOR TYPE Filed Nov. 1, 1946 8 Sheets-Sheet 2 II II ec. 9, 11952 H. R. NILSSON 2,620,968

MACHINE OF THE SCREW-COMPRESSOR TYPE Filed Nov. 1, 1946 8 Sheets-Sheet 5 Dec. 9, H952 H. R. NILSSON 2,520,968

MACHINE OF THE SCREW-COMPRESSOR TYPE Filed Nov. 1, 1946 8 Sheets-Sheet 4 Dec. 9, 1952 H. R. NILSSON 2,620,968

MACHINE OF THE SCREW-COMPRESSOR TYPE Filed NOV. 1, 1946 8 Sheets-Sheet 5 Patented Dec. 9, 1952 UNITED STATES PATENT OFFICE MACHINE OF THE SCREW-COMPRESSOR TYPE Wis., trustees Application November 1, 1946, Serial No. 707,084 In Sweden November 3, 1945 Claims.

The present invention relates to rotary ma chines of the helical screw type in which two or more intermeshing rotors provided with helical lands and grooves and working between end walls of a co-operating casing, form working chambers of variable volume as the rotors revolve.

In such machines, there are formed in the beginning of each suction period as well as at the end of each compression period trapped spaces between the cooperating rotor lands and the inlet and outlet end wall, respectively. At the inlet these trapped spaces increase from a volume practically equal to zero to appreciable value, before the communication with the inlet is opened. This increase in volume thus corresponds to a certain work done in evacuation, which involves pure loss.

At the outlet the extent of the trapped spaces will decrease during the rotation to substantially zero volume, which theoretically would correspond to a compression to an infinitely high pressure and render operation of the machine impossible. That this is not the case in practice, is due to the fact that there is always a certain clearance between the rotors, through which the trapped medium, when the pressure rises, leaks to an adjacent space having a lower pressure. Thus, also the work done in compression in the trapped spaces at the outlet end brings about a pure loss of power, besides which there comes in as well a shock effect which causes irregular operation of the machine. Moreover, due to the trapped spaces at the outlet end machines of this type could not be employed as liquid pumps, since the pressure in these spaces, if they were filled with liquid, would rise to impractically high values.

The general object of the present invention is to provide an improved form of construction which shall eliminate the deficiencies arising from the creation of the trapped spaces or pockets and which shall also turn such spaces to useful account by the provision of arrangements which will permit compressed elastic fluid to be discharged simultaneously at different pressures from the same machine.

The invention will be more fully described with reference to the accompanying drawings, where- Figures 1 and 2 are sectional views; through a screw compressor of a type previously known having its rotors in a somewhat different position in relation to each other.

Figure 3 is a work diagram of a compressor of the type shown in Figures 1 and 2.

Figure 4 is a side elevational view partly in section showing one embodiment of a compressor according to the invention.

Figure 5 is a sectional view taken on the line VV of Figure 4, and Fig. 6 a longitudinal sectional view taken on the line VI-VI of Fig. 5 through one end of the compressor, as viewed from above.

Figures 7 and 8 are cross sectional and longitudinal sectional views, respectively showing, a special embodiment in which particular measures have been taken for enlarging the trapped spaces and discharging the medium compressed therein to a higher pressure through a special outlet, Figure 7 being a section taken on the line VII VII of Figure 8, and Figure 8 being a section taken on the line VIII-VIII of Figure 7.

Figure 9 is a view similar to Figure 4 of another embodiment. Figure 10 is a sectional view taken on the line X-X of Figure 9 and Figure 11 is a longitudinal sectional view taken on the line XIXI of Figure 10 through one end of the compressor, as viewed from above.

Figures 12 and 13 are views similar to Figs. 7 and 8 but showing a modified arrangement.

Figures 14-19 are fragmentary elevational views on a larger scale showing some suitable embodiments of the grooves at the end faces of the rotors as well as of the channel at the end wall of the casing.

All the figures show compressors having symmetrically formed threads or lands. In Figures e-S and 14-17 the channels l5 are provided in the ends of male rotors, whereas the channels in Figures 9-13 and 18-19 are provided in the end faces of female rotors.

