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Claims  |
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What is claimed is:
1. A method of testing a fluid environment for a propensity to deposit
foulant upon a surface immersed therein comprising,
subjecting a test surface to said fluid environment whereby foulant may be
deposited thereon, subjecting both a reference surface and said test
surface to fluid environments having a known relation of heat flow
parameters,
subjecting said reference surface to said first mentioned fluid environment
during said first mentioned step,
varying the propensity of said first mentioned fluid environment to deposit
foulant upon said test surface relative to its propensity to deposit
foulant upon said reference surface during said first mentiond step,
transferring heat between said surfaces and their respective fluid
environments, and
comparing the heat transfer characteristics of said surfaces adjacent their
respective fluid environments.
2. The method of claim 1 including the step of causing said surfaces to
have a predetermined relation of thermal characteristics prior to said
first mentioned step, and wherein said step of comparing comprises
comparing thermal characteristics of said surfaces to thereby determine
whether or not said test surface thermal characteristic has changed
because of foulant deposited thereon by said first mentioned fluid
environment.
3. The method of claim 3 wherein said surfaces are caused to have
substantially the same thermal characteristics prior to said first
mentioned step.
4. The method of claim 1 wherein said step of comparing comprises applying
thermal fluxes to said test and reference surfaces, and comparing the
temperatures of said surfaces.
5. The method of claim 1 wherein said step of varying includes the step of
protecting said reference surface from deposit of foulant thereon by said
first mentioned fluid environment during said first mentioned step.
6. A method of testing a fluid environment for a propensity to deposit
foulant upon a surface immersed therein comprising,
subjecting a test surface of said fluid environment whereby foulant may be
deposited thereon, subjecting both a reference surface and said test
surface to fluid environments having a known relation of heat flow
parameters,
comparing the heat transfers between said surfaces and their respective
fluid environments,
said step of comparing including sensing the difference in temperature of
said test and reference surfaces, and
applying to said surfaces respectively first and second heat fluxes
differentially adjusted to decrease variation of said difference in
temperature with variation of the fluid environments to which said test
and reference surfaces are subjected.
7. A method of testing a fluid environment for a propensity to deposit
foulant upon a surface immersed therein comprising,
subjecting a test surface to said fluid environment whereby foulant may be
deposited thereon,
subjecting both a reference surface and said test surface to fluid
environments having a known relation of heat fluid parameters,
comparing the heat transfers between said surfaces and their respective
fluid environments, said step of comparing comprising applying thermal
fluxes to said test and reference surfaces,
comparing the temperatures of said surfaces, and including the step of
differentially adjusting the thermal fluxes applied to said test and
reference surfaces so as to decrease variation of the temperature
comparison with variation of the fluid environments to which said surfaces
are both subjected.
8. A method of testing a fluid environment for a propensity to deposit
foulant upon a surface immersed therein comprising,
subjecting a test surface to said fluid environment whereby foulant may be
deposited thereon,
subjecting both a reference surface and said test surface to fluid
environments having a known relation of heat flow parameters,
comparing the heat transfers between said surfaces and their respective
fluid environments, said step of comparing comprising applying mutually
different thermal fluxes to said test and reference surfaces to maintain
mutually equal temperatures at said surfaces, and
comparing the respective thermal fluxes applied to said surfaces
respectively.
9. A method of testing a fluid environment for a propensity to deposit
foulant upon a surface immersed therein comprising,
subjecting a test surface to said fluid environment whereby foulant may be
deposited thereon,
subjecting both a reference surface and said test surface to fluid
environments having a known relation of heat flow parameters,
comparing the heat transfers between said surfaces and their respective
fluid environments,
subjecting said reference surface to said first mentioned fluid environment
during said first mentioned step, and
varying the propensity of said first mentioned fluid environment to deposit
foulant upon said test surface relative to its propensity to deposit
foulant upon said reference surface during said first mentioned step.
