U.S. patent number 3,740,926 [Application Number 05/098,429] was granted by the patent office on 1973-06-26 for portable electronic precipitator.
This patent grant is currently assigned to Texas Electronic Precipitator Co.. Invention is credited to Teddy A. Duval.
United States Patent |
3,740,926 |
Duval |
June 26, 1973 |
PORTABLE ELECTRONIC PRECIPITATOR
Abstract
A two-stage type electrostatic precipitator mechanism is carried
within a rectangular cabinet having an air intake at one end and an
air outlet at the other. A blower mechanism is carried within the
cabinet for moving air through the cabinet and past the ionizing
wires and collecting plates of the precipitator mechanism. A power
supply circuit provides high voltages for the ionizing wires and
collecting plates and an indicator lamp is connected in circuit
with such power supply circuit for providing an alarm signal when
the collecting plates become too dirty. The spacing between the
collecting plates and the value of the high voltage supplied
thereto are of an unconventional nature and provide improved
operating performance. The precipitator unit as a whole is
constructed so that it can be operated in either a normal or an
upside-down position.
Inventors: |
Duval; Teddy A. (Garland,
TX) |
Assignee: |
Texas Electronic Precipitator
Co. (Garland, TX)
|
Family
ID: |
22269241 |
Appl.
No.: |
05/098,429 |
Filed: |
December 15, 1970 |
Current U.S.
Class: |
96/26; 55/481;
96/82 |
Current CPC
Class: |
B03C
3/32 (20130101) |
Current International
Class: |
B03C
3/00 (20060101); B03C 3/32 (20060101); B03c
003/66 () |
Field of
Search: |
;55/104,105,106,139,136,137,138,140,141,143,145,270,274,481,126,129,151 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Electro-Air Electronic Air Cleaners Slim Line units," Electro-Air
Cleaner Company, Inc., Olivia & Sproul Streets, McKees Rocks,
Pa. 4 pages, received in Patent Office Oct. 26, 1967..
|
Primary Examiner: Talbert, Jr.; Dennis E.
Claims
I claim
1. A portable electronic precipitator for cleaning the air in a
room or other relatively enclosed environment frequented by human
beings comprising:
a rectangular cabinet having top, bottom, front and back panels and
an air intake at one end and an air outlet at the opposite end;
blower means carried within the cabinet for moving air from the
intake to the outlet;
precipitator means carried within the cabinet and including a group
of spaced electrically conductive elements located in the air flow
path for removing airborne particles from the air; and,
power supply means carried within the cabinet for producing a
substantial electrical potential difference between different ones
of the conductive elements;
the blower means, precipitator means and power supply means being
mounted within the cabinet so that the precipitator can be operated
either with the top panel on top and the bottom panel on bottom or
vice versa;
first mounting means secured to the top panel and second mounting
means secured to the bottom panel of the cabinet so that the
cabinet may be mounted either with the top panel up and the bottom
panel down or vice versa;
indicator means connected in the circuit with the power supply
means for providing an alarm signal when the conductive elements
become too dirty for desired operational efficiency;
said front panel including a hinged door for providing access to
the interior of the unit, the direction in which said front panel
faces being capable of being made constant as the direction of air
flow is reversed by reversing the one said mounting means used for
mounting said precipitator; and
a panel plate mounting the controls for said precipitator and
releasably secured to a fixed portion of said front panel whereby
as the mounting is changed from one of said mounting means to the
other, the panel plate means can be rotated 180.degree. to maintain
the printed legends on the plate in a non inverted condition.
2. A portable electronic percipitator in accordance with claim 1
wherein the indicator means is constructed and connected to sense
the leakage current passing between the electrically conductive
elements.
3. A portable electronic precipitator in accordance with claim 1
wherein the power supply means includes a step-up transformer and
rectifier circuit means coupled to the secondary thereof for
supplying a relatively large unidirectional voltage to some of the
electrically conductive elements and wherein the indicator means
includes a current sensing means connected in series with the
transformer secondary winding and the rectifier circuit means.
4. A portable electronic precipitator in accordance with claim 3
wherein the current sensing means includes an indicator lamp for
providing a visual alarm signal when the electrically conductive
elements become too dirty.
5. A portable electronic precipitator in accordance with claim 3
wherein the current sensing means includes a current sensing
resistor connected in series with the transformer secondary winding
and the rectifier circuit means and further includes an indicator
lamp and current limiting resistor connected in series across the
current sensing resistor.
