TRIBHUVAN UNIVERSITY

INSTITUTE OF ENGINEERING

A

Report

On

Temperature

Measurement in Vertical Brick Kiln

SUBMITTED BY: - SUBMITTED TO:- Shankar Singh Dhami Dr. Riddhi Ratna Sthapit (066MSR516) Department of Mechanical Engineering

November 20, 2010

Experiment 1: Temperature measurement of Vertical Brick Kiln

Objective To measure the temperature of the Vertical Brick Kiln

Condition for Experiment Periodic steady State condition Measurement covering one full cycle Insert the probe to the place where the temperature is measured Flue gas temperature, brick temperature, Ground temperature are measured Measure the temperature of Brick at the firing zone, heating and cooling zone The temperature is around the (500-1100)oC

Planning of Experiment and Experimental Setup

Care taken not to introduce any intermediate metal: Extension cables and connectors of

chromel/alumel. Extension cable insulation: polyester film, fiberglass braid with varnish impregnation

(protection from moisture). After connecting thermocouple assemblies with extension cable, checked for reverse

polarity at the thermocouple – extension wire junction. Temperature scanner: Resolution = 1oC; Accuracy = ±2oC; cold junction compensation

using electronic circuit. (Frequency of temp. measurement = 900 s) > (Time constant for SSring = 359 s for

h=10W/m2K) Temperatures manually recorded; frequency of measurement 15/ 30/60 min.

The brick firing process consists essentially of increasing the temperature of bricks progressively over a period of time, holding it at a peak temperature (at about 800-1100 º C), and then cooling back to the ambient temperature. Over the years brick kilns have basically evolved from rudimentary "intermittent" kilns to more complex energy-efficient “continuous " kilns. In intermit-tent kilns, bricks are fi red in batches. Generally, bricks and fuel are stacked in layers and the entire batch is fired at once; the fire is allowed to die out and the bricks allowed to cool after they have been fired. In a continuous kiln, on the other hand, the fire is always burning and bricks are being warmed, fired and cooled simultaneously in different parts of the kiln. The heat in the flue gases is utilized for heating and drying of green bricks and the heat in the fired bricks is used for preheating air for combustion.

Selection of instrument The temperature measurement instruments that are used in the brick kiln are

Thermocouple: Pyrometer Temperature Plugs Thermister Resistance temperature detectors (RTD) Fine wire resistance thermometer Spectroscopy Hot wire Anemometry (HWA) Infrared Thermometer

1. Thermocouple: Definition: A thermocouple is a junction between two different metals that produces a voltage

related to a temperature difference. In 1821, the German–Estonian physicist Thomas Johann Seebeck discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect.

Principle of operation: Any attempt to measure the generated voltage necessarily involves connecting a conductor to the "hot" end. Another additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement. That difference increases with temperature, and is between 1 and 70µV/°C for standard metal combinations

2. Pyrometer Definition: A pyrometer is a non-contacting device that

intercepts and measures thermal radiation, a process known as pyrometry. Modern pyrometers became available when the first disappearing filament pyrometer was built by L. Holborn and F. Kurlbaum in 1901. This device superimposed a thin, heated filament over the object to be measured and relied on the operator’s eye to detect when the filament vanished. The object temperature was then read from a scale on the pyrometer.

Principle of operation: A pyrometer has an optical system and detector. The optical system focuses the thermal radiation onto the detector. The output signal of the detector (Temperature T) is related to the thermal radiation or irradiance j* of the target object through the Stefan–Boltzmann law, the constant of proportionality σ, called the Stefan-Boltzmann constant and the emissivity ε of the object.

This output is used to infer the object's temperature. Thus, there is no need for direct contact between the pyrometer and the object, as there is with thermocouple and Resistance temperature detector (RTDs).The temperature inside a brick furnace is too high (appprox 1100 °C). In order to measure this temperature using thermocouples, RTDs by an operator could be lethal so using pyrometer will be safe.

Brick temperature inside the surface may be measured using a thin film thermocouple. The

thermocouple consists of a plug located inside the temperature location being measured. A

coated nickel wire is passed through the central axis of the plug. The coating on the wire is

nickel oxide which insulates the wire from the main body of the plug. The nickel wire is then

connected to the plug by a 0.5 micron layer of nickel coating on the exposed surface of the plug.

