US3520354A

US3520354A - Method of improving heat exchanger performance

Info

Publication number: US3520354A
Application number: US772302A
Authority: US
Inventors: Benjamin Lawrence
Original assignee: Procter and Gamble Co
Current assignee: Procter and Gamble Co
Priority date: 1968-10-31
Filing date: 1968-10-31
Publication date: 1970-07-14
Anticipated expiration: 1987-07-14

Description

July 14, 1970 a. LAWRENCE 3,520,354

METHOD OF IMPROVING HEAT EXCHANGER PERFORMANCE Filed Oct. 31, 1968 Fig. 4

INVENTOR. Benjamin Lawrence ATTORNEY United States Patent O 3,520,354 METHOD OF IMPROVING HEAT EXCHANGER PERFORMANCE I Benjamin Lawrence, Springfield Township, Hamilton County, Ohio, assignor to The Procter & Gamble Company, Cincinnati, Ohio, a corporation of Ohio Filed Oct. 31, 1968, Ser. No. 772,302 Int. Cl. F28f 27/02 US. Cl. 165-1 Claims ABSTRACT OF THE DISCLOSURE A method of operating a heat exchanger within which a hot, crystal-forming melt is being cooled with resultant crystallization within the heat exchanger of at least some of the melt. The direction of flow of the melt through the heat exchanger is periodically reversed to extend the effective operating time thereof.

BACKGROUND OF THE INVENTION This invention relates to a method of operating heat exchangers. More particularly, the invention relates to an improved method of operating heat exchangers within which hot, crystal-forming melts are being cooled to a temperature at or below the point where crystallization takes place within the heat exchanger.

The usual procedure for operating a heat exchanger to the point where crystallization takes place is to permit within which a hot, crystal-forming melt is being cooled flow of the crystal-forming melt until the crystal buildup reaches an intolerable level such that the efficiency of the heat transfer process is significantly impaired. The buildup of crystals within the exchanger has two separate effects: one is the constriction of the flow passage, thereby reducing the flow rate of the material which is passing therethrough; the second is the formation of an insulating layer of crystals on the surface through which the heat transfer occurs, thereby reducing both the heat transfer rate and the overall heat transfer coefficient of the system. When the crystal buildup reaches such a point the heat exchanger can be cleaned by passing a hot liquid through it to melt or dissolve the crystals and restore the heat transfer surface to its original, clean condition. This hot liquid can either be hot water or it can be the hot, crystal-forming melt itself, which can be circulated through the exchanger while the coolant flow is shut off. Either of these liquids, provided they are sufficiently hot, will rapidly remove the crystals and thereby clean the interior surfaces of the heat exchanger. The operation of a process in this fashion, that is, frequently shutting down the process to permit clean-out of the heat exchanger, is a very inefiicient and costly method of operation.

Although the prior art has recognized that the reversal of flow in heat exchangers is effective to dislodge and flush debris from the heat transfer surfaces of surface condensers, it does not teach the application of a flow reversal technique to heat exchangers wherein crystal-forming melts are being cooled to the crystallization stage. While debris and sediment may be removed by sudden flow reversal, the buildup of crysta s cannot be so removed and thus the flow reversal technique would seem inapplicable to that type of system. The prior art is careful to distinguish between sediment or debris carried by the cooling liquid (which is usually water) and scale, in that the sediment or debris can be disturbed and removed by means of flow reversal, whereas scale cannot.

M 3,520,354 Patented July 1970 Examples of such prior art teachings are US. Pat. 2,136,087, issued Nov. 8, 1938, to William F. Schroth, and US. Pat. 3,211,217, issued Oct. 12, 1965, to W. H. McKee et al.

Additionally, the prior art does not teach the periodic reversal of flow for extended periods to remove the crystal buildup that rapidly takes place when crystal-forming melts are being cooled. Rather, the flow reversal taught by the prior art is apparently intended to be a sudden and sporadically applied measure since the buildup of such debris and sediment does not take place rapidly, but slowly over a relatively protracted time period.

