Method for controlling a first reference temperature in a device for compressing gas

Abstract

A method for controlling a first reference temperature in a device (1) for compressing gas, the device (1) including an oil-injected element (2) for compressing the gas; an oil injection pipe network (6) for injecting oil into the oil-injected element (2), including: an apportioning means (8) for apportioning the oil into a first part and into a second part; an oil cooler (10) cooled by a fan (9), for cooling the first part; and a bypass (11) for diverting the second part past the oil cooler (10). An apportioning proportion of the first part is controlled to a required apportioning proportion, and subsequently a speed of the fan (9) is controlled to a required speed optionally on the basis of the apportioning proportion, wherein the apportioning proportion is controlled by a control unit (15) on the basis of a non-fuzzy logic algorithm.

Claims

1. A method for controlling a first reference temperature in a device (1) for compressing gas to a first desired temperature value, wherein the device (1) comprises: an oil-injected element (2) for suctioning the gas at an inlet (3) of the device (1) and compressing this gas to an operating pressure at an outlet (4) of the oil-injected element (2); an oil injection pipe network (6) with a discharge (7) for injecting oil into the oil-injected element (2), comprising: an apportioning means (8) for apportioning the oil into a first part and into a second part; an oil cooler (10) cooled by a fan (9), for cooling the first part; and a bypass (11) for diverting the second part past the oil cooler (10), wherein the method comprises: determining a required apportioning proportion of the first part for directing a second reference temperature in the device (1) to a second desired temperature value; controlling an apportioning proportion of the first part to the required apportioning proportion; determining a required speed of the fan (9) for directing the first reference temperature to the first desired temperature value, wherein, if the first reference temperature is the same as the second reference temperature, determining the required speed on the basis of the second desired temperature value and the apportioning proportion; and controlling a speed of the fan (9) to the required speed, wherein the apportioning proportion is controlled using a control unit (15) on the basis of a non-fuzzy logic algorithm with the following as input: a first current value al for the second reference temperature; and the second desired temperature value.

2. The method according to claim 1, wherein the second desired temperature value is determined on the basis of a highest temperature value in a group of one or more temperature values.

3. The method according to claim 2, wherein a first temperature value T1 in the said group is representative of a value Tcond of the second reference temperature at which a temperature of the compressed gas at the outlet (4) is equal to a first condensation temperature of the compressed gas at the outlet (4); or the first condensation temperature plus a first safety margin.

4. The method according to claim 3, wherein the first temperature value T1 is limited according to a first temperature interval between a first minimum temperature limit value Tmin,1 and a first maximum temperature limit value Tmax,1.

5. The method according to claim 2, wherein a second temperature value T2 in the said group is representative of a value of the second reference temperature at which a specific energy requirement of the device (1) is minimal.

6. The method according to claim 5, wherein the second temperature value T2 is determined on the basis of at least a second current value 2 representative of the operating pressure; and a third current value 3 representative of a temperature of the gas at the inlet (3).

7. The method according to claim 6, wherein, in the case that the oil-injected element (2) is driven by a variable-speed motor, the second temperature value T2 and/or the first speed value v1, respectively, is further determined on the basis of a tenth current value 10 and eleventh current value 11, respectively, which are representative of a rotational speed of the variable-speed motor.

8. The method according to claim 5, wherein the second temperature value T2 is limited according to a second temperature interval between a second minimum temperature limit value Tmin,2 and a second maximum temperature limit value Tmax,2.

9. The method according to claim 2, wherein the second reference temperature is controlled from an old temperature value to the second desired temperature value; and to determine the second desired temperature value, the said highest temperature value is limited according to a third temperature interval between, on the one hand, the old temperature value minus a maximum temperature decrease value Tmax,down and, on the other hand, the old temperature value plus a maximum temperature increase value Tmax,up.

10. The method according to claim 9, wherein the second reference temperature is controlled from the old temperature value to the second desired temperature value in a predefined time interval t, and that the maximum temperature decrease value Tmax,down and the maximum temperature increase value Tmax,up are positively dependent on a length of the predefined time interval t.

