8 replies to this topic

Hello friends,
I am designing a plate and frame heat exchanger (PHE). The NTU calculated is 0.54. According to standard performance limits for Alpha-Laval PHE, the pressure drop is 30 kPa per NTU. Hence the pressure drop amounts to 16.2 kPa but the pressure drop I have calculated is 39.2 kPa. Can anyone explain why my answer is different?

[djack77494]

the pressure drop I have calculated is 39.2 kPa. Can anyone explain why my answer is different?

I can follow where your estimated dP using Alpha-Laval PHE methods yields 16.2 kPa. But how do you calculate 39.2 kPa? You haven't given us any information about where this number comes from. With enough information supplied, I'm sure you'll get an explanation.

Thanks for your response.

By the way, the pressure drop on the cold side is higher. (∆p = 501 kPa). Below is how I have calculated for the cold side.
Pressure drop was calculated from:
∆pis = f (M*M)/ρ

f = 0.0892*106* (Re to the power of -0.10) …… Re = 3383 (cold side, ∆p = 501 kPa)
φis and φp were ignored.
Therefore, ∆p = ∆pis

[djack77494]

Pressure drop was calculated from:
∆pis = f (M*M)/ρ

I'm sorry, Madhy, but you've reached (exceeded) the limits of my knowledge regarding PHE's. It does sound as if the equation you used to calculate the value of 16.2 kPa was very general-purpose, and therefore perhaps not very accurate. It would be hard for me to imagine that you could closely estimate a PHE pressure drop merely from knowing the NTU's. Hopefully someone more knowledgable than I can add to this dialogue.

Thanks for your consideration, djack77494.

[Qalander (Chem)]

Thanks for your consideration, djack77494.

Dear Madhy Hello and Good afternoon,
May I suggest that If you may provide complete Equation utilized for your calculations and values of critical variables in question . This may help in re-analyzing the issue and find out the ways and means to support you.
Regards
Qalander

[Jiten_process]

hi,

I would say, you have to apply the same formulae to calculate pressure drop across the PHE.
Let me explain you, you first fix the velociy between two plates (use recommended velocity considering the fouling and all, coz there is no well astablished fouling values for PHE), calculate the the flow across each cassett (by determining no.of plates for the given duty) and find the spacing between two plates which is nothing but the flow area.

Now calcuate the pressure drop exactly the same manner using darcy equation, but make sure here the diameter is equivalant diameter.

The pressure drop value which you get if it is exceeding your assumed cutoff value reassume lower velocity and follow the same procedure.

I dont see anything wrong in this,
what other have to say on this?
to be continued....

[nghiah5]

Hello friends,
I am designing a plate and frame heat exchanger (PHE). The NTU calculated is 0.54. According to standard performance limits for Alpha-Laval PHE, the pressure drop is 30 kPa per NTU. Hence the pressure drop amounts to 16.2 kPa but the pressure drop I have calculated is 39.2 kPa. Can anyone explain why my answer is different?

Hello Madhy. I'm trying to calculate the pressure drop of a PHE too. Can you show me how to solve this problem. I really hope receving from you a letter or your calculated files. Thanks you very much.
My email: commando_200687@yahoo.com

[breizh]

Hello Madhy ,
Let you try Perry's chemical engineering handbook , you will find correlation .
regards
breizh