System and method for monitoring and controlling oil return to compressor

A system and method for monitoring and controlling oil return to a compressor, characterized by monitoring and controlling oil level, oil temperature, and compressor outlet pressure, so as to determine whether the oil level is lower than a predetermined level threshold, whether the temperature in the lubricating oil box is lower than a predetermined temperature threshold, and whether the compressor outlet pressure exceeds a predetermined pressure threshold, so as to control the flow of lubricating oil returned to the oil box by controlling the valve opening of the oil return valve according to a non-segmentation principle or a segmentation principle, ensure sufficient lubricating oil in the oil box, and enhance efficiency of the system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technology for monitoring and controlling oil return to a compressor, and more specifically, to a method and a system for monitoring and controlling quantity of a lubricating oil returned to an oil box of the compressor by controlling valve opening of an oil return valve at an oil return inlet of the oil box.

2. Description of the Related Art

Chillers are familiar refrigerating equipment in a central air conditioning system. Ice water produced by chillers efficiently lowers indoor temperature by heat exchange and via pipelines. In recent years, chillers are getting popular. A compressor is central to a chiller as far as the operation thereof is concerned. The compressor is a special air pump. The compressor in operation has to be continuously lubricated with lubricating oil in order to minimize friction. During a compression process of the compressor, a copious discharge of coolant (i.e., refrigerant) usually accompanies a loss of a trace of lubricating oil (known as “escaping oil”), which is inevitable. More badly, when discharged from the compressor, lubricating oil does not return to the oil box of the compressor in the end. As a result, the compressor tends to run out of lubricating oil. A shortage of lubricating oil causes friction to the detriment of bearings and then the compressor itself, thus damaging the chiller. Hence, the control of the lubricating oil return to the oil box is of vital importance to a central air conditioning system.

To return the otherwise discharged lubricating oil to the oil box, manufacturers usually put an oil return valve at the oil return inlet of the oil box, leave the oil return valve open to the full, that is, an opening of 100%, and drive the lubricating oil back to the oil box through the valve under a high pressure provided at a high-pressure outlet of the compressor at the cost of extra electrical power. However, the level of lubricating oil is not monitored and the number of the oil return is not controlled in the way as above. Even though the oil return is monitored, it still not ensures the lubricating oil return to oil box fully. Thereby the lubricating oil is decreasing with time. The operation of the compressor is hardly smooth because of low oil level. In addition, the system wastes a lot of electric energy because the oil return valve stays open. Hence, the system efficiency is going down.

In order to avoid low oil level and improve the system. U.S. Pat. No. 6,834,514 (FIGS. 6 and 7) and U.S. Pat. No. 6,993,920 (FIGS. 8 and 9) have disclosed an oil level sensor disposed in the oil box to detect the oil level and control the valve opening (0% or 100%, that is, by ON-OFF control) of an oil return valve provided at an oil return inlet of the oil box. The air conditioning system shuts down when the oil level is lower than an alert threshold. But the prior art still has drawbacks, for example, valve opening is switched between 0% and 100%, which is a rather great increment or decrement of valve opening. The compressor wastes a lot of electric energy and deteriorates the efficiency when the opening is 100%.

In conclusion, there is an urgent need for a method for overcoming the drawbacks of the prior art, so as to ensure a sufficiently high oil level, reduce high-pressure power loss, and render the system safe, reliable and efficient.

BRIEF SUMMARY OF THE INVENTION

Therefore, an objective of the present invention is to provide a more convenient way to use a system for monitoring and controlling oil return to a compressor so as to ensure the oil in the oil box is enough to protect the compressor and the bearings.

In another embodiment, the present invention provides a method and system for monitoring and controlling oil return to the compressor so as to reduce the high-pressure power loss and enhance the efficiency of the system.

In another embodiment, the present invention provides a system and method for monitoring and controlling oil return to the compressor in a way that the system and method are safe to operate.

To achieve the above objectives, the present invention provides a method for monitoring and controlling oil return to the compressor which are applied in a system for monitoring and controlling the quantity of the returned oil in the oil box by controlling valve opening of an oil return valve provided at an oil return inlet of an oil box. The system comprises: an oil return valve provided at the oil return inlet; a pressure sensor provided at an outlet of the compressor, and an oil level sensor and a temperature sensor provided in the oil box. The method for monitoring and controlling oil return to the compressor comprises: providing a lowest oil level of oil in the oil box; providing a pressure required for oil return; providing an initial oil return flag; providing the oil level of oil return corresponding to the initial oil return flag; providing a maximum temperature value and providing a minimum temperature value.

