I'd try an in situ measurement; Maybe a 3omega metod fits well since it needs only a Pt thin wire. If you need any more specs. please do ask!

You can estimate it numerically knowing the dry analysis of the gas and the composition of the fuel.

Hello, maybe tuned diode laser techniques (TDLAS instruments) can be suitable to your application to have an in-situ measurements of the water vapor pressure with which you can derive your desired humidity parameter. There are also a number of commercial instruments based on TDLAS techniques. Please let me know if you need major information.

Denis

The best method is using TESTO portable GC analyzers. If you introduce the fuel composition accurately to the machine, it will correct the measured values according to anticipated calculated values. Our experience in using this machines in industrial fired heaters with flue gas temperatures higher than 1200 C shows good agreement with theoretical anticipations. Contact TESTO company and ask for practical solutions since they are the best in flue gas measurements.

http://www.macinstruments.com/mac125_moisture-analyzer.html

Here is another instrument option.

Thank you all for your advices.

Dr. Denis Smorgon , Would you please tell me which company in UK can provide the service /or sell the equipment for measuring water vapor of air in elevated temperature using tuned diode laser techniques

It will be necessary to compensate for furnace temperature if measurement is required over a greater temperature range.

If volume is not restricted heating a gas will cause it to expand and reduce the number of gas molecules in a given space, i.e. the laser beam path length. So it is necessary to compensate for furnace temperature if measurement is required over a temperature range greater than + - 15 degrees C.

We will run different temperature profile. For same temperature profile the temperature in our furnace may vary less than 15 degrees. But we will change temperature profile quite often so we will face a problem with compensation for the furnace temperature.

Dr. Babak Maghbooli we want measure the water vapor of air in our furance and not measure flue gas .Can the TESTO portable GC analyzers measure he water vapor of air at high temperature ?

However our furance is a gas burning Radiant tube furnace , there may be some leak of flue gas. but our main interest is to measure the the water vapor of in our furance

Dr. Don Hargis thank you for your information

Temperature Range 40° F / 4° C to 1200° F / 650° C - Above 1200° F requires special mounting tube.

please tell me the option which can direcly measure the air above 800°C without special mounting tube

Thank you

Hello,

you can take a look at

www.spectrasensors.com

http://www.yokogawa.com/eu/an/analytical/EXAxt450Series/Truepeak-TDL.htm

But I think you can find also other sources on the web.

Denis

Dr. Don Hargis thank you for your information

You can use an FTIR device, Fourier Transform Infra Red. The measurement is direct. Previously you must cool the gas up to 180ºC before inputing in the FTIR.

This question and evaluative issue has persisted in my mind for days, as it presents several underlying questions, some of which could only be broached with more information concerning the gaseous environment of the furnace and the available technologies for evaluating a sampling. After considering the nature of 'humidity' and searching for detection technology, I discussed the problem with an Emeritus Professor of Chemistry with industrial experience.

Given that the H2O vapor in the furnace is mixed with other gases at 800C, and will remain in a gaseous state when kept well above H2O's boiling point (will not condense until it approaches 100C), the pipe used to contain a vaporized sample of the 'air' could be heated during the process to some range above 100C to below 200C, and maintained at the range for evaluating with you humidity sensor.

If possible, upon removal of the air sample, the gas mixture could be transferred to a Gas Chromatograph for analysis of the gas mixture. This would provide greater detail of the components of the mixture and further data for analysis of the H2O vapor content.

The limitations of this suggested process could be many and restrict completion, such as engineering the heated pipe and assembly, transporting the heated pipe containing the gas sample to the GC device, et cetera.The pipe containment assembly would have need of pressure adjustment or maintained at a comparable pressure to the furnace.

I would be very interested to know your results.

Following the comments of Mr. Hargis, I am very interested to know your result, too.

Thank you for all for your comments

From all of your comments and also discussed with other experts, now I have a few solutions listed below

1. The high temperature humidity measurement system: using humidity measuring instrument based on the principle of chilled mirrors. The air sample will pumped out of the furnace through a pipe. There is an issue to keep the air warm enough so it will not condense before it reach the chilled mirror.

2. Using humidity sensor. The hot air will be pump out of furnace to a tube (tunnel). When the hot air go though the tube its temperature will cool down to some degree (for example 50°C) and use self-regulate trace cable ( or other insulations ) to keep temperature nearly constant ( above the dew point to avoid condensation) at a certain length of the tube. At that position the humidity sensor will be fixed to measure the humidity of the air, which will represent the humidity of the atmosphere in the furnace.

