Pottery and Ceramics Tools
Venting a Kiln
I am using this Enviro Vent 2 article for my page on venting a kiln because in my career of ceramics,
SAME QUALITY VENTINGThe EnviroVent 2 is a Downdraft Ventilation System designed to extract fumes from your electric kiln and vent them outdoors before they have a chance to enter the room. Because it is a DownDraft Ventilation System the EnviroVent 2 also improves the firing atmosphere in the kiln while it is protecting you from the odorous fumes that can be emitted from certain clay bodies and glazes when they are fired.
Here's How It WorksA designated number of holes are drilled in the lid and floor of the kiln either in the factory or on location. A spring loaded plenum cup (see diagram) is placed under the kiln and tensioned between the floor of the kiln and the floor under the kiln. The holes in the floor of the kiln are drilled in a tight pattern so the plenum cup can cover them all. The other end of the cup is connected by 3" ducting to a fan which is generally mounted on the wall. On the opposite end of the fan the air is ducted through the wall to the outside air. The plenum cup has 3 additional holes from which it draws room temperature air to mix with the heated air from the kiln. This serves 2 purposes. Since the kiln air is mixed with cooler room air the air coming out of the end of the vent is never hotter than an average hair dryer. Secondly, the holes in the plenum cup are sized to reduce the vacuum on the kiln chamber so only a small amount of air is moving through the kiln. This ensures the firing time is not affected and the ware is not at risk.
The first two pictures show venting on the bottom of the kiln and a diagram of how the fumes travel out of the kiln, the last two pictures shows a vent hood application also with a diagram of the path the fumes travel out of the kiln. With the vent hood the kiln lid must be left open 4 or 5 inches to let the fumes escape. A fan in the top of the hood draws the fumes up and out the hood duct.
As your kiln heats up the brick expands. When it expands the hot face of the kiln floor, the inside, expands more than the cold face or underside. This causes the slab to cup, or become slightly concave on the bottom. The result is that the center of the kiln floor moves upward at higher temperatures. The spring loaded cup follows the floor ensuring at tight seal throughout the firing process.
This is a close up diagram of the under side of the kiln in the picture on the left above.
Better Heat UniformityMost people are aware of the fact that heat rises. The EnviroVent 2 helps compensate for this rising heat by creating a flow of air moving back down the kiln chamber. As the air moves in a downward direction it is also deflected by ware and shelves causing turbulence. This turbulence helps move heat to cooler areas of the kiln like the center.
Better Kiln AtmosphereWhen you are firing in an electric kiln you are firing in an Oxidation atmosphere. The EnviroVent 2 helps bring in more oxygen and flush out fumes which can form a reduction atmosphere. A reduction atmosphere can be desirable in a gas or wood fired kiln however it only causes problems in an electric kiln. Your elements will last longer, your glazes will be clearer and brighter and you will help prevent glazes migrating between pieces.
Better Working EnvironmentPrior to the introduction of venting systems it was still necessary to vent the kiln. This was done by propping the lid open with a wedge of brick until the kiln reached 1000°F (538 °C). Not only was this inconvenient, it also radiated the fumes right into the room.
A Better VentA new benefit of the EnviroVent 2 is that it is now has a Negative Pressure System. This means that since the motor is pulling the air instead of pushing the air, if there is ever a hole in the ducting the fumes will not escape into the room.
The Skutt EnviroLinkThe EnviroLink is an electrical switching device that can be controlled by an automatic kiln to determine when a vent turns on and off.
Certified safeCeramic and glass fusing materials, including greenware, glazes and luster's, contain organic compounds. The first phase of the firing process removes these gases or vapors from the wares before the higher temperature changes take place. These contaminants need to be removed from the kiln, and replaced with fresh air. This is especially important with red, orange, yellow and brown glazes and luster's, because they need oxygen for good results.
Some specific problems related to poor venting include:
• Grayish inner areas (black core) when bisque firing earthenware.
• Unburned carbon in bisque ware that causes crazing, pinholes or blisters during glaze firing.
• Dulled surfaces and cloudy colors caused by sulfur gases.
• Weak colors in red-orange-yellow glazes.
• Color migration caused by glaze fumes depositing on nearby ware.
• Cones which fall improperly due to inadequate oxygen.
If your firing chamber is not properly vented, your work is likely to suffer.
Q.Can you describe my kiln venting options?
A.There are several options, ranked here in order of sophistication:
• Natural ventilation from open doors and windows.
• Room ventilation fans.
• Convection canopy collection hoods.