Description
BACKGROUND OF THE INVENTION
This invention relates to electronic precipitators and,
particularly, to portable electronic precipitators for cleaning the
air in an interior space frequented by human beings.
It has been heretofore proposed to use a compact portable
electronic precipitation in a people-inhabited room or interior
space for purposes of cleaning the air in the room. Such a
precipitator is particularly useful for removing solids and liquid
such as smoke, oil mist and the like from the air. The previously
proposed precipitators, however, suffer from various limitations
and disadvantages. For one thing, the operation of the precipitator
causes the air-borne particles to be deposited on and adhere to the
surfaces of the collecting plates therein. As the use continues,
the particle layer or film become thicker and thicker, thus
reducing the air space between the collecting plates. Eventually,
the air gap between collecting plates becomes sufficiently small
such that electrical arcing occurs between plates. Arcing reduces
the voltage across the plate, substantially reducing the efficiency
of the unit and increasing the amount of ozone produced. This can,
of course, be prevented by periodic cleaning of the collecting
plates. Unfortunately, however, there are many variables which
enter into determining the length of time required for the particle
deposit to build up to a dangerous thickness. Thus, it is difficult
to know how often the collecting plates should be cleaned in order
to maintain efficient operation.
Another problem with previously proposed portable type
precipitators is that they tend to produce more ozone than is
desired. If such ozone concentration should become very
significant, it could have a toxic effect on the human inhabitants
in the room. Also, the smell of such ozone may be disagreeable to
some of the inhabitants.
A further problem with some previously proposed portable type
precipitators is that their construction allows some of the room
air to pass through the precipitator without passing between the
collecting plates. This undesired blow-by reduces the overall
efficiency of the precipitator.
It is an object of the invention, therefore, to provide a new and
improved portable electronic precipitator which substantially
overcomes one or more of the limitations of the portable
precipitators heretofore proposed.
It is another object of the invention to provide a new and improved
portable electronic precipitator which substantially avoids
excessive ozone production and the like and in which an indication
is provided if the collecting plates become excessively dirty.
For a better understanding of the present invention, together with
other and further objects and features thereof, reference is had to
the following description taken in connection with the accompanying
drawings, the scope of the invention being pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings:
FIG. 1 is a perspective view of a portable electronic precipitator
constructed in accordance with the present invention;
FIG. 2 is a perspective view similar to FIG. 1 except that the
precipitator unit is turned upside-down so that the direction of
air flow is reversed;
FIG. 3 is an elevational view taken along section line 3--3 of FIG.
1 and showing the precipitator with the front panel removed;
FIG. 4 is a cross-sectional view taken along section line 4--4 of
FIG. 3 and showing further details of the collecting cell;
FIG. 5 is a top view of the collecting cell used in the FIG. 1
precipitator unit;
FIG. 6 is an electrical circuit diagram for a first embodiment of a
power supply unit carried within the FIG. 1 precipitator; and
FIG. 7 is a circuit diagram for a second embodiment of a power
supply unit.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
Referring to FIG. 1, there is shown a portable electronic
precipitator for cleaning the air in a room or other interior
environment frequented by human beings. The precipitator includes a
rectangular metal cabinet 10 having a top panel 11, a bottom panel
12, a back panel 13 and a front panel 14. The cabinet 10 also
includes an air intake 15 located at the right hand end in FIG. 1
and an air outltet 16 located at the left hand end in FIG. 1. The
front panel 14 includes a hinged door 17 having a handle 18, the
door hinge being indicated at 19. The remainder of the front panel
14 is permanently secured to the top and bottom panels 11 and 12.
An on-off switch 20, a pair of indicator lamps 21 and 22 and a fuse
holder 23 are mounted on a panel plate 14a releasably secured to
the non-door portion of the front panel 14.
FIG. 2 shows the precipitator cabinet 10 of FIG. 1 turned
upside-down so that the top panel 11 is on the bottom and the
bottom panel 12 is on the top. This reverses the direction of air
flow, the air moving from left to right in FIG. 2. At the same
time, the front panel 14 is still facing in the forwardly
direction. During installation, the panel plate 14a is unfastened,
rotated 180.degree. and then refastened. This keeps the printed
legends on plate 14a in a non-inverted condition for easier reading
of same.