10. The method of claim 9 wherein said step of varying propensity to
deposit foulant comprises providing different temperatures at said test
and reference surfaces
11. A method of testing a fluid environment for a propensity to deposit
foulant upon a surface immersed therein comprising,
subjecting a test surface to said fluid environment whereby foulant may be
deposited thereon,
subjecting both a reference surface and said test surface to fluid
environments having a known relation of heat flow parameters, and
comparing the heat transfers between said surfaces and their respective
fluid environments, said last mentioned fluid environments being
substantially identical to each other and being different than said first
mentioned fluid environment.
12. A method of testing a fluid environment for a propensity to deposit
foulant upon a surface immersed therein comprising,
subjecting a test surface to said fluid environment whereby foulant may be
deposited thereon,
subjecting both a reference surface and said test surface to fluid
environments having a known relation of heat flow parameters, and
comparing the heat transfers between said surfaces and their respective
fluid environments, including the step of applying a first thermal flux to
said test surface during said first mentioned step to enhance the
propensity to deposit foulant and applying second thermal fluxes to both
said test and reference surfaces during said step of comparing.
13. A method of testing a fluid environment for a propensity to deposit
foulant upon a surface immersed therein comprising,
subjecting a test surface to said fluid environment whereby foulant may be
deposited thereon,
subjecting both a reference surface and said test surface to fluid
environments having a known relation of heat flow parameters, and
comparing the heat transfers between said surfaces and their respective
fluid environments, including the step of varying the foulant forming
conditons present at the interface between said test surface and said
fluid environment to thereby change the rate of foulant deposited upon
said test surface, said last mentioned step being carried out prior to
said step of comparing.
14. A method of testing a fluid environment for a propensity to deposit
foulant upon a surface immersed therein comprising,
subjecting a test surface to said fluid environment whereby foulant may be
deposited thereon,
subjecting both a reference surface and said test surface to fluid
environments having a known relation of heat flow parameters,
transferring heat between each said surface and its respective fluid
environment,
sensing the difference in temperatures of said surfaces,
sensing a heat transfer characteristic of said last mentioned fluid
environments, and
compensating said sensed difference in temperature in accordance with said
sensed heat transfer characteristic.
15. The method of claim 14 wherein said step of compensating comprises
combining with said sensed difference in temperature a quantity that is a
function of the product of a first quantity representing said sensed
temperature difference and a second quantity representing said sensed heat
transfer characteristic.
16. The method of detecting deposition of adherent precipitate upon a
surface which comprises:
providing first and second surfaces,
exposing said surfaces to a fluid which may deposit an adherent precipitate
thereon,
varying the propensity of said first mentioned fluid environment to deposit
foulant upon said test surface relative to its propensity to deposit
foulant upon said reference surface during said second mentioned step,
exposing said surfaces to fluid environments having a known relation of
heat flow parameters,
enhancing heat flow between said surfaces and said fluid environments, and
comparing heat transfer characteristics of said first and second surfaces
adjacent said fluid environments having a known relation of heat flow
parameters.
17. The method of detecting deposition of adherent precipitate upon a
surface which comprises:
providing first and second surfaces,
exposing said surfaces to a fluid which may deposit an adherent precipitate
thereon,
enhancing the deposition rate on said first surface with respect to the
deposition rate on said second surface to thereby induce more deposition
of adherent precipitate on said first surface than on said second surface,
and
comparing heat transfer of said first and second surfaces to fluid
environments having a known relation of heat flow parameters, said step of
enhancing comprising providing different temperatures at said first and
second surfaces.
18. The method of detecting deposition of ahderent precipitate upon a
surface which comprises:
providing first and second surfaces,
exposing said surfaces to a fluid which may deposit an adherent precipitate
thereon,
enhancing the deposition rate on said first surface with respect to the
deposition rate on said second surface to thereby induce more deposition
of adherent precipitate on said first surface than on said second surface,
comparing heat transfer characteristics of said first and second surfaces,
and
providing heat inputs to said first and second surfaces relatively adjusted
to unbalance temperature difference at said surfaces in a sense to
decrease sensitivity to changes in fluid environment.