3. The water vapor of furnace air can also be measurement using tuned diode laser techniques (TDLAS instruments) but it will be expensive.

4. My team has proposed ‘An unconventional and straightforward methods: Pump the hot air out of furnace to a cooling coil to condense the water vapour into liquid water, collect and measure the condensed water as a measure of humidity. But I am not sure if this method will works and delivery a reliable result

Any comment and suggestion are highly appreciated.

Thank you

The 4th method is extremely risky, because you must know the exact humidity of combustion air ( accurate calculations based on relative humidity, ambient pressure and ambient temperature is mandatory) also no amount of water should be condensed up stream of collecting and measuring system since it may not fall down due to liquid water capillary force. But still it seams the cheapest way. By the way how do you measure the amount of combustion air, especially excess air amount? Is it controllable and precise?

If the hot air pumped out of the furnace is condensed in a manner that isolates the condensed water vapor from any 'outside' source of H2O, the condensed liquid water should be representative of the H2O content, to wit: a measurement of humidity, Option 4.

I would suggest as a reference, for fine tuning your method of measurement, Psychromtry (12-3), Chemical Engineers' Handbook, McGraw-Hill Book, ISBN 0070494797. For liquid-gas systems, which is implied by option 4, Section 18-70 to 18-88 of the Handbook discusses in detail phase separations and collection mechanisms.

Again, depending on the chemical components of the air (vapor mixture), and the level of precision demanded by your experts and measuring apparatuses, you may wish to give some thought to Liquid-Vapor Equilibrium and the azeotropic characteristics of the condensed water in which pressure is now a critical parameter, for example (Matter in Equilibrium, Berry, Oxford University Press, 2002, Sec. 25.4). Yet this could be going too far, though the instruments for ascertaining pressure are far less expensive than Option 3, would entail collection and processing data through electronic spreadsheets or other computer assisted tools.

I believe Option 4, though 'unconventional' is straightforward and provides the method with the ability to extend precision with engineering designed limitations, as stated above, with the emphasis that the condensed H2O should be sufficiently isolated.

What are the components of your excess air (combustion air)? Possibly, you should consider a very small volume for sampling and measurement, which will afford you increased control of pressure and temperature. A manageable quantity, sufficiently isolated, should represent the overall gaseous environment of the furnace, save for such parameters as turbulent flow, et cetera, but eliminating otherwise extraneous parameters to humidity should not be an undue concern.

If the gas mixture (sample) contains combustible compounds, such as organics, at ambient temperature, you will need to replace oxygen containing air with an inert gas, such as helium or nitrogen. This 'carrier' gas will replace combustible air and provide the safety you are trying to achieve.

Good point

The method I proposed is expensive. We use a portatile FTIR device to measure the analysis of flue gases of small boilers. We measure several compounds, CO2, CO, H2O, NO2, NO, SO2, HCl, CxHy, etc. O2 is not measured by this device.

Another method I remember is the measurement of the O2 content in the gas wet basis (zirconium cell technique) and dry basis (paramagnetic technique) . Then it is easy to calculate de moisture of the gas. The instruments that measure them are commercial and are less expensive than an FTIR.

A measurement of humidity, Option 4.

Don I think the components of the air in our furnace are much close to the components of air in atmosphere apart from the water vapour content may be different.

There are few problems in Option 4 need to overcome

1. To condense the water fully ( 100% of the water vapour condense to liquid water)

2. As Don mentioned , to totally isolate the condensed water vapour from any 'outside' source of H2O

3. To achieve the high sensitivity and reasonable speed of the measurement

If the furnace air is sufficiently similar to atmospheric air there should be less concern over combustible air. Engineering your pipe for sample retrieval and containment could be accomplished to avoid condensation of H2O vapor by maintaining the temperature of the pipe assembly well above 100C. The humidity sensor could be affixed to the pipe, prior to taking the sample, with a heat shield similar to an optical chopper.

Assuming the humidity sensor range has a ceiling of approximately 200C the pipe assembly could be heated to between 120C and 180C without adversely affecting the sensor. With a heat shield between the sampling pipe and the fixed sensor you can control how long to take the measurements. My reference to an optical chopper merely infers a mechanical device for controlling the exposure of the sensor to the sample in the pipe.

