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Unfortunately, as we’ve seen in California, an uninterrupted power supply is not always guaranteed. However, there are several strategies that you can implement to protect your kilns from power problems.
Types of Power ProblemsFor the purposes of this article, we will only consider two types of power outages: momentary and extended.
Momentary outages are events that last less than 15 seconds. The sources for these power “blips” vary from the automatic resetting of power transmission equipment during a lightning storm to a maintenance technician throwing the wrong breaker in the substation. While these momentary delays are short, their effects can be as damaging as a longer delay. Even a short delay can cause the entire kiln control system to shut down. If an operator does not notice the problem and restart the kiln, this momentary shutdown can turn into an extended shutdown.
Extended outages are events that last more than 15 minutes. These events are usually brought on by a failure in the power transmission system to the factory, but in recent months, extended outages have also been brought on by rolling blackouts because of insufficient power supplies. During an extended outage, the kiln can easily lose several hundred degrees in temperature.
Both of these types of power problems can occur with little or no warning. Even with the “planned” rolling blackouts that have been occurring in California, the utilities give notice only about five to 10 minutes before the outages begin. On a kiln with a 24-hour cycle, little can be done to prepare for an hour-long shutdown with five or 10 minutes’ notice.
Strategies for Momentary OutagesThe best approach to protecting your kiln depends on the type of outage that you expect to have and the power demands of your kiln. Keeping a gas-fired shuttle kiln running through a momentary outage is a very different problem than keeping an electric-heated pusher kiln going through an extended outage.
For gas-fired kilns, the first priority is to protect the kiln’s safety circuit. The NFPA (formerly the National Fire Protection Association) and other governing bodies require all kilns built after 1989 to have a safety circuit that monitors the system for safe operation variables. This safety circuit monitors the incoming gas pressure, combustion air pressure and kiln temperature to be sure all of these are within their “safe” limits. Additional safety checks may be tied into other fans on the kiln if they are present. When this safety circuit is complete, the main gas valves are opened. If this safety circuit is turned off, the kiln must be allowed to purge for three complete air changes before the gas valve can be reopened.
If momentary outages are the primary concern, the kiln can be protected to “ride through” these outages. On most gas-fired kilns, the largest demand is the power for the blowers and fans. The fans have a large amount of energy stored in the inertia of their rotation. If the power to a fan is shut off for a few seconds, it will not stop its rotation instantly but will instead gradually wind down. In many cases, the fan will still produce enough pressure to maintain its pressure switch. Thus, if power is maintained to the safety circuit, the kiln can be kept in a condition where it is ready to run the instant that the power comes back on again.
If the burners of a gas-fired kiln are equipped with flame safety, another high priority is the flame relays. A flame relay is a small piece of electronics that manages the ignition and monitors the condition of the flame at the burners. A sensor installed on each burner with flame safety constantly detects if a flame is present on the burner. If the flame is not present, the flame relay shuts off the gas supply for the burner or burners it monitors. Most flame safety systems need to be manually reset. Adding the flame relays to the backup circuit will keep the burners lit as long as the remainder of the safety circuit stays enabled.
For electric kilns, fewer items require protection. Electric kilns tend to have less airflow than gas-fired kilns, so they tend to lose heat more slowly. Electric kilns also have another advantage in that they usually do not need to be purged. Generally, the only elements that need to be protected in an electric kiln are the instruments and any electrically controlled components for maintaining atmosphere.
Protecting against momentary outages is usually done using an uninterruptible power supply (UPS). UPS manufacturers sell their products based on their VA ratings—the measure of the power consumed by a system. In single-phase systems, VA is equivalent to watts.
For example, consider a kiln that has a 110V, single phase control circuit. The system control power breaker is 5A, the safety circuit breaker is 5A, the flame relays take 10A, and the starter for the combustion blower has a 5A breaker. A total of 25 amps therefore need to be protected on this kiln. To find VA, multiply the amps times the volts. In this example, the VA rating would be 2750 (written as 2750VA). To protect this circuit, you would need a 3 kVA UPS.
Most modern UPS systems use a battery to back up their power supply. Incoming power is converted to DC and is used to charge a battery. An electronic circuit monitors the power coming into the UPS, and if it drops outside of certain preset limits, the UPS will supply power from its battery bank. The DC energy stored in the batteries must be converted back to AC for the equipment. This is done through an inverter, which uses a series of electronic switches to reconstruct single-phase or three-phase AC from the DC signal.
One downside to the battery backup UPS are the batteries. There is no reliable way to tell if a battery is good other than connecting a load to the battery. If you suspect that you have a bad car battery, for instance, the battery can be tested with a device containing an array of heaters. The same is true with the batteries on a UPS—they must be tested periodically with a load equal to the load they back up. However, even with rigorous testing, there is still no guarantee that the batteries will support the load when called upon.
