Steam's 'Roll' In Papermaking
A paper corrugator uses as much -- or more -- steam as a brewery, refinery or textile mill. Steam heat is crucial for the production of high quality corrugated board. Perfectly produced corrugated is durable, consistent and very flat, providing a surface that is readily printable. A constant temperature must be maintained throughout the manufacturing process to achieve this desired flatness.
Steam Use In Corrugated ProductionThe final corrugated paper product is composed of two liner papers bonded to a corrugated or fluted paper layer in the middle. The liner layers usually are heavier than the corrugated layer.
The first machine in the corrugated line is called the single facer. It actually flutes the paper medium and glues one liner to it. A typical system might run a range of paper weights from 23 lb to 69 lb; paper widths are usually 70 to 110". The paper is pulled over the six to eight steam-heated cylinders of the single facer. The cylinders are typically 12 to 18" in diameter. The cylinders heat and condition the Kraft paper as it rides over the rolls of the single facer, making it more receptive to the adhesive when the product is bonded together. Then, the paper is pulled flat as it goes over steam chests -- which are hollow blocks of steel, filled with steam -- that help cure the adhesive.
The weight of the paper being run determines the amount of steam being used at any given time -- the heavier the paper, the greater the demand for steam. Heavier paper absorbs more heat. When light paper is run, there is less demand for steam. However, the demand for steam is fairly constant because fluctuations are gradual. In fact, this steady steam demand often is only interrupted by the weekend shutdown.
The starch adhesive used to bond the paper layers is heated in large, steam-heated mixers that typically hold 600 gal of adhesive. The glue is warmed to 105 to 110oF (40 to 43oC). The glue stays hot because the bonding machines are heated. So, when the adhesive is recovered and goes into storage, it is still hot and ready for recirculation. Unlike the adhesive, maintaining the temperature of the circulating water as it changes its state from steam to condensate is a much more complex matter.
Cascading Condensate RecoveryThe corrugated industry has employed various systems to recover and reheat the hot water that is formed as steam emits its heat and condenses.
Cascade was the original system going back to the first corrugators. It was a holdover from the paper mills who had bought up the corrugators. Paper mills weren't concerned about energy losses because they were burning bark and other waste product. But, increasingly, corrugators started to take a hard look at ways to save energy.
In a cascade system, steam feeds into the single facer at a single point, feeds all of the rolls from that point, and then collects condensate from all of the rolls into one vessel. Though the cascade system worked, it was more complex than a system developed in the '60s called a high pressure return system.
The high pressure return system employs traps instead of the orifices used in the cascade system. To create the high pressure return, a pump took high temperature, high pressure condensate from the corrugator and pumped it directly into the boiler. The trap that was being used “knew” the difference between a gas and a liquid. However, it could not distinguish the differences among air, noncondensable gases that formed in the steam system, and flash steam, or the condensate that flashes back to steam. As all are gases, the trap could not tell the difference among them, so it would stay shut until it vented all those gases.
One problem caused by traps that cannot differentiate between gases is the speed at which air in the corrugator can be displaced by steam. When steam cools and becomes liquid condensate, there is a 1,600-to-1 volume change. So, when a corrugator shuts down on Friday night, all the steam that is in the corrugator condenses, producing a vacuum that pulls in air through the rotary joints and other vacuum-release valves. Thus, the whole corrugator is filled with air. When it is restarted on Sunday night, the air has to be removed before the corrugator can get hot enough to run double-walled board. With the old trap system, it would take about two days to remove all of the air and get hot enough, so corrugators tried not to schedule the production of double-wall corrugated until Wednesday.
High Pressure Return RefinedA trap that can distinguish between gases was developed in the early '80s. It contains a float that senses the condensate and a thermic element that senses temperature. Flash steam is created at a lower temperature. That temperature difference causes the trap to open and blow out the gas. The trap becomes a temperature and condensate slave. A constant draw into the trap is caused by a venturi tube, which is also designed to vent noncondensable carbon dioxide, nitrogen and oxygen gases.
The new traps stay open and release all the air in the system so that steam can rapidly replace it. With the newer high pressure return trap system, a plant can be up and running within an hour if it have one type of straight watertube boiler.
In a high pressure return system, steam pressure for the normal American made corrugator is about 170 lb, or 380oF (193oC). When the steam gives up its energy, it becomes water, but it is still at 380oF. The system immediately, sends that water-condensate back to the boiler room and feeds it to the boiler -- losing only 50oF (28oC), the feedwater temperature normally being in the area of 330oF (166oC).
