EP0607101A1

EP0607101A1 - A lubrication oil returning system for refrigeration compressors

Info

Publication number: EP0607101A1
Application number: EP94610002A
Authority: EP
Inventors: Johansen Bent
Original assignee: BIRTON AS
Current assignee: BIRTON AS
Priority date: 1993-01-14
Filing date: 1994-01-14
Publication date: 1994-07-20
Also published as: DK4093A; DK4093D0

Abstract

Compressors 1, 2, 3, and 4 run in parallel have common suction line, pressure line, and oil separator 9. Dependent on the requirement for cooling only the required number of refrigerating compressors are made to operate. The oil from the oil separator is sucked into the sumps of the compressors, and the compressor(s) which is/are running suck required oil through oil level equalization pipes E1, E2, E3, E4, and E from the sumps of the other compressors. When the oil level in these has sunk to the level where the pipe ends are exposed, no more oil may be removed from the non-operative compressors, and these will have the amount necessary available for starting operation when required. This is obtained without individual float valves on the sumps by ensuring a lower pressure in the sump of the operative compressors than in the suction line. This difference in pressure is obtained by means oil a difference pressure element which may be an orifice, a calibrated pipe, or a throttle valve.

Description

The invention concerns an apparatus for the control of the oil level in refrigerating compressors run in parallel and comprising an oil separator.
Refrigerating compressors run in parallel normally comprise a number of refrigerating compressors mounted on a common chassis and are put in operation in a number commensurate with the instant cooling capacity requirement. Thus they are to be regarded as a capacity controlled refrigerating machine. The compressors are switched on by an automatic control device, and the starting sequence of the compressors is changed according to a predetermined plan in order that all compressors will have the same total running time, whereby a uniform wear will be achieved.
The refrigerating compressors may be of the type having the shaft passing through a stuffing box for belt drive or direct coupling to a motor, or in semi-hermetic or hermetic construction with a built-in motor. The refrigerating compressors have a certain amount of lubricating oil in the sump/motor casing or in the crank case which must not fall below a certain level on order to ensure correct lubrication and/or oil pressure.
Since the refrigerating compressor will always have a certain oil consumption during running, which is carried via the refrigerant from the compressors into the cooling circuit the cooling plant is always constructed in order to conduct the oil back to the crank case or sump of the refrigerating compressors by means of suitable oil separating means.
In refrigerating compressors run in parallel it will frequently occur that the oil is distributed unfavourably so that some compressors are over-filled with oil while others may lack oil. In order to prevent this refrigerating compressors run in parallel are supplied with an installation for distributing the oil to the individual compressors. The distributing installation may consist of an oil separator which carries the oil separated out of the refrigerant to an oil reservoir and returns it via a float controlled valve fitted to the sump to that particular compressor which lacks oil and which therefore has opened the float valve. It will be realized that this construction is complicated and hence expensive, because it requires not only an oil reservoir but also an individual float valve for each compressor. Another construction joins the sumps of the compressors with large-diameter pipes whereby oil level equalizing will occur, but it depends on all compressors being at the same level in the chassis. There is, so to speak, a common sump.
There is in consequence a need for an apparatus for controlling the oil level in refrigerating compressors run in parallel in which the said disadvantages are avoided and which is furthermore suitable for expansion of an existing plant of compressors with further compressors. This is obtained in an apparatus according to the invention which is particular in that the return of the oil takes place to the suction side of the refrigerating compressors run in parallel which is connected via a small pressure drop to the crank case/sump of each compressor, and in that a small diameter tube is connected between pipe stubs placed in the sump of each compressor at a level corresponding to the desired oil level.
In a further advantageous embodiment the difference pressure element is an orifice of reduced diameter.
In a further advantageous embodiment the difference pressure element is a calibrated tube.
In a further advantageous embodiment the difference pressure element is an adjustable valve whereby the pressure drop may be easily adjusted when the plant is extended.