19. The method of monitoring foulant accumulation upon a surface that has
been exposed to a fluid that may be foulant comprising:
exposing said surface and a reference surface to a common fluid,
comparing the heat transfer between said common fluid and each said
surface, and
compensating for initial differences in thermal transfer characteristics of
said surfaces by applying compensatory differential heating thereto when
said surfaces are in mutually similar conditions of surface accumulations.
20. The method of monitoring foulant accumulation upon a surface that has
been exposed to a fluid that may be foulant comprising:
exposing said surface and a reference surface to a common fluid,
comparing the heat transfer between said common fluid and each said
surface, and
compensating said comparing of heat transfer according to variation of heat
transfer characteristics of said common fluid.
21. The method of monitoring foulant accumulation upon a surface that has
been exposed to a fluid that may be foulant comprising:
exposing said surface and a reference surface to a common fluid,
comparing said heat transfer between said common fluid and each said
surface, and
controlling said common fluid according to variations of its heat transfer
characteristics, thereby to decrease said variations.
22. A foulant probe comprising:
first and second surfaces,
means for heating said surfaces,
means for measuring temperature of said surfaces,
a probe sheath having first and second areas thereof forming said test and
reference surfaces respectively,
said means for heating comprising a heater having first and second heater
portions mounted within said sheath and positioned at said first and
second areas respectively,
said means for measuring temperature comprising first and second sleeves
interposed between each of said heater portions and said first and second
areas respectively, and
first and second heat sensing elements connected with said first and second
sleeves adjacent said first and second sheath areas respectively.
23. The probe of claim 22 wherein at least one of said heat sensing
elements is a thermoelectric wire joined to one of said sleeves, said wire
and at least one of said sleeve and sheath being formed of different
materials that generate an electrical signal indicative of the temperature
of the junction thereof.
24. The probe of claim 23 wherein each of said sleeves is formed with a
longitudinally extending groove and said thermoelectric wires lie in said
grooves.
25. The probe of claim 23 wherein said sheath is formed with an aperture,
and including a material having a low thermal resistivity in said aperture
and between at least parts of said sheath and one of said sleeves to
provide a low thermal resistivity path from said sleeve to the exterior of
said sheath.
26. A thermally sensitive foulant probe comprising
an elongated cartridge having first and second heater elements mounted
thereto at first and second areas thereof spaced axially along said
cartridge,
a first sleeve circumscribing said cartridge in close thermal contact with
said first area thereof,
a second sleeve circumscribing said cartridge in close thermal contact with
said area second thereof,
an elongated probe sheath circumscribing said cartridge and said sleeves in
close thermal contact with said sleeves, and
first and second temperature sensing devices fixed to and between said
probe sheath and said first and second sleeves respectively.
27. The probe of claim 26 wherein said temperature sensing devices are
thermoelectric wires.
28. The probe of claim 26 including means for forming a wire receiving
conduit between said sheath and at least one of said sleeves, and at least
one of said wires of one of said sensing devices extending from the other
of said sleeves through said conduit.
29. The probe of claim 26 wherein at least one of said sleeves is formed
with a plurality of longitudinally extending slots on the exterior surface
thereof, a plurality of wires extending through some of said slots between
said sleeve and said sheath and having their ends connected to said
sleeve, the other of said sleeves having a plurality of longitudinally
extending slots formed in an exterior surface thereof, said plurality of
wires extending through some of the slots in said second sleeve between
the sleeve and said sheath.
30. The probe of claim 26 wherein at least one of said sensing devices is a
thermoelectric junction formed by a single thermoelectric wire and said
sleeve and sheath.
