US 20050103035 A1 Abstract An innovative oil observer for estimating oil concentration and oil amount in a refrigerant compressor in a vapor compression cycle is described. The invention ensures the safe operation of the compressor by ensuring that adequate lubricant is present in the compressor. This oil observer is based on oil models for components of air conditioning and refrigeration systems. Oil models for HVAC components estimate oil mass and refrigerant mass in each component. With all component oil models and heat exchanger observers which provide the estimation of inner geometric lengths of two-phase flow heat exchangers, a system-level oil observer is established by integrating all component models. Experimental testing has been conducted to verify the performance of this oil observer for steady state operation and dynamic processes. The invention has direct applications in residential and commercial air conditioning and refrigeration systems.
Claims(20) 1. A method of monitoring a parameter related to lubricant in a first component of a vapor compression cycle system, comprising:
estimating a parameter related to retained lubricant in a plurality of other components of the vapor compression cycle system; and subtracting the estimate of the parameter related to retained lubricant from a parameter related to a known total lubricant in the vapor compression system. 2. The method of 3. The method of 4. The method of 5. A refrigeration apparatus comprising a device for monitoring a parameter related to lubricant in a first component of the refrigeration apparatus, the device estimating a parameter related to retained lubricant in a plurality of other components of the refrigeration apparatus and subtracting the estimate of the parameter related to retained lubricant from a parameter related to a known total lubricant in the refrigeration apparatus. 6. The refrigeration apparatus of 7. The refrigeration apparatus of 8. The refrigeration apparatus of 9. A method of monitoring a parameter related to lubricant in a component of a vapor compression cycle system, comprising:
detecting a parameter related to a state of the component; and using the parameter related to the state of the component, estimating the parameter related to lubricant in the component. 10. The method of clam 9, further comprising using the parameter related to lubricant in the component to determine an amount of lubricant in the component. 11. The method of clam 9, further comprising using the parameter related to lubricant in the component to determine a concentration of lubricant in the component. 12. The method of 13. A refrigeration apparatus comprising a device for monitoring a parameter related to lubricant in a component of the refrigeration apparatus, the device detecting a parameter related to a state of the component and, using the parameter related to the state of the component, estimating the parameter related to lubricant in the component. 14. The refrigeration apparatus of clam 13, wherein the apparatus uses the parameter related to lubricant in the component to determine an amount of lubricant in the component. 15. The refrigeration apparatus of clam 13, wherein the apparatus uses the parameter related to lubricant in the component to determine a concentration of lubricant in the component. 16. The refrigeration apparatus of 17. A method of monitoring a parameter related to lubricant in a heat exchanger of a vapor compression cycle system, comprising:
determining a length of a two-phase portion of the heat exchanger; and using the length of the two-phase portion of the heat exchanger, estimating the parameter related to lubricant in the heat exchanger. 18. An apparatus for monitoring a parameter related to lubricant in a heat exchanger of a vapor compression cycle system, the apparatus determining a length of a two-phase portion of the heat exchanger and, using the length of the two-phase portion of the heat exchanger, estimating the parameter related to lubricant in the heat exchanger. 19. A method of monitoring a parameter related to lubricant in a heat exchanger of a vapor compression cycle system, comprising:
determining a length of a single-phase portion of the heat exchanger; and using the length of the single-phase portion of the heat exchanger, estimating the parameter related to lubricant in the heat exchanger. 20. An apparatus for monitoring a parameter related to lubricant in a heat exchanger of a vapor compression cycle system, the apparatus determining a length of a single-phase portion of the heat exchanger and, using the length of the single-phase portion of the heat exchanger, estimating the parameter related to lubricant in the heat exchanger.Description This application is based on U.S. Provisional Application Ser. No. 60/523,447, filed on Nov. 19, 2003, the contents of which are incorporated herein in their entirety by reference. Lubricating oil in the compressor of a heating, ventilation and air conditioning (HVAC) system provides lubrication for moving parts in the compressor. Good lubrication ensures the safe operation of the compressor. For a refrigerant compressor, the oil lubricating capability decreases when the oil is mixed with liquid refrigerant. For example, this may happen when the