US2933826A

US2933826A - Heat pump system for paper machine dryers

Info

Publication number: US2933826A
Application number: US632075A
Authority: US
Inventors: Edgar J Justus
Original assignee: Beloit Iron Works Inc
Current assignee: Beloit Iron Works Inc
Priority date: 1957-01-02
Filing date: 1957-01-02
Publication date: 1960-04-26
Anticipated expiration: 1977-04-26

Description

April 26, 1960 E. J. JUSTUS 5 9 HEAT PUMP SYSTEM FOR PAPER MACHINE DRYERS Filed Jan. 2, 1957 J My [04:41? 1/ (/wras' operation of the dryers.

HEAT PUMP SYSTEM FOR PAPER MACHINE DRYERS Edgar .I. Justus, Beloit, Wis., assignor to Beloit Iron Works, Beloit, Wis, a corporation of Wisconsin Application January 2, 1957, Serial No. 632,075

8 Claims. (Cl. 34-86) The instant invention relates to an improved paper machine dryer arrangement, and more particularly, to an improved heating system for paper machine dryers.

In a paper machine dryer system of the type customarily used, there are mounted a plurality of rolls, usually in a pair of tiers, and the wet paper web is fed into the dryer beneath the roll in the lower tier, then over a roll in the upper tier thereabove and down beneath the next roll in the lower tier, and so on in a serpentine path through the entire dryer. Each of the rolls is a hollow drum rotatably mounted receiving steam through a coaxial inlet and each of the rolls is provided with a coaxially aligned condensate removal line. The steam heated rolls cause moisture in the paper web to be evaporated in order to obtain the desired moisture content in the final paper. In the usual arrangement, the condensate is returned to a boiler and steam is regenerated therefrom and returned to the dryer drums or rolls.

The steam entering into the dryer drums is steam of about to 160 pounds per square inch pressure ordinarily, and the moisture driven out of the web is approximately at the temperature of the boiling point of water (212 F.) at atmospheric pressure.

I In order to protect the operators, prevent overheating of the dryer room, and control the rate of evaporation of the moisture in the web passing through the dryers, it is customary to enclose the dryer in a hood which permits controlled flow of incoming air up over the dryer drums .and which controls the flow of the off-running stream of air and water vapor coming off the top of the dryer. Heretofore, the customary practice has been to vent the hood to the atmosphere outside the building.

Theinstant invention involves a unique saving in the In the instant arrangement the condensate returned from the dryer drums is converted into steam, but the heat employed for such conversion is taken from the air-vapor mixture ordinarily vented from the hood. Actually, the bulk of the heat is obtained by condensation of the vapor carried with the air out of the hood. The dryer drum condensate is converted to steam, using such heat, by subjecting the condensate to vacuum conditions, so as to obtain therefrom steam under subatmospheric pressure. Such steam is then converted to superheated steam at a greater (usually, superatmospheric) pressure by passing the same through a compressor or pump. The compressor converts the steam to superheated steam at the operating pressure for the :dryer'drum (which may be about -5 to 160 pounds per .square inch in a typical operation).

Preferably, the heat in the vented air-vapor mixture is recovered by subject- 'ing this mixture to a water spray which brings the temperature of the air-vapor mixture substantially below its 'dew point to effect condensation of most of the vapor.

Patenit' vert the dryer drum condensate (under vacuum) to steam which is then fed into the compressor suction. In the instant method and apparatus great saving results in the use of mechanical energy (via the compressor) in combination with the recovery of the heat from the vented air-vapor mixture.

It is, therefore, an important object of the instant invention to provide an improved apparatus and method for paper machine dryer operation.

It is a further object of the instant invention to provide an improved heat recovery system for paper machine dryers and an improved method of utilizing heat and mechanical energy in the operation of paper machine dryers.