To detect the temperature value of the oil box and determine the initial oil level value, the compressor is shut down when the lowest oil level exceeds the initial oil level value, otherwise analyze the pressure of the returned oil, oil level in the oil box, and temperature value and to detect the temperature value of the oil in the oil box and determining the temperature value, the oil box is to be warming up when the temperature value is not equal to a minimum temperature value, otherwise the temperature value is to be between the minimum value and the maximum value; and detect the pressure value of the returned oil and determine the pressure value, and maintain full opening of the oil return valve when the pressure exceeds a pressure value necessary for oil return, otherwise maintain full opening of the oil return valve. Hence, valve opening of the oil return valve is controlled by a non-segmentation principle or a segmentation principle.

Proportional-integral-derivative (PID) controller algorithm applies to the non-segmentation principle. The control flow comprises: retrieving the initial oil return flag of oil and making a true value of the oil level value to a first oil level value and a false value of oil level value to a second oil level value; calculating the difference between the measured oil level value and the oil level of the returned oil and obtaining the range of valve opening of the oil return valve and resetting the oil return flag to the true value when the valve opening is increasing, or resetting the oil return flag to the false value when the valve opening is not increasing; and controlling the valve opening by converting the range of valve opening to an electric signal instruction. The range of valve opening is defined and divided into standard segments according to the segmentation principle. The segments match different valve openings of the oil return valve, respectively. The segments and the valve openings are identified and determined according to the measured oil level values. Then, the segments confirmed are converted into electric signals to control the valve opening.

The method further comprises determining whether the oil level exceeds the lowest oil level value, analyzing the pressure of the returned oil and the temperature of the oil box so as to perform the periodical analysis when the oil level exceeds the lowest oil level value, otherwise, the compressor is shut down.

The temperature sensor detects the temperature value, the pressure sensor detects the oil pressure value and the heater warms up the temperature of the oil box.

The steps of warming up comprise: setting the oil temperature to the minimum temperature and to determine the pressure of oil return, controlling valve opening of the oil return valve and reducing quantity of the cooling water when the pressure of oil is lower than the pressure required for oil return, otherwise maintain full opening of the oil return valve. The electric signal instruction is a voltage signal, from 0 to 10 Volts, or a current signal, from 4 to 20 mA.

On the other hands, the oil return flag is one of the first and second values, the first value is the false value and the second value is the true value. Alternatively, the first value is the true value, and the second value is the false value. Alternatively, the first oil level value is the high oil level value, and the second oil level value is the low oil level value, and the initial oil return flag is the true value. The five segments are determined to control the valve opening of the oil return valve.

The present invention provides a system for monitoring and controlling oil return to a compressor, so as to monitor quantity of the returned oil in the oil box by controlling the valve opening of the oil return valve. The system for monitoring and controlling oil return to the compressor comprises: a pressure sensor provided at an outlet of the compressor and detects the pressure of the returned oil; an oil return valve provided at an oil return inlet and coupled to the pressure sensor by controlling the valve opening of the oil return valve according to a segmentation principle or a non-segmentation principle; an oil level sensor provided in the oil box to detect the temperature of oil in the oil box and detect the pressure of the oil returned to the oil box, oil level in the oil box and temperature periodically and convert the measurement result to the oil return valve, so as to monitor and control quantity of the returned oil in the oil box by controlling the valve opening of the oil return valve according to the segmentation principle or the non-segmentation principle.

The measurement result is converted into an electric signal instruction according to the segmentation principle or the non-segmentation principle. The electric signal instruction is a voltage signal ranging from 0 to 10 volt or is a current signal ranging from 4 to 20 mA.

The system for monitoring and controlling oil return to a compressor further comprises the jet pump provided at an oil return inlet of the oil box and connected to the oil return valve. It can be a power source that sends feedback to the outlet of the compressor and the heater provided in the oil box.

Regarding the system for monitoring and controlling oil return to a compressor, PID controller algorithm applies to the non-segmentation principle.

If the segmentation principle is adopted, the oil level in the oil box is divided into a plurality of segments, and the segments correspond to different valve openings, respectively, so as to control the valve opening of the oil return valve by the oil level. Preferably, the oil level is divided into five segments.

