WO1986002714A1

WO1986002714A1 - Solution heat pump apparatus and method

Info

Publication number: WO1986002714A1
Application number: PCT/US1985/002160
Authority: WO
Inventors: Michael L. Lane; Lowell T. Whitney; Charles Paul Beck
Original assignee: Adolph Coors Company
Priority date: 1984-11-02
Filing date: 1985-10-31
Publication date: 1986-05-09
Also published as: AU5098285A; EP0200782A1

Abstract

Solution heat pump systems, improvements in the heat exchangers thereof, methods for utilizing waste heat and for improving the efficiency of heat exchange are provided, to up-grade waste heat by temperature boosting. Waste heat is fed to the evaporization zone of a first heat exchanger (2) of an absorption heat pump apparatus to vaporize a first fluid (3) of a binary fluid, the vaporized first fluid (3) is then absorbed by the binary fluid in the absorber zone (5) releasing heat. A second heat exchanger (40) in the absorber zone (5) accepts the released heat from the binary fluid, thereby upgrading the temperature of the fluid to produce steam in the heat exchanger (40). After the release of heat from the binary fluid, the first fluid of the binary fluid is vaporized by use of waste heat and the vapor is condensed (35). The condensate is transferred to the first heat exchanger (2) for revaporation and the desorbed binary fluid is transferred to the absorber for further absorption of fluid. Improved heat exchanging contact between a source of waste heat and a binary working solution is obtained by providing a uniform film of binary working solution on the outside surfaces of a vertical tube heat exchanger.

Description

SOLUTION HEAT PUMP APPARATUS AND METHOD This invention relates to solution heat pump systems and to methods for utilizing waste heat and more particularly to waste heat powered solution heat pump applications to up-grade waste heat by temperature boosting for use in various industrial applications for producing steam.

This invention also relates to improvements in the heat exchanger apparatus in solution heat pump systems and in particular to apparatus and methods for improving the efficiency of heat exchange in the desorber and absorber in a waste heat powered solution heat pump application to up-grade waste heat by temperature boostin . Many solution heat pump apparatus and methods have been developed. One of the first proposed practical uses of an absorption heat pump was reported by D.A. Williams and J. B. Tredemann at the Intersociety Energy Conversion Engineering Conference, 9th Proceedings, August, 1974 in a paper entitled Heat Pump Powered by Natural Thermal Gradients.

Additional work has been reported in various recent patents including the following:

U.S. Patent No. 4,333,515, issued June 8, 1982, inventor William H. Wilkinson et al entitled Process and System for Boosting The Temperature of Sensible Waste Heat Sources; U.S. Patent ^"No. 4,338,268, issued July 6, 1982, inventor William H. Wilkinson et al, entitled Open Cycle Thermal Boosting System;

U.S. Patent No. 4,402,795, issued September 6, 1983, inventor Donald C. Erickson, entitled Reverse_, .Absorption Heat Pump Augmented Distillation Process.

The foregoing patents and the references cited therein represent the current state of the temperature boosting art using solution heat pump technology.

In general, waste heat from industrial or other sources can be boosted to higher temperature levels by combining at least one relatively high pressure Rankine vapor generation cycle with at least one solution heat pump cycle. In a typical system, waste heat is utilized to boil off a fluid termed a refrigerant in the Rankine cycle evaporator to provide a source of vapor to an absorber in the solution heat pump. In the absorber, the refrigerant vapor is contacted with a binary working solution containing absorbent and refrigerant. As the refrigerant vapor is absorbed into the binary absorbent solution, its latent heat of condensation and heat of solution are given off to a heat exchanger at a temperature higher than the temperature of the waste heat source. The work- ing solution is then throttled to reduce the pressure and introduced into a relatively low pressure desorber where a portion of the refrigerant is desorbed as vapor from the binary solution by the addition of more waste heat through a heat exchanger. The desorbed refrigerant vapor is then condensed by contact with a colder heat exchanger at a temperature less than the temperature of the vapor, and the condensed refrigerant is then pumped to the evaporator for reuse. The concentrated working solution is recycled from the desorber to the absorber preferably through a heat exchanger where sensible heat is exchanged with the dilute working solution being con¬ veyed from the absorber to the desorber. Waste heat sources which have been used to power solution or absorption heat pumps, as described, can be obtained from either sensible heat, latent heat or both. Utilization of a sensible waste heat source has been maximized by extracting successive portions of heat for use first in the Rankine cycle evaporator section and then in the heat pump cycle desorber section of solution or absorption heat pump. Multiple cycle systems can also be employed to boost the temperature of a portion of the waste heat to even higher levels.

Many industries must dispose of large amounts of heat produced for or resulting from chemical pro¬ cessing and the like, which generally cannot be recovered using conventional heat exchange equipment because that heat is at too low a temperature for further use.

