TECHNICAL FIELD

This invention relates to positive-displacement compres¬ sors, rotary, like e.g. he tooth and screw types as well as oscillating, like e.g.the scroll type and reciprocating like e.g. the piston type.

BACKGROUND ART

A compressor of this type would be the ordinary tooth com¬ pressor having two rotors with parallel shafts disclosed in for instance GB-A-l 321 485. This type of compressor is usually used for providing oilfree compressed air in the low capacity range. A typical characteristic of this type of machine resides in the fact that the absolute volume change as a function of angle of rotation is about constant which means that the relative change in the volume of the compressed air is increasing continuously during the com- presβion phase. This creates throttling losses especially at the beginning and the end of the discharge phase. The other types of compressors mentioned above, suffer from similar problems.

A BRIEF DESCRIPTION OF THE INVENTION

The general inventive idea will be described in detail be- low mainly with reference to one of the types of compres¬ sors according to the above, the tooth compressor .

one object of the present invention iβ to provide a solu¬ tion to said problem with the output losses. The Invention as applied to tooth compressors is characterised in that the angular speed of at least one of the rotors is arranged to be varied during the compression cycle which comprises the suction, compression and discharge phases, in this way it will be possible to control the relative volume change in time. This effect would be especially interesting to achieve at the discharge phase when the output port is open.

one obvious advantage with this solution would be that the throttellng losses at the output port could be signifi- cantly decreased. This also means that the compression work will be decreased. Additionally the torque on the driving shaft will be more uniform during the cycle which of course takes down the load on the coupling and bearingB and de¬ creases mechanical vibrations and the noise level.

Due to the fact that the pressure peaks in the compression chamber are decreased we also have the advantage of de¬ creased leakage losses.

Other characteristics and advantages of the invention will be apparent from the following description which will be accompanied by appropriate drawings.

BRIEF DESCRIPTION OF THE FIGURES

Figure l shows the principle of an ordinary tooth compres¬ sor

Figure 2 shows a diagram with several characteristics re¬ lating to an ordinary tooth compressor _.

Figure 3 shows the corresponding characteristics relating to a compressor according to the invention

Figure 4 shows the idea of the invention applied to a com¬ pressor according to Figure 1

Figure 5 shows the principle of a ultitooth compressor

DETAILED DESCRIPTION OF A FEW EMBODIMENTS

Figure 1 shows the general principle of an ordinary tooth compressor, six consecutive positions of the two rotors during one compression cycle are illustrated in the figures a-f. The female and male rotors 3 and 4 respectively, are arranged in two intersecting bores 1 and 2. The inlet and outlet ports 5 and 6 respectively, are both illustrated as axial openings in the rear end wall. Each rotor has a hub portion and, in this example, one tooth extending therefrom radially. The hub of the female rotor controls the opening of the outlet port 6.

Assuming atmospheric pressure at the inlet 5 in Figure la, we will have the same pressure in the entire interior of the compressor. The outlet port 6 is closed, in Figure lb the interior of the compressor has Just been divided into two chambers, a low pressure chamber 8 and a high pressure chamber 7, without mutual communication. In the interval between the rotor positions illustrated in the Figures lb and lc the volume of the high pressure chamber 7 has been gradually decreased while the outlet port is still closed. In Figure Id the output port 6 is partially open making it

possible for the compressed fluid to escape. The low pres¬ sure chamber θ is constantly at atmospheric pressure. At the end-phase of the compression cycle, illustrated in the figures le and If, the last pocket of the high pressure chamber 7, now confined between the two teeth of the ro¬ tors, is squeezed out through the gradually closing output port.

As indicated above this end-phase of the compression cycle is the critical part of the cycle when it comes to losses.

Figure 2 shows a diagram with a few characteristics relat¬ ing to an ordinary tooth compressor, cf. the above, having two synchronised rotors.

Curve b illustrates the variations of the absolute pressure in the high pressure chamber during one complete compres¬ sion cycle corresponding to one complete revolution of each rotor in the machine.

