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Guru
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What is the Meaning of %RH at 180oC?

08/15/2015 1:16 AM

What is the meaning of %RH at 180oC? See this tender and give your comments.

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Guru

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#1

Re: What is the meaning of %RH at 180oC?

08/15/2015 1:30 AM

I read it as Temperature -40°C to 180°C: Relative humidity 0 to 100% RH: and Flow 0 to 30 m/s.

I don't see %RH at 180°C anywhere.

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Guru

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#4
In reply to #1

Re: What is the meaning of %RH at 180oC?

08/15/2015 9:04 AM

sounds hot and dripping wet

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Guru

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#10
In reply to #4

Re: What is the meaning of %RH at 180oC?

08/17/2015 3:52 AM

more details please..!

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Guru
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#7
In reply to #1

Re: What is the meaning of %RH at 180oC?

08/16/2015 8:56 PM

The subject line says RH and Temperature of Flue Gas. Obviously RH sensor is exposed to same wide range of temperature. That is the worry point of my question.

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#2

Re: What is the meaning of %RH at 180oC?

08/15/2015 6:24 AM

It should not be mixed up. The first specs are for temperature limits. The second is humidity limits. You find a device at ranging operation on those limits-then it would be acceptable to owner and/or qa/qc.

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Guru
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#3

Re: What is the meaning of %RH at 180oC?

08/15/2015 6:45 AM

Dear Mr.Shyam,

What I understand from your data is a tender document calling for a system with a transmitter to design, fabricate, erect, instal and measure the Relative Humidity from -40 Deg C to 180 Deg.C and range of Relative Humidity is 0 to 100% for which the specifications are given with quantity.

The Relative Humidity will vary with reference to the Temperature. Relative Humidity is indirectly proportional to the Temp. i.e., Higher the Temp. lower will be the Relative Humidity and vise versa.

It is to be noted that below 0 Deg.C, water in the air will e in the form of Ice.

The water in the air which will remain in the form of vapour at super heated condition and degree of super heat will be as per DALTON's PARTIAL LAW of PRESSURE(S)

DHAYANANDHAN.S

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Guru
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#13
In reply to #3

Re: What is the meaning of %RH at 180oC?

08/19/2015 11:48 PM

Perhaps the pressure conditions in the duct may make the dynamic range of RH measurement with larger range of temperature very different. I am not sure if it is the intention but technically all feasible in nuclear power plants where highly pressurized high temperature near saturated steams are often used. Thanks for clarity.

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#5

Re: What is the meaning of %RH at 180oC?

08/15/2015 12:13 PM

I read it as a single instrument with 3 interchangeable probes: one temp, one RH, one anemometer (air velocity) to measure flow in a duct.

Although the RH measurement is in percentage, it needs to be able to withstand an exposure temperature range of -40 to 180 Deg C.

But in the end, those specs are likely covered by the specific brand and model that uses "LCC-S configuration and calibration software" where the appropriate probe(s) will meet those specs.

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#6

Re: What is the Meaning of %RH at 180oC?

08/16/2015 12:00 PM

I think we are being confused by the the phase changes that will occur during the process. In an open system ,like a flue, condensing water will be lost as the temperature falls below the frost point. Other phase changes will occur as the mixture is heated. Better description of the heating and cooling parameters is required.

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#8

Re: What is the Meaning of %RH at 180oC?

08/17/2015 12:24 AM

It does seem a bit odd

lyn whilst it doesn't specifically state that it needs t read at 100% RH at 180C thats how I would normally read a spec that its required to perform at the full range on all criteria.

However clearly in this case there is an anomaly. I am somewhat shocked to see 120C dry bulb temp on the psychometric chart in Perry (Chem Eng Handbook) which flummoxes me.

My general feeling of 100%RH is that its a vapour stream and if you add another water molecule that molecule will appear as a liquid droplet in the vapour stream. If the general stream is at 180C then my initial thought is that any liquid that came out of the stream would vapourise.

But it would only vaporise if there was enough sensible heat in the system - vaporising the liquid droplet would cool the stream potentially releasing more liquid etc. If there is a heat source keeping the vapour at 180C then the liquid will vaporise so in theory you culd have an infinite amount of water in th stream at 180C

So I think this is where the question is heading

For temps below 100C the device should measure 100%RH

The devices should be resistant to 180C temps and capable of measuring full performance once temperature falls below 100C

But really this is all whistling in the dark by me trying to kick some ideas around.

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Guru
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#9
In reply to #8

Re: What is the Meaning of %RH at 180oC?

08/17/2015 1:34 AM

Is system like duct where flue gas is pressurized then things will be different but user has not stated any pressure values in the duct so I believe it may be just around atmospheric pressure.

It is always said about RH sensor to have a performance range for temperature which is usually below 70C unless specially needed for some reason as most of the RH sensors fail to perform on higher temperatures. This 180C range is mind bogging as both temperature and humidity sensors are for same flue gas monitoring in some duct.

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Guru
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#11

Re: What is the Meaning of %RH at 180oC?

08/17/2015 5:19 PM

It means that the partial pressure of the water vapour in the stream being measured is at or below the boiling pressure at that temperature. The stream is therefore superheated as far as its H2O content is concerned, with a mixture of non-condensable components also present in that stream. Were the stream at 100% RH then it would be saturated.

A glance in a set of steam tables - a highly recommended possession for any Engineer - will reveal all.

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Guru
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#12
In reply to #11

Re: What is the Meaning of %RH at 180oC?

08/19/2015 11:44 PM

"steam tables" idea was very interesting one. Thanks.

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Guru
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#14

Re: What is the Meaning of %RH at 180oC?

09/30/2024 10:08 AM

If this is a flue gas and the exhaust is to atmosphere then the chances of its being at 100 <...%RH...> and <...180oC...> at the same time are nil. So that condition is meaningless. The saturation pressure of steam at <...180oC...> is around 9 barG/130psiG.

  • Query the need for a <...%RH...> sensor in such a duct. It would make more sense to install a %O2 instrument instead.
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