U.S. patent number 3,590,215 [Application Number 04/810,976] was granted by the patent office on 1971-06-29 for clinical fluid warmer.
This patent grant is currently assigned to Thermolyne Corporation. Invention is credited to Cleophas E. Anderson, Darrle D. Moore, William J. Walsh.
United States Patent |
3,590,215 |
Anderson , et al. |
June 29, 1971 |
CLINICAL FLUID WARMER
Abstract
A dry heat clinical blood warmer comprising first and second
heater plates, pivotally mounted relative to each other; a thin,
flat, channeled sac, having an inlet and an outlet spaced from each
other, is mounted between the plates, the sac contacting both
plates when the plates are closed. The electrical heater elements
for the plates are effectively tapered, by positioning or by power
relationships, from inlet end to outlet end, in proportion to the
diminishing rate of heat absorption of blood or other fluid flowing
through the sac.
Inventors: |
Anderson; Cleophas E. (Dubuque,
IA), Moore; Darrle D. (Dubuque, IA), Walsh; William
J. (Dubuque, IA) |
Assignee: |
Thermolyne Corporation
(Dubuque, IA)
|
Family
ID: |
25205199 |
Appl.
No.: |
04/810,976 |
Filed: |
March 27, 1969 |
Current U.S.
Class: |
392/470; 219/494;
219/525; 165/46; 219/506; 392/479; 604/114; 607/106 |
Current CPC
Class: |
F24H
9/2014 (20130101); A61M 5/44 (20130101); A61M
2205/3653 (20130101) |
Current International
Class: |
F24H
9/20 (20060101); A61M 5/44 (20060101); A61j
001/00 (); F24h 001/20 (); H05b 001/02 () |
Field of
Search: |
;219/296--299,301--309,311,310,313,327,328,521,524,525
;128/272,214,399,400 ;165/46,171,80,74,86 ;222/146,146HE |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bartis; A.
Claims
We claim:
1. A dry heat clinical fluid warmer for use in warming refrigerated
blood and other fluids to a safe usable transfusion temperature, at
any flow rate from zero to a given maximum rate, comprising:
a first thermally conductive heat plate;
a second thermally conductive heater plate;
mounting means mounting said first and second heater plates for
pivotal movement, relative to each other, between a closed position
in which said heater plates are disposed in closely spaced parallel
relation to each other and an open position in which said heater
plates are disposed at a relatively wide angle to each other;
a thin, flat, disposable fluid sac having inlet and outlet openings
at opposite ends thereof;
support means for removably supporting said sac in predetermined
position on one of said heater plates, when the heater plates are
in open position, opposite sides of said sac contacting said first
and second heater plates, respectively, when said heater plates are
in closed position and said sac is filled with fluid;
first and second electrical heating elements, individually
associated with said first and second heater plates, respectively,
each of said heating elements being effectively tapered from the
end thereof adjacent the sac inlet to the end adjacent the sac
outlet so that the heat output of said heating elements gradually
diminishes form the inlet end of said sac to the outlet end thereof
in direct proportion to the diminishing rate of heat absorption by
fluid moving from the sac inlet to the sac outlet, each heating
element comprising at least two sections, arranged in sequence
between the sac inlet and the sac outlet; and
precision thermal control means, thermally connected to at least
one of said heater plates and electrically connected to said
heating elements, for controlling operation of said heating
elements in accordance with the temperature of said heater plate,
said control means including separate thermostats for controlling
the respective sections of each heating element.
2. A dry heat clinical fluid warmer according to claim 1 in which
said mounting means mounts said heater plates in vertical alignment
with one of said heater plates pivotally movable about a vertical
axis, in which the sections of each heating element are mounted one
above the other, and in which the sac inlet is located at the
bottom of the heater plates and the sac outlet is located at the
top of the heater plate.
3. A dry heat clinical fluid warmer according to claim 1 in which
the taper of said heating elements is determined in accordance with
the basic relationship
where
T = fluid temperature at a given time
T.sub.o = plate temperature above initial fluid temperature
D = thermal conductivity of sac
A = contact area of one plate
D = thickness of sac
S = maximum liquid flow rate
P = ratio of effective contact area to actual plate area
all expressed in the c.g.s. system.