It is another object of the instant invention to provide an improved paper machine dryer system comprising a dryer part having a steam inlet and condensate outlet, a hood surrounding the drums having an air inlet and an air-vapor outlet, first heat exchange means connected to said hood outlet and having a cold water inlet and hot water outlet, second heat exchange means having one pass with an inlet connected to said first means water outlet and a second pass, separated from the first, provided with an inlet connected to said condensate outlet and an outlet, and a pump having an intake connected to said second pass outlet and a discharge connected to said steam inlet.

Still another object of the instant invention is to provide an improved method which comprises condensing a stream of steam against a surface contacting a wet paper web to vaporize moisture therein, withdrawing condensate from said surface, subjecting such condensate to subatmospheric pressure and heat to vaporize the same, compressing the vaporized condensate to produce superheated steam, directing such superheated steam into said stream of steam, sweeping a stream of air over said web to pick up vaporized moisture therefrom, and withdrawing heat from the air stream carrying the vaporized moisture to heat the condensate at subatmospheric pressure.

Othere and further objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed disclosure thereof and the drawing attached hereto and made a part hereof.

defining an air-vapor outlet at 12d (the fiow of air and vapor being indicated by arrows). Actually, a plurality of dryer drums 11 mounted in a pair of tiers are employed within the hood 12, but for simplicity only a single dryer drum 11 is here shown. The hood substantially encloses the dryer drums 11, 11. A small amount of air may enter the hood (at the web inlet and outlet) by diffusion against a positive small pressure difference between the hood interior and ambient atmosphere.

In the instant invention a conduit He carries the vapor mixture (which may contain substantially all vapor) from the outlet 12d to a first heat exchange system 13. In the heat exchanger 13 relatively cool water is sprayed through a sparger 13a into the air-vapor stream wherein the cool water is heated (primarily by the heat of vaporization of the vapor which is condensed thereby) and hot water flows out a bottom exit 13b. The gas remaining (which may be mostlyair with substantially all of the vapor pickedv up in the hood 12 removed therefrom) continues through an outlet duct 12f leading away from the heat exchanger 13 and, if desired, this gas may then be vented to the atmosphere through a purge valve 14; or (as will be explained hereinafter) this gas may continue around through a duct 12g into the suction port 15a of a blower 15 and out the discharge port 15b thereof back into the hood 12 through a conduit 1211. As will be appreciated, if this gas is released to the atmosphere through the purge valve 14 at the end of the conduit 12f, then atmospheric air enters into the hood inlet conduit 12h through a valve 16. When the blower 15 is in operation the valves 14 and 16 are closed. An advantage in the use of the blower 15 is that the gas recycled is warmer than atmospheric air (which would come through the valve 16) and a heat loss involved in heating atmospheric air up to operating temperatures in the hood12 is eliminated, at the cost of the operation of the blower 15.

As here shown, the water circulated through the first heat exchanger 13 is continuously recycled. The water passes through the sparger 13a, the air-vapor mixture and finally the hot water outlet 13b at the bottom of the heat exchanger 13 into the inlet 17a of a'second heat exchanger 17. In contrast to the first heat exchanger 13 which permits actual contact between the fluids undergoing the heat exchange, the second heat exchanger 17 is provided with a first or outside pass 17b which receives the water from the first exchanger 13 through the inlet 17a and a second or inside pass which is maintained separate from the first pass' 17b because it is enclosed within a bundle of tubes 170 which define the same. As will be appreciated, other forms of heat exchange structure may be employed, but the heat exchanger 17 is provided with separated passes 17b and 170. The hot water flowing through the outside pass 17b flows over the tubes of the inside pass 170 collecting in the bottom 17d of the heat exchanger 17 from which it is withdrawn through a valve 18 and conduit 19 into a reservoir 20. The water in the reservoir 20 has lost some of its heat which was removed by the second pass 170 and this water is then returned to the sparger 13a through conduit 21 and pump 22. Since this cycle will continuously gain in water content because of the continuous removal of vapor from the air-vapor stream, an overflow valve 23 is connected to the reservoir for main,- taining the total amount of water in the system substantially constant;