Compared with the prior art, in the embodiment of the present invention, the oil level is determined by monitoring and controlling the temperature value and oil level value of the lubricating oil in the oil box, as well as the pressure value at the high-pressure outlet of the compressor, so as to maintain the temperature of the lubricating oil at between the predetermined minimum and maximum temperature. At the same time, the pressure value at the high-pressure outlet of the compressor is monitored and controlled, by controlling the valve opening of the oil return valve according to the segmentation principle or the non-segmentation principle.

As disclosed in the present invention, consideration is given to three parameters, namely the temperature value and oil level value of the lubricating oil, and the pressure value at the high-pressure outlet of the compressor. As disclosed in the present invention, valve opening of the oil return valve is controlled in a continuous manner (without segmentation) or in a segmented manner (with segmentation), so as to prevent a waste of power and promote the efficiency of the system. Also, the system of the present invention is safe, because the present invention discloses the lowest oil level value and thereby prevents the oil box from running out of lubricating oil to the detriment of the operation of the compressor.

DETAILED DESCRIPTION OF THE INVENTION

Please refer toFIG. 1.FIG. 1is a schematic diagram of a system for monitoring and controlling oil return to a compressor for use with a centrifugal chiller. As illustrated in the drawing, the system for monitoring and controlling of oil return to a compressor comprises a jet pump32, a pressure sensor35, an oil level sensor33, a temperature sensor34, and a high-pressure gas oil return valve31. In the embodiment of the present invention, the jet pump32gives feedback to the outlet pressure of the compressor as a power supply. The pressure sensor35is directly installed at the outlet of the compressor. The oil level sensor33and the temperature sensor24are installed in the oil box. Three parameters, namely oil level in the oil box, temperature of the oil box, and pressure at the high-pressure outlet of the compressor, are measured simultaneously. Signals are sampled periodically so as to determine whether the quantity of the returned oil is appropriate. The result of the determination is sent to the high-pressure gas oil return valve as a basis for the instructions that control the valve opening. In an embodiment of the present invention, the quantity of the lubricating oil returned to the oil box is efficiently monitored and controlled by controlling the valve opening of the high-pressure gas oil return valve31.

At the very beginning, allowable minimum temperature value T1and allowable maximum temperature value T2of lubricating oil stored in the oil box, allowable lowest oil level value Lmin of the lubricating oil in the oil box, pressure value Pset required for oil return, an oil return flag (as shown inFIG. 5A), and the oil level d of the returned oil corresponding to the oil return flag (as shown inFIG. 5A) are predetermined. In an embodiment of the present invention, in view of the characteristics of coolant, the maximum temperature value T2is set to 55° C., the minimum temperature value T1to 40° C., the lowest oil level value Lmin to 6 cm, and the pressure value Pset required for oil return to 7 kg*f/cm2(R-134A).

Next, the step of “detecting the oil level value of the lubricating oil in the oil box in real time” is performed, and then the detected oil level value is outputted. The oil level value is detected by the oil level sensor33in the oil box. Then, the oil level value detected and outputted is received. If the current oil level of the lubricating oil is less than the lowest oil level value Lmin, the compressor is shut down, and the monitoring and controlling process ends.

Upon determination that the current oil level of the lubricating oil exceeds the lowest oil level value Lmin, the three parameters (oil level in the oil box, temperature of the oil box, and pressure at the high-pressure outlet) are ready for analysis as shown in the flow chart inFIG. 2.

Referring toFIG. 2, upon determination that preheating of the system is done, the step of “detecting the current temperature T of the lubricating oil in the oil box” is performed, and then the measured value is outputted. The temperature value T is detected by the temperature sensor34in the oil box. Afterward, the temperature value T detected and outputted is received. Next, the temperature value T is determined to see whether it is equal to the minimum temperature value T1or not.

If the temperature value T is not equal to the minimum temperature value T1(false), the step of warming up is performed. The lubricating oil in the oil box is heated until the temperature value T is equal to the minimum temperature T1. Then, the pressure value P at the high pressure source is checked and determined to see whether the pressure value P is sufficient to effectuate oil return (P>Pset). If the pressure value P is higher than the pressure value Pset, the high-pressure gas oil return valve31will have to be fully opened (100%), and this step will end, otherwise, quantity of the cooling water should be decreased (by 5%), and (100%) full opening of the high-pressure gas oil return valve31is controlled and maintained. The lubricating oil in the oil box is preheated by a heater (as shown inFIG. 3) installed in the compressor.