SUMMARY OF THE INVENTION Briefly stated, in accordance with one aspect of the present invention, a method is provided which permits an increase in the effective operating time of a heat exchanger within which a hot, crystal-forming melt is being'fcooled with resulting crystallization and with buildupof crystals on the interior surfaces thereof. The crystalforming melt is caused to flow first in one direction and then in a second direction opposite to the first and, if desired, then is caused to again flow in the first direction.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevation, partially in cross section, showing the internal construction of a typical scraped surface heat exchanger Within which a hot, supersaturated, crystal-forming melt can be cooled.

FIG. 2 is a sectional view taken along the line 22 of FIG. 1.

FIG. 3 is a flow diagram which illustrates diagrammatically the arrangement of a suitable system, including the directions of coolant and hot melt flow for a normal counter-current flow process wherein the direction of the coolant fiow is opposite that of the hot melt flow.

FIG. 4 is a flow diagram similar to FIG. 3 but showing the same apparatus arranged for reverse flow wherein the coolant and the hot melt both flow in the same direction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, and particularly to FIG. 1 thereof, there is shown, in partial cross section, a scraped surface heat exchanger 10 which is suitable for use with the method of the present invention. Although the method will be explained in connection with a scraped surface heat exchanger, it is to be understood that it is not limited to that specific type of apparatus and can be used with other types of heat exchangers. However, the scraped surface heat exchanger is a preferred type of heat exchanger in connection with the cooling of crystal-forming melts since it is an efficient heat transfer device and provides a homogeneous cooled product.

Heat exchanger 10 comprises an outer cylindrical barrel 11 which is closed at each end by

end caps

12 and 13. A cylindrical layer of insulation 14 overlies the exterior surface of outer cylindrical portion 11 to minimize heat transfer losses therethrough. An inner cylindrical barrel 15 is disposed within outer cylindrical barrel 11 and positioned within inner cylindrical barrel 15 is a cylindrical shaft 16 which is rotatably supported in

journal bearings

17 and 18 in

end caps

12 and 13, respectively. Shaft 16 is usually referred to as the mutator shaft and carries a plurality of fixed scraper blades 19 and 20 (see FIG. 2) which scrape the cooling melt from the inner Walls of inner cylindrical barrel in a mannerto be hereinafter explained. In FIG. 1, scraper blade has been shown partially broken away for clarity. The combination of the outer cylindrical barrel 11, inner cylindrical barrel 15 and mutator shaft 16 defines a pair of coaxial

annular passageways

21 and 22 within the exchanger. Outer passageway 21, Which surrounds inner passageway 22, carries the cooling medium and has inlot 23 and outlet 24 associated therewith. Inner passage way 22, which carries the hot melt that is to be cooled, also has a suitable inlet 25 and outlet 26 associated therewith.

Scraper blades

19 and 20 are attached to mutator shaft 16 by means of bolts 27 which securely hold

blades

19 and 20 against a series of radially positioned, aligned bosses 28 which serve to

space scraper blades

19 and 20 from mutator shaft 16. The connection of

scraper blades

19 and 20 with relation to bosses 28 and mutator shaft 16 is shown more clearly in FIG. 2 of the drawing.

Blades

19 and 20 are made of spring steel in such a length and width that they are in continuous contact with the inner surface of inner cylindrical barrel .15. When mutator shaft 16 is rotated,

blades

19 and 20 serve to scrape from the inner surface of inner cylindrical barrel 15 the product which adheres thereto and simultaneously to agitate the melt as it passes through inner passageway 22 to provide a homogeneous cooled product.

The method forming the present invention has found particular utility in the cooling of melts such as sugar solutions from which crystal-containing icings and fondants are prepared. As used herein, the term melt refers to a liquid composition essentially free from solids or crystals. Thus, to form the melt the material must be heated to a temperature greater than its melting points, i.e., the point at which it is essentially free from solids or crystals. The ensuing discussion will be based on the use of the method to cool hot sugar solutions, although it is to be understood that the method herein described can also be used in connection with other hot, crystalforming melts which are to be cooled to provide such products as, for example, soap or detergent bars, peanut butter, plastic shortenings, icings, and the like.