31. A foulant probe adapted to be immersed in a fluid environment of which
the foulant propensity is to be measured, said probe comprising:
first and second measuring surfaces,
means for heating said surfaces,
means for measuring the temperature of said surfaces to generate a
measurement signal,
a source of heater power,
means for applying said power to said heating means, and
means for compensating said measurement signal in accordance with
variations of said heater power.
32. A foulant probe adapted to be immersed in a fluid environment of which
the foulant propensity is to be measured, said probe comprising:
first and second measuring surfaces,
means for heating said surfaces,
means for measuring the temperature of said surfaces to generate a
measurement signal,
said heating means comprising means for providing relative heating to said
surfaces in a predetermined relation that yields a decreased change in
difference between said temperatures for changes in said fluid
environment.
33. The probe of claim 32 including means for compensating said measurement
signal for changes in said fluid environment when said surfaces are in
like condition.
34. The probe of claim 32 including means for compensating said measurement
signal for changes in fluid environment when said surfaces are in
different conditions.
35. A foulant probe adapted to be immersed in a fluid environment of which
the foulant propensity is to be measured, said probe comprising:
first and second measuring surfaces,
means for heating said surfaces,
means for measuring the temperature of said surfaces to generate a
measurement signal, and
means responsive to said fluid environment for compensating said
measurement signal in accordance with a characteristic of said fluid
environment, said characteristic of said fluid environment being one of
the characteristics of viscosity and flow velocity.
36. A foulant probe adapted to be immersed in a fluid environment of which
the foulant propensity is to be measured, said probe comprising:
first and second measuring surfaces,
means for heating said surfaces,
means for measuring the temperature of said surfaces to generate a
measurement signal,
means responsive to said fluid environment for compensating said
measurement signal in accordance with a characteristic of said fluid
environment,
said means for compensating said measurement signal comprising means for
sensing a characteristic of said fluid environment that varies its thermal
effect upon said test and reference surfaces,
means responsive to said last named sensing means for generating a fluid
environment signal, and
means for adding said fluid environment signal to said measurement signal.
37. A foulant probe adapted to be immersed in a fluid environment of which
the foulant propensity is to be measured, said probe comprising:
first and second measuring surfaces,
means for heating said surfaces,
means for measuring the temperature of said surfaces to generate a
measurement signal,
means responsive to said fluid environment for compensating said
measurement signal in accordance with a characteristic of said fluid
environment,
said means for compensating comprising means for sensing a characteristic
of said fluid environment that varies its thermal effect upon said test
and reference surfaces, means responsive to said last named sensing means
for generating a fluid environment signal representative of changes in
said fluid environment,
means for combining said measurement signal with said fluid environment
signal,
means for generating a compensation signal that is a function of said
combined measurement and fluid environment signal, and
means for combining said compensation signal with said measurement signal
to provide an indication of foulant on said test surface as compared to
said reference surface, said indication being compensated for variation of
said fluid environment.
38. A foulant probe adapted to be immersed in a fluid environment of which
the foulant propensity is to be measured, said probe comprising:
first and second measuring surfaces,
means for heating said surfaces,
means for measuring the temperature of said surfaces to generate a
measurement signal that indicates foulant propensity, and
including means responsive to said fluid environment for varying said fluid
environment to decrease variation of a heat flow parameter thereof.
39. The method of monitoring foulant propensity of fluid of a fluid system
comprising the steps of
flowing fluid from said system over reference and test surfaces,
enhancing the propensity of said fluid to deposit foulant upon said test
surface during a foulant period,
flowing fluids having a known relation of heat flow parameters over said
test and reference surfaces respectively during a measuring period,
applying heat flux to both said surfaces during said measurement period to
enhance transfer of heat therefrom to said fluids, and
measuring transfer of heat from said test and reference surfaces to said
fluids during said measuring period.
40. The method of claim 39 wherein said step of flowing fluid during a
measuring period comprises flowing fluid of a temperature lower than the
temperature of fluid flowing during said foulant period.
