Abstrict
An electric heater employs plate-shaped heating elements, each
consisting of a core of semiconductor material whose opposing faces
are coated with conductive material and having a multiplicity of
apertures to permit passage of air. The heater includes a housing
with a rear air inlet and a forward air outlet. A fan having a fan
venturi causes air flows forwardly through the housing. A two-piece,
ceramic holder maintains a number of the plate-shaped heating elements
in generally coplanar, spaced-apart relationship, in front of a
forward venturi opening. The holder has a large recess positioned
over the forward venturi opening which receives substantially all
air flow from the venturi, and a number of passages, one associated
with each heating element, which direct air from the recess through
the cores towards the forward air outlet. Each of the passages flares
radially outwardly and rearwardly from the rear face of the associated
heating element, and opens into the recess and the recess flares
radially outwardly and rearwardly from about the passage openings
to the forward venturi opening. The arrangement results in improved
heat transfer and much quieter operation than would otherwise be
achieved. To accommodate thermal expansion and contraction of the
heating elements, the two pieces of the holder are clamped together
by spring-loaded bolts which permit the spacing between the holder
members to vary, while still securing the heating elements.
Claims
I claim:
1. An electric heating unit, comprising:
a plate-shaped heating element having a core of semiconductor material
with a positive resistance-temperature coefficient, the core having
a pair of opposing core faces including a first core face and a
second core face and a multiplicity of apertures extending between
the first and second core faces whereby air can flow through the
core, the heating element having a first conductive coating over
the first core face and a second conductive coating over the second
core face;
a heating element holder formed of an electrically and thermally
insulating material, the heating element holder including first
and second separable holder members, the first holder member overlaying
the first core face and the second holder member overlaying the
second core face, the first and second holder members being apertured
adjacent the first and second core faces respectively to define
a passage through the heating element holder permitting air flow
through the core;
electrical contact means for electrically contacting the heating
element, including coating contact means between the first and second
holder members for separately contacting each of the first and second
conductive coatings, and including terminal means accessible exernally
of the heating element holder and connected to the coating contact
means for defining a first terminal electrically connected to the
first conductive coating and a second terminal electrically connected
to the second conductive coating and electrically isolated from
the first terminal; and,
attachment means for drawing together the first and second holder
members, the attachment means including biasing means for urging
the first and second holder members towards one another in elastically
displaceable relative relationshsip so that the heating element
is secured between the holder members in contact with the contact
means.
2. An electric heating unit as claimed in claim 1 in which:
the first holder member has a first holder face internal to the
heating element holder;
the second holder member has a second holder face internal to the
heating element holder;
the first and second holder faces are contoured together to define
a pocket containing the heating element.
3. An electric heater unit as claimed in claim 2 in which the coating
contact means comprise a pair of conductive metal plates located
between the first and second holder faces and shaped to seat against
the first and second holder faces, the pair of conductive metal
plates including a first metal plate overlaying the first conductive
coating and a second metal plate overlaying the second conductive
coating, the first and second metal plates being apertured to permit
passage of air through the apertures of the core of the heating
element.
4. An electric heating unit as claimed in claim 3 in which the
first and second holder members are formed of a ceramic material.
5. An electric heating unit as claimed in claim 1 in which the
first and second holder members have a multiplicity of pairs of
aligned apertures, one aperture of each a pair of aligned apertures
being formed in the first holder member and the other aperture of
each pair of aligned apertures being formed in the second holder
member, and in which the attachment means comprise:
a multiplicity of fasteners, one fastener associated with each
of the pairs of aligned apertures;
each fastener having a shaft extending through the associated pair
of aligned apertures with a first shaft end portion extending externally
of the first holder member and a second shaft end portion extending
externally of the second holder member, a first abuttment member
attached to the first shaft end portion, a second abuttment member
attached to the second shaft end portion and bearing against the
second holder member, and coil spring means acting between the first
abuttment member and the first holder member for urging the first
holder member against the second holder member.
6. An electric heating unit as claimed in claim 5 in which the
first and second holder members are formed of a ceramic material.
