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Because of incorrect information on superheat, some mill steam systems have been poorly designed. These poor designs have been costly, often with the end result of lower paper machine efficiency and production rates. Poor control of superheat in the dryer section of a paper machine is especially problematic and can directly cause production losses and runnability problems.
Superheated steam is steam that has been heated above saturated steam temperature. Steam that is superheated will not condense, and steam will only condense and give up its latent heat after the superheat has been reduced to the saturated steam temperature. In paper machine drying systems, the ideal amount of superheat is to have no superheat as the steam enters the dryer. This is because it can cause a delay in condensing and heat transfer.
This article addresses some of the incorrect information that has been circulated about superheat in the paper machine dryer section. In addition, it describes issues surrounding the presence of either too much or too little superheat. The article also provides examples of how superheat control has helped specific paper machines increase production and efficiently use heat in the dryer section.
MISINFORMATION ABOUT SUPERHEAT. A variety of incorrect information exists about the effects of superheat in paper machine dryers, as well as on the sheet itself. The following section discusses some of this misleading information.
1. Superheat causes the dryer surface to get very hot. This is the most often quoted, repeated, and misunderstood effect of superheat. If this were true, the steam tables would have to be rewritten. Not only is it untrue, it is not even possible. Heat transfer across a dryer shell depends on the temperature of the condensate that is in contact with the shell inner surface. It is the condensate that is responsible for the heat transfer through the shell, not the steam temperature in the dryer.
If condensate is responsible for the heat transfer, we know that condensate in a dryer cannot exist above the saturation temperature of the pressure inside that dryer. Condensate will stay at the temperature of the pressure, or slightly less, if heat is extracted—no matter what the amount of superheat going into the dryer.
For a given operating condition, the only way to change the temperature of the condensate that affects the temperature of the shell is to change the steam pressure. As the temperature of the condensate drops below saturating temperature against the shell and turbulence in the condensate returns the cooler condensate to the steam surface, more steam will condense to bring the condensate in contact with the steam back to the saturating temperature.
2. During a break, all the condensate inside a dryer will flash into steam, leaving a bare cylinder wall, and the surface temperature will rise close to that of the superheat. This situation is impossible, since, in order to flash all the steam inside a dryer, we would have to pass all the steam going to the machine through one dryer. Once the steam is de-superheated, it will condense before more superheated steam is admitted into the dryer.
During a break, there is always condensate inside the dryer. Also, during a break, the dryer surface temperature rises not because of superheat, but because there is no heat sink (the sheet) to take away the heat from the dryer surface. The rise in surface temperatures during a break is equal to the condensate temperature less the temperature drop across the shell. The dryer surface temperatures during a break are about 15°F to 20°F below the saturating steam temperature. This is why the dryer surface is hotter, not because of the superheat.
3. Low-pressure steam results in cooler surface temperatures than high-pressure steam when used at the same pressure in the dryers. This is not possible. Either low-pressure or high-pressure steam will give the same condensate temperature for a given pressure inside the dryer. Dryer surface temperatures will depend on pressure in the dryer, condensate rimming or not rimming, felts or no felts, the addition of dryer bars, varying the condensate film thickness with or without bars, and the amount of water in the sheet presented to the surface of the dryer.
4. Superheat causes breaks. So far, no relationship has been found to link superheat to sheet breaks. Any such correlation is apparently coincidental.
WHEN IS SUPERHEAT PROBLEMATIC? Superheat can cause severe drying and production problems under certain conditions, and either too much or too little superheat can be a problem. The normal recommendation is for 20°F to 30°F of superheat. This assumes that the superheat control is at the powerhouse, and there is a long pipeline to the machine that will reduce the superheat to nearly zero by the time it reaches the dryer entrance. If the superheat control is at the paper machine, the superheat can be set at 2°F to 5°F.
There are some designs where the de-superheater can be located after the main steam control valve to the main steam section of a paper machine. This is very effective for cascading systems.
Also, steam conditioning valves are available that will de-superheat as the steam is admitted into each dryer steam section. These are well suited for recirculating thermocompressor steam sections. Steam conditioning valves are not worth considering unless the grades are dryer limited or the makeup steam valves are operating in the wide-open position.
Excessive superheat. The ideal amount of superheat is to have no superheat as the steam enters the dryer. Superheated steam has to be de-superheated before condensing in the dryers. Superheat in a dryer causes a delay in condensing and heat transfer. It appears that the loss in drying rate for a dryer without bars is equivalent to raising the steam temperature 1°F to compensate for each 15°F to 20°F of superheat.
