1、474 1 An Oil Circulation Observer for Estimating Oil Concentration and Oil Amount in Refrigerant Compressors Xiang-Dong He, Ph.D. Member ASHRAE Shinichi Kasahara ABSTRACT This paperpresents an innovative oil circulation observer to estimate oil concentration and oil amount in refiigerant compressors
2、. This model-based dynamic observer is based on oil models for the components of an air-conditioning or refrig- eration system. Oil models for HVAC since the fan is shut off and the liquid in the evaporator may not be evaporized, large amounts of liquid refrigerant may enter the compressor chamber a
3、nd mix with the lubricating oil. To quanti9 how much liquid refrigerant is mixed with the oil in the compressor, an important index under investigation is the oil concentration. For reliable operations, the oil concentration needs to be above a certain level such that the viscosity of the Tao Cheng
4、Harry H. Asada, Ph.D. oilhefrigerant 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 the oil be guaranteed to return to the compressor. However, in an eva
5、porator when the superheat temperature is large and the evaporating tempera- ture is low, the oil viscosity may become high because the liquid refrigerant becomes vapor in the superheat range. If the vapor velocity is not sufficient to transport the oil, some oil may remain in the evaporator. For th
6、e suction line, the oil retention may be a problem if the 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 compresso
7、r. When significant amount of oil remains in the evaporator-condenser-gas line circuit or the accumulator, the oil amount in the compressor will not be sufficient to provide reliable lubrication. The oil concentration and the oil amount in the compres- sor cannot directly be measured without special
8、 sensors, Only for the purpose of research and development, a viscosity sensor can be placed at the bottom of the compressor with a special design to measure the viscosity of the oilkefrigerant mixture in the compressor, and the oil concentration is calcu- lated from the values of viscosity and oil
9、temperature. Through the glass window installed at the side of the compres- sor, the oilrefrigerant mixture liquid level can be measured. Without a viscosity sensor or a special oil concentration meter that is not available in actual air-conditioning and refrigeration machines, oil concentration and
10、 oil amount in the compressor cannot be determined. This paper provides an innovative method to determine the oil concentration and the oil amount Xiang-Dong He is a senior researcher at Daikin U.S. Corporation and visiting scientist of mechanical engineering, MIT, Cambridge, Mass. Tao Cheng is a gr
11、aduate research assistant of mechanical engineering and Harry H. Asada is a professor and director of d Arbeloff Laboratory for Information Systems and Technology, MIT, Cambridge, Mass. Shinichi Kasahara is a research leader at Daikin Air Conditioning R however, the fractional thickness increase was
12、 smaller than that of flow rate. Recently, several research works have been done in this field. Certain oil retention characteristics in a CO, air- conditioning system is experimentally clarified by Jun-Pyo et al. (2002). Some results of their experiments indicate that the oil retention volume incre
13、ases as the oil circulation rate increases. The higher the mass flow rate, the lower the oil retention. Temperature is also a main factor for the oil retention. In the paper by Xuan and Newel1 (2002), a shear stress model is built to estimate the oil film thickness in the suction and discharge lines
14、. The results are similar to those before. The more the oil mass flow rate, the thicker the oil film is. The higher the vapor velocity, the thinner the oil film is. The temperature also has influence on the film thickness. COMPONENT OILIREFRIGERANT MODELS In this section, we present models to estima
15、te the oil mass and the refrigerant mass in the evaporator, the condenser, the gas line, the liquid line, and the accumulator. To estimate the oil mass and the refrigerant mass in the evaporator and the condenser accurately, the three most important factors are (1) proper void fraction model, (2) ac
16、curate volumes for the one- phase subcooled liquid section length in the condenser and the two-phase section lengths, and (3) oil circulation rate. Condenser Oil Model: Refrigerant and Oil Mass in Condenser The condenser can be divided into three sections as shown in Figure 1: the superheated sectio
17、n (Lci), the two-phase section (Lc), and.the subcooled section (Lc3). Lc2 andLc3 are obtained from a condenser observer that will be discussed later in the paper. At i = 1, the vapor quality x = O; at i = N, x = 1. For the condenser, assume the heat flux from the heat exchange is constant; then the