11. The method according to claim 2, wherein the required apportioning proportion is determined according to a first ratio 1 between the first current value 1 and the second desired temperature value.

12. The method according to claim 11, wherein the required apportioning proportion, between a minimum zero value and a maximum value of 100%, is dependent on the first ratio 1 according to a first monotonically increasing function.

13. The method in accordance with claim 11, wherein the required apportioning proportion has a maximum value of 100% when the first current value 1 is higher than the second desired temperature value or the second desired temperature value plus a second safety margin; or is higher during a first period than the second desired temperature value or the second desired temperature value plus the second safety margin; and has otherwise a minimum zero value.

14. The method according to claim 1, wherein the second reference temperature is a temperature of the gas at the outlet (4) of the oil-injected element (2); or is a temperature of the oil at the discharge (7) of the oil injection pipe network (6).

15. The method according to claim 1, wherein the required speed is determined on the basis of a highest speed value in a set of one or more speed values.

16. The method according to claims 15, wherein a first speed value v1 in the said set is representative of a value for the speed of the fan (9) required to achieve the second desired temperature value for the second reference temperature.

17. The method according to claim 16, wherein, when a fourth current value 4 for the second reference temperature is higher than a predefined minimum temperature; and a fifth current value 5 for the apportioning proportion is higher than a predefined minimum apportioning proportion and the fourth current value 4 is higher than the second desired temperature value, the first speed value v1 is determined on the basis of at least a sixth current value 6 representative of the operating pressure; and a seventh current value 7 representative of a temperature of the gas at the inlet (3).

18. The method according to claim 17, wherein, when the fourth current value 4 is higher than the second desired temperature value plus a first tolerance value; or during a second period, the fourth current value 4 is higher than the second desired temperature value plus the first tolerance value; or the fourth current value 4 is lower than the second desired temperature value minus a second tolerance value; or during a third period, the fourth current value 4 is lower than the second desired temperature value minus the second tolerance value, the first speed value v1 is further determined on the basis of at least the fifth current value 5 for the apportioning proportion; and a second ratio 2 between the fourth current value 4 and the second desired temperature value.

19. The method according to claim 18, wherein the first speed value v1 is dependent on the second ratio 2 according to a second monotonically increasing function.

20. The method according to claim 18, wherein the first speed value v1 is dependent on the fifth current value 5 according to a third monotonically increasing function.

21. The method according to claim 15, wherein, when the device (1) is provided with an aftercooler (25) for cooling the compressed gas downstream of the oil-injected element (2), when an eighth current value 8 for a lowest available temperature in the aftercooler (25) is higher than a value for a required lowest available temperature, a second speed value v2 in the set is determined on the basis of the first speed value v1; and a third ratio 3 between the eighth current value 8 and the value for the required lowest available temperature; and otherwise, the second speed value v2 is set equal to zero.

22. The method according to claim 21, wherein the required lowest available temperature is equal to a value for a second condensation temperature of the gas in the aftercooler (25) plus an offset.

23. The method according to claim 21, wherein the second speed value v2 is dependent on the third ratio 3 according to a fourth monotonically increasing function.

24. The method according to claim 15, wherein a third speed value v3 in the set is determined on the basis of a ninth current value 9 for the first reference temperature; and a predefined maximum value for the first reference temperature, wherein the third speed value v3 is equal to zero when the ninth current value 9 is lower than the predefined maximum value; and a value representative of a maximum speed of the fan (9) when the ninth current value 9 is higher than the predefined maximum value.

25. A computational control assembly comprising a first computational control unit (13) provided with a control unit (15) for controlling a second reference temperature in a device (1) for compressing gas to a second desired temperature value; and a second computational control unit (22) for controlling a first reference temperature in the device (1) to a first desired temperature value for performing a method according to claim 1.

26. A device for compressing gas provided with a computational control assembly according to claim 25.

Description

(1) To better illustrate the features of the invention, the following describes, by way of example without any restrictive character, a number of preferred embodiments of a method, computational control assembly and device according to the invention, with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a device provided with a computational control assembly according to the invention;

(3) FIG. 2 shows a schematic overall view of a method according to the invention;

(4) FIG. 1 shows a device 1 for compressing gas, which device 1 comprises an oil-injected element 2 for suctioning the gas at an inlet 3 of the device 1 and compressing this gas to an operating pressure at an outlet 4 of the oil-injected element 2.