In the application of the method of the present invention to crystal-forming melts, the melt is caused to,flow through inner passageway 22 and the coolant is caused to flow through outer passageway 21, preferably in'a direction opposite that of the product flow in order to permit the greatest heat transfer. Simultaneously, mutator shaft 16 is caused to rotate by means of a source of motive power (not shown), which can, for example, comprise an electric motor and an associated gear reduction system. In the course of flowing through inner passageway 22, the melt is simultaneously cooled, continuously scraped from the surface of inner cylindrical barrel 15 by

scraper blades

19 and 20, and continuously agitated by mutator shaft 16 to insure a uniformly cooled product that is homogeneous in nature. The flow diagram of FIG. 3 shows this mode of operation, which can be characterized as the normal flow mode, with the coolant and the hot melt flowing in opposite directions as indicated by the arrows. Also shown are an inlet valve 29 and an outlet valve 30, each of which is so arranged that the hot melt flows in a direction opposite that of the coolant flow.

In FIG. 4 there is shown the arrangement wherein the reverse flow mode is in effect. In this particular mode the direction of coolant flow remains unchanged, while that of the hot melt has been reversed by rotating both inlet and outlet valves one quarter turn to cause the hot melt to enter the heat exchanger through what was formerly the hot melt outlet and to leave the heat exchanger from what formerly was the hot melt inlet.

Valves

29 and 30 are four-way valves and the details of their construction will be familiar to those skilled in the art.

Since the crystal-forming melt hereinabove referred to is a hot, saturated or supersaturated liquid, when it is cooled crystals tend to form throughout the solution. A number of the crystals adhere to the inner surface of the innermost cylinder wall but are scraped therefrom by

scraper blades

19 and 20 and intermixed with the bulk of the cooling melt. Other crystals adhere to the portions of the

scraper blades

19 and 20, to spacer bosses 28 on mutator shaft 16, and to mutator shaft 16 itself. Since there is no way to remove these crystals from the mutator shaft or any of its associated parts while the hot melt is flowing therethrough, the crystals tend to build up on those surfaces. If left unchecked, the buildup proceeds to the point where the flow passage is considerably restricted, thus impeding flow and adversely affecting the efliciency of the heat transfer process.

For a reason which is not precisely known, the greatest crystal buildup occurs on the portion of mutator shaft 16 closest to the point at which the hot melt is introduced and does not uniformly distribute itself along the entire axial length of the shaft. Therefore, when the direction of flow of the hot melt is reversed, the crystal buildup occurs on the opposite end of mutator shaft 16 than the end on which it originally took place, thereby distributing the crystal buildup somewhat evenly along the axial length of mutator shaft 16 and its associated parts. However, because of the tendency for the buildup to be greatest at the hot melt inlet, the center portion of mutator shaft 16 has a smaller degree of crystal buildup than do each of the ends. It is thought that in the course of the flow reversal process some of the crystals that are formed at the upstream end of the heat exchanger tend, to some degree, to erode some of the crystal buildup that has previously taken place at the opposite end of the mutator shaft. Thus, by periodically reversing the flow direction the usual uneven distribution of the crystal buildup is altered to the extent that it is more uniformly distributed. Furthermore, since as indicated above some of the subsequently formed crystals appear to partially remove some of the previously built-up crystals that have formed On mutator shaft 16, this further tends to make the buildup uniform in distribution. While it would appear that the method of the present invention permits only a doubling of the effective operating time of the heat exchanger by reason of the change in direction of product flow, it has been found in practice that the effective operating time is increased by a factor of at least 3 in comparison with operating the heat exchanger without periodically reversing the flow direction.