• Mechanical fan collection hoods.
• Mechanical downdraft vent systems.
Q.What are the main differences between venting systems?
A.Window ventilation and convection hoods are passive—the warm fumes rise, and hopefully, are diluted or escape. This requires very large volumes of fresh air, which is costly in the winter.
Collection hoods with electric fans do a better job of removing fumes, but fumes still enter the room air before they are collected. There is usually still a significant odor. External hoods do not help internal kiln performance.
Mechanical downdraft systems, such as the Skutt EnviroVent, were developed to address both needs:
• Improved kiln firing chamber heat distribution and circulation for more consistent firing results; and
• Removal of fumes directly from the firing chamber before they can post a health hazard. Little or no odor can be detected.
Q.How much does heat distribution vary?
A.A one-cone difference is common from top to bottom of an unvented kiln. You can observe this difference with cone groups set at different levels. The difference can grow great with larger electric kilns, especially when firing to low temperatures. For example, cone 03 has bent on a top shelf when only cone 05 has bent on a bottom shelf. Tests have shown a 30° difference top to bottom on high fires and 80° difference when low firing.
Q.How much does the EnviroVent help heat distribution?
A.Heat naturally rises therefore the bottom of the kiln can fire cooler than the top, especially at lower temperature ranges. On tests conducted on a 10 cu. ft. kiln with pyrometers at six levels, it was found that the EnviroVent can cut temperature differences to about half of normal. The top-to-bottom differential was cut from 79° to 45°. Some users have reported even greater improvement in heat distribution.
Q.How does it work?
A.Developed jointly with the Edward Orton Jr. Ceramic Foundation, the EnviroVent is protected under U.S. patent 4,863,374. It is unique because it creates a downdraft venting effect, immediately mixing fumes with cooling air in a closed plenum chamber at the bottom of the kiln. The venting requires a few very small holes to be drilled in the lid and floor of the kiln . The fan motor mounts to the wall and vents the fumes to the outside much like a household dryer.
Q.Does this require major Modifications to my kiln?
A.No. You can use the twist drills we provide to bore though the soft firebrick by hand. The holes are quite small, shown her at actual size. As an example, the Skutt 1018 needs only two 15/64” holes in the floor and lid. We can pre-drill the kiln in the factory at the time it is ordered. Be sure to tell your distributor when ordering your new kiln that you want it drilled.
Q.Is installation difficult?
A.No. You drill the few holes needed for your kiln, place the EnviroVent 2 spring loaded plenum cup under your kiln and mount the motor to the wall. We supply the aluminum drier ducting that is used between the plenum cup and the motor. Skutt even supplies the stainless sleeve that is needed to pass through the wall.
It runs on 115V household current. If you install the EnviroVent on a kiln with a metal floor plate, you will need an electric drill.
Q.Must I always turn on the EnviroVent when I fire?
A.Yes. The airflow is necessary to cool the EnviroVent's electric motor. However, you will want to use the EnviroVent for all types of firings in any case.
Q.Can I do burn-out firings?
A.The EnviroVent was designed for conventional pottery techniques. A large load of burn-out pieces may momentarily overload the EnviroVent. We recommend smaller loads of burn-out pieces.
Q.Can I rearrange my kiln area easily?
A.Certainly, The vented air is cool enough to allow you to use flexible ducting away from the kiln. There are no overhead brackets or pulleys to restrict -your locations.
Q.Does the EnviroVent change firing times?
A.There may be a slight increase of 10 to 15 minutes to shut-off in an 8 hour firing. However, your total kiln firing cycle will probably be shorter because the EnviroVent removes heat from the kiln after shut-off without causing thermal shock.
Q.What does the UL listing of a kiln vent system mean?
A.It means that the manufacturer has submitted the vent unit, properly installed on specific kiln models, to Underwriters' Laboratories for rigorous testing under actual high-heat firing conditions. When installed on Skutt UL kilns, the EnviroVent is the only L listed downdraft vent system.
Some kiln vents use an electric motor which may have been UL listed for general room temperature use. This does not make their vents UL listed.
Q.What is The EnviroLink?
A.The EnviroLink is an accessory item that can be ordered for the EnviroVent and Envirovent 2 that works in conjunction with the Skutt KilnMaster or GlassMaster controller. It allows you to program the controller to automatically turn the vent on and off throughout a firing program. You simply enter an on/off setting for each segment of a Ramp and Hold program while you are programming the kiln. There are preset “vent programs” that can be run in conjunction with a Cone Fire Mode or Glass Fire Mode program as well.
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