Patent Number: 052157040
Section: description

DESCRIPTION OF THE INVENTION Water-to-water heat exchangers typically are constructed in two forms. In the "U-tube" configuration shown in FIG. 1, a plurality of tubes 10 are formed into the shape of a "U" with their ends fitted into holes in a tube sheet 11. A bonnet 12 having an inlet port 13 and an outlet port 14 for the service fluid is bolted or otherwise secured to the periphery of tube sheet 11, and a divider plate 15 is positioned inside the bonnet to separate the inlet ports and outlet ports of the tubes. A shell 16 having an inlet port 17 and outlet port 18 for the process fluid is also fastened to the periphery of tube sheet 11. In a "straight-tube" type of heat exchanger (not shown), the tubes are straight and their ends are fitted into two separate tube sheets, each having a bonnet attached at its periphery. The service fluid is admitted into one of the bonnets, flows through the tubes, and exits through an outlet port in the other bonnet. A shell having inlet and outlet ports for the process fluid surrounds the tubes and is attached to the periphery of each tube sheet. FIG. 2 illustrates an end view of the heat exchanger of FIG. 1 with the bonnet removed and shows schematically the fittings and instrumentation necessary to conduct a heat transfer test in accordance with this invention. A water reservoir 20 is connected to an inlet port 21 of a tube in the heat exchanger via an inlet hose 22 and a metered pump 23. An inlet fitting 24 forms the connection between hose 22 and port 21. An outlet port 25 of the same tube is connected to reservoir 20 via an outlet fitting 26 and an outlet hose 27. Reservoir 20 contains cooling coils 22a and baffles 22b which assure that the water is mixed and at a uniform temperature before it is returned to hose 22. A vent 22c allows the escape of any air that is initially in hoses 22 or 27 or the tube being tested. A microprocessor 28 is fed signals indicating the temperature (t.sub.i) of the water at inlet port 21, the temperature (t.sub.o) of the water at the outlet port , the temperature (T) of the process fluid, the pressure differential (.DELTA.P) between the water at inlet port 21 and outlet port 27, and the flow rate provided by pump 23. FIG. 3 illustrates inlet fitting 24 in detail. Fitting 24 contains a tubular body 30 attached at one end to a centering guide 31 which is inserted into the tube. A rubber seal 32 provides a leakproof seal between centering guide 31 and tube sheet 11. An 0-ring 33 seals body 30 and centering guide 31. Inlet hose 22 fits over a hose connection 34 at the other end of body 30. Inserted through the wall of body 30 are a temperature detector 35 (e.g., a resistance temperature detector, thermistor or thermocouple) and a pressure detector 36, both of which are connected to microprocessor 28 as shown in FIG. 2. Outlet fitting 26 is similar in construction to inlet fitting 24. The heat transfer performance of an individual tube in a heat exchanger is measured by U, which is its actual heat transfer coefficient in operation. Its optimal heat transfer coefficient when it is clean is represented by U.sub.c. r.sub.f, the fouling resistance of the layer or layers of contamination, is equal to: EQU r.sub.f =1/U-1/U.sub.c If there is more than one layer of deposit on the inside and/or outside of the tube, r.sub.f is the summation of the fouling resistance of the layers: EQU r.sub.f =r.sub.1 +r.sub.2 - - - r.sub.a The actual heat transfer coefficient U is derived by equating (i) the loss of heat from the fluid as it flows through the tube to (ii) the heat flow through the wall of the tube and any deposit layers on the surface of the tube. The loss of heat from the fluid is represented by: EQU Q=mC.sub.p (t.sub.o -t.sub.i) (1) where, Q is the heat flow in Btu/hr PA0 m is the mass flow rate of the fluid in lbs/hr PA0 C.sub.p is the specific heat of the fluid at constant pressure in Btu/lbs-.degree. F. PA0 t.sub.i is the temperature at the inlet of the tube PA0 t.sub.o is the temperature at the outlet of the tube PA0 U is the actual heat transfer coefficient of the heat exchanger in Btu/hr-ft.sup.2 -.degree. F. PA0 A.sub.o is the area of the outside surface of the tube in ft.sup.2 PA0 LMTD is the log mean temperature difference between the service water in the tube and the process fluid in .degree. F., which in turn is equal to ##EQU1## where T is the temperature of the process fluid, which is assumed to be a constant. The heat flow through the tube wall and fouling layers is represented by: EQU Q=UA.sub.0 (LMTD) (2) where, Solving equations (1) and (2) for U yields: ##EQU2## Thus U is expressed in terms of the characteristics of the service water (C.sub.p), the dimensions of the tube (A), temperatures of the water and process fluid (t.sub.o, t.sub.i, T), and the rate of flow of the service water (the mass rate of flow (m) is easily computed from the flow meter on pump 23, e.g., for water, m=496.79 times the flow rate in gals/min). All of these quantities are either known or are obtainable from the meter and detectors associated with the heat exchanger and pump 23. Microprocessor 28 can easily be programmed to provide a continuous indication of U. Alternatively, U can be computed manually. The arrangement described above contemplates that the service fluid would be chilled in reservoir 20. If reservoir 20 includes a heater instead of a cooler, the same calculations can be performed except that t.sub.i and t.sub.o are reversed in each of the equations. The number of heat exchanger tubes that need to be tested depends on the statistical distribution of fouling in the individual tubes and is expected to vary between six tubes and 10% of the total number of tubes. It appears that the tubes to be tested should be selected randomly. Once the heat transfer coefficient U has been determined, it can be compared with U.sub.c, the heat transfer coefficient of the heat exchanger in a clean condition, to calculate r.sub.f, the fouling resistance. The heat transfer capability of the heat exchanger decreases as the fouling resistance increases. The value of U.sub.c is can be obtained or derived from the technical specifications and design data for the heat exchanger. If it is not available, one of the tubes can be cleaned and the test can be performed on the clean tube. The pressure differential between the inlet and outlet ports 13 and 14 of bonnet 12 may be recorded at the time of each test, and a correlation between pressure differential and the heat transfer coefficient of the exchanger can be developed. If the correlation appears reliable, then the pressure differential can be monitored in lieu of future direct measurements of the heat transfer coefficient, saving considerable time and expense. The method and structure described above can be used both with liquid-to-liquid heat exchangers and liquid-to-gas heat exchangers. If the tubes are finned, the outside area can be determined from the number and geometry of the fins. The above description is intended to be illustrative and not limiting. Other methods and embodiments will be apparent to those skilled in the art all of which are within the broad principles of this invention.