Sources of this wasted heat include heat losses from boilers, drying equipment, chemical reactors, and fractionation equipment; low pressure steam which would otherwise be vented or condensed using air or cooling water and the like; and other low quality heat derived from a wide variety heat exchange equipment. In many cases, substantial amounts of increasingly expensive fuel must be burned only to result in much of the heat produced being lost in a low grade form of waste heat. If a portion of this waste heat could be upgrade for further use, energy would be conserved and fuel cost savings realized.

Several types of heat pumps can be used to increase the temperature of waste heat such as can be obtained from low pressure steam to a useable level. An absorption cycle heat pump process such as previously described may be utilized for this purpose.

A modification of an absorption cycle heat pump is described in U.S. Patent No. 4,167,101 as a means to elevate the temperature of a waste heat source. In that substantially isobaric process, a vapor is absorbed into a liquid phase solvent in an absorption zone which subsequently releases its heat of solution to an external heat receiving medium. The solution is then taken to a stripping zone where a stripping gas desorbs the vapor from solution. The resulting gaseous mixture is then fractionated by partial liquefaction and phase separa¬ tion. The stripping gas is then recycled to the stripping zone while the liquid fraction is vaporized and then recycled to the absorber where the process is repeated.

The proposed use of a waste heat powered absorption cycle heat pump in a wide variety of indus¬ trial applications is most useful if the output of such a device is in the form of ,low to medium pressure pro- cess steam, since such steam is universally useful and easily conveyed within most processing plants without additional equipment. A temperature booster, to be economically useful in a variety of industrial appli¬ cations where process steam is desired, should be able to exhibit a thermal efficiency of at least 40% per stage of temperature boost. In order to produce usable medium pressure process steam (i.e. up to 250 psig and 406°F.), a temperature booster should also be capable of providing a maximum temperature boost up to nine-tenths of the temperature difference between the waste heat and the low temperature heat sink used for waste heat rejection. For example, if waste heat with an average temperature of 220°F. and cooling water at 90°F. were available, the maximum boosted output would be 340°F. This relationship must generally hold true in order to produce an economi¬ cally useful device for most steam producing industrial medium pressure steam producing applications. Generally, the waste heat that drives a temperature booster machine is energy that is not hot enough to be useful with con- ventional technology. It is therefore an objective of the present invention to provide an absorption cycle heat pump booster system in a method which is capable of economically upgrading waste heat to useful levels and in particular, to provide a system and method for pro¬ ducing high quality, low to medium pressure process steam for a wide variety of applications using relatively low quality waste heat as the waste heat source.

Waste heat at a temperature between about 180°F. (82^QC.) and 300^qF. (149°C), such as from spent process steam or other sources, is fed into the evaporization zone of a first heat exchanger of an absorption heat pump apparatus where optionally the first heat exchanger in the evaporization zone utilizes the waste heat to vaporize a first fluid of a binary fluid at a relatively high pressure, the vaporized first fluid is then absorbed by the binary fluid in the absorber zone releasing both a heat of condensation and the heat of solution. A second heat exchanger in the absorber zone accepts the released heat from the binary fluid, thereby upgrading the temperature of the fluid, i.e. water, to produce low to medium pressure steam in the heat exchanger. The binary fluid after removal of some of its heat, as described, is then transferred to a second pressure vessel maintained at a pressure lower than the pressure of the evaporation and absorption zones of the first pressure vessel, wherein the first fluid of the binary fluid is vaporized by contact with another source of waste heat, preferably in the same temperature range as the temperature of the waste heat source for the evaporator. The resultant vapor is placed in contact with another, colder heat exchanger where it is condensed. The resultant condensate is transferred to the first heat exchanger for re-vaporation and the desorbed binary fluid is transferred to the first pressure vessel for further absorption of the first fluid vapor into the binary fluid after first evaporation. Another heat exchanger can be provided to exchange heat between the two binary streams flowing between the absorber and desorber. In this manner, it is possible, for example, to utilize low quality heat in the form of 10 - 68 psia steam at 193^βF. to 300°F. (149°C.) to produce 20 - 250 psia saturated steam by the use, preferably, of a LiBr - Water binary system for the working fluid of the absorber and desorber described, where the concentration of the working solution as it enters the absorber is between 40% by weight of LiBr and 70% by weight LiBr and exits the absorber at between about 1% to about 10% less concentrated than initially and the concentration change of the working solution in the desorber is the same as in the absorber.

Additionally, where low pressure steam is the waste heat source, the steam can be absorbed directly in the absorber section of the first pressure vessel and condensed in the desorber heat exchangers. This steam condensate can be used as feed water to a waste heat boiler, or utilized as feed water to the absorber heat exchanger and converted to process steam. In absorption systems, not only must efficient heat transfer occur in the absorber and desorber sections but also efficient mass transfer of refrigerant into and out of solution must occur. In prior absorption refrigeration systems, the desorber section of the system typically consisted of a chamber having heat exchange tubes immersed in a pool of binary solution. Heat transfer was limited by the surface area of the tubes, residence time of the solution, and back mixing, which occurred as new solution was fed into the chamber and as convective recirculation occurred in the pool. Mass transfer was similarly limited by what typically was the relatively small surface area of the pool of solution.