4. A dry heat clinical fluid warmer according to claim 3 in which
each of said heating elements comprises a series of n transverse
segments each centered in a segmental area of its associated plate,
from the sac inlet end of the plate to the sac outlet end, said
segmental areas of said plates being determined in accordance with
the series relationship: ##SPC2##
where
A.sub.1 = first segmental plate area, at inlet end
A.sub.2 = second segmental plate area
A.sub.n = last segmental plate area, at outlet end
T' = fluid temperature rise per segment all expressed in the c.g.s.
system.
5. A dry heat clinical fluid warmer according to claim 4, in which
said mounting means mounts said heater plates in vertical
alignment, in which each electrical heating element comprises two
sections, one above the other, with each heating element section
distributed in accordance with said series relationship, and in
which said precision thermal control means includes a first main
thermostat for control of both of the lower heat element sections
and a second main thermostat for control of both of the upper
heating element sections.
6. A dry heat clinical fluid warmer according to claim 3 in which
each of said heating elements comprise a series of n transverse
segments centered in nsegmental areas of equal size on its
associated plate, from the sac inlet end of the plate to the sac
outlet end, the power inputs to said heating element segments being
determined in accordance with the series relationships
W.sub.1 =4.18 LST.sub.1,
W.sub.2 =4.18 LST.sub.2,...
W.sub.n =4.18 LST.sub.n
and
T.sub.1 =T.sub.o M,
T.sub.2 =(T.sub.o -T.sub.1) M,...
T.sub.n =(T.sub.o -T.sub.1 -...T.sub.n-1) M
wherein
and where
T.sub.1...T.sub.n = fluid temperature rise in each segment
L = constant to compensate for low-line voltage and thermal
losses
W.sub.1... W.sub.n = power input for each segment
all expressed in the c.g.s. system.
7. A dry heat clinical fluid warmer according to claim 6, in which
said mounting means mounts said heater plates in vertical
alignment, in which each electrical heating element comprises tow
sections, one above the other, with each heating element section
distributed in accordance with said series relationships, and in
which said precision thermal control means includes a first main
thermostat for control of both of the lower heating element
sections and a second main thermostat for control of both of the
upper heating element sections.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This invention is an improvement upon the clinical fluid warmer
disclosed and claimed in the copending application of Germain G.
Pins, Ser. No. 589,262, filed Oct. 25, 1966 and assigned to the
same assignee as the present, now U.S. Pat. No. 3,475,590, issued
Oct. 28, 1969 invention
BACKGROUND OF THE INVENTION
Blood is ordinarily refrigerated and stored at temperatures of the
order of 4.degree. C. For utilization, it must be warmed to
approximate body temperature of 37.degree. C. For a given surgical
procedure, it is customary practice to warm enough blood to meet
all anticipated transfusion needs, frequently resulting in a
substantial loss when some of the anticipated needs fail to occur.
The excess blood cannot be refrigerated a second time and must be
destroyed.
Continuous blood heating devices have been proposed, using a liquid
heating bath and a heat transfer coil immersed in the bath, the
blood being heated by passage through the coil. Devices of this
kind, however, tend to produce substantial overheating or
underheating of the blood depending upon the rate of flow of the
blood. Ideally, a blood warmer should be able to supply blood for
use in a quite restricted temperature range despite large
variations in flow rate.
The blood warmer of the aforesaid Pins U.S. Pat. No. 3,475,590
affords a controlled temperature output for the blood over a
practical range of flow rates from 0 to 150 cubic centimeters per
minute. That is, the Pins blood warmer assures a supply of blood at
a temperature within an acceptable rang for transfusion purposes,
starting with blood refrigerated to 4.degree. C. or even lower,
even though the flow rate may be changed from a maximum of 150 cc.
per minute to a complete stop at any time during the transfusion
procedure. However, the Pins device does present some difficulties
with respect of cleaning and accessibility and is susceptible of
some improvement with respect to the precise control of the blood
temperature.