7. An electric heating unit, comprising:
a multiplicity of plate-shaped heating elements in generally coplanar
spaced-apart relationship, each heating element having a core of
semiconductor material with a positive resistance-temperature coefficient,
the core having a pair of opposing core faces including a first
core face and a second core face and a multiplicity of apertures
extending between the first and second core faces whereby air can
flow through the core, each heating element having a first conductive
coating over the respective first core face and a second conductive
coating over the respective second core face;
a heating element holder formed of an electrically and thermally
insulating material, the heating element holder including first
and second separable holder members, the first holder member overlaying
the first core faces of the heating elements and the second holder
member overlaying the second core faces of the heating elements,
the first and second holder members being apertured adjacent the
first and second core faces to define passages through the heating
element holder permitting air flow through the cores;
electrical contact means for electrically contacting the heating
elements, including coating contact means between the first and
second holder members for separately contacting the first conductive
coatings and the second conductive coatings, and including terminal
means accessible externally of the heating element holder and connected
to the coating contact means for defining a first terminal electrically
connected to the first conductive coatings and a second terminal
electrically connected to the second conductive coatings and electrically
isolated from the first terminal; and,
attachment means for drawing together the first and second holder
members, the attachment means including biasing means for urging
the first and second holder members towards one another in elastically
displaceable relative relationship so that the heating elements
are secured between the holder members in contact with the contact
means.
8. An electric heating unit as claimed in claim 7 in which:
the first holder member has a first holder face internal to the
heating element holder;
the second holder member has a second holder face internal to the
heating element holder;
the first and second holder faces are contoured together to define
a multiplicity of pockets each containing one of the heating elements.
9. An electric heater unit as claimed in claim 8 in which the coating
contact means comprise a pair of conductive metal plates located
between the first and second holder faces and shaped to seat against
the first and second holder faces, the pair of conductive metal
plates including a first metal plate overlaying the first conductive
coatings and a second metal plate overlaying the second conductive
coatings, the first and second metal plates being apertured to permit
passage of air through the apertures of the cores of the heating
elements.
10. An electric heating unit as claimed in claim 9 in which the
first and second holder members are formed of a ceramic material.
11. An electric heating unit as claimed in claim 7 in which the
first and second holder members have a multiplicity of pairs of
aligned apertures, one aperture of each a pair of aligned apertures
being formed in the first holder member and the other aperture of
each pair of aligned apertures being formed in the second holder
member, and in which the attachment means comprise:
a multiplicity of fasteners, one fastener associated with each
of the pairs of aligned apertures;
each fastener having a shaft extending through the associated pair
of aligned apertures with a first shaft end portion extending externally
of the first holder member and a second shaft end portion extending
externally of the second holder member, a first abuttment member
attached to the first shaft end portion, a second abuttment member
attached to the second shaft end portion and bearing against the
second holder member, and coil spring means acting between the first
abuttment member and the first holder member for urging the first
holder member against the second holder member.
12. An electric heating unit as claimed in claim 11 in which the
first and second holder members are formed of a ceramic material.
13. An electric heater comprising:
a housing having a rear air inlet and a forward air inlet;
a fan mounted inside the housing for producing an air flow forwardly
from the rear air inlet to the forward air outlet, the fan having
a fan venturi for directing the air flow including a forward venturi
opening, a rear venturi opening and a longitudinal venturi axis;
a multiplicity of plate-shaped heating elements each having a core
of semiconductor material with a positive resistance-temperature
coefficient, each core having a pair of opposing core faces including
a forward core face and a rear core face and a multiplicity of apertures
extending between the forward and rear core faces whereby air can
flow through the cores, each heating element having a forward conductive
coating over its forward core face and a rear conductive coating
over its rear core face;
a heating element holder mounted in the housing intermediate of
the forward venturi opening and the forward air outlet and having
forward and rear holder faces, the heating element holder including
holding means for holding the heating elements in generally coplanar,
spaced-apart relationship and for orienting each of the cores generally
perpendicular to the longitudinal venturi axis and spaced forwardly
of the forward venturi opening, the rear holder face having a recess
positioned over the forward venturi opening to receive substantially
all air flow from the forward venturi opening, the holder having
a multiplicity of passages each having a forward passage opening
in the front face of the heating element holder and a rear passage
opening in the recess spaced forwardly of the forward venturi opening,
each passage having an associated one of the heating elements positioned
intermediate of its forward and rear passage openings and so oriented
that all air flow in the passage flows through the associated heating
element, each passage flaring radially outwardly and rearwardly
from the rear core face of the associated heating element to its
rear passage opening, the recess flaring radially outwardly and
rearwardly from about the rear passage openings to the rear face
of the heating element holder; and,
electrical contact means extending into the heater element holder
for use in placing the forward and rear conductive coatings of the
heating element cores and the fan in contact with a source of electric
power.