In particular, heat exchanger apparatus com- prising vertical tubes with the waste heat source inside the vertical tubes and the concentrated LiBr-water work¬ ing fluid in heat exchanging contact with the outside of the tubes has exhibited inefficiencies in practice as previously described.

Likewise, the prior absorber section heat exchanger with the heat being transferred from the working fluid on the outside of the tubes during absorption to the fluid on the inside of the tubes have also not provided the requisite efficiency heat exchange for economic utilization of such systems. Different heat exchanger designs, as previously shown, have attempted to solve some of these problems by structural arrangements to increase residence time of the working fluid by designs which formed smaller pools of working fluid in contact with the outer surface of the vertical tubes. Such apparatus were, however, large and expensive and has enjoyed only limite success.

Therefore, a further objective of the invention is to provide an improved heat exchanger design and method for exchanging heat between a waste heat source and a binary working fluid preferably in an absorption heat pump unit and in particular the desorber section thereof.

It is also an objective of this invention to provide an improved heat exchanger design, and method for exchanging heat between a binary working fluid and another fluid preferably in an absorption heat pump unit and in particular in the absorber section thereof.

It is a further objective of the present invention to incorporate improved design of the present invention in a vertical tube heat exchanger where the "waste heat source is contained in the vertical tubes and the binary working fluid uniformly contacts the outside, of the vertical tubes whereby a refrigerant, typically water, is desorbed from the binary fluid and the water vapor is subsequently condensed and the reconcentrated binary fluid is circulated to the absorber section of the solution heat pump.

It is still a further objective of the present invention to incorporate the improved design of the present invention in a vertical tube heat exchanger where the binary working fluid during absorption contacts the outside of the vertical tubes and efficiently trans- fers the heat of absorption to the fluid inside of the tubes.

The present invention provides improved heat exchanging contact between a source of waste heat and a binary working solution by providing a uniform film of binary working solution on the outside surfaces of a vertical tube heat exchanger.

In the accompanying drawings:

Fig. 1 is a schematic of one embodiment a solution heat pump system for producing low to medium pressure process steam according to the present invention.

Fig. 2 is a schematic of another embodiment of solution heat pump, system for producing low to medium pressure process steam according to the present invention.

Fig. 3 is _.a schematic of a test apparatus for the evaluation of a vertical tube heat exchanger used as the desorber section of an absorption heat pump temperature boosting system.

Fig. 4 is a cross-sectional view of the vertical tube heat exchanger of Fig. 3, taken along the lines and arrows 4-4 of Fig. 3.

Fig. 5 is a partial broken cross-sectional view of one tube mounting embodiment of the present inventio .

Fig. 6 is a partial broken cross-sectional view of another tube mounting embodiment of the present invention.

Fig. 7 is -a partial broken cross-sectional view of still another tube mounting embodiment of the present invention. In the schematic shown in. Fig. 1, low temperature steam is introduced into a first heat exchanger 2 in - pressure vessel 10, through line 1. Heat exchanger 2_ heats a water fluid 3 in the first zone of pressure vessel 10 to produce water vapor which is absorbed in a second zone of pressure vessel 10, at 5 into a LiBr water binary fluid. The steam condensate from heat exchanger 2 passes through.a vapor-liquid separator trap 4 and is then passed to the condensate receiver 30. A portion of the steam that goes into line 1 is also directed via line 11 to the first zone of a pressure vessel 20 where it passes through heat exchanger 12 before being sent via line 14 through trap 17 to the condensate receiver 30. The heat exchanger 12, when heated by the steam, evaporates water from the binary working fluid passed over the heat exchanger 12. The water evaporated is condensed by heat exchanger 35 in a second zone of pressure vessel 20 and collects at 15. Concentrated binary solution at 13 is then transferred via line 16, preferably through a recuperative heat exchanger 18 and into the second zone or absorber zone of pressure vessel 10 where it is sprayed or otherwise placed in heat exchange relationship with heat exchanger 40 in the presence of the vapor from the first zone of the pressure vessel 10.

The heat extracted by heat exchanger 40 is used to produce steam from the feed water. Preferably, the condensate from the condensate receiver 30 can be used as feed water. Also preferably, the hot working fluid 5 from the pressure vessel 10 is further used in the recuperative heat exchanger 18 to heat the con¬ centrated binary fluid 13 before introduction into the absorber zone of pressure vessel 10. Under appropriate conditions, which will be more fully described herein¬ after, the steam generated in heat exchanger 40 is passed by line 41 into a steam drum 50 before eventual use. Cooling media is used in the heat exchanger 35 in the condenser zone of pressure vessel 20 to condense the water vapor at 15 desorbed from the binary fluid 13 by heat exchanger 12 in the desorber zone of pressure vessel 20. The condensed water 15 is transferred via line 22 into the evaporator zone of pressure vessel 10 to be evaporated by the low temperature steam passing through heat exchanger 2. An alternative configuration of this process could be equally effective if heat exchangers 2 and 40, and also heat exchangers 12 and 35, were in separate pressure vessels that were in vapor communi¬ cation between the respective pairs.