defrost operation is turned on during the heating season, since under such conditions, the indoor fan is typically shut down, and liquid in the evaporator may not be evaporated. As a result, large amounts of liquid refrigerant may enter the compressor chamber and mix with the lubricating oil. To quantify how much liquid refrigerant is mixed with oil in the compressor, an important index under investigation is oil concentration. For reliable operations, oil concentration needs to be above a certain level such that the viscosity of the oil/refrigerant mixture is large enough to guarantee sufficient lubrication for moving parts in the compressor. All refrigerant compressors circulate some amount of oil through the system. It is essential that oil be returned in the system. However, in an evaporator, when superheat is large and evaporating temperature is low, oil viscosity may become high because liquid refrigerant becomes vapor in the superheat range. If vapor velocity is not sufficient to transport the oil, some oil may remain in the evaporator. Similarly, in suction lines, oil retention may be a problem if refrigerant vapor velocity is not sufficient or the refrigerant temperature is low. For a multi-evaporator system with a vertical gas line, if the vapor velocity is not high enough, the oil cannot be pushed upward and return to the compressor. When a significant amount of oil remains in the evaporator-condenser-gas line circuit or accumulator, the oil in the compressor will be not sufficient to provide reliable lubrication. Conventionally, the amount and concentration of oil in the compressor cannot be directly measured without special sensors. For purposes of research and development on the system, special designs can be used to place costly viscosity sensors at the bottom of the compressor to measure the viscosity of the oil/refrigerant mixture in the compressor, and oil concentration is calculated from the value of viscosity and oil temperature. Through a glass window installed at the side of compressor, the oil/refrigerant mixture liquid level can be measured. Without a viscosity sensor or a special oil concentration meter that is not available in actual application of air conditioning and refrigeration systems, the amount and concentration of oil in the compressor cannot be determined in conventional systems. This invention provides an innovative method to determine the amount and/or the concentration of lubricant in the compressor of an HVAC system based on HVAC component oil models and heat exchanger observers. In accordance with a first aspect, the invention is directed to an apparatus and method for monitoring a parameter related to lubricant in a first component of a vapor compression cycle system. In accordance with the invention, a parameter related to retained lubricant in a plurality of other components of the vapor compression cycle system is estimated. The estimate of the parameter related to retained lubricant in the plurality of other components is subtracted from a parameter related to a known total lubricant in the vapor compression system. In one embodiment, the first component of the vapor compression cycle system is a compressor. The plurality of other components of the vapor compression cycle system can comprise at least one of an evaporator, an accumulator, a suction gas line, a discharge gas line, a condenser, a liquid line and a receiver. The parameter related to retained lubricant in the plurality of other components of the vapor compression cycle system can be determined using one or more parameters related to the state of each component of the vapor compression system. In accordance with another aspect, the invention is directed to an apparatus and method for monitoring a parameter related to lubricant in a component of a vapor compression cycle system. In accordance with the invention, a parameter related to a state of the component is detected. The parameter related to lubricant in the component is estimated using the parameter related to the state of the component. In one embodiment, the parameter related to lubricant in the component is used to determine an amount of lubricant in the component. In one embodiment, the parameter related to lubricant in the component is used to determine a concentration of lubricant in the component. In one embodiment, the component of the vapor compression cycle system is one of an evaporator, an accumulator, a suction gas line, a discharge gas line, a condenser, a liquid line and a receiver. In accordance with another aspect, the invention is directed to an apparatus and method for monitoring a parameter related to lubricant in a heat exchanger of a vapor compression cycle system. In accordance with the invention, a length of a two-phase portion of the heat exchanger is determined. The parameter related to lubricant in the heat exchanger is estimated using the length of the two-phase portion of the heat exchanger. In accordance with another aspect, the invention is directed to an apparatus and method for monitoring a parameter related to lubricant in a heat exchanger of a vapor compression cycle system. In accordance with the invention, a length of a single-phase portion of the heat exchanger is determined. The parameter related to