14. An electric heater as claimed in claim 13 in which:
the heating element holder includes forward and rear separable
holder members, the forward holder member overlaying the forward
core faces and the rear holder member overlaying the rear core faces;
the electrical contact means include coating contact means between
the forward and rear holder members for separately contacting each
of the forward and rear conductive coatings, and including terminal
means accessible externally of the heating element holder and connected
to the coating contact means for defining a first terminal electrically
connected to the forward conductive coatings and a second terminal
electrically connected to the rear conductive coatings and electrically
isolated from the first terminal; and,
attachment means for drawing the forward and rear holder members
together in elastically displaceable relative relationship whereby
the heating elements are secured in the heating element holder and
the forward and rear holder members are separable with expansion
of the heating elements.
15. An electric heater as claimed in claim 14 in which:
the forward holder member has a face internal to the heating element
holder;
the rear holder member has a face internal to the heating element
holder;
the internal faces of the holder members are contoured together
to define a multiplicity of pockets, each pocket containing one
of the heating elements.
16. An electric heater as claimed in claim 15 in which the coating
contact means comprise a pair of conductive metal plates located
between the forward and rear holder members and shaped to seat against
the internal faces of the forward and rear holder members, the pair
of conductive metal plates including a forward metal plate overlaying
the forward conductive coatings and a rear metal plate overlaying
the rear conductive coatings, the forward and rear metal plates
being apertured to permit passage of air through the apertures of
the cores of the heating elements.
17. An electric heater as claimed in claim 16 in which the forward
and rear holder members have a multiplicity of pairs of aligned
apertures, one aperture of each pair of aligned apertures being
formed in the forward holder member and the other aperture of each
pair of aligned apertures being formed in the rear holder member,
and in which the attachment means comprise:
a multiplicity of fasteners, one fastener associated with each
of the pairs of aligned apertures;
each fastener having a shaft extending through the associated pair
of aligned apertures with a forward shaft end portion extending
externally of the forward holder member and a rear shaft end portion
extending externally of the rear holder member, a forward abuttment
member attached to the forward shaft end portion, a rear abuttment
member attached to the rear shaft end portion, and coil spring means
acting between one of the forward and rear abuttment members and
a corresponding one of the forward and rear holder members for urging
the holder member together.
18. An electric heater as claimed in claim 17 in which the forward
and rear holder members are formed of a ceramic material.
Description FIELD OF THE INVENTION
The invention relates to forced-air electric heaters, and more
particularly to improvements in the construction of electric heaters
incorporating semiconductor heating elements.
BACKGROUND OF THE INVENTION
Electric heaters employing semiconductor heating elements represent
a marked improvement over prior resistance-wire type heaters. These
new heaters include heating elements which consists of a planar
core of semiconductor material formed with a multiplicity of apertures
that permit passage of air to draw heat from the core. Opposing
faces of the core are coated with electrically conductive coatings
that produce a fairly even distribution of current flow in the core
when a voltage difference is applied to the coatings. These heaters
tend to be more durable than prior resistance-wire type heaters,
tend to be more compact for a given heat requirement, and have the
additional advantage that very hot air flows can be produced while
the temperature of the associated heating element remains below
about 200 degrees centigrade, thereby reducing the risk of fire,
particularly where there are flammable materials in the heated environment.