The complete cycle described is capable of using low temperature steam of from about 180°F. (82^βC.) to about 300°F. (149°C.) and about 9 psia (62 kPa) to about 68 psia (469 kPa) to produce steam of from about 230°F. (110°C.) to about 400°F. (205°C.) and about 20 psia (138 kPa) to about 250 psia (1.72 MPa) , when pressure vessel 10 is operated at between about 8 psia (60 kPa) and 65 psia (448 kPa) and pressure vessel 20 is operated at between about 1 psia (7 kPa) and 14 psia (96 kPa) using a binary system of LiBr and water where the concentration of the binary system in the desorber at 13 is about 45% by weight LiBr to about 70% by weight LiBr and the binary fluid concentration is changed from 1% to 10% in the absorber at 5.

In the embodiments shown in Fig. 2, a source of waste heat such as a fractionation tower 60 is used to heat a waste heat boiler 70 to produce low temperature vapor which is absorbed by a binary fluid in a pressure vessel 80 directly in contact with the absorber heat exchanger 75. The feed water for the waste heat boiler 70 is taken from the condenser zone of second pressure vessel 90, which is desorbed and condensed refrigerant . from pressure vessel 90. The binary working fluid and vaporized refrigerant are then introduced into pressure vessel 80 via line 74 and 71. The vaporized refrigerant is also introduced into the desorber zone of pressure vessel 90 vial line 72.

The concentrated binary fluid 76 in pressure vessel 90 is transferred via line 74 through heat exchanger 78 to the absorber zone of pressure vessel 80. The dilute fluid 81 in pressure vessel 80 is cooled by the recuperative heat exchanger 78 before introduction via line 82 into the desorber zone of pressure vessel 90. The foregoing system description utilizing a waste heat powered boiler eliminates the need for a separate evaporator zone in the first pressure vessel 80. Depending on the quality of the waste heat source, the pressure developed in the waste heat boiler can be from about 10 psia (69 kPa) to about 68 psia (469 kPa) at a temperature of about 193°F. (89°C.) to about 300°F. (149°C.)

The refrigerant condensate 77 in the pressure vessel 90 which is then vaporized in 70, when added to the desorbed binary fluid 76 in the pressure vessel 80, will produce a binary working fluid 81 at a temperature of about 230°F. (110°C) to about 420°F. (215°C), having about 45% by weight LiBr to about 70% by weight LiBr in the pressure vessel 80 which will, when dsscrbed, produce a working fluid solution 76 containing 1% to 10% by weight less water than solution 81.

The pressure maintained in pressure vessel 80 will equal the pressure of steam 71. The pressures maintained in the pressure vessel 90 will be less and should be between about 1 psia (7 kPa) to about 15 psia

(103 kPa) , as determined by the temperature of the cooling media and approach temperature of heat exchanger 79.

The typical sources of waste heat suitable for use with the present invention include: distillation and stripping towers or columns in oil refineries, chemical processing and the like; waste heat recovery from stack gases; blow heat recovery from pulp and paper processes; toasting and drying processes in the food industry; and exhaust gases from internal combustion engines and gas turbines.

It has been determined that it is necessary in a vertical tube heat exchanger, which is useful in either the absorber or desorber sections of a solution heat pump for temperature boosting, that there be a uniform film, particularly in the case of a desorber, of the binary working fluid on the outer surfaces of tubes con- taining a waste heat source for there to be efficient desorption of the excess refrigerant from the working solution or in the case of the absorber section efficient absorption of the refrigerant vapor and efficient heat transfer from the working solution to whatever media is used in the interior of the tubes.

In addition to maintain a uniform film of binary fluid in contact with the vertical tubes, it has been discovered that it is essential that there be a closely selected temperature difference between the binary working fluid and the waste heat source and a sufficient flow rate of binary solution down the sur¬ face of the tubes, to provide for. up to two times the heat transfer efficiency than has been obtained utilizing convention design criteria. In addition, it has been learned that there are several secondary factors that are important to the practice of the present invention.

The system pressure, the heat transfer coeffi¬ cient of the tube and the range of heat transfer rates, the tube composition and surface condition and configuration, and the total surface area of the tubes, the length of the tubes and the manner in which the working fluid is held and initially introduced onto the surfaces of the tube, must all be considered when employing the concepts of the present invention in the design of an improved heat exchanger.

To exemplify the interrelationships between the temperature difference and the flow rate necessary to the practice of the present invention, the heat exchanger apparatus shown in Figs. 3 and 4 was constructed.