lubricant in the heat exchanger is estimated using the length of the single-phase portion of the heat exchanger. The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In this invention, a dynamic nonlinear observer to estimate the oil concentration and the amount of oil in a refrigerant compressor is described. This oil observer is based on 1) integrated oil/refrigerant distribution and circulation model to estimate oil mass and refrigerant mass in each HVAC component, 2) heat exchanger observers to estimate the two-phase section lengths of the evaporator and condenser, and the subcool section length of the condenser, 3) mass conservation for oil and refrigerant in the whole machine. Dynamic simulation based on the oil/refrigerant model requires the initial conditions for all state variables. However, most of the initial conditions for the state variables cannot be measured by sensors. The dynamic nonlinear observer of the invention described herein can estimate unmeasured variables such as oil concentration and oil amount in the compressor using available sensor information such as evaporating temperature, condensing temperature, etc. To synthesize an oil observer, integrated models for oil/refrigerant circulation and distribution have been developed for each main component of an air conditioning and refrigeration system. Components include evaporator, condenser, gas line, liquid line, accumulator and compressor. Oil retention and refrigerant mass in each component are estimated based on void fraction model and estimated geometry of heat exchangers such as length of two-phase section in evaporator and lengths of two-phase section and sub cooled one-phase liquid section in condenser. There are no sensors to measure length of the two-phase section in the evaporator and the lengths of the two-phase section and sub cooled one-phase liquid section in the condenser. In accordance with the invention, a dynamic evaporator observer is used to estimate length of the two-phase section in the evaporator based on available sensor information of evaporating temperature. A dynamic condenser observer is used to estimate lengths of the two-phase section and sub cooled one-phase liquid section in the condenser based on available sensor information of condensing temperature. This invention is applicable to reciprocating compressors, scroll compressors, rotary and swing compressors, centrifugal compressors, and screw compressors. This invention is applicable to residential air conditioners and heat pumps, commercial air conditioners and heat pumps, chillers, multi-evaporator systems, refrigerators, refrigeration systems and other types of machines working on the vapor compressor cycle principle. This invention is applicable to all combinations of micible oil and refrigerant. 1. Oil Observer Structure. An oil observer described herein in accordance with the invention is based on oil models that will be described below in Section The condenser oil model With the estimation of oil mass in the evaporator, condenser, gas line, liquid line and accumulator, the oil mass in the compressor is obtained since total oil mass in the machine is constant. With the estimation of refrigerant mass in the evaporator, condenser, gas line, liquid line and accumulator, refrigerant mass in the compressor is obtained since total refrigerant mass in the machine is constant. Furthermore, the liquid refrigerant in compressor can be estimated and then the oil concentration can be calculated based on estimated oil mass and liquid refrigerant mass in the compressor. 2. Component Oil/Refrigerant Models In this section, models for estimating oil mass and refrigerant mass in the evaporator, the condenser, the gas line, the liquid line and the accumulator are described. To estimate the oil mass and refrigerant mass the in evaporator and condenser accurately, important factors are 1) proper void fraction model, 2) accurate volumes for one-phase subcool liquid section length in the condenser and two-phase section lengths, 3) and oil circulation rate. 2.1 Condenser Oil Model: Refrigerant and Oil Mass in Condenser The two-phase section The length of each element is dl For each element in the two-phase section The vapor refrigerant mass in that element can be obtained by:
The liquid refrigerant mass in that element can be obtained by:
Then, the liquid refrigerant mass in the entire condenser can be obtained by:
The vapor refrigerant mass in the condenser can be obtained by:
The oil mass in the condenser can be obtained by
The total refrigerant mass in condenser is
The above condenser oil model obtains information including Lc2, Lc3 from the condenser observer 2.2 Evaporator Oil Model: Refrigerant and Oil Mass in Evaporator The length of each element is dl For each element in the two-phase section, the void fraction is calculated based on the Hughmark's void fraction mode using the parameters in that element. Then, the liquid refrigerant mass in the evaporator can be obtained by:
The oil mass in the evaporator can be obtained by:
The vapor refrigerant mass in the evaporator can be obtained by
The total refrigerant mass in evaporator is