Such a heater was proposed by me in my Canadian Pat. No. 1,119,579.
Heaters incorporating semiconductor heating elements of the type
described above are still relatively novel, and a number of problems
have arisen in the construction of such heaters. In particular,
a convenient, inexpensive mechanism is required to hold the heating
elements. The exact manner in which the heating elements are held
tends to be more critical in the new type heaters than in the old
resistance-wire type, as the heating elements tend to be smaller,
and air flows from an associated fan must consequently be more tightly
constrained to achieve the full benefit of employing such heating
elements. As well, because of the size and nature of the heating
elements, considerable care must be taken to ensure simultaneous
electrical and thermal isolation of the heating elements from any
associated housing, and proper electrical contact with a supply
of line voltage. Moreover, it is desirable to improve the heat transfer
efficiency of such a heater, and to provide quieter operation, operation
tending to be noisy because of the extent to which air flows are
affected by the limited cross-sectional area of the new heating
elements.
Accordingly, it is one object of the present invention to provide
a heater construction employing semiconductor heating elements of
the type described above which results in improved heat transfer
and quiet operation.
It is another object of the invention to provide a convenient,
inexpensive mechanism for holding and electrically contacting semiconductor
heating elements of the type described above.
BRIEF SUMMARY OF THE INVENTION
The invention provides an electric heater which includes a housing
having a rear air inlet and a forward air outlet. A fan is mounted
inside the housing intermediate of the rear air inlet and the forward
air inlet to produce an air flow through the housing. The fan is
of a type which includes a fan venturi that directs air flow generated
by the fan from a rear venturi opening (which is placed in communication
with the rear air inlet of the housing) to a forward venturi opening.
The fan air flows are heated by a multiplicity of disk-shaped heating
elements. Each heating element includes a generally planar core
with a pair of opposing forward and rear core faces, electrically
conductive coatings covering each of the opposing faces, and a multiplicity
of apertures which permit air flow through the core. The cores are
formed of a semiconductor material with preferrably a positive resistance-temperature
coefficient (PTC), the significance of which will be discussed below
in connection with a preferred embodiment of the invention.
A heating element holder formed of an electrically insulating and
heat-insulating material is mounted in the housing intermediate
of the forward fan venturi opening and the forward air outlet of
the housing. The holder includes holding means which maintain the
heating elements in generally coplanar, spaced-apart relationship
relative to one another. When the holder is in an operative position,
the heating elements are oriented generally perpendicular to the
longitudinal axis of the fan venturi and spaced forwardly therefrom,
with each rear core face facing toward the forward venturi opening.
The holder is adapted to direct air flows escaping from the forward
fan venturi opening through the cores. Accordingly, the holder comprises
air flow receiving means, including a recess formed in a rear face
of the holder and positioned at the forward fan venturi opening,
which receive substantially all air flow from the forward venturi
opening in the recess. A multiplicity of passages in the holder
guide air received in the recess through the heating elements. Each
passage extends between a forward face and the rear face of the
holder, and each passage has a forward opening in the forward face
of the holder and a rear opening in the recess, spaced forwardly
of the forward fan venturi opening. Each passage is associated with
a different one of the heating elements, the associated heating
element being positioned intermediate of the forward and rear passage
openings, and positioned so that all air flow in the passage is
constrained to flow through the core.
The passage and the recess are shaped or dimensioned to provide
a smooth air flow through the cores. In particular, each of the
passages has a minimum cross-sectional area which corresponds substantially
to the cross-sectional area of the associated heating element. Each
passage is flared rearwardly and radially outwardly from the rear
face of the associated heating element to guide air flows smoothly
towards the heating element. The recess is similarly flared, radially
outwardly and rearwardly, from about the rear passage openings to
the rear face of the holder. The flaring which may be either convex
or concave result in less turbulent air flows and consequently quieter
operation.
Electrical connection means are provided which extend into the
holder for placing the coatings of each heating element in contact
with the source of electric power, and also serve to place the fan
in contact with the source of power.
Other aspects and advantages of the present invention, particularly
a novel heating element holder, will be apparent from the description
below of a preferred embodiment.
