US3824778A

US3824778A - Heating procedure in a false twist texturizing process

Info

Publication number: US3824778A
Application number: US00291585A
Authority: US
Inventors: H Flanders
Original assignee: Burlington Industries Inc
Current assignee: Burlington Industries Inc
Priority date: 1972-09-25
Filing date: 1972-09-25
Publication date: 1974-07-23
Anticipated expiration: 1991-07-23

Abstract

An improved heating procedure in a yarn texturizing process by heat setting a false twist therein which comprises confining a sonic flow of superheated steam in closely adjacent surrounding relation along a predetermined longitudinal extent of the yarn continuously moving between the upstream position of twist restraint and the position at which the false twist is imparted to the yarn. The procedure makes possible the texturizing of yarn at production rates of 275 ypm utilizing a steam source at a temperature of 470*F and pressure of 55 psig with a sonic flow confining surface of approximately 6 inches in length and 0.083 inches in diameter.

Description

United States Patent [191 Flanders, Jr.

[ July 23, 1974 1 HEATING PROCEDURE IN A FALSE TWIST TEXTURIZING PROCESS [75] Inventor: Harry E. Flanders, Jr., Greensboro,

[73] Assignee: Burlington Industries, Inc.,

Greensboro, N.C.

22 Filed: Sept. 25, 1972 211 App]. No: 291,585

[52] US. Cl. 57/157 TS, 28/1.4, 28/62, 28/72.12, 57/157 F [51] Int. Cl 002g 1/02 [58] Field of Search 57/34 HS, 140 R, 157 F, 57/157 TS, 157 R; 28/62, 1.4, 72.12, 72.17

[56] References Cited UNITED STATES PATENTS 3,069,836 12/1962 Dahlstrom et a1 57/157 F 3,303,169 2/1967 Pitzl 57/157 F 3,382,658 5/1968 McIntosh et al.... 57/34 l-IS 3,423,809 l/1969 Schmidt 28/72.17 3,447,210 6/1969 Barlow 28/62 X 3,457,610 7/1969 Williams et a1. 28/62 X 3,529,413 9/1970 Marrinan et a1. 57/140 R 3,608,299 9/1971 Dugas 57/157 F 3,611,698 10/1971 Horn 57/140 R 3,640,063 2/1972 Schmid et a1. 57/140 R 3,720,079 3/1973 Katsumata et a1. 28/62 X Primary Examiner--John Petrakes Attorney, Agent, or Firm-Cushman, Darby & Cushman [57 ABSTRACT An improved heating procedure in a yarn texturizing process by heat setting a false twist therein which comprises confining a sonic flow of superheated steam in closely adjacent surrounding relation along a predetermined longitudinal extent of the yarn continuously moving between the upstream position of twist restraint and the position at which the false twist is imparted to the yarn. The procedure makes possible the texturizing of yarn at production rates of 275 ypm utilizing a steam source at a temperature of 470F and pressure of 55 psig with a sonic flow confining surface of approximately 6 inches in length and 0.083 inches in diameter.

10 Claims, 2 Drawing Figures 1. HEATING PROCEDURE IN A FALSE TWIST TEXTURIZING PROCESS This invention relates to texturizing thermoplastic yarn and more particularly to improved heating procedures for setting a false twist in thermoplastic yarns.

The practice of texturizing synthetic yarn by heat setting a false twist therein, which has achieved such widespread use today in connection with polyester yarns, has been known since at least the 1930s. The US. Patent to Finlayson et al., 2,1 11,211, dated Mar. 15, 1938, discloses an apparatus and procedure for texturizing or crimping synthetic yarn by heat setting a false twist therein, which embodies the basic apparatus components and procedural steps which are still in use today. The procedure, as disclosed in the Finlayson et al., patent, involves the feeding of a supply of thermoplastic yarn continuously into and out of a texturizing or crimping zone within which the yarn is moved past a twisting device. The twisting device serves to build up and maintain in the yarn passing through the texturizing zone a twist in one direction which extends upstream from the twisting device to a first position of restraint. The twist in the yarn passing between the first position of restraint and the twisting device is heat set therein by elevating and then reducing the temperature of the yarn before it reaches the twisting device. The F inlayson et al., patent discloses the utilization of steam, hot air or heated solvent vapors as the setting medium. Insofar as the twist setting structure is concerned, Finlayson et al., disclose the provision of a main steam pipe to which wet steam is supplied, the pipe having a nipple through which the yarn enters the pipe and an offset member provided with an outlet nipple through which the yarn passes out of the steam pipe.