The above evaporator oil model obtains information of two-phase section length L 2.3 Liquid Line Oil Model: Refrigerant and Oil Mass in Liquid Line When the void fraction value α is obtained, the total liquid volume (liquid refrigerant+oil) in the liquid line is Q The vapor refrigerant mass in the liquid line can be obtained by:
The liquid refrigerant mass in the liquid line can be obtained by:
In the gas line, it is assumed that the void fraction is the same as the superheated section and there is no liquid refrigerant. Assuming that V is the total volume of the gas line, α is mean void fraction of gas line. The vapor refrigerant mass in the gas line can be obtained by:
Gas line and liquid line oil models use information of averaging vapor quality, averaging refrigerant temperature, oil circulation rate C 2.5 Accumulator Oil Model: Refrigerant and Oil Mass in Accumulator Assuming that T The vapor refrigerant mass in the accumulator can be obtained by:
The oil mass in the accumulator is
In Section 2 above, models to estimate oil mass and refrigerant mass in condenser, evaporator, gas line, liquid line and accumulator were described. In this section, estimation of the oil mass and refrigerant mass in the compressor based on the conservation of oil and refrigerant mass inside the machine is described. Oil concentration in the compressor can be derived in accordance with the following. Oil mass conservation for the entire machine is
Based on oil mass conservation, the estimated oil mass in the compressor {circumflex over (M)} Refrigerant mass conservation for the entire machine is
Based on refrigerant mass conservation, the estimated refrigerant mass in the compressor {circumflex over (M)} In order to estimate the oil concentration in the compressor, the liquid refrigerant mass in compressor is estimated. With the estimation of {circumflex over (M)} Based on the estimated oil mass from Equation (27) and the estimated liquid refrigerant mass from Equation (29), the estimated oil concentration in the compressor can be expressed by
In one embodiment, the invention is achieve Assuming a uniform temperature throughout the evaporator tube wall, the heat transfer equation of the tube wall is as follows:
The first term on the right hand side represents the heat transfer rate per unit length from the room to the tube wall. The second term represents the heat transfer rate per unit length from the tube wall to the two-phase refrigerant. Assuming the mean void fraction {overscore (γ)} is invariant, the liquid mass balance equation in the two-phase section of the evaporator is
In equation (33), the left hand side is the liquid mass change rate in the evaporator. On the right hand side, q/h The inlet refrigerant mass flow rate {dot over (m)} Assuming that the vapor volume is much larger than the liquid volume in the low-pressure side, the vapor mass balance equation in an evaporator is:
Equation (36) can be written as
Based on equations (32), (33) and (39), the state space representation for the low order evaporator model is as follows, where T Where T Since only T The following are the dynamics of the non-linear observer described herein.
From the observer dynamics, if {circumflex over (T)} 4.2 Model-based Nonlinear Observers for Condenser In the oil observer described in Section In this section, the model of the condenser is described. The condenser model is similar to the evaporator model. The vapor balance equation is as follows
The heat transfer equation is as follows:
The liquid mass balance equation in the condenser model is expressed in equation (44). It is a little bit different from the evaporator model. Two sections have liquid refrigerant. One is the liquid phase section, the other is the two-phase section.
It can be seen that there are four unknowns but three equations. One of the unknowns is eliminated for the observer design. One assumption made is that the length change of the superheated phase is very slow
Therefore the liquid mass balance equation can be written as
Equations (42), (43) and (44) are the condenser model. The model-based observer for the condenser is described as follows
The length of the superheated portion of the evaporator can be calculated by subtracting the length of the two-phase portion of the evaporator from its total length. With regard to the condenser, the length of the superheated portion of the condenser can be calculated by subtracting the length of its two-phase and subcool portions from its total length. 4.3 Gas Line Observer and Liquid Line Observer During system start-up, steady state and other transient operations, if the refrigerant at the exit of the evaporator is at the two-phase state, the gas line is filled with two-phase flow. When the refrigerant at the exit of the evaporator is superheated vapor, the gas line is filled with superheated vapor flow. The gas line observer is used to detect whether the refrigerant in the gas line is at two-phase state or superheated state based on the length of the two-phase section of the evaporator. If the length of the two-phase section of the evaporator is smaller than the total length of the evaporator, then the gas line observer will indicate that the gas line is filled with superheated vapor, and the oil mass and refrigeration mass in the gas line will be estimated accordingly. If the length of the two-phase section of the evaporator is