Archive:New Draft of the Week: Difference between revisions
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=== Nomination === | === Nomination === | ||
To nominate an article: | |||
# Before nominating an article, you must make the changes listed at [[Template:Featured Article Candidate]]. | |||
# Then, you can add the article to the list above using the {{tl|Featured Article Candidate}} template. | |||
# The list is sorted by score, with the highest at the top, and then by name, in alphabetical order. | |||
The following rules apply: | |||
* To nominate a draft, simply create a new row at the bottom of the [[#Current nominees|current nominees]] list. Use the {{tl|NAOTW row}} template. Alphabetize your nomination in relation to the other nominees with the same score. | * To nominate a draft, simply create a new row at the bottom of the [[#Current nominees|current nominees]] list. Use the {{tl|NAOTW row}} template. Alphabetize your nomination in relation to the other nominees with the same score. | ||
* Any Citizen may nominate a draft. | * Any Citizen may nominate a draft. | ||
Revision as of 05:08, 7 June 2009
The New Draft of the Week is a chance to highlight a recently created Citizendium article that has just started down the road of becoming a Citizendium masterpiece. It is chosen each week by vote in a manner similar to that of its sister project, the Article of the Week.
Current nominees
The next New Draft of the Week will be the article with the most votes at 1 AM UTC on Thursday, 11 June 2009. I did the honors this time. Milton Beychok 07:06, 5 June 2009 (UTC)
| Nominated article | Supporters | Specialist supporters | Date created | Last date eligible | Score | |||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
 Air preheater: A general term to describe any device designed to preheat the combustion air used in a fuel-burning furnace for the purpose of increasing the thermal efficiency of the furnace. [e]
 (PD) Image: Milton Beychok Schematic diagram of a steam generator in a conventional coal-fired power plant. An air preheater (APH) is a general term to describe any device designed to preheat the combustion air used in a fuel-burning furnace for the purpose of increasing the thermal efficiency of the furnace. In particular, this article describes the combustion air preheaters for the large fuel-burning furnaces used to generate steam in thermal power plants. The air preheater increases the steam generator's thermal efficiency by preheating the combustion air with heat recovered from the hot combustion flue gases (see the adjacent diagram). TypesThe two most often used categories of air preheaters in thermal power plants are regenerative air preheaters and tubular air preheaters.[1][2][3][4] Regenerative air preheatersThe two most common types of regenerative air preheaters are
Regenerative air preheaters may also be categorized as recuperators, which are special types of heat exchangers designed to recover or reclaim heat in order to reuse or recycle it. Rotating-plate regenerative air preheater (PD) Image: U.S. Environmental Protection Agency A typical rotating-plate regenerative air preheater.[2] (PD) Image: U.S. Environmental Protection Agency A typical rotating-plate regenerative air preheater.[2]
In the tri-sector design, the steam generator's hot flue gas flows through the largest sector (usually spanning about half the cross-section of the casing) and transfers some of its heat into the heat-absorbing material within the rotating wheel element. The cooled flue gas is then routed to further treatment in dust removal and other equipment before being vented from the flue gas stack. Ambient air is blown through the second, smaller sector by a centrifugal fan and absorbs heat from the heated material as it rotates through that smaller sector. The heated air then flows into the steam generating furnace as combustion air. The third sector is the smallest one and it heats a portion of the ambient air which is then routed into the coal pulverizations and is used to transport the coal-air mixture to coal burners. Thus, the total air heated in the RAPH provides: heated primary combustion air, heated air to remove moisture from the pulverized coal and carrier air for transporting the pulverized coal to the coal burners. Since the flue gas pressure is lower than the pressure of the air being heated, there is some small leakage (between the sectors) of flue gas into the air. The bi-sector design is used in thermal power plants burning fuels (such as oil or gas) that do not require pulverizing or removal of moisture and therefore have need for heated air other than for combustion air. The quad-sector design has a large sector heated by flue gas and three air-heating sectors: one is for the combustion air and that sector is flanked by two smaller air sectors. In applications such as circulating fluid bed (CFB) combustion systems where the differential between the air pressure and the flue gas pressure is even higher than in a conventional coal-fired steam generator, flue gas pressure, such a design is ideal since it acts to reduce the leakage of air into the flue gas.