U.S. patent application number 10/967941 was filed with the patent office on 2005-05-19 for oil circulation observer for hvac systems.
This patent application is currently assigned to Massachusetts Institute of Technology. Invention is credited to Asada, H. Harry, Cheng, Tao, He, Xiang-Dong.
Application Number | 20050103035 10/967941 |
Document ID | / |
Family ID | 34577108 |
Filed Date | 2005-05-19 |
United States Patent
Application |
20050103035 |
Kind Code |
A1 |
He, Xiang-Dong ; et
al. |
May 19, 2005 |
Oil circulation observer for HVAC systems
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.
Inventors: |
He, Xiang-Dong; (Belmont,
MA) ; Asada, H. Harry; (Lincoln, MA) ; Cheng,
Tao; (Malden, MA) |
Correspondence
Address: |
MILLS & ONELLO LLP
ELEVEN BEACON STREET
SUITE 605
BOSTON
MA
02108
US
|
Assignee: |
Massachusetts Institute of
Technology
|
Family ID: |
34577108 |
Appl. No.: |
10/967941 |
Filed: |
October 19, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60523447 |
Nov 19, 2003 |
|
|
|
Current U.S.
Class: |
62/192 ;
62/126 |
Current CPC
Class: |
F25B 31/002 20130101;
F25B 2700/03 20130101; F25B 2500/16 20130101 |
Class at
Publication: |
062/192 ;
062/126 |
International
Class: |
F25B 049/00; F25B
031/00 |
Claims
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 claim 1, wherein the first component of the vapor
compression cycle system is a compressor.
3. The method of claim 1, wherein the plurality of other components
of the vapor compression cycle system 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.
4. The method of claim 1, wherein the parameter related to retained
lubricant in the plurality of other components of the vapor
compression cycle system is determined using one or more parameters
related to the state of each component of the vapor compression
system.
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 claim 5, wherein the first
component of the refrigeration apparatus is a compressor.
7. The refrigeration apparatus of claim 5, wherein the plurality of
other components of the refrigeration apparatus 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.
8. The refrigeration apparatus of claim 5, wherein the parameter
related to retained lubricant in the plurality of other components
of the refrigeration apparatus is determined using one or more
parameters related to the state of each component of the
refrigeration apparatus.
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 claim 9, wherein 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.
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 claim 13, wherein the component
of the refrigeration apparatus is one of an evaporator, an
accumulator, a suction gas line, a discharge gas line, a condenser,
a liquid line and a receiver.
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
RELATED APPLICATION
[0001] 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.
BACKGROUND OF THE INVENTION
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
SUMMARY OF THE INVENTION
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] 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.
[0015] FIG. 1A contains a schematic block diagram of a vapor
compression refrigeration system in accordance with one embodiment
of the present invention.
[0016] FIG. 1B contains a schematic block diagram of the overall
structure of an embodiment of the oil observer in accordance with
the invention.
[0017] FIG. 2 contains a schematic functional block diagram of one
embodiment of an evaporator observer in accordance with the
invention.
[0018] FIG. 3 contains a schematic functional block diagram of one
embodiment of a condenser observer in accordance with the
invention.
[0019] FIG. 4 contains a schematic diagram of the element model for
the condenser two-phase flow region in accordance with the
invention.
[0020] FIG. 5 contains a schematic block diagram of the element
model for the evaporator two-phase flow region in accordance with
the invention.
[0021] FIG. 6 contains a schematic block diagram of a liquid line
in accordance with the invention.
[0022] FIG. 7 contains a schematic diagram of a low-order
evaporator model in accordance with the invention.
[0023] FIG. 8 contains a schematic diagram of a low-order condenser
model in accordance with the invention.
[0024] FIG. 9 contains a graph of the outdoor air temperature
profile over time for an experiment performed in accordance with
the invention.
[0025] FIG. 10 contains a graph of the compressor oil viscosity
profile over time for the experiment.
[0026] FIG. 11 contains a graph of the compressor oil temperature
profile over time for the experiment.
[0027] FIG. 12 contains a graph of the discharge pressure profile
over time for the experiment.
[0028] FIG. 13 contains a graph of the suction pressure profile
over time for the experiment.
[0029] FIG. 14 contains a graph of the mass flow rate profile over
time for the experiment.
[0030] FIG. 15 contains a graph of the evaporating temperature
profile over time for the experiment.
[0031] FIG. 16 contains a graph of the condensing temperature
profile over time for the experiment.
[0032] FIG. 17 contains a graph illustrating compressor oil mass
estimation error in accordance with the invention.
[0033] FIG. 18 contains a graph illustrating estimation error of
oil concentration in the compressor in accordance with the
invention.
