'ncinerator Temperature Measurement How, What and Where

J. L. LAUER

Bailey Meter Company New York, New York

Abstract

used and accepted device for measuring temperature

in incinerators. The thermocouple develops a dc sig..

The obj ect of this paper is to describe and discuss

nal propo�tional to temperature changes between the

the various methods currently available for tempera­

junction point of two dissimilar metals and a cold, or

ture measurement, with particular emphasis on those

reference, junction. This signal is then received,

applicable to incinerators. It will also indicate the

amplified, and used for recording, control, or both. A

various special conditions, commonly found in in­

Chromel-Alumel Thermocouple has an effective range

cinerators, which these instruments must meet. The

to 2000F, with the up-per limit approximately 2300 F

scope of this paper will be confined to the temperature

for very short periods of time.

sensors, their characteristics and application require­ ments, such as location and protection, which are

A second type of electrical sensor is the radiation

pyrometer. This instrument detects the level ohadiant

"peculiar to incinerators". How

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I I I I I I I I I I I I I I I I I I

energy emitted from a hot object and, in conjunction

with an electrical receiver for the voltage output, con..

Temperature Meas uring Devices

verts the radiation measurements into temperature

There arc several types of sensors or temperature measuring devices available for use in an incinerator.

measurements. It has an effective range from 1000 F to 3000F, but can be used for temperatures as high as

cinerators fall into two general classifications, the

measure the actual fire temperature in the primary

4000F. This is the only device which could properly

Temperature sensing elements for application in in�

combustion chamber of an incinerator.

electrical type, and the filled-bulb type. The selection of the element to be used for a specific application is

Another general type of electrical sensor is the re­

sistance type temperature detector. This device func ..

usually influenced by the maximum temperature to be

tions by using the principle that the resistance of a

measured, environmental conditions at the meaSl1re� ment location, accuracy, and tile desired temperature

platinum wire varies along a known curve in Rccord·

span. Other considerations which may or may not be

ance with the temperature variation. The current output

speed of response, and linearity of the sensing device.

electrical circuit in a manner such that a voltage

from a resistance temperature detector is used in an

im'portant, depending upon application, include cost,

The first type, under the electrical category of sensors, is the thermocouple. It and its associated

proportional to temperature is produced. The appli­

couples arranged in series to produce a higher output

suring devices includes the filled-bulb system. This

cable range is from -400 F to 1000 F.

The second general category of temperature mea·

forms, such as the thermopile (6 number of thermo­

system consists of a bulb connected through small

signal than a single thermocouple) is the most widely

165

diameter capillary tubing to a pressure sensing cle­

cient rises and the rate with which the heat is trans­

ment such as a bourdon tube, capsule, or bellows.

ferred increases. This is due La the foster rate at

The hourdon tuhe, hecause of its small volume, is

which the flowing medium in direct contact with the

the most widely used pressure sensing clement. The hulh, capillary tuhing, and hourdon tuhe arc filled with a substance wllich expands or contracts

any detecting element properly, avoiding stagnant

change is transmitted through the capillary tuhing to

hrought ahout hy increased velocity, can he fully

element, or protecting well, is removed from the ele� ment. This emphasizes the importance of locating

wpen subjected to a cllange in temperature. This

areas, so that the increased speed of response,

the bourdon tube to produce a motion in the bourdon

used to indicate a more truly representative tempera­

tuhe. The system may he filled with liquid, vapor, gas

ture.

or mercury.

Since the majority of sensi�g clements must be

Filled systems generally offer two types of sensing

inserted in a protecting well, this additional mass

hulhs: the straight hulh, often used with a protecting

and thickness of metal must be considered in the

well, and a coiled hulh. The coiled hulh is designed

over-all response factor. As the wall thickness of

to measure the average temperature in a large pipe or

the well increases, the response time also increases.

duct, and is stretched completely across the duct to

Finally, the infernal time constant, involving the

eliminate stratification problems. However, unusual

time to transfer the heat from the protecting well

conditions of corrosion, erosion, and the deposition of

metal to the sensing element itself, will affect the

material on the hulh will generally preclude the usc ·of

over�aIl response factor. The smaH mass of electric

a _coiled bulb in incinerators. The various types of

detecting units gives them some advantage over the

filled systems are generally suited for a maximum con­

more massive requirements of the filled hulh sys­

tinuous temperature up to 1200F.

tems. Proper transmission of the heat is essential

thermal hulh directly into the measured media hecause

insure sulficient metal to metal contact.