(5) In the scope of the invention, the device 1 is to be interpreted as a complete compressor or vacuum pump installation including, inter alia, the oil-injected element 2 in the form of a compressor or vacuum pump element, respectively, all typical connecting pipes and valves, a possible housing of the device 1 and a motor 5 driving the oil-injected element 2.

(6) In the context of the present invention, the oil-injected element 2 is to be understood as an element housing in which the gas is compressed by means of a rotating rotor movement or by a reciprocating piston movement.

(7) In this respect, as a non-limiting example, the oil-injected element 2 may comprise one or more screw rotors, gear rotors, baffles, lobes or pistons.

(8) When the device 1 comprises a compressor element, the inlet 3 of the device 1 is typically fluidically connected to an atmospheric environment of the device 1. When the device 1 comprises a vacuum pump element, the inlet 3 is typically fluidically connected to a user network or an enclosed space at sub-atmospheric pressure.

(9) Further, the device 1 also comprises an oil injection pipe network 6 having a discharge 7 for injecting oil into the oil-injected element 2.

(10) In this regard, it is not precluded within the scope of the invention that the oil injection pipe network 6 comprises multiple discharges 7 for injecting oil into the oil-injected element 2.

(11) The compression of the gas in the oil-injected element 2 generates compression heat which heats up the gas. In order to keep a temperature of the compressed gas at the outlet 4 of the oil-injected element 2 below a certain maximum safety limit, a temperature of the injected oil should be below a maximum level corresponding to this safety limit. On the other hand, the temperature of the compressed gas at the outlet 4 must also not fall below a first condensation temperature of the gas at the outlet 4 or below the first condensation temperature plus a first safety margin in order to avoid formation of condensate at the outlet 4. Consequently, the temperature of the injected oil must be above a minimum level corresponding to this first condensation temperature or to this first condensation temperature plus the first safety margin. The temperature of the gas at the outlet 4 of the oil-injected element 2 and accordingly the temperature of the oil at the discharge 7 of the oil injection pipe network 6 should thus be controlled to a value within a temperature interval correspondingly limited at both ends.

(12) For this purpose, the oil injection pipe network 6 comprises an apportioning means 8 for apportioning the oil into a first part and into a second part, such as, for example, a thermostatic control valve; an oil cooler 10 cooled by a fan 9, for cooling the first part; and a bypass 11 for diverting the second part past the oil cooler 10.

(13) The fan 9 has a variable speed and is driven by means of a second motor 12. This makes it possible, for example, to control the cooling of the first part of the oil to be injected by adjusting the speed of the fan 9.

(14) More generally, in the present invention, the speed of the fan 9 is adjusted such that a first reference temperature in the device 1 is controlled to a first desired temperature value.

(15) The apportioning means 8 and the bypass 11 are provided for diverting a second part of the oil to be injected past the oil cooler 10, and thus limiting more or less the cooling by the oil cooler 10 of the oil to be injected by controlling an apportioning proportion of the first part of the oil. In this manner, a second reference temperature in the device 1 may be controlled to a second desired temperature value, wherein the second reference temperature is, for example, a temperature of the compressed gas at the outlet 4 of the oil-injected element 2 or a temperature of the oil at the discharge 7 of the oil injection pipe network 6. The first reference temperature, which is controlled by the fan 9, may be the same as the second reference temperature, wherein the first desired temperature value is thus also equal to the second desired temperature value.

(16) For controlling the apportioning proportion, the device 1 is provided with a first computational control unit 13. This first computational control unit 13 comprises a computational unit 14 for determining the second desired temperature value; and a control unit 15 for adjusting the apportioning proportion of the first part to the second desired temperature on the basis of a first current value for the second reference temperature.

(17) In this case, the control unit 15 is designed as, for example, a PID controller or an ON/OFF controller.