The column apparatus shown in Figs. 3 and 4 was operated according to the parameters hereinafter _ described. Hot water to stimulate a waste heat source was introduced at 1' , Fig. 3, and conveyed into the interior of the vertical pipes 2' shown in Fig. 4. A fluid tight seal was provided at flange 3¹ to insure that the waste heat ^"containing water passed only vertically downward through the tubes 2' , shown in Fig. 4. A dilute solution, of typically LiBr and water from the absorber section of a solution heat pump, is introduced into the column 10' at 4¹ to contact only the outside of the tubes, shown in Fig. 3.

Baffle plates 20', Fig. 5, are provided at several locations along the lengths of the column 10" and constructed in a manner to provide an open annulus through which the dilute LiBr-water solution will flow by gravity downwardly onto the surface of the tubes 2*.

Centering device 28' is provided to maintain the relative position of tubes 2' and baffel plate 20'. The dilute solution will evaporate water as it picks up -waste heat from the water introduced at l¹, into the intexior of the tubes 2¹ through the tube walls. The water vapor produced can be removed from the column 10' at various locations, such as points 11' and 12'. In the configuration shown in Fig. 3, a condenser is provided to condense the water vapor to liquid water. The concentrated Li-Br-water solution reaching the bottom of the column 10^* is removed at 15', normally to be recirculated to the absorber section of a solution heat pump apparatus as described hereinbefore. In the experimental set-up shown, the concentrated solution is removed at 15' and is introduced into a mixing tank 30' where it is diluted to the typical concentration of a dilute solution from the absorber section heat pump for reintroduction at 4' into the column 10' . The operation of the described exemplary apparatus has produced the following criteria for obtaining the results of the pre¬ sent invention.

The main vertical section of the heat exchanger 10', outside of the tubes 2' is preferably maintained at a pressure of between 1 psia and 10 psia and more preferably between about 1 psia and 3 psia for best results. Typically, the temperature of the waste heat containing water is between about 180°F. (82°C.) and about 250°F. (121°C.) and preferably between about 200°F. (93°C.) and about 220°F. (104°C).

Preferably the temperature difference between the waste heat source and the binary working fluid should be in the range of from about 5°F. (2.8°C) and about 25°F. (13.9°C), more preferably between about 10°F. (5.6°C.) and about 20°F. (11.1°C.) and most preferably less than about 15°F. (8.3°C.). To insure a uniform heat exchanging film of dilute binary solution on the exterior surfaces of the tubes 2' it is preferable to provide a sufficient flow rate of dilute solution such that under the conditions of heat exchange provided there is a uniform falling film of sufficient binary solution on the exterior surfaces of the tubes 2' and a temperature difference selected to provide for complete wetting of the tube surfaces and a minimum of sputtering or splattering of liquid from the tube surface. The wide range of appli- cability of the present invention, has been determined to be optimized by a flow rate of binary working fluid in the range of at least about 0,10 gallons per minute per inch of circumference and preferably from about 0.10 to about 0.40 gallons per minute per inch of circumference of the vertical tube used in order to achieve the improved efficiency of heat transfer of the present invention. As previously described, it is preferred to design the tube and baffle structure with the annular space 21' so as to distribute the binary working fluid as uniformly as possible onto the outer surface of the tubes within the flow rates previously described. -_; Several designs will function in this regard including . those shown in Figs. 5, 6, and 7, where tubes 2' and distribution plates 20' are arranged with an annulus 21' and the plate 20' either provided with centering device 28', a porous pad 25* or a screen 26* . Preferably the centering device 25' will consist of rods of approximately the same dimension as the space between tubes 2' and installed in such a manner as to firmly hold tubes 2' in center of hole in distribution plate 20', thus forming a uniform annular space 21'. Preferably, the pad 25' should be open enough in construction to permit sufficient free flow of the binary solution introduced above the distribution plate 20' through the pad 25' and the annulus 21' onto the tube 2' to achieve the identified flow rates. Most preferably, the pad 25' or screen 26* are forced into and partially through the annulus 21' to provide wicking and centering actions and better direct the fluid uniformly onto the outside surfaces of the tubes. An optimum design can be selected following the foregoing principles to achieve the functionality described without undue experimentation. In addition to the foregoing, additional design features can be utilized such as vertically splined tubes, and tubes with other special surface pre¬ paration including coatings and the like, if selected to minimize interference with the heat transfer from between the source of heat and the desired medium for receiving that heat and still promote uniform wetting of the exterior surface of the tube with binary working fluid. Any surface preparation or surface coatings should also be selected for their resistance to chemical attack by the binary fluid to minimize long-term maintenance problems in the design.