In a compressor of the design shown in Figures 1 and 2 a pair of lands commences the creation. of a Vacuum and flow of the working medium in that position of the rotors which is designated in Figure 1. After a certain movement of the rotors into the position shown in Figure 2 there has been formed a space 2, which, so to speak, constitutes the beginning of the suction period. This volume is confined by two rotor lands and a surface S which at the same time forms the bottom of the space between a compressing pair of lands and, therefore, must be covered by the end wall plate according to the contour l, see Figure 2, the dotted contour.

In view of the fact that the end wall covers the end plane of the rotors, there is no communication between the enclosed volume 2 and the inlet, before the rotors have performed a certain turning movement with simultaneous enlargement of the volume 2. Thi increase of volume Without admission of fresh medium to the enclosed volume must, of course, involve a. work of evacuation and thus a loss of work in, for instance, a compressor. When finally the communication between the enclosed volume 2 and the inlet is opened, fresh medium will, on account of the existing vacuum, enter at a relatively high velocity.

At the outlet a corresponding process will take place. Thus between the rotor chambers the lands which coact in the position shown in Figure 1, enclosed spaces I are formed, the volume of which during the continued turning movement of the rotors will decrease with continued compression of the medium enclosed in said spaces as a result.

The operating process in a compressor of the type described above is illustrated also by the work diagram shown in Figure 3. The hatched surfaces correspond to the loss of power which takes place because of the closed spaces at the inlet and outlet and which the present invention in intended to overcome.

The embodiment of the invention shown in Figures 4 to 6 and 9 to 11 is constituted by a compressor having two rotors II], II, engaging one another, the limiting surfaces of the lands of the rotor I being substantially concave and those of the lands of the rotor II being substantially convex. The rotors I0, II are arranged in a compressor casing I2 having end walls I3 and I4 at the inlet end and the outlet end, respectively. Provided in the end walls of the rotor lands having convex limiting surfaces as in Figure 4 or concave limiting surfaces as in Figure 9 are grooves or channels I5 which, when a closed chamber is formed between the rotors and one of the end walls I3, I4, place this chamber in open communication with a channel or port I6 in the appertaining end wall, which channel in turn communicates with the inlet I! and the outlet I8, respectively. In the drawings, the channels I5 and I6 are only shown at the outlet end.

Since, as described above, at the outlet end there is a certain portion of the compressed working medium under a higher pressure, the invention also contemplates such control of the outlet that this highly compressed medium may be discharged separately instead of being simply supplied to the main portion of the medium which is discharged away at a lower pressure. Such an arrangement is shown in Figures '7 and 8, 12 and 13, according to which the channels I5 in the end walls of the rotors connect the enclosed spaces with an auxiliary outlet passage terminating in an auxiliary port in the end wall separated from the main outlet port.

When that portion of the medium which has been compressed to a higher pressure, is discharged separately in order to be used for special purposes, it may sometimes also be desirable that this portion form a higher percentage of the total amount than may be obtained in connection with the use of the standard type of compressor. To this end, it is possible to insert at the outlet a partition or the like (Figures 7 and 8; 12 and 13), against which the lands of the rotors I0 and II seal at a previous stage of the exhaust, so that the enclosed chamber is appreciably enlarged and, consequently, receives a higher peroentage of the total amount of the compressed medium discharged.

Devices analogous to those above described in connection with the outlet end may also be arranged at the inlet, so that the work of evacuation produced at this location may be utilized for special purposes. By this means, the entire machine becomes symmetrical and may thus operate in either direction.

It is also important that the channels I5 be of the right dimensions. If they are too small, an unreasonably high pressure difference is required between the enclosed spaces and the outlet. If, on the contrary, they are too large, the loss caused in by-passing the residual medium remaining in the channel I5 through locks to the inlet becomes excessive.

The channels I5 and I6 should be located in such a manner that communication between them is interrupted when the enclosed volume is zero or immediately before that time. Otherwise, communication may be established between inlet and outlet. Furthermore, the channel I5 should be made as short as possible, thus reducing the quantity of trapped fluid in the channel itself. Figures 14-19 show how the channel length may be varied.

Below there is shown an example of the increase in efficiency which may be obtained by inserting these grooves or channels.

As an example there has been chosen a compressor for a compression ratio of 1:1.5 and a diameter of the screw of 180 millimetres, length 312 millimetres, 3 lands on each rotor and an embracing angle of the lands of In the calculation of leakage there is assumed to be a play or clearance of 0.1 millimetre.

Number of revolutions per minute=5000.