As indicated in FIG. 1, a first set of mounting members 24 is
secured to the top panel 11, while, as indicated in FIG. 2, a
second set of mounting members 25 is secured to the bottom panel
12. These mounting members 24 and 25 are in the form of hollow
square-shaped metal channel members welded or bolted to the top and
bottom panels 11 and 12. Various metal eyelets 26 may be screwed
into the mounting members 24 for purposes of suspending the cabinet
10 from the ceiling of a room by way of wires 27. Similarly, when
the upside-down position of FIG. 2 is desired, various eyelets 28
may be screwed into the bottom panel mounting members 25 for
suspending the cabinet 10 by means of wires 29. Alternatively, the
cabinet 10 may be set on top of a shelf (shown in FIG. 3) or some
form of horizontally extending support brackets which extend
outward from a wall of the room.
As will be seen, the various units, components and mechanisms
located within the cabinet 10 are constructed so that the
precipitator operates perfectly satisfactorily either with the top
panel 11 on top and the bottom panel 12 on bottom or vice-versa.
This upside-down feature gives the user a greater degree of freedom
in the manner of placement of the cabinet 10 in the room. In
particular, the cabinet 10 can be mounted so that the air flow is
from right to left or vice-versa, as desired, and yet the front
panel 14 will still face in the same direction for purposes of
allowing access to the service door 17 for enabling periodic
replacement of the precipitator mechanism within the cabinet 10 by
clean mechanisms. This is particularly important where it is
desired to have the back panel 13 located against or near a wall of
the room. In other words, the cabinet 10 can be mounted against a
wall of the room and the user will still have a choice as to the
direction of air flow.
Referring now to FIG. 3, there is shown a front elevational view of
the precipitator unit of FIG. 1 with the front panel 14 (including
door 17) removed. As seen in FIG. 3, there is carried within the
cabinet 10 a blower unit 30 for moving air through the cabinet 10
from the intake 15 to the outlet 16. Blower unit 30 includes a
squirrel cage type fan blade mechanism 31 and an electric motor 32
mounted thereon and coupled to the squirrel cage rotor by way of
pulley belt 33. Motor 32 is mounted on a plate 32a, one end of
which is pivotally pinned to the housing of fan 31 and the other
end of which is supported by a tension adjustment screw 32b for
adjusting the tension of pulley belt 33. An adjustable linkage 32c
running to the base plate for the fan housing maintains the proper
belt tension when the unit is operated in the inverted position of
FIG. 2. The outlet opening 34 of the fan blade mechanism 31 is
mounted in an exhaust port formed in an otherwise solid partition
36 which extends completely across the left-hand end of the cabinet
10 a short distance inside the cabinet outlet 16. A louver
structure 38 is mounted in the air outlet 16 for covering the
opening at the left-hand end of the precipitator and, at the same
time, allowing the discharge of the cleaned air back into the
room.
A mechanical air filter 40 is removably mounted immediately inside
the cabinet intake opening 15. Filter 40 may be of either the glass
wool or metal excelsior type and serves to prevent larger size
solid particles or materials from entering the interior of the
cabinet 10. Filter 40 also serves to impart a pressure drop across
the inlet so that a more uniform velocity distribution is
achieved.
Also carried within the cabinet 10 is a precipitator mechanism
located in the air flow path for removing the smaller airborne
particles (tobacco smoke, oil mist, etc.) from the air as it passes
through the cabinet 10. In the present embodiment, the precipitator
mechanism is a two stage type of electrostatic precipitator and, as
such, includes an ionizer unit 42 and a collecting cell unit 44
located downstream of the ionizer unit 42. Each of units 42 and 44
extends across the width and height of the interior of the cabinet
10 and each is removably mounted within the cabinet 10 by means of
slideways formed by channels 46 which are secured to the top and
bottom panels 11 and 12 and which run horizontally from the front
to the back of the cabinet 10. Thus, the ionizer and collecting
cell units 42 and 44 (as well as the filter 40) can be removed from
the cabinet 10 by opening the service door 17 and pulling them in
the forwardly direction. Collecting cell unit 44 is provided with a
handle 48 for facilitating its removal.