According to the invention it is unnecessary to use an oil reservoir as no floats are used for controlling the return of oil. It is only required that the plant is constructed with an oil separator with oil return to the suction side of the refrigerating plant which is connected via a small pressure drop to the crank case/sump of each compressor, and in that a small diameter tube is connected between pipe stubs placed in the sump of each compressor at a level corresponding to the desired oil level. Since the pressure in the suction side of each compressor is communicating freely with the crank case/sump of the compressor, and since there is a pressure difference between the suction side of the suction side of the plant and the crank case because of the pressure difference element, those compressors which are running will suck oil through the small pipes which are connected. When the oil level in the compressors which are not running has been lowered to the level at which the pipe stub is fitted, the oil level will sink no further, and there will only be sucked refrigerant vapours through the pipe of small diameter. Thus it is obtained that the oil is distributed evenly in all compressors whithout overfilling and completely without any mechanical level controls. By this system it is not required that the compressors be at the same level on the chassis.
The invention will be explained in greater detail in the following with reference to the drawing which schematically shows a plant of refrigerating compressors run in parallel with an oil level equalization according to the invention.
The drawing shows compressors 1, 2, 3, and 4 which are connected to suction lines 5, 6 and pressure lines 7, respectively. The compressors are fitted into a rack which is not shown, and the compressors 1 and 2 are physically disposed above the compressors 3 and 4. During operation of the cooling plant one or several of the compressors such refrigerant vapours from the cooling plant through the suction lines 5, 6 and compress the vapours and send them along in a gaseous state to the pressure lines 7, 8 to a condensor, not shown, in which the refrigerant is condensed to the liquid state. As mentioned in the introduction lubrication occurs in that there is an amount of oil in the sump or crankcase of each compressor S, as shown at compressor 2. During operation part of the lubricating oil is drawn along through the pressure lines and it is separated out again in an oil separator 9. The oil is deposited at the bottom and opens a float valve when a certain level has been reached. It thereby opens to an oil return pipe 11 which is connected to the suction side of the compressors. Since there is a free passage between the suction side and the crankcase of the compressor the oil will return for lubricating purposes little by little. However, there may be a large distance between the inlet end of the oil return pipe 11 and the suction side of e.g. compressor 4. In order to ensure that there is a reasonably equal distribution of oil between the various compressors their crankcases S are connected by pipes E, E1, E2, E3, E4 of narrow gauge, wherethrough the oil may run.
During the control of the compressor plant it is possible that only compressor 2 is running and that it is lacking oil. Since the pressure in the suction lines 5, 6 is the same everywhere and furthermore equal to the pressure in the crankcases, the compressor 2 will not be supplied with oil through the pipe E2 unless the pressure in the crankcase is higher to an amount corresponding to the head of an oil column in the pipe E. In order to obtain this, there is fitted a difference pressure element C into the suction line V2 belonging to the compressor 2 which ensures that there is a slightly lower pressure in the crankcase S than in the suction line V2 and hence in the whole of the pipe system 5, 6 and in the other crankcases. When the oil level has sunk to the level in which the pipe stubs E1, E3, and E4 are fitted, in the other compressors 1, 3, and 4 which are not in operation, the oil level will not sink further and only refrigerant vapours will be sucked through the pipes E, E1, E2, E3, and E4. This is a negligible amount which will participate in the refrigerant circulation.
When fitting several compressors in a rack at different levels the pressure drop in the difference pressure element must be adjusted to the suction head of the oil to each individual compressor. In order to check the oil level it is advantageous to fit a sight glass in the sump of each compressor at the same level as the pipe stub for the oil return pipe.

Claims

An apparatus for the control of the oil level in refrigerating compressors run in parallel and comprising an oil separator,
characterized in that the return of the oil takes place to the suction side of the refrigerating compressors run in parallel which is connected via a small pressure drop to the crank case/sump of each compressor, and in that a small diameter tube is connected between pipe stubs placed in the sump of each compressor at a level corresponding to the desired oil level.
An oil level control apparatus according to claim 1,
characterized in that the difference pressure element is an orifice of reduced diameter.
An oil level control apparatus according to claim 1,
characterized in that the difference pressure element is a calibrated tube.
An oil level control apparatus according to claim 1,
characterized in that the difference pressure element is an adjustable valve.