Scaling Pressure drop in Heat Exchanger

[Reinoud]

Hi All,

I am building a pressure drop model to calculate my maximum mass flows through a large network of piping. Now there are a few Heat exchangers implemented from which I have no information on how they are constructed. I only get the pressure drop values that are measured at given properties:

Pressure drop: DeltaP_1
Operating Pressure: P_1
Temperature when tested: T_1
Mass flow when testing: m_1

Where the 1 stands for values at test.

I was wondering how to calculate the pressure drop of these HE's (Heat Exchanger's) in other operating conditions. I tryed with the following (very simple) relation:

DeltaP_2 = DeltaP_1 * (Density_1/Density_2)

where the 2 stands for the situation I am calculating.

This obviously does not work. I also tryed putting in the friction coefficient but it only flew way out of scale and is hard to determine without the detailed overview of the construction of the HE.

So, does anybody have experience with these kind of problems and would like to share his experience? Any help would be appreciated?

Thanx in Advance,
Reinoud

 

[fvincent]

Dear Reinoud

First of all check whether the HE operates in turbulent region. If yes I would use a very similar co-relation as yours, to which I just included the square of mass flow ratio:

deltaP_2 = [ deltaP_1 * density_1 / m_1^2 ] X

[ m_2 ^2 / density_2]

However I suggest you to use density_1 and density_2 to express the average densities (inlet/outlet) for the respective cases.

The equation above is valid for cases where viscosity plays no special role at all, that is, for fluids whose viscosity at T_1 or T_2 are quite similar.


If you can use Perfect Gas behaviour for the fluid through the HE, you can employ the following simplification:

deltap2 = [ deltaP_1 * P_1 / m_1^2 / T_1] X

[ m_2 ^2 * T_2 / P_2]


Here again I would use T_1 and T_ 2 to express the average temperature of the fluid along the HE, and the same criterion for P_1 and P_2


fvincent
Figener S/A

 

[TBP]

The HX manufacturer should be able to provide accurate pressure drop information.

 

[Reinoud]

Thx for the help allready provided!!

The first thing I did was to get the information from the supplier but they only have the pressure drop values at one state where I am (trying to) scaling from.

The thing is I put all my piping & HE (>1000) in a model where I used a electrical-equivalent setup to calculate the total pressure drop over the system. Then with this value I can get the individual mass flows through the pipes and HE's. Now for the HE it is a bit hard because I will not know what the mass flow will be for them. I think this should be done with trail and error to get appropriate results. I do not like building model like that, so does someone have experience with this or has another way of tackling the problem??

Thx in advance,
Reinoud

 

[abeltio]

What I've used in the past with success was the following relationship (for liquids):
Considered the press drop vs. flow as 2nd degree curve (valid in the fully turbulent region):
So:
dp_2 = dp_1 * (Q_2/Q_1)^2

Units:
[dp] = press or head
[Q] = volumetric flow

This was valid when the flow changes were < 30% of the original flow, always in the turbulent region (or at least we never noticed the difference...)
HTH
Saludos.
a.

 

[25362]

If the exchangers are inside a wider web of units and parallely connected to inlet and outlet headers the friction drop may not be determined by the exchangers but by the overall system. Then the exchangers would be passing liquid in a manner as explained by Abeltio (chau, amigo).