[0034] FIG. 19 contains a graph illustrating a comparison between
experimental and estimated oil mass in the compressor in accordance
with the invention.
[0035] FIG. 20, contains a graph of refrigerant mass inventory in
the condenser in accordance with the invention.
[0036] FIG. 21 contains a graph of condenser subcool section length
for one pass in accordance with the invention.
[0037] FIG. 22 contains a graph of the oil mass in the condenser in
accordance with the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0038] FIG. 1A contains a schematic block diagram of a vapor
compression refrigeration system in accordance with one embodiment
of the present invention. Referring to FIG. 1A, the vapor
compression system includes a condenser 1 connected to a compressor
3 via a gas discharge line 2. An accumulator 4 collects refrigerant
flowing through the system and is connected to the compressor 3. An
evaporator 6 is connected to the accumulator 4 via a gas suction
line 5. An expansion valve 8 is connected to the evaporator 6 via a
liquid line 7. A receiver 9, which receives and stores liquid
refrigerant flowing through the system is connected between the
condenser 1 and the expansion valve 8. It should be noted that
although not shown in the drawing of FIG. 1A, the vapor compression
system of the invention can also include additional components such
as an oil separator, a gas cooler, an internal heat exchanger or
other components. The invention is applicable to systems that
include these and other components of vapor compression
refrigeration systems.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 1. Oil Observer Structure.
[0044] An oil observer described herein in accordance with the
invention is based on oil models that will be described below in
Section 2 and uses a sensor measurement such as evaporating
temperature, condensing temperature, superheat, subcool, etc., to
estimate the oil concentration and oil amount in the compressor,
without an expensive viscosity sensor.
[0045] FIG. 1B is a schematic block diagram of the overall
structure of an embodiment of the oil observer 10 in accordance
with the invention. In FIG. 1B, the evaporator oil model 12 is used
to estimate oil mass and refrigerant mass in the evaporator. The
two-phase length of the evaporator Lei is obtained from the
evaporator observer 14, which is described in Section 4 below. The
structure of the evaporator observer 14 is shown in FIG. 2, which
is a schematic functional block diagram of the evaporator observer
14. The evaporator observer is a dynamic observer taking
evaporating temperature Te as an input, and whose output is the
two-phase length Lei (that is I in the model equation).
[0046] The condenser oil model 16 is used to estimate oil mass and
refrigerant mass in the condenser. The two-phase length of the
condenser L.sub.c2 and the subcool section length L.sub.c3 are
obtained from the condenser observer 18, which is described in
Section 4 below. FIG. 3 is a schematic functional block diagram of
the condenser observer 18. The condenser observer 18 is a dynamic
observer that is similar to the evaporator observer 14, taking
condensing temperature Tc, subcool SC, etc., as input, and having
two-phase length L.sub.c2 and subcool section length L.sub.c3 as
outputs. The gas line oil model 20 is used to estimate oil mass and
refrigerant mass in the gas line using parameters provided by the
gas line observer 60. The liquid line oil model 22 is used to
estimate oil mass and refrigerant mass in the liquid line using
parameters provided by the liquid line observer 62. The accumulator
oil model 24 is used to estimate oil mass and refrigerant mass in
the accumulator. The accumulator oil model 24 receives input
information including liquid volume in the accumulator, which is
measured from the accumulator glass window or other
measurement/estimation methods 26. For an air conditioning or
refrigeration system without an accumulator, the accumulator oil
model 24 in FIG. 1B is not used. The receiver oil model 64 is used
to estimate oil mass and refrigerant mass in the receiver.
[0047] 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.
[0048] 2. Component Oil/Refrigerant Models
[0049] 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.
[0050] 2.1 Condenser Oil Model: Refrigerant and Oil Mass in
Condenser
[0051] FIG. 4 is a schematic diagram of the element model for the
condenser two-phase flow region. The condenser can be divided into
three sections as shown in FIG. 4: the superheated section 28
having length L.sub.c1, the two-phase section 30 having length
L.sub.c2 and the sub cooled section 32 having length L.sub.c3.
Lengths L.sub.c2 and L.sub.c3 are obtained from the condenser
observer 18.
[0052] The two-phase section 30 of the condenser can be divided
into N elements i. At i=1, the vapor quality x=0; at i=N, x=1. For
a condenser, it is assumed that the heat flux from the heat
exchange is constant. Then the vapor quality decreases linearly.
The two-phase region is divided into N elements as shown in FIG.
4,0so that within each element the thermodynamic property
differences in each phase are negligible.