The ionizer unit 40 includes a four-sided frame-like metal chassis
50 having a vertically spaced series of horizontal metal plates or
fins (not shown) which extend from the front panel end 51 to the
rear panel end (not visible) of the chassis 50. Located
intermediate these horizontal fins are horizontally-extending
ionizer wires (not visible) which are strung between the ends of
inwardly extending metal fingers 52 which form parts of and extend
toward the left from a pair of side-located vertically running
channel members 53 (the rearward one of which is hidden by the
forward one). The channel members 53 are spaced from the chassis 50
by means of electrical insulators 54. The application of a
relatively high positive voltage to the ionizer wires causes a
corona discharge between such wires and the grounded fins, the
latter being grounded by way of the ionizer chassis 50 and the
cabinet 10. The gaseous ionization thus produced by such corona
discharge serves to charge the suspended airborne particles as they
move through the ionizer unit 40.
FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 3
and shows in greater detail the construction of the collecting cell
unit 44. Such collecting cell unit 44 includes a group of spaced
electrically conductive elements located in the air flow path for
removing from the air the airborne particles charged by the ionizer
unit 40. As seen in FIG. 4, these electrically conductive elements
comprise two interleaved sets of spaced parallel plates, the plates
in the first set being designated by reference numeral 56 and the
plates in the second set being designated by reference numeral 58.
Plates 56 will be referred to as the positive plates, while plates
58 will be referred to as the negative plates. When in place in
cabinet 10, the positive plates 56 are connected to a source of
high positive voltage, while the negative plates 58 are grounded to
the cabinet. In FIG. 4, the plates 56 and 58 are being viewed in an
edgewise manner, such plates being supported between and extending
parallel to a pair of end plates 60 and 62, the former of which is
seen in elevation in FIG. 3. Thus, the collecting cell plates 56
and 58 have their larger surfaces disposed in line with the
direction of air flow.
The positive plates 56 are supported near their four corners by
four spacer assemblies 64 which pass through end plate holes 64a
(FIG. 3) and are bolted to insulator blocks 66 which are, in turn,
bolted to the end plates 60 and 62. A further positive plate spacer
assembly 67 extends through the middle of the collecting cell
plates. As seen in FIG. 4, such center spacer assembly 67 comprises
an elongated metal bolt or shaft 68 having a series of metal spacer
rings 69 mounted thereon. Shaft 68 passes through small,
tight-fitting holes in the positive plates 56. Spacer rings 69
clamp the plates 56 in position and maintain the desired spacing
therebetween. Such rings 69 pass through holes in the negative
plates 58 and end plates 60 and 62 of larger diameter than the
rings 69 such that there is no electrical contact therebetween. The
ends of spacer shaft 68 are bolted to insulator blocks 70 which
are, in turn, bolted to the end plates 60 and 62. The construction
is such that all of the positive plates 56 are in electrical
contact with the center shaft 68 and the plates 56 and shaft 68 are
electrically insulated from both the negative plates 58 and the end
plates 60 and 62.
Negative plates 58 are supported in a somewhat similar manner by
means of spacer assemblies 72 located near the four corners
thereof. The metal center shafts of these spacer assemblies 72 are
bolted directly to the end plates 60 and 62 for purposes of
grounding the negative plates 58 to the cabinet 10. The negative
plates 58 are somewhat longer in the direction of air flow than
positive plates 56 so that spacer assemblies 72 may be located
clear of the positive plates 56.
Electrical contact is made with the positive plates 56 by means of
a nut 74 on the rearward end of the center spacer shaft 68. When
the collecting cell unit 44 is in place in the cabinet 10, this nut
74 engages a contact spring 76 in a contact assembly 78 mounted on
the inner wall of the back panel 13 of the cabinet 10. Contact
assembly 78 includes an insulator block 80 which is bolted to the
back panel 13 and a support plate 82 which is bolted to the front
side of the insulator block 80. Contact spring 76 is a strip of
resilient metal material bent to provide a forwardly extending hump
portion which extends through an opening 83 in the support plate
82. The lower end of contact spring 76 is secured to the plate 82
by screws 84. The lugged end of an insulated conductor wire 85 is
looped over and makes contact with one of the screws 84. Conductor
wire 85 runs to an electrical power supply unit to be considered
hereinafter. A vertical slot or channel 86 is cut into the
insulator block 80 for accommodating the contact spring 76.