If the temperature value T is equal to the minimum temperature value T1(true), the lubricating oil is normally preheated according to the temperature value T, and the temperature value T is set to between the minimum temperature value T1and the maximum temperature value T2(T1<T<T2); meanwhile, the step of “detecting the current pressure value P at the high-pressure outlet of the compressor in real time” is performed, and then the detected pressure value P is outputted. The pressure value P is detected by the pressure sensor35provided at the high-pressure outlet of the compressor.

Next, the pressure value P is compared with the predetermined pressure value Pset so as to determine whether the pressure value P is higher than the predetermined pressure value Pset. If the pressure value P is higher than the predetermined pressure value Pset, the standard quantity of the cooling water is maintained, and the valve opening of the high-pressure gas oil return valve31is controlled (as shown inFIG. 5A); upon a negative determination, the quantity of the cooling water is decreased, and the valve opening of the high-pressure oil return31is set to full (100%).

Next, the oil level value is compared with the lowest oil level value Lmin so as to determine whether the oil level value is less than the lowest oil level value Lmin. If the oil level value is less than the lowest oil level value Lmin (true), the system is shut down. If no (false), end this process.

Referring toFIG. 3Aand FIG.3A′, a process for controlling valve opening of the high-pressure gas oil return valve31is shown. Options, namely a segmentation principle and a non-segmentation principle, are available to the step of controlling the valve opening of the high-pressure gas oil return valve31. As shown inFIG. 3A, proportional-integral-derivative (PID) controller algorithm applies to the non-segmentation principle to retrieve the oil return flag and determine the oil return flag. If the oil return flag is true, the oil level value d is set to a first oil level value dh. If the oil return flag is false, the oil level value d is set to a second oil level value dl. The oil return flag is one of a first value and a second value. The first value and the second value are logical negation of each other. The oil return flag represents one of the first oil level value dh and the second oil level value dl, that is, an oil level of the lubricating oil in the oil box, so as to give initial definition of the oil return flag. In the embodiment of the present invention, the first value is set to true, and the second value is set to false. Alternatively, the first value is set to false and the second value to true. Similarly, the first oil level value dh (corresponding to the first value) is a high oil level value, and the second oil level value dl (corresponding to the second value) is a low oil level value. Therefore, the user sets the oil return flag to true or false initially according to the real-time oil level value.

Next, the range of valve opening of the high-pressure gas oil return valve31is calculated according to the current oil level, oil level value d (the goal value) and the predetermined PID controller algorithm. Then the calculated range of valve opening is converted into electrical signal instructions for controlling the valve opening of the high-pressure gas oil return valve31.

The oil level value d is a standard value for PID controller algorithm. The difference between the current oil level value and the standard value for PID controller algorithm is calculated. The adjustable range of valve opening of the high-pressure gas oil return valve31relative to the current valve opening is calculated by PID controller algorithm. The PID controller algorithm is well known by persons ordinarily skilled in the art, and thus it is not described in detail herein for brevity.

The electric signal instruction is a voltage signal (from 0 to 10 V) or is a current signal (from 4 to 20 mA). Conversion of the values (range of valve opening) into an electric signal instruction is well known by those of ordinary skill in the art, and thus it is not described in detail herein for brevity.

Next, the tendency of the range of valve opening to vary is determined according to the calculated range of valve opening. If the range of valve opening increases, then reset the oil return flag to true; otherwise, the oil return flag is reset to false. At this point, the step of controlling the valve opening of the high-pressure gas oil return valve31ends.

As shown inFIG. 3A, the adjusting opening signal is produced by PID controller algorithm (i.e., according to the non-segmentation principle) with opening of high pressure gas oil return valve31that is adjusted by an electric signal instruction. Compared with the prior art (being left open all the time or using on-off oil return control), the non-segmentation principle adopted in the present invention is more reasonable. The valve opening of the high-pressure oil return valve31is accurately controlled by PID-based optimization of real-time oil level value of the lubricating oil, wherein the valve opening does not stay at 100% or switch between 0% and 100%, thereby enhancing efficiency of the system.