[0053] The length of each element is dl.sub.2=Lc.sub.2/N and vapor
quality x(i) in section i can be evaluated: 1 x ( i ) = i - 1 N - 1
( 1 )
[0054] It is assumed that Lc.sub.2 is the length of two-phase
section, x is the vapor quality of the oil/refrigerant mixture at a
location of condenser, C.sub.oil is the oil circulation rate (wt %)
defined as the ratio of oil mass flow rate and total
oil/refrigerant mixture mass flow rate, and a is void fraction
which can be estimated by different void fraction models. The
following equations (2) through (4) are used in accordance with one
embodiment of the invention to estimate mean void fraction a based
on Hughmark's void fraction model that is dependent on mass flow
rate. 2 = K H ( 2 ) = x / g ( x / g ) + ( 1 - x ) / l ( 3 )
[0055] where the parameter K.sub.H has been fitted to a polynomial:
3 K H = 0.7266477 - 3.481988 .times. 10 - 4 Z k - 0.845427 Z k +
0.0601106 Z k 1 / 3 ( 4 )
[0056] while Z.sub.k depends on viscosity, averaged Reynold number
Re (depending on mass flux etc.), the Froude number Fr, and the
liquid volume fraction Y.sub.L. 4 Z k = Re 1 / 6 Fr 1 / 8 y L 1 / 4
Re = DG l + ( v - l ) Fr = 1 gD ( Gx v ) 2 y L = 1 -
[0057] where D is tube hydraulic diameter, G is refrigerant mass
velocity, g is acceleration due to gravity, and .mu..sub.v is
dynamic viscosity of refrigerant vapor, and .mu..sub.l is viscosity
of liquid mixture.
[0058] For each element in the two-phase section 30, the void
fraction is calculated based on the Hughmark's void fraction mode
using the parameters in that element. For a certain element with
vapor quality x and calculated void fraction value .alpha., the
total liquid volume (liquid refrigerant+oil) in that element is
dQ.sub.liquid=dV(1-.alpha.)=A- .sub.cdz(1-.alpha.). Assuming that
the oil is well mixed with the liquid refrigerant, the following
equation (5) is used to estimate the oil mass retention in that
element. 5 dM oil = dV ( 1 - ) C oil ( 1 - x ) liquid ( 5 )
[0059] where .rho..sub.liquid is the density of oil/refrigerant
mixture and is calculated as follows: 6 liquid = oil 1 + 1 - x - C
oil 1 - x ( oil / R - 1 ) ( 6 )
[0060] where .rho..sub.oil is pure oil density and .rho..sub.R is
density of refrigerant liquid and 7 1 - x - C oil 1 - x
[0061] is mass fraction of refrigerant in the oil/refrigerant
mixture.
[0062] The vapor refrigerant mass in that element can be obtained
by:
dM.sub.ref,vapor=dV.alpha..rho..sub.g (7)
[0063] The liquid refrigerant mass in that element can be obtained
by: 8 dM ref , liquid = dV ( 1 - ) 1 - x - C oil 1 - x liquid ( 8
)
[0064] Then, the liquid refrigerant mass in the entire condenser
can be obtained by: 9 M ref , liquid = ( 1 - C oil ) A c L c3
liquid + z = 0 z = L c2 ( 1 - ( x ( z ) ) ) liquid 1 - x ( z ) - C
oil 1 - x ( z ) A c z = ( 1 - C oil ) A c L c3 liquid + i = 1 N ( 1
- i ) liquid 1 - x i - C oil 1 - x i A c l 2 ( 9 )
[0065] The vapor refrigerant mass in the condenser can be obtained
by: 10 M ref , vapor = z = 0 z = L c2 ( x ( z ) ) g A c z + N A c L
c1 g ( 10 ) = i = 1 N i g A c l 2 + N A c L c1 g
[0066] The oil mass in the condenser can be obtained by 11 M oil =
A c L c3 liquid C oil + z = 0 z = L c2 ( 1 - ( x ( z ) ) ) liquid 1
-- C oil 1 - x ( z ) A c z + ( 1 - N ) A c L c1 oil = A c L c3
liquid C oil + i = 1 N ( 1 - i ) liquid 1 - x o - C oil 1 - x i A c
l 2 + ( 1 - N ) A c L c1 oil ( 11 )
[0067] The total refrigerant mass in condenser is
M.sub.ref=M.sub.ref,vapor+M.sub.ref,liquid (12)
[0068] The above condenser oil model obtains information including
Lc2, Lc3 from the condenser observer 18, condensing temperature
T.sub.c, oil circulation rate C.sub.oil, and mass flow rate for
calculating void fraction. Mass flow rate can be estimated based on
the compressor mass flow model. It should be noted that the length
of the subcool section Lc3 is the key value for accurate estimation
of refrigerant mass inventory in the condenser, since in the
subcool section, all refrigerant is high quality liquid that has
much higher density than vapor refrigerant density. Another
important factor is the selection of void fraction model. The
Hughmark model is selected in embodiment of the invention because
some other models tend to underestimate the liquid mass. Generally,
the condenser can hold about 40% to 48% of total refrigerant
charge.