41. The method of claim 39 wherein said step of flowing fluid during a
measuring period comprises flowing fluid from a source other than said
fluid system.
42. The method of claim 41 including the step of heating fluid from said
system before flowing such fluid over said surfaces during said foulant
period.
43. The method of claim 39 including heating said test and reference
surfaces during said measuring period and wherein said step of measuring
temperature comprises measuring the temperature of said test surface with
respect to the temperature of said reference surface.
44. A thermal probe adapted to be immersed in a fluid environment to detect
foulant propensity thereof, said probe comprising
test, fluid environment, and reference surfaces, test and reference heaters
adjacent said test and reference surfaces respectively,
means for differentially measuring temperature at said test and reference
surfaces, and
means responsive to temperature of said fluid environment surface for
compensating the differentially measured temperature of said test and
reference surfaces.
45. A thermal probe adapted to be immersed in a fluid environment to detect
foulant propensity thereof, said probe comprising:
test and reference surfaces,
test and reference heaters adjacent said test and reference surfaces
respectively,
means for differentially measuring temperature at said test and reference
surfaces,
means for providing heating power to said heaters, and
means for effecting a difference in the heating power applied to said
heaters to compensate for differences in thermal characteristics of the
heat path including said reference surface and its associated heater and
the heat path including said test surface and its associated heater.
46. A thermal probe adapted to be immersed in a fluid environment to detect
foulant propensity thereof, said probe comprising:
test and reference surfaces,
test and reference heaters adjacent said test and reference surfaces
respectively,
means for differentially measuring temperature at said test and reference
surfaces,
said surfaces comprising first and second portions of a probe body of
thermoelectric material,
said means for differentially measuring temperature comprising a first wire
joined to said first probe body portion,
a second wire connected to said second probe body portion, said wires being
formed of a thermoelectric material different than the material of said
probe body, and
means for measuring the voltage difference between said wires at points
thereof remote from their respective junctions with said probe body to
thereby measure the temperature difference between said junctions.
47. A foulant probe adapted to be immersed in a fluid environment of which
the foulant propensity is to measured, said probe comprising:
first and second measuring surfaces,
means for heating said surfaces,
means for differentially measuring the temperature of said surfaces to
generate a measurement signal, and
means responsive to said fluid environment for compensating said
measurement signal in accordance with a characteristic of said fluid
environment.
48. A method of testing a fluid environment for a propensity to deposit
foulant upon a surface immersed therein comprising:
subjecting a test surface to said fluid environment whereby foulant may be
deposited thereon,
subjecting both a reference surface and said test surface to fluid
environments having a known relation of heat flow parameters,
comparing the heat transfers between said surfaces and their respective
fluid environments,
sensing a heat transfer characteristic of said last mentioned fluid
environments, and
compensating said comparison of heat transfers in accordance with said
sensed characteristic.
49. A thermally sensitive foulant probe comprising
an elongated probe sheath,
first and second heater elements spaced axially along said sheath,
a first sleeve circumscribing said first heater element in close thermal
contact therewith,
a second sleve circumscribing said second heater element in close thermal
contact therewith, said elongated probe sheath circumscribing said heater
elements and said sleeves in close thermal contact with said sleeves, and
first and second temperature sensing devices fixed to said first and second
sleeves respectively adjacent said sheath. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to detection, measurement and control of the
formation of adherent precipitates such as scale, paraffin, wax, etc. on
various surfaces.
The formation of aherent precipitates on equipment surfaces immersed in
liquids is a long standing, widespread and costly problem in the industry.
Such deposits reduce the rates of heat transfer, increase corrosion and
erosion, clog flow lines and interfere with the proper functioning of
instruments and control systems.
