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Selection Of Heater For Instrument Air Heating

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#1 ushakuna

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Posted 28 October 2015 - 12:05 AM

I have a task to design heat exchanger to heat 30 m3/h (192 kg/h) of instrument air (6 bar, 45°C) to 85°C. Heating media available are Sat steam at 3.5 barg and 9 barg. I have tried estimating Coil heater with steam in shell and air in tube. Area requirement comes to about 16 m2 with U of about 3 W/m2C (2.6 kcal/h/m2/C). The area seems to be higher and also difficult to accommodate.

 

Request expert advice on type of design I need to select. Will Double Pipe Heat Exchanger work here? Can I use formula for HTC for condensation on horizontal tube given in Kern for HTC calculation on steam side?

 



#2 msaexcel@gmail.com

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Posted 28 October 2015 - 09:15 AM

Heat of compression (HOC) dryer may be a good option.

#3 breizh

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Posted 28 October 2015 - 06:34 PM

hi ,

Why do want to heat instrument air ?  if it is to remove moisture , you should consider to ask for air dyer , air compressor vendors will help you .

 

Hope this helps

 

Breizh



#4 Art Montemayor

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Posted 28 October 2015 - 08:45 PM

Ushakuna:

 

What our experienced and clever senior Forum member, Breizh, asks is the obvious: why are you trying to heat instrument air?  This question precedes the obvious follow up: if indeed you want to heat instrument air, why not just take it directly from the compressor’s discharge?

 

The discharge temperature of an air compressor at 6 barg is approximately 130 oC.

You should tell us all the background for your application.  If indeed you need to heat compressed air, is it absolutely dry, saturated with water vapor, or does it have reduced water content?  Of course you can use a double pipe heat exchanger - if you can tolerate the physical size and the space it will occupy.

 

An overall U value of 2.6 kcal/m2-h-oC is equal to 0.532522 Btu/ft2-h-oF.  This is not only a very low value for a U, it is ridiculously low.  Where or how did you obtain this value for U?  For this application I would expect a U value of 10-20 Btu/ft2-h-oF for a BEU, horizontal, TEMA exchanger with steam on the tube side.  With steam on the shell side, this might go as high as 50 Btu/ft2-h-oF.  My reference is Kayode Coker’s book, “Ludwig’s Applied Process Design for Chemical and Petrochemical Plants, Vol 3; 4th Edition.

 

Your large calculated heat transfer area (172 ft2) is due to the ridiculously small U you have employed.



#5 ushakuna

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Posted 16 November 2015 - 06:56 AM

Hello all,

 

Sorry for keeping quite for long time..and thank you for your valuable suggestions.

 

We have installed ammonia analyzer on top of Urea prill tower, near axial flow ID fans for analysis of ammonia emission. The analyzers require dry air (at 1 kg/cm2g) for continuous purging. We checked with supplier, who demands dry air.

 

Meanwhile I have assumed pipe in pipe (DPHE) exchanger with 3.5 barg saturated LP steam at annular side and compressed air in pipe and calculated the size as follows. Sizes assumed are inner pipe 1" and outer pipe 2". 

 

Air requirements have changed as follows:

Air requirement = 320 m3/h at 1 kg/cm2g (~640 Nm3/h = 828 kg/h), heating from 40°C to 85°C. Heat duty = 9030 kcal/h (10.5 kW)

So LP steam requirement is estimated to be 18 kg/h.

 

 

I estimated steam side condensing HTC using equation :

For condensation on vertical tubes or plane surfaces hv = 0.94[(k3r2g/m) x (l/LDT)]0.25  (got from a website)

(all properties of condensate at wall temperature about 105°C, L= assumed as 5m for initial calculation), dT = average comp air temperature & wall temp (105-65°C = 40°C).

I got HTC, ho= 3188 W/m2K

 

For air side, used eqn 6.2 in Kern and got hi = 1227 W/m2K, or hio = 1227*24.2/33 = 900 W/m2K

(Calculation: Mass velocity G = 500 kg/s/m2

 

Overall HTC = hio x ho / (hio + ho) = 900 x 3188 / (900 +3188) = 702 W/m2K

 

SO area required = Q / UxLMTD = 10500 / (702 x 85) = 0.176 m2

Area per unit length of 1" pipe = pi x d x L = 3.14 x 0.033 x 1 = 0.103 m2

 

So total length of DPHE = 0.176 / 0.103 = 1.71 say 2m.

 

Is it OK? Please suggest.



#6 srfish

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Posted 16 November 2015 - 02:55 PM

Ushakuna,

 

Have you checked the pressure drop inside the tube. I get an extremely high velocity and pressure drop.






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