[0069] 2.2 Evaporator Oil Model: Refrigerant and Oil Mass in
Evaporator
[0070] FIG. 5 is a schematic block diagram of the element model for
the evaporator two-phase flow region. The evaporator can be divided
to two sections as shown in FIG. 5: a superheated section 34 having
length Le2, and a two-phase section 36 having length Le1. Two-phase
section length Lei is obtained from evaporator observer 14. The
two-phase region is divided into N elements i. At i=1, the vapor
quality x=x.sub.0 at i=N, x=1-C.sub.oil. The calculation for the
evaporator is similar to that of the condenser. It is assumed that
the vapor quality decreases linearly. The two-phase region is
divided into N elements as shown in FIG. 5, so that within each
element the thermodynamic property differences in each phase are
negligible.
[0071] The length of each element is dl.sub.l=L.sub.el/N, and vapor
quality in section i can be evaluated: 12 x ( i ) = x 0 + i - 1 N -
1 ( 1 - x 0 ) ( 13 )
[0072] 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: 13 M ref , liquid = z = 0 z = L
e1 ( 1 - ( x ( z ) ) ) liquid 1 - x ( z ) - C oil 1 - x ( z ) A c z
= i = 1 N ( 1 - i ) liquid 1 - x i - C oil 1 - x i A c l 1 ( 14
)
[0073] The oil mass in the evaporator can be obtained by: 14 M oil
= z = 0 z = L e1 ( 1 - ( x ( z ) ) ) liquid 1 -- C oil 1 - x ( z )
A c z + ( 1 - N ) A c L e2 oil = i = 1 N ( 1 - i ) liquid 1 - x o -
C oil 1 - x i A c l 1 + ( 1 - N ) A c L e2 oil ( 15 )
[0074] The vapor refrigerant mass in the evaporator can be obtained
by 15 M ref , vapor = z = 0 z = L e1 ( x ( z ) ) g A c z + N A c L
e2 g = i = 1 N i g A c l 1 + N A c L e2 g ( 16 )
[0075] The total refrigerant mass in evaporator is
M.sub.ref=M.sub.ref,vapor+M.sub.ref,liquid (17)
[0076] The above evaporator oil model obtains information of
two-phase section length L.sub.el from the evaporator observer 14,
evaporating temperature T.sub.c, inlet vapor quality x.sub.0, oil
circulation rate C.sub.oil, and mass flow rate for calculating void
fraction. Generally, the evaporator can hold about 10% to 16% of
total refrigerant charge.
[0077] 2.3 Liquid Line Oil Model: Refrigerant and Oil Mass in
Liquid Line
[0078] FIG. 6 is a schematic block diagram of a liquid line. It is
assumed that V is the total volume of the liquid line, x is the
average vapor quality of oil/refrigerant mixture of the liquid
line, and .alpha. is mean void fraction of the liquid line and can
be estimated by different void fraction models. In accordance with
the invention, the Hughmark model or the following equation is used
to estimate mean void fraction .alpha. 16 = 1 1 + 1 - x x ( g l ) 2
3
[0079] where .rho..sub.g is the saturated vapor density and
.rho..sub.l is the saturated liquid density
[0080] When the void fraction value .alpha. is obtained, the total
liquid volume (liquid refrigerant+oil) in the liquid line is
Q.sub.liquid=V(1-.alpha.). Assuming that the oil is well mixed with
the liquid refrigerant; the following equation is used to estimate
the oil mass retention in the liquid line. 17 M oil = V ( 1 - ) C
oil ( 1 - x ) liquid ( 18 )
[0081] The vapor refrigerant mass in the liquid line can be
obtained by:
M.sub.ref,vapor=.alpha.V.rho..sub.g (19)
[0082] The liquid refrigerant mass in the liquid line can be
obtained by: 18 M re , liquid = V ( 1 - ) 1 - x - C oil 1 - x
liquid ( 20 )
[0083] 2.2 Gas Line Oil Model: Refrigerant and Oil Mass in Gas
Line
[0084] 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, .alpha. is
mean void fraction of gas line. The vapor refrigerant mass in the
gas line can be obtained by:
M.sub.ref,vapor=.alpha.V.rho..sub.g (21)
[0085] the oil mass retention in the gas line is
M.sub.oil=V(1-.alpha.).rho..sub.oil (22)
[0086] Gas line and liquid line oil models use information of
averaging vapor quality, averaging refrigerant temperature, oil
circulation rate C.sub.oil, and total mass flow rate.