Referring now to the steam-condensate system associated with the dryer drum 11, itwill be seen that the condensate is withdrawn from the drum 11 through a condensate outlet line 11b and returned to a reservoir or hot well 24. From the hot well 24 water vapor and/ or condensate is drawn (or optionally pumped by a pump not shown) through a conduit25 and valve 26 into the inlet side of the second pass 170 of the heat exchanger 17 at 17c, by a compressor 27 having its intake 27a connected to the second pass outlet 17,. The compressor 27 maintains subatmospheric pressure within the second pass 17c, so that completevolatilization of the condensate therein takes place to form steam at subatmospheric pres sure. The compressor 27 then converts the steam at subatmospheric pressure to superheated steam at superatmospheric pressure coming from the compressor discharge 27b.

The compressor discharge 27b feeds to the steam inlet 11a for the drum 11, but preferably the superheated steam at superatmospheric pressure is converted to saturated steam at superatmospheric pressure at a mixing station 28 whereat condensate and/or water vapor is introduced from the hot well 24 through a conduit 29 by a pump 30. By the addition of a predetermined amount of condensate to the mixing station 28 saturated steam isobtained'at the steam inlet 11a for the -drum 11.

per square inch gauge. predetermined amount of condensate is not sent through Considering a specific set of operating conditions, it will be seen that a substantial economic advantage is obtained in the practice of the instant invention. At a production rate of 233 pounds per minute paper, the paper entering the dryers is 33.9% dry and the paper leaving the dryers is 91.3% dry. The water removed per pound of paper is thus 1.693 pound per pound of paper produced. The water evaporated at this production rate is thus 395 pounds per minute. The heat load on the dryers may thus be computed as follows:

B.t.u./min. (a) Amount required to heat paper and residual Water from F. to 212 F. 10,848 (b) Amount required to heat and evaporate 395 lbs. of water from 85 to 212 F. 433,749 (0) Heat loss from dryer and hood by radiation and convection 6,710 (d) Miscpfriction losses 276 Total 451,582

In operating the present system, the water comes through the sparger 13a at 190 F. and exits from the exchanger 13 at 200 F., wherein this water has gained 387,200 B.t.u./min. (through condensation of 391 pounds per minute of vapor at 212 F.). The inside pass is maintained at 9.3 pounds per square inch (absolute) so 427.5 lbs. per minute of condensate is vaporized by the energy gained in the exchanger 13. The vaporized condensate is then converted to 40 pounds per square inch (ga.) steam into which 71.2 pounds per minute of condensate is fogged at the mixing station 28 to obtain saturated steam.

It is apparent from the foregoing that the bulk of the 'heat load on the dryer is taken up by the amount required to evaporate the 395 pounds of water per minute. In the specific embodiment here involved, the hood 12 is not sealed but only small openings are permitted resulting in a loss of about 1% of the vapor produced within the hood 12. The remainder of this vapor, which is about 391 pounds per minute is then fed into the heat exchanger 13 at about 212 F. Relatively cool water at about F.'is fed into the exchanger 13 through the spargers 13a and the resulting water coming out the exit 13b of the heat exchanger 13 is 200 F. In the heat exchanger 13 (which is a cooling tower embodiment here' employed) and bulk of the heat of vaporization imparted to the system by the drum 11 is recovered. The amount of vapor carried out of the cooling tower 13 is a negligible amount, even though the air leaving the cooling tower 13 is substantially saturated (at about 190 F.). In the instant operation, the amount of air passing over the dryer 11 is only about 0.183 pounds per minute and this can carry away only a negligible amount of the 391 pounds of water vapor in the conduit 12 The amount ofheat actually recovered in the second heat exchanger 17 is in excess of 375,000 B.t.u./minute (actually it is 387,200 B.t.u./min.) and the power required to operate the compressor 27 is 76,400 B.t.u./minute. The compressor power is thus the only energy added to the system. i v