A typical curve showing the exposed outlet area from th high pressure chamber is designated c. When the output por opens and the exposed area is still small, it can be see from the curve b that the absolute pressure in the hig pressure chamber continues to build up and reaches a pea value PIP. As the outlet area continues to increase th high pressure fluid escapes more easily which causes th following pressure decrease and with some lag after th outlet area maximum we will have a minimum value in th pressure. The ideal would have been a minimum value equa to the constant back pressure, cf.curve f.

The area of the output port is then decreasing, cf. Figur le, while the relative volume change is still increasing as mentioned above. This fact creates a pressure peak P2 in the high pressure chamber as illustrated in curve b.

The said two peaks in the pressure cause the peaks P1TO and P2TO in the torque on the rotor shafts, curve d, as well as the peaks P1TE and P2TE in the gas temperature, curve e. The fact that the pressure, even at the moment of maximum opening of the output port, is higher than the back pres¬ sure shifts the other peaks to higher values which means that the average torque and pressure and corresponding losses are raised.

As mentioned above one of the objectives of the present in¬ vention is to decrease the dynamic losses of positive dis¬ placement compressors, here represented by a tooth compres¬ sor. With synchronised rotors, as in the present case, the relative volume change of the high pressure chamber as a function of angle of rotation would not be possible to vary. However, the same parameter as a function of time would be possible to control by varying the angular speed of the rotors with time, e.g. decreasing the speed at the end of the compression phase in each compression cycle. One of the effects of such a control would be that the opening time of the outlet port would be increased.

The most simple solution to the problem would be to drive the rotors in synchronism but with varying angular speed.

Figure 1 shows, however, that with the shape of the rotors of our example It would be possible to drive the two rotors at independent speeds during about half of the cycle from about the position illustrated in Figure lb to the one il- lustrated in Figure Id.

One of the greatest advantages of the invention resides in the fact that ordinary tooth compressors designed for syn¬ chronous operation could be converted according to the idea of the invention to decrease the dynamic losses.

How the control of the angular velocity could be achieved will be described more in detail below..

Figure 3 shows the same characteristics as figure 2 relat¬ ing to a compressor in which the velocity of the rotors is controlled according to the invention. As can be seen from the curve b, the first peak PIP in the absolute pressure is lowered as well as the following minimum value which is now close to the ideal value equal to the back pressure.

Due to the fact that the relative volume change with time has now been decreased, according to the idea of the inven¬ tion, especially at the end of the compression cycle, the peak pressure P2P Is now much lower. This fact causes the peak values as well as the mean values of the torque and gas temperature to decrease favorably as can be seen from the curves d and e.

An advantageous positive displacement compressor on which the idea of the invention is favorably applied is, as men¬ tioned above, the ordinary tooth compressor. This type of compressor is once again illustrated in Figure 4. The same designations as in Figure 1 has been used for corresponding parts. In order to illustrate a simple driving mechanism, according to the invention, part of the gear box with two elliptical gears has been shown schematically in the same Figure.

The initial positions of the rotors are the same as in Fig- ure la. If we assume that the male rotor, i.e the rotor 4, is driven clockwise with constant angular speed, e.g. di¬ rectly from a motor shaft, then the female rotor which con¬ trols the opening of the outlet port 6 will have its mini¬ mum angular velocity in the illustrated position which is about the position in which the last pocket of the high

pressure chamber 7 is squeezed out through the closing out¬ put port 6.

It is understood that different designs of the gears offer a very wide range of possibilities when it comes to the ve¬ locity control of the female rotor, in the illustrated ex¬ ample we will get another minimum velocity in the middle of the compression phase which is of less interest in an ap¬ plication like in Figure 4. For a double tooth compressor as the one Illustrated in Figure 5 this type o£ drive mech¬ anism would, however, be required.

A gearbox having two circular but excentric gears would give a mechanism having only one velocity minimum each com- pression cycle.

Thus, suitable drive mechanisms could be designed making use of non-circular, e.g elliptical, concentric and excen¬ tric gear wheels as well as circular excentric gear wheels.

In the example of Figure 4 the rotations of the two rotors are not synchronised. However, synchronous rotation of the rotors is also within the scope of the present invention. A gear box comprising a combination of gear wheels according to the above could easily be envisaged giving the two ro¬ tors synchronous but varying angular speed giving the same effect at the end of the discharge phase.