U.S. patent application number 11/345511 was filed with the patent office on 2007-08-02 for method for controlling a pulsed expansion valve.
Invention is credited to Robert Walter Redlich.
Application Number | 20070175229 11/345511 |
Document ID | / |
Family ID | 38320660 |
Filed Date | 2007-08-02 |
United States Patent
Application |
20070175229 |
Kind Code |
A1 |
Redlich; Robert Walter |
August 2, 2007 |
Method for controlling a pulsed expansion valve
Abstract
A method of controlling the duty cycle of a pulse width
modulated expansion valve in order to achieve low and stable
evaporator superheat is disclosed. Duty cycle is
incremented/decremented if superheat is above/below a dead band,
and, in a preferred embodiment, incremented/decremented if the time
derivative of superheat is above/below a preset value.
Inventors: |
Redlich; Robert Walter;
(Athens, OH) |
Correspondence
Address: |
KREMBLAS, FOSTER, PHILLIPS & POLLICK
7632 SLATE RIDGE BOULEVARD
REYNOLDSBURG
OH
43068
US
|
Family ID: |
38320660 |
Appl. No.: |
11/345511 |
Filed: |
February 1, 2006 |
Current U.S.
Class: |
62/225 |
Current CPC
Class: |
Y02B 30/70 20130101;
F25B 2600/21 20130101; F25B 41/347 20210101; F25B 41/31
20210101 |
Class at
Publication: |
062/225 |
International
Class: |
F25B 41/04 20060101
F25B041/04 |
Claims
1. A method for stable control of evaporator superheat in a vapor
compression refrigerator, evaporator superheat symbolized herein by
SH, SH defined as the difference between evaporator outlet and
inlet temperatures, the refrigerator including a compressor, a
condenser, a pulse width modulated expansion valve, an evaporator,
and a thermostatic control for turning the compressor on when the
temperature of the interior of the refrigerator rises above a
preset value and turning it off when the interior temperature falls
below a preset limit, the method consisting generally of
controlling the duty cycle of the pulse width modulated expansion
valve in response to evaporator superheat, duty cycle defined as
the ratio of the time the expansion valve is open to the time
interval between successive openings of the expansion valve, duty
cycle symbolized herein by DCY, the method specifically comprising
the following elements, 1) temperature sensors responsive to SH, 2)
an electronic control which periodically responds to SH signal from
the temperature sensors by adjusting DCY as follows: if SH is
within a preset range referred to herein as the "dead band", the
control does not change existing DCY; if SH is above the dead band,
the control increments DC; and if superheat is below the dead band,
the control decrements DCY.
2. A method according to claim 1, in combination with the following
additional operations, if .times. .times. d ( SH ) d t .gtoreq.
first .times. .times. preset .times. .times. value , increment
.times. .times. DCY , .times. if .times. .times. d ( SH ) d t
.ltoreq. second .times. .times. preset .times. .times. value ,
decrement .times. .times. DCY , .times. ##EQU5## where, in the
above, t represents time.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to vapor compression
refrigeration in which the flow rate of refrigerant is controlled
by a pulse width modulated expansion valve.
[0003] 2. Description of Related Art
[0004] It is well known in the field of the invention that
refrigeration efficiency increases as evaporator superheat
decreases. To realize high efficiency, expansion valves are
commonly used in feedback systems that achieve low superheat by
increasing the flow of refrigerant when superheat increases above a
design value and decreasing flow when superheat falls below the
design value. Such systems are prone to superheat oscillation
because of the thermal time constant of evaporator temperature in
response to changing flow rate, and also because of transit time
delay between a change in flow rate at the evaporator inlet and
consequent change in flow at the evaporator outlet. Stability
commonly requires relatively expensive controls, and may only be
achieved if superheat is relatively high. The present invention is
a method according to which an inexpensive control, used with a
pulse width modulated expansion valve, can provide superheat that
is both stable and lower than is realized in existing art.
BRIEF SUMMARY OF THE INVENTION
[0005] A pulse width modulated expansion valve, when used to
control refrigerant flow, is opened at constant frequency and held
open for a controllable time. "Duty Cycle", abbreviated hereafter
by DCY, is the ratio of the time the valve is open to the interval
between successive openings of the valve. Generally, evaporator
superheat control with a pulse width modulated expansion valve
comprises lowering DCY when superheat decreases and conversely.
