600¦F velocity and wet bulb measurement in air

[dvd]

I am interested in hearing about your knowledge of commercially available portable probes for measuring air velocity and wet bulb temperature (not with the same instrument) inside of a large industrial enclosure with maximum temperature of 600°F and maximum velocity of 4000 ft/min. I would appreciate vendor names if possible.

Thanks

 

[quark]

DVD!

Fischer Rosemount make Annubar type DP meters are rugged and will suit your application for flow measurement and will be a cost economic solution.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

 

[MortenA]

I dont think anubar would be useable in an "enclosure" (not a pipe i guess?

Is this "enclosure" a furnance?

Best Regards

Morten

 

[dvd]

Thank you for your interest. The enclosure would be an oven up to 200 feet long, 14 feet wide, and 12 feet tall. There are upper and lower conveyors inside. Access would be through access holes, say 2" pipe welded into oven side panels. The flow inside is somewhat turbulent.

The ideal velocity sensor would have directional sensing capabilities. I have used pitot tubes and S-tubes with good results in some areas of the oven and poor results in other areas. I have been thinking in terms of hot-film anemometers, but I would like to hear of any suggestions.

Any experiences with high-temp wet bulb? My own experiences have been to place a wick on a thermocouple probe and put this into boiling water then insert it into the oven. The temperature drops until the wet bulb temperature is reached and then climbs again. Sometimes it works okay and sometimes not. A more reliable method would be appreciated.

 

[quark]

MortenA!

There are annubars of Fischer Rosemount make for HVAC applications which are meant for air velocity checking in ducts and we have used them with satisfactory results. But you cannot check flow direction with them.

DVD!

I feared to advise you to put a wick to a thermocouple probe because I thought it as non technical practice. (But that was what exactly we are doing). Do you intend to check only wet bulb temperature or humidity?

Regards,
Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

 

[dvd]

Quark!

I am only interested in the wet bulb temperature.

Thanks

 

[rjw57]

dvd-

The dewpoint at 600degF can be measured with this device from Vaisala -

http://www.vaisala.com/page.asp?Section=5544

Wet bulb can be estimated from all the quantities you will measure by the methods found here -

http://www.snowball.frogspace.net/js/wxalgs2.html#calc_wb

The velocity would probably be best left to a rake type pitot. This is territory I think that would be best left to the people whom design them. Couple it with a dP cell from any instrumentation manufacturer and you're good to go. I've dealt (a long time ago) with United Sensor Corp.

3 Northern Blvd.
Amherst, NH 03031 2329
USA
Tel: 603-672-0909
Fax: 603-672-0037

Product Description
Mfr. Of Pitot, Kiel, 2 & 3 Dimensional Directional Probes

Hope this helps. I assume that you are doing an industrial dryer or some type of heat treatment system?

 

[dvd]

rjw57:

Thanks for the information. I am looking into the dewpoint sensor. You are correct in assuming that I am working with an industrial dryer.

dvd

 

[witchdoc]

Measuring wetbulbs using the method that you describe is at best a guess. Radiation from the oven walls can produce errors. We have had much better success with a "chilled mirror" type sensor.

The best solution (highly accurate, but cumbersome to handle) is to use EPA method 5 stack sampling device (normally used to measure stack gas velocity and composition). This method uses a flowmeter to pull a sample (isokinetically) of the flue gas. Moisture content is measured by condensing the moisture in the sample and weighing the condensate.

A good reference for velocity measurement techniques is "Industrial Ventilation Guide" published by the American Conference of Governmental Industrial Hygienist.

 

[dvd]

Witchdoc:

Please tell me more about the "chilled mirror," I am not familiar with that terminology. I agree that Industrial Ventilation is a great reference. I have a copy on my desk.

Thanks

 

[witchdoc]

I understand the chilled mirror sensor is a mirror that is immersed in the gas stream (typically, a sample of the gas stream is pumped through the sensor). The mirror is cooled using external cooling until condensation is detected on the mirror. The more elaborate instruments use a photo detector to sense the onset of condensation. I think Omega engineering still offers one. They are quite pricey, and require regular cleaning. This technology has been around for a number of years.

 

[rjw57]

dvd-

First, I want to let you know that I am no psychrometry expert so take what I have said with a grain of salt. (I used to work for a manufacturer of industrial dryers). As far as I know, most thermoelectrically cooled chilled mirror systems are good to about 85degC. If you need to use this system, you would require a sampling system which would cool the gas stream to a usable temperature, so your dewpoint better be below this value and there had better be pressure compensation if the sample is at some other pressure than that in your oven. As far as radiation being a source of error (as pointed out by Witchdoc) in the gas temperature measurement method I had proposed, it would certainly be necessary to properly shield the temperature sensor from radiation souces (

http://www.unitedsensorcorp.com/temperature.html).

I don't know how your oven is designed, but normally, the energy source is mounted so as not to directly fire or radiate into the conveyor space. Radiation should not be a big source of error in this case.

In your original post, you asked for ways to to measure "wet-bulb" temperature. If so, your only means of doing this directly that I'm aware of is with a sling type or wetted wick/fan combo. Anything other than this method involves some form of humidity sensor with equations for indirect measurement of wetbulb temperature. Even the EPA method described by Witchdoc does not give wet bulb temperature. You can derive (admittedly accurately) moisture content from this method, but you still need to do a calculation to obtain wet bulb temperature (which is still just that, a calculation potentially riddled with mathematical innaccuracy if not intended for use at these high temperatures). I would suggest a quick phone call to ASHRAE to ask them if they have any literature on high temperature pshychrometry. All I know of is ASHRAE's RP1060. The stated ranges of interest for ASHRAE RP-1060 are:

Temperature: 200-320°C (400-600°F),

Pressure: Low corresponding to altitude of 2250 meters
(7500 feet) to high of 5 MPa, and

Humidity Ratio: 0 to 1 (max.)

There is also a source of work by Hyland & Wexler as well as a practical book entitled "Psychrometrics: Theory & Practice" by Joseph B. Olivieri (Editor), Mildred Geshwiler (Editor), T. Singh (Editor)


Good luck...