18、vapor quality decreases linearly. The two- phase region is divided into N elements as shown in Figure 1, so that within each element the thermodynamic property differences in each phase are negligible. The length of each element is d12 =Lc2 /Nand the vapor quality x(i) in section i can be evaluated:
19、 i- 1 x(i) = - N- 1 490 ASHRAE Transactions: Aesearch Element division in two phase region refrigerant vapor; pris the viscosity of liquid mixture; and x is For each element in the two-phase section, the void frac- tion is calculated based on Hughmarks void fraction mode using the parameters in that
20、 element. For a certain element with vapor quality x and calculated void fraction value a, the total liquid volume (liquid refrigerant + oil) in that element is deliquid = dV( 1 - a) = AJz(1- a), where dVis the volume of that element. Assume that the oil is well mixed with the liquid refngerant; the
21、 following equation is used to estimate the oil mass retention in that element. 1,2,3,n i-l,i,i+l N the vapor quality. Figure 1 Element model for the condenser two-phaseflow region. dMoi, = dV(1 - Assume that Lc2 is the length of the two-phase section, x is the vapor quality of the oil/refrigerant m
22、ixture at a certain location of the condenser, Coir is the oil circulation rate (wt%) defined as the ratio of the oil mass flow rate and the total oil/ where pliquid is the density of the oivrefrigerant mixture and is calculated as follows: (6) refrigerant mixture mass flow rate, and a is the void f
23、raction - Poil and can be estimated by different void fraction models. The following equation is used here to estimate the void fraction a Pliquid - 1 -.-coi, 1+ (Poi/PR - 1) 1 -x based on Hughmarks void fraction model that is dependent on the mass flow rate (Hughark 1962; Chen and hse 1995). where
24、poil is the pure oil density and pR is the density of refrig- erant liquid and a = KH (2) (3) 1-x-Co; 1 -x where pg is the is the saturated liquid density. The parameter KH has been fitted to a polyno- mial, vapor density and is the mass fraction of the refrigerant in the oikefrigerant mixture. The
25、vapor refrigerant mass in that element can be obtained by KH = 0.7266477 - 3.481988 x 10-4Z,- 0.845427 + 0.0601106Z3 dMref,vapor = dapg . k (4) (7) while Z, depends on the viscosity, averaged Reynold number Re (depending on mass flux, etc.), the Froude number Fr, and the liquid volume fraction yL. R
26、e 1 /6Fr 1 18 1 /4 z, = YL L(%)* PP, The liquid refrigerant mass in that element can be obtained by Then, the liquid refrigerant mass in the entire condenser can be obtained by z=L I z=o where the velocity V, is defined as that for the two-phase trav- eling at the same velocity, i.e., no slip betwee
27、n the phases; D is the tube hydraulic diameter; Gis the refrigerant mass veloc- ity; g is the acceleration due to gravity; pg is the viscosity of (9) The vapor refrigerant mass in the condenser can be obtained by ASHRAE Transactions: Research 49 1 The oil mass in the condenser can be obtained by .-
28、z = L, i=l The total refrigerant mass in the condenser is Mref = Mref,vopor+ Mref,liquid. (12) The above condenser oil model needs information of Lc2, Lc3 from the condenser observer, the condensing temperature Tc, the oil circulation rate Coil, and the mass flow rate for calculating the void fracti
29、on. The mass flow rate can be esti- mated based on the compressor mass flow model. It should be noted that the length of the subcooled section Lc3 is the key value for accurate estimation of the refrigerant mass inventory in the condenser (Harms 2003), since in the subcooled section, all refrigerant
30、 is high quality liquid that has much higher density than the vapor refrigerant density. Another important factor is the selection of a void fraction model. The Hughmark model is selected in this research because some other models tend to underestimate the liquid mass. Generally, the condenser could
31、 hold about 40% to 48% of the total refrigerant charge. Evaporator Oil Model: Refrigerant and Oil Mass in Evaporator The evaporator can be divided to two sections as shown in Figure 2: the superheated section (Le2) and the two-phase section (Lel). The two-phase section length Le1 is obtained from th
32、e evaporator observer that will be discussed later in the paper. At i = I, the vapor quality x = no; at i = N, x = 1 - Coil. The calculation for the evaporator is similar to that of the condenser. Its assumed that the vapor quality decreases linearly. The two-phase region is divided into N elements
33、as shown in Figure 2, so that within each element the thermody- namic property differences in each phase are negligible. The length of each element is dll = LelLW, and the vapor quality in section i can be evaluated: (13) i- 1 x(i) = x+(-x) For each element in the two-phase section, the void frac- t
34、ion is calculated based on Hughmarks void fraction mode using the parameters in that element. Then, the liquid reffig- erant mass in the evaporator can be obtained by Z=O (14) The oil mass in the evaporator can be obtained by i= 1 (15) The vapor refrigerant mass in the evaporator can be obtained by
