Knowledge Brief

How is temperature measurement performed in a brick kiln?

Measurement of temperatures in a brick kiln, such as the temperatures of gases in the flue ducts and the chimney, kiln outer walls and final products, are necessary to locate undesired heat losses in the kiln. Reducing undesired heat losses in the kiln reduces consumption and wastage of solid fuel, improves the quantity of good quality bricks, and increases revenue. Measurement of temperature of bricks in the firing zone is necessary for proper controlling and monitoring of kiln operation.

What are the basic instruments required to measure temperature in a kiln?

Temperatures in a kiln can be measured by two types of thermometers:

1. Contact-type thermometers
2. Non-contact type thermometers.

1. Contact-type thermometers

These thermometers measure temperature by coming in direct contact with the heat source. A thermocouple (with portable indicator) is the most common ‘contact-type’ device used to measure gas temperature in a brick kiln since it is relatively inexpensive and easy to use.

2. Non-contact type thermometers

These thermometers measure temperature without coming in direct contact with the heat source. An infrared detector is a ‘non-contact type’, hand-held, lightweight and easy to use thermometer which is used to measure the temperature of hot, hazardous and hard-to-reach surfaces in the kiln. They have faster response time and high accuracy. This instrument can be used only to measure the temperature of a surface.

What is the proper way to measure temperature by a thermocouple?

While measuring the surface temperature, the junction or tip of the thermocouple probe must touch the hot surface adequately and uniformly to provide good thermal contact with the surface. This ensures lower measurement error in determining the actual surface temperature.

While measuring hot gas/ air temperature, the thermocouple must be positioned in such a way that the hot gas/ air flows over the tip of the probe. It should be made sure that the thermocouple is in contact with the hot gas/ air till the temperature reading on the indicator is stable. This ensures lower measurement error in determining the temperature.

What is the proper way to measure temperature by an infrared (IR) thermometer?

The accuracy of the handheld IR thermometer is primarily determined by the distance-to-spot ratio (D/S Ratio). This ratio is the size of the area being evaluated by the IR thermometer as it relates to distance. In other words, the area being measured becomes larger as the distance increases. The smaller the target, the closer you should be to it. This ratio will have a significant impact on the accuracy or precision of the reading.

If the target you are measuring is a feed-hole cover which is six inches in size, and your handheld infrared thermometer has a D/S ratio of fifty to one (as shown in the figure), then the maximum distance at which you can reliably measure the temperature of the feel hole cover is 300 inches (50 x 6 = 300). Beyond this distance the temperature measurements will not be accurate.

The table below provides maximum distances at which the temperature of various kiln parts can be reliably measured by using an IR thermometer of 50:1 D/S ratio–

How to measure brick temperature in the firing zone of a kiln?

Measurement of temperature of bricks in the firing zone is necessary for proper controlling and monitoring of kiln operation. A Type K thermocouple is used to measure temperature in the firing zone. This thermocouple is commonly used in brick kilns as it can measure temperatures up to 1400°C which is suitable for brick kiln applications.

The thermocouple probe of sufficient length is inserted deep into the firing zone from the fuel feeding holes in the kiln roof so that it touches the hot brick surface to be measured. The indicator attached at the other end will give the temperature of the hot surface.

The various steps to be kept in mind while measuring temperature of bricks are as follows –

1. Temperature of bricks should be measured at the center of trench width, for all six chambers in the firing zone.
2. Temperature should be measured at three depths- top, middle and bottom, at all six measuring points.
3. The temperature of the most recently opened chamber (chamber-6) governs the start of coal feeding in that chamber. For e.g., if the temperature of the sixth chamber is more than 500-650 deg C, only then feeding of coal should be started and at temperatures below, biomass should be fed.
4. The temperature of the 2^nd, 3^rd & 4^th chamber is the highest across all chambers. This temperature varies from 950 to 1050 deg C. A periodic temperature monitoring can be used to a good indicator to assess the product quality.
5. The temperature of the chambers should be monitored every, twice a day and at a fixed time. For e.g. every day at 7AM and at 7PM.

How to measure temperature of flue gases in a kiln?

While measuring gas temperature in flue ducts, temperature gauges are installed on the shunts. These gauges provide the temperature of flue gas entering the central ducts from the side ducts. In an FCBTK and NDZK, the temperature of flue gas and the height of the chimney determine the draught available to operate the kiln.

For FCBTK and NDZKs, the gas temperature in flue ducts should be around 80 to 150 deg C to create sufficient draught to operate the kiln; temperatures higher than 150 deg C, results in undesired heat losses via flue gases. The commonly used gauges are of the type mercury filled temperature gauges and bimetallic temperature gauges.

How to measure surface temperatures in a kiln?

Surface temperature in a kiln can be measured using infrared thermometer by focusing the ‘red spot’ on the hot surface. The indicator screen at the front will display the temperature of the surface. Front lens of the thermometer must be kept clean and the body must be kept cool to ensure minimum measurement error.