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smathur72
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Selection of Relative Humidity at Constant Pressure & Temp

01/10/2014 12:02 AM

Recently I have encountered with a problem on selection of Relative humidity at constant Temp & Pressure for design of Shell & tube heat exchanger.

The parameter are given as under Design DBT= 41 C and humidity levels are mentioned as 93% & 27%.the exchanger has fluid of air on shell side ( 135/50 c)and cooling water on tubes side (32/40 c).

I have selected higher humidity which has given more heat duty but when I Checke the calculation with lower RH the duty has reduced but area has increased and net Overdesign margin will go on negative sides.

My question is there what is the safe rule for selection of RH at constant Pressure & Temp ,because higher RH selection gives Higher duty and low area, while lower RH selection provide Low Duty and higher area.
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Tornado
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#1

Re: Selection of Relative humidity at constant pressure & Temp

01/10/2014 2:53 AM

The heat conductivity factor (K or U) would be higher when more water vapor condensation is involved, resulting in less surface area for a given duty.

To be safe, check both conditions, and use the larger of the resulting areas.

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Shyam
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#5
In reply to #1

Re: Selection of Relative humidity at constant pressure & Temp

01/11/2014 3:24 AM

Tornado:

Is the calculation CODE takes into account the rate of condensation or you go by some rule of thumb with experimental data? Interesting one.

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#6
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Re: Selection of Relative humidity at constant pressure & Temp

01/11/2014 3:40 AM

I don't have the type of software now used by heat exchanger mfrs, but I suspect the OP does. My comments are really "old school". In the U.S., Crane Valves used to have a catalog with quite a bit of "rule of thumb" information such as steam tables, K factors for various shell-and-tube situations, Williams-Hazen pipe flow calcs, etc.; it was affectionately known as the "orange Bible."

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#7
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Re: Selection of Relative humidity at constant pressure & Temp

01/11/2014 5:19 AM

Thanks.

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GameJudge
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#10
In reply to #6

Re: Selection of Relative humidity at constant pressure & Temp

01/13/2014 7:29 PM

Crane Technical Paper No. 410
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smathur72
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#8
In reply to #1

Re: Selection of Relative humidity at constant pressure & Temp

01/12/2014 11:52 PM

you have mentioned to check both the condition and use Larger of the resulting area it means we should ignore heat duty and focus on larger area only because in higher RH we get higher Heat duty and low Area while on Lower RH we get lower Heat duty & Higher surface area .
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JP76
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#2

Shell around COIL Re: Selection of Relative Humidity at Constant Pressure & Temp

01/10/2014 7:01 PM

www.AquaSystemsInc.com

Ernie has an idea

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#4
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Re: Shell around COIL Re: Selection of Relative Humidity at Constant Pressure & Temp

01/11/2014 1:45 AM

Irrelevant. The OP is talking about air to water exchangers, not liquid to liquid.

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dhayanandhan
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#3

Re: Selection of Relative Humidity at Constant Pressure & Temp

01/11/2014 1:33 AM

Dear Mr.smathur 72.

A RH of 60% is CONSIDERED as Standard for Design Purpose.

DHAYANANDHAN.S
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smathur72
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#9
In reply to #3

Re: Selection of Relative Humidity at Constant Pressure & Temp

01/12/2014 11:55 PM

Thanks ,but please let me know is there any basis or guide line to use 60% RH as standard for Design.?
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dhayanandhan (1); GameJudge (1); JP76 (1); Shyam (2); smathur72 (2); Tornado (3)

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