Compressor assembly

BACKGROUND OF THE INVENTION

The present invention refers to a compressor system with a water-injection cooled compressor in a cooling water circuit.

Such compressor assemblies are employed to compress gaseous media, especially air, and to provide this as a pressure gas. Water is used for sealing, lubricating and cooling the compressor, which has the advantage over oil not to be detrimental to health or the environment. DE 44 47 097 describes a water-cooled compressor system in which the compressor is formed as a screw-type compressor. The cooling water is injected in the area of the rotors of the compressor and reseparated from the compressed gas after leaving the compressor. The water heated in the compressor is then guided towards a cooling means. Then the cooled cooling water is filtered and re-supplied to the compressor. In this process, various parameters of the cooling water can be changed by evaporation of the cooling water or by absorption of humidity from the air into the cooling water: if the relative salinity of the water increases because of evaporation of cooling water, an increased amount of deposit precipitates, which might cause damages and defects in the narrow sealing gaps and to the sealing rings. Deposit formation is kept low by a polarization means, which does not, however, prevent the salinity from rising. If, on the other hand, the salinity of the cooling water decreases because of humidity absorbed from the air, the buffer ability of the cooling water to absorb free carbon dioxide is reduced. Yet free carbon dioxide in the cooling water which is not buffered (bound) is very aggressive and corrosive. The Ph-value of the cooling water might also be changed by evaporation of cooling water, absorption of humidity from the air into the cooling water or absorption of copper or iron ions, so that the cooling water has a corrosive effect.

From DE 821 993 and US 722 524, compressors are known that use distilled water for cooling. Distilled water is rather expensive and is very delicate with a view to its pH value.

SUMMARY OF THE INVENTION

It is the object of the invention to improve the water quality in an injection-cooled compressor system.

In a compressor system according to the invention, the cooling water circuit is provided with a measuring device to determine the values of conductivity of the cooling water and a water supply source to supply non-desalinated water and desalinated water, i.e. non-deionized and deionized water. A controlling means causes the introduction of desalinated water, when the conductivity exceeds an upper conductivity limit, and the introduction of non-desalinated water into the cooling water circuit, when the conductivity falls below a lower conductivity limit. Thus, the salinity of the cooling water is controlled such that the conductivity of the cooling water always remains within a predetermined range. As the conductivity also is an approximation for the pH-value of the water, the pH-value of the cooling water can be controlled such that it does not shift into the acid or alkaline range but remains within the neutral range by controlling the conductivity of the water.

When the conductivity falls below the lower conductivity limit, non-desalinated water is added to the cooling water, thereby increasing the conductivity of the cooling water so that the value of conductivity of the cooling water again rises above the lower conductivity limit. By controlling the conductivity of the cooling fluid to be above a lower conductivity limit, a strong decline in the pH-value of the water is avoided. Thereby, the pH-value of the water is maintained above a pH-value of 6.5, for example. Thus, the cooling water is not aggressive, so that the corrosion because of the cooling water within the coolant circuit is greatly reduced.

When the upper conductivity limit is exceeded, desalinated water is added to the cooling water. Thereby, the conductivity of the cooling water is reduced. After the conductivity falls below the upper conductivity limit, the introduction of desalinated water is stopped again. By means of the salts dissolved in the cooling water, the cooling water can absorb carbon dioxide, which is absorbed when air is compressed and has a very corrosive effect. The salinity of the cooling water, however, is not arbitrary, as the salts start to deposit when a certain concentration of salts is exceeded, i.e., particles form in the cooling water, which can lead to damages in sealing gaps, bearing seals, sliding bearings etc. Therefore, the salinity of the cooling water has to be limited, which is determined by the upper conductivity limit.

Thus, two effects are achieved by controlling the conductivity of the cooling water within a determined range of conductivity: firstly, a super-acidity of the cooling water is avoided, thereby in turn avoiding corrosion within the cooling water circuit. Secondly, the salinity of the cooling water is maintained below a limit so that no deposition occurs, whereby damages or defects of movable parts within the cooling circuit can be avoided. Thereby compressor systems can be put into practice in which less corrosion-resistant materials can be used in the cooling water circuit and in the compressor. As the cooling water is nearly free of deposits, a compressor can be designed with very narrow sealing gaps, and therefore, for example, with sliding bearings. Thus, the cooling water control means according to the invention provides the preconditions for increasing the effectivity and longevity of a gas compressor system.