The wires from thermocouples located on the piston are conveyed out of the engine using a

linking mechanism.

3. Templugs Templugs are small screwed plugs made from special alloy steels. These may be used to find steady state temperatures of components within the engine. As the templugs are exposed to elevated engine temperatures over a period of time the special steel alloys will change in

hardness, the hardness remains after the plug has cooled. Since the temperature-hardness characteristics of the templugs are already known, the temperature can be derived by measuring the hardness. The templugs are screwed into the component whose temperature is being measured and left for a fixed period of time under steady engine running conditions. The temperature is worked out from measuring the hardness at point of contact between the plug and component. A sufficient number of plugs may be used to calculate the heat flow through the component using a computer by modeling the isothermals. Another method of analyzing the heat flow is by using the electrical analogue method.

4. Thermisters Thermistors are made from various nonmetallic conductors (i.e. metal oxides). The types of

thermistors found in a vehicle exhaust environment will typically produce a negative

temperature coefficient (NTC), meaning the resistance will decrease with increasing

temperature. Thermistors offer a high sensitivity over a smaller range in temperature than

either thermocouples or RTDs. At 0 oC the resistance can be over 100,000 Ω, at 200 oC 200 to

500 Ω, and at 800 oC 50 Ω. Thus, thermistors can achieve very high sensitivities over a

particular range of temperatures. However, achieving nearly the same accuracy over a large

range in temperatures is not possible (unless several pull up resistors are used) due to the

highly nonlinear characteristic response. Thermistors can be made very small for quick

response. However, they are not able to withstand even mild vehicle exhaust environments

without being protected by a metal or ceramic insulated sheath thus causing the sensor

response to be relatively slow.

Tolerance of a thermistor depends on its intended range of use. Thermistor tolerances in

manifold air temperature sensors (MAT) or coolant sensors are very tight over the relatively

narrow range of measurement (ex. 0.6 oC from 0 o to 100 oC). However in a vehicle exhaust that

can vary between -40 oC and 1000 oC, thermistors have a fairly poor tolerance depending on

the temperature range (2% to > 6% of temperature). As previously mentioned, thermistors

typically have a very high resistance below 100 oC. This makes it difficult to meet requirements

of being able to read the sensor at -40 oC, or being able to perform OBD II start up diagnostics at

20 oC.

5. Fine Wire Resistance Thermometer The fine wire resistance thermometer may be used to measure temperatures within the exhaust system. The wire is made from tungsten, typically in the region of 6.9 microns diameter and about 10mm long. Generally the smaller the size of the wire the faster the response, however the wire must be sufficiently strong to withstand conditions in the exhaust flow. To preserve the life of the wire, some probes are designed to enable the wire to be retracted when not in use. This is particularly helpful in preventing damage from solid particles during startup. The tungsten wire thermometer is connected up to some electronic apparatus making up one element of an a.c bridge and suitably offset to enable measurement of the transient temperature. The picture below shows a typical probe for measuring exhaust temperatures. The tungsten wire is passed through the hollow nickel sleeves and welded into place between the two nickel tips.

6. Spectroscopy Spectroscopy is used to measure the intensity of light emitted at wavelengths in the infra red, ultra violet or visible light spectrum by gases in the combustion chamber of kiln to determine temperature and species concentration. The graphs below shows the wavelength emissions of various species present in the combustion cycle and a typical emissions levels at different wavelengths for a spark ignition and diesel internal combustion engines.

Temperatures can be derived from spectroscopic measurements applying emission/absorption methods according to Planck's or Wiens and Kirchhoff's laws, provided there is local thermal equilibrium in the system. The emissivity and absorption of radiating gases varies strongly with wavelength. Therefore spectroscopic temperature measurements are based on the radiative quantities at specific wavelengths. The temperature of soot can be found because it is almost a blackbody source (the emissivity ~1). The temperature can be found using Planck’s law that relates spectral radiance of a blackbody radiator to the temperature. For flame temperature measurements the temperature of the soot is often assumed to be equivalent to the flame temperature

7. Infra-red spectroscopy For this particular set-up, the firing zone is viewed through a sapphire window and IR-transparent fibre optic cable. Two liquid nitrogen cooled (to eliminate external thermal influences) detector arrays are then used to resolve spectra covering wavelength regions between 1.2 to 3.2 and 2.5 to 5.3 microns. The two channels are connected to amplifiers capable of scanning the spectrum in 1 millisecond which translates to a crank angle of 12 degrees at 2000 r.p.m. The diagram below shows a typical set-up for Infrared spectroscopy.