The US. Patent to Kunzle, 2,790,298, dated Apr. 30, 1957, discloses an improved steam heating structure for use in heat setting false twisted thermoplastic yarns which embodies a length of tube of approximately 39 inches having labyrinth seals of almost 3 inches in length at each end thereof, the arrangement being operable to withstand and maintain steam pressures therein of approximately I atmosphere and up to 5 atmospheres while yarn is passed therethrough. I

The machines in use today generally rely upon electricity as a heat source, utilizing either air or a yarn contacting structure as the yarn heating medium.'These present day heating procedures are the equivalent of those provided in the Finlayson patent disclosure in the sense that the heating medium employed to impart the desired heat to the yarn is generally maintained in a stationary condition within the system and the yarn is passed thereby. With procedures of this type, the attaining of the proper yarn temperature in the process is dependent upon the temperature of the heating medium and the time period in which the heating medium is in heat exchange relation to the yarn. The heating time is, in turn, dependent upon the linear speed of the yarn through the processing equipment or the production rate. Experience has shown that in order to obtain increased production rates, it has been necessary to extend the length of the heating equipment in order to accurately control the yarn temperature. In other words, increased production rates could not be accommodated simply by increasing the temperature of the heating medium. Consequently, the electrical heaters used in most machines today have attained lengths of the order of the 39 inch length mentioned above in connection with the Kunzle patent. Contrary to the horizontally inclined disposition of the steam tubes disclosed in Kunzle, the electrical heaters are disposed vertically and, in many installations, represent a practical limit to the height of the overall machine.

An object of the presentinvention is to provide an improved heating procedure in a false twist yarn texturizing process which permits increased production rates without increasing apparatus size.

In accordance with the principles of the present invention, this objective is obtained by subjecting the twisted yarn to a sonic flow of superheated steam confined in closely surrounding relation to the yarn. This procedure not only obviates the above-noted limitations of existing electrical heaters, but differs materially from the steam heating procedures heretofore proposed by Finlayson et al. and Kunzle. The present procedure is similar to the Finlayson et al. and Kunzle and the existing non-contact electrical heaters in that the yarn is heated primarily by convection of a surrounding fluid heating medium. This basic mode of heat transfer is inherently more efficient than the conduction heating carried out by contact heaters because it is more uniform. The procedure of the present invention presents the convection heating medium to the yarn in the form of a continuously changing relatively small body of fluid heating medium flowing with a sonic velocity. In terms of effiCiency,-the convective heat transfer coefficient of turbulent high speed steam against the entire surface of the yarn is greater than that of conduction through the interface of the yarn and contact heater surfaces or that provided by the non-turbulent convection of either Finlayson et al. or the existing non-contact electrical heaters, or Kunzle. Thus, the present procedure has the advantage of more efficiently delivering more heat in a shorter period of time than prior art procedures.

Present machines which have the capability of running at production rates of ypm require heaters of nearly 40 inches in height. With the present heating procedures, the same production rate can be achieved with a sonic fiow confinement length of 3 7% inches and rates as high as 275 ypm can be obtained with only a 6 inch length of sonic flow confinement.

It is further an object of the present invention to provide an improved heating procedure in a process of heat setting a false twist in thermoplastic yarn which can be effectively carried out with the use of simple low profile equipment, which equipment is economical to manufacture, operate and maintain.

These and other objects of the present invention will become more apparent during the course of the following detailed description and appended claims.

The invention can best be understood with reference to the accompanying drawings wherein an illustrative device for carrying out the invention is shown.

In the drawings:

FIG. 1 is a yarn flow diagram illustrating the procedural steps of the false twist process to which the present improvement relates and, schematically, the apparatus used in carrying out the process; and

FIG. 2 is a vertical sectional view of the heater structure used in carrying out the improved heating procedure of the present invention.