SUMMARY OF THE INVENTION
It is a principal object of the present invention, therefore, to
provide a new and improved dry heat clinical blood warmer that
utilizes the structural features and operating attributes of the
Pins blood warmer to the fullest extent, but provides much greater
convenience and accessibility in changing from one blood supply to
another or from one patient to another.
Another object of the invention is to provide a new and improved
dry heat clinical blood warmer which is readily and conveniently
cleaned, even in those instances in which an accident may occur and
some spillage of blood or other transfusion fluid may take
place.
Another principal object of the invention is to afford a new an
improved dry heat clinical blood warmer in which thermal control is
made more effective by precise proportioning of the electrical
heater elements in accordance with the diminishing rate of heat
absorption of the blood or other fluid flowing through the
heater.
Accordingly, the invention relates to a dry heat clinical fluid
warmer for use in warming refrigerated blood and other fluids to a
safe, usable transfusion temperature, at any flow rate from zero to
a given maximum rate. The warmer of the invention comprises first
and second thermally conductive heater plates and means for
mounting those plates for pivotal movement between a closed
position in which the plates are in closely spaced parallel
relation and an open position in which the plates are rather widely
spaced from each other. The warmer further comprises support means
for removably supporting a thin, flat, channelled fluid-conducting
sac on one of e heater plates, the sac having inlet and outlet
openings at opposite ends thereof. When the heater plates are in
closed position and the sac is filled with fluid, the opposed sides
of the sac contact the first and second heater plates,
respectively. First and second heating elements are provided for
the first and second heater plates, respectively. Each heating
element is tapered from the inlet end to the outlet end so that the
heat output diminishes in proportion to the diminishing rate of
heat absorption of fluid moving from the sac inlet to the sac
outlet. Precision thermal control means are provided for
controlling the operation of the heating elements in accordance
with the temperature of one of the heater plates.
Other and further objects of the present invention will be apparent
from the following description and claims and are illustrated in
the accompany drawings which, by way of illustration, show
preferred embodiments of the present invention and the principles
thereof and what is now considered to be the best mode contemplated
for applying these principles. Other embodiments of the invention
embodying the same or equivalent principles may be made as desired
by those skilled in the art without departing from the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front perspective view of a clinical fluid warmer
constructed in accordance with one embodiment of the present
invention;
FIG. 2 is a detail sectional view of a part of the fluid warmer of
FIG. 1 illustrating the engagement of the heater plates with the
sac that carries fluid through the warmer;
FIG. 3 is a perspective view of the clinical fluid warmer of FIG. 1
with the door nearly closed and with a door cover removed to show
the thermostatic controls;
FIG. 4 is a schematic diagram of the heating elements and
electrical controls for the clinical fluid warmer of FIGS. 1--3;
and
FIG. 5 is a schematic illustration of the heating elements for a
different embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1--4 illustrate a dry heat clinical fluid warmer 10, for use
in warming refrigerated blood and other fluids to a safe usable
transfusion temperature, constituting a first embodiment of the
present invention. The clinical fluid warmer 10 comprises a base 11
and an upwardly extending stationary frame 12. Base 11 projects
forwardly of the frame 12 and, at the upper end of the frame, there
is a forwardly projecting housing 13.
A first thermally conductive heater plate 21 is fixedly mounted on
frame 12. Heater plate 21 is formed in two sections, a lower
section 21A and an upper section 21B. A second thermally conductive
heater plate 22 is included in fluid warmer 10. Like heater plate
21, plate 22 is constructed in two sections, a lower section 22A
and an upper section 22B. Heater plate 22 is mounted upon door 14
that is pivotally mounted upon the base and frame 11--13 of the
fluid warmer device. Thus, heater plates 21 and 22 are pivotally
movable, relative to each other, between an open position (FIG. 1)
and a closed position; in the closed position the two heater plates
are disposed in closely spaced parallel relation to each other
(FIG. 2).