8. Hot wire anemometry The basis for hot wire anemometry is the heat balance equations which can be applied to the anemometric signal to calculate the flow velocity. Hot wire anemometry experiments use a thin wire stretched between the tips of two prongs. The hot wire is introduced into the combustion chamber using a probe. The anemometric probes consist of a thin wire held across the tips of two prongs (there are a variety of different arrangements), the anemometric probes may also be operated as thermometric sensors. The probes consist of prongs insulated from the exterior by inserting them axially into ceramic rods encased in a steel pipe. The sensing element is a couple of mm long and typically 10 micrometers in diameter, though this is fairly large (poor frequency response) it must be strong enough to survive the particularly harsh environment of the combustion chamber. The criteria for the dimensions of the probe are based on, strength, spatial resolution and the frequency response of the prong and wire combination. The heat exchange between the hot wire and the firing zone environment affect the output voltage of the anemometric signal. For hot wire anemometry it is necessary to measure both the gas temperature and prong tip temperature in addition to the anemometric signal. This is so that during flow measurements the thermal capacity and thermal inertia of the hot wire and prongs are taken into account and may be balanced by a feedback system when processing anemometric data. The feedback system keeps the mean temperature and resistance of the wire constant. Partially shielded probes may used to resolve any directional ambiguity in the signal. The schematic below shows a typical anemometric probe inserted into the combustion bowl of a diesel engine, z, r and s are the axial, radial and tangential velocity components respectively.

9. Infrared The infrared method works on the following principle, a black body source is placed one side of a cylinder, on the opposite side directly in line is an optical filter, lens and infrared detector. The cylinder is fitted with two quartz windows either side, in line with the detector and black body source (see diagram below). The black body temperature may be set using a potentiometer. If the temperature in the cylinder is lower than the black body temperature, there will be a decrease in intensity as some of the black body radiation is absorbed by the gas. As the temperature inside the cylinder reaches the same as the black body temperature, the rate of absorption by the cylinder gas will equal the rate of emission. If the temperature in the cylinder is above the black body temperature then there will be an increase in recorded intensity. The system is calibrated to detect infra red emissions at 2.6 microns, one of the wavelengths for water vapour. The optical filter is designed to absorb all of the unwanted wavelengths.

During the intake stroke there is minimal absorption of infrared by the cylinder gases and so the line AB is recorded on the detector. The intensity level only corresponds to the black body temperature, not the cylinder gas temperature and is recorded by a thermocouple device. The black body temperature is initially set above the intake temperature, so that as the compression stroke starts and the cylinder temperature rises the intensity received will decrease. As the cylinder temperature increases further the intensity will reach a minimum, and then begin to rise as the cylinder temperature approaches the black body temperature. Once cylinder gas temperature has reached the black body temperature (point C on the graph), the crank angle is noted. By taking a series of readings setting different black body temperature values, a temperature crank angle plot may be constructed. The accuracy of this method is based on the assumption that the temperature cycle is the same for each cycle. It also depends upon the condition of the quartz windows and their radiation and absorption qualities.

Selected Thermometer Among the all thermometers the thermocouple thermometer is used to measure the temperature of the brick kiln due to Can be inserted inside the ground Easily and widely available Reasonable chief Data is required for hourly so that it will be in thermal equilibrium Long life by proper use. Can be standardized

Advantages with thermocouples Capable of being used to directly measure temperatures up to 2600 oC. The thermocouple junction may be grounded and brought into direct contact with the

material being measured. Disadvantages with thermocouples Temperature measurement with a thermocouple requires two temperatures be

measured, the junction at the work end (the hot junction) and the junction where wires meet the instrumentation copper wires (cold junction). To avoid error the cold junction temperature is in general compensated in the electronic instruments by measuring the temperature at the terminal block using with a semiconductor, thermistor, or RTD.

Thermocouples operation is relatively complex with potential sources of error. The materials of which thermocouple wires are made are not inert and the thermoelectric voltage developed along the length of the thermocouple wire may be influenced by corrosion etc.

The relationship between the process temperature and the thermocouple signal (millivolt) is not linear.