Referring now more particularly to the drawings, there is shown in FIG. 1 thereof a yarn flow diagram including a schematic showing of the apparatus for carrying out the process of the present invention. As shown, a supply of thermoplastic yarn in the form of a yarn package is provided from which yarn 12 is drawn, as by a first or upstream set of nip rolls 14 or the like. The nip rolls l4 serve to continuously feed the yarn 12 into a texturizing or crimping zone, indicated at 16, from which the yarn is continuously drawn by a second or downstream set of nip rolls 18 or the like. The yarn 12 passing through the texturizing zone 16 moves past a false twisting device, schematically indicated at 20, at a position intermediate to the ends thereof so as to impart and maintain a twist in one direction, which extends upstream to the upstream nip rolls 14. The twist V turizing thermoplastic yarn by heat setting a false twist therein is conventional.

The present invention is more particularly concerned with improvements in the procedure for elevating the temperature of the twisted yarn and the cooperation of this procedure with the other steps of the conventional process as outlined above. It will be understood that the term texturizing as used herein is inclusive of the known process of one-zone draw-texturizing, wherein the yarn is simultaneously drawn and false-twisted, as well as the known two-zone draw-texturizing process. It will be further understood that after the yarn 12 passes the downstream nip rolls 18, it may be subjected to further conventional procedures, the diagram of FIG. 1 illustrating simply the subsequent forming of a package 22 of the texturized yarn as by a take-up roll 24. a

In accordance withthe principles of the present invention, the heating of the yarn in the texturizing process is accomplished by confininga sonic flow of superheated steam in closely adjacent surrounding relation along a predetermined length of the yarn l2 continuously moving between the upstream nip rolls 14 and the false twisting device 20. FIG. 2 illustrates a device, generally indicated at 26, for carrying out the heating procedure. As shown, the device 26 includes a plenum chamber 28 having a superheated steam inlet pipe 30 communicating therewith. It will be understood that the inlet pipe 30 communicates with a source of superheated steam (not shown) which may be of any conventional construction capable of delivering a continuous supply of superheated steam at an accurately controlled temperature and pressure.

Disposed in communication with the plenum chamber 28 is a sonic flow confining member 32. Formed in the end of the member 32 which communicates with the plenum chamber is a converging nozzle 34 formed the axis of which is aligned with the axis of the cylindrical surface forming the throat 36.

When the device 26 is used to carry out the improved heating procedure of the present invention in the texturizing process previously described, the device 26 is positioned within the texturizing zone 16 between the upstream nip rolls l4 and the false twisting device so that a predetermined longitudinal extent of the yarn continuously moving therein will pass through the longitudinally aligned throat 36 and passage 40 provided by the device. The passage 40 is smaller than the throat 36 so as to establish a continuous flow of superheated steam from the plenum chamber 28 through the converging nozzle 34 and outwardly to the atmosphere through the throat 36.

In accordance with the principles of the present invention, the temperature and pressure of the source of superheated steam applied to the plenum chamber 28 are so related to the size of the throat 36 as to establish a sonic flow of superheated steam through the throat 36. The term sonic-flow as used herein relates to a fluid flow velocity equal in magnitude to the speed of sound waves within the flowing medium. These conditions are also interrelated to the longitudinal forces imposed on the yarn as it passes through the various stages within the texturizing zone 16, the heating stage. The longitudinal forces transmitted to the yarn by the present heating procedure differ from those transmitted by known heating procedures in that in the present procedure the heating medium is in sonic motion rather than being stationary. Thus, with the present invention the forces applied to the yarn by the heating procedure are susceptible to being reversed in direction by reversing the orientation of the device 26 with respect to the direction of yarn movement. On the one hand, where the device 26 is oriented so that the sonic flow of superheated steam within the throat 36 is in the same direction as the direction of yarn movement, the action of the sonic flow is to tend to pull the yarn from the upstream nip rolls l4 and to push the yarn toward the downstream nip rolls 18. This action tends to increase the tension of the yarn between the device 26 and the upstream nip rolls and to decrease the tension of the yarn between the device and the downstream nip rolls. On the other hand, where the-device 26 is oriented so that the sonic flow of super-heated steam within the throat 36 is in'a direction opposed to the direction of yarn movement, the action of the sonic flow is to tend to push the yarn toward the upstream nip rolls 14 and to pull the yarn from the downstream nip rolls 18. This action tends to decrease the tension of the yarn between the upstream nip rolls l4 and the device 26 and to increase the tension of the yarn between the device 26 and the downstream nip rolls 18. In either event, the action of the heating medium in sonic movement with respect to the yarn establishes a turbulent environment not presented by other prior art procedures necessitating the maintenance of the yarn in afairly taut condition as it passes through the heating stage in order to secure good crimp stability and prevent undue breakage. For this reason, particularly when texturizing commercially available fully drawn yarns such as polyester which exhibit low shrinkage tension during annealing, it is usually desirable to operate with low to even slightly negative overfeeds, the preferred range being somewhat variable depending on the yarn speed, in the general range of plus 4 to minus 3 percent overfeed. In general, higher yarn speeds require lower overfeeds because of the increased breaking tendencies of fast-running slack yarns. Higher overfeeds are also believed possible with yarns which shrink more than polyester during 'texturizmg.

