Heat exchanger performance problem

[sheiko]

Hello

The performance of one of our shell and tube heat exchanger has changed in a significant manner.

The service is crude oil in the tubes and diesel in the shell.
It has 1 shell pass and 4 tubes passes.
Tube side equipped with turbotal
Surface is 197 m2.

Before a shut down, performance was:
LMTD = 70C
F = 0.88
Duty = 1550 Gcal/h (heat balance closure = 1%)
Temperature cross = 14C

During the shutdown, tube bundle have been replaced by a new one. New Turbotal as well.

After shutdown, in service, performances are:
Before a shut down, performance was:
LMTD = 50C
F = 0.34
Duty = 1800 Gcal/h (heat balance closure = 5%)
Temperature cross = -21C

What could be the possible causes?

@Lionel_Sheiko / geniechimiquefacile.com

 

[sheiko]

Additionnal data:
C = crude
D = diesel
I = in
O = out
M = mass flowrate
Cp = specific heat

Before shutdown:
Tci = 152C
Tco = 205C
Tdi = 280C
Tdo = 219C
Mc = 47.5 t/h
Md = 37.9 t/h
CPc = 0.62
CPd = 0.68

After shutdown:
Tci = 143C
Tco = 207C
Tdi = 272C
Tdo = 186C
Mc = 45.7 t/h
Md = 32.8 t/h
CPc = 0.62
CPd = 0.66

@Lionel_Sheiko / geniechimiquefacile.com

 

[marty007]

If I am reading the information correctly it looks like your performance has improved from 1550 to 1800 Gcal/h.

What were the reasons for requiring the bundle replacement?

Had tubes been plugged in the original unit? If so the new unit could now have more effective surface area than the original unit as all tubes would be available.

How badly fouled was the bundle on the original unit when the data was collected? The new unit would be in the cleanest state possible with no fouling resistance when first put in service.

Just a couple of thoughts.

 

[srfish]

The performance after shutdown is impressive. In years as a heat exchanger consultant, I have never seen an LMTD correction factor achieved that low (0.34). The lowest has been 0.66. The charts in TEMA have the lowest correction factor as 0.50.

 

[sheiko]

Thanks Dale
Do you mean a decrease of F means better performances?

@Lionel_Sheiko / geniechimiquefacile.com

 

[srfish]

It means the performance is much better than anyone would expect. Congratulations.

 

[sheiko]

Thanks
But it's not MY success..
I have just noticed that (the only one unfortunately).

@Lionel_Sheiko / geniechimiquefacile.com

 

[EdStainless]

You don't mention the pressure drops across each side.
Watching these is a good way to keep tabs on fouling.
So unless extracting more heat from the diesel is a problem you are good to go.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[sheiko]

Guys I have an additionnal question if you allow me.

During the design stage of a new heat exchanger, temperature cross (hot outlet minus cold outlet is negative) means low F which in turns means that some part of the surface is useless (no heat transfer).

so why during operation (once the heat exchanger is built and in operation), low F does mean good performance?

Could please help me clarify this confusion (difference of understanding of the same concept between design and operation)?

Thanks!

@Lionel_Sheiko / geniechimiquefacile.com

 

[ione]

Are you messing LMTD correction factor and fouling factor?
The lower the fouling factor the higher the performance of the heat exchanger.
The higher the LMTD correction factor the higher the performance of the heat exchanger.

 

[sheiko]

Ione thanks but I am not talking about fouling factors. I do know what it is.
Please read srfish answer to understand what I mean.

@Lionel_Sheiko / geniechimiquefacile.com

 

[ione]

This seems really to elude me as well. I've always known that for an ideal counter-current heat exchanger the correction factor was equal to 1.0. And values lower than 1.0 accounted for multi-pass or crossflow configurations. Perry's states that LMTD correction factors below the 0.8 threshold indicate inefficient heat exchanger design.

 

[ione]

I’ve tried to re-think at it in a different way, and taking into account the data you’ve posted.

Being:

Q = U*A*(LMTD)*F

If you've noticed an increase in Q with a decreased LMTD and F, as A is constant, U must have increased. A better overall heat transfer coefficient translates in a better performance.