Thermal measuring and testing – Calorimetry – Gain or loss of heat by heat utilizing load in path of heat...
Reexamination Certificate
1999-01-27
2001-06-05
Bennett, G. Bradley (Department: 2859)
Thermal measuring and testing
Calorimetry
Gain or loss of heat by heat utilizing load in path of heat...
C374S029000, C374S135000
Reexamination Certificate
active
06241383
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a method for long term monitoring of heat exchangers in which the rate of flow of the heat exchange fluid and the temperature change that the fluid flow produces on a heated surface exposed to the fluid flow is measured, and further to a means for electrolytically altering the fluid environment of the heated surface.
2. Discussion
This invention arises from the need to measure the performance degradation, caused by surface fouling, of heat transfer surfaces in heat exchangers used in various applications such as power generation heat exchanger applications which may result from fouling of those surfaces. Water is often used as the active heat exchange fluid and if used untreated or inadequately treated, can promote both scale formation and micro-bio-fouling which can drastically reduce heat transfer efficiency. It is often impractical to adequately treat such water because of the large quantities used. Instead, the heat exchanger active surfaces must often be cleaned periodically as needed. In some heat exchanger operations cleaning is performed continuously. For example, in some applications cleaning is performed with circulating abrasive coated balls. In other operations, the exchangers are temporarily removed from service, opened and cleaned directly.
Both of the above methods have their related costs and mix of advantages and disadvantages which, in either case, leaves much improvement to be desired. For example, the procedure using the abrasive balls may not perform the cleaning thoroughly enough while the direct cleaning method incurs the expense of down time and high labor costs. By determining the effectiveness of the cleaning operation and when cleaning is needed, a heat exchanger can be more optimally maintained and its overall cost of operation reduced.
In some applications, for example, aboard ocean going ships, chemical treatment of the ocean water used as the heat exchange fluid is one of the ways, at least in part, of maintaining the heat exchange surfaces at high efficiency. Chlorination of the ocean water to control bio-fouling of the heat exchanger surfaces is commonly employed. However, careful control of the chlorination is required to achieve the desired maintenance without corrosively damaging the related plumbing or producing undesirable environmental impacts.
Previously developed devices and/or methods have typically determined the degradation of the heat transfer efficiency of heat exchangers from long term measurements of the exchanger's inlet and outlet temperatures, and its flow. Refinements in this art include taking multiple flow and temperature measurements directly at the internal sites of the heat exchanger, as in U.S. Pat. No. 5,429,178. While this means can be effective, it is somewhat expensive. Furthermore, if the flow sensor is of the paddle wheel type for example, the precision of measurement, particularly at low flow rates, is poor and the moving vaned paddle wheel element is itself subject to accumulations and blockage which can result in poor reliability.
Ideally, a device which is used to monitor the condition of a heat exchanger would, in addition to being cost effective to purchase, install and maintain, simulate the heat exchanger in miniature and be subject to the same operating environment. The instrument would experience the same heat exchange fluid at similar heat exchange temperatures and would use the same material heat transfer surfaces. Then, by measuring the heat transfer of the heat exchange surface within the device, the heat transfer characteristic of the heat exchanger can be reasonably inferred and maintenance procedures more optimally performed. As a further provision, the monitoring device would preferably include a means for exposing some of its active surfaces to controlled chemical exposure, for example, chlorination.
SUMMARY OF THE INVENTION
The above and other objects are provided by a heat transfer monitoring apparatus and method in accordance with the preferred embodiments of the present invention. The apparatus provides for the flow rate measurement of the heat exchange fluid and the measurement of the temperature rise experienced by a heated temperature responsive sensing element exposed to that flowing fluid. The temperature rise of the sensing element is affected by the efficiency of its heat transfer surface to that fluid and to the flow rate of the fluid.
In a preferred embodiment a magnetic flow sensor configured as an insertion flow probe senses the fluid flow rate. Mounted below it, two tubes, each wound with temperature responsive wire, comprise the heated temperature responsive sensing elements. Electrical current flowing through the wires causes the temperature of each flow tube to rise. Fluid flow through the flow tubes removes heat from the flow tubes, thereby reducing the temperature rise of each. Since clean flow tubes will transfer their heat energy to the fluid with higher efficiency for the same flow rate, the sensing elements will experience a relatively small temperature rise compared to that which would be experienced if the flow tubes contained a thermally insulating coating on their heat exchange surfaces. By measuring the flow rate, electrical power to and the temperature rise of the sensing elements over that of the fluid, and the flow tube dimensions, a heat transfer constant factor may be determined. Alternatively, as the change in heat transfer over a period of heat exchanger operation is typically the primary requirement for maintenance, only a relative indication of heat transfer is often needed. As a result, the flow tube dimensions, as long as they remain reasonably constant, need not be of concern.
Many types of flow sensors may be used for making the fluid flow measurement although the magnetic type is preferred. The magnetic type has the ability to operate reliably in difficult conditions over a wide range of flow rates and has no moving parts in the flow passage. Since long term repeatability rather than absolute accuracy is the principal requirement of the flow sensing component of the monitor, a relatively inexpensive and small insertion probe configured flow sensor is usually satisfactory. However, until recently all known commercially available magnetic flow meters, whether of full port or probe style, were relatively expensive, large and somewhat complex, often requiring fine tuning during installation to achieve acceptable results. This has now changed with the introduction of a new magnetic flow sensing technology described in a U.S. Pat. No. 5,691,484, issued Nov. 25, 1997, the disclosure of which is hereby incorporated by reference. This technology enables a small and relatively inexpensive insertion flow probe to be made which uses permanent magnets and simplified electronics to generate flow related signals with a favorable balance of sensitivity, long term stability and reliability, which makes it the sensor type of choice for the present invention.
In operation, the flow monitor's magnets are physically relocated periodically so as to reverse the polarity of the magnetic flux through the fluid flow passages. This causes the generated voltage at the electrodes to be an alternating voltage with a magnitude proportional to the fluid flow rate. The magnets are relocated in such a way that the common mode signals generated by their movement cancel leaving only the signal responsive to the fluid flow. Furthermore, the mechanical effort to move the magnets is very small since the magnetic flux pattern is stable and the magnetic fields are not exposed to variable magnetic reluctance in the magnetic circuit. The total power consumption for the magnetic flow sensor of the present invention is in the tens of milliwatts compared to tens of watts for other conventional types of magnets flow sensing devices.
Although a single heated temperature responsive sensing element is able to provide a useful heat transfer measurement by having it “time share” the fluid temperature measurement a
Feller Murray F.
Garey John F.
Bennett G. Bradley
Kiewit David
Verbitsky Gail
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