US2192168A - Paper drying system and apparatus therefor - Google Patents

Description

PAPER DRYING SYSTEM AND APPARATUS THEREFOR Filed Sept. 17. 1937 Inventor John W.Bell

His Attor ey Patented Mar. 5, 1940 PAPER DRYING SYSTEM AND APPARATUS THEREFOR John W. Bell, St. Catharines, Ontario, Canada Application September 17, 1937, Serial No. 164.417

3 In' Canada October 6, 1936 3 Claims.

My invention relates to the drying of webs of paper or the like and more particularly to 'apparatus and a system whereby economy of heat required for the drying is attained.

In the operation of continuous paper mills the sheet of paper formed in and issuing from the wet end of the machine contains about 60 percent moisture. This moisture is removed by passing it over a series of rolls or drums heated by steam above atmospheric pressure whereby the moisture is converted into steam. 'I'his steam rises from the paper and mixes with the atmosphere surrounding the paper machine. It is customary to erect hoods over the machine and install fans to withdraw this mixture of air and steamfrom the machine room. The mixture of air and steam is usually expelled to the outside with-a complete loss of the heat units contained in the steam evaporated from the paper. For each pound of dry paper obtained there is evaporated from the paper about 2 pounds of water and to evaporate this water requires from 1 to 2 pounds of steam delivered to the interior of the drying rolls. The-expenditure of this drying, steam represents a very material item in the cost of paper production and many schemes have been suggested to make use of the heat units contained in the steam arising from the paper and eliminate this loss. However, regardless of the fact that the paper industry has been actively seeking meansto overcome this loss, no system as far as I am aware has found practical acceptance. Systems .have been devised and are in practical use for the withdrawal of the mixture of air and steam from the machine room primarily for the purpose of improvement of working conditions therein, but with no material recovery of the heatunits.

My invention is directed to a means whereby a large proportion of the heat energy present .in the steam coming off the paper web in the drier is recovered and made available for use in the plant. In accordance with my invention I collect the steam liberated from the paper in such a manner that it is free from air and is therefore in a condition whereby the latent heat units contained therein may be utilized in the same manner as any other source of steam.

Myinvention further contemplates a system whereby. the steam driven off from, the paper by paper web enters and leaves the chamber and the whereby steam liberated from the paper collects Y in the chamber. I withdraw from the chamber the collected steam at a lesser rate than it is evolved, whereby the steam evolved in excess of the steam withdrawn escapes through the open- 5 lugs and prevents the ingress of air therethrough, I Preferably the openings are in the form of slots near the-bottom of the casing through which the escaping steam prevents or removes air which 9 tends to be drawn into the chamber by the entering paper web. I withdraw the steam from the chamber at a point remote from the escape openings, compress the withdrawn steam, and deliver the compressed steam as heating medium for the rolls, the heat losses in the system being made up by steam supplied from a boiler supply. The drawing is a diagrammatic representation ofm'yinvention. Reference characters Ill represent a series of drying rolls of the type customarily used in paper machine driers. Each roll consists of a hollow drum mounted for rotation about its axis and connected to driving means. Each roll is provided with means whereby heating steam may be introduced to its interior and condensate removed therefrom. Steam at a pressure above atmospheric is used in the rolls whereby the temperature of the rolls is main- V tained sumciently above 212 F. so that the water in the paper may be rapidly evaporated at atmospheric pressure. To keep the drier within practical limits of size, considering the high speed at which paperforming machines operate, it is necessary to raise the temperature of the web rapidly to the boiling point of water and keep it there throughout its travel by supplying a sufficient quantity of heat to make up the latent heat of evaporation. In a practical example, a paper mill operating at a speed of 1,000 feet per minute\ utilizes rolls of 60 inches diameter offering about 40 320 linear feet of contact with the paper. These rolls are heated by steam at about '8 pounds pressure, having a corresponding condensing temperature of 235 degrees Fahrenheit. This s temperature of 235 degrees, maintained .in the interior'of the roll, is sufiicient to overcome the temperature drop' of the roll and keep the wet web 'at the boiling point and supply the latent heat necessary for evaporation. Customarily the heating steam is introduced axially at one end of the roll and condensate removed axially at the other end. Reference character ll represents a web of wet paper. delivered from the wet end of the paper making machine (not shown).- a