[0087] 2.5 Accumulator Oil Model: Refrigerant and Oil Mass in
Accumulator
[0088] Assuming that T.sub.r is refrigerant temperature in
accumulator, the saturated liquid density .rho..sub.l and saturated
vapor density .rho..sub.g can be determined from T.sub.r based on
thermodynamics properties. Oil density .rho..sub.oil is a function
of T.sub.r. It is assumed that the total volume of accumulator is
V, and the liquid volume V.sub.L can be calculated based on liquid
level measurement through the glass window at the side of
accumulator.
[0089] The vapor refrigerant mass in the accumulator can be
obtained by:
M.sub.ref,vapor=(V-V.sub.L).rho..sub.g (23)
[0090] The oil mass in the accumulator is
M.sub.oil=V.sub.L.rho..omega..sub.oil (24)
[0091] where .rho. is the density of oil/liquid refrigerant mixture
and is expressed by 19 = oil 1 + ( 1 - oil ) ( oil / R - 1 ) ( 25
)
[0092] and .omega..sub.oil is the oil concentration of oil/liquid
refrigerant mixture in accumulator. The liquid refrigerant mass in
the accumulator can be obtained by:
M.sub.ref,liquid=V.sub.L.rho.(1-.omega..sub.oil) (26)
[0093] 3. Oil Observer for Estimation of Oil Concentration and Oil
Mass in Compressor
[0094] 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.
[0095] Oil mass conservation for the entire machine is
M.sup.cir.sub.oil+M.sup.accu.sub.oil+M.sup.comp.sub.oil=M.sup.total.sub.oi-
l
[0096] where M.sup.total.sub.oil is the total oil mass charged into
the machine and has a known value,
M.sup.cir.sub.oil=M.sup.evap.sub.oil+M.sup-
.cond.sub.oil+M.sup.gas.sup..sub.--.sup.line.sub.oil+M.sup.liquid.sup..sub-
.--.sup.line.sub.oil is the total oil retention in the refrigerant
circuit including the condenser, evaporator, gas line and liquid
line. M.sup.accu.sub.oil is the oil retention in the accumulator.
If there is no accumulator in a machine, this value is zero.
M.sup.cir.sub.oil and M.sup.accu.sub.oil are estimated based on oil
models described in Section 2 above.
[0097] Based on oil mass conservation, the estimated oil mass in
the compressor {circumflex over (M)}.sup.comp.sub.oil from the oil
observer shown in FIG. 1B can be expressed by
{circumflex over
(M)}.sup.comp.sub.oil=M.sup.total.sub.oil-M.sup.cir.sub.o-
il-M.sup.accu.sub.oil (27)
[0098] Refrigerant mass conservation for the entire machine is
M.sup.cir.sub.ref+M.sup.accu.sub.ref+M.sup.comp.sub.ref=M.sup.total.sub.re-
f
[0099] where M.sup.total.sub.ref is the total refrigerant mass
charged into the machine and has a known value, M.sup.cir.sub.ref
is the total refrigerant mass inventory in the refrigerant circuit
including the condenser, evaporator, gas line and liquid line.
M.sup.accu.sub.ref is the refrigerant mass in the accumulator. If
there is no accumulator in a machine, this value is zero.
M.sup.cir.sub.ref and M.sup.accu.sub.ref are estimated based on
models described in Section 2 above.
[0100] Based on refrigerant mass conservation, the estimated
refrigerant mass in the compressor {circumflex over
(M)}.sup.comp.sub.ref from the oil observer shown in FIG. 1B can be
expressed by
{circumflex over
(M)}.sup.comp.sub.ref=M.sup.total.sub.ref-M.sup.cir.sub.r-
ef-M.sup.accu.sub.ref (28)
[0101] 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)}.sup.comp.sub.ref from
Equation (28), the liquid refrigerant mass {circumflex over
(M)}.sup.comp.sub.ref,Liquid is equal to
{circumflex over (M+EE.sup.comp.sub.ref,Liquid={circumflex over
(M)})}.sup.comp.sub.ref-.rho..sub.g(V.sup.comp-V.sup.comp.sub.Liquid)
(29)
[0102] where the second term of Equation (29) is vapor refrigerant
mass in the compressor, .rho..sub.g is the density of vapor
refrigerant in the compressor, V.sup.comp is the total compressor
volume where refrigerant presents, V.sup.comp.sub.Liquid is the
liquid volume of oil/liquid refrigerant mixture and can be
determined by the measurement of liquid level at the glass window
of the compressor.