This refinement of the high pressure condensate return system was invented by John Donahue of Donahue and Associates International Inc., Milford, Ohio. Donahue describes his invention. “In 1982, when I first started in business, we designed our own trap just for the corrugated industry. Our system feeds the steam into each roll but then each roll is trapped individually. The trap then becomes a slave to the roll and holds a constant temperature on every roll. That's why we have the thermostatic element as part of the trap. It also has a float that senses condensate, a thermostatic element that senses temperature, and a venturi tube [a stainless steel tube with a very small hole in it] for noncondensable gases.”
Boiler Technology Improves ReliabilityBoilers were always the source of heat for the corrugated process, but early on, firetube boilers fell out of favor because of their size and space consumption. When a coiled watertube boiler was developed, it had some advantages over the firetube boilers. The coiled watertube boilers took up much less space than the huge drum-like water vessels of firetube boilers. They were compact and more efficient than the firetubes and their design eliminated the danger of pressure-vessel explosion that was ever-present with firetube boilers.
Coiled watertube boilers rely on a pump that maintains constant pressure and circulates the water through the coils. This high pressure circulation is critical for both steam production and for returning the condensate product back into the semi-closed receiver. It is a positive-displacement pump with a constant output. For instance, a 300 hp boiler might require a minimum of 20 gal/min. But the pump may pump 25 gal/min with the rest being returned via the trap on the steam separator back to the semi-closed receiver.
This diaphragm hydraulic pump is subject to constant wear and tear to the seats, disks, springs and diaphragms. In the corrugated industry, the pump lasts five to 10 years, but it must be regularly repaired, usually twice a year. This is normal maintenance. Each maintenance session costs $1,000 to $1,500, depending on the manhours involved. (If the boiler is hot, there are more manhours than if it is cold.)
Case Study: One CorrugatorOne Midwest corrugator with 188,000 ft2 under roof puts heavy demands on its steam boilers for processing corrugated products. (The boilers are not used for heating the plant.) The maintenance superintendent, who has been with the company for 18 years, recalls a critical time, three years ago. “Our coiled watertube boilers were old and not reliable any more -- and parts were expensive.” The Midwest corrugator found the pump created production problems as well as repair expenses. As a result, three years ago the company decided to replace their coiled watertube boilers.
The maintenance superintendent recalls, “The coiled watertube boilers have their own special pump, and it's a problematic pump. It created reliability problems.” The corrugator was looking for reduced maintenance expenses and ease of operation. In addition to the pump, the coil, heat exchanger or pressure-vessel replacement on the coiled watertube boilers could run $30,000. The Midwest corrugator had gone through a number of coils, so the company considered straight watertube boilers, which have few moving parts: the damper motor, blower and some valves.
The corrugator's maintenance superintendent describes the new boilers. “We purchased and installed from M&M Services, two 250 hp high-pressure boilers [250 psi] three years ago. You were always pumping water with the coiled watertube boilers, whereas you just draw water when you need it with the straight watertube boilers. It's a different system that seems to work better. It has a conductivity sensor that automatically blows down as the conductivity rises. You just set a point for conductivity and it maintains it at that level.”
John Malinowski of M&M Services adds, “The blower in coiled watertube boilers needs constant cleaning. The blowers on the straight watertube boilers do not need much maintenance. They might need the screen cleaned once in a while, but that is relatively easy.”
The straight watertube boilers start up and shut down as required by the load, producing another in-service efficiency saving. If there is no load, they will completely shut down whereas the coiled watertube boilers constantly run, allowing the fan to cool the watertubes as it is idling. PH
Sidebar: Semi-Closed SystemJohn Malinowski of M&M Services, West Chicago, Ill., explains how a semi-closed system of circulation increases system efficiency. “Most corrugators have a semi-closed system, where the heat is retained in a high pressure receiver. Either the hot well tank [also called a makeup tank or condensate tank] or deaerator feeds the semi-closed receiver tank, which is under about 80 to 120 lb of pressure. At that pressure, it stays at about 300 to 330oF [149 to 166oC]. Because we're feeding that water at about 300 to 330oF to the boiler, the boiler only has to bring it up to 380oF [193oC], which is a 50 to 80oF [28 to 44oC] rise.”
Malinowski continues, “If they take it from a hot well tank or deaerator and don't have the semi-closed receiver tank, the boiler has to take it from about 212oF [100oC] degrees up to 380oF [193oC]. So, there is quite a bit of efficiency savings there.”
When the steam goes out and gets consumed, it comes back to the receiver, which is under 80 lb of pressure, and that heat is all retained. It is not going back down to the ambient temperature -- 212oF -- and bringing it up again.