By maintaining the sample well above H2O boiling point you avoid condensation and allow an extended measurement time. Speed of measurement is not a unyielding factor if the pipe is maintained well above 100C, or if the sample is isolated from ambient air as it condenses.

The pipe could be heated by a heating coil with a temperature regulator, as a simple option and insulated within another larger piping assembly or enclosure. At this point you would be fabricating a potentially marketable furnace humidity measurement instrument, though I do not know its marketable value for industry.

Since I do not have the precise specs for your radiant heater, my observations are only piecemeal. I could assume that you view the furnace's interior through a view port, possibly partially obstructed by a thick high temperature glass or the view port has a removable gate/shield. The furnace may also have a pressure release nozzle. The sampling pipe could be engineered to retrieve the air sample from the view port or nozzle without allowing ambient air to enter. This is accomplished through mechanical components such as valves, pumps, et cetera. Retrieving gas or liquid samples from containment areas has been a part of chemical engineering for many years. Selecting or engineering the right device and mechanism to fit your furnace's interior access port or nozzle is a critical engineering design element.

Achieving high sensitivity will be dependent on the precision and sensitivity of your humidity sensor. If you choose to allow the H2O vapor to condense within the pipe assembly other approaches to determining H2O liquid content could be used, then extrapolating back to humidity, through calculations of pressure, volume, et cetera. You may still want to fix and seal the humidity sensor to the sampling pipe. In this scenario I do not immediately see a problem with isolating the sample, if the pipe is evacuated and dry prior to attaching for retrieving the sample. Of course ambient air will condense on the exterior pipe surface, yet how would this interfere with interior measurement readings?

If you weigh the evacuated, dry sampling pipe prior to and after taking a sample, depending on the precision of your weighing device, you can again calculate the H2O content as condensation occurs and reaches liquid-vapor equilibrium. This is not a direct procedure for determining humidity, as you well know. Yet you are unfolding new information and new problems in response.

If I were given this problem from inception, with all the specifications and operational parameters, I would layout the options in detail with possible limitations and necessary controls. From that point I would formulate hypothetical scenarios based on the real-world parameters to see which option might provide the more reasonable results. In view of this engineering process, I would most likely utilize at least two options and compare the results.

Here is a company that performs well with high temperature furnaces and may be of assistance, though this may be with considerable cost:

Enertechnix

Box 469

Maple Valley, WA 98038

Phone: (425) 432-1589 Fax: (425) 432-1557

www.enertechnix.com

Research and Development: [email protected] 

By measuring the resistance of air present in the furnace

Dear Sunil Thank you for your suggestion

Please tell me what resistance we should measure and how to measure ?

Thank you

A megger can be use to measure the moisture contents in the air.

Dear Mudassir

Can a megger be used to measure the moisture contents in hot air( up to 800 oC) ?

Thanks

To measure the humidity of air in furnace by directly measure the electricity resistance of air has many difficulties.

First, the electrical resistance of the air is very high (up to 3X1016 .m) so it will be very difficult to detect any current by Megger on a reasonable voltage. If apply a very high voltage, then there is a problem of generate arc and also has a health and safety issue.

The ground of the furnace is not insulated so it may be possible to cause a short circuit. Also the product, steel sheet (passing through the furnace) is a good conductor. it will also make it difficulty to the measure the resistance of air and relate the measurement to the humidity of air.

The other facture such as the density of dust or micro particles in the air is not constant and the temperature of the furnace is not constant etc. Since the temperature co-efficiency of the air in furnace is unknown so it will be difficult to compensate the influence of the temperature.

Any ideas to overcome these difficulties?

If you are worried about the humidity of the air in your furnace, you already have a phenomena which can be used to measure it. Just do a careful calibration in the laboratory. Good luck.

A very powerful approach is Tunable Diode Laser Absorption Spectroscopy. In principle this method can be implemented in situ, although in practice you may have a hard time keeping the optics clean enough. The other problem you may have is that the water content is probably high, so you may saturate the signal. You may be able to overcome this by tuning to a weak absorption line. This method is similar to FTIR, but more expensive.

As an alternative to sampling, you can use a high temperature zirconia-based humidity analyser. You can find several commercial models on the market (e.g. Yokogawa ZR202) that can work up to 700-800 °C of gas temperature.