[0103] 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 20 w oil
comp = M ^ oil comp M ^ oil comp + M ^ ref , Liquid comp ( 30 )
[0104] In one embodiment, the invention is achieve 20% estimation
error for oil concentration, that is 21 w oil comp - w oil sensor w
oil sensor * 100 % < 20 % ( 31 )
[0105] where W.sup.sensor.sub.oil is experimental oil concentration
that is correlated from measurement of viscosity by a viscosity
sensor installed at the bottom of the compressor.
[0106] 4. Heat Exchanger Observers
[0107] 4.1 Model-based Nonlinear Observers for Evaporator
[0108] FIG. 7 contains a schematic diagram of the low-order
evaporator model in accordance with the invention. T.sub.e is the
evaporating temperature. l is the length of the two-phase section.
T.sub.w is the wall temperature of the tube. T.sub..alpha. is the
room air temperature. {dot over (m)}.sub.in and {dot over
(m)}.sub.out are the inlet and outlet refrigerant mass flow rates,
respectively. q is the heat transfer rate from the tube wall to the
two-phase refrigerant. q.sub..alpha. is the heat transfer rate from
the room to the tube wall.
[0109] Assuming a uniform temperature throughout the evaporator
tube wall, the heat transfer equation of the tube wall is as
follows: 22 ( c p A ) e T w t = D o o ( T a - T w ) - D i i ( T w -
T e ) ( 32 )
[0110] 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.
[0111] Assuming the mean void fraction {overscore (.gamma.)} is
invariant, the liquid mass balance equation in the two-phase
section of the evaporator is 23 l ( 1 - _ ) A l ( t ) t = - q h 1 g
+ m . i n ( 1 - x 0 ) ( 33 )
[0112] In equation (33), the left hand side is the liquid mass
change rate in the evaporator. On the right hand side, q/h.sub.lg
represents the rate of liquid evaporating into vapor, and {dot over
(m)}.sub.in(1-x.sub.0) is the inlet liquid mass flow rate.
[0113] The inlet refrigerant mass flow rate {dot over (m)}.sub.in
is dependent on the expansion valve opening A.sub.v, the low
pressure P.sub.e and high pressure P.sub.c, and can be expressed
by
{dot over (m)}.sub.in=A.sub.v.sup..alpha.g.sub.v(P.sub.e,P.sub.c)
(34)
[0114] where .alpha. and g.sub.v (P.sub.e, P.sub.c) can be
identified for a given expansion valve. P.sub.e and P.sub.c can be
measured by two pressure sensors. For the two-phase section, the
pressure is an invariant function of the temperature. Therefore,
the inlet refrigerant mass flow rate {dot over (m)}.sub.in can be
expressed as
{dot over (m)}.sub.in=A.sub.v.sup..alpha.g.sub.v(T.sub.e,T.sub.c)
(35)
[0115] 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: 24 M v t = v g ( T e ) T e T e t = m
. i n x o + q h 1 g - m . out ( 36 )
[0116] where M.sub.v is the total vapor mass and V is the total
volume of the low-pressure side. h.sub.g-h.sub.l=h.sub.lg, where
h.sub.l and h.sub.g are refrigerant saturated liquid and vapor
specific enthalpies. The outlet refrigerant mass flow rate is the
same with the compressor mass flow rate which is dependent on the
compressor speed, the low pressure P.sub.e and high pressure
P.sub.c, and can be expressed by
{dot over (m)}.sub.out =.omega.g(P.sub.e,P.sub.c) (37)
[0117] where g(P.sub.e,P.sub.c) can be identified for a given
compressor. As described above, the pressure is an invariant
function of the temperature for the two-phase section. Therefore,
the outlet refrigerant mass flow rate can be expressed as
{dot over (m)}.sub.out =.omega.g(T.sub.e,T.sub.c) (38)
[0118] Equation (36) can be written as 25 T e t = D i i kh 1 g l (
T w - T e ) + x o k m . i n - 1 k m . out where k = V g ( T e ) T e
. ( 39 )
[0119] Based on equations (32), (33) and (39), the state space
representation for the low order evaporator model is as follows,
where T.sub.e, l and T.sub.w are the three states of the model. 26
( T . e T . w l . ) = ( D i i kh 1 g l ( T w - T e ) + x o k m . i
n - 1 k m . out 1 ( C p A ) e ( D o o ( T a - T w ) - D i i ( T w -
T e ) ) 1 l ( 1 - _ ) A ( - D i i l ( T w - T e ) h 1 g + m . i n (
1 - x o ) ) ) ( 40 )
[0120] Where T.sub..alpha., {dot over (m)}.sub.in, and {dot over
(m)}.sub.out are the inputs to the system.