DESCRIPTION OF THE DRAWINGS
The invention will be better understood with reference to drawings
illustrating a preferred embodiment, in which:
FIG. 1 is a perspective view of an electric heater embodying the
invention;
FIG. 2 is an exploded perspective view of the heater;
FIG. 3 is a perspective view of a heating element holder incorporated
in the heater;
FIG. 4 is a cross-sectional view along the lines 4-4 of FIG. 1;
and,
FIG. 5 is a schematic drawing of the electrical control circuitry
associated with the heater.
DESCRIPTION OF PREFERRED EMBODIMENT
FIGS. 1 and 2 illustrates the major components (excepting wiring)
of an electric heater 10 embodying the invention. The heater 10
includes a housing constructed of sheet metal. The housing includes
a main housing member 12 whose front face has a forward air outlet
14 circumscribed by abuttment flanges 16 (only one specifically
indicated in FIG. 2). The housing also includes a rear housing insert
18 which fits tightly into the back of the main housing member 12
and which is attached to the main housing member 12 by means of
screws (not illustrated) threaded through holes (only one hole 20
specifically indicated) provided in the sidewalls of the main housing
member 12. The rear housing insert 18 has a rear air inlet 22 which
is normally covered by a removable sheet 24 of appropriate air filtering
material, retained by means of brackets 26 (two such brackets being
illustrated in the cross-sectional view of FIG. 4). The construction,
function and assembly of many of the components will be readily
apparent, and consequently only details of the various components
relating specifically to aspects of the invention will be provided
in order to better highlight the invention.
The heating function of the electric heater 10 is provided by four
disk-shaped heating elements 28, 30, 32, 34. The heating elements
are substantially identical, and consequently only the heating element
28 will be described in detail, with particular reference to FIG.
5 where components of the heating element 28 better illustrated.
The heating element 28 includes a planar core 40 of semiconductor
material having a forward face 42, and an opposing rear face 44.
The thickness of the core 40 is about 1/4 inch, and the diameter
of the core 40 is about 1 and 15/16 inches. Conductive silver coatings
48, 50 are deposited on the forward and rear core faces 42, 44,
and serve to produce a distributed current flow between the opposing
core faces 42, 44 when a voltage difference is applied to the coatings
48, 50. A multiplicity of apertures (not specifically indicated)
extending between the forward and rear core faces 42, 44 permit
air flow through the core 40 to draw heat from the core 40 during
operation. The relative orientation of the cores is that illustrated
in FIG. 2, namely, a generally coplanar, spaced-apart relationship.
The semiconductor material of the cores is preferrably a mixture
of lead titanate and barium titanate, although a variety of other
semiconductor materials may be used. The semiconductor material
has a relatively high positive resistance-temperature coefficient
(PTC), which is normally about 7.65 ohms/degrees centigrade (as
measured between the opposing faces of all cores and considering
the cores as parallel resistances). The resistance as measured between
the opposing faces of the cores (once again considering the cores
to be resistances connected in parallel) totals about 8 ohms at
165 degrees centigrade and about 161 ohms at 185 degrees centigrade.
With a line voltage of 110 v. RMS applied to the opposing faces
of the cores, the cores will have a nominal power consumption of
about 1,500 W at an operating temperature of 165 degrees centigrade,
and a nominal power consumption of 75 W at an operating temperature
of 185 degrees centigrade. It will be readily apparent that the
power consumption of the cores drops markedly as the operating temperature
of the cores rises. Accordingly, in most circumstances, with no
air flow through the cores to draw heat, the cores may be expected
to reach an equilibrium temperature below about 200 degrees centigrade.