- Quite often it is impossible to immerse the bare

in hoth cases, and evel)' effort should he made to

of high pressure or the corrosive effect of the media

What - The Incineration Process

on the hulh. The latter is most often a factor in in­

cinerator applications. A protecting well must th en be .

A brief description of tile conditions existing in an

used.

incinerator, tracing the gas passage from the primary

Protecting wells, or sockets, are installed in pipe

comhustion chamher to the stack, will he presented

lines or ducts when required to protect the temperature

prior to describing the application of these instruments.

threaded into the pipe. They are availahle in a num­

cinerator, the actual fire temperature is approximately

ing upon the severity of the environment which they

hetween 1800 F and 2000 F. Normally, this temperature

reduces the speed of response of the system, and

operation. If care is not exercised in the mixing of re­

sensing element. The wells may either he welded or

Inside the primary combustion chamber of the in­

her of wall thicknesses and alloy materials, depend­

2400F. The comhustion chamher exit temperature is

must withstand. However, the use of a well greatly

is the one used as the master indication for furnace

should only he used when ahsolutely necessary.

luse in the pits, this temperature can become as high

There are many factors which affect the ahility of

as 2200 or 2300 F in a very short time, due to the igni­

the sensing element to quickly detect temperature

tion of a charge of high Btu refuse. A load of sawdust

changes. The detection of any temperature change hy

in a charge, for example, could increase the normal

a sensing device requires some heat transfer, either

operating temperature of 1800 F to over 2000F in ap­

lrom the medium to the sensing device, or from tile de­

proximately 15 seconds. This effect is similar to light.

vice to the medium. A number of resistances to this

ing off an oil hurner from hot refractory. While this

heat transfer are always present, and should be ana­

temperature fluctuation may be most prevalent in batch feed furnaces, it also exists in continuous feed furnaces. .. . The expreSSIon "Furnace Temperature ..IS commonly used by operators and others to mean combust ion

lyzed as to their ultimate effect on the performance of the temperature measuring device.

The nature of the medium itself plays a part in the

over-all response of the measuring device. Since it

chamber exit temperature. The actual lIame temperature

is usually easier to transfer heat from a liquid to

is very seldom measured. It is evident that when us­

metal than it is from a gas to metal, we would gcn�

ing the phrase "Furnace Temperature", one should

crally find a slower response from a sensing device

always state where the measurement was taken. Other·

in air than from one in water.

wise, the phrase is not definite.

As the velocity of the medium passing over the

After leaving the combustion chamber, the gases enter the secondal)' flues. These are the flues ente ring

sensing erement increases, the heat.transfer coe££i�

166

I I the waste heat boiler, if one is provided,

Of

First, we will consider the actual fire, or flame,

the spray

temperature. As stated previously, the temperature

and fly Rsh removal facilities. In this area, the tem�

range of the actual £ire is of the order of 2400F,

perature ranges from 1400 to 1800 F. II a waste heat

which is above the practical limit of a Chromel-Alumel

boiler is provided, the temperature in the flue between

Thermocouple. The radiation pyrometer is the only

the boiler and the fly ash removal facilities ranges

practical measuring device with which to properly ob­

from 500 to 700 F.

tain this temperature. The sighting tube of the pyro­

The gases leaving the fly ash removal faeilities

meter might best be located in the crown, or roof, of

enter the chimney flue and are discharged out the

stack. The temperature expected in this area is

the furnace, sighting down on the fire. These devices

upstream from the point of measurement.

cooling. The air purge keeps the sighting tube clear,

generally are provided with an air purge, to aid in

usually less than 1000F, depending on the equipment

and relatively cool, but does not allect the tempera­

The environment existing in the gas passage of an

ture reading, since the pyrometer responds to the level

incinerator is a very important consideration when

choosing the temperature sensing element to be used.

of radient energy emitted by the hot target object. The

tains many substances which tend to contaminate the

mize slagging and its elfect upon the reading, and

location in the furnace crown is recommended to mini.