This constitutes negative feedback since reduced DCY raises
superheat. Thus, if the system is stable, superheat will be
maintained near a constant value. Unless measures are taken to
prevent it, instability will occur, particularly at low superheat
because the ratio [(change in superheat/change in DCY)] increases
rapidly as superheat approaches zero. Thus, in existing art, it is
difficult to maintain average superheat below 5.degree. C. A
controller according to the method of the invention achieves stable
average superheat of about 3.degree. C. with the following basic
method; [0006] a) Creation of a superheat "dead band", typically
from 2.5.degree. C. to 4.5.degree. C. If superheat is within this
dead band, the controller maintains the existing DCY. [0007] b) If
superheat is outside the dead band, the controller, at time
intervals .DELTA.t, increments existing DCY if superheat is above
the dead band and decrements existing DCY if superheat is above the
dead band.
[0008] It can be shown that a system controlled according to the
above method is stable within wide ranges of .DELTA.t, increments
of DCY, and decrements of DCY, and when used in a conventional
refrigerator in which the compressor is turned on and off by a
thermostat, can maintain average superheat close to the center of
the dead band.
[0009] Superheat excursions outside the dead band can be reduced in
amplitude and number of occurrences by augmenting the basic method
with a "rate correction" as follows; if .times. .times. d ( SH ) d
t .gtoreq. first .times. .times. preset .times. .times. value ,
increment .times. .times. DCY .times. .times. ( t = time , SH =
superheat ) ##EQU1## if .times. .times. d ( SH ) d t .ltoreq.
second .times. .times. preset .times. .times. value , decrement
.times. .times. DCY ##EQU1.2##
g) BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0010] FIG. 1 illustrates operation of a pulse width modulated
expansion valve, and defines "duty cycle".
[0011] FIG. 2 illustrates response of superheat to an increment and
a decrement of duty cycle.
[0012] FIG. 3 illustrates a basic embodiment of the method of the
invention, in the form of an operations flow chart that can be
implemented with a microprocessor.
[0013] FIG. 4 shows how application of the basic method of the
invention causes superheat to decrease from a value above the dead
band to a value within the dead band.
[0014] FIG. 5 illustrates a preferred embodiment of the method of
the invention, in the form of an operations flow chart that can be
implemented with a microprocessor.
[0015] FIG. 6 shows how application of the preferred method of the
invention causes superheat to decrease from a value above the dead
band to a value within the dead band.
[0016] In describing the preferred embodiment of the invention
which is illustrated in the drawings, specific terminology will be
resorted to for the sake of clarity. However, it is not intended
that the invention be limited to the specific term so selected and
it is to be understood that each specific term includes all
technical equivalents which operate in a similar manner to
accomplish a similar purpose.
h) DETAILED DESCRIPTION OF THE INVENTION
[0017] FIG. 1 illustrates the definition of the "duty cycle"
(symbolized herein as DCY) of a pulse width modulated expansion
valve. Referring to FIG. 1, the valve is opened at successive times
separated by a constant interval T, and is held open for a
controllable interval T (open). DCY is defined as; DCY = T .times.
.times. ( open ) T ##EQU2##
[0018] FIG. 2 illustrates how evaporator superheat, symbolized
herein by SH, and defined as; [0019] SH =(Temperature of Evaporator
Outlet--Temperature of Evaporator Inlet), responds to decrements
and increments of DCY.
[0020] Referring to FIG. 2, DCY is decremented at time t(dec).
Decrementing DCY causes the average refrigerant flow at the
evaporator inlet, where the expansion valve is located, to
decrease. Decreased flow at the evaporator outlet commences after a
transit time interval required for refrigerant to move from the
inlet to the outlet of the evaporator. Following the transit time
interval, SH rises exponentially toward a new equilibrium, with a
thermal time constant dependent on the heat capacity of the
evaporator.