(18) In this case, the first current value for the second reference temperature is provided by measurement using a temperature sensor, for example a first temperature sensor 16 at the outlet 4 of the oil-injected element 2 or a second temperature sensor 17 at the discharge 7 of the oil injection pipe network 6.

(19) The second desired temperature value is determined by the computational unit 14 on the basis of at least: a second current value representative of the operating pressure, which second current value is provided, for example, by measurement using a first pressure sensor 18 at the outlet 4 of the oil-injected element 2; and a third current value representative of a temperature of the gas at the inlet 3, which third current value is provided, for example, by measurement using a third temperature sensor 19 at the inlet 3 of the device 1.

(20) Additionally, it is also possible to take into account a measurement of an atmospheric pressure at the inlet 3, which measurement is provided, for example, using a second pressure sensor 20 at the inlet 3 of the device 1. However, it is also possible to simply assume an absolute standard value of 1 bar or 1 atmosphere for the atmospheric pressure, which means that measurement of this atmospheric pressure and consequently the second pressure sensor 20 are not strictly necessary for the invention.

(21) It is likewise possible to take into account a measurement of a relative humidity at the inlet 3, for example using a humidity sensor 21 at the inlet 3. Alternatively, a worst-case relative humidity value of 100% may also be assumed for this gas at the inlet 3. In the latter case, the measurement of the relative humidity at the inlet 3 and consequently the humidity sensor 21 are not strictly necessary for the invention.

(22) On the basis of the second desired temperature value determined by the computational unit 14 and the first current value for the second desired temperature value, the control unit 15 will determine the required apportioning proportion and control the apportioning proportion of the first part of the oil to this required apportioning proportion.

(23) In the case of FIG. 1, the apportioning means 8 is positioned downstream of the oil cooler 10 and the bypass 11. However, in the context of the invention, the apportioning means 8 is not precluded from being positioned upstream of the oil cooler 10 and/or the bypass 11, for example at a point where a pipe to the oil cooler 10 and the bypass 11 branch off from each other.

(24) For controlling the speed of the fan 9, the device 1 is provided with a second computational control unit 22.

(25) The second computational control unit 22 forms, together with the first computational control unit 13, a computational control assembly according to the invention.

(26) Control of the fan 9, like control of the apportioning proportion of the first part of the oil as already described above, may have the purpose of controlling the second reference temperature to the second desired temperature value. In that case, the first reference temperature will therefore be the same as the second reference temperature and the first desired temperature value will be equal to the second desired temperature value.

(27) When, in that case, a fourth current value for the second reference temperature is higher than the second desired temperature value and a fifth current value for the apportioning proportion is higher than a predefined minimum apportioning proportion, the required speed of the fan 9 is then determined by the second computational control unit 22 on the basis of at least: a sixth current value representative of the operating pressure, which sixth current value is provided, for example, by measurement using the first pressure sensor 18 at the outlet 4 of the oil-injected element 2; and a seventh current value representative of a temperature of the gas at the inlet 3, which seventh current value is provided, for example, by measurement using the third temperature sensor 19, respectively.

(28) The fourth current value may be provided, for example, by measurement using the first temperature sensor 16 or the second temperature sensor 17.

(29) The second desired temperature value is obtained by the second computational control unit 22 from the computational unit 14.

(30) In order to then be able to take into account the apportioning proportion of the first part of the oil when controlling the speed of the fan 9, the fifth current value for the apportioning proportion can also be taken into account for determining a specific value for the required speed of the fan 9. This fifth current value can be provided by measurement using a position or flow sensor 23 in the apportioning means 8 by which the degree of opening of the apportioning means 8 and consequently the apportioning proportion of the first part of the oil can be measured.

(31) It is of course not impossible in the context of the invention for the second computational control unit 22 to obtain the fifth current value directly from the control unit 15 (not shown in FIG. 1). In that case, the position or flow sensor 23 is no longer necessary and can be dispensed with.

(32) FIG. 1 also shows that the gas compressed by the oil-injected element 2 can be passed, for example, through an oil separator 24 in which the compressed gas is purified by separating the oil previously injected into the oil-injected element 2 from the compressed gas, before the thus purified compressed gas leaves the device 1.

