GB2111662A

GB2111662A - Heat transfer apparatus comprising a refrigerant circuit

Info

Publication number: GB2111662A
Application number: GB08235603A
Authority: GB
Inventors: Ludek Kucera; August Vogeli
Original assignee: Sulzer AG; Gebrueder Sulzer AG
Current assignee: Sulzer AG
Priority date: 1981-12-17
Filing date: 1982-12-14
Publication date: 1983-07-06
Also published as: SE8207121D0; JPS58107888A; DK490882A; IT8224707A0; SE8207121L; ES8400190A1; ZA827971B; FR2518662A1; IT1156133B; DE3238241A1; ES516979A0

Abstract

The apparatus for the oil supply to a screw compressor (3) comprises a closed circuit for the oil. In this circuit, the delivery line (16) of the compressor (3) is connected to an oil separator (4) in which refrigerant dissolves in the oil. The resulting oil/refrigerant solution is divided into two sub-flows, one sub-flow being cooled in a cooler (8) and fed to the bearings and to the shaft seal (12) of the compressor (3). The second uncooled sub-flow is injected into the compression chamber of the compressor (3) via a line (14). To control the outlet temperature of the refrigerant/oil mixture leaving the compressor (3), a temperature sensor (17) feeds a signal to a final control element (21) which so controls a three-way valve (20) connected to receive either of the sub-flows through lines (19) and (14) that an appropriate quantity of the cooled oil/refrigerant solution is injected into the compression chamber in dependence on the outlet temperature of the refrigerant/oil mixture. By means of these steps, the viscosity of the oil/refrigerant solution injected into the bearings of the compressor (3) does not fall below a required value. <IMAGE>

Description

SPECIFICATION Heat transfer apparatus comprising a refrigerant circuit This invention relates to heat transfer apparatus comprising a refrigerant circuit containing a screw compressor and an oil circuit interleaved with the refrigerant circuit. Such apparatus may be used for the purpose of refrigeration or heating, i.e. as a heat pump, and the invention is particularly but not exclusively applicable to heat pump apparatus.

In screw compressors, which may be constructed with one or two shafts, it is known to inject oil under pressure into the compressor in order to lubricate the bearings, rotor or rotors, shaft seals and the transmission gear if provided, to seal off the gaps between the rotors in the case of a two-shaft construction, and between the rotor or rotors and and the housing, and dissipate some of the compression heat.

The oil is separated from the compressed refrigerant/oil mixture in an oil separator and cooled in a cooler and re-injected into the compressor.

For conventional screw compressor applications in heat transfer apparatus, particularly refrigeration plants but also possibly heat pump plants, in which relatively low condensation temperatures are required, e.g. low-temperature heating installation, this type of oil supply does not involve any particular difficulty.

Since in these cases the viscosity of the oil separated in the oil separator remains in an acceptable range, the lubricating oil films can form satisfactorily in the compressor.

Figure 1 diagrammatically illustrate a construction of a refrigerating plant of this kind, which can also be operated as a heat pump under the conditions indicated.

The plant comprises a refrigerant circuit containing a screw compressor 3 driven by an electric motor 1 via a coupling 2, an oil separator 4, a condenser 5, and an evaporator 6. Ammonia may be used as the refrigerant, for example.

The refrigerant vapour which is compressed in the compressor3 and which contains oil as a constituent, is introduced into the oil separator 4. The oil/refrigerant solution forming in the oil separator is taken out of the separator and fed by a pressure-raising pump 7 through a cooler 8 and then injected via branch lines 9, 10a, 10b and 11 into the compression chamber. and fed into the bearings (not shown) and the shaft seal 12 of the compressor.

In this case water, for example, may be used as a coolant for the cooler 8 and the condenser 5, the coolant being fed in the direction of the arrow first through the cooler 8 and then through the condenser 5. The heating medium for the evaporator 6 may, for example, be water, brine or air.

If, however, screw compressors are used in heat pump installations, e.g. for high-temperature heating installations, high condensation temperatures are usually required, e.g. of the order of 65-120"C.

Because of the high condensation temperature in the refrigerant circuit of the plant, the compressor must provide a relatively high final pressure. Consequently, refrigerant concentrates considerably in the lubricating oil, which is at the final pressure in the oil separator, particularly if the refrigerant consists of fluoro-chloro-hydrocarbons (e.g. R 12 and R 22), hydrocarbons or hydrocarbon mixtures. The increased refrigerant content in the oil may reduce the viscosity of the oil/refrigerant solution to such an extent that lubricating oil films can no longer reliably form in the compressor.