In the instant operation, the specific operating conditions involve'maintaining a pressure in the inside pass 17c of the heat exchanger 17 of 9.3 pounds per square inch absolute (at which water boils at 190 F.) and the compressor 27 discharges superheated steam at 40' pounds As previously mentioned, a small the heat exchanger 17 but instead is fogged into the superheated steam in the mixing chamber 28 to convert the same to saturated steam. It will thus be seen that, assuming mechanical and thermal energy to be substantially equal in cost (on the basis of B.t.u./min.) the saving in the instant operation is 83%. 1

If the operation is changed so as to produce 100 pounds 6 per square inch steam at the discharge of the compressor 27, the compressor power required is 126,000 B.t.u'./minute and the saving is thus 72%. In the embodiments just described, the amount of air employed in the system is so small that there is no need to operate the blower 15, and there is consequently no mechanical energy input at this point. If, however, greater amounts of air are used, then the mechanical energy input at thecompressor may be balanced against the heat energy required to heat fresh incoming air from room temperature to the dryer temperature, also taking into consideration the heat retained in the system in the form of Water vapor carried by the air'through the cooling tower 13.

Since the hood pressure is maintained most advantageously at atmospheric pressure, the air-vapor mixture taken therefrom well always be less than 212 F. It is thus necessary to operate the inside pass 170 of the heat exchanger 17 at subatmospheric pressure sufiicient to permitboiling or water therein at a temperature below 212 F. There is, thus a maximum permitted pressure within the inside pass 17c which is required in order to obtain evaporation of the condensate therein, but the minimum operating pressure is primarily a practical consideration. The discharge pressure of the compressor 27 is, of course, greater than the intake pressure (and preferably atmospheric pressure) which is the pressure at which the dryer drum 11 is maintained. This pressure may also vary within practical limits. The primary consideration here is that of increasing temperature of the steam coming out of the second pass or inside pass 17c of the heat exchanger 17 to the extent necessary to make up for heat not recovered from the system to the operation of the first heat exchanger 13.

It will be understood that modifications and variations maybe efi'ected without departing from the spirit and scope of the novel concepts of this invention.

I claim as my invention:

1. In a paper machine dryer system, a dryer drum having a steam inlet and condensate outlet, a hood surrounding the drum having an air inlet and an air-vapor outlet, first heat exchange means connected to said hood outlet and having a cold water inlet and hot water outlet, second heat exchange means having one pass with an inlet connected to said first means water outlet and a second pass, separated from the first, provided with an inlet connected to said condensate outlet and an outlet, and a pump having an intake connected to said second pass outlet and a discharge connected to said steam inlet, said pump converting water vapor at low pressure in said second pass to vapor at higher pressure in said steam inlet, thereby adding heat to the system.

2. In a paper machine dryer system, a dryer drum having a steam inlet and condensate outlet, a reservoir connected to said condensate outlet, a hood surrounding the drum having an air inlet and an air-vapor outlet, first heat exchange means connected to said hood outlet and having a cold water inlet and hot water outlet, second heat exchange means having one pass with an inlet connected to said first means water outlet and a second pass, separated from the first, provided with an inlet connected to said reservoir and an outlet, and a pump having an intake connected to said second pass outlet and a discharge connected to said steam inlet, said pump maintaining water vapor at subatmospheric pressure in said pass and maintaining water vapor at superatmospheric pressure in said steam inlet by compressing water vapor passing through the pump so as to add heat to the system.

3. In a paper machine dryer system, a dryer drum having a steam inlet and condensate outlet, a reservoir connected to said condensate outlet, a hood surrounding the drum having an air inlet and an air-vapor outlet, first heat exchange means connected to said hood outlet and having a cold water inlet and hot water outlet, sec-. ond heat exchange means havingone pass with an inlet connected to said first means water outlet and a second pass, separated from the first, provided with an inlet connected to said reservoir and an outlet, a pump having an intake connected to said second pass outlet and a discharge connected to said steam inlet, and a conduit interconnecting said pump discharge and said reservoir, said pump converting water vapor at low pressure in said second pass to vapor at higher pressure insaid steam inlet, thereby adding heat to the system.