The experimental desorber shown in Figs. 3 and 4, was designed as a -vertical shell and tube heat exchanger. The outer shell was fabricated from 8-inch schedule 40 pipe (carbon steel) with a tube sheet/flange at top and bottom. The shell was also provided with = ^' several viewing ports for observation of the flow and wetting of the falling film of solution. Twenty-one copper alloy tubes (0.75 inch OD) , were used in this design. The total heat transfer length was 7 feet 10 inches. Two baffle plates or flow distribution plates were used to direct the flow of the dilute solution onto the tubes. One plate, such as shown as plate 20' in Fig. 5, was located 4 inches below the top tube sheet; the second plate was located 27 inches above the bottom tube sheet. The lower baffle was included to redirect any solution that may have splattered into the shell back to the tubes. To quantify the amount of solution splattering, a 1-inch drip ring was welded to the inside of the shell, 4 inches from the bottom tube sheet.

The purpose of the desorber experiments was to measure the tube outside heat transfer coefficient (h ) Btu/hr ft 2°F^* . The experimental runs were carried out for-the range of operating characteristics employed. For each test run, the log mean temperature difference (LMTD) between the heating water and solution was calculated from measured temperatures according to:

_TLM_MΦT_TD. ₍₁₎

where the subscripts indicate: w - water s - solution o - outlet i - inlet

The total heat transfer rate (Q) Btu/hour was calculated from the measured heating water flow rate and temperature difference according to:

where m = mass flow rate (Ib/hr) cp = specific heat (Btu/lb°F.)

Using the design values, an overall heat transfer co-

2 efficient (U) Btu/ft hr°F.), based on the tube outsi heat transfer area (A), was then calculated using:

U = Q (3)

(LMTD) (A)

The tube outside heat transfer coefficient was calculated using the previously calculated value of U and the calculated value for the inside heat transfer coefficient

(h. ) based on the Dittus-Boelter correlation for turbulent flow in tubes or pipes. The tube wall conduction term for copper was neglected since it has a minor resistance to heat flow to calculate ho from:

h ^•***- (4)

1 - 1 u ε.

where: h_± = 0.023 (Re)°-⁸Pr^0,3

D - Tube inner diameter K - thermal conductivity for water

Re - Reynolds number based on D Pr - Prandtl number for water The outside, inside, and overall heat transfer coefficients were calculated along with heat transfer rate, LMTD, and solution flow rate for each experimental run.

During the experiments, the binary solution was observed to exhibit blowing or sputtering off the tubes due to the high heat transfer rate at low flow rates and vacuum outside of the range previously described. When the vacuum is lower, more gas, such as nitrogen, is dissolved in solution, which tends to impede the desorption process. The effect of this is to reduce Q and LMTD, which seems to provide for adequate desorption to occur over the full tube length, which has the effect of raising h . Thus, the degree of vacuum affects h only in an indirect way. The low vacuum slowed down the total heat transfer by acting as an additional resistance in desorption of the vapor. It is significant to note that the LMTD tests closely simulate the intended conditions for a desorber. Thus, the high h tests are the ones of greatest design interest.

The vertical desorber described has a high heat transfer performance if operated under specific constraints identified. The tube outside heat transfer

2 coefficient can be greater than 700 Btu/hr ft ^βF. provided the LMTD is limited to a maximum of 15°F. (i.e., the

2 heat flux per tube is limited to 4,500 Btu/hr ft ). Additionally, with the flow distribution plate design employed, the tubes were remarkably well-wetted under all circumstances frαntop to bottom under the conditions employed. Even when sputtering occurred, the tubes would tend favorably to rewet. Other tests were conducted to establish the proper design .and operating criteria for the generation and maintenance of a uniform film coverage of the LiBr- water binary solution on the surfaces of the heat exchanger tubes. As previously shown, the beneficial improved heat exchanging capability of the heat exchanger depends upon the maintenance of a uniform film of liquid covering the entire surface of each of the tubes containing the waste heat containing water.

The present invention is then directed to a heat exchanger where the flow rate into a vertical tube heat exchanger, as described, is between 0.10 to about 0.40 gallons per minute per inch of tube circumference at a pressure for a desorber of about 19.5 in.Hg (5.2 psia or 35.8 kPa) and a temperature difference of less than about 15°F.

The present invention has been described with respect to the presently known preferred embodiments thereof. It is contemplated, however, that the inventive concepts disclosed may be otherwise variously embodied and it is intended that the appended claims be construed to include alternative embodiments of the invention except insofar as limited by the prior art.

