Virtual sensor for water content in oil circuit

Abstract

A method for monitoring an oil-injected screw compressor configured to compress aspirated air by returning oil from an oil separator vessel (11) to a compression chamber (12) of a compressor block (30), for condensate formation in the oil circuit due to a too low compression discharge temperature (VET), determines a water inlet mass flow {dot over (m)}.sub.ein(t) and a water outlet mass flow {dot over (m)}.sub.aus(t) for a point in time t and determines generated condensate flow Δ{dot over (m)}.sub.w(t)={dot over (m)}.sub.ein(t)−{dot over (m)}.sub.aus(t) on the basis of difference formation.

Claims

1. A method for monitoring an oil-injected screw compressor, the method comprising: determining at points in time over a specified time interval condensate mass flows in an oil circuit of the oil-injected screw compressor that is configured to compress aspirated air, each condensate mass flow determined at a given point in time, t, by: determining a water inlet mass flow {dot over (m)}.sub.ein(t) at the given point in time; determining a water outlet mass flow {dot over (m)}.sub.aus(t) at the given point in time, t; and calculating the condensate mass flow, Δ{dot over (m)}.sub.w(t), at the given point in time as a difference between the water inlet mass flow and the water outlet mass flow, Δ{dot over (m)}.sub.w(t)={dot over (m)}.sub.ein(t)−{dot over (m)}.sub.aus(t); using all the condensate mass flows over the specified time interval to determine a total condensate mass, m.sub.K, in the oil circuit; comparing the total condensate mass, m.sub.K, to a limit value m.sub.K,max1; and intervening in a control unit of the screw compressor when the total condensate mass exceeds the limit value,
m.sub.K>m.sub.K,max1.

2. The method according to claim 1, wherein using all the condensate mass flows over the specified time interval to determine the total condensate mass, m.sub.K, comprises calculating a summation of the total condensate masses at each point in time over the specified time interval, by calculating for a regular time interval, dt, the condensate mass at a previous point in time, m.sub.K(t−dt), and a condensate mass flow, Δ{dot over (m)}.sub.w, over the regular time interval, dt, m.sub.K(t)=m.sub.K(t−dt)+Δ{dot over (m)}.sub.w*dt.

3. The method according to claim 1, wherein using all the condensate mass flows over the specified time interval to determine the total condensate mass m.sub.K, comprises calculating the condensate mass as a sum of a known initial value at an initial time, t.sub.0, of the condensate mass m.sub.K (t.sub.0) and a numerical integration of the condensate flow Δ{dot over (m)}.sub.w over the specified time interval, m.sub.K=m.sub.K(t.sub.0)+∫.sub.di 0.sup.tΔ{dot over (m)}.sub.w(τ)dτ.

4. The method according to claim 1, wherein intervening in a control unit of the screw compressor comprises increasing a target value for the compression discharge temperature (VET) is increased by a predetermined temperature value.

5. The method according to claim 1, wherein the method further comprises: comparing the total condensate mass m.sub.K to a limit value m.sub.K,max2; and issuing an error message when the total condensate mass exceeds the limit value
m.sub.K>m.sub.K,max2.

6. The method according to claim 1, wherein determining the water inlet mass flow, {dot over (m)}.sub.ein (t), comprises calculating the water inlet mass flow from an inlet air mass flow {dot over (m)}.sub.Luft(t) and an assumed relative humidity of 100%.

7. The method according to claim 1, wherein determining the water inlet mass flow, {dot over (m)}.sub.ein (t), comprises calculating the water inlet mass flow using an inlet air mass flow {dot over (m)}.sub.Luft(t) and a relative humidity with a fixed value, F, of between 70% and 100%, 70%≤F<100%, or a relative humidity dependent on an intake temperature, T.sub.Ans, of the screw compressor.

8. The method according to claim 6, wherein the method further comprises determining the inlet air mass flow {dot over (m)}.sub.Luft(t) using a system-specific delivery characteristic, which depends on a current speed, n, of the screw compressors, pressure p.sub.akt, in an oil separator tank fed by the screw compressor, ambient pressure p.sub.amb, and intake temperature T.sub.Ans.

9. The method according to claim 8, wherein determining the water inlet mass flow, {dot over (m)}.sub.ein (t), further comprises using the inlet air mass flow {dot over (m)}.sub.Luft(t), the intake temperature T.sub.Ans, the ambient pressure p.sub.amb, and the relative humidity of 100%, to determine the water inlet mass flow.

10. The method according to claim 8, wherein determining the water outlet mass flow, {dot over (m)}.sub.aus (t), further comprises using the inlet air mass flow, {dot over (m)}.sub.Luft(t), the pressure, p.sub.akt, in the oil separator tank a temperature after the oil separator tank T.sub.2, and the relative humidity, of 100% are included to determine the water outlet mass flow.

