By Bob Gieselman, chief engineer at Paragon Industries, LP
The kiln is carefully loaded with the previous week’s work and attention is given to every detail to ensure a successful firing. Then, the unthinkable happens. The kiln continues to fire well beyond the planned temperatures and all the contents of the kiln are destroyed. Worse yet, the kiln itself is destroyed.
Post mortem analysis of kilns that have experienced this kind of catastrophic failure have revealed several weaknesses in the design and manufacture of kilns. Understanding these weaknesses will help every artist to ensure a correct firing. After all, a kiln is nothing more than a tool in the artist’s arsenal to create the most renowned masterpiece.
The most obvious weakness is the failure of the artist to properly program the controller. This mistake may be the symptom of inexperience but must be addressed regardless. However, it is our experience that this weakness very rarely causes an over fire as artists are very competent in their art and the operation of a kiln.
A second weakness is the failure of the temperature sensor. This is a very subtle condition that even the most experienced artists will miss. Fortunately, this does not happen very frequently. Normally, a thermocouple or temperature sensor will fail in the open condition as age oxidizes the sensor and material disappears. Once a thermocouple is open, the controller senses this condition and aborts the firing. However, if the thermocouple has been shorted due to misrouted wires in the control box or foreign material melted on the thermocouple, an over fire is possible.
By far, the most common cause of an over fire is a defective relay that has its contacts welded closed. This condition applies power to the elements and continues to heat the kiln regardless of the controller’s signal to the relay. If this condition exists before a firing, it may be detected by the kiln heating up while the controller is in an idle condition. Otherwise, the condition is very difficult to detect. The most popular relay used in kilns is the electro-mechanical dry relay. This relay is economical and tested at full rated load to over 300,000 cycles. It is the standard work horse which will complete the task at a reasonable cost. However, since it is a electro-mechanical (with the emphasis on mechanical), it will fail over time.
A search for substitutes for the electro-mechanical dry relay will reveal several choices. The most obvious is the solid state switch which contains no mechanical parts. These switches will reliably operate at the power levels required by the kilns. However, as a solid state device, they are extremely temperature sensitive. Even in the best of environments, the solid state switch will convert a certain amount of power to heat. This in turn must be dissipated through heat sinks, fans, or other cooling methods to ensure the device does not experience a thermal runaway. If the solid state device ever gets overheated, it will fail, and it will fail in the shorted or “On” condition. Since the control boxes for most kilns present a hot environment, it is very difficult, if not impractical, to maintain a cool environment for the solid state switch. However, solid state switches are very popular in industrial situations where the controller is remote to the kiln and where zero crossover or 4-20ma control is desirable. These remote control consoles are usually fan cooled and in environmentally controlled areas. In addition, these controls are very expensive and the requirement for the precise process control should be analyzed to justify the cost.
Another obvious replacement for the electro-mechanical relay is the wet, or mercury, relay. This device consists of a plunger with a small amount of mercury on top. As the device is activated the plunger moves up and compresses the mercury to the second contact. The advantage of this mechanical device is that there is no arc formed when the contacts are closed. This arc is the main reason dry electro-mechanical contacts are welded shut. The mercury relays have been tested over 1,000,000 cycles before failure and have proven themselves to be more reliable than the dry relays. However, they are still a mechanical device and they will fail. Fortunately, when they do fail, it is normally in the open or “Off” condition where gravity tends to pull the plunger away from the contacts. There are several disadvantages with mercury relays: cost, environmental restrictions to mercury, and disposal. In addition, the mercury relay must be operated in an upright position or the mercury will short the contacts in a horizontal position. Another disadvantage is the power required to activate the coil. The low voltage controllers are not capable of activating a mercury relay directly. Usually the controller will activate a dry electro-mechanical relay which in turn will activate the mercury relay. Now, how does this help if a dry electro-mechanical relay is required to activate a mercury relay? By far, the majority of the current is passed through the contacts of the mercury relay and only a very small amount of current passes through the dry relay. The amount of current through the dry contacts is not enough to produce the destructive arc. This in effect increases the amount of cycles the dry relay can perform before failure, actually to the quality level of the mercury relay.
If the kiln manufacturer is to continue to use dry electro-mechanical relays, there are other strategies than can be followed to protect from an over fire situation. The primary strategy is to limit the amount of power any one relay controls to an amount that will not let the kiln over fire. If there are multiple relays in the kiln, and no one relay controls the majority of the power, it would take multiple relay failures to cause an over fire. Multiple relay failures during a single firing is highly improbable and unlikely. The majority of kilns will fit into this condition.
Certain kilns, especially those with only one element and one relay, require a different strategy. Paragon Industries has been exploring the concept to use a controller with two outputs. One output is to control the temperature of the kiln, and the other output (a safety output) will be turned off if a certain, programmed, temperature is exceeded. These two outputs control relays that are in series with each other. Unfortunately, these two outputs are not enough as the safety relay could be in the shorted condition and the operator will not know it. A visual indicator will have to be added to warn the operator of a defective condition in the safety relay. Another strategy requires the use of a redundant or additional controller in series with the main controller. The primary duty of the second controller is to monitor for an over temperature condition and remove the power from the kiln if the over temperature is seen. This second controller must be latched in a safe mode such that it does not reapply power once the kiln cools down. While this second controller will solve the problem, the total cost of the control system is doubled. One novel approach is to use a kiln sitter as the second controller. While the initial expense is reduced, the recurring maintenance with cones will be required. Paragon Industries continues to explore methods to prevent over fire. As electrical control devices become available in the market, we constantly evaluate their suitability to control a kiln. We also strive to meet our customer kiln requirements and maintain fiscal restraint. We want to present the best performance for the minimum cost.