The present process improvements would have applicability with false twist processes as combined with drawing processes include both sequential or two-zonel draw texturizing and concurrent or one-zone draw texturizing. In the case of one-zone draw texturizing of undrawn feed yarn, the feed ratio of the upstream and down-stream nip rolls may be varied taking into account the disclosure with respect to feed ratios contained in British Pat. No. 1,263,055. Where draw-spun partially oriented feed yarns are utilized in accordance with the disclosure contained in the commonly. assigned U.S. Patent application in the name of Ronald J. Small, Ser. No. 349,930, filed Apr. 11, 1973, the feed ratio utilized is varied taking into account the feed ratio formula disclosed therein.

The direction of sonic flow with respect to the direction of yarn travel has an effect on the tenacity and crimp development of the processed yarn due to the action noted above. Steam flow in an upstream direction with respect to the yarn travel as shown inFIG. 2 is preferable because of the somewhat better crimp development. However, where greater tenacity with less crimp development is desired in the finished yarn, a downstream flow can be utilized.

With the above in mind, the variables in the heating procedure of the present invention can best be understood by first considering the heat transfer characteristics of the heating medium to the yarn and then the heat conduction within the yarn.

In analyzing the heat transfer it is assume 1. that the steam entering the nozzle from the plenum chamber is at zero velocity,

2. that the steam expands ideally (no losses) through the converging nozzle to sonic velocity and travels through the throat with no friction losses,

3. that the steam system is operating at steady flow equilibrium adiabatically, that is, the heat loss through the insulated jet body is negligible compared with the total energy flow,

4. that the turbulent flow of steam around the yarn surface is developed at the converging section of the nozzle andremains constant along the nozzle throat, and

5. that the heat transfer due to radiation is negligible.

For a steady state, adiabatic process the entropy remains constant; thus S S,. An ideal fluid expanding through a converging nozzle has an increase in velocity with a decrease of the exhaust pressure until sonic velocity at the throat is reached. The pressure ratio at a sonic velocity is called the critical pressure ratio and is given by,

P2/P1 /k 1) The heat transfer, q, to an adiabatic process is zero, and this leaves the change of internal energy Ah, and change of kinetic energy, AKE. Rewriting the equation, Ah AKE 0. With these three equations and the initial steam conditions, the velocity, temperature, and density of the steam in the jet nozzle can be calculated. The surface convection heat transfer coefficient of superheated steam, without condensation, flowing inside pipes is given by McAdams, Heat Transmission, page 172 from experimental data. An equation in terms of dimensionless ratios was fitted to the data and is N 0.021 (N where the Biot Modulus is, N hD/K, and the Reynolds number is, N V p D/u. Using the state of the steam in the jet nozzle, the Reynolds number can be calculated. Thus, the value of the Biot modulus is known, and the surface convection heat transfer coefficient, h, can be calculated.

In analyzing the heat conduction in the yarn, it is assumed 1. that the yarn is twisted to such an extent that it is treated as a monofilament for internal heat conduction,

tofth e jet leng t h divided by the yarn. speed,

3. that the heat conduction equation is solved for the condition of a long cylinder subjected to a sudden change in environmental temperature,

4. that the initial temperature distribution in the cylinder (yarn) is uniform and equal to T 5. that at the time 6 0 the yarn is exposed to steam whose temperature is T and 6. that the convection heat transfer coefficient, h, between the surface of the body and the fluid is uniform and does not change with time.