For a dryer with bars, each 1°F rise in steam temperature will compensate for 30°F to 40°F of superheat. For grades that are not dryer limited, the pressure is simply raised to compensate for the degree of superheat. On dryer-limited grades, the loss in production can be noticeable, especially at higher pressures where the incremental temperature increase for each pound of pressure increase is small compared to lower pressures. At 12 psig, each 1 psig rise in pressure increases the steam temperature by 2°F. At 145 psig, each 1 psig rise in pressure increases the steam temperature only 1°F. For example, a dryer-limited linerboard machine could be loosing production the equivalent of 10 psig to 15 psig if considerable superheat exists and some of the pressure is needed to compensate for the superheat.
In addition, a problem that should be of major concern, but one that is normally ignored, is the maximum amount of superheat or temperature allowed by the ASME code to reach any part of a dryer. This code calls for a maximum of 450°F for cast iron dryers. When this temperature is reached for steam going into dryers, the machine should be shut down until the superheat is brought under control and below 450°F. Several machines have been found to operate with steam above 450° F. Superheated steam will heat the dryer journal, steam entrance pipes, and parts of the dryer heads to its own temperature.
Not enough superheat. Total lack of superheat, on the other hand, has also been documented to be very detrimental to heat transfer and production. Systems with no superheat, along with inoperative or small steam trap lines, allow condensate to build up in the main steam lines and discharge into the dryers. Dryers close to the steam inlet will get the larger amount of condensate from steam lines and, in many cases, from flood dryers that are operating close to the marginal flooding point. This has been documented on several high-speed paper machines. Condensate in steam lines is also very detrimental to eroding valve seats and piping elbows.
Superheat can also reduce production noticeably on machines with wide-open steam-section makeup valves. This is a very serious problem on some machines. Wide-open valves will pass approximately the same volume at equal pressure and pressure drop across the valve. If the volume of steam is superheated, less pounds of steam will pass into the dryers for the same volume, resulting in less BTUs entering the dryer and lower operating pressure. The loss in production can also be great when the sheet comes back on the dryers. The condensing load is very high, resulting in reduced pressure and loss in production until the condensing load is stabilized and the normal operating pressure is regained.
A major mill problem is that many de-superheaters are not operating due to either maintenance problems or due to the fact that de-superheaters are just shut off for no apparent reason. Also, several de-superheaters have been found to be very ineffective because of low velocity through the de-superheater.
EXAMPLES OF CONTROLLED SUPERHEAT. As the following case histories describe, the control of superheat in the dryer section of a paper machine offers significant benefits, including increased production and more available heat for drying paper.
Newsprint machine benefits from superheat. A mill had been told that they could not use 150 psig steam in the dryers on its high-speed newsprint machines. The mill was forcing itself to use 40 psig saturated reboiler steam because of all the horror stories it had been told about using high-pressure (high superheated) steam. The results were severe upsets between machines whenever one machine had a break. With so many upsets, runnability problems, and lost production, the mill was willing to run a trial to check the effect of superheat on its machines.
On the initial trial day, the low pressure 40 psig steam was left in the dryers at an operating pressure of 20 psig and dryer surface temperatures were taken. On the second day, only 150 psig steam reduced to the operating pressure of 20 psig was used. The dryer surface temperatures at the same pressure inside the dryer showed an equal dryer surface temperature for either high- or low- pressure steam supply. The low-pressure steam had very little superheat after the control valve to the dryer section. The high-pressure steam had 150°F to 175° F of superheat. The conclusion was that superheat and initial steam header pressure had no effect on the dryer surface temperature at the same operating dryer pressure.
Figure 1: Effects of superheat on steam volume/1,000 BTU of latent heat.
De-superheating increases production. Figure 1 shows the effects of superheat on steam volume/1,000 BTU of latent heat. These curves were drawn for a production superintendent in order to explain why his paper machine was loosing production due to the high degree of superheat. The machine was operating with a wide open make-up valve and 35 psig in the main header. The steam temperature was 400°F. Saturating temperature for 35 psig is 281°F, resulting in a volume of 8.5 ft3/1,000 BTU of latent heat. From Figure 1, we can see that at 400°F, the volume for 1,000 BTU is 9.5 ft3. By de-superheating the steam on this machine to 300° F, production increased by 10%.
De-superheater helps linerboard production. An older linerboard machine had no pressure control. One steam valve was opened at the powerhouse that allowed steam at the same pressure to all sections of the paper machine. The steam valve was wide open and the machine pulled all the steam it could. Also, the de-superheater had not worked for some 20 years. The results were 200°F of superheat to the machine in a long steam line from the powerhouse and a wide-open valve.
The mill was convinced to repair the de-superheater and return it to service. As a result, production increased from 10% to 15% on all grades. By reducing the volume of steam, it was possible to provide many more pounds of steam to the dryers and more BTUs were available to dry paper.
Robert D. Perrault is the president and CEO of Perrault Drying Systems Inc., Newnan, Ga.
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