Warmer 10 further comprises support means for removably supporting
a thin, flat , channelled fluid-conducting sac 16 in predetermined
position upon heater plat 21. This support means comprises four
pins 15A, 15B, 15C and 15D mounted on frame 12 and positioned to
project through four corresponding apertures in sac 16. Pins
15A--15D are aligned with four corresponding apertures 17A--17D in
heater plate 22. The support means for the fluid-conducting sac 16
further comprises individual slots 18A and 18B in heater plates 21A
and 22A, respectively, for receiving an inlet opening tube 18 that
is a part of sac 16. Corresponding slots 19A and 19B in heater
plate sections 21B and 22B, respectively, receive the outlet tube
19 of the sac.
As can be seen from FIG. 1, when door 14 is open heater plate 22 is
disposed at a relatively wide angle with respect to heater plate
21. Complete accessibility is thus provided for mounting or
demounting the plastic fluid-conducting sac 16 in device 10. But
when door 14 is closed, the two heater plates 21 and 22 are
disposed in closely aligned parallel spaced relation to each other,
as shown by the detail sectional view of FIG. 2. Thus, when door 14
is closed and the heater plates are in their closed positions,
relative to each other, the opposite sides of the channelled sac 16
contact heater plates 21 and 22 respectively. This provides for a
rapid and efficient heat transfer from the heater plates to the
fluid in sac 16.
Heater plates 21 and 22 are each provided with electrical heating
elements. The positioning and alignment of the heating elements for
warmer 10, in relation to the heater plates, is best illustrated in
FIG. 4. As shown therein, heater plate sections 21A and 21B are
provided with individual electrical resistance heaters 25A and 25B,
respectively. The heating elements 25A and 25B are each effectively
tapered from the bottom of the heating element to the top of the
heating element so that the heat output diminishes in an upward
direction across the face of heater plate 21. MOre specifically,
the heating elements are effectively tapered so that the heat
output diminishes gradually from that portion of the heater plate
that is adjacent to the inlet end 18 of the sac 16 to the outlet
end 19 of the sac, in proportioned to the diminishing rate of heat
absorption of fluid moving through the sac from its inlet to its
outlet.
To understand the requirement for tapering of the heating elements
25A and 25B, consideration may be given to the thermal changes of
fluid flowing through the channelled sac 16. The flood or other lid
gradually increases in temperature as it traverses the sac from the
bottom inlet 18 to the top outlet 19. The rate of heat absorption
of the fluid is proportional to the temperature differential
between the blood and the heater plates 21, 22. The heater plates
are of high thermal conductivity (relatively heavy cast aluminum is
preferred) and hence generally uniform in temperature.
Consequently, the fluid absorbs less heat near the outlet end of
the sac 16 than near the inlet end because the temperature
difference is much smaller at the outlet end of the sac.
The second heater plate 22, comprising sections 22A and 22B,
includes two sectional heating elements 26A and 26B. EAch of the
heating elements 25A, 25B, 26A and 26B is of sinuous configuration
and each includes four main horizontally disposed segments. For
practical purposes, the end connections between the horizontal
segments can be disregarded. Furthermore, the rate of heat output
for heating element 25A is constant throughout its length. But the
four horizontal segments of heater element 25A are not equally
spaced with respect to the vertical height of heater plate section
21A.
Thus the first or lower horizontal segment 25A1 of heating element
25A is centered in a relatively small area A1 of heater plate
section 21A. Note that this area A1 is aligned with the inlet 18.
The next horizontal segment 25A2 of the same heating element is
centered in a somewhat wider segmental area A2 of the heater plate.
the next segmental area, in which the heating element segment 25A3
is centered, is still wider, and the widest of the segmental areas
of the heater plate 25A is the uppermost area A4 containing the
last heating element segment 25A4. The same distribution pattern is
utilized for each of the heating element sections 25B, 26A and
26B.
The electrical control for device 10 is shown in FIGS. 3 and 4. It
comprises four thermostats 31, 32 33 and 34 mounted in cover 14 on
the rear surfaces of heater plate 21, as shown in FIG. 3.