Preferably, the water supply source comprises a desalinization device which is supplied with non-desalinated water. Furthermore, a bypass is provided bypassing the desalinization device, and valves are provided connecting either the desalinization device or the bypass to the compressor. The valves are switched such that water desalinated in the desalinization device is supplied to the compressor or the cooling circuit when the conductivity falls below a lower limit. If non-desalinated water is to be supplied to the cooling circuit, the valves are switched such that the non-desalinated water is guided past the desalinization device through the bypass and supplied to the compressor or the cooling water circuit in a non-desalinated state. The non-desalinated water supplied to the desalinization device or the bypass can, for example, be drinking water from the public drinking water net. Preferably, the desalinization device is provided as an ion exchanger or a reversed osmosis device.

The overall control of the conductivity can occur in the form of a flicker control or continuously. Preferably, the conductivity can be controlled in a range from 10 to 20 &mgr;S/cm at 25° C.

In a preferred embodiment, there is provided a return pipe from the cooling water circuit to a water supply source inlet through which water of the cooling water circuit can be supplied to the water supply source. The cooling water coming from the cooling water circuit can be desalinated in the desalinization device of the water supply source when needed. This results in a closed coolant circuit the cooling water thereof is desalinated in a leg parallel to the compressor in the desalinization device and resupplied to the cooling water circuit. Thus, the cooling water is recycled. Only little strain is put on the desalinization device by not desalinating water supplied from the outside, e.g. drinking water, as the water of the cooling water circuit generally only has to be desalinated to a relatively small extent.

In a preferred embodiment of the invention, the water of the water supply source is led into a gas take-in duct of the compressor. Thus, water coming from the water supply source is not led into the cooling water circuit, which approximately provides the pressure of the gas compressed by the compressor, but is led into the gas take-in duct of the compressor where there is an approximately atmospheric pressure. In this manner, an introduction of the water of the water supply source into the cooling water circuit can be achieved without a compression thereof.

In a preferred embodiment, the compressor comprises sliding bearings, the water of the water supply source being injected directly into the sliding bearings. The injection of water into the sliding bearings is performed before the compressor starts working. Thus it is ensured that the sliding bearings are filled with water, i.e. are lubricated, when the compressor starts working. An abrasion-intensive mixed friction involving dry friction and sliding friction when the compressor starts working can thereby be avoided. After the compressor starts working, the pressure in the cooling circuit increases to the compressor pressure. That is why the water coming from the water supply source, which only has a slight excess pressure, cannot be led directly into the cooling water circuit any more. Therefore, the water coming from the water supply source is led into the gas take-in duct after the compressor starts working.

In a preferred embodiment, the cooling water circuit provides a temperature sensor to compensate for the temperature when measuring the conductivity. As the conductivity of water strongly depends on the temperature, a temperature compensation of the measured conductivity is required for a normalized measurement. Therefore, the temperature sensor is arranged near the position of the conductivity sensor.

Preferably, the water supply source is connected to the drinking water net. Thereby, non-desalinated water to be supplied to the water supply source is available at any place of installation, by means of which water the salinity and therefore the conductivity of the cooling water can be increased. The system water pressure of the drinking water net is sufficient to supply the water to the water supply source and to inject it, in a non-desalinated or desalinated state, into the gas take-in duct or, before the compressor starts working, into the compressor.

An embodiment of the invention will now be described in detail with reference to the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is an illustration of a compressor system

10

for generating oil-free pressure air.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The compressor system

10

includes a compressor

12

driven by an electric motor

13

and provided as a water-injection cooled screw-type compressor. The screw-type compressor

12

draws in air via a gas take-in duct

14

and compresses it to approximately 8 to 10 bars. The air compressed in the compressor

12

is led into a precipitation or settling vessel

16

from which it can be taken out via a pressure air conduit

18

for further use.