A fan 52 is mounted inside the main housing member 12 intermediate
of the rear inlet 22 and the forward air outlet 14. The fan 52 has
a fan casing 54 which defins a fan venturi 56. The fan venturi 56
has a forward venturi opening 58, a rear venturi opening 60 and
a central longitudinal axis 62. The fan 52 has a central hub 64
mounted in the fan venturi 56 in general alignment with the venturi
axis 62 by means of a spider 66 which has four legs 68 (only one
specifically indicated) connecting the hub 64 to the fan casing
54. The hub 64 contains an electric motor (not illustrated) and
a fan blade assembly 70 which can be rotated by the motor to produce
an air flow forwardly through the fan venturi 56. Power to operate
the motor is conducted through one of the legs of the spider 66,
in a manner which will not be described. A suitable fan is sold
by Rotron Inc. of Woodstock, N.Y., U.S.A. under the trademark MUFFIN.
The heater 10 also includes a heating element holder which holds
the heating elements 28, 30, 32, 34 in the generally coplanar, spaced-apart
relationship referred to above. The heating element holder also
orients the cores of the heating elements generally perpendicular
to the longitudinal axis 62 of the fan venturi 56, spaced about
1 inch forwardly of the forward venturi opening 58 with each rear
core face facing towards the forward venturi opening 58. The holder
has a two-piece separable construction involving a forward holder
member 72 and a rear holder member 74, each of which is formed of
a ceramic material. The term "ceramic material" as used
in this specification is intended to denote a clay-like material
such as fired greenware, porcelain or any other material that can
be molded in an initial condition and then fired to produce a hard
durable product and that is both thermally and electrically insulating.
Such ceramic materials are generally inexpensive and easily molded
into the shapes required. It will be appreciated, however, that
the resultant product tends to be very brittle.
The forward holder member 72 defines a forward face 76 of the holder
and has an opposing inner face 78. The rear holder member 74 defines
a rear face 80 of the holder and has an opposing inner face 82 which
is positioned adjacent the inner face 78 when the heating element
holder is fully assembled.
The inner holder member faces 78, 82 are contoured to define pockets
(two specifically indicated by reference numerals 84, 86 in the
cross-sectional view of FIG. 4) in which the heating elements are
retained. The contouring in this instance consists of generally
triangular projections (only two projections 88, 90 specifically
indicated in FIG. 3 where the contouring is best shown) and central
circular projections 92, 94 best illustrated in the view of FIG.
3 where the holder members 72, 74 are shown separated. When the
holder is assembled, as in FIG. 4, the pockets closely receive the
heating elements, gripping the opposing faces of each heater element
core. Although the pockets defined are relatively loose (in radial
directions), the arrangement has proven sufficient for the purposes
of the invention.
The heating element holder also has four passages, one associated
with a different one of the four heating elements, which direct
air flows through the heating elements. Each passage extends between
the forward and rear holder faces 76, 80, and has a forward passage
opening (not specifically indicated) in the forward holder face
76 and a rear passage opening (not specifically indicated) in the
rear holder face 80, more specifically, in a recess formed in the
rear holder face 80 and described more fully below. Two such passages
96, 98 are specifically indicated in the cross-sectional view of
FIG. 4. It will be apparent from FIG. 4 that each of the passages
96, 98 intersects the pocket in which the associated heating element
28 or 32, respectively, is contained, and that each of the passages
96, 98 has a minimum cross-sectional area perpendicular to the venturi
axis 62 which is substantially the same as the cross-sectional area
of the associated heating element, thereby ensuring retention of
the associated heating element without significantly impeding air
flow through the associated heating element. The heating elements
are of course so positioned in the associated passages that substantially
all air flow in the passages is constrained to flow through the
cores of the heating elements.
The holder is also adapted to receive substantially all air flow
from the forward venturi opening 58. In particular, the rear holder
member 74 has a rectangular circumferential lip 100 dimensioned
to fit about a corresponding lip 102 of the fan casing 54. Air flow
from the forward venturi opening 58 is consequently directed against
the rear holder face 80, and a large recess 104 is formed in the
rear holder face 80 adjacent to and in general axial alignment with
the forward venturi opening 58, to receive the air flows. Each of
the passages (such as the passages 96, 98) in which the heating
elements are positioned has its rear passage opening located within
the recess 104, spaced forwardly of the forward venturi opening
58. As will be apparent in FIG. 4, where the cross-sectional shape
of the passages 96, 98 is shown, the passages 96, 98 flare radially
outwardly and rearwardly from the rear face of each associated heating
element 28, 32 respectively to the associated rear passage opening.