To provide insight into the non-segmentation principle disclosed in the present invention, exemplification is as follows: the first value is set to true, the second value to false, the first oil level value (high oil level value) dh to 25 cm, the second oil level value (low oil level value) dl to 15 cm, and the oil return flag is initially defined as the first value (true). Specifically speaking, the oil return flag is retrieved, and the oil return flag is checked and determined whether the oil return flag is true, when the current temperature T of the lubricating oil in the oil box is between the predetermined minimum temperature T1and the maximum temperature T2and the pressure value P at the high-pressure outlet of the compressor exceeds the predetermined pressure value Pset. Since the oil return flag is initially defined as true, the oil level value d is set to high oil level value (25 cm). Next, the high oil level value is treated as the standard value for PID controller algorithm, and the difference between the current oil level value and the standard value for PID controller algorithm is calculated according to the current oil level value detected and outputted. The PID controller algorithm calculates the adjustable range of valve opening of the high-pressure gas oil return valve31relative to the current valve opening thereof, and the calculated range of valve opening is converted into an electric signal instruction for (electrically) controlling the valve opening of the high-pressure gas oil return valve31.

Next, the oil return flag is reset according to the calculated tendency of the range of valve opening to vary. The oil return flag is reset to true when the range of valve opening tends to increase. Next, go back to the step of “detecting the current oil level value of the lubricating oil in the oil box in real time” prior to analysis of the three parameters, and reset the oil level value d required for the next PID controller algorithm according to the flag value of the oil return flag. The oil return flag is reset to false when the range of valve opening tends to decrease. Next, go back to the step of “detecting the current oil level value of the lubricating oil in the oil box in real time” prior to analysis of the three parameters, and reset the oil level value d required for the next PID controller algorithm according to the flag value of the oil return flag.

Please refer toFIG. 3B.FIG. 3Bis a table showing parameters used in controlling the valve opening of the high-pressure gas oil return valve31according to a segmentation principle. As shown in the drawing, like components applied to the segmentation principle and the non-segmentation principle are denoted alike, and detailed description of the components are omitted herein for brevity.

According to the segmentation principle, the current temperature value T of the lubricating oil in the oil box is set to between the predetermined minimum temperature T1and maximum temperature T2. If the pressure value P at the high-pressure outlet of the compressor exceeds the predetermined pressure value Pset, the oil level values of the lubricating oil are divided into segments, and the oil level values of the lubricating oil correspond to the valve openings of the high-pressure gas oil return31, respectively. The predetermined range of the oil level values are searched so as to determine the predetermined range of the oil level values within which the oil level values fall. The valve opening is determined in accordance with the relationship of the oil level values determined and the valve openings.

As shown inFIG. 3B, under the segmentation principle, the quantity of the returned oil is controlled according to oil level, and thus five segments are defined, namely Segment1(H1≦oil level L), Segment2(N2≦oil level L<H1), Segment3(N1≦oil level L<N2), Segment4(L1≦L<N1) and Segment5(L<L1). The control mechanism for the high-pressure oil return valve31is regulated by detecting the segments and their respective outputs.

Next, the measured valve opening is converted into the electric signal instruction (0˜10V or 4-20 mA) for controlling valve opening (0-100%). The valve opening of the high-pressure gas oil return valve31provided at the oil return inlet of the oil box is electrically controlled in a precise manner by the segmentation principle, so as to prevent the oil box from running out of the returned oil to the detriment of the compressor.

For example, the segments is H1=25 cm, N2=23 cm, N1=20 cm and L1=15 cm. The oil level L is searched and determined to be falling within Segment3when oil level L=22 cm is detected. Hence, the current valve opening is 30%, and the current valve opening is converted into the electric signal instruction to control the valve opening. In so doing, drawbacks of the prior art are overcome.

In conclusion, in the embodiment of the present invention, the oil level and the lowest oil level value are determined based on the oil level, temperature and pressure in the oil box. If the oil level is less than the lowest oil level value, the compressor is shut down, otherwise, the temperature of the lubricating oil in the oil box is monitored; if the temperature is less than the predetermined minimum temperature, then the lubricating oil is to be warmed up until the temperature value equals the minimum temperature value, and then full valve opening is maintained, otherwise, the temperature of the lubricating oil is set to between the minimum and maximum temperatures. At the same time, the pressure value at the high-pressure outlet of the compressor is monitored. If the pressure value exceeds the predetermined pressure value, the valve opening of the oil return valve of the oil box is controlled according to a segmentation principle or a non-segmentation principle. The lubricating oil is returned to the oil box. Hence, the bearings are sufficiently lubricated with the lubricating oil so as for the bearings to operate smoothly.

Moreover, in the embodiment of the present invention, the valve opening of the high-pressure oil return valve is controlled by the continuous control (without segments) or segmented control (with segments) so as to enhance efficiency of the system. The lowest oil level is defined, so as to avoid a shortage of the returned oil.