Kiln Connection / Columns

KILN CONNECTION: Breaking the Rules

Now—more than ever—when selecting a kiln firing system, look at the design very critically before the price.

Lately, I have been visiting companies that are “on the edge” financially, due to a variety of reasons—foreign competition, poor yields and process practice, or bad equipment. These companies always have one thing in common: kilns that are liabilities rather than assets. These kilns are owned by large and small companies alike; while the larger companies can absorb the losses inflicted by poor firing equipment, the smaller companies often cannot recover.

Kiln designs from around the world are fairly diverse, and consequently, they have varying levels of performance and upkeep. Sometimes, poorly performing kilns can be dramatically improved with modest capital expenditure; in other cases, the cost of repairing a kiln properly and fixing the design deficiencies can be higher than the cost of replacing it with a new kiln.

Following are a few case histories to demonstrate what can happen when poorly designed systems are purchased, usually at a low first cost.

Case 1

In the 1990s, one foreign kiln manufacturer enjoyed remarkable success selling tunnel kilns worldwide. Unfortunately, the company’s designs and execution were not nearly as good as its salesmanship. Some attributes of its designs were innovative, but most of its design concepts were just plain terrible. I have worked on a large number of these kilns and can say without qualification that they have cost their owners millions of dollars in lost ware, downtime and maintenance.

Ceramic companies that purchased these kilns, at low prices, have found that their continued maintenance amounts to incredible expense—as much as 10% of the original capital cost. They have found that many of the most basic elements of the kiln must be repaired or renewed frequently. Some problems include:

• The kiln rails wear so severely that they must be replaced every three years. Comparatively speaking, the life of kiln rails should be greater than the life of the kiln.

• Very poor kiln car design, with no overlapping seals between cars, results in heat leakage, air infiltration and high under-car temperatures. These faults lead to overheated wheel bearings, increased fuel consumption and air infiltration defects.

• The combustion systems and burners are unstable and inaccurate. Venturi blocks located in front of each burner overheat, slump and then cause kiln wall melting when the burner combustion gases cannot escape into the kiln.

• A remarkable variety of kiln wall refractory failures guarantee that sections of the kiln will have to be re-bricked every few years.

• Pushing systems are overly complicated, inaccurate and interrupt the pushing sequence several times daily.

Repairing a kiln such as this requires an enormous investment in the kiln’s critical systems—car movement, refractories, combustion and air handling, and kiln cars. The low first price would not have been justified even if it had been zero!

Case 2

In other plants, I have seen periodic kilns with a number of problems, including:

• No sandseals, which means that there will be plenty of air infiltration to leak into the kiln, raise fuel consumption and reduce temperature uniformity.

• Updraft fluing directly into a collection hood, assuring that there will be dirt losses from particles dropping into the flue, along with plenty of radiation losses to increase fuel consumption.

• A single control regulator for all eight burners, which causes poor burner turndown and extremely fast kiln heating in the beginning of the cycle.

• Burners stacked four high and firing in a narrow firelane. This insures that the high-velocity burners are restricted in traveling the length of the firing lane, and minimal recirculation of internal gases causes terrible temperature uniformity.

• A single zone of control in a kiln with numerous burners. While the average temperature in the kiln will match the desired setpoint, the range of temperatures in the kiln makes it unsuitable for firing products that must have uniform properties.

So, what’s the point? Now—more than ever—when you select a kiln firing system, look at the design very critically before you even look at the price page. Remember that first cost has little to do with the total cost of ownership. If you want to make premium quality products, realize that it won’t happen if you use the cheapest equipment to process them. In other words, “beauty is only skin deep, but ugly goes right to the bone.” Never, ever, handicap your facility with low-cost, poorly designed equipment—unless you enjoy seeing your company struggle!

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