Similarly, the recess 104 flares radially outwardly and rearwardly
from about the rear passage openings to the rear holder face 80.
Accordingly, substantially all air escaping from the forward venturi
opening 58 is delivered in a relatively smooth fashion to the heating
elements.
The forward holder member 72 is identical to the rear holder member
74, and is effectvely the rear holder member 74 rotated through
180 degrees about a vertical axis. Accordingly, the forward holder
member 72 defines a large recess 106 in the forward holder face
76 into which each of the forward passage openings opens. Accordingly,
each of the holder passages such as the passages 96, 98 which are
exemplary and illustrated in FIG. 4) flares radially outwardly and
forwardly from the front faces of each associated heating element,
and the recess 106 flares radially outwardly and forwardly from
about the forward passage openings to the forward holder face 76.
This arrangement produces a smoothing of the heated air flows escaping
from the forward air outlet 14, but is not strictly essential to
obtaining improved heat transfer efficiency and quieter operation.
The arrangement does, however, eliminate the need for an additional
distinct component in the heater 10.
Forward and rear conductive metal plates 108, 110 provide electrical
contact with the forward and rear faces of the heating element cores
inside the holder. It will be apparent from the view of FIG. 3 that
the forward conductive plate 108 seats in a recess defined by the
triangular projections on the inner face 78, and has a central aperture
112 which permits clearance of the central projection 92. The plate
108 overlays the forward faces of the heating element cores to provide
the required electrical contact. Four apertures 114 (only one specifically
indicated) with a circular periphery are formed in the plate 108,
and are positioned over the forward faces of the heater element
cores to permit passage of air. The diameter of the apertures 114
on the plate 108 is marginaly smaller than that of the heating elements
to ensure electrical contact. A metal terminal strip 116 is spot
welded to the plate 108, and extends to points external of the heater
element holder, when assembled, to permit line voltage to be conveniently
applied to the plate 108. To accommodate the terminal strip 116,
a groove 118 is formed in the inner face 78 of the forward holder
member 72, and is dimensioned to receive the terminal strip 116
loosely. The conductive plate 110 provides electrical contact with
the rear faces of the heating element cores, and seats against the
inner face 82 of the rear holder member, in a manner analogous to
the seating of the plate 108. The structure and function of the
two plates 108, 110 is substantially identical, and consequently
no further detail will be provided regarding the plate 110.
The holder members 72, 74 are drawn together to contain the heating
elements in the pockets by means of four fasteners. Since the fasteners
are identical, only one fastener 120 has been specifically illustrated
and indicated, in FIG. 2. The fastener 120 consists basically of
a bolt 122, and coil spring 124 and a nut 126. The heating element
holder is first loosely assembled, for example, by seating the plate
110 against the inner face 82 of the rear holder member 74, positioning
the heater elements appropriately over the apertures in the plate
110, seating the plate 108 against the inner face 78 of the forward
holder member 72, and then carefully bringing the holder member
72, 74 together in an operative orientation. The holder members
72, 74 then have four pairs of aligned apertures, only the apertures
128, 130 being specifically indicated, which extend fully between
the forward and rear holder faces 76, 80. The coil spring 124 is
located about the shaft 132 of the bolt 122, and the shaft extended
fully through the aligned apertures 128, 130. The nut 126 is then
threaded onto the rear shaft end portion 134 until the nut 126 abuts
the rear holder face 80. The coil spring 124, which has a larger
diameter than the apertures 128, 130, locates in the process about
a forward shaft end portion 136, and acts between the bolt head
and the forward holder face 76 to draw the holder members 72, 74
together. To ensure positive seating of the coil spring 124 against
the forward holder member 72, a shallow well 140 is located about
the aperture 128 in the forward holder face 76 and dimensioned to
receive a portion of the coil spring 124. In the preferred embodiment
illustrated, a metal grill 141 is interposed between the bolt head
of each fastener and the associated coil spring to fasten the grill
directly to the forward holder member 72, but the grill 141 could
equally well be attached to the abutment flanges 16 of the main
housing member 12.
