How To Measure Wall Losses By Infra Red Pyrometer

How to measure wall losses by Infra Red Pyrometer Pyroprocessing

PROCESS MEASUREMENT TECHNIQUES - PR-PYR P06-25 V1.0

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Introduction ƒ

When making a thermal heat balance on equipment (kiln, cooler…), heat losses (wall losses) by radiation and convention heat transfer need to be taken into account. The principle is to measure the average surface temperature of the equipment.

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This procedure explains how to measure the wall losses on the kiln shell as an example. All other wall losses such as cooler, preheater, etc can be measured following the same principle

Safety aspects ƒ ƒ ƒ ƒ ƒ

Hot surfaces Rotating equipment Dust Radiative heat Trips when looking at equipment

Prerequisites ƒ

The most important tool is an Infra Red Pyrometer as shown in the picture. The measuring point on the shell is indicated by a laser beam and the surface temperature can be read immediately.

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Further the basic dimensions of the equipment need to be known in order to calculate the surface area.

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Measure the local ambient air temperature around the surface to be measured For surfaces exposed to wind check local wind speed with an anemometer Avoid conducting this measurement during rainfall

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Procedure Template developed by

How to measure wall losses by Infra Red Pyrometer Pyroprocessing

PROCESS MEASUREMENT TECHNIQUES - PR-PYR P06-25 V1.0

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Action Steps 1.

Make meter marks on the kiln shell ƒ

2.

Using a measuring tape, make a mark every meter along the kiln shell. Use the hand rail of the kiln walkway or any other convenient equipment for the marks.

Measure the kiln shell temperature at every mark ƒ

Set the emissivity to a value corresponding to the colour of the shell, see the table in the appendix. Note that it is important to set this properly otherwise incorrect readings of temperature will be obtained.

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Typically for the kiln shell a value in the range of 0.90 - 0.95 can be used depending upon the discolouration.

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At every mark, take a temperature reading of the shell. If for example the kiln length is 60 m, there will be 60 temperature readings.

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At every mark, point the pyrometer on the shell for an entire kiln revolution. Record the average shell temperature.

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Measure and record the ambient temperature and the wind speed. In areas with running shell cooling fans a simple approach is to increase the ambient wind speed by 2 m/s for this shell section.

For other surfaces like cyclones:

3.

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A good practice for non rotating equipment is to measure the surface temperature with a contact thermocouple or by just attaching a small thermocouple on the surface. Measure the same spot with the Pyrometer and adjust the emissivity accordingly to achieve the same temperature. Carry out that procedure at least twice for every type of surface structure.

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In areas with many neighbouring hot surfaces (for example a narrow preheater/calciner structure or tertiary air duct close to kiln) ambient temperature should be measured for the specific location.

Calculate the wall losses ƒ

Calculate the losses due to radiation only as follows: 4

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ƒ Determine the average of Tshell – Tambient (in Kelvins) for the entire shell then calculate ƒ Radiation losses

Qradiation= α x x σ x A x ( Tshell4 – Tambient4) Where Qradiation is radiation loss in Watts α is the view factor (normally taken as 1 provided there are no close obstructions that could prevent the heat being radiated to the surroundings is the emissivity of the surface σ is the Stefan Boltzmann Constant 5.6697 x 10-8 W/m2.K4

Procedure Template developed by

How to measure wall losses by Infra Red Pyrometer Pyroprocessing

PROCESS MEASUREMENT TECHNIQUES - PR-PYR P06-25 V1.0

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Action Steps A is the surface area in m2 Tshell is the shell temperature in Kelvins Tambient is the ambient temperature in Kelvins ƒ

Calculate the losses due to convection only as follows:

ƒ Determine the average temperature difference between the shell and the ambient tshell– tambient (in Celcius), then calculate convection losses by:

ƒ If there is no wind, i.e. natural convection use the flowing equation:

Qconvection= 1.7794 x A x ( tshell– tambient)1.25 ƒ In the case that the shell loss is affected by wind, i.e. forced convection, then use the alternative equation:

Qconvection= (7 + 0.514 x V) x A x ( tshell– tambient)

Where

Qconvection is the convection loss in Watts 2 A is the surface area in m tshell is the shell temperature in Celcius tambient is the ambient temperature in Celcius V is the velocity of the wind in m/s ƒ

The total heat loss from the shell is sum of both the radiation and convection losses

QTotal = Qradiation + Qconvection This calculation approach is recommended to be used for all surfaces (preheater / cooler / kiln / …). Theoretically different formulas are required depending on the surface shape. Nevertheless the error of using one common formula is small compared to measurement errors. For example calculating the kiln using the Anselm curves or by above formula shows a difference of below 5%.

Procedure Template developed by

How to measure wall losses by Infra Red Pyrometer Pyroprocessing

PROCESS MEASUREMENT TECHNIQUES - PR-PYR P06-25 V1.0

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Appendix 1.

Typical Values for Emissivity: ∈

Material Bricks Steel For oxidized steel For dusty kiln shell For silica bricks Other data Iron oxide Zinc galvanized sheet bright Iron polished Steel dense shiny oxide layer

∈ 0.8 0.95 ∈ =0.996-2.88*10-4.(ts-100) ∈ =0.96-5.2*10-4.(ts-100) ∈ =0.81-6.08*10-4.(ts-200) ts 500C 28C 425C 25C

∈ 0.78 0.23 0.144 0.82

Steel oxide Steel oxide Steel polished Steel pipe

ts 40C 370C 770C 200

∈ 0.94 0.97 0.52 0.8

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