The �nvironment at the combustion chamber exit con�

sensing element. Among these arc slag, which is prim­

this location may also provide the clearest sighting

alumina, with amounts of iron oxide, manganese oxide,

the greatest problem with the crown installation is one

path to the flame. Experience to date has shown that

arily a glass containing major amounts of silica and

of wear, or erosion, due to the abrasive action of fly

lime and titania also present. Slag tends to build up on

ash caught up in the furnace gas turbulence. The tern.

the protecting weHs, mechanicaHy gripping them. When the temperature changes, differential expansion will

perature of the actual fire is seldom measured, how­

point include the abrasive action of fly ash, and the

system required. As the relation between flame

ever, due to the relatively high cost of the sensing

break these wells. Othcr problems existing at this

temperature and refractory life and proper combustion

corrosive and contaminating action of gases containing

becomes better understood, the radiation pyrometer,

metal vapor and other substances produced in the

and the flame temperature measurement may become

furnace.

part of the instrumentation in new plants.

The environment in the secondary flues is 'Very

The next area to be considered is the primary com.

likely to contain slag, while contaminating gases and

bustion chamber. The primary combustion chamber

vapors, as well as fly ash, may also be present. The

temperature is approximately 1800 F, and, in fact, in­

fly ash at this point is most abrasive, since it has

cinerator control systems are generally based upon

cooled somewhat and hardened.

controlling this temperature at a relatively constant

After thc waste heat boiler and fly ash separators,

1800 F. A Chromel-Alumel Thermocouple can be con­

or the spray and fly ash removal facilities, a stucco­

sidered for this measuring sensor, as it has an ef­

like gray coating may exist. This coating may be

fective range between 100F and 2000F, with an upper

deposited on the Induced Draft Fan, the stack, and

limit of approximately 2300 F for short periods of

over any openings. It is very difficult to remove and

time.

in some incinerators, a periodic lye bath is used, in

When considering the use of a thermocouple to

order to prevent excessive build-up. Slag generally does not present a problem at this point. Fly ash

measure primary combustion chamber temperature, the

the fly ash removal facilities, still exists. However,

The Chromcl-Alumel Thermocouple is attacked and

existing environmental conditions play a major role.

abrasion, in proportion to the fly ,ash not removed by since the velocities and concentrations arc usually

con taminated by the gases present in the combustion

siol) from fly ash is not considered a problem at this

Protecting wells presently available, constructed of

�hamber. Therefore, a protecting well is required.

much lower than those previously encountered, abra­ " '.

stainless steel, or other adcquate alloy, all will re­

poi�i: in the gas passage.

sist the gases, but are subject to corrosion from slag.

Where - Application to Incinerators

Hence, a protecting tube, or sheath, is required over

the protecting well. The Silicon-Carbide protecting

The application of the various temperature measur­

ing devices described above to an incinerator plant

tube is recommended as the best available at the

of the area where the measurement is to be made, the

but is porous to gases. Therefore, to effectively with­

present time. This tube will resist slag and fly ash,

requires that consideration be given to the environment

stand the various conditions exis ting at this location,

proper location of the sensor, and the use to be made

the best protection would be afforded by a Silicon-

of the measurement.

/

167

these areas, since the maximum temperature expected

Carbide protecting tube, a stainless steel protecting

I,

well, and the Chromel-Alumel thermocouple element.

is 1000F, a resistance temperature detector or a filled

disintegrate from oxidation. The Chromel wire will

be used in these areas. From the point of view of ob­ taining the correct average temperature in the duct, a

system is recommended. A thermocouple could also

Eventually, the Chromel-Alumel thermocouple will

,

disintegrate first, followed closely by failure of the

coiled-bulb filled system is best. However, serious

Alumel wire. A swaged, magnesia packed, sheathed ! ther mocouple is generalJy recommended to protect the

questions concerning corrosion, erosion and deposi­ tion of matter on the bulb have been raised hy de­