The most common form of such troublesome foulant coatings is adherent
inorganic scale which often precipitates from water used in industrial
equipment. For example, insoluble deposits of alkaline earth metal
carbonates and sulfates frequently precipitate on the surfaces of heat
exchanger tubes, thus reducing by major amounts the rates of heat
transfer. The fact that the tubes are hot is a primary reason for such
scale formation.
Although adherent inorganic scale is the most common form of foulant, it is
emphasized that adherent organic deposits are also major problems in
certain industries. Thus, the formation of harmful precipitates is not
confined to aqueous systems. For example, in the refining of oil sticky
adherent deposits form on metal surfaces of the reactors, heat exchangers
or transfer lines. These deposits are often the result of heating of the
oil being processed, which heating changes or decomposes asphaltic
constituents, asphaltines or similar substances to form undesired adherent
coatings. In other instances cooling, instead of heating, is the cause of
the problem. For example, crude petroleum oil will deposit adherent
coatings of paraffin wax when the temperature of the oil or of the
surfaces over which it passes is lowered sufficiently.
Where a liquid is treated with chemicals to control corrosion, bacteria or
other characteristics of the liquid, adherent scale derived from such
chemicals may also be formed.
Scale or other deposited foulant coating is also a troublesome occurrence
in many systems containing organic liquids. For example, deposits
frequently occur in high wattage electrical transformers in which the
windings are immersed in hydrocarbons or in halogenated aromatic compounds
and the like; in hydraulic oil systems containing polyols, ethers and
other organics; in heat transfer liquid systems such as heavy oil,
bisphenol A or similar high boiling organics; and in numerous organic
chemical processing units.
Scale and other harmful foulant coatings are likewise found in two-phase
systems. For example, in the processing of freshly produced crude oil, the
fluid is heated in a "heater-treater" unit to separate the unwanted salt
water. Alkaline earth metal carbonates and sulfates are often present as
adherent scale in such treating systems, the scale being sometimes mixed
with various amounts of organic material.
There exists a major need for a practical, commercial method of determining
whether or not a system is forming significant scale or other adherent
precipitates, of determining the conditions under which scale might form,
and of determining the conditions under which such formation can be
prevented either by addition of chemical scale inhibitors or by control of
process variables. It is highly important that the method be capable of
implementation by commercial instruments, which function at all times and
which do not require trained chemists or scientists for their operation.
It is also extremely important that the method be so sensitive that the
propensity of a system to develop scale will be detected without waiting
until the foulant has created substantial harm in the commercial system
being monitored.
In the past, physical inspection of plant equipment has been the common
method of ascertaining the presence and existence of adherent scale and
other precipitates. Another common method has been to measure changes in
heat transfer rates (or in required liquid flow velocities to maintain a
certain heat transfer rate). Both of these common methods suffer from the
fatal deficiency that the harm which it is desired to prevent (for
example, lowered heat transfer rate) must occur before "preventive"
measures can be taken.
Because of the great difficulty of making physical inspections of the
industrial equipment itself, one method of making heat exchanger studies
is to specially design, construct and operate a laboratory model heat
exchanger. Such a model usually includes windows for visual inspection, or
includes means for withdrawing heat exchanger tubes so that they can be
inspected and analyzed. Similarly, it is known to design laboratory heat
exchangers wherein the heat transfer rates are monitored in relation to
electrical power input, or steam condensation rates. Obviously, the
construction and operation of such laboratory models is expensive and
time-consuming and the data obtained with them may not be truly
representative of what is occurring in the actual industrial equipment.
Furthermore, reliance on changes in heat transfer rates, or on macroscopic
inspection of surfaces, produces a fatal insensitivity.
In addition to constructing and operating models of heat exchangers or
other industrial equipment, there are frequently employed, in the
laboratory, chemical methods related to formation of scale and similar
substances. For example, test solutions are prepared which are basically
unstable and will, in response to heating or standing, and to the passage
of time, yield precipitates of alkaline earth metal carbonates or
sulfates. Different chemicals are added to such test solutions, and the
degree to which such additives prevent or inhibit precipitation is
determined. It is, however, emphasized that such tests do not provide
continuous monitoring of an actual commercial system, nor do they
necessarily produce significant data relative to formation of adherent
scale in the actual system. It is to be noted that adherent scale or other
precipitate is extremely harmful, but that those precipitates which are
not adherent may be relatively harmless.