(33) Oil separated in the possibly present oil separator 24 may in this case preferably be reinjected into the oil-injected element 2 via the oil injection pipe network 6.

(34) Optionally, the compressed gas, whether purified or not, may also be sent through an aftercooler 25 before leaving the device 1. The compressed gas may be cooled in this aftercooler 25 by the same fan 9 as is used for the oil cooler 10. In that case, it may be that the speed of the fan 9 is controlled such that a lowest available temperature of the gas in the aftercooler 25 is below a required lowest available temperature. The first reference temperature in that case is thus equal to the lowest available temperature of the gas in the aftercooler 25. The fan 9 is controlled on the basis of the required lowest available temperature and an eighth current value for the lowest available temperature, which eighth current value is measured, for example, using a fourth temperature sensor 26 at a suitable location in the aftercooler 25.

(35) The speed of the fan 9 may also be controlled on the basis of a predefined maximum value for the first reference temperature, for example at a location in the device 1 where the temperature is typically relatively high and should remain below the maximum value for safety reasons. Here, the first reference temperature is, for example, a temperature of the motor 5, the second motor 12 or a frequency converter of the device 1. The first reference temperature may also be a temperature of the gas coming out of the aftercooler 25.

(36) The speed of the fan 9 is then controlled using, as input, a ninth current value for the first reference temperature, which ninth current value is then measured, for example, using a fifth temperature sensor 27.

(37) In the context of the invention, it is not impossible for this fifth temperature sensor 27 to coincide with, for example, the first temperature sensor 16 or the second temperature sensor 17.

(38) If the motor 5 is a variable-speed motor, the computational unit 14, when determining the second desired temperature, also takes into account a tenth current value representative of a rotational speed of the motor 5, and the second computational control unit 22, when determining the required speed of the fan 9, may also take into account an eleventh current value representative of the rotational speed of the motor 5.

(39) A schematic overall view of a method according to the invention is illustrated in FIG. 2.

(40) As already described, a second desired temperature value for the second reference temperature is determined in the computational unit 14.

(41) In this case, the second desired temperature value is determined on the basis of a highest temperature value in a group of two temperature values. This is illustrated in FIG. 2 with a first maximization operator MAX.sub.1.

(42) A first temperature value T.sub.1 in the said group is thus representative of a value of the second reference temperature at which a temperature of the compressed gas at the outlet 4 of the oil-injected element 2 is equal to the first condensation temperature of the compressed gas at the outlet 4 of the oil-injected element 2 or this first condensation temperature plus the first safety margin.

(43) The first condensation temperature may be determined in a manner known by a person skilled in the art as described, for example, in WO 2018/033827 A1.

(44) When determining the first temperature value, a value T.sub.cond representative of the first condensation temperature plus or not plus the first safety margin can in this case still be limited according to a first temperature interval between a first minimum temperature limit T.sub.min,1 and a first maximum temperature limit T.sub.max,1. This limitation of the first condensation temperature plus or not plus the first safety margin is performed in a first limitation operator LIM.sub.1.

(45) If the second reference temperature is the temperature of the gas at the outlet 4 of the oil-injected element 2, a value for the first minimum temperature limit value T.sub.min,1 and the first maximum temperature limit value T.sub.max,1 may vary, for example, between 0 C. and 120 C., and this value may be set with an accuracy of, for example, 1 C.

(46) A second temperature value in the said group is representative of a value T.sub.SER of the second reference temperature at which a specific energy requirement of the device 1 is minimal.

(47) When the motor 5 is a fixed-speed motor, this value T.sub.SER of the second reference temperature can be calculated on the basis of the second current value .sub.2 representative of the operating pressure and the third current value .sub.3 representative of the temperature of the gas at the inlet 3, for example according to the following equation:

(48) $\begin{array}{cc}{T}_{\mathrm{SER}}=B.Math.{}_3+C.Math.{}_2+D& \left(\mathrm{equation}1\right)\end{array}$

(49) When the motor 5 is a variable-speed motor, this value T.sub.SER of the second reference temperature can be calculated on the basis of the second current value .sub.2 representative of the operating pressure, the third current value .sub.3 representative of the temperature of the gas at the inlet 3 and the tenth current value .sub.10 representative of the rotational speed of the motor 5, according to the following equation, for example:

(50) $\begin{array}{cc}{T}_{\mathrm{SER}}=A.Math.{}_{10}+B.Math.{}_3+C.Math.{}_2+D& \left(\mathrm{equation}2\right)\end{array}$

(51) Here, the current value .sub.10 is a value for the rotational speed of the motor 5 determined as a percentage of a maximum rotational speed of the motor 5.