11. An oil-injected screw compressor comprising a control unit adapted to perform the method according to claim 1.

12. The method according to claim 7, wherein the method further comprises determining the inlet air mass flow {dot over (m)}.sub.Luft(t) using a system-specific delivery characteristic, which depends on a current speed, n, of the screw compressor, pressure, p.sub.akt, in an oil separator tank (11) fed by the screw compressor, ambient pressure, p.sub.amb, and intake temperature, T.sub.Ans.

13. A method for monitoring an oil-injected screw compressor, the method comprising: determining at points in time over a specified time interval condensate mass flows in an oil circuit of the oil-injected screw compressor that is configured to compress aspirated air, each condensate mass flow determined at a given point in time, t, by: determining a water inlet mass flow {dot over (m)}.sub.ein(t) at the given point in time; determining a water outlet mass flow {dot over (m)}.sub.aus(t) at the given point in time, t; and calculating the condensate mass flow, Δ{dot over (m)}.sub.w(t), at the given point in time as a difference between the water inlet mass flow and the water outlet mass flow, Δ{dot over (m)}.sub.w(t)={dot over (m)}.sub.ein(t)−{dot over (m)}.sub.aus(t); using all the condensate mass flows over the specified time interval to determine a total condensate mass, m.sub.K, in the oil circuit by calculating the condensate mass as a sum of a known initial value at an initial time, t.sub.0, of the condensate mass, m.sub.K(t.sub.0), and a numerical integration of the condensate flow Δ{dot over (m)}.sub.w over the specified time interval, m.sub.k=m.sub.K(t.sub.0)+∫.sub.t.sub.0.sup.tΔ{dot over (m)}.sub.w(τ)dτ; comparing the total condensate mass, m.sub.K, to a limit value m.sub.K,max1; and intervening in a control unit of the screw compressor when the total condensate mass exceeds the limit value, m.sub.K>m.sub.K,max1.

14. A method for monitoring an oil-injected screw compressor, the method comprising: determining at points in time over a specified time interval condensate mass flows in an oil circuit of the oil-injected screw compressor that is configured to compress aspirated air, each condensate mass flow determined at a given point in time, t, by: determining a water inlet mass flow {dot over (m)}.sub.ein(t) at the given point in time by calculating the water inlet mass flow using an inlet air mass flow {dot over (m)}.sub.Luft(t) and a relative humidity with a fixed value, F, of between 70% and 100%, 70%<F<100%, or a relative humidity dependent on an intake temperature, T.sub.Ans, of the screw compressor; determining a water outlet mass flow {dot over (m)}.sub.aus(t) at the given point in time, t; and calculating the condensate mass flow, Δ{dot over (m)}.sub.w(t) at the given point in time as a difference between the water inlet mass flow and the water outlet mass flow, Δ{dot over (m)}.sub.w(t)={dot over (m)}.sub.ein(t)−{dot over (m)}.sub.aus(t); and using all the condensate mass flows over the specified time interval to determine a total condensate mass, m.sub.K, in the oil circuit by calculating the condensate mass as a sum of a known initial value at an initial time, t.sub.0, of the condensate mass, m.sub.K(t.sub.0), and a numerical integration of the condensate flow Δ{dot over (m)}.sub.w over the specified time interval, m.sub.Km.sub.K(t.sub.0)+∫.sub.t.sub.0.sup.tΔ{dot over (m)}.sub.w(τ)dτ; comparing the total condensate mass, m.sub.K, to a limit value m.sub.K,max1; and intervening in a control unit of the screw compressor when the total condensate mass exceeds the limit value,
m.sub.K>m.sub.K,max1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other objects and features of the invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention.

(2) In the drawings,

(3) FIG. 1 shows a block diagram of an oil-injected screw compressor with oil separator tank according to the prior art;

(4) FIG. 2 shows a flowchart depicting a preferred embodiment of the present invention; and

(5) FIG. 3 shows a block diagram of an oil-injected screw compressor according to the invention with oil separator tank.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(6) FIG. 3 shows a block diagram of a screw compressor 10 comprising a compressor block 30 and an oil separator 11. The screw compressor 10 also has a compression chamber 12 in the compressor block 30. Compressed air reaches the oil separator 11 via a first compressed-air line 13. An oil filter 14 is provided in the oil separator tank 11. The compressed air, which is fed via the first compressed-air line 13 into the oil separator tank 11, passes through the oil filter 14 and is fed via a second compressed-air line 15 to an aftercooler 16 and from there via a third compressed-air line 17 to a useful application, e.g. a line network buffered via a compressed-air tank in an industrial plant.