The solubility of the refrigerant in the oil depends upon the type of refrigerant, type of oil, compressor final pressure, and outlet temperture of the refrigerantloil mixture after the compressor.

The higher the pressure in the oil separator, the greater the solubility of the refrigerant in the oil, and the higher the temperature the lower the solubility.

The invention has as an object a system for the oil supply to a screw compressor, whereby the viscosity is kept under control by reducing the refrigerant content in the oil.

According to the present invention, heat transfer apparatus comprises: a refrigerant circuit containing a screw compressor and an oil/refrigerant separator; and an oil circuit comprising the screw compressor, the separator and means for returning the oil/ refrigerant solution forming in the separator to the compressor, the returning means comprising a first path directing one part of the returned solution into the compression chamber of the compressor at least without substantial cooling and a second path directing a second part of the returned solution through cooling means and then into the compressor bearings.

With this arrangement, the cooled part of the returned solution used for the bearings has a viscosity which ensures that the oil films form to the required degree, while on the other hand the injection of the uncooled part of the returning solution directly into the compression chamber keeps the outlet temperature appropriately high so that the minimum amount of refrigerant is dissolved in the oil.

The compressed refrigerant/oil mixture outlet temperature varies according to the compressor compression ratio, which depends (a) on the temperature of the heat-transfer medium in the refrigerant circuit evaporator and (b) the temperature to be produced in the heat-absorbing medium in the condenser.

On a change of compression ratio the amount of oiürefrigerant solution injected into the compressor also varies.

If, for example, the compression ratio is reduced, e.g. if the pressure in the evaporator rises and the pressure in the condenser remains constant, or if the pressure in the evaporator remains constant and the pressure in the condenser drops, the amount of oil/refrigerant solution injected also decreases. In the optimum case, with a reducing compression ratio, the amount of oil/refrigerant solution injected is reduced to such an extent that the outlet temperature remains constant or varies within a range that the viscosity of the cooled part of the returned solution is not critically affected.

There may, however, be operating conditions in which a change of the compression ratio results in such a change in the amount of oil injected that the outlet temperature falls to such an extent that the viscosity of the oil/refrigerant solution becomes inadequate, and there may also be cases in which the outlet temperature rises above a value resulting, for example, in carbonization or some other deterioration of the properties of the lubricating oil.

In order in such cases to ensure reliable operation of the plant in respect of the screw compressor oil supply, the plant preferably includes a sensor for the outlet temperature of the refrigerant/oil mixture from the compressor and means for controlling the rate of flow in at least one of the first and second paths in dependence on the sensed temperature.

Alternatively, the apparatus may include a sensor for the outlet temperature of the refrigerant/oil mixture from the compressor, the sensor controlling a three-way valve connected to two feed lines for the oil/refrigerant solution, one feed line branching off before, and the other after, the cooler, the outlet line from the three-way valve being connected to the compression chamber.

This control of the outlet temperature may also be used if inadmissible conditions in respect of the screw compressor oil supply also occur at part-load operation.

The invention may be carried into practice in various ways but three heat pump plants embodying the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 2 is a flow - diagram of an oil supply system for a screw compressor in a refrigerant circuit of a heat pump plant in which the outlet temperature is controlled by controlling the flow of the uncooled part of the return flow; Figure 3 is a similar illustration of a second plant in which the outlet temperature is controlled by controlling the temperature of the oil/refrigerant solution injected into the compression chamber; and Figure 4 is a flow diagram of a third plant in which the outlet temperature is controlled by controlling the flow of oil/refrigerant solution injected into the compression chamber.

Those parts of the plant shown in Figure 1 which correspond in Figures 2 to 4 have been given like references.

The refrigerant used is, for example, a fluoro chloro - hydrocarbon, e.g. R12 or R22.

Referring to Figure 2, the flow path 13 of the oil and refrigerant solution formed in the oil separator is divided into two parts or sub -flows downstream of the pump 7 and upstream of the cooler 8, a first sub - flow being cooled 8 and being fed via the branch lines 10a, 10b and 11 to the bearings (not shown) and shaft seal 12 of the compressor. The second uncooled sub -flow is injected into the compression chamber of the compressor via a line 14 cooling a control valve 15.