The web passes about the series of rolls as in- 5 of the Chamber l4.

dicated and leaves the drying rolls at l2 dried to the desired extent. I

I provide heat insulated walls l3 completely enclosing the drying rolls and forming a. chamber It the bottom of which is shown constituted by the floor on which the drier rests. Narrow slots l5 and "5 are provided in the end walls close to the bottom through which the paper web enters and leaves the chamber respectively. I! represents a source of steam supply such as a steam boiler. A pipe I8 leads from the boiler and from the pipe l8 branch pipes l9 lead to each of the rolls. A compressor has its delivery side connected to the pipe l8. The intake of the compressor is connected to a pipe2l which leads from the pipes 22 communicating with the top A pipe 23 delivers steam from the boiler supply I 1 to the pipe 2| leading to the compressor as indicated at 24. Valves 25 and 26 in the pipe l8 control the steam supply to rolls from the boiler and the compressor respectively. A control valve 21 is provided in the compressor intake and a regulating valve 28 is provided in the pipe 23.

In preparation for operation the drier in accordance with my invention is brought up to operating temperature by admitting steam from the boiler supply. This may be done by opening valves 25'and 26 and closing valves 21 and 28. The entering'steam will raise the temperature of the rolls and the compressor and the temperature of the air within the chamber l4. Valve 28 is then opened which admits steam from the boiler to the top of the casing through pipe 22.

The entering steam forces out the air from within the chamber through the slots near the bottom thereof and eventually raises the temperature of the chamber to operating temperature and fills the chamber with steam. 7

To operate the rolls are started and the paper web passes through the chamber in heat transferring relation to the rolls. The web is raised to the boiling point of water quickly and as it continues over the rolls the contained water is converted into steam at 212 F. which escapes into the chamber Id. The compressor is started and valve 25 is closed and valve 21 is opened. Valve 28 is adjusted to admit a limited amount of steam from the boiler. Steam is drawn from the chamber I4 by the compressor which com presses it to the required amount to give thg steam temperature required in the rolls, and delivers it to the rolls through the pipes l8 and I9. It is essential that the steam in the chamber drawn off, by the compressor be free from air. otherwise the contained air would accumulate in the condensing rolls l0 and stall the system. The steam is collected free from air by preventing the entrance of air through the slots ,l5 and I6 by allowing excess steam to escape therethrough.

The valve 28 in the pipe leading from the boiler supply is adjusted to admit sufflcient steam so that the compressor draws somewhat less steam from the chamber than is liberated from the drying paper. The excess of steam liberated by the paper escapes through the slots I5 and I6 thus tending to prevent the admission of air at this point. Any small amount of air dragged into the chamber by the incoming paper web is immediately mixed with steam contained in the chamber at the point. of entrance. A mixture oi!v steam and air at a given temperature and pressure is heavier than pure steam at the same temperature and pressure, and therefore the air will remain momentarily at the point where it enters and will be entrained by the outgoing stream of steam and expelled through the slots.

It is particularly to be noted that the steam drawn from the chamber by the compressor is drawn from the top of the chamber at a point remote from the slots, the only point where air can enter the chamber. A plurality of steam outlets 22 are provided to assure that the steam equipment inside the hood at a temperature somewhat higher than 212 F. the steam will pick up a slight degree of superheat and condensation within the chamber is minimized. Any condensate incidentally formed will collect in the sump at the bottom of the drier and may be drained oil in any suitable manner.

The heat losses in the system described are made up by (1) Radiation from the housingand accessories.

. (2) Heat carried away in the dried paper leaving the drier.

(3) Heat in the condensate leaving the drying rolls.

(4) Heat in the steam escaping from the slots 15 and l6.

These losses are made up by the steam conthe valve 28 will be set to deliver sufflcient make up steam so that a sumcient amount of the steam liberated from the paper will escape through the openings l5 and Hi ,to prevent the ingress of air through these openings and to take care of minor variations in the rate of steam liberated from the paper, due for example to variations in the quantity of water carried into the drier by the web in a given time. A Hoods 29 and 30 are provided over the slots I and I6 respectively. Conduits 3| lead from the hoods whereby the steam escaping from the slots l5 and I6 are conducted from the drier room.