FIG. 5 shows a top view of the collecting cell unit 44. A pair of
vertically-extending resillient sealing strips 88 are secured to
the inside of door 17 for engaging end plate 60 of collecting cell
unit 44, while a second pair of vertically-extending resillient
sealing strips 89 are secured to the inside of back panel 13 for
engaging the other end plate 62 of unit 44. These strips 88 and 89
may be made of felt or other suitable material and serve to prevent
air flow on the exterior sides of the end plates 60 and 62. Upper
and lower baffle plates 89a (only the lower of which is visible in
FIG. 4) prevent air flow past the collecting cell unit 44 in
regions above and below the effective areas of collecting plates 56
and 58. Thus, undesired blow-by is prevented and the efficiency of
the precipitator unit is increased.
Referring to FIG. 6 there is shown the circuit diagram for a power
supply unit 90 carried within the cabinet 10 for producing the
desired electrical potential differences between the conductive
elements in the ionizer unit 42 and the collecting cell unit 44.
Physically, the power supply unit 90 is secured to the bottom panel
12 at the location indicated in FIG. 3. As seen in FIG. 6, the
power supply unit 90 includes a volgage step-up transformer 91
having a primary winding 92 and a secondary winding 93. Primary
winding 92 is coupled to a pair of terminals 94 which are adapted
to be connected to a source of alternating-current power such as,
for example, an alternating-current power line. A rheostat 95 is
connected between one of the terminals 94 and one end of the
primary winding 92. A fuse 23a (located in fuse holder 23) is
connected between the other terminal 94 and one end of the primary
winding 92. A red-colored indicator lamp 21 is connected in
parallel with the rheostat 95 and a secondary winding 92 for
indicating that the high-voltage power supply unit 90 is being
energized.
Connected to the secondary winding 93 of the step-up transformer 91
is a rectifier circuit 100 of the voltage quadrupler type. A
bleeder resistor 96 is also connected across secondary winding 93
to stabilize the loading on transformer 91 and thus the voltage
across secondary 93. Quadrupler circuit 100 includes a first
voltage doubler 101 connected in cascade with a second voltage
doubler 102. Voltage doubler 101 includes a pair of semiconductor
diodes 103 and 104 and a pair of capacitors 105 and 106. The second
voltage doubler 102 includes a pair of semiconductor diodes 107 and
108 and a pair of capacitors 109 and 110. High-voltage output
terminals for the quadrupler circuit 100 are indicated at 111 and
112. A common ground return terminal is indicated at 113, such
terminal 113 being grounded to the cabinet 10.
The operation of the quadrupler circuit 100, which is well known in
the electrical arts, is such that there is produced between the
first high-voltage output terminal 111 and the ground terminal 113
a direct-current voltage difference of approximately (slightly less
than) twice the peak value of one-half cycle of the
alternating-current voltage appearing across the secondary winding
93 of the step-up transformer 91. There is produced between the
second high-voltage output terminal 112 and the ground terminal 113
a direct-current voltage difference equal to (slightly less than)
twice the voltage difference between the first high-voltage
terminal 111 and the ground terminal 113. The voltages at terminals
111 and 112 are of positive polarity. In the present embodiment,
the turns ratio of the step-up transformer 91 is constructed so
that, with an input voltage of 90 volts at terminals 94, the
magnitude of the positive direct-current voltage appearing at the
first high-voltage terminal is approximately 4.4 kilovolts, while
the magnitude of the positive direct-current voltage appearing at
the second high-voltage terminal 112 is approximately 8.4
kilovolts, both being measured with respect to the ground terminal
113.
As indicated in FIG. 6, the first or lower-value high-voltage
terminal 111 is connected to the positive plates 56 in the
collecting cell 44 by way of the conductor wire 85 and the contact
assembly 78 previously considered. The second or higher-value
high-voltage output terminal 112, on the other hand, is connected
to the ionizing wires in the ionizer unit 42. An important feature
of this FIG. 6 power supply embodiment is that the circuit is
constructed so that the ionizer 42 cannot operate unless the
collecting cell unit 44 is also operating. This results from the
fact that units 42 and 44 are connected directly to the voltage
doublers 101 and 102 and from the further fact that the second
doubler 102 cannot function unless the first doubler 101 is
operating. This interlock feature prevents the undesired escape of
ionized particles into the room in the event the collecting cell
unit 44 should become inoperative.