[0121] Since only T.sub.e can be easily measured using a
thermocouple, an observer in accordance with the invention is used
in estimating the value of l, the length of two-phase section of
the evaporator.
[0122] The following are the dynamics of the non-linear observer
described herein. 27 ( T ^ . e T ^ . w l ^ . ) = ( D i i kh 1 g l ^
( T ^ w - T ^ e ) + x o k m . i n - 1 k m . out 1 ( C p A ) e ( D o
o ( T a - T ^ w ) - D i i ( T ^ w - T ^ e ) ) 1 l ( 1 - _ ) A ( - D
i i l ^ ( T ^ w - T ^ e ) h 1 g + m . i n ( 1 - x o ) ) ) - ( L 1 L
2 L 3 ) ( T ^ e - T e ) ( 41 )
[0123] where L.sub.1, L.sub.2, and L.sub.3 are observer gains. For
the observer, the contraction theory is used in accordance with the
invention to ensure that the estimated state variables will
converge to the actual states in the plant The contraction theory
states that the system {dot over (x)}=f(x, t) is said to be
contracting if 28 f x
[0124] is uniformly negative definite. All system trajectories then
converge exponentially to a single trajectory, with convergence
rate .vertline..lambda..sub.max.vertline., where .lambda..sub.max
is the largest eigenvalue of the symmetric part of 29 f x .
[0125] Therefore, if we can make sure the ax actual states are
particular solutions of the observer and the observer is
contracting, then we can conclude that all the trajectories of the
observer will converge to the particular solutions that are the
actual states.
[0126] From the observer dynamics, if {circumflex over (T)}.sub.e
is equal to T.sub.e, the observer dynamics are the same with the
system dynamics. So the actual states that are the solutions of
this set of equations are particular solutions of the observer. If
the symmetric part of the Jacobian matrix of the observer dynamics
is uniformly negative definite, then the trajectory of the observer
dynamics will converge to the particular solution which means the
observed states are the same as the actual states.
[0127] 4.2 Model-based Nonlinear Observers for Condenser
[0128] In the oil observer described in Section 1 above, the length
of two-phase and length of subcooled liquid section of a condenser
are used to estimate the oil in the condenser, since the oil
calculation models are different for different phase sections.
[0129] In this section, the model of the condenser is described.
The condenser model is similar to the evaporator model. FIG. 8
contains a schematic diagram of a low-order condenser model in
accordance with the invention.
[0130] The vapor balance equation is as follows 30 M v , c t = V c
g ( T c ) T c T c t = m . i n , c - D i , c i , c L c2 ( T c - T w
, c ) h 1 g ( 42 )
[0131] where T.sub.c is the condensing temperature, L.sub.c2 is the
two phase length and T.sub.w,c is the wall temperature of the
condenser.
[0132] The heat transfer equation is as follows: 31 ( c p A ) c T w
, c t = D i , c i , c ( T c - T w , c ) - D o , c o , c ( T w , c -
T o , c ) ( 43 )
[0133] where T.sub.a,c is the outdoor air temperature.
[0134] 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. 32 A c (
1 - _ ) l L c2 t + A c l L c3 t = D i , c i , c L c2 ( T c - T w ,
c ) h 1 g - m . out , c ( 44 )
[0135] where L.sub.c3 is the subcool liquid phase length.
[0136] 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 33 L c1 t 0.
[0137] We have 34 A c ( 1 - _ ) l L c2 t + A c l ( L - L c1 - L c2
) t = A c ( 1 - _ ) l L c2 t - A c l L c2 t = - A c _ l L c2 t = D
i , c i , c L c2 ( T c - T w , c ) h 1 g - m . out , c L c3 = L - L
c1 - L c2 ( 45 )
[0138] Therefore the liquid mass balance equation can be written as
35 L c2 t = - D i , c i , c L c2 ( T c - T w , c ) A c _ l h 1 g (
46 )
[0139] Equations (42), (43) and (44) are the condenser model. The
model-based observer for the condenser is described as follows 36 (
T ^ c T ^ w , c L ^ c2 ) = ( m . in , c / k - D i , c i , c L ^ c2
( T ^ c - T ^ w , c ) k h 1 g 1 ( C p A ) c ( D i , c i , c ( T ^ c
- T ^ w , c ) - D o , c o , c ( T ^ w , c - T a , c ) ) - 1 A c _ l
( D i , c i , c L ^ c2 ( T ^ c - T ^ w , c ) h 1 g - m . out , c )
) - ( L 1 L 2 L 3 ) ( T ^ c - T c ) ( 47 )
[0140] where L.sub.1, L.sub.2, and L.sub.3 are observer gains.
