Kiln Jam Detector – Gypsum Board Application

Introduction – Model KJD-7000

For as long as continuous gypsum board dryers have existed, one of the most serious potential problems is a kiln jam (board jam).

Blockages most frequently occur when a warped board-end is caught by the high-velocity airstream lifting and skewing a board into the path of other boards. Paper blows may also lead to kiln jams. A less common but nevertheless real cause occurs if the board is not fully “Hard” or hydrated, resulting in board sagging between rollers.
Regardless of the cause of blockage, the costs can be significant due to:

  • Product loss inside dryer
  • Lost production during clean-up
  • Kiln damage resulting directly from kiln-jam and indirectly during clean-up
  • SAFETY – a frantic effort to clear a blockage and resume production as soon as possible may result in injuries to production personnel.

All the above costs are reduced significantly with the rapid indication of the blockage

Types of Jam Detectors

Mechanical: The first type of kiln jam detector was a mechanical design. Board movement lifted a lever which connected via push rods to actuate switches located on top of the dryer. If board flow ceased due to an upstream blockage, the switch would open, triggering an alarm after a set period. This type of system was not very reliable and users familiar with it have sometimes stated that more jams were caused by these mechanisms than were prevented.

Photo Detector: Non-contact photo detectors, often incorporating fiber optics, replaced many mechanical systems. In recent years, siloxane has largely replaced wax, as an ingredient in water-resistant boards. A down-side of siloxane is the deposit it leaves on everything inside the gypsum kiln. This deposit seriously impairs the performance of optical sensors.

Radio Frequency: RF sensing has been successfully used for more than half a century for the in-kiln measurement of board moisture. Sensortech Systems, the World leader in this type of measurement has long understood the potential for this technology to detect the presence or absence of product without board contact. The relatively high cost of incorporating these sensors under every board-stream in, perhaps, multiple zones of a dryer was thought to have been cost-prohibitive. The gypsum board industry has, nonetheless, persisted in its demand for such a system to detect kiln jams.

In response, Sensortech has now developed a significantly lower-cost sensor making such a system possible.

RF In-Kiln Product Detector

As previously mentioned, the well-proven ST-2200 and, more recently, ST-3300 in-kiln moisture transmitters were considered too expensive to incorporate in large quantity throughout a gypsum board dryer. Most of the sensor cost is due to the use of custom manufactured high temperature coaxial cables of which a total of three are used both for measurement and internal referencing. The sensing of presence or absence of a board is a much less demanding requirement than that for detecting fractions of percent moisture changes in a board.

Sensortech has designed a custom high temperature cable capable of withstanding temperatures exceeding 375°C (700°F).

The standard cable length is projected to be 5m (16-foot), sufficient to reach across a triple-wide dryer deck and exit the dryer on each deck via a short rigid conduit to a small junction box containing the sensor electronics. The sensor itself had to be built in the most economic form, to be able to withstand temperatures exceeding 325°C (650°F) and rugged enough to survive a tough environment. The sensor leverages much of its design from the open-frame planar sensors used for moisture measurement.

Kiln-Jam Sensor


The outer frame is formed from 16ga stainless steel; ceramic insulators isolate a stainless-steel center electrode to which the high temperature cable is attached.

The electronics unit is located outside the dryer at each deck level. An on-board 32-bit ARM-Based micro-controller processes signals from up to 3 sensors and networks via RS-485 to a dedicated PLC. A user-friendly program sets operating parameters.

Alarm Generation

The system software will trigger an alarm, based on either of the following conditions:

  1. No product over sensor for a pre-defined time interval – blockage upstream.
  2. Failure to detect gaps between boards over a pre-defined time – blockage downstream.

In the event of start-up, changeovers and shut-down, any false alarms will be explained by board tracking software and resets automatically sent to the jam detector from process supervisory software.

The second case is slightly less easy. Any small break between boards, less than 2.5mm (1/10-inch) will be sufficient to create a disturbance in the sensor output frequency sufficient to detect. If boards are even butted together, the slightest end warp will still produce an adequate disturbance. User software allows the nominal time between gaps to be programmed and the number of missed disturbances (gaps) required to generate an alarm.

Any small break between boards, less than 1mm (1/25-inch) will be detected. If boards are even butted together, the slightest end warp will still produce an adequate signal. User software allows the nominal time between gaps to be programmed and the number of missed gaps required to generate an alarm.