(52) In the preceding equations 1 and 2, the value T.sub.SER of the second reference temperature is expressed in C., the second current value .sub.2 is determined as the operating pressure in bar, and the third current value .sub.3 is determined as the temperature of the gas at the inlet 3 in C.

(53) If the second reference temperature is the temperature of the gas at the outlet 4 of the oil-injected element 2, possible value intervals for the constants A, B, C and D in the preceding equations 1 and 2 are:

(54) $\begin{array}{cc}\left\{A.Math.0C.A64000C.\right\}& \left(\mathrm{equation}3\right)\end{array}$$\begin{array}{cc}\left\{B.Math.0B1000\right\}& \left(\mathrm{equation}4\right)\end{array}$$\begin{array}{cc}\left\{C.Math.0C./\mathrm{bar}C255C./\mathrm{bar}\right\}& \left(\mathrm{equation}5\right)\end{array}$$\begin{array}{cc}\left\{D.Math.-32000C.D32000C.\right\}& \left(\mathrm{equation}6\right)\end{array}$

(55) When the second temperature value T.sub.2 is determined, the value T.sub.SER can then still be limited according to a second temperature interval between a second minimum temperature limit value T.sub.min,2 and a second maximum temperature limit value T.sub.max,2. This limitation of the value T.sub.SER is performed by a second limitation operator LIM.sub.2.

(56) If the second reference temperature is the temperature of the gas at the outlet 4 of the oil-injected element 2, a value for the second minimum temperature limit value T.sub.min,2 and the second maximum temperature limit value T.sub.max,2 may vary, for example, between 0 C. and 120 C., and this value may be set with an accuracy of, for example, 1 C.

(57) Optionally, when the second reference temperature is to be controlled from an old temperature value to the second desired temperature value, the said highest temperature value resulting from the first maximization operator MAX.sub.1 can be limited according to a third temperature interval between, on the one hand, the old temperature value minus a maximum temperature decrease value T.sub.max,down and, on the other hand, the old temperature value plus a maximum temperature increase value T.sub.max,up. This allows an excessive decrease or increase in the second reference temperature to be avoided. This limitation of the highest temperature value is performed by a third limitation operator LIM.sub.3.

(58) Here, it is possible for a predefined time interval t to be determined for control of the old temperature value to the second desired temperature value, wherein the maximum temperature decrease value T.sub.max,down and the maximum temperature increase value T.sub.max,up are positively dependent on a length of this predefined time interval t.

(59) Optionally, the second desired temperature value can still be limited according to a fourth temperature interval between a third minimum temperature limit value T.sub.min,3 on the one hand and a third maximum temperature limit value T.sub.max,3 on the other hand.

(60) If the second reference temperature is the temperature of the gas at the outlet 4 of the oil-injected element 2, the third minimum temperature limit value T.sub.min,3 may be set as a value between, for example, 20 C. and 80 C. with an accuracy of, for example, 1 C. to prevent condensate formation at the outlet 4.

(61) Alternatively, the third minimum temperature value T.sub.min,3 may be set as a high value of, for example, 105 C. if the oil injection pipe network 6 is further provided with a heat recovery system (not shown in FIG. 1) by which heat can be recovered from the oil separated by the oil separator 24 to a heat absorbing fluid. Such a high value for the third minimum temperature value T.sub.min,3 allows the heat recovery system to recover a higher amount of heat from the oil in the oil injection pipe network 6 even at relatively high temperatures for the heat absorbing fluid.

(62) The third maximum temperature limit value T.sub.max,3 can be set as a value between, for example, 100 C. and 120 C. with an accuracy of, for example, 1 C.