[0141] 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.
[0142] 4.3 Gas Line Observer and Liquid Line Observer
[0143] 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.
[0144] 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.
[0145] 5. Experimental Comparison
[0146] 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%.
[0147] 5.1 Experimental Set-Up
[0148] 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.
[0149] 5.2 Experimental Testing
[0150] 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.
[0151] 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:
[0152] 1) Outdoor temperature: 35 C
[0153] 2) Indoor temperature: 20 C
[0154] 3) Compressor Speed: 70 Hz
[0155] 4) Expansion Valve: 15 Steps
[0156] 5) Indoor fan speed: 1250 rpm
[0157] 6) Outdoor fan speed: 630 rpm
[0158] 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 FIG. 9, which is a graph of the outdoor air temperature profile
over time for the experiment.
[0159] FIG. 10 is a graph of the compressor oil viscosity profile
over time for the experiment. FIG. 10 illustrates that the oil
viscosity is changed from about 0.0035 Pa.s to 0.0039 Pa.s. FIG. 11
is a graph of the compressor oil temperature profile over time for
the experiment. As shown in FIG. 11, oil temperature is changed
from about 57 C to 52 C. FIG. 12 is a graph of the discharge
pressure profile over time for the experiment. As shown in FIG. 12,
discharge pressure is changed from 2.8 MPa to 2.4 MPa. FIG. 13 is a
graph of the suction pressure profile over time for the experiment.
FIG. 14 is a graph of the mass flow rate profile over time for the
experiment. FIG. 15 is a graph of the evaporating temperature
profile over time for the experiment. FIG. 16 is a graph of the
condensing temperature profile over time for the experiment.
[0160] 5.3 Comparison results
[0161] 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.
[0162] Measurement at the Initial Condition:
[0163] 1) Evaporating temperature: 7.8 C
[0164] 2) Evaporating pressure: 0.8 MPa
[0165] 3) Condensing temperature: 48.2C
[0166] 4) Condensing pressure: 2.77 MPa
[0167] 5) Subcool: 6 C
[0168] 6) Mass Flow Rate: 0.0185 kg/s
[0169] 7) Oil Temperature: 57 C
[0170] 8) Oil Viscosity: 0.0035 N/m{circumflex over ( )}2*s
(Pa*s)
[0171] 9) Liquid Volume in Accumulator: 130 cc
[0172] 10) Liquid Volume in Compressor: 330 cc
[0173] Comparison
[0174] 1) Estimation Error of Oil Concentration in
Compressor=13%
[0175] 2) Estimation Error of Oil Mass in Compressor: 10%
[0176] It is assumed that
[0177] 1) Oil Circulation Rate in the Circuit is 0.5% (no sensor to
measure)
[0178] 2) Refrigerant Concentration in accumulator is 65% (no
sensor to measure)
[0179] 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 FIGS. 17-19. That is, FIG. 17 is a
graph illustrating compressor oil mass estimation error. FIG. 18 is
a graph illustrating estimation error of oil concentration in the
compressor. FIG. 19 is a graph illustrating a comparison between
experimental and estimated oil mass in the compressor in accordance
with the invention. The estimation error is smaller than 10% in
most cases.
[0180] 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 FIG. 20,
which is a graph of refrigerant mass inventory in the condenser for
the experiment. FIG. 21 is a graph of condenser subcool section
length for one pass in the experiment. The length of the subcool
section L.sub.c3 estimated from the condenser observer is shown in
FIG. 21.
[0181] FIG. 22 is a graph of the oil mass in the condenser for the
experiment. With the subcool section length being changed from 1.9
m to 2.62 m, the refrigerant mass inventory is increased from 42.2%
of total refrigerant mass to 46.7% of total refrigerant mass.
[0182] Table 1 shows the estimated oil distribution in the air
conditioning machine at time t=10 minutes in accordance with the
invention.
1TABLE 1 Estimated Oil Mass in Each Component Component % of Total
Oil Mass Compressor 68.2% Condenser 5.6% Evaporator 2.8% Gas Line
(Suction) 8.1% Gas Line (Discharge) 2.9% Accumulator 12.3% Liquid
Line 0.1% Total 100% Table 2 shows the estimated refrigerant mass
distribution in the air conditioning machine at time t = 10
minutes.
[0183]
2TABLE 2 Estimated Refrigerant Mass in Each Component % of Total
Refrigerant Component Mass Compressor 19.2% Condenser 46.8%
Evaporator 14.7% Gas Line (Suction) 1.5% Gas Line (Discharge) 2%
Accumulator 11.7% Liquid Line 4.1% Total 100%
[0184] 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.
[0185] 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.
* * * * *