Connected in circuit with the power supply means represented by
step-up transformer 91 and rectifier circuit 100 is indicator means
114 for providing an alarm signal when the electrically conductive
plates 56 and 58 in the collecting cell unit 44 become too dirty.
This indicator means includes a current-sensing element represented
by a current-sensing resistor 115 connected in series with the
transformer secondary winding 93 and the rectifier circuit 100 for
sensing the leakage current passing between the collecting cell
plates 56 and 58. The indicator means further includes an
amber-colored indicator lamp 22 and a current-limiting resistor 116
connected in series across the current-sensing resistor 115. When
the leakage current flowing between the positive and negative
collecting plates 56 and 58 indicates that the build up of the
particle film on such plates has reached the critical level, the
voltage drop across the current-sensing resistor 115 reaches a
value sufficient to cause a lighting of the indicator lamp 22. This
provides a visual alarm signal which indicates that the collecting
cell plates 56 and 58 have become too dirty.
The collecting cell unit 44 is constructed so that the spacing
between neighboring ones of the positive and negative collecting
cell plates 56 and 58 is not more than 0.205 inches. The
high-voltage power supply unit 90 is constructed so that the
voltage difference or potential difference between negative and
positive plates 56 and 58 is less than 5,000 volts. This collecting
cell spacing and collecting cell potential difference are
significantly less than those used in conventional precipitator
apparatus. It has been found, however, that such reduced spacing
and voltage parameters provide an improved air cleaning action
while, at the same time, decreasing the amount of ozone produced.
The desired spacing factor between neighboring collector cell
plates is obtained by the proper choice of lateral dimensions for
the various spacer rings used on the spacer assemblies 64, 67 and
72. The desired potential difference value is obtained primarily by
proper proportioning of the turn ratio in the step-up transformer
91. Rheostat 95 enables some adjustment of the high-voltage output
values but is included primarily for purposes of compensating for
differences in alternating-current power line voltages at different
places of use.
FIG. 7 shows a modified form of construction for the power supply
unit 90. In FIG. 7, the bleeder resistor 96 of FIG. 6 is omitted.
In place thereof, bleeder resistors 121 and 122 are connected
between the output terminals of voltage doublers 101 and 102,
respectively, and chassis ground. Resistor 121 is of the
potentiometer type and collecting cell 44 is connected to the
sliding tap thereof. Ionizer unit 42 is connected to the output
terminal of the second doubler 102 by way of a current limiting
resistor 123. The sliding tap on resistor 121 enables proper
adjustment of the collecting cell voltage relative to the ionizer
voltage. The FIG. 7 circuit has the advantage that less heat is
generated by the bleeder resistors but suffers from the
disadvantage that the interlock feature of the FIG. 6 circuit is
not as complete.
Cabinet 10 and service door 17 are constructed in an air-tight
manner such that when the door 17 is closed, air can enter or leave
the cabinet 10 only by way of intake 15 and outlet 16. The latching
mechanism associated with door handle 18 is provided with a safety
interlock mechanism (not shown) such that the high-voltage power
supply 90 is automatically turned off whenever the service door 17
is open. Also, the power cord and plug used for connecting the unit
to the power line is of the three-wire and three-prong type with
one wire and prong being used to ground the cabinet 10.
In use, the blower unit 30 serves to cause air in the room to be
drawn into the intake 15, to pass through the cabinet 10 and to be
discharged from the outlet 16. As the particle-laden air passes
through the corona discharge produced by the ionizer unit 42, the
airborne particles acquire an electrical charge. Thereafter, as
such charged particles move through the collecting cell unit 44,
they are electrically attracted to the plates 56 and 58 and deposit
themselves thereon. As this deposit or film on the collecting
plates 56 and 58 builds up, the leakage current between plates 56
and 58 increases. When the magnitude of such leakage current
reaches the dangerous level such that arcing may begin to occur,
the voltage drop across the current-sensing resistor 115 becomes
sufficient to light the amber-colored indicator lamp 22. This
visual indication warns the user that the collecting plates 56 and
58 have become too dirty and need cleaning.
While there has been described what is at present considered to be
a preferred embodiment of the invention, it will be obvious to
those skilled in the art that various changes and modifications may
be made therein without departing from the invention, and it is,
therefore, intended to cover all such changes and modifications as
fall within the true spirit and scope of the invention.
* * * * *