The following example will illustrate the economy of my system. The heat required, exclusive of radiation and incidental losses, to bring about the required evaporation during the production 01' 2000 lb. .of acceptable finished paper is made up as follows:

p Y B. t. u. (I) Heat required to raise 4000 lb. water "from room temperature (65. F.) to

Item (II) constitutes the latent heat of evaporation and represents about 83% of the total heat required. In accordance with my system I return say 98% or more of the steam as heating medium for the rolls in which this latent heat is recovered. Consequently I save about 98% of 83%, or 81% of the total heat necessary to meet the three requirements listed above. The figure of 98% given above for the amount of steam is merely illustrativefor purposes of the example representing the..case'where about 2% of evolved steam is permitted to escape through the slots l5 and I6 for the purpose of excluding air. The amount of steam allowed to escape should of course be as small as possible and will be regulated in practice to as small an amount as is possible to accomplish the results aimed at, i. e. the exclusion of air from the chamber and will vary with thecloseness of control exercised and operating conditions.

' In addition to the above another source of loss is present in systems in which the steam is drawn from the drier room together with air. The scavenging air comes in contact with the heated rolls and other heated parts of the drier and picks up heat therefrom which is lost to the system. This source of loss is eliminated in my system as air is excluded from the chamber surrounding the rolls.

.The loss of heat in condensate from the interior of the rolls has not been referred to as it is of the same order of magnitude in present systems and in my system. It is moreover in a form which readily leads itself to recovery in heat exchange apparatus.

It is evident from the foregoing that I have provided paper drying apparatus whereby a large proportion of the steam given off from the drying paper may be collected free from airand is available for use as a heating medium. It will also be seen that I have invented a system for the drying of continuous web material such as paper wherein a large proportion of the heat necessary to remove the moisture from the paper is reclaimed and used for drying the web.- An installation embodying my invention also makes for satisfactory conditions in the drier room as steam is not liberated into the room. As the steam is collected free from air, the loss of heat units in scavenging air present in customary systems is. absent.

In describing my invention I have diagrammatically shown the principle of operation. It will be apparent to those skilled in the art that ports closed during operation may be provided in the walls of the housing whereby the paper may be cleared in the event of a break in the web during operation.

While I have shown in illustrating my invention a particular manner in' which the compressed steam is returned to the drier rolls together with make-up" steam, it is apparent that modifications of the particular way in which ,the compressed evolved steam and the make up steam already under pressure may be returhed to the rolls will occur to those skilled in the art. Other modifications within the spirit of my invention will readily occur to those skilled in the art and it is understood that I do not desire to be limited to specific details and steps shown but claim as my invention all modifications.

which come within the scope of the appended claims.

What I claim as new and desire 'to secure by Letters Patent of the United States, is:

l. The method of drying a web of paper or the like, which comprises passing the web about steam together with additional 'steam from an-' other source to the rolls as heating medium therefor.

2. The method of drying a web of paper or the like, which comprises passing the web about rolls heated by steam at a pressure above atmospheric and enclosed in a chamber provided with slots located near the bottom of the chamber and through which the web enters and leaves the chamber whereby the steam evolved from the drying web fills the chamber, withdrawing the evolved steam from the top of said chamber at a rate less than it is evolved from said paper and delivered to said chamber, whereby part of the steam escapes through the slots and prevents the entrance of air to the chamber, compressing said withdrawn steam anddeiivering the compressed steam together with additional steam from another source to the rolls as heating medium therefor.

3. The method of drying a web of paper or the like, which comprises passing the web aboutsteam heated condensing rolls enclosed in a chamber provided with restricted slots through which the paper passes to and leaves the chamber, supplying steam to said rolls at a pressureabove atmosphericpressure by a compressor, admitting steam to the intake of said compressor from said chamber and from another sourceoi' steam, regulating the amount of steam admitted from said other source to an amount whereby the compressor draws steam from said chamber at a less rate than it is liberated from the web and delivered to the chamber and whereby excess steam in said chamber escapes through said slots and prevents the entrance of air therethrough.

' JOHN W. BELL.

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