(7) The oil separated in the oil separator tank 11 is returned via a return line 18 to the screw compressor 10 where it is injected into the compression chamber 12 for cooling, sealing and lubrication. The return line is divided into a first partial line 20 and a second partial line 21 at a branching point 19. The first partial line 20 and the second partial line 21 will be reunited in a unification point 22.

(8) An oil cooler 23 is provided in the first partial line 20, which cooler extracts heat from the recirculated oil and for this purpose is cooled with an air stream generated by a fan 24 for better heat dissipation. The second partial line 21 forms a bypass through which oil can be guided past the oil cooler 23. With an electrically controllable, infinitely adjustable regulating valve 25, the ratio between the oil guided through the oil cooler 23 and the oil guided past the oil cooler 23 can be infinitely adjusted. This allows the temperature of the oil injected into compression chamber 12 to be set to a desired value.

(9) During normal operation, the oil temperature of the oil injected back into compression chamber 12 is set to such a temperature value that the compression discharge temperature (VET) at an outlet 26 of the screw compressor 10 is at such a temperature level that condensate accumulation in the oil separator tank 11 is avoided. At the same time, a minimum compression discharge temperature of e.g. 60° C. is targeted. Also, a maximum compression discharge temperature must not be exceeded, let alone due to legal requirements. In this respect, a compression discharge temperature of no higher than 95° C. is the target for most screw compressors.

(10) During normal operation, the compression discharge temperature should be set as low as possible in the aforementioned areas in order to avoid unnecessary stress of the oil; on the other hand, as already mentioned, condensate formation should be prevented with certainty.

(11) The screw compressor 10 further has a central control unit 40 or interacts with such a central control unit 40. The central control unit 40 can receive data from various sensors in a preferred embodiment, namely from a first temperature sensor which is designed and arranged to detect the intake temperature T.sub.Ans, a second temperature sensor 42 which is arranged to detect the compression discharge temperature at an outlet of the compressor block 30 and a third temperature sensor 43 which is designed and arranged to detect the temperature T.sub.2 after the oil separator 11. The central control unit 40 also receives data from two pressure sensors, namely a first pressure sensor 44 which is arranged and provided for the detection of a pressure P.sub.akt in the oil separator tank 11 and a second pressure sensor 45 which is intended for the detection of a network pressure P.sub.N after the aftercooler. A further third pressure sensor (not shown) may be located on the central control unit to detect and transmit an ambient pressure P.sub.amb. However, the ambient pressure can also be taken into account as an assumed value or received externally, for example via a data network, for example from a weather station. In addition, a fixed assumed temperature value can be used instead of the temperature T.sub.Ans, which is recorded by the temperature sensor 42, for example. A fixed temperature value of 25° C. or 30° C. may optionally also be used, for example, depending on the installation location. Instead of a pressure P.sub.Akt in the oil separator tank 11, it is also possible to recalculate the pressure in the oil separator tank P.sub.Akt from the data supplied by the pressure sensor 45, i.e. from the network pressure P.sub.N, or the pressure in the oil separator tank P.sub.Akt can be estimated from the network pressure P.sub.N. Furthermore, a fixed value for the pressure in the oil separator tank P.sub.Akt can also be assumed, for example the maximum value which is preset on the operating side for the screw compressor or a maximum value which is preset by an operator at the control unit of the screw compressor.

(12) According to the present invention, a method or a virtual sensor is proposed to monitor or determine the correct functioning of the above-mentioned regulation if condensate accumulation appears to be inadvertently possible. For this purpose, water mass flows are compared with each other at respectively predetermined sampling times and a currently generated condensate flow is determined from this. From an addition of the value of the currently generated condensate flow, which can assume different signs, and the condensate mass already present from previous calculations, a currently existing condensate mass is calculated.

(13) Since a worst-case scenario can be assumed when determining the water inlet mass flow, for example that the ambient air has a 100% relative humidity, certain values, such as condensate flow or condensate mass, are always to be understood as “currently possible generated condensate flow or currently possible condensate mass”, i.e. also as a worst-case scenario. With the water outlet mass flow, on the other hand, it is assumed that there is 100% saturation anyway if condensate is to be discharged, so that here the maximum possible water outlet mass flow should at least approximately coincide with the actually discharged water outlet mass flow.

(14) On the basis of the flowchart according to FIG. 2, the preferred embodiment of the method according to the invention is again explained in detail below. In step 100 the method is started and in step 101 it is checked whether the screw compressor is running under load and one second has already passed. If the result of this test is negative, this test step must be repeated. If the result is positive, a possible air mass flow or inlet air mass flow is calculated in a step 102 as follows:
{dot over (m)}.sub.Luft(t)={dot over (m)}.sub.L(p.sub.amb,p.sub.aktn,T.sub.Ans . . . )   (1)

(15) The calculation of the possible air mass flow includes a system-specific delivery characteristic, which depends on the current speed n for screw compressors regulated by frequency converters, the pressure p.sub.akt in the oil separator tank 11, the ambient pressure p.sub.amb, and the intake temperature T.sub.Ans after the oil separator tank 11.