4."1n a paper machine dryer system, a dryer drum having a steam inlet and condensate outlet, a hood surrounding the drum having an air inlet and an air-vapor outlet, first heat exchange means connected to said hood outlet and having a cold water inlet and hot water outlet, second heat exchange means having one pass with an inlet connected to said first means waterv outlet and having'a water outlet connected tothe cold water inlet of said first means and a second pass, separated from the first,- provided with an inlet connected to said condensate outlet and an outlet, and a pump having an intake connected to said second pass outlet and a discharge connected to said steam inlet, said pump maintaining water vapor at subatmospheric pressure in said pass and maintaining water vapor at superatmospheric pressure in said steam inlet by compressing water vapor passing through the pump so as to add heat to the system.

5. In a paper machine dryer system, a dryer drum having a steam inlet and condensate outlet, a hood surrounding the drum having an air inlet and an air-vapor outlet, first heat exchange means connected to said hood outlet and having a cold water inlet spraying water into an air-vapor stream passing therethrough and a hot water outlet, a blower receiving air passing through said first heat exchange means and discharging air into the hood air inlet, second heat exchange means having one said second pass outlet and a discharge connected to said steam inlet, said pump maintaining water vapor at subatmospheric pressure in said pass and maintaining water vapor at superatmospheric pressure in said steam inlet by compressing water vapor passing through the pump so as to add heat to the system.

6. In a paper machine dryer system, a dryer drum having a steam inlet and condensate outlet, a reservoir connected to said condensate outlet, a hood surrounding the drum having an air inlet and an air-vapor outlet, first heat exchange means connected to said hood outlet and having a cold water inlet spraying water into an airvapor stream passing therethrough and a hot water outlet, a blower receiving air passing through said first heat exchange means and discharging air into the hood air inlet, second heat exchange means having one pass with an inlet connected to said first means water outlet and a second pass, separated from the first, provided with an inlet connected to said reservoir and an outlet, a pump having an intake connected to said second pass outlet and a discharge connected to said steam inlet, and a conduit interconnecting said pump discharge and said reservoir, said pump maintaining water vapor at subatmospheric pressure in said pass and maintaining water vapor at superatmospheric pressure in said steam inlet by compressing water vapor passing through the pump so as to add heat to the system.

7. In a paper machine dryer system, a dryer drum having a steam inlet and condensate outlet, a reservoir connected to said condensate outlet, a hood surrounding the drum having an air inlet and an air-vapor outlet, first heat exchange means connected to said hood outlet and having a cold water inlet spraying Water into an air-vapor stream passing therethrough and a hot water outlet, 9.

blower receiving air passing through said first heat exchange means and discharging air into the hood air inlet, second heat exchange means having one pass with an inlet connected to said first means water outlet and having a water outlet connected to the cold water inlet of said first means and a second pass, separated from the first, provided with an inlet connected to said reservoir and an outlet and a pump having an intake connected to said second pass outlet and a discharge connected to said steam inlet, and a conduit interconnecting said pump discharge and said reservoir, said pump maintaining water vapor at subatmospheric pressure in said pass and main: taining Water vapor at superatmospheric pressure in said steam inlet by compressing Water vapor passing through the pump so as to add heat to the system.

8. In a paper machine dryer system, a dryer drum having a steam inlet and condensate outlet, a reservoir connected to said condensate outlet, a hood surrounding the drum having an air inlet and an air-vapor outlet, first heat exchange means connected to said hood outlet and havinga cold Water inlet and hot water outlet, second heat exchange means having one pass with an inlet con! nected to said first means water outlet and having a Water outlet connected to the cold water inlet of said first means and a second pass, separated from the first, provided with an inlet connected to said reservoir and an outlet, a pump having an intake connected to said second pass outlet and a discharge con'..ected to said steam inlet, and a conduit interconnecting said pump discharge and said reservoir, said pump maintaining water vapor at subatmospheric pressure in said pass and maintaining water vapor at superatmospheric pressure in said steam inlet by compressing water vapor passing through the pump so as to add heat to the system.

2,718,710 Spooner Sept. 27, 1955' may