Claims

C L A I M S 1. A method of upgrading waste heat with a modified solution heat pump system to produce steam having a temperature of from 230°F. (110°C.) to about 400°F. (205°C.) at a pressure of from about 20 psia (138kPa) to about 250 psia (l.^'72MPa) comprising the steps of: a. establishing heat exchanging contact between a source of waste heat with the water condensate from a binary working fluid comprising LiBr and water, at a temperature of at least 180°F. (82°C.) and a preselected pressure in an evaporator whereby the water condensate is at least partly vaporized to form water vapor; b. contacting said water vapor with said binary working fluid of LiBr and water in an absorber wherein the concentration of LiBr by weight is from 40% to 70% by weight and is diluted from 1% to 10% by weight by absorption of said water vapor; c. separately concentrating said binary working fluid in a desorber at a preselected lower pressure by heat exchanging contact with a source of waste heat to desorb the water by evaporation from said binary working fluid to produce a more concentrated solution by vaporizing from 1% to 10% by weight of the water; d. returning said concentrated binary to said absorber for re-dilution preferably after being heated in a recuperative heat exchange where heat is exchanged with the dilute binary from said absorber; e. condensing by heat exchanging contact with a lower temperature condenser, the water vapor so removed to form said condensate; f. Returning said condensate to heat exchanging contact with the waste heat in said evaporator; g. Providing heat exchanging contact between the binary working fluid in the absorber, and a source of feed water in said absorber_.heat exchanger whereby the feed water is converted to low to medium pressure steam at a higher temperature than the source of waste heat.

2. A method of upgrading waste steam with a modified solution heat pump system to produce steam having a temper¬ ature of from 230°F. (110°C.) to about 400°F. (205°C.) at a pressure of from about 20 psia (138kPa) to about 250 psia (1.72 MPa) from steam having a temperature of about 193°F. (89°C.) to about 300°F. (149°C.) and a pressure of about 10 psia (69kPa) to about 68 psia (469 kPa) com¬ prising the steps of: a. introducing low temperature steam into a first pressure vessel in a manner to contact said steam with a water-LiBr binary fluid of a predetermined con¬ centration whereby water vapor is at least partially absorbed by said binary fluid in a manner to release heat wherein the initial concentration of LiBr

40% to 70% by weight and is diluted from 1% to 10% by weight by absorption of the water vapor; b. transferring the diluted binary fluid after absorption to a desorber section of a solution heat pump; c. separately concentrating said binary work¬ ing fluid in the desorber at a preselected lower pressure by heat exchanging contact with a source of heat to desorb at least some of the water from the binary fluid by evaporation from said binary working fluid to produce a more concentrated solution by vaporizing from 1% to 10% by weight of the water; d. Condensing the water vapor desorbed by heat exchanging contact with a lower temperature condenser, to form a condensate; e. Circulating said binary working fluid from said desorber to the absorption section of said first pressure vessel into contact with said low temperature steam; f. Providing heat exchanging contact between the binary working fluid in the first pressure vessel and a source of feed water to said absorbing section whereby the feed water is converted to low to medium pressure steam at a higher temperature than the source of waste heat.

3. A method of improving the transfer of heat in a vertical tube heat exchanger comprising the steps of intro¬ ducing a heat source into the interior of the tubes of a vertical tube heat exchanger, introducing and distributing a fluid, into which heat is to be transferred onto the exterior surfaces of said vertical tubes in a manner to provide for a uniform falling film of said fluid on sub¬ stantially all of the exterior surfaces of said vertical tubes so as to exchange the heat imparted to the tubes by said heat source to the falling film; and recovering the heated fluid.

4. The method of claim 3, wherein the flow rate of the falling film is sufficient to wet substantially all of the surface area of the vertical tubes in the vertical tube heat exchanger.

5. The method of claim 3, wherein the temperature difference between the heat source and the said fluid is less than about 15°F. (8.3°C).

6. The method of claim 3, wherein said fluid is kept at a pressure of from 1 psia to 10 psia during contact with the vertical tubes.

7. The method of claim 3, wherein the flow rate of the falling film is maintained at at least 0.10 gallons per minute per inch of tube circumference.

8. A method of desorbing refrigerant from a binary working fluid in the desorber section of a temperature boost¬ ing-solution heat pump comprising the steps of: providing a heat source in the vertical tubes of a vertical tube heat exchanger apparatus; distributing the binary working fluid from which a refrigerant is to be desorbed as a uniform falling film of fluid in heat exchanging contact on the exterior surfaces of the vertical tubes containing the heat source at a sufficient flow rate to maintain tube wall wetting and a preselected temperature differential to minimize sputtering and splattering of the film of fluid and maximize the desorbing of the refrigerant by evaporation.

9. The method of claim 8, wherein the pressure over the binary working fluid during heat exchange and desorption is between 1 psia and 10 psia.

10. The method of claim 9, wherein the flow rate of the binary working fluid during heat exchange desorption is at least 0.10 gallons per minute per inch of circumference of the vertical tubes.

11. The method of claim 10, wherein the temperature difference between the waste heat source and the binary working fluid is in the range of from about 5°F. to about 25°F.

12. A method of improving the transfer of heat in a vertical tube heat exchanger comprising the steps of intro¬ ducing a heat receiving medium into the interior cf the tubes of a vertical tube heat exchanger, introducing and distribut¬ ing a fluid, from which heat is to be extracted onto the exterior surfaces of said vertical tubes in a manner to provide for a uniform falling film of said fluid on substan¬ tially all of the exterior surfaces of said vertical tubes so as to exchange heat from said fluid into the vertical tubes and into the heat receiving medium contained therein.