The compressor system

10

comprises a cooling water circuit

20

in which the cooling water cooling the compressor

12

is cooled and purified. During compression, the cooling water is injected in the area of the rotors of the screw-type compressor via water injection nozzles to seal the sealing gap in the compressor

12

and to absorb and dissipate the heat produced during compression in the compressor

12

.

The cooling water is reseparated from the pressure air in the settling vessel

16

and led to a cooling means

24

via a cooling water conduit

22

. In the course of the cooling water conduit

22

between the settling vessel

16

and the cooling means

24

, a particulate filter

26

is arranged in which impurities and fine particles are filtered out of the cooling water. The cooling water flows on to the cooling means

24

in which the cooling water heated in the screw-type compressor

12

is cooled again. After that, the cooled cooling water passes through a polarization means

27

in which the voltage occurring between two electrodes transforms the natural minerals dissolved in the cooling water into the corresponding hydroxides, thereby avoiding precipitations and incrustations. In the further course of the cooling water conduit

22

, there are arranged a temperature sensor

28

and a conductivity sensor

30

. The cooling water passing the conductivity sensor

30

flows on through the cooling water conduit

22

to the compressor

12

, in which it is injected via nozzles in the area of the rotors of the screw-type compressor

12

on the one hand, and via a bearing conduit

32

into sliding bearings of the screw-type compressor

12

on the other hand. In the course of the bearing conduit

32

, a reflux valve

34

and a current relay

36

are arranged.

Cooling water can be drained from the cooling water conduit

22

via a drain pipe

23

with a drain valve

24

, when too much cooling water circulates in the cooling water circuit

20

.

A pH-value sensor

31

is provided on the settling vessel

16

measuring the pH-value of the cooling water in the settling vessel

16

.

The compression system

10

further comprises a water supply source

40

wherefrom recycled water can be supplied to the cooling water when required. The water supply source

40

serves to provide desalinated and non-desalinated water which is introduced into the coolant circuit

20

via an outlet

42

of the water supply source

40

.

Water from a conduit

46

connected to the drinking water net or cooling water from a branch conduit

48

is optionally fed to the water supply source

40

via a supply conduit

44

. The branch conduit

48

branches between the polarization means

27

and the temperature sensor

28

from the cooling water conduit

22

so that a part of the cooling water can be deviated via the branch conduit

48

bypassing the compressor

12

to the water supply source

40

. A control valve

50

, a reflux valve

34

with a passage towards the water supply source as well as a particulate filter are arranged in the course of the branch conduit

48

. The drinking water conduit

46

comprises a stop valve

52

and also a reflux valve

34

continuous towards the direction of the water supply source

40

.

The drinking water conduit

46

and the cooling water branch conduit

48

discharge into the supply conduit

44

in the course of which a further particulate filter

35

is arranged. The supply conduit

44

finally discharges into a desalinization device

54

, which can be formed as a mixed bed ion exchanger. The desalinization device

54

can also be formed as a reversed osmosis device. In the mixed bed ion exchanger

54

, strongly acid and strongly alkaline exchangers are employed which withdraw almost all the cations and anions from the water to be treated. The water thus deionized, i.e. desalinated, has a conductivity of about 0.1 &mgr;S/cm to 0.2 &mgr;S/cm at 25° C. Thus, the supplied non-deionized water is deionized by the desalinization device

54

, i.e. desalinated to more than 90%. Therefore, the deionized water exiting from an output conduit

55

of the desalinization device

54

has a low salinity and therefore a low conductivity as well as a low pH-value.

A control valve

56

is arranged in the course of the desalinization output conduit

55

, by means of which the output conduit

55

may be opened or closed.

A bypass

58

is provided parallel to the desalinization device

54

, which bypass connects the supply conduit

44

of the water supply source

40

to the output conduit

42

of the water supply source

40

, bypassing the desalinization device

54

. In the course of the bypass

58

, there also is arranged a control valve

60

. The water flowing towards the water supply source

40

through the drinking water supply conduit

44

or the cooling water branch conduit

48

can be led either through the desalinization device

54

or past the desalinization device

54

directly to the output conduit

42

according to the position of the valves

56

,

60

.

The output conduit

42

in turn branches into two branch conduits: firstly, the output conduit

42

is connected to the bearing conduit

32

of the cooling circuit

20

via a connection conduit

62

and discharges upstream of the current relay

36

into the bearing conduit

32

. In the connection conduit

62

, there is also provided a reflux valve

34

the direction of passage thereof has been chosen to be the direction towards the bearing conduit

32

. Secondly, an inlet conduit

64

leads from the output conduit

42

of the water supply source

40

to a water inlet

65

in the air take-in duct

14

. In the course of the inlet conduit

64

, there also is provided a reflux valve

34

as well as a control valve

66

.

A pH-sensor

31

can be used to measure the pH-value in the settling vessel

16

. However, the results obtained by measuring the pH-value are not always reliable and can therefore not be used as the primary control variable.

The membrane filters

26

prevent an unchecked development of germs in the cooling system and in the desalinization device

54

.

All four control valves

50

,

56

,

60

,

66

are controllable, i.e. switchable, by a controlling means

70

via electric control lines

68

. The four sensors

26

,

28

,

30

,

36

of the compressor system

10

are connected to the controlling means

70

via measuring lines

72

, so that the measuring data of the sensors

26

,

28

,

30

,

36

are received and inputted by the controlling means

70

.

During the operation of the compressor

12

, the cooling water circulating in the cooling water circuit

20

is subject to a lot of influences which keep changing the composition and the properties of the cooling water. Chemical, electrochemical as well as physical processes are responsible for this. Thereby, especially the pH-value as well as the salinity of the cooling water are changed. For example, in the summer months and especially in tropical regions, a lot of the humidity in the air taken in precipitates as a condensate, which gets into the cooling water circuit in the compressor

12

and in the settling vessel

16

. Thereby, the cooling water might be diluted with respect to its salinity, so that the relative salinity of the cooling water is reduced. However, when very dry cold air is compressed, evaporated water can be withdrawn from the cooling water and be absorbed and dissipated by the compressed heated air. Thereby, the relative salinity of the water is increased. A salinity which is too high leads to a deposition, i.e. a precipitation of deposits in the cooling water. The deposits can cause damages or defects to sealing gaps, seals, valves etc. of the compressor

12

. If the salinity is too low, the buffer ability of the cooling water with respect to the absorption of carbon dioxide contained in the air taken in is reduced. If the buffer ability of the cooling water is low, not enough carbon dioxide can be absorbed from the compressed air. The free carbon dioxide not buffered in the cooling water in turn lowers the pH-value, i.e., the cooling water acquires aggressive, corrosive properties.

A cooling water which is as neutral as possible what the corrosion is concerned should have a pH-value of about 7. In no case, however, should the pH-value be outside of the range between 6.5 and 7.5, as corrosion can occur even at these values.

Generally, the salinity of water is determined by measuring the conductivity of the water. In the present example, the conductivity of the cooling water should always be between 10 and 20 &mgr;S/cm (at 25° C.). The conductivity is measured by the conductivity sensor

30

and received, evaluated and converted into corresponding control measures and instructions by the controlling means

70

.

The value of conductivity measured by the conductivity sensor

30

is compensated, i.e. normalized, in the controlling means

17

according to the temperature measured by the temperature sensor

28

. This is necessary as the conductivity of the water depends highly on the temperature.

If the upper conductivity limit of 20 &mgr;S/cm is exceeded, the controlling means

70

opens the control valve

56

in the output conduit

55

of the desalinization device

54

. Thereby, non-desalinated water can flow through the supply conduit

44

and the desalinization device

54

and be desalinated. The desalinated, i.e. deionized water flows via the water supply source output conduit

42

and the inlet conduit

64

to the water inlet

65

where the desalinated water is introduced into the air take-in duct

14

. The desalinated water thus introduced gets into the compressor

12

together with the air taken in and is finally reseparated from the compressed air in the settling vessel

16

. By this route, it gets back into the cooling water circuit

20

, where it changes the salinity. This way, desalinated water is introduced into the cooling water circuit

20

until the conductivity measured by the conductivity sensor falls back again below the upper conductivity limit of 20 &mgr;S/cm.

If the value falls below the lower conductivity limit of 10 &mgr;S/cm, the non-desalinated water from the drinking water conduit

46

is not led via the desalinization device

54

but via the bypass

58

directly to the output conduit

42

by opening the control valve

60

. The non-desalinated water is then also introduced into the gas take-in duct

14

via the inlet conduit

64

.

If the value falls below the lower conductivity limit of the water supply source

40

and desalinated water has to be available, either the drinking water from the conduit

46

or the cooling water led through the branch conduit

48

can be desalinated. If the upper conductivity limit is exceeded, i.e., if the salinity in the cooling water increases in an undesired manner, in the case of a normal operation, the cooling water introduced via the branch conduit

48

of the water supply source

40

is desalinated. In this process, little strain is put on the desalinization device

54

, and the periods for its regeneration are prolonged. The inlet of water to the water supply source

40

is controlled by correspondingly controlling the control valves

50

,

52

of the drinking water conduit

46

and the branch conduit

48

.

The pH-value sensor

31

continuously measures the pH-value of the cooling water in the settling vessel

16

. If the pH-value falls below 6.5, the water supply source

40

supplies non-desalinated water to the cooling water circuit, until the pH-value exceeds 6.5 again. However, if the pH-value exceeds 7.5, water desalinated in the desalinization device

54

is supplied to the cooling water circuit

20

. The pH-value is, however, always only corrected to such an extent as to maintain the preset conductivity of the cooling water. This means that maintaining the preset conductivity has priority over controlling the pH-value.

Before the compressor

12

starts working, i.e., before the rotors start rotating, the sliding bearings of the compressor

12

have to be filled with water in order to avoid an abrasion-intensive mixed friction when the compressor starts working. However, the sliding bearings cannot be filled with water from the cooling water circuit

20

, as the cooling water circuit

20

is not yet pressurized.

In order to fill the sliding bearings of the compressor

12

before the compressor

12

starts working, the drinking water supplied by the drinking water conduit

46

is used, which water generally has a pressure of about 3.5 bars. The drinking water is prepared in the water supply source

40

in a known manner such that it has a conductivity in the range from 10 to 20 &mgr;S/cm approximately. It is only after the sliding bearings of the compressor

12

have been filled with water in this manner that the rotors of the compressor

12

can start working.

As the working pressure of 8 to 10 bars quickly sets within the compressor, the prepared water from the drinking water net cannot be fed to the sliding bearings after the compressor has started working, as the pressure thereof of 3.5 bars is too low for this purpose. Therefore, after the compressor

12

has started working, the sliding bearings are lubricated by the cooling water fed to the sliding bearings of the compressor

12

via the bearing conduit

32

. This is possible as the cooling water always provides a pressure of approximately the working pressure of the compressor

12

.

As sliding bearings are very sensitive to a failure of the water lubrication, the flow towards the sliding bearings via the bearing conduit

32

is supervised by the current relay

36

. As soon as the current relay

36

indicates that the flow of the water is too small, the controlling means

70

switches off the motor

13

, so that the rotors of the compressor

12

come to a standstill.

If within a preset timeframe no sufficient change in conductivity occurs when the upper conductivity limit of 20 &mgr;S/cm is exceeded and desalinated water is added, it can be presumed that the capacity of the desalinization device

54

is exhausted. In this case, the controlling means

70

indicates the exhaustion of the desalinization device

40

via a display device (not shown) and, if necessary, switches off the motor

13

of the compressor

12

.

By controlling the salinity of the cooling water via measuring the conductivity, as has been described, a compressor system is provided fulfilling high demands on the quality of the cooling water irrespective of climatic conditions, the quality of the drinking water, the place of installation etc. Thereby, compressors with very narrow sealing gaps can be put into practice. Furthermore, a corrosion of the compressor system is avoided by indirectly controlling the pH-value of the cooling water. Therefore, less noble and less expensive materials can be used while increasing the service life of the system at the same time.

Although a preferred embodiment of the invention has been specifically illustrated and described herein, it is to be understood that minor variations may be made in the apparatus without departing from the spirit and scope of the invention, as defined the appended claims.