Other examples of laboratory procedures relative to scale, etc., involved
determining the stability of the water in aqueous systems. Stability is
ascertained by measuring or calculating from composition analysis, the
minimum amount of acid or base required to effect precipitation. The
amount of reagent tolerated by the solution without precipitation is taken
as being proportional to stability and thus as being inversely
proportional to the scale-forming tendency of the liquid. Such periodic
tests can, at best, only be indirectly and uncertainty related to the
tendence of an actual system to form adherent scale or other deposits.
In our prior U.S. Pat. Nos. 3,848,187 and 3,951,161, we describe extremely
precise high sensitivity methods of employing electrical contact
resistance to sense incipient precipitation of a foulant coating such as
an adherent scale, paraffin wax or the like. The methods and apparatus
described in these patents are useful, effective and of high sensitivity,
but require moving parts that could adversely affect operation over long
periods of time. Further, moving parts also add complexity and cost.
Detection and measurement of foulant coatings employing variations in heat
transfer caused by a buildup of a foulant coating have been known in the
past and avoid problems of moving parts. However, all of these methods
lack sensitivity required for rapid and real time evaluation and, in
addition, are subject to major errors due to various changes that may
occur in the fluid during or between measurements.
In one such method, a test surface is heated electrically while monitoring
the temperature of its surface that is in contact with the fluid. After a
period of immersion in the fluid of which the foulant propensity is to be
detected, temperature is again monitored and the temperature difference
between the first and second measurements is employed as an indication of
the change in foulant coating between the times of the first and second
measurements. Prior methods employing this principle of detecting changes
in heat transfer characteristics caused by changing foulant coatings, are
useful as a practical matter only for detection of large changes in
foulant coatings. By the time such a prior art system can provide a useful
measurement, serious foulant deposit may have already occurred. Such
systems are unable to measure relatively small changes in foulant coatings
because the readings vary widely as sensitivity is increased. A problem
with such prior systems is the fact that the measured temperature varies
with many different parameters of the fluid in which the test surface is
immersed. In some systems flow rate through a test cell is increased in
order to stabilize cell temperature at the entering fluid temperature.
With such high flow velocities, the flow velocity itself becomes most
critical. Thus for an instrument of high sensitivity, relatively small
variations in any one of a number of parameters of the fluid may cause an
output reading to vary from zero to full scale even with only a slight
disturbance in a parameter such as flow rate. Fluid parameters that affect
this temperature measurement include fluid velocity, viscosity,
temperature, composition, thermal conductivity, flow pattern at the
surface (which may vary with varying roughness due to increasing foulant
coating), and other flow patterns, among others. Therefore, with prior
measurements based upon monitoring of changes in heat transfer due to
changes in foulant coating, it is necessary to maintain all of these fluid
parameters the same at each measuring period so that the fluid at the test
surface has the same effect upon surface temperature at one measuring
period as it does during a subsequent measuring period. Even under
laboratory conditions, such identity of fluid characteristics is
exceedingly difficult to achieve. In practical circumstances and in field
situations, particularly where an instrument is to be used for long term
monitoring of an actual system, control of such fluid characteristics is
not feasible.
In summary, previous methods known for monitoring scaling, other than our
prior U.S. Pat. Nos. 3,848,187 and 3,951,161, do not detect or measure
accumulation of foulant in an actual system before such foulant has built
up to a degree sufficient to cause significant damage, nor do such prior
systems provide a way to test a particular liquid in order to determine in
a relatively short time its foulant propensity.