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Reprinted from the July/August, 2010 issue of Glass Art Magazine
The kiln is carefully loaded with the previous week’s work and attention is given to every detail to ensure a successful firing. Then, the unthinkable happens. The kiln continues to fire well beyond the planned temperatures and all the contents of the kiln are destroyed. Worse yet, the kiln itself is destroyed.
Post mortem analysis of kilns that have experienced this kind of catastrophic failure have revealed several weaknesses in the design and manufacture of kilns. Understanding these weaknesses will help every artist to ensure a correct firing. After all, a kiln is nothing more than a tool in the artist’s arsenal to create the most renowned masterpiece.
The most obvious weakness is the failure of the artist to properly program the controller. This mistake may be the symptom of inexperience but must be addressed regardless. However, it is our experience that this weakness very rarely causes an over fire as artists are very competent in their art and the operation of a kiln.
A second weakness is the failure of the temperature sensor. This is a very subtle condition that even the most experienced artists will miss. Fortunately, this does not happen very frequently. Normally, a thermocouple or temperature sensor will fail in the open condition as age oxidizes the sensor and material disappears. Once a thermocouple is open, the controller senses this condition and aborts the firing. However, if the thermocouple has been shorted due to misrouted wires in the control box or foreign material melted on the thermocouple, an over fire is possible.
By far, the most common cause of an over fire is a defective relay that has its contacts welded closed. This condition applies power to the elements and continues to heat the kiln regardless of the controller’s signal to the relay. If this condition exists before a firing, it may be detected by the kiln heating up while the controller is in an idle condition. Otherwise, the condition is very difficult to detect. The most popular relay used in kilns is the electro-mechanical dry relay. This relay is economical and tested at full rated load to over 300,000 cycles. It is the standard work horse which will complete the task at a reasonable cost. However, since it is a electro-mechanical (with the emphasis on mechanical), it will fail over time.
A search for substitutes for the electro-mechanical dry relay will reveal several choices. The most obvious is the solid state switch which contains no mechanical parts. These switches will reliably operate at the power levels required by the kilns. However, as a solid state device, they are extremely temperature sensitive. Even in the best of environments, the solid state switch will convert a certain amount of power to heat. This in turn must be dissipated through heat sinks, fans, or other cooling methods to ensure the device does not experience a thermal runaway. If the solid state device ever gets overheated, it will fail, and it will fail in the shorted or “On” condition. Since the control boxes for most kilns present a hot environment, it is very difficult, if not impractical, to maintain a cool environment for the solid state switch. However, solid state switches are very popular in industrial situations where the controller is remote to the kiln and where zero crossover or 4-20ma control is desirable. These remote control consoles are usually fan cooled and in environmentally controlled areas. In addition, these controls are very expensive and the requirement for the precise process control should be analyzed to justify the cost.
Another obvious replacement for the electro-mechanical relay is the wet, or mercury, relay. This device consists of a plunger with a small amount of mercury on top. As the device is activated the plunger moves up and compresses the mercury to the second contact. The advantage of this mechanical device is that there is no arc formed when the contacts are closed. This arc is the main reason dry electro-mechanical contacts are welded shut. The mercury relays have been tested over 1,000,000 cycles before failure and have proven themselves to be more reliable than the dry relays. However, they are still a mechanical device and they will fail. Fortunately, when they do fail, it is normally in the open or “Off” condition where gravity tends to pull the plunger away from the contacts. There are several disadvantages with mercury relays: cost, environmental restrictions to mercury, and disposal. In addition, the mercury relay must be operated in an upright position or the mercury will short the contacts in a horizontal position. Another disadvantage is the power required to activate the coil. The low voltage controllers are not capable of activating a mercury relay directly. Usually the controller will activate a dry electro-mechanical relay which in turn will activate the mercury relay. Now, how does this help if a dry electro-mechanical relay is required to activate a mercury relay? By far, the majority of the current is passed through the contacts of the mercury relay and only a very small amount of current passes through the dry relay. The amount of current through the dry contacts is not enough to produce the destructive arc. This in effect increases the amount of cycles the dry relay can perform before failure, actually to the quality level of the mercury relay.
If the kiln manufacturer is to continue to use dry electro-mechanical relays, there are other strategies than can be followed to protect from an over fire situation. The primary strategy is to limit the amount of power any one relay controls to an amount that will not let the kiln over fire. If there are multiple relays in the kiln, and no one relay controls the majority of the power, it would take multiple relay failures to cause an over fire. Multiple relay failures during a single firing is highly improbable and unlikely. The majority of kilns will fit into this condition.
Certain kilns, especially those with only one element and one relay, require a different strategy. Paragon Industries has been exploring the concept to use a controller with two outputs. One output is to control the temperature of the kiln, and the other output (a safety output) will be turned off if a certain, programmed, temperature is exceeded. These two outputs control relays that are in series with each other. Unfortunately, these two outputs are not enough as the safety relay could be in the shorted condition and the operator will not know it. A visual indicator will have to be added to warn the operator of a defective condition in the safety relay. Another strategy requires the use of a redundant or additional controller in series with the main controller. The primary duty of the second controller is to monitor for an over temperature condition and remove the power from the kiln if the over temperature is seen. This second controller must be latched in a safe mode such that it does not reapply power once the kiln cools down. While this second controller will solve the problem, the total cost of the control system is doubled. One novel approach is to use a kiln sitter as the second controller. While the initial expense is reduced, the recurring maintenance with cones will be required. Paragon Industries continues to explore methods to prevent over fire. As electrical control devices become available in the market, we constantly evaluate their suitability to control a kiln. We also strive to meet our customer kiln requirements and maintain fiscal restraint. We want to present the best performance for the minimum cost.
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Reprinted from the July/August, 2010 issue of Glass Art Magazine
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