[6] The rotating wheel element rotates quite slowly (around 3-5 revolutions per minute) to allow optimum heat transfer first from the hot exhaust gases to the element and then, as it rotates, from the element to the air in the other sectors. Construction featuresThe heat-absorbing material in the rotating wheel element consists of vertical corrugated plates pressed into steel baskets, with sufficient space between the plates for the flue gas to pass through. The plates are corrugated to provide more surface area for the heat to be absorbed and also to provide needed rigidity. The baskets are designed to be replaceable as needed. The vertical shaft that rotates the wheel is supported on thrust bearings at the lower end lubricated with an oil bath that is cooled by water circulating in coils inside the oil bath. Cooling of the bottom end of the shaft is needed since that is where the hot flue gas enters the preheater. The top end of the shaft has a simple roller bearing to hold the shaft in a vertical position. Radial supports and cages for holding the corrugated plate baskets in position are attached to the rotating shaft. Radial and circumferential seal plates are also provided to minimize leakage of flue gas or air between the sectors. For cleaning of the baskets while in operation, steam jets are provided to blow any fly ash (deposited by the flue gas) into an ash hopper below the preheater. The rotating shaft is driven by a motor and gearing. To avoid uneven thermal expansion and contraction resulting in damage to the rotating wheel, the rotation must be started before starting the steam generator and must also be kept in rotation for some time after the steam generator is shut down. The baskets of corrugated plates are subject to abrasive and corrosive wear from the fly ash and corrosive gases in the flue gas. Hence frequent replacements are required and new baskets are always kept on hand and ready for use. Stationary-plate regenerative air preheaterThe heat absorbing element in this type of regenerative air preheater is stationary rather than rotating. Instead, the air ducts in the preheater are rotated so as to alternately expose sections of the heating absorbing element to the upflowing air. The hot flue gas enters at the top of the preheater and flows down through those exposed sections of the stationary heat-absorbing element that are not blocked by the rotating air outlet ducts, thus heating those sections of the stationary element. As the air ducts slowly rotate around, they pass over the heated sections and the incoming air is heated as it flows upward through those heated sections. As indicated in the adjacent drawing, there are rotating inlet air ducts (inside the outer casing) at the bottom of the stationary heat absorbing element as well as the rotating outlet air ducts at the top of the stationary element. The basic heat transfer principals of the stationary-plate regenerative preheater are the same as for the rotating-plate regenerative preheater. The table below provides a comparison of some design parameters between the rotating-plate and stationary-plate preheaters:
Tubular type Tubular air preheaters may have a number of configurations:[2][8][9]
A number of new circulating fluid bed (CFB) and bubbling fluid bed (BFB) steam generators are using tubular air preheaters, which avoids the air leakage associated with regenerative air preheaters. Dew point corrosionThe water dew point of air or any other gas containing water vapor usually refers to the temperature (for a given pressure) at which the air or gas is saturated with water vapor. That means that the air or gas is at the point where the water vapor will start to condense into liquid water if the temperature is lowered beyond that point. As a broad generality, the combustion flue gases from steam generators fueled by coal, fuel oil, natural gas, or biomass are composed of carbon dioxide (CO2) and water vapor (H2O) as well as nitrogen and excess oxygen remaining from the intake combustion air. Typically, more than two-thirds of the flue gas is nitrogen. The combustion flue gases may also contain small percentages of air pollutants such as particulate matter, carbon monoxide, nitrogen oxides, and sulfur oxides in the form of gaseous sulfur dioxide (SO2) and gaseous sulfur trioxide (SO3). The SO3 is present because a portion of the SO2 formed in the combustion of the sulfur compounds in the steam generator's furnace fuel is further oxidized to SO3 as the flue gas travels through the superheater and reheater sections of the steam generator (see the above diagram of a steam generator). The gas phase SO3 then combines the vapor phase H2O to form gas phase sulfuric acid H2SO4:[10]
Because of the presence of gaseous sulfuric acid, the dew point of most flue gases is much higher than the water dew point of air and the flue gas dew point is referred to as the acid dew point. That is the flue gas temperature at which acid will begin to condense out of the flue gas if the temperature is lowered beyond that point. For example, a flue gas with 5 volume % water vapor and containing no acid gases has a water dew point of about 32 °C (90 °F). The same flue gas with the addition of only 0.01 volume percent of SO3 will have an acid dew point of about 118 °C (244 °F).[11] The acid dew point of a combustion flue gas depends upon the composition of the specific fuel being burned and the resultant composition of the flue gas. Given a flue gas composition, its acid dew point can be predicted fairly closely. As an approximation, the acid dew points of flue gases from thermal power plants range from about 120 °C to about 150 °C (250 to 300 °F). All of the air preheater types incur erosion problems to some extent from the fly ash particles in the flue gas. If the hot flue gas temperature in an air preheater is lowered to below its acid dew point, then the air preheater also incurs corrosion problems which can be quite severe. That is especially true of the tubular air preheaters. For that reason, many air preheaters have a means for the air to partially bypass the air preheater (see above tubular air preheater diagram) so that the amount of heat exchange can be controlled to avoid lowering the flue gas temperature below the acid dew point. To mitigate dew point corrosion, the tubular air preheaters may use ceramic or Teflon-coated tubes and the regenerative air preheaters may use special corrosion-resistant steels or enameled materials. The addition of limestone (CaCO3) into circulating fluidized bed (CFB) steam generators results in capturing 95% or more of the gaseous SO2 in the combustion product gases as solid calcium sulfate (Ca2SO4) and that occurs before the SO2 has time to be further oxidized to SO3. Thus, the acid dew point of the flue gas from a CFB steam generator is higher than from conventional thermal power plant steam generators. That means that the air preheaters in CFB units have considerably less dew point corrosion problems. That may also be one of the reasons why a number of the newer CFB units are using tubular preheaters. References