(63) The second desired temperature value thus determined in the computational unit 14 is further used in the control unit 15 to determine the required apportioning proportion on the basis of a first ratio .sub.1 between the first current value .sub.1 for the second reference temperature and this second desired temperature value.

(64) The required apportioning proportion can be determined as a continuous proportion between a minimum zero value and a maximum value of 100% depending on the first ratio .sub.1 according to a first monotonically increasing function.

(65) On the other hand, the required apportioning proportion can also be determined as a binary proportion which, during operation of the device 1 is a maximum value of 100% when the first current value .sub.1 is higher than the second desired temperature value or the second desired temperature value plus a second safety margin; or is higher during a first period than the second desired temperature value or the second desired temperature value plus the second safety margin; and is otherwise a minimum zero value.

(66) Here, the second safety margin can be set to a value between, for example, 0 C. and 20 C. with an accuracy of, for example, 0.1 C.

(67) The first period can be set to a value between, for example, 0 seconds and 255 seconds.

(68) On the basis of the required apportioning proportion determined by the control unit 15, the apportioning means 8 is then actuated to actually achieve this required apportioning proportion.

(69) A required speed of the fan 9 for controlling the first reference temperature to the first desired temperature value is determined using the second computational control unit 22.

(70) For this purpose, the required speed is selected as a highest speed value from a set of, in this case, three speed values. This is illustrated in FIG. 2 with a second maximization operator MAX.sub.2.

(71) A first speed value v.sub.1 in the said set is in that case representative of a speed value of the fan 9 required to achieve the second desired temperature value for the second reference temperature.

(72) In a first operating regime in which the device 1 is yet to warm up, that is, when a fourth current value .sub.4 for the second reference temperature is lower than a predefined minimum temperature of, for example, 90 C. needed to end this first warm-up operating regime, the first speed value v.sub.1 is equal to a zero value.

(73) In a second operating regime of the device 1 wherein the fourth current value .sub.4 is higher than the predefined minimum temperature, the first speed value v.sub.1 is still equal to a zero value when the apportioning proportion is lower than a predefined minimum apportioning proportion or the fourth current value .sub.4 is lower than the second desired temperature value.

(74) The predefined minimum apportioning proportion can, for example, be set as a value between, for example, 0% and, for example, 100% with an accuracy of, for example, 1%.

(75) On the other hand, in the second operating regime, when the fifth current value as for the apportioning proportion is higher than the predefined minimum apportioning proportion and the fourth current value .sub.4 is higher than the second desired temperature value, the first speed value v.sub.1 is determined on the basis of at least the sixth current value .sub.6 representative of the operating pressure; and the seventh current value .sub.7 representative of the temperature of the gas at the inlet 3.

(76) When the motor 5 is a variable-speed motor, when the first speed value v.sub.1 is determined, the eleventh current value .sub.11 representative of the rotational speed of the motor 5 is also taken into account, for example according to the following equation:

(77) $\begin{array}{cc}{v}_1=v_{1,\mathrm{raw}}=E.Math.{}_{11}+F.Math.{}_7+G.Math.{}_6+H& \left(\mathrm{equation}7\right)\end{array}$

(78) Here, the current value .sub.11 is a value for the rotational speed of the motor 5 determined as a percentage of the maximum rotational speed of the motor 5.

(79) In the preceding equation 7, the first speed value v.sub.1 is determined as a percentage of a maximum speed of the fan 9, the sixth current value .sub.6 as the operating pressure in bar, and the seventh current value .sub.7 as the temperature of the gas at the inlet 3 in C.