Another downside to battery backup is the potential environmental hazard associated with storing the batteries. For high-power UPS applications, isolating the batteries is not a trivial task. The batteries must be isolated in a vented room with air filtration. The size of the battery array for a high-power application is also significant, and protection from acid spills and lead contamination must be provided.
UPS systems that use other methods, such as a flywheel, to store their energy are currently being developed. While such systems are not yet commercially available, they promise to eliminate the potential battery problems that exist with current UPS systems. The systems are more compact than the battery-based systems, and the amount of stored energy is quantifiable by other means, such as measuring the rotation speed of the flywheel.
Strategies for Extended OutagesProtecting your kiln from an extended outage begins by protecting your kiln from a momentary outage, then adding a generator to supply all the power to the system. The UPS is still needed because most generators need 15-30 seconds before they are ready to produce full power.
The generator-based system adds two additional components: the generator and an automatic transfer switch. The automatic transfer switch functions like the electronics in the UPS. Inside the switch are components that monitor the incoming voltage. If the voltage drops out of the preset specification, the transfer switch sends a signal to start the generator. Once the generator is up to speed, the power supply is automatically switched to the generator. When the transfer switch detects that line power has returned, it switches the system back and shuts down the generator.
Modern generators are available that run on nearly every fossil fuel, including natural gas. It is therefore very convenient to power the generator using the existing natural gas lines in the factory. No additional fuel supply or storage is needed.
Sizing a full backup system is more difficult than sizing a UPS since most kilns have a three-phase power supply. The inductive load from the motors on the system adds further complexity to the design, as capacitors may be required on the system to correct its power factor. It is best to consult a qualified electrical engineer when attempting a project such as this.
An alternative strategy that is possible with today’s powerful computers is modeling the sintering reaction as it takes place in the ceramic body. A detailed profile of the kiln is made, and a mathematical model of the product as it passes through the kiln is kept. The temperature within the product is predicted, and from this, the degree that the product has sintered can be measured.
With this kind of model, the temperature of the product is continuously calculated during a power failure, and the degree of sintering is still tracked. When power is returned to the kiln, the heating profile and the push rate can be varied to complete the sintering that was started. Systems with this kind of functionality have been available for the metals industry for a several years.
The Changing U.S. Power IndustryIn the past, power generation and transmission have been the sole domain of publicly regulated utilities. That is now changing. To date, 24 states have passed legislation deregulating their utilities, and another 18 are actively considering deregulation.
To be eligible for the competitive benefits of deregulation, power utilities must choose one of three roles: They can generate power, transmit power or distribute power. The “power company” that sends your monthly statement is your distribution company. To keep your business, they compete with the other distribution companies to provide you with power at the lowest cost. They are constantly negotiating with generating companies to purchase the least expensive power, and have it delivered to you in the most economical way.
One thing that the deregulated utilities promise is “premium power” contracts for their consumers willing to pay for this service. In exchange for higher power rates, utilities promise 100% uptime for these customers. This will be done by first maintaining these customers’ power at the expense of other customers’ power. Additional strategies include providing the customer with a generation plant right on their property to fill in when the power grid is down.
One side effect of deregulation is that since the power company no longer controls all aspects of generating power, customers can more easily generate their own power, in effect becoming their own generating company. Excess power can be sold back to the power company at whatever rate they are able to negotiate.
For the ceramic manufacturer, this change could be significant since a large amount of excess heat is available in most facilities. This heat could be used to generate electricity.
Analysts expect a proliferation of “distributed generation” facilities to begin emerging in upcoming years. Small natural gas turbine facilities will be the first to emerge, followed by fuel cell, solar and wind farms. Keeping the generator close to the end user makes economic sense, since eliminating most of the transmission lines reduces the system maintenance costs and eliminates resistance losses in the transmission lines.
Minimizing Power ProblemsOperating a plant in the U.S. has given most companies the advantage of inexpensive, dependable, readily available electricity. However, power supplies aren’t always guaranteed. Momentary outages still exist due to natural disasters and other situations, and for the residents of California, extended outages from the regulatory climate have become more commonplace. Time will tell if more outages such as this are in store for other areas of the U.S.
Analyzing the power problems your plant has experienced in the past and is likely to see in the future will give you an idea of the protection method needed for your type of power problems. With the right preparation, you can keep the effects of power problems to a minimum.
For More InformationFor more information about kiln operation issues, contact EISENMANN Corp., 150 E. Dartmoor Dr., Crystal Lake, IL 60014; (815) 455-4100; fax (815) 455-1018; chris.pilko@EISENMANN.com; or visit www.EISENMANN.com.
SIDEBAR: Momentary Outage ProtectionFollowing is the bare minimum that you must protect on a kiln to prevent damage from a momentary outage:
- The safety circuit
- The gas train valves
- The flame relays and valves
- The starters for any fans and blowers
- Any circuits that supply power to the fans and blowers
- The chart recorder and temperature controllers