Accordingly, it is an object of the present invention to detect and/or
measure foulant or foulant propensity of a fluid before such foulant will
adversely affect operation of a system. Another object of the present
invention is the detection and measurement of foulant in a system by means
of measurement of heat transfer characteristics and without the necessity
of removing a test surface from the fluid in which it is immersed. Another
object of the invention is to determine quickly and readily conditions
under which foulant of various types will precipitate from various fluids.
SUMMARY OF THE INVENTION
In carrying out principles of the present invention in accordance with a
preferred embodiment thereof, there is provided an unique thermal bridge
in which a test surface is subjected to a fluid environment of which
propensity to deposit foulant upon a surface immersed therein is to be
detected or measured. Subsequently in a measuring period, both said test
surface and a reference surface are subjected to fluid environments having
a known relation of heat flow parameters, and heat transfers between both
surfaces and their respective fluid environments are compared. Preferably,
the fluid environments to which the test and reference surfaces are
subjected have mutually identical heat transfer characteristics for
simplicity of mechanization. According to a feature of the invention, the
test and reference surfaces are initially thermally adjusted by applying
thereto a differential heat input that causes the indicated relative heat
transfer characteristics to exhibit a minimum change over a range of
variations of the fluid environment. According to another feature of the
invention, improved insensitivity to variation of fluid environment during
a measuring period, even at very high levels of measurement sensitivity,
is achieved by compensating the output indication according to a selected
function of sensed variations in the fluid environment, and/or by
controlling the fluid environment at the test and reference surfaces so as
to decrease such variations.
The present invention, having no moving parts, and being relatively
insensitive to variations in fluid environment, is readily embodied in a
simple probe that may be exposed to actual fluid of a system to be
monitored so as to provide either intermittent or continuous readout and
record of foulant or foulant propensity with such a sensitivity as to
signal said adverse fouling before the monitored system is damaged by
fouling.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating a single path heat transfer measurement of
prior art;
FIG. 2 is a functional diagram of a thermal bridge employed in carrying out
principles of the present invention;
FIG. 3 is a side view of a simple mechanization of principles of the
present invention;
FIG. 4 is an end view of the instrument of FIG. 3;
FIG. 5 is an electrical circuit diagram of the instrument of FIGS. 3 and 4;
FIG. 6 is a sectional view of a probe embodying principles of the present
invention;
FIG. 7 is an exploded perspective view, with parts broken away,
illustrating components of the probe of FIG. 6;
FIG. 8 is a section taken on line 8-8 of FIG. 6;
FIG. 9 is an enlarged fragmentary longitudinal section of the probe of FIG.
6;
FIG. 10 is a diagram of electrical circuits used in conjunction with the
probe of FIGS. 6-9;
FIG. 11 is a diagram of other electrical circuits that may be used with the
probe of FIGS. 6-9;
FIG. 12 shows still another modification of electrical circuits for use
with the probe of FIGS. 6-9;
FIG. 13 shows a modified heating circuit for the probe of FIGS. 6-9; and
FIG. 14 illustrates a typical application of a scale sensitive probe to an
exemplary fluid system.
DETAILED DESCRIPTION
The present invention derives significant advantages from use of an unique
thermal bridge to detect or measure foulant coating by means of its heat
transfer characteristics. In order to fully appreciate advantages derived
from this type of measurement, there will first be described aspects of
prior art arrangements that attempt to measure foulant by means of heat
transfer characteristics.
It is known that deposited scale and other foulant form a coating on a
surface that changes its heat transfer characteristics. Thus, if one heats
the surface and measures the heat transfer from the surface to a fluid in
contact with the surface, one can, theoretically, obtain an indication of
the foulant coating in terms of change in the heat transfer characteristic
of the surface. Such a previously known arrangement is functionally
illustrated in FIG. 1 wherein a foulant coated surface represented by box
10 is heated from a heat source 12.
Box 10 represents part of a foulant test element heated at one side by heat
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