The yarn is idealized as a long cylinder, and the heat transfer in a long cylinder is governed by the basic differential heat equation where a is the thermal diffusivity} The initial and boundary conditions for the solution are:

1. that at time 0 0 the cylinder temperature is uniform atT=T 2. that at time 0 0 the cylinder is exposed to a fluid at temperature T and 3. that the unit surface conductance, h, between the surface of the body and fluid is uniform and does not change with time. Thus, the heat flow at the surface is equal to q Ah (T, T The solution to the differential equation evaluated at these boundary conditions is given in dimensionless temperature ratios (Tr/r T)/(T,, T as a function of the Fourier modulus, (all/r and the reciprocal of the Biot modulus, (klhr The solution is given in Principles of Heat Transfer by Kreith, pp. 141-142, FIG. 4-10.

From the above, it can be seen that with an increase of only the steam pressure, holding the superheated temperature constant, the temperature and velocity inside the throat remain almost constant. The only change is the increase of steam density. Thisincrease in density produces an increase of Reynolds number resulting proportionally in a larger convection heat transfer coefficient. When this larger coefflcient iscarried through the temperature calculation.'thefinal'result is-a decrease in the temperature difference .of the steam and yarn. Thus, it is seen that the steam pressure governs the heating rate while the input steam temperature controls the heater temperature. With this analysis it is possible to determine if the steam jet heater is heating in the transient zone where yarn temperature will fluctuate or in the stable or near steady state zone where the yarn temperature is uniform.

Within the parameters of steam temperatures and pressures necessary to obtain a yarn temperature required to heat set the false twist within the particular yarn being processed, it is further to be noted that as the steam temperature is increased, crimp development of the processed yarn increases but the tenacity thereof decreases. The exact temperature and pressure conditions of the steam utilized are therefore dependent upon the balance of these two characteristics desired in the end product.

The process of this invention may be applied in conjunction with any of several standard means of introducing false twist. Spindle twisting is used herein only as an example. As noted before, the overfeed (or underfeed) which may be used varies with both the nature of the feed yarn and the type and speed of texturizing. A low-shrinkage yarn such as commercial, drawn, 150- denier polyester may be treated at overfeeds ashigh as about 4 percent at conventional 160 ypm production rates. At higher yarn speeds, breakage resulting apparently from increased ballooning at the twister begins to be observed. In generaLfor commercial drawn polyes-- ter, in speed ranges of 160 to 275 ypm, underfeeds of 0.5 to 1.5 percent are preferred, with about 1 percent being optimum. Necessary adjustments for drawtexturizing are discussed hereinbefore.

Another factor in establishing the optimum overfeed, essentially apart from the shrinkage factor, is the need to assure good crimp and crimp stabilization. Too taut a yarn cannot twist to its maximum potential and thus falls short of maximum crimp. Too loose a yarn twists without adequate frictional control at the twister, and this lack of friction permits the twist to slip past at intervals before the crimp becomes heat-set. The balancing and control of these and other operational features is well-known in the art of false-twist texturizing.

Although current interest in false-twist texturizing lies primarily in low-shrinkage polyester, the process of the invention is believed equally applicable to other fibers, such as nylon. Operating parameters for higher shrinkage drawn fibers will obviously need adjustment from those for commercial polyester, particularly in' terms of permitting or even requiring higher ranges of overfeed, ranging as high as about 8 percent. Other factors such as yarn diameter, thermal conductivity, and specific heat will also affect the temperature and dwell time in the heat-setting zone. One skilled in the art, given these principles and the following specific details for polyester, will readily be able to apply the invention to other polymer types, yarn sizes, and false-twist tex-' turizing systems.

An illustrative example of texturizing multifilament Dacron polyester yarn (regular, drawn, ISO-denier Type 56) utilizes a steam source at a temperture of 470F and a pressure of 55 psig. At conventional production rates of 160 ypm with a 1 percent underfeed as previously noted, the heating procedure of the present invention can be carried out utilizing a flow confining member 32 having a throat 36 approximately 3.50

inches long and 0.083 inches in diameter (member 40 being 0.021 inches in diameter). By following the analysis previously set forth, this steam pressure and temperature condition produces a temperature of 346.6F and pressure of 23.3 psig with sonic velocity of 1,682 ft/sec. inside the throat 36. These throat conditions produce a heating effect which results in a calculated yarn surface temperature of 324F and yarn center temperature of 297F. The temperature differential between the heating medium and yarn is 22.6F (346.6 324F) which is within the stable heating range.