(16) In a step 103, a water inlet mass flow is calculated from the possible air mass flow according to the following formula (2):
{dot over (m)}.sub.ein(t)={dot over (m)}.sub.w(T.sub.Ans,p.sub.amb, {dot over (m)}.sub.Luft,φ=100%)   (2)

(17) This calculation or determination is based on a worst case scenario, i.e. it is assumed that the aspirated ambient air has a relative humidity of 100%.

(18) In a step 104, the current water outlet mass flow is calculated with the following formula (3)
{dot over (m)}.sub.aus(t)={dot over (m)}.sub.w(T.sub.2,p.sub.akt,{dot over (m)}.sub.Luft,φ=100%)   (3)
in which the temperature T.sub.2 after the oil separator tank 11, the pressure in the oil separator tank p.sub.akt, the inlet air mass flow {dot over (m)}.sub.Luft and a relative humidity of 100% are included in this calculation. It should be clear to the person skilled in the art that steps 103 and 104 can also take place in reverse order or at the same time.

(19) From the water inlet mass flow determined in step 103 and the water outlet mass flow determined in step 104, a current water mass change rate, i.e. a currently generated condensate mass, is calculated in step 105 according to equation (4) as follows:
Δ{dot over (m)}.sub.w(t)={dot over (m)}.sub.ein(t)−m.sub.aus(t)   (4).

(20) If the currently generated water mass change rate or the currently generated condensate mass is positive, condensate is actually produced and an existing condensate mass is increased. If, on the other hand, the value of the currently generated water mass change rate or the currently generated condensate mass is negative, condensate is discharged, i.e. a current condensate mass is reduced.

(21) The currently present condensate mass m.sub.K is calculated according to equation (5) in step 106, so that the currently given condensate mass is updated taking into account the current water mass change rate calculated in step 105.

(22) The method could theoretically start again from scratch at this point if the determination of the currently given condensate mass m.sub.K is only to take place in the sense of a virtual sensor.

(23) However, other measures may also be considered. For this purpose, step 107 checks whether the currently given condensate mass m.sub.K≤0. In this case, the currently given condensate mass is set to zero in one step 108 and the process is started from the beginning. If the currently given condensate mass m.sub.K is positive, on the other hand, the process is restarted in a step 109 with the currently given condensate mass m.sub.K, and on the other hand in a step 110 it is checked whether the currently given condensate mass m.sub.K is above a limit value m.sub.K,max1, i.e. above a certain amount of condensate. If this is the case, in a further step 111 the target value of the compression discharge temperature is increased by 5K until the currently given condensate mass has reached the value 0 again. However, it is also taken into account that the compression discharge temperature does not exceed a maximum value of 95° C. However, if it is determined in step 110 that the currently present condensate mass has not exceeded the specified limit value of m.sub.K,max1, the target value of the compression discharge temperature is not affected.

(24) Although above a mass flow or a mass change rate was represented as a quantity characterizing the water flow or the condensate accumulation, a volume flow at a suitable reference condition or also a mass flow can be used instead. Alternatively, it would also be possible to calculate a quantity characterizing the condensate mass change approximately by the product of a quantity characterizing the air mass flow (e.g. mass, volume, quantity) and the difference of the water vapor pressures of inlet and outlet each converted to a common reference pressure.

(25) Although only a few embodiments of the present invention have been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.

LIST OF REFERENCE SIGNS

(26) VET Compression discharge temperature {dot over (m)}.sub.Luft Inlet air mass flow {dot over (m)}.sub.ein,max Maximum possible water inlet mass flow {dot over (m)}.sub.aus,max Maximum possible water outlet mass flow Δ{dot over (m)}.sub.w Currently generated condensate flow m.sub.K(t) Condensate mass 1′ Suction point 10 Screw compressor 11, 11′ Oil separator tank 12 Compression chamber 13 First compressed-air line 14, 14′ Oil filter 15 Second compressed-air line 16, 16′ Aftercooler 17 Third compressed-air line 18 Return line 19 Branching point 20 First partial line 21 Second partial line 22 Unification point 23, 23′ Oil cooler 24 Fan 25, 25′ Regulating valve Outlet (screw compressor) 27, 27′ Non-return valve 30, 30′ Compressor block 40, 40′ Central control unit 41, 41′ First temperature sensor (T.sub.Ans) 42, 42′ Second temperature sensor (VET) 43, 43′ Third temperature sensor (T.sub.2) 44, 44′ First pressure sensor (P.sub.akt) 45, 45′ Second pressure sensor (P.sub.N)