[0021] Again referring to FIG. 2, DCY is incremented at time
t(inc). Incrementing DCY causes the average refrigerant flow at the
evaporator inlet, where the expansion valve is located, to
increase. Increased flow at the evaporator outlet commences after a
transit time interval required for refrigerant to move from the
inlet to the outlet of the evaporator. Following the transit time
interval, SH falls exponentially toward a new equilibrium, with a
thermal time constant dependent on the heat capacity of the
evaporator.
[0022] From the above, it follows that SH can be controlled by
periodically either incrementing DCY if SH is above a specified
value SH(design) or decrementing DCY if SH is below SH(design).
However, such a control would not cause SH to converge to SH
(design). Rather, SH would oscillate around SH (design) because of
the thermal time constant of the evaporator and the evaporator
transit time.
[0023] The basic invention overcomes instability by periodically,
at time intervals .DELTA.t, incrementing or decrementing DCY if SH
is respectively above or below a range of SH referred to herein as
the "dead band", and maintaining existing DCY if SH is within the
dead band. For example, if the dead band is 2.5.degree. C.
.ltoreq.SH .ltoreq.4.5.degree. C., SH is incremented if SH exceeds
4.5.degree. C., decremented if SH is less than 2.5.degree. C., and
maintained at the existing DCY if SH is within the dead band. It
can be shown that SH will stabilize within the dead band over wide
ranges of .DELTA.t, DCY increment, and DCY decrement.
[0024] FIG. 3 shows an operations flow chart implementing the basic
invention. The operations shown can be controlled by an
appropriately programmed microprocessor. In FIG. 3: [0025] dead
band is 2.5.degree. C. .ltoreq.SH .ltoreq.4.5.degree. C. and;
[0026] .DELTA.t=10 seconds, [0027] Decrement of DCY=0.005.times.DCY
[0028] Increment of DCY=0.005.times.DCY
+[0.001.times.(SH-4.5)].times.DCY
[0029] The term [0.001.times.(SH-4.5)].times.DCY is included in the
increment of DCY to hasten reduction of SH from high values such as
15-20.degree. C. to the dead band.
[0030] FIG. 4 shows reduction of SH from a value SH(0) to values
within the dead band. DCY is incremented at points A.B, and C,
causing SH to overshoot the lower limit of the dead band. To
correct the overshoot, DCY is decremented at points E, F, and G,
causing SH to overshoot the upper limit of the dead band. This
overshoot is corrected by incrementing DCY at points H and I, thus
bringing SH into the dead band at point J. The value of DCY that
exists at point H is maintained, and SH drifts downward as the
interior of the refrigerated space cools. Eventually SH will reach
the lower limit of the dead band, and will be decremented to bring
it within the dead band (this process is not shown in FIG. 4).
[0031] A preferred method which will reduce the number of
occurrences and the amplitude of overshoots is shown in FIG. 5. In
addition to the basic method, it incorporates "rate correction" as
follows; if .times. .times. d ( SH ) d t .gtoreq. first .times.
.times. preset .times. .times. value , increment .times. .times.
DCY .times. .times. ( t = time ) ##EQU3## if .times. .times. d ( SH
) d t .ltoreq. second .times. .times. preset .times. .times. value
, decrement .times. .times. DCY ##EQU3.2##
[0032] In FIG. 5, the first preset value is 0.05 degrees per second
and the second preset value is -0.05 degrees per second.
[0033] Rate correction corrects for trends in SH. Its effect is
shown in FIG. 6, which may be contrasted with FIG. 4. A rate
induced decrement is applied at point C, where .times. .times. d (
SH ) d t .ltoreq. - .05 , and .times. .times. a .times. .times.
rate .times. .times. increment .times. .times. is .times. .times.
applied .times. .times. at .times. .times. point .times. .times. D
, .times. where .times. .times. d ( SH ) d t .gtoreq. .05 .
##EQU4## The effect of the rate correction is to substantially
reduce time during which SH is outside the dead band.
[0034] Wide ranges of values of values of the parameters .DELTA.t,
increment, decrement, and rate correction will result in acceptable
control of superheat according to the method of the invention. Any
practically useful set of values of these parameters, when used
with the method of the invention, is considered to be within the
scope of the invention.
* * * * *