Thermostat 31 is a principal control thermostat for the electrical
heating elements 25A and 26A. Thermostat 33 is a main control
thermostat for the heating elements 25B and 26B. Thermostats 32 and
34 are limiting thermostats for controlling an alarm system.
As shown in the schematic diagram of FIG. 4, the heating elements
of device 10 are energized for a conventional AC supply having
terminals 41 and 42. One line terminal 41 is connected through
thermostats 32 and 34, in series, and through thermostat 32 to the
input terminal of heating element secton 25B. The other terminal of
heating element section 25B is connected across to the
corresponding heating element section 26B in the other heater plate
and the latter is returned to the other line terminal 42.
The control circuit for the lower heating element sections 25A and
26A is similar but is made specifically different to handle higher
currents. Thus, the energizing circuit for heating element sections
25A and 26A, beginning at line terminal 41, again extends through
the two series-connected thermostats 32 and 34. From thermostat 34,
the circuit extends to the input electrode of a triac 36, the
output electrode of the triac being connected to heating element
section 25A, which is connected in series with heating element
section 26A back to line terminal 42. The control thermostat at 31
is connected, in series with a current-limiting resistor 37,
between the input electrode of triac 36 and the gate electrode of
the triac.
In normal operation, thermostat 33, which is mounted on the upper
heated plate section 21B (FIG. 3) directly controls the
energization and deenergization of the two series-connected upper
heating element sections 25B and 26B. Thermostat 34 is also mounted
upon the upper heater plate section 21B, but does not serve a
primary control function for the upper heating elements. It serves
only as an limit control to actuate an alarm 38 in the event of
some failure on the part of thermostat 33 or some other malfunction
of the warmer that leads to substantial overheating of the plate
section 21B.
Similarly, thermostat 32 does not perform a major control function
with respect to either heating element section 25A or section 26A,
although it is mounted on heater plate section 21A. Like thermostat
34, thermostat 32 is set for a temperature slightly above the
desired operating temperature for the heater plated and functions
only in the event that the main control for heating element
sections 25A and 26A 31 and triac 36) fails to perform properly.
The alarm 38, which may comprise an audible, visual or other type
of alarm, is connected in parallel with thermostats 32 and 34. The
alarm being normally shunted by the two thermostats, does not
operate unless one of the two thermostats is heated beyond its
setting, opening and allowing energization of the alarm.
From the foregoing description and with particular reference to
FIG. 4, it is seen that warmer 10 provides two pairs of heater
plate sections, each controlled by a single thermostat mounted don
one of the plate sections. Thus, the input to the entire heating
surface of each pair of heater plate sections is controlled in
accordance with the conditions of thermal equilibrium at only one
point. If the heat supply preceeds or falls below the rate of heat
absorption at some other point, on either pair of heater plate
sections, an undesirable temperature variation can result.
The design of the heating elements cannot be optimized for more
than one flow rate unless the heater plates are subdivided into
minute incremental areas, each with its own heating element and
control. But good results can be obtained with two equalized
heating areas, separately controlled as in device 10. That is, the
heating elements of the two sections of the heater plated can be so
proportioned that the heat supply equal the heat demand over each
area at some maximum rate of flow. A maximum flow rate is selected
as the control criterion because the danger of shock to a patient,
from blood transfused below body temperature, increases as the flow
rate increases and because the temperature increase in the fluid
processed by the warmer decreases as the flow rate increases. That
is, by optimizing the "tapering" of the heating elements for the
maximum permissible flow rate of the fluid warmer, the temperature
increase in the blood at that rate is placed under maximum
control.
In warmer 10, in any comparable heat exchanger in which a liquid
medium is heated in a thin thermally conductive bag sandwiched
between relatively closely spaced heating plates, the eat
differential can be represented by the relationship:
(1) dH=KAP(To-T)dt/D
in which
dH = heat differential
T.sub.o = temperature of the plates, in degrees, above initial
liquid temperature
T T33 temperature of liquid at given time
dt = time differential
D = thickness of sac
K = thermal conductivity of sac
A = contact area of one plate
P = effective contact area to actual plate area
The increase in temperature dT is an incremental time dt is:
(2) dT=d.sub.H /V
in which dT = temperature increase in time dt
V = fluid volume in sac.