Although the method of the present invention is suitable for use with other hot, crystal-forming melts, in its preferred form it is used in connection with the cooling of hot sugar solutions which have a monosaccharide content which ranges from 0% to about 15% by weight, a disaccharide content which ranges from about 50% to about by weight, and the remainder, or from about 10% to about 50% by weight, water. Of course additional ingredients in the form of flavoring agents, colorants, gelling agents, and the like could also be added to the solution in small amounts, if desired. Preferably, the abovedescribed hot sugar solutions are cooled in a scraped wall heat exchanger from an initial temperature of from about 180 F. to about 270 F. through a temperature range of from about 40 F. to about 220 F. to a final temperature of from about 50 F. to about F. to provide a homogeneous cooled product. More preferably, the hot solution is cooled from an initial temperature of from about 200 F. to about 240 F. through a temperature range of from about 90 F. to about F. to a final temperature of from about 80 F. to about 110 F.

The flow reversal should be effected before the flow passage is restricted to the extent that it adversely affects the efliciency of the heat transfer process. Preferably, the flow is allowed to continue in a given direction for from about 5 to about 60 minutes whereupon the flow is reversed so that it flows in the reverse direction for from about 5 to about 60 minutes. Any number of additional flow reversals can be effected until the flow stops because of excessive buildup or until the process is stopped because of poor efficiency.

The operation of the method in conjunction with a hot sugar solution which is being cooled to form a fondant is described in the following examples.

EXAMPLE I A 2-foot x 3-inch diameter scraped-wall heat exchanger manufactured by the Votator Division of Chemetron Corp. was installed in a fondant processing line to cool, a continuously flowing hot sugar solution. The hot sugar solution comprised 77.3% sucrose, 15.5% water, 7.0% dextrose, and 0.2% hydrophilic colloids (carragheen gum and algin) and was at an initial temperature of 220; F. The coolant used was a brine solution having a specific gravity of 1.25 and was at an initial temperature of -14 F. The hot sugar solution was pumped through the heat exchanger in the inner channel at a rate of 300 pounds per hour. The mutator shaft carrying the scraper blades was rotated at a constant speed of 300 rpm. The coolant flow was initially in a direction opposite that of the direction in which the hot sugar solution flowed and was at a rate to provide a cooled sugar solution temperature of 90 F. at the product outlet of the heat exchanger. The system was operated at these conditions for 30 minutes, after which the flow direction of the hot sugar solution was reversed. The rate and direction of coolant fiow remained unchanged. The heat exchanger was operated at this condition for 30 minutes, at which point the fiow direction of the hot sugar solution was again reversed without changing the coolant flow rate or direction.

The above-described flow reversal cycle was repeated at 30 minute intervals until a total operating time of 4 hours had elapsed (8 flow reversals). The initial overall heat transfer coefficient of the system was calculated to be 129.5 B.t.u./hr./ft. F. At the conclusion of the 4 hour period of operation the calculated overall heat transfer coefficient of the system had decreased to a value of 123.5 B.t.u./ft. F., a decrease of 4.6%. The mutator shaft was removed from the heat exchanger and exhibited a considerable but relatively uniform buildup of sugar crystals along its axis.

When the same system and operating conditions described above was repeated without the flow reversals and while maintaining the direction of fiow of the hot sugar solution in a direction opposite that of the coolant, the effective operating time was only 1 hour and 15 minutes because the crystal buildup had proceeded to a point where inefficient operation resulted. Within that period, the calculated overall heat transfer coefficient of the system varied from an initial value of 137.4 B.t.u./hr./ft. F. to a final value of 109.2 B.t.u./hr./ft. F., a decrease of 20.5%.

When the initial temperature of the sugar solution ranges from about 180 F. to about 270 F. and the final 'temperature of the solution ranges from about 50 F. to about 140 F., substantially similar results are obtained in that periodic flow reversals significantly increase the effective operating time of the heat exchanger when cooling such solutions.

When the hot sugar solution has a monosaccharide content which ranges from to about 15% by weight, a disaccharide content which ranges from about 50% to about 90% by weight and the remainder, or from about 10% to about 50% by weight, water, substantially similar results are obtained in that periodic flow reversals significantly increase the effective operating time of the heat exchanger when cooling such solutions.