A production rate of 275 ypm can be obtained utilizing the same steam supply conditions (470F'and 55 psig) by extending the length of the flow confining member 32 to obtain the same dwell time of 0.036 seconds. The amount of length extension required is approximately 2.5 inches making the member 32 a total of 6 inches in length. The yarn produced at the 275 ypm rate with the conditions indicated above had approximately 3.73 gm/denier tenacity,y 18,2 percent elongation, 31.8 percent crimp development and 5.2 percent shrinkage. The false twist device utilized was a conventional Fagg spindlette capable of over 600,000 rpm run at 614,000 rpm at the 275 ypm production rate. Still greater texturizing speeds may be made possible by employment of higher temperatures and longer jets.

It will be understood that while the improvements of the present invention have been disclosed in connection with the production of texturized yarns having a uniform crimp throughout the longitudinal extent thereof, the underlying principles of the present'invention and the improved heating procedures thereof have applicability to the production of false twist texturized yarns of the novelty configurations disclosed and claimed in the commonly-assigned US. Pat. application in the name of Joe F. London, Jr., Ser. No. 291,586, filed concurrently herewith.-

, It thus will be seen that the objects of this invention have been fully and effectively accomplished. It will be realized, however, that the foregoing preferred specific embodiment has been shown and described for the purpose of illustrating the functional and structural principles of this invention and is subject to change without departure from such principles. Therefore, this invention includes all modifications encompassed within the spirit and scope of the following claims.

I claim:

1. In a process of texturizing thermoplastic yarn I which includes the steps of continuously feeding yarn into and out of a texturizing zone, maintaining a false twist in yarn within said texturizing zone by continuously imparting a twist to the yarn at a twisting position spaced downstreamfrom an upstream position of restraint, and raising and then lowering the temperature of the yarn during the continuous movement thereof between said upstream position of restraint and said twisting position to heat set the twist therein, the improvement which comprises effecting the raising of the temperature of the yarn by confining a sonic flow of superheated steam in closely adjacent surrounding relation along a predetermined longitudinal extent of the yarn continuously moving between said upstream position of restraint and said twisting position.

2. The improvement as defined in claim 1 wherein the sonic flow of superheated steam is in a longitudinal direction opposed to the longitudinal direction of movement of the predetermined longitudinal extent of yarn contacted thereby.

3. The improvement as defined in claim 1 wherein the confinement of said sonic flow of super-heated steam is along a cylindrical surface throughout said predetermined longitudinal extent.

4. The improvement as defined in claim 1 wherein said sonic flow is established by communicating a source of superheated steam along a frustoconical surface converging with one end of the cylindrical surface of confinement.

5. The improvement as defined in claim 1 wherein the sonic flow of superheated steam is in a longitudinal direction the same as the longitudinal direction of movement of the predetermined longitudinal extent of yarn contacted thereby.

6. The improvement as defined in claim 1 wherein the yarn is made of polyester and the feeding of the yarn into and out of said texturizing zone is accomplished with an underfeed of between 0.5 percent and 1.5 percent.

7. The improvement as defined in claim 6 wherein the source of superheated steam is maintained at a temperature of the order of 470F and at a pressure of the order of 55 psig.

8. The improvement as defined in claim 7 wherein the feeding of said yarn into and out of said texturizing zone is at a rate of approximately 275 ypm.

9. The improvement as defined in claim 8 wherein said predetermined longitudinal extent is of the order of 6 inches.

10. The improvement as defined in claim 9 wherein the sonic flow of superheated steam is of the order of 1,682 ft/sec.

Claims

1. In a process of texturizing thermoplastic yarn which includes the steps of continuously feeding yarn into and out of a texturizing zone, maintaining a false twist in yarn within said texturizing zone by continuously imparting a twist to the yarn at a twisting position spaced downstream from an upstream position of restraint, and raising and then lowering the temperature of the yarn during the continuous movement thereof between said upstream position of restraint and said twisting position to heat set the twist therein, the improvement which comprises effecting the raising of the temperature of the yarn by confining a sonic flow of superheated steam in closely adjacent surrounding relation along a predetermined longitudinal extent of the yarn continuously moving between said upstream position of restraint and said twisting position.

7. The improvement as defined in claim 6 wherein the source of superheated steam is maintained at a temperature of the order of 470*F and at a pressure of the order of 55 psig.