equal to the total length of the evaporator, the gas line observer will indicate that the refrigerant in the gas line is at the two-phase refrigerant state, and the oil mass and refrigeration mass in the gas line will be estimated accordingly. During the start up, steady state and other transient operations, if the refrigerant at the exit of the condenser is at the two-phase state or the subcooled liquid state, the liquid line is filled with two-phase flow. When the refrigerant at the exit of the condenser is superheated vapor, the liquid line is filled with superheated vapor flow. The liquid line observer is used to detect whether the refrigerant in the liquid line is at the two-phase state or the superheated state based on the length of the superheated vapor section of the condenser. If the length of the superheated vapor section of the condenser equals the total length of the condenser, then liquid line observer will indicate that the liquid line is filled with superheated vapor, and the oil mass and refrigeration mass in the liquid line will be estimated accordingly. Otherwise, the length of superheated vapor section of the condenser is smaller than the total length of the condenser, and the liquid line observer will indicate that the refrigerant in the liquid line is at the two-phase refrigerant state, and the oil mass and refrigeration mass in the liquid line will be estimated accordingly. 5. Experimental Comparison In order to verify the oil observer and oil models described herein, experimental testing has been conducted. The comparison results show the error for estimation of oil concentration in accordance with the invention is less than 20%. 5.1 Experimental Set-Up The machine under testing was a split type residential air conditioner. The refrigerant used in this machine is R410A. The total refrigerant charge is 900 g. The lubricating oil is FVC50K, and 400 ml of oil was charged into the machine (about 370 g). The cooling capacity of the machine is 2.8 kW. All sensors (temperature, pressure, and two mass flow meters, viscosity sensor) are all connected to National Instrument data acquisition board and then connected to a PC. 5.2 Experimental Testing Experimental testing was done for several dynamic processes such as change of outdoor temperature by removing several insulation boards of the container, change of compressor speed, and change of expansion valve opening, etc. Experimental data for dynamic process with the outdoor temperature change from 35 C to 27 C is described in details in this sub-section, and the comparison for oil concentration in the compressor described in the following sub-section is based on this testing. After the start-up and running of the testing machine for more than 30 minutes, the operation is almost steady state. The operation condition is as follows: -
- 1) Outdoor temperature: 35 C
- 2) Indoor temperature: 20 C
- 3) Compressor Speed: 70 Hz
- 4) Expansion Valve: 15 Steps
- 5) Indoor fan speed: 1250 rpm
- 6) Outdoor fan speed: 630 rpm
The outdoor temperature is changed from 35 C to 27 C and then the system is allowed to reach another steady state as shown in 5.3 Comparison results The comparison between oil concentration estimated by the oil observer of the invention and experimental measurement of oil concentration by a viscosity sensor are set forth in this subsection. The first comparison is made under the initial condition. Measurement at the Initial Condition: 1) Evaporating temperature: 7.8 C 2) Evaporating pressure: 0.8 MPa 3) Condensing temperature: 48.2C 4) Condensing pressure: 2.77 MPa 5) Subcool: 6 C 6) Mass Flow Rate: 0.0185 kg/s 7) Oil Temperature: 57 C 8) Oil Viscosity: 0.0035 N/m{circumflex over ( )}2*s (Pa*s) 9) Liquid Volume in Accumulator: 130 cc 10) Liquid Volume in Compressor: 330 cc Comparison 1) Estimation Error of Oil Concentration in Compressor=13% 2) Estimation Error of Oil Mass in Compressor: 10% It is assumed that 1) Oil Circulation Rate in the Circuit is 0.5% (no sensor to measure) 2) Refrigerant Concentration in accumulator is 65% (no sensor to measure) For the dynamic conditions in which the outdoor temperature is changed from 35 C to 27 C described in subsection 5.2, the comparison results are shown in During this dynamic process, the refrigerant mass in the condenser is changed from 380 g (42.2% of total refrigerant mass) to 420 g (46.7% of total refrigerant mass), as shown in Table 1 shows the estimated oil distribution in the air conditioning machine at time t=10 minutes in accordance with the invention.
Hence, the present invention includes a system-level oil observer to estimate oil concentration and oil mass in the compressor of a vapor compression system. The invention includes oil distribution and refrigerant distribution models for the condenser, evaporator, gas line, and liquid line to estimate oil mass and refrigerant mass in each component. Heat exchanger observers (for evaporator and condenser) and oil models are integrated into the compressor dynamic oil observer. Experimental testing was performed to validate the oil observer and develop comparison results that illustrate less than 10% error. While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Referenced by
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