When the period between successive flow reversals ranges from about to about 60 minutes, substantially similar results are obtained in that the effective operating time of the heat exchanger is significantly increased.

6 EXAMPLE n A storage-stable, non-firming, aerated icing having improved air dispersion stability can be prepared from the following ingredients:

Percent y Ingredients weight Fondant 83. 355

Water 15. 505 Dextrose. 5. 510 Sucrose 78. 900 Hy rophilic Colloid (Carragheen gum and a1- gin). 0. 085

Shortening 14. 550

Basestock (refined, bleached soybean oil partially hydrogenated to an iodine value of about 85 and a solids content index of 11 at 70 F.) 58. 000 Hardstoek (substantially completely hydrogenated soybean oil and substantially completely hydrogenated rapeseed oil in a weight ratio of 2.511 and having an iodine value of 8) 35. 000 Monoand diglycerides of partially hydrogenated soybean oil having an iodine value of about 80 4. 000 Polyoxyethylene sorbitan mouostearate (Tween 60") 3. 000

Minors 2. 095

S 0. 500 50% citric acid solution. 0. 026 Ascorbic acid 0.020 Potassium sorbate- Flavoring, dyes The fondant can be prepared by slowly adding dextrose and sucrose to a dispersion of hydrophilic colloid in water at F. while mixing in a steam-jacketed paddletype mixer. The shortening components can be added to the fondant, the temperature raised to F., and mixing continued for about five minutes until all the shortening solids are dissolved. At this point, the mixture is a slurry of sugar crystals in liquid shortening and sugar. The slurry is passed through a shell and tube heat exchanger to a temperature of about 230 F. and held for three minutes at this temperature while agitating in a picker box mixer until all sugar crystals are dissolved to form a melt.

The melt, at 230 F., is rapidly chilled in a conventional scraped wall heat exchanger (Votator A unit) in which the coolant, coolant temperature, melt flow rates and mutator rotation are the same as described in Example I. The coolant flow rate is adjusted to reduce the melt temperature to about 91 F., at the outlet of the heat exchanger. Subsequently, the melt is further processed to result in any icing of the type desired. Such subsequent operations are well known and play no part in the present invention and are therefore not described herein.

As can be seen from the foregoing, the operation of the heat exchanger in the reverse flow condition is intended to take place for an extended time period, i.e. from about 5 minutes to about 60 minutes, and not instantaneously. Although the prior art teaches sudden reversals of flow in surface condensers to remove deposits of debris and sediment, the nature of those deposits is completely different from the deposits which result when crystals are formed on the heat transfer surface from cooling crystal-forming solutions.

While particular embodiments of the invention have been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention and it is intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.

What is claimed is:

1. A method of cooling a hot crystal-forming melt within a heat exchanger having scraper blades attached to a shaft rotatably supported in the said heat exchanger with attendant crystallization of the melt, said method comprising:

(a) flowing said melt through said heat exchanger in a first direction for a period of time ranging from -60 minutes;

(b) reversing the flow of said melt in a second direction opposite to said first direction for a period of time ranging from 560 minutes;

(c) continuously during the flow of said melt in the said first direction and said second direction forming on the interior surfaces of the said heat exchanger a crystal buildup due to crystallization of the said melt; and

(d) scraping the Walls of said heat exchanger with said scraper blades being in continuous scraping contact with the said Walls and simultaneously agitating the melt while it is being cooled therein.

2. The method of claim 1 wherein said melt is a sugar solution.

3. The method of claim 2 wherein said sugar solution comprises, on a weight basis, a monosaccharide content of from 0% to about 15%, a disaccharide content of from about to about and a water content of from about 10% to about 50%.

4. The method of claim 3 wherein said melt is cooled from an inlet temperature which ranges from about 180 F. to about 270 F. to an outlet temperature which ranges from about 5 0 F. to about F.

5. The method of claim 4 including the additional step of again reversing the flow of said melt to flow in said first direction.

References Cited UNITED STATES PATENTS 2/1951 Feldstein et al. 1'6 592 10/1968 Leach -94 U.S. Cl. X.R.