35、z = Le, Mref,vapor = 1 a(x(z)/cdz + aNce2g Z=O (16) N = aif$icdli + aNAcLe2Pg i= 1 The total refrigerant mass in the evaporator is Mref = Mrej,vapor+ Mref,iiquid. (17) The above evaporator oil model needs information of the two-phase section length Le, from an evaporator observer, the evaporating te
36、mperature Te, the inlet vapor quality xo, the oil circulation rate Coi, and the mass flow rate for calculating the Element division in two phase region 1,2,3,0 i -1, i, i+l N ,til III1 Figure 2 Element model for the evaporator two-phase flow region. 492 ASHRAE Transactions: Research Vapor volume =ay
37、 / / I / I Liquid volume = (1 - a)V Fgure 3 A schematic of liquid line. void fraction. Generally, the evaporator can hold about 10% to 16% of the total refrigerant charge. Liquid Line Oil Model: Refrigerant and Oil Mass in Liquid Line A schematic of the liquid line is shown in Figure 3. Assume that
38、V is the total volume of the liquid line, x is the average vapor quality of the oilrefrigerant mixture of the liquid line, and a is the mean void fraction of the liquid line and can be estimated by different void fraction models. The Hughmark model or the following equation can be used to esti- mate
39、 mean void fraction a: 1 a= I+- -g I*(ps23 When the void fraction value a is obtained, the total liquid volume (liquid refrigerant + oil) in the liquid line is Qliquid = V(1- a), where V is the volume of the liquid line. Assume that the oil is well mixed with the liquid refrigerant; the following eq
40、uation is used to estimate the oil mass reten- tion in the liquid line. (18) Coil Moil = -a)p)Piiquid The vapor refrigerant mass in the liquid line can be obtained by MreAvapor = avpg . (19) The liquid refrigerant mass in the liquid line can be obtained by Gas Line Oil Model: Refrigerant and Oil Mas
41、s in Gas Line In the gas line, it is assumed that the void fraction is the same as the superheated section and there is no liquid ASH RAE Transactions: Research refrigerant. Assume that Y is the total volume of the gas line, and a is the mean void fraction of the gas line. The vapor refrigerant mass
42、 in the gas line can be obtained by Mrehvapor = aVpg. (21) The oil mass retention in the gas line is Moil = V( 1 - a)Poil . (22) The gas line and the liquid line oil models use information of the averaging vapor quality, the averaging refrigerant temperature, the oil circulation rate Coii, and the t
43、otal mass flow rate. Accumulator Oil Model: Refrigerant and Oil Mass in Accumulator Assuming that Tr is the refrigerant temperature in the accumulator, the saturated liquid density pl and the saturated vapor density pg can be determined from Tr based on the ther- modynamics property. The oil density
44、 poil is a function of Tr. Assume the total volume ofthe accumulator is I: and the liquid volume VL can be calculated based on the liquid level measurement through the glass window at the side of the accu- mulator. The vapor refrigerant mass in the accumulator can be obtained by MreAvapor = (V- VL)P
45、. (23) Moil = V,POoil (24) The oil mass in the accumulator is where p is the density of oil/liquid refrigerant mixture and is expressed by and wail is the oil concentration of the oillliquid refrigerant mixture in the accumulator. The liquid refrigerant mass in the accumulator can be obtained by Mre
46、f,iquid = VLP(* - moil) . (26) OIL OBSERVER TO ESTIMATE OIL CONCENTRATION AND OIL MASS IN COMPRESSOR In the above section, we presented the models to estimate the oil mass and the refngerant mass in the condenser, the evaporator, the gas line, the liquid line, and the accumulator. In this section, w
47、e estimate the oil mass and the refrigerant mass in the compressor based on the conservation of oil and refrigerant mass inside the machine. The oil concentration in the compressor can be derived. 493 Evaporator Measurement L r ; Observer M “ Gas Line -b Compressor oil Model M 9“ Oil Estimator 1pdl,
48、* M Oll * LiquidLine Oil Model f-6 M OY Accumulator VITd -b Figure 4 Overall oil observer structure. GlassWindow * Accumulator or Observer OilMcdel M“ The oil observer that we present here is based on the oil models discussed above and uses the sensor measurements such as the evaporating temperature
49、, the condensing temper- ature, the superheat, the subcool, etc., to estimate the oil concentration and oil amount in the compressor, without an expensive viscosity sensor. The overall structure of the proposed oil observer is shown in Figure 4. In Figure 4, the evaporator oil model estimates the oil mass and the refrigerant mass in the evaporator. The two-phase length of the evaporator Lei is obtained from an evaporator observer that will be described in a separate publication for heat exchanger observers. It is a dynamic observer taking the evaporating temp
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