(80) If the second reference temperature is the temperature of the gas at the outlet 4 of the oil-injected element 2, possible value intervals for the constants E, F, G and H in equation 7 are:

(81) $\begin{array}{cc}\left\{E.Math.0E64000\right\}& \left(\mathrm{equation}8\right)\end{array}$$\begin{array}{cc}\left\{F.Math.0{\left(C.\right)}^{-1}F1000{\left(C.\right)}^{-1}\right\}& \left(\mathrm{equation}9\right)\end{array}$$\begin{array}{cc}\left\{G.Math.0{\left(\mathrm{bar}\right)}^{-1}G255{\left(\mathrm{bar}\right)}^{-1}\right\}& \left(\mathrm{equation}10\right)\end{array}$$\begin{array}{cc}\left\{H.Math.-32000H32000\right\}& \left(\mathrm{equation}11\right)\end{array}$

(82) When in this case the fourth current value .sub.4 is higher than the second desired temperature value plus a first tolerance value; or during a second period, the fourth current value .sub.4 is higher than the second desired temperature value plus the first tolerance value; or the fourth current value .sub.4 is lower than the second desired temperature value minus a second tolerance value; or during a third period, the fourth current value .sub.4 is lower than the second desired temperature value minus the second tolerance value,
the first speed value v.sub.1 is further determined on the basis of at least the fifth current value .sub.5 for the apportioning proportion; and a second ratio .sub.2 between the fourth current value .sub.4 and the second desired temperature value.

(83) The first tolerance value and the second tolerance value can, for example, be set between a value of, for example, 0 C. and, for example, 20 C. with an accuracy of, for example, 0.1 C.

(84) The second period and third period can, for example, be set between a value of, for example, 0 seconds and, for example, 255 seconds.

(85) The first speed value v.sub.1 is in this case preferably dependent on the second ratio .sub.2 according to a second monotonically increasing function, and alternatively or additionally preferably dependent on the fifth current value .sub.5 according to a third monotonically increasing function, for example according to the following equation:

(86) $\begin{array}{cc}{v}_1=v_{1,\mathrm{raw}}.Math.{}_5.Math.P.Math.{}_2.Math.Z& \left(\mathrm{equation}12\right)\end{array}$

(87) In this equation 12, the fifth current value is determined as the percentage apportioning proportion of the first part of the oil.

(88) Possible value intervals for the constants P and Z in equation 12 are:

(89) $\begin{array}{cc}P=0-4& \left(\mathrm{equation}13\right)\end{array}$$\begin{array}{cc}Z=0-4& \left(\mathrm{equation}14\right)\end{array}$

(90) A second speed value v.sub.2 in the said set is determined as follows: when the eighth current value .sub.8 for the lowest available temperature in the aftercooler 25 is higher than the value for the required lowest available temperature, the second speed value v.sub.2 is determined on the basis of the first speed value v.sub.1; and a third ratio .sub.3 between the eighth current value .sub.8 and the value for the required lowest available temperature; otherwise, the second speed value v.sub.2 is set equal to zero.

(91) The required lowest available temperature is equal to a value for a second condensation temperature of the gas in the aftercooler 25 plus an offset.

(92) The second speed value v.sub.2 is preferably dependent on the third ratio .sub.3 according to a fourth monotonically increasing function. When the eighth current value .sub.8 for the lowest available temperature in the aftercooler 25 is higher than the value for the required lowest available temperature, the second speed value v.sub.2 is calculated, for example, according to the following equation:

(93) $\begin{array}{cc}{v}_2=v_1.Math.{}_3.Math.P& \left(\mathrm{equation}15\right)\end{array}$

(94) In the preceding equation 15, the second speed value v.sub.2 is determined as a percentage of the maximum speed of the fan 9.

(95) A possible value interval for the constant P is already given in equation 13.

(96) A third speed value v.sub.3 in the said set is determined on the basis of the ninth current value .sub.9 for the first reference temperature; and the predefined maximum value for the first reference temperature,
wherein the third speed value .sub.9 is equal to zero when the ninth current value .sub.9 is lower than the predefined maximum value; and a value representative of the maximum speed of the fan 9 when the ninth current value .sub.9 is higher than the predefined maximum value.

(97) The predefined maximum value can, for example, be set between a value of, for example, 90 C. and, for example, 120 C. with an accuracy of, for example, of 1 C.

(98) Finally, on the basis of the required speed determined by the second computational control unit 22, the second motor 12 is actuated to actually run the fan 9 at the required speed.

(99) The present invention is by no means limited to the embodiments described as examples and shown in the figure, but a method, computational control device or device according to the invention can be implemented in all kinds of variants without departing from the scope of the invention as defined in the claims.