As a working approximation, it may be assumed that the specific
gravity of the fluid being heated is unity and that the specific
heat is also unity. All units are assumed to be in the cgs system.
With these assumptions, and on the further consideration that the
volume of the sac 16 is proportional to its surface area, the
foregoing expressions (1) and (2) can be developed to afford a
fundamental heat exchange equation for warmer 10, at a maximum
fluid flow rate S, as follows:
In a typical design, it may be assumed that the total surface are
of sac 16 is 400 sq. cm. and that the effective sac area is a total
of 200 sq. cm. for each of the heater plates, the reduction being
due to the channelled construction of the sac (see FIG. 2). For a
typical plastic material suitable for sac 16, the thermal
conductivity may be taken as 0.00035, the thickness of the sac may
be assumed to be 0.0254 cm. and the maximum rate of flow may be
taken as 150 cc. per minute 2.5 cc. per second. On this basis, and
starting with blood refrigerated to a temperature of 4.degree. C.
to be transfused at a temperature of 37.degree. C., the temperature
rise T for the lower heater plate sections 21A and 22A, as derived
from equation (3), is 22.degree. C. One-fourth of this total
temperature rise is to be contributed by each of the segmental
areas A1 through A4. If equation (3) is solved for the area A, it
results in the expression
and the individual areas A1 through A4 may be calculated in
accordance with the series relationship: ##SPC1## in which T' =
fluid temperature rise per segment
A.sub.1 ...A.sub.n = segment areas, inlet end to outlet end.
For the construction specifically illustrated in FIG. 1--4 with the
parameters assumed above, the calculations for the segmental areas
A1--A4 are as follows:
A.sub.1 =182 1n 33/33-5.5=33.2 sq. cm.
A.sub.1 +A.sub.2 =182 1n33/33-11=73.7 sq. cm.
A.sub.1 +A.sub.2 +A.sub.3 =126 sq. cm.
A.sub.1 +A.sub.2 +A.sub.3 +A.sub.4 =200 sq. cm.
and the actual areas are:
A1 =33.2 sq. cm
A2 =40.5 sq. cm.
A3 =52.3 sq. cm.
A4 =74 sq. cm.
With similar assumptions, and calculations on the same basis, the
incremental areas for the upper heater plate sections, segmental
areas A5 through A8, can be shown to be
A5 =33.2 sq. cm.
A6 =40.5 sq. cm.
A7 =52.3 sq. cm.
A8 =74 sq. cm.
Even though segmental areas A5 through A8 are equal to areas A1
through A4, respectively, heating element 25B is not the same as
heating element 25A because it is required to produce only
one-third of the heat output of heating element 25A. That is,
heating element 25B contributes only one-third of the total
temperature rise that is contributed by heating element 25A.
At the maximum flow rate for fluid through warmer device 10, the
two lower heating elements 25A and 26A require a total heat output
of approximately 230 watts. However, it is necessary to provide for
peripheral heat losses and to compensate for possible drops in line
voltage. In a practical design, a total capacity of 450 watts for
the two heating elements 25A and 26A is adequate. The upper two
heating elements 25B and 26B need only a third of this heating
capacity and may total 150 watts. These heat capacities are divided
equally between the two heating element sections in each pair.
The division of heater plate section 21A into the four segmental
areas A1 through A4 is a matter of practical design considerations
and is not critical to the present invention. That is, the total
number of transverse legs or segments to the heating element
section 25A can be varied. For example, in a given construction it
may be desirable to use six transverse segments or legs for the
heater element, which would produce six segmental areas of
progressively increasing size, form bottom to top of the heater
plate section. Calculated as described above, the segmental areas
would be as follows:
A11 =21.5 sq. cm.
A12 =24.2 sq. cm.
A13 =28.0 sq. cm.
A14 =33.3 sq. cm.
A15 =40.8 sq. cm.
A16 =52.2 sq. cm.