13. The method of claim 12, wherein the flow rate of the falling film of fluid is sufficient to wet substantially all of the surface area of the vertical tubes in the vertical tube heat exchanger.

14. The method of claim 12, wherein said fluid is kept at a pressure of from 10 psia to 100 psia during contact with the vertical tubes.

15. The method of claim 12, wherein the flow rate of the falling film is maintained at at least 0.10 gallons per minute per inch of vertical tube circumference.

16. A method of absorbing refrigerant.into a binary

U& s _ .! i a i ^'.__!___:£___ I working fluid in the absorber section of a temperature boosting solution heat pump comprising the steps of: providing a heat receiving medium in the vertical tubes of a vertical tube heat exchanger apparatus; distributing the binary working fluid into which a refrigerant is to be absorbed as a uniform falling film of fluid in heat exchanging contact on the exterior surfaces of the vertical tubes containing the heat receiving medium at a sufficient flow rate to maintain tube wall wetting and a preselected temperature differen¬ tial to maximize the heat exchange from the absorbent.

17. The method of claim 16, wherein the pressure over the binary working fluid during heat exchange and absorption is between 10 psia and 100 psia.

18. The method of claim 17, wherein the flow rate of the binary working fluid during heat exchange from absorption is at least 0.10 gallons per minute per inch of vertical tube circumference.

19. A waste heat recovery system capable of producing steam at a higher temperature than the temperature of the waste heat source comprising: a. means for evaporating water utilizing heat exchanging contact between said waste heat and water at a first preselected pressure; b. absorbing means including a means for contact¬ ing the- water vapor produced in the evaporating means with a binary liquid solution of water and LiBr to absorb at least a portion of said water vapor whereby heat is given off, including means in heat exchanging communication with said contacting means for transferring said heat to a source of feed water to produce steam; c. means for desorbing said water from the water- LiBr solution which was received from said absorbing means, including means for supplying heat to said solution at a second preselected pressure lower than said first preselected pressure; d. means for condensing at least a portion of the water vapor evaporated by said desorbing means; e. first conduit means for recycling said solution between said desorbing means and said absorbing means and, second conduit means for transferring said solution from said absorbing means to said desorbing means and third conduit means connecting said condensing means with said evaporating means.

20. A waste heat recovery system capable of producing steam at a higher temperature than the temperature of a waste heat source comprising: a. means for contacting low temperature steam with a binary liquid solution of water and LiBr to absorb at least a portion of said steam whereby heat is given off; b. means in heat exchanging communication with said contacting means for transferring said heat to a source of feed water to produce steam; c. means for desorbing at least a portion of said water from the water-LiBr solution received from said absorb¬ ing means, including means for supplying heat to said solution at a second preselected pressure lower than said first preselected pressure; d. means for removing at least a portion of the water vapor from the desorbing means; e. first conduit means for recycling said solution between said desorbing means and said absorbing means and, second .conduit means for transferring said solution from said absorbing means to said desorbing means.

21. A vertical heat exchanger apparatus for exchanging heat comprising: vertical tube means for containing a heat source from which heat is to be extracted; container means surrounding said vertical tube means for containing fluid to be heated by said vertical tube means; means for introducing said fluid into said container means; means for exhausting said fluid from said container means ; perforated baffle means horizontally in said container means and surrounding said vertical tube means providing an open annulus between said baffle means and said vertical tube means to provide an opening through which said fluid must flow downward over the vertical tube means.

22. The heat exchanger apparatus of claim 21, additionally comprising: fluid control means associated with said baffle means and said annulus and adapted to provide a substantially uniform falling film of said fluid on the surfaces of said vertical tube means.

23. The heat exchanger of claim 22, wherein the fluid control means includes centering, means comprising intersect¬ ing structural member means wherein the intersection defined by said structural member means receives in contact relation¬ ship each verticle tube of said vertical tube means, spacing each vertical tube of said vertical tube means centrally of each opening in said perforated baffle means defining said open annulus.

24. The heat exchanger of claim 23, wherein each of the intersecting structural member means are approximately the size of the space between said vertical tube means.

25. A vertical heat exchanger apparatus for exchanging heat comprising: vertical tube means for containing a medium into which heat is to be transferred; container means surrounding said vertical tube means for containing a heating fluid in contact with said vertical tube means; means for introducing said hea.ting fluid into said container means; means for exhausting said heating fluid from said container means after contact with said vertical tube means; perforated baffle means horizontally in. said. con-. tainer means and surrounding said vertical tube means providing an open annulus between said baffle means and said vertical tube means to provide an opening through which said heating fluid must flow downward over the vertical tube means.

26. The heat exchanger apparatus of claim 25, additionally comprising: fluid control means associated with said baffle means and said annulus and adapted to provide a sub¬ stantially uniform falling film of said fluid on the surfaces of said vertical tube means.