The four fasteners being spring-loaded permit separation of the
holder members 72, 74 in response to thermal expansion of the heating
elements. This arrangement is critical, as the ceramic holder members
72, 74 are very brittle, and could otherwise be easily cracked during
operation of the heater 10.
The heating element holder and grill 141 when assembled togeher
are then attached to the fan casing 54. This attachment is effected
by passing the rear shaft portions of the various fasteners through
apertures in the fan casing 54 and attaching additional nuts to
the rear shaft end portions. For example, with the fastener 120
illustrated, an aperture 142 in the fan casing 54 is aligned with
the previously aligned holder member apertures 128, 130 and the
rear shaft end portion 134 passed through the aperture 142. A nut
144 is then threaded onto the rear shaft end portion 134 to complete
connection to the fan casing 54. The fan casing is in turn attached
to the rear housing insert 18 by means of screws which are threaded
through aligned apertures (only one pair of aligned apertures 146,
148 specifically indicated) in the fan casing 54 and rear housing
insert 18. A strip of insulating material 150 (fragmented) is wrapped
around the periphery of the heating element holder, and the entire
assembly so formed is inserted into the main housing member 12 until
the grill 141 engages the abutment flanges 16. Securement is completed
by threading screws through the holes 20 in the main housing member
12 to secure the rear housing insert 18 to the main housing member
12, as mentioned above.
The electrical wiring and control circuitry associated with the
heater 10 is schematically illustrated in FIG. 5. Line power is
delivered via a power cord 152 (the two lines of the power cord
being indicated with the same reference numeral 152) to the conductive
plates 108, 110. The electrical connection so formed is direct,
involving no switching circuitry to control the amount of power
delivered, except for a simple double pole single throw switch 154
which serves to turn the power to the heater 10 off and on. The
power consumed by the heating elements, and the heat consequently
delivered is controlled entirely by varying fan speed with a fan
control 156. The fan control 156 includes as a primary switching
element a bidirectional silicon controlled rectifier which permits
substantially continuous variation of fan speed. By increasing fan
speed the temperature of the heating elements drops, but the resultant
increase in conductivity of the heating element cores results in
a marked increase in power consumption. Thus, without effectively
upwardly or downwardly scaling the line voltage applied to the heating
element cores, the quantity of heat delivered can be varied. The
required control circuitry and switches can be conveniently mounted
to the rear housing insert 18 with appropriate wiring, as schematically
illustrated, conducting power to the fan 52 and heating elements.
These matters will be readily apparent to one skilled in the art.
A test was performed to determine whether a heater substantially
identical to the preferred embodiment 10 exhibited improved heat
transfer efficiency and quieter operation. The test involved a comparison
with a prototype device that had an identical housing, identical
heating elements, an identical fan, and identical control circuitry.
The principal difference between the preliminary prototype and the
preferred version of the heater resided in the heating element holder.
The four heating elements were contained between two metal plates,
bolted together, each of which was apertured to permit passage of
air through the heating elements in a manner similar to that of
the conductive plates 108, 110. The assembly so formed was bolted
to the associated fan with the heating elements positioned at about
1 inch from the forward fan venturi opening. Power consumption of
the cores was monitored by means of a watt meter. With the prototype
version coupled to a 110 v. RMS line source, an ambient temperature
of about 25 degrees centigrade, and the fan operating at full speed,
the heating elements had a total power consumption of about 1,200
W. A noticeable leakage of air backscattered through the rear fan
venturi opening was also noted. The preferred version substantially
identical to the preferred embodiment 10 was operated under similar
conditions, and a heating element power consumption of about 1,380
W was noted with no apparent leakage of air through the rear fan
venturi opening. This represents about a 15% improvement in energy
transfer to fan air flows. Additionally, on a purely qualitative
basis, it was noted that the preferred version was considerably
quieter in operation.
It will be appreciated that a preferred embodiment of the invention
has been described and that modifications may be made therein without
departing from the spirit of the invention and the scope of the
appended claims.
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