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Daniel Mietchen | Milton Beychok | 3
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| Proteus mirabilis
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Milton Beychok | Daniel Mietchen | 12 May 2009 | 11 June 2009 | 3 | |||||||||||||||||||||
| Wrench (tool)
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Howard C. Berkowitz | 31 May 2009 | 1 July 2009 | 3 | ||||||||||||||||||||||
| Three Week Hero
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Milton Beychok; Meg Ireland | 18 May 2009 | 19 June 2009 | 2 | ||||||||||||||||||||||
| Immigrant Song
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Daniel Mietchen; Meg Ireland | 11 May 2009 | 10 June 2009 | 2 | ||||||||||||||||||||||
Current Winner
To change, click edit and follow the instructions, or see documentation at {{}}. Template:Featured Artice
Previous winners
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Rules and procedure
Eligibility
- The primary criterion of eligibility for a new draft is that it must have been created no more than one month before the date of the next selection (Currently every Thursday).
- Articles must be ranked 1 or 2 (developed or developing).
Nomination
To nominate an article:
- Before nominating an article, you must make the changes listed at Template:Featured Article Candidate.
- Then, you can add the article to the list above using the {{Featured Article Candidate}} template.
- The list is sorted by score, with the highest at the top, and then by name, in alphabetical order.
The following rules apply:
- To nominate a draft, simply create a new row at the bottom of the current nominees list. Use the {{}} template. Alphabetize your nomination in relation to the other nominees with the same score.
- Any Citizen may nominate a draft.
- No Citizen may have nominated more than one article listed under "current nominees" at a time.
- The article's nominator is indicated simply by the first name in the list of votes (see below).
- At least for now--while the project is still small--you may nominate and vote for drafts of which you are a main author.
- An article can be the New Draft of the Week only once. Nominated articles that have won this honor should be removed from the list and added to the list of previous winners.
- Comments on nominations should be made on the article's talk page.
Voting
- To vote, simply add your name and date next to an article title, after other nominations in the "Supporters" area, by signing ~~~~ (four tildes). (The date is necessary so that we can determine when the last vote was added.)
- You may vote for as many drafts as you wish, and each vote counts separately, but you can only nominate one at a time; see above. You could, theoretically, vote for every nominated article on the page.
- Add your name in the "Specialist supporters" column only if you are an editor who is an expert about the topic in question. Your vote will be counted as three.
Ranking
- Nominations are listed by number of votes first, then alphabetically.
- Project Admins should make sure the votes are correctly tallied, but anyone may do this. Remember that "Specialist Votes" are worth three.
Deleting
- Any draft will be deleted when it is past its "last date eligible". Don't worry if this happens to your article; consider nominating it as the Article of the Week.
- If an editor believes that a nominee in his or her area of expertise is ineligible (perhaps due to obvious and embarrassing problems) he or she may remove the draft from consideration. The editor must indicate the reasons why he has done so on the nominated article's talk page.
Choosing the weekly winner
- Each Tuesday (at 7 PM UTC; 2 PM EST; 11 AM PST; 4 AM Sydney), one of the program admins--the first one who has the honor--moves the article that is at the top of the list (i.e., the one with the most votes) to the front page, announces the winner on Citizendium-L and updates the "previous winning drafts" section accordingly.
- If you are volunteering to change the New Draft of the Week on the front page, then say so above the "article nominees" table ("I'm doing the honors this time" -- Jane Doe) so that no one else does it.
In the event of a tie
Where two or more articles have achieved an equal number of votes, the following procedure shall be used to break the deadlock:
- Number of specialist supporters: the article with most specialist supporters goes through.
Should this fail to produce a winner:
- Alphabetical order: The article appearing first by English alphabetical order shall be named New Draft of the Week.
- The remaining winning articles are guaranteed this position in the following weeks, again in alphabetical order. No further voting would take place on these, which remain at the top of the table with notices to that effect.
- Further nominations and voting take place to determine future winning articles for the following weeks.
- Winning articles may be named New Draft of the Week beyond their last eligible date if their circumstances are so described above.
Adding the New Draft of the Week to the home page
To add the winning article to the home page, edit the Current Winner section of this page, above.
Administrators
The Administrators of this program are the same as the admins for CZ:Article of the Week.
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