thermocouple from oxidation and extend its service

signers and plant personnel. Recalling that a stucco­

life. A 14-gage thermocouple in a 3/B-in. diameter,

like gray coating, tending to coat all exposed areas

1/32-in. wall stainless steel protecting well has

and openings, exist at this point, it is expected that this huild-up would caUse a gradual lagging of the

rec"ently been introduced in incinerators to take the

place of the widely used bare 8-gage wire couple. It

bulb, with correspondingly lower readings. It would,

is the heaviest thermocouple presently available, a �d

however, be easier to remove this coating from a proa

is expected to extend the service lif e period over the

tecting well, during periodic maintenance, than it

bare B-gage type. To minimize the ellects of chunks

would be from a coiled bulb stretched completely

of slag breaking 011 and dropping on the thermocouple,

aCrOSs the duct.

a vcrtic . al installation in the furnace roof, Of crown, is

the recommended location. It is recognized that stratification is a major prob�

The resistance temperature detector would offer

greater accuracy over the thermocouple at a higher

lem in the location of a temperature sensor. The opti� mum solution is to take a temperature traverse, under

cost. If this measurement were to be used in a control

system to provide low temperatures to protect the

various load conditions, to determine a correct, rep­

Induced Draft Fan, this detector could be economi­

resentative location. The installation of the tempera­

cally justified. If the measurement is only of secon­

tUre transmitting device may be made, but the location of the thermocouple should be withheld until a traverse

dary interest, a thermocouple would suffice.

is made. This is expensive, but may be justified in

i

large plants where refractory maintenance, due to

A stainless steel or alloy protecting well is sulfi­

improper temperature control resulting from an incor-

cient to protect the detector, thermocouple, or filled

, rect se nsor location, may run into many thousands of

system bulb to provide satisfactory service for a year

dollars.

or longer. The sensor could he installed vertically or

cally justified, an alternate would he the installing of

too, several temperature traverses may be required to

n the above recommendation cannot be economi­

in a convenient side location in the gas stream. Here, obtain a representative location in the gas stream.

the thermocouple, with provision being made for

various openings. A traverse should be made later through the openings provided, with the calibration of

Aside from the physical factors, such as tempera­

the instrument set up to reflect the average tempera­

ture range desired,. gases, slag, etc. which aflect the

tUre conditions, as detennined by this traverse.

sensor and its protection device, we must consider

The next area to be considered is the combustion

the operation and maintenance capabilities of each

chamber outlet, or secondary flues. The temperature

incinerator. Which personnel in the plant will he re­

ranges frolll 1400F to 1800F in this area, and a Chromel-Alumel thermocouple is again recommended,

sponsible for maintaining the instrumentation equip­

ment? In many cases, the instrumentation maintena

since this expected temperature is above the upper "

,

ance capabilities of a plant consist merely of the replacement of a thermocouple and protecting well,

operating limits of either resistance temperature de-

"' tectors or filled systems. Basically the same' environ­ mental conditions encountered in the primary combus­

quickly and simply done by means of a quick discon­ nect polarized plug and receptacle. Because of this

recommended that the thermocouple he installed

in the smaller and medium sized plants, a simple,

in tll.iS area, and since fly �sh is most abrasive at

may be preferred over more complex, and therefore

Carbide protecting tube and alloy protecting well are

automatic control of the incinerator process is desired.

tion chamher will he found here, and it is again

lack of properly trained maintenance men, particularly

vertically through the crown. Since slag is still likely

rugged system, while not as accurate and informative,

this point, having cooled and hardened, a Silicon­

more delicate, instrumentation. However, as greater

also recommended.

as the "state of the art" improves, more complex 8)'8" tems will he needed. This will necessitate better trained personnel, and nn ellective, conscientiously

The final areas under consideration are the flues

between the waste heat boiler and the fly ash re­

moval facilities, the chimney lIue and the stack. In

applied program of preventative maintenance.

168

!

1

the range desired and the physi cal effects of slag,

Conclusion

gascs, etc. must be considered when selecting types

An attempt has heen made in this paper to suggest

and locations of the various available sensors.

he proper type of instrumentation and the best loca­

Another important factor which should also be con­

tion for the various temperature measurements to be

sidered is the capability of the personnel of the parti­

taken in the incinerator system. We have seen that

cular incinerator to maintain the equipment.

)

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169

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