12 replies to this topic

[curious_cat]

How different is the HTC for a Wide Gap Design as opposed to the conventional narrow gap design? I'm assuming the wider gaps cause less turbulence & hence poorer HTCs. But how much poorer approximately? Any ideas?

 

All the estimation guidelines I see are for the regular design & not the wide Gap one. Even APVs propreitory design tool has  no wide gap option. 

 

The narrow gap models have a 2-4 mm spacing whereas in WG models it is 11 mm to 17 mm. 

 

Of course, for the final specs. I'd email the vendors for an accurate area estimate but right now I'm only evaluating some options & hence only need a very approximate estimate for derating a conventional design. 

 

The HEX has steam on one side and aqueous medium on the other side to be heated up. 

[breizh]

Hi Curious-cat ,

I 've attached literature from APV . You may consider to contact them .

 

Note : a few other resources for Plate heat exchangers to support your work..

 

Good luck.

Breizh

[PingPong]

There are many correlations floating around for single phase heat transfer Nusselt number and Friction Factor in PHE's.

They are based on the Reynolds and Prandtl numbers, not unlike the correlations for S&T exchangers. Reynolds and Nusselt are then based on either the Hydraulic Diameter D_h or the Equivalent Diameter D_e

A larger plate spacing obviously results in a larger D_h and D_e

 

See for example section 7 in: https://books.google...epage&q&f=false

and section 11.6 in: https://books.google...epage&q&f=false

 

Using Google you can find articles with similar correlations.

 

[latexman]

Characterize the fluids, especially the one on the wide gap size.  Maybe it's close to someone's experience here, then you might get a "go-by" number.

[curious_cat]

Characterize the fluids, especially the one on the wide gap size.  Maybe it's close to someone's experience here, then you might get a "go-by" number.

 

Right. 

 

Fluid 1: Saturated Steam at approx. 1.5 barg 

 

Fluid 2: Saturated NaCl brine solution (dilute slurry). Approx. 25% w/w. With 5% max suspended NaCl solids.

Density 1190 kg/m3. 

Viscosity 1.8 cP

Specific Heat: 3200 J/kg K

Thermal Conductivity  0.62 W/m K

 

 

The NaCl slurry will be on the wide gap side. 

 

Even an approximate HTC will do very well. This is only an initial estimation exercise. 

 

Anyone? 

Edited by curious_cat, 13 September 2015 - 09:36 PM.

[latexman]

Fluid 2 average density, heat capacity, viscosity, and thermal conductivity, if known?   

[curious_cat]

Fluid 2 average density, heat capacity, viscosity, and thermal conductivity, if known?   

 

Density 1190 kg/m3. 

Viscosity 1.8 cP

Specific Heat: 3200 J/kg K

Thermal Conductivity  0.62 W/m K

 

I've now edited the post above to include these details. 

[latexman]

The majority of my experience is with plate and frame heat exchangers used as coolers with cooling water.  The process side is usually 0.9-1.1 kg/m3, 10-2500 cP, 2.5-4.2 J/kg K, and 0.33-0.67 W/m K.

 

That's pretty different from your application.  I will say though that our P&F HTC ~ 3.33 x S&T HTC in our application.  I hope this helps.

[curious_cat]

The majority of my experience is with plate and frame heat exchangers used as coolers with cooling water.  The process side is usually 0.9-1.1 kg/m3, 10-2500 cP, 2.5-4.2 J/kg K, and 0.33-0.67 W/m K.

 

That's pretty different from your application.  I will say though that our P&F HTC ~ 3.33 x S&T HTC in our application.  I hope this helps.

 

Very helpful. I'm seeing a HTC of approx. 1500 W/m2 C in a very similar application but with a S&T HEX.  Is the 3.3x with a narrow gap design plate & frame?

 

Also. how high are the pressure drops? Have you used any correlations to predict them? Is there a certain nominal velocity you try to shoot for? ( e.g. 1.5 to 4 m/s seems typical for tube side of S&Ts. 

[latexman]

No, the 3.3x is for the wide gap.

 

Our design case for the process side is laminar flow.  Therefore pressure drops are not high, IMO, mainly because we are using the wide gap.  We usually limit the process dP to 10 psi.  Our operating pressure is 40 psi.  I have no heat transfer correlations; we give the vendor a performance spec. and let them handle the sizing, velocity rules-of-thumb, etc. 

[breizh]

Hi ,

I remember using PHEX on ammoniated Brine from Alpha laval when I was producing Soda ash . I remember that we installed automatic basket filter to prevent clogging .

 

As I wrote earlier big names like Alpha laval , APV or Graham may support you.

 

Good Luck

 

Breizh

[curious_cat]

The vendor GEA has a pretty nice online tool & I fed my duty specs into it & it creates for me a design where the overall HTC comes to 7000 W/m2 K. 

 

I wasn't expecting a coefficient this high. Is this a reasonable coefficient for the steam-aqueous stream heating duty I described? 

 

The individual heat transfer coefficients on the steam and fluid side are predicted to be both approx. 15,000 W/m2K

Edited by curious_cat, 15 September 2015 - 09:06 PM.

[latexman]

Yes, looks reasonable to me.

Edited by latexman, 15 September 2015 - 09:22 PM.