The heating elements 25A, 25B, 26A and 26B, constructed as
described above, are each effectively tapered from the lower or
inlet end to the upper or outlet end so that the heat output of the
two heater plates diminishes gradually, form inlet end to outlet
end, in proportion to the diminishing rate of heat absorption of
fluid moving through sac 16. This is accomplished by the thermal
control, and particularly thermostats 31 and 33, even through the
thermostats control operation of the heating elements in accordance
with the temperature of only one heater plate. Relatively precise
control is possible because the equilibrium conditions at
corresponding positions on the front and back heater plates are
essentially identical. Moreover, since sac 16 is filled with a
liquid medium, there is effective heat conduction, through the sac
and the liquid, between the front and back heater plates.
The same basic operating effect, with the heating elements
conforming to the basic relationship set forth in equation (3), can
also be obtained in a construction in which the transverse segments
of the heating elements are centered in segmental areas of equal
size. A construction of this kind is illustrated in FIG. 5, showing
a heater plate 121 comprising lower and upper sections 121A and
121B with heating elements 125A and 126A. The incremental areas A21
through A28 of heater plate 121 are all equal in size and each
contains a single resistive heat element.
From equations (1) through (3), it can be demonstrated that the
construction shown in FIG. 5 produces the same operational effect
as the construction of FIG. 4 if the power outputs of the
individual heating element segments are determined in accordance
with the series relationships
(6) W.sub.1 =4.18 LST.sub.1,
W.sub.2 =4.18 LST.sub.2,...
W.sub.n =4.18 LST.sub.n and
(7) T.sub.1 =T.sub.o M,
T.sub.2 =(T.sub.o -T.sub.1) M,...
T.sub.n =(T.sub.o -T.sub.1 -...T.sub.n.sub.-1) M
in which
n = total number of segments (8 in FIG. 5)
T.sub.1...T.sub.n = fluid temperature rise in each segment
L = constant to compensate for low line voltage and thermal
losses
W.sub.1...W.sub.n 31 power input for each segment
and in which
This construction is practical and effective, but does require
fabrication of individual legs for the heater element segments that
are different from each other, and thus entails a larger number of
individual parts than the variable-displacement construction of
FIG. 4.
Applying the parameters set forth above to the construction
illustrated in FIG. 5, the power requirements for the eight heating
element segments in plate areas A21 through A28 (FIG. 4) are as
follows:
W.sub.1 =162 watts
W.sub.2 =123 watts
W.sub.3 =94 watts
W.sub.4 =71 watts
W.sub.5 =54 watts
W.sub.6 =41 watts
W.sub.7 =31 watts
W.sub.8 =24 watts
It may be noted that the power requirements W.sub.1 --W.sub.4 for
the first four segments (areas A21--A24) total 450 watts and that
the power requirements W.sub.5 --W.sub.8 for the next four segments
(areas A25--A28) total 150 watts. Thus, the Power requirements are
the same for the construction of FIG. 5 as for that of FIG. 4, as
long as the basic operational requirements remain unchanged. It can
also be shown that the temperature rise and thermal conditions for
the heater plates are essentially the same for both heater element
constructions.
The fluid warmer to the invention is highly convenient and
accessible in use. It requires but a moment to open door 14, to
remove sac 16 from the fluid warmer, and to place a new sac in the
fluid warmer and close the door. This is the total action required
to change the fluid warmer from operation with one blood pumping
system to another. If there is any leakage from sac 16, or
contamination from any other source, all of the heating surfaces
are fully accessible and available for cleaning. Thus, the fluid
warmer presents no sanitation problem, an important consideration
in emergency room and like applications.
The precision tapering of the electrical heating elements in the
fluid warmer of the present invention enable the warmer device to
maintain a substantially constant output temperature over a wide
range of fluid flow rates. That is, the proportioning of the
electrical heater elements in accordance with the heat absorption
rate of the blood or other clinical fluid affords a precise thermal
control that gives the blood warmer a substantially margin of
safety in operation, without requiring an elaborate multithermostat
circuit or other complex controls.
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