Per cent Without channels the increase of pressure in relation to the total work is 3.9 With channels the increase of pressure in relation to the total work is 0.2

Increase in efliciency 3.7 The leakage from outlet to inlet is smaller with channels than without channels.

The volumetric efiiciency is 0.36 From this is deducted for loss caused by passing through locks 0.2

Increase in efficiency 0.16

The total increase in eficiency at the outlet end 3.86

To this is to be added the increase in efliciency at the inlet end.

In reversible compressors the channels function alternately for inlet or outlet depending on the direction of rotation.

Of course, the invention is not limited to the embodiments described above and shown in the drawings but may in its details be varied in many ways without on that account going outside the scope of the invention.

I claim:

1. In a rotary device of the character described, a casing having an end wall, a pair or rotors rotatably mounted in said casing and having interineshing helical lands and grooves cooperating with each other and the casing to form working chambers defined in part by said end wall and additional trapped spaces defined in part by said end wall, said trapped spaces varying in volume to or from substantially zero volume as the rotors revolve, a main outlet port in said casing for venting said working chambers and means for venting said trapped spaces as the volumes thereof decrease upon rotation of the rotors comprising an auxiliary outlet port in said end wall separate from said main port and grooves in the ends adjacent to said end wall of the lands of at least one of said rotors located to provide communication between said trapped spaces and said auxiliary port.

2. In a rotary device of the character described, a casing having an end wall, a pair or rotors rotatably mounted in said casing and having intermeshing helical lands and grooves cooperating with each other and the casing to form working chambers defined in part by said end wall and additional trapped spaces defined in part by said end wall, said trapped spaces varying in volume to or from substantially zero volume as the rotors revolve, a main outlet port in said casing for venting said working chambers and means for venting said trapped spaces as the volumes thereof decrease upon rotation of the rotors comprising an auxiliary outlet port in said end wall separate from said main port, said main port comprising at least a portion located in said end wall in spaced relation to said auxiliary port, and grooves in the ends adjacent to said end wall of the lands of at least one of said rotors located to provide communication between said trapped spaces and said auxiliary port.

3. In a rotary device of the character described, a casing having an end wall, a pair or rotors rotatably mounted in said casing and having intermeshing helical lands and grooves cooperating with each other and the casing to form working chambers defined in part by said end wall and additional trapped spaces defined in part by said end wall, said trapped spaces varying in volume to or from substantially zero volume as the rotors revolve, a main outlet port in said casing for venting said working chambers and means for venting said trapped spaces as the volumes thereof decrease upon rotation of the rotors comprising an auxiliary outlet port in said end wall separate from said main port, a projection carried by said casing extending axially inwardly from said end wall and cooperating with the rotor lands to increase the maximum volumes of the trapped spaces communicating with said auxiliary port, and grooves in the ends adjacent to said end wall of the lands of at least one of said rotors located to provide communication between said trapped spaces and said auxiliary port.

4. In a rotary device of the character described. a casing having an end wall, a pair or rotors rotatably mounted in said casing and having intermeshing helical lands and grooves, the lands of one of said rotors being of generally convex profile and the lands of the other of said rotors being of generally concave profile and said lands and grooves cooperating with each other and the casing to form working chambers defined in part by said end wall and additional trapped spaces defined in part by said end wall, said trapped spaces varying in volume to or from substantially zero volume as the'rotors revolve, a main outlet port in said casing for venting said working chambers and means for venting said trapped spaces as the volumes thereof decrease upon ro tation of the rotors comprising an auxiliary outlet port in said end wall separate from said main port and grooves in the ends adjacent to said end wall of said lands of geneally convex profile.

5. In a rotary device of the character described, a casing having an end wall, a pair of rotors rotatably mounted in said casing and having intermeshing helical lands and grooves, the lands of one of said rotors being of generally convex profile and the lands of the other of said rotors being of generally concave profile and said lands and grooves cooperating with each other and the casing to form working chambers defined in part by said end wall and additional trapped spaces defined in part by said end wall, said trapped spaces varying in volume to or from substantially zero volume as the rotors revolve, a main outlet port in said casing for venting said working chambers and means for venting said trapped spaces as the volumes thereof decrease upon rotation of the rotors comprising an auxiliary outlet port in said end wall separate from said main port and grooves in the ends adjacent to said end wall of said lands of generally concave profile.

HANS ROBERT NILSSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS