EP0758054A1 - Oil circulation system for screw compressors - Google Patents

Abstract

In an oil separator (3) under outlet pressure, a delivery flow to the screw compressor (1) is divided into a gas flow (31) and an oil flow (32). The oil flow reaches the screw compressor via a throttle position (25) and an oil inlet (10). Before the throttle position via a branch (11) a part oil flow (35) is fed for lubrication of bearings and/or shaft seals. The branch passes through an evapn. container (24), which on the gas side is connected via a conduit (23) with an intermediate pressure connection (14) on the compression path of the compressor, in order to feed the part flow with a pressure corresp. to the intermediate pressure to a lubrication connection (15) on the screw compressor.

Description

The invention relates to a screw compressor device for refrigerants with refrigerant-soluble oils, preferably for ammonia with polyalkylene glycol soluble therein, a feed stream downstream of a screw compressor being divided into a gas stream and an oil stream in an oil separator under outlet pressure, and the oil stream via a throttle point and an oil inlet gets into the screw compressor.

In screw compressors, two helical rotors, the main and secondary rotor, roll in one housing. During the suction process, the tooth gap between the rotors increases during the rotation, gas is sucked in. As the rotors continue to rotate, this tooth gap closes when the intake control edge is passed over. When turning further, the counter wheel engages in the gap and continuously reduces the enclosed gas space, the gas is compressed until finally the exit control edge is reached and the compressed gas is pushed out.

A relatively large amount of oil is injected into the gas flow to be pumped in the compression chamber in order to achieve a better seal and thereby an improvement in the degree of delivery and to dissipate part of the heat of compression with the oil. This oil must be separated from the gas flow on the pressure side of the compressor by an oil separator, because otherwise it would undesirably burden the refrigeration cycle.

1 shows such a known device:

From the screw compressor 1, a gas stream 30 interspersed with oil particles is conveyed via a pressure line 2 to an oil separator 3. From there, the deoiled gas stream 3l is led to a condenser in the sense of the refrigeration cycle. The oil separated in the oil separator 3 passes via a line 4 to a water- or air-cooled oil cooler 5, in which the heat of compression is removed. From there, the oil is fed back to the compressor 1 via a line 6, an oil filter 7, a check valve 8, a solenoid valve 9 and an oil inlet 10, the pressure difference between the pressure and suction sides on the compressor being generally used for the oil delivery. A part of this returned oil is used for the lubrication of the bearings and for so-called "open" compressors, in which the drive shaft is led outwards, for the lubrication and cooling of the shaft seal. The rotating shaft seal is used to seal the compressor drive shaft against the atmosphere.

For example, NH3 refrigeration systems used to be operated almost exclusively with so-called flooded evaporators and with NH3-insoluble mineral oils. This Mineral oils could not accumulate with NH3 due to their insolubility, so that largely pure oil was available again for the lubrication of the bearings and the shaft seal. With NH3 screw compressors, supplying the shaft seal and the drive-side bearing on the suction side with lubricating oil has become problematic since such screw compressor refrigeration systems have recently been operated increasingly with NH3-soluble oil, a polyalkylene glycol, also called PAG oil for short. These NH3-soluble oils are a prerequisite for a so-called dry expansion evaporation, with which the NH3 filling quantity in the refrigeration cycle can be significantly reduced compared to flooded operation. Due to the applicable accident prevention regulations, modern refrigeration technology makes great efforts to manufacture NH3 refrigeration systems with the smallest possible fill quantities.

The NH3-soluble oils accumulate due to the dissolution behavior according to the pressure and temperature conditions given in the oil separator with a certain amount of NH3, at a normal operating point, for example with approx. 6% NH3 in the oil. When supplying oil to the shaft seal and the drive-side bearing, the oil is released to suction pressure. The NH3 absorption capacity of the oil decreases, for example to about 3% NH3 in the oil at a normal operating point, so that the difference of approximately 3% NH3 inevitably evaporates from the oil. Due to the very large steam volume of NH3, this evaporation process creates a large volume of oil foam (at a normal operating point approx. 11 times the volume compared to pure oil), the lubricating effect of which is much lower than that of pure oil. As a result, this often leads to very rapid wear on the shaft seal and sometimes also on the drive end Bearing due to insufficient lubrication. Facilities are known to help to a limited extent to remedy this.

FIG. 2 shows a known device for refrigerant-soluble oils on screw compressors, which relaxes the oil provided for lubrication and sealing via a throttle point 25 and passes through an evaporation tank 21, the steam chamber of which is connected via a line 12 to the suction side 29 of the screw compressor 1. As a result, the oil is "degassed" and can be supplied to the shaft seal and the drive-side bearing with a better lubricating effect, the necessary pressure difference being generated by an oil pump 13. This arrangement has the disadvantage that the screw compressor depends on the functionality of an oil pump during its operation. Another disadvantage is that with the start of such a screw compressor device, the oil foams up behind the throttle point 25 and reaches the lubrication points in part as foam.

The object of the invention is to improve these circumstances.

This object is achieved in that a partial oil flow for the lubrication of bearings and / or shaft seals is branched off in front of the throttle point, which is led through an evaporation tank which is connected on the gas side via a line to an intermediate pressure connection on the compression path of the screw compressor to supply the partial oil flow with a pressure corresponding to the intermediate pressure to a lubrication connection on the screw compressor.

This arrangement has the advantage that the lubrication pressure does not collapse in the event of a power failure during the run-down of the screw compressor, since the pressure difference between final pressure and intermediate pressure slowly, if at all, then slowly decreases after the screw compressor has come to a standstill. Furthermore, no oil pump is necessary. In addition, the steam discharged from the lubricating oil is fed in at intermediate pressure, so that it does not have to be compressed from suction pressure to intermediate pressure, thus improving the cooling capacity of the cycle.

Further advantageous developments of the invention are shown in the dependent claims 2 to 8. It has proven to be advantageous, in addition to the intermediate pressure, to adjust the partial oil flow with a throttle or control element. In addition, it makes sense to heat the oil in the evaporation tank in order to expel more gas at a higher temperature with less gas solubility. The reduction in the gas content more than compensates for the reduced viscosity in order to obtain useful lubrication. In addition, the partial oil flow between the evaporation tank and the lubrication points could still be cooled in order to obtain a higher viscosity and, due to the higher solubility, hardly any gas could be expelled at the lubrication points even when the pressure was reduced. Furthermore, for a start and further operation, it makes sense to arrange the evaporation tank above the screw compressor and to prevent the evaporation tank and the supply line from being emptied at a standstill with shut-off valves, so that at least the geodetic gradient is available. If the pressure difference between the final pressure and the intermediate pressure in the starting phase is too small, an oil pump can also be provided after the evaporation tank, which is switched off after the start, ie if the pressure difference is sufficient.

Many of the screw compressor designs have the option of additionally sucking in a secondary gas flow into the already partially compressed gas. An opening in the housing is arranged so that an intermediate pressure of suction and final pressure is reached in the tooth gap at this point. In the case of a two-stage expansion, the gas of the first expansion stage is drawn into the compression space via this opening. This "economizer circuit" improves the efficiency of the refrigeration system. An advantage of the above device is that the screw compressors, which have such an economizer connection, do not have to be changed in construction in order to make use of the device.

Fig.3

schematisch eine erfindungsgemässe Einrichtung, bei der ein Teilölstrom nach dem Durchlaufen eines Oelkühlers entnommen wird;

Fig.4

schematisch eine erfindungsgemässe Einrichtung, bei der ein Teilölstrom ungekühlt vor dem Oelkühler entnommen wird; und

Fig.5

schematisch eine Einrichtung analog zu Figur 3, bei der eine Hilfspumpe die Schmierung beim Start des Schraubenverdichters unterstützt.

The invention is described below using exemplary embodiments. Show it:

Fig. 3

schematically a device according to the invention, in which a partial oil flow is removed after passing through an oil cooler;

Fig. 4

schematically a device according to the invention, in which a partial oil flow is taken uncooled before the oil cooler; and

Fig. 5

schematically a device similar to Figure 3, in which an auxiliary pump supports the lubrication when starting the screw compressor.

The figures show an improvement in the lubrication of bearings and shaft seals on screw compressors for refrigerants with refrigerant-soluble oils. A typical application arises, for example, for ammonia with polyalkylene glycol soluble therein. Because the oil flow returned to the screw compressor Before the pressure is reduced to approximately the compressor suction pressure, a partial oil flow at an intermediate pressure is removed and passed through an evaporation tank, substantial portions of dissolved refrigerant can be fed in at an intermediate pressure connection on the compression path of the screw compressor. The lubricating properties of the remaining partial oil flow at the lubrication connection are correspondingly more favorable, and the disadvantages of pressure collapse or foam delivery are eliminated with an oil pump, since no oil pump is necessary with the appropriate intermediate pressure.

In FIG. 3, a screw compressor 1 conveys ammonia in gaseous form from a suction line 29 and compresses it, polyalkylene glycol being injected at an oil inlet 10 in order to improve the sealing effect between the compression chambers. A throttle point 25 is symbolically shown for the resistance of the nozzles or orifices during injection. The flow 30 emerging from the screw compressor 1 against a final pressure is fed via a pressure line to an oil separator 3, which has a gas space 3a, from which a gas flow 31 is fed to a condenser (not shown), while there is an oil supply 3b at the bottom of the oil separator 3 which an oil flow 32 is passed through a line 4 through an oil cooler 5. The cooled oil flow 32 arrives in a line 6 via an oil filter 7, a check valve 8 and a solenoid valve 9 to the throttle point 25 and the oil inlet 10. Before the throttle point 25, a partial oil flow 35 is branched off in a branch 11 for the lubrication of bearings and shaft seals and led into an evaporation tank, which is under the pressure of an intermediate pressure connection 14 on the compression path of the screw compressor 1. The pressure at the inlet into the branch 11 must therefore be somewhat higher than the pressure in the Intermediate pressure port 14 to restrict the partial oil flow 35 with an orifice 26. In the evaporation tank 24, ammonia is outgassed by the dwell time in the tank and by vacuum zones at the edges of the orifice 26, which ammonia is fed in via a line 23 at the intermediate pressure connection 14. The outgassing can be supported by a heating device 19, as shown in FIGS. 4 and 5. The degassed partial oil flow passes through a line 15 and a solenoid valve 17 to a lubrication connection 16 and after passing through bearings and shaft seals in a suction chamber 29 at the compressor inlet back into the gas flow. A so-called economizer connection on the screw compressor 1 is used as the intermediate pressure connection. This "economizer" connection is available on every modern screw compressor and opens into the screw compressor at a point on the compression path at which the suction chamber is already closed by the screw profiles. The amount of gas supplied at this point therefore no longer strains the gas volume drawn in and is therefore largely neutral in terms of performance. In addition, there is an approximately 1.5 to 2 bar higher pressure at the economizer connection 14 than at the shaft seal of the compressor which is under suction pressure, so that this pressure difference can be used for oil delivery and an oil pump is therefore unnecessary in most cases.

The evaporation tank 24 is arranged above the screw compressor 1 and the solenoid valve 17 is closed at a standstill in order to have a supply of oil under gravity at the start.

In FIG. 5, only the circuit of the partial oil flow 35 has been expanded by further components compared to FIG. 3. In the branch 11 there is a solenoid valve 18 is provided, which prevents oil from flowing back from the higher areas when it is at a standstill, and the partial oil flow 35 is limited by a control valve 20 which, for example, keeps the oil level in the evaporation tank 24 constant. A heating device 19 promotes the degassing of refrigerant. After the solenoid valve 17, the line 15 bifurcates into a line 15a, in which an oil pump 22 with a check valve 27 is installed as a starting aid, and into a line 15b with a check valve 28, in order to convey past the shutdown oil pump 22 into the lubrication connection 16 . Such a booster pump 22 could always run when the intermediate pressure is insufficient for lubrication. The control valve 20 would track the partial oil flow 35 according to the pumping capacity of the pump 22.

FIG. 4 shows, compared to FIGS. 3 and 5, an arrangement in which the branch 11 for the partial oil flow 35 goes away in front of the oil cooler 5. The partial oil flow reaches the evaporation tank 24 much hotter via a solenoid valve 18 and a control valve 20. A heating device 19 attached to the evaporation tank 24 will therefore only be necessary in exceptional cases. The oil also reaches the lubrication connection 16 via a line 15 and a solenoid valve 17, the solenoid valve 17 holding the oil supply in the higher-level evaporation tank 21 at a standstill.

Claims (11)

Screw compressor device for refrigerants with refrigerant-soluble oils, preferably for ammonia with polyalkylene glycol soluble therein, a feed stream (30) downstream of a screw compressor (1) in an oil separator (3) under outlet pressure into a gas stream (31) and into an oil stream (32) is divided and the oil flow (32) reaches the screw compressor (1) via a throttle point (25) and an oil inlet (10),
characterized in that a partial oil flow (35) for the lubrication of bearings and / or shaft seals is branched off in front of the throttle point (25) and is guided through an evaporation tank (24) which is connected on the gas side via a line (23 ) is connected to an intermediate pressure connection (14) on the compression path of the screw compressor (1) in order to supply the partial oil flow (35) to a lubrication connection (16) on the screw compressor at a pressure corresponding to the intermediate pressure. Screw compressor device according to claim 1, characterized in that the evaporation tank (24) is arranged geodetically above the lubrication points on the screw compressor (1) in order to take advantage of a gradient for the lubrication pressure. Screw compressor device according to claim 2, characterized in that between the evaporation tank (24) and the lubrication connection (16) A shut-off valve (17) is arranged to prevent oil from escaping from the evaporation tank (24) when it is at a standstill. Screw compressor device according to one of Claims 1 to 3, characterized in that a shut-off valve (18) is arranged in the branch (11) in order to prevent oil from running back when it is at a standstill. Screw compressor device according to one of claims 1 to 4, characterized in that a heating device (19) is attached to the evaporation tank (24) to support the evaporation capacity, which prevents the oil from being enriched with refrigerant when it is at a standstill. Screw compressor device according to one of claims 1 to 5, characterized in that a throttle element (26) is provided in the branch (11) in order to restrict the partial oil flow (35). Screw compressor device according to claim 6, characterized in that a control valve (20) is provided in the branch (11) as a throttle member in order to regulate the partial oil flow (35) in the branch (11) according to a predetermined setpoint. Screw compressor device according to one of claims 1 to 7, characterized in that two parallel connecting lines (15a, 15b) are provided between the evaporation tank (24) and the lubrication connection (16), one of which (15a) is a feed pump (22) with a check valve ( 27) and the other (15b) have a check valve (28) in order to start the Screw compressor (1) to support the delivery of oil into the lubrication connection (16). Screw compressor with a device according to one of claims 1 to 8, characterized in that it has, as an intermediate connection (14), an economizer connection, as is provided for an intermediate feed of a partial gas flow with multiple expansion of the gas flow (31). Screw compressor with a device according to one of claims 1 to 8, characterized in that the intermediate pressure connection (14) has no connection to the suction space over the entire performance range, in particular even under extreme partial load, in order to maintain an intermediate pressure necessary for lubrication. Screw compressor with a device according to one of claims 1 to 8, which screw compressor has in particular a shaft seal and has a drive-side bearing on the suction side.

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