All sensors in an array are node addressable on an RS-485 network. If an anomaly occurs at any sensor, the exact position will be known including which deck and its position on the deck (left, right, center). As the blockage grows, additional sensors on adjacent decks give alarm indication.

Sensor Array Location

Every plant is unique and where kiln jams occur will vary accordingly. Our own experience together with input from many of our gypsum board client companies would suggest a common area for blockage occurs at zone#1 inlet nozzle where airflow is counter to board travel. Since all inlet nozzles have potential for board disturbance, the closeness of zones #1 & #2 inlet nozzles would indicate an economic justification for placing a single sensor array following zone #2 inlet. If zone #1 is the most troublesome region and the fastest response is required, then the array could be located in the section between inlet manifolds #1 and #2. This region is likely to provide easiest access for cabling to the sensors.

Generally, a sensor array should be located slightly downstream of any area known or expected to be a candidate for blockages. The sensors will alarm upstream or downstream of a blockage. The preference for a downstream location is to protect the sensors when clearing a blockage. The sensors are more likely to be damaged while clearing board directly on top of them.

Return on Investment (ROI)

The introduction to this paper outlines some of the more significant costs associated with a kiln-jam.

  • SAFETY – No price is generally too high to achieve a safe working environment.
  • Product loss inside dryer.
  • Lost production during clean-up.
  • Kiln damage resulting directly from kiln-jam and indirectly during clean-up.

A reliable kiln jam detector will reduce the size of dryer obstruction. First and foremost, among the subsequent benefits is the lowered likelihood of injury to workers clearing the blockage. It is very difficult to put a price on the benefit of improved safety.

Gypsum board dryers have evolved over the past 100-years to the degree that today’s latest kilns would be beyond the imagination of the earliest pioneers in this industry. 25 years ago, a dryer was typically 2-boards wide with 8-decks resulting in a kiln speed 1/16th of the line speed. This provided sufficient drying time for typical line speeds of 60 – 100 m/min (200 – 300fpm). An ever-increasing number of dryer decks and parallel board streams are today employed, providing adequate drying time with line speeds exceeding 200m/min (650fpm). A side-effect of the increased number of board streams is a proportional increase in total kiln capacity and subsequent product loss when the board line is stopped to clear a kiln-jam.

Without a kiln jam detector, a blockage may be detected by other process anomalies such as erratic zone temperature control and possibly abnormal sounds emanating from the dryer. If no other warning is noted, the ultimate indicator will be a lack of board exiting the dryer on one or more decks. At this point the damage could be grave. The board line must first be stopped, and the dryer switched off. Some time is required prior to and opening kiln doors. Once access to the blockage is made, frantic product clearing ensues with the use of heavy bars, boat-hook type tools and heavy gloves. Little care is exercised with surroundings and collateral damage and injury may result.

In extreme cases, damage to drive chain, rollers and air nozzles may occur resulting in even longer down-times and further lost production.

The key to minimizing losses is the rapid detection of a kiln jam condition. An early indicator allows the operator to stop feeding board into the dryer and to run out all the good board inside the dryer.

Historical plant data will indicate the cost of kiln jams. Plants producing lighter boards are likely to experience a higher frequency of jams. The trend toward lighter weight board products, innovative fillers and heavier papers increases kiln jam occurrences.

The price of a kiln jam system is proportional to the number of board streams and subsequent number of sensors. Thus, a triple-wide 16-deck dryer (48 streams) would be more expensive to equip than a double-wide 10-deck dryer (20 streams) but loss of an entire kiln full of board would be proportional also. It is felt that the cost of a single sensor array may be less than the cost of a single major kiln jam resulting in the loss of kiln contents together with an 8-hour loss of production.

Additional Potential Measurements

As the name implies, the primary purpose of the kiln jam detector is to provide early warning of kiln jams with subsequent reduction in magnitude of blockage and all associated losses. There are other potential uses, however, which may be implemented with additional software.

  • Paper Blow Detector – A significant paper blow (>12-inch width) will create a signal in the same way as a gap between boards. What identifies it as a blow is the unexpected interval of the condition relative to an anticipated board end. This may provide time to make process changes before blows lead to a board jam.
  • Gap Indicator – A numeric value proportional to board end gaps may be output and used for better gap control.
  • Board Presence – Modern board lines track board position with good accuracy. The kiln jam detector senses the presence or absence of product and may complement board line supervisory control software to verify board position within fractions of an inch.
  • There may be other possible uses for the sensor. Sensortech welcomes customer feedback to improve the product.