The delivery line 16 of the compressor 3 contains a temperature sensor 17 acting on a control valve 15 via a final control element 18.

As already described hereinbefore, the outlet temperature of the compressed refrigerant/oil mixture affects the solubility of the refrigerant in the oil in the oil separator 4 and hence the viscosity of the lubricant. On the other hand, the outlet temperature must not exceed a top critical value.

If the outlet temperature deviates from the desired value due to varying operating conditions, the temperature sensor 17 opens or closes the control valve 15 the appropriate amount by means of a signal fed to the final control element 18.

While in the above described plant the outlet temperature is controlled via a flow control system, the outlet temperature in the plant shown in Figure 3 is controlled by controlling the temperature of the sub -flow injected into the compression chamber. In this case, in the event of an outlet temperature deviation from the set - value, a signal fed by the temperature sensor 17 to the final control element 21 adjusts a three - way valve 20 connected to two feed lines for the oil and refrigerant solution, one line 14 branching off upstream of the cooler 8 while the other line 19 branches from a junction line 22 downstream of the cooler 8. The outlet line 14a from the three - way valve is connected to the compression chamber of the compressor.

In the plant shown in Figure 4, the outlet temperature is controlled by controlling the rate of flow of some of the uncooled sub - flow, while the throughput of uncooled oil/refrigerant solution injected into the compression chamber via line 14 remains constant for a given state of operation.

In the event of the outlet temperature deviating from the set- value, a temperature sensor 17 by way of a final control element 23 controls a two - way valve 24, the feed line 25 of which is connected to the junction line 22 and the ouput line 26 of which is taken to the compression chamber. The two - way valve is in an intermediate position at the required outlet temperature so that a partial quantity of cooled oil/refrigerant solution is constantly admixed with the uncooled oil/refrigerant solution. As soon as the outlet temperature exceeds the set - value, the amount of cooled oil/refrigerant solution injected into the compression chamber is increased while, if the outlet temperature set - value falls, the amount of cooled oil/refrigerant solution is throttled accordingly. In this case the uncooled and cooled oil/ refrigerant solutions are mixed inside the compression chamber.

It should be pointed out that, in each of the plants shown in Figures 2 to 4, the uncooled sub - flow can be injected into the compression chamber at more than one place.

It may also be advantageous to feed uncooled oil/refrigerant solution to the shaft seal, instead of cooled solution.

Claims

1. Heattransfer apparatus comprising: a refrigerant circuit containing a screw compressor and an oil/refrigerant separator; and an oil circuit comprising the screw compressor, the separator and means for returning the oil/refrigerant solution form.

ing in the separator to the compressor, the returning means comprising a first path directing one part of the returned solution into the compression chamber of the compressor at least without substantial cooling and a second path directing a second part of the returned solution through cooling means and then into the compressor bearings.

2. Apparatus as claimed in Claim 1 in which the plant is arranged to be operated as a heat pump.

3. Apparatus as claimed in Claim 1 or Claim 2 which includes a sensor for the outlet temperature of the refrigerant/oil mixture from the compressor and means for controlling the rate of flow in at least one of the first and second paths in dependence on the sensed temperature.

4. Apparatus as claimed in Claim 1 or Claim 2 which includes a sensor for the outlet temperature of the refrigerant/oil mixture from the compressor, the sensor controlling a three-way valve connected to two feed lines for the oil/refrigerant solution, one feed line branching off before, and the other after, the cooler, the outlet line from the three - way valve being connected to the compression chamber.

5. Heat transfer apparatus constructed and arranged to operate substantially as described herein with reference to Figure 2 or Figure 3 or Figure 4 of the accompanying drawings.

6. Apparatus for the oil supply to a screw compressor for a refrigerating plant, the oil circulating in a closed circuit and being separated from the compressed refrigerant and oil mixture in an oil separator and a partial quantity of the refrigerant from the separated oil being dissolved in the oil in dependence upon pressure and temperature, and compression heat absorbed by the oil then being dissipated in a cooler and the oil/refrigerant solution being injected under pressure into the screw compressor, the flow path of the oil/refrigerant solution forming in the oil separator being divided into two sub - flows upstream of the cooler, a first sub - flow being injected into the compression chamber in the uncooled state at at least one place, while compression heat is dissipated from the second sub - flow in the cooler and the cooled sub - flow is injected at least into the compressor bearings.