The calibration of the thermocouple should be carried out while it is in use by comparing it to a nearby comparison thermocouple. If the thermocouple is removed and

placed in a calibration bath, the output integrated over the length is not reproduced exact. Temperature Range of Thermocouples:

Thermocouple Maximum Temperature (oC) Continuous Spot

Copper-Constantan 400 500 Iron-Constantan 850 1,100 Chromel-Constantan 700 1,000 Chromel-Alumel 1,100 1,300 Nicrosil-Nisil 1,250 - Tungsten-Molybdenum* 2,600 2,650

Experimental Setup: 27 thermocouple thermometers (18 for the measurement of Brick Temperature and 9 for the gas) are used. Thermocouple probes coated with the insulating material except the tip. The locations of thermometers are shown in the diagram below.

Observation and Procedures In one pole three thermocouples are kept and inserted into the ground below the surface of furnace. There are all together six poles or sticks attached with the extension wire of thermocouples. The tips of thermocouples are kept at the fixed height from the surface into the pole. The readings of the all thermometers are measured at every ten hour and reported. Table 1: For the Brick Temperature Thermocouples

(oC) Time (hr)

T1

T2

T3

T4

T5

T6

T7

T8

T9

10 20 .. ..

440 450

Table 2: For the Gas Temperature

Thermocouples (oC)

Time (hr)

T10

T11

T12

T13

T14

T15

T16

…..

…..

T26

T27

10

20 …. …..

430 440

Measure the temperature at the top, middle and bottom of the furnace by the selected thermocouples attached to the pole. Interpolate the data for other heights from the surface.

a. Top level (2300mm from Ground, 350 mm from the surface) e.g. T1, T4, T7, b. Middle Level (1400 mm from ground, 1250 mm from the surface) e.g. T2, T5, T8, c. Bottom Level (at the ground, 2650 mm from the surface)e. g. T3, T6, T9,

Variation of Brick Temperature with Time and Ground Temperature with depth:

Plot the graphs between

a) Temperature and time of thermocouples for different level b) Temperature and Depth at different time

Analysis the result and you can calculate the heat loss and transfer by the gases and can improve the energy efficiency of the coals and kilns.

Possible Errors in Measurement: Drift in the Reference Voltage: When using such instruments one must be careful to check to see that the battery supply is within the prescribed limits. It is always a good practice to check the reference voltage output by measuring the temperature of several thermocouples in an ice bath at 0 oC. Fabrication of the Thermocouple: In fabricating the thermocouple one must avoid making two basic mistakes. The first mistake may be in the selection of the gauge (diameter) of the thermocouple wire. The junction may end up contacting the walls of the container and movement of the shelves, e.g. during stoppering may cause the wire to remove closures from some of the vials. Second, one should exercise care in the formation of the thermocouple junction. One can make a junction by simply twisting the two wires together to form a braid. Without Clean Contacts:

It is important the connection at the copper - copper and constantan - constantan junctions are kept clean and free of any oxide or any other form of contamination. This will be particularity troublesome for those systems which require steam sterilization. The presence of such oxides or contamination can alter the “electron work function” of the surface. This can lead to the formation of a second junction at the contact and an error in the thermocouple reading. Contamination of the contacts can lead to an error in the temperature reading as much a 2 oC. The same would be true if the wire used to form the thermocouple sensor junction also becomes corroded or contaminated. It is best that the junction be protected from any direct contact with the formulation where in time corrosion can take place. Effect on the Cake Structure: It has been reported in the literature by several investigators that the metal surface of the junction or the bare wire can act as a nucleation site for ice formation and thus greatly alter both the degree of supercooling and the ice structure in the cake. To eliminate the contact between the thermocouple and the solution use a Teflon protective coating or sheath. Believing the temperature reading: Broken wire or poor contacts are common causes for a thermocouple to read an open circuit. However, should the thermocouple indicate a temperature that is not constant with the other sensors can we believe the temperature reading. The sensor may be misplaced but the temperatures are correct within the given accuracy of the thermocouple. Poor Calibration: The correct calibration of thermocouples can be tedious and difficult. It is always essential thermocouple calibration be done next to another--already calibrated--thermocouple. During the calibration bath, the output is not reproduced exactly as it was, making correct calibration essential. The causes of errors in brief are Poor junction connection

Shunt impedance and galvanic action

Thermal shunting

Noise and leakage currents

Thermocouple specifications

Documentation