HEI Fouling resistance in Heat Exchangers

[MechMad1]

Hi!
Just trying to define the fouling resistance for a tube/shell heat exchangers.
I'm following "HEI Standards for closed feedwater heaters", para. 2.2.2.
As I understand, I should consider 0.0002 units in the inside tubes (for all the zones) and 0.0003 units in the outside tubes (only desuperheating and subcooling zones).
Am I right?
Some supplier is telling that I should consider 0.0002 units in the inside tubes and 0.0005 in the outside tubes ...
As you see, we do not interpret the standard the same way ...

Thank you!

 

[rmw]

First, is it boiler feedwater heaters that you are referring to as tube/shell heat exchangers?

If so, go with the HEI numbers. If it is some other type of Hx, use fouling factors common for the type of service that the Hx will see.

Boiler feedwater is typically a higher purity than lots of other types of liquids found in Hx's. If your fluid is not as pristine as BFW, you need to use different fouling factors as appropriate.

rmw

 

[MechMad1]

Hi rmw!
I didn't explain myself correctly.
Both, me and the supplier, are trying to select the fouling resistance according to HEI. The problem is that we do not interpret the standard the same way.
The supplier's interpretation is that we should consider 0.0002 units in the inside tubes and 0.0005 in the outside tubes ...
I interpret that it should be 0.0002 units in the inside tubes (for all the zones) and 0.0003 units in the outside tubes (only desuperheating and subcooling zones).
In the end it's a matter of money.
And yes, I am reffering to a boiler feedwater heater.

Thanks!!

 

[vpl]

What is the fluid outside the tubes? Or are you just looking for everyone to agree with you blindly?

Why is the supplier recommending the 0.0005? -- after all he has a lot of experience with sizing heat exchangers and you're in part paying for his expertise.

Are you being "penny wise and pound foolish" in that you will save money up front in buying the heat exchanger and then spend money like crazy in keeping it clean over the operating life? I know a plant where they ended up having to derate one of their heat exchangers and still descale and clean it every outage because the original design was a bit too agressive on getting "right on" the heat transfer capability with no operational margin.

Patricia Lougheed

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[rmw]

I don't have access to HEI FWH standards. The condenser standards don't use fouling factors but rather a cleanliness factor.

Your situation is interesting in that the vendor's position will make their heater larger meaning that they will be less competitive than a supplier who uses your numbers.

Their competitor on the other hand, using the .0002 and .0003 values in the zones you indicate will have less surface area and hence less cost, so the first vendor is taking a risk of putting their competitor in a more competitive position.

I would ask myself why they would want to do that. Could it be as Patricia has postulated that they have some emperical data that leads them to put themselves in that position and risk the job?

Another thing is that since the DSH and drains cooler zones are a fairly minor part of the heater (depending on which heater it is of course), the overall oversurface due to the additional fouling factor your way won't be as large as applying their numbers their way.

Advantage: Competition.

rmw

 

[MechMad1]

The fluid used outside the tubes is steam (an extraction of the HP turbine). I think is fairly reasonable to use the fouling resistance recommended by the HEI standard.
If the supplier had given the values according to its experience, we would have re-considered the values, but it seems that is only a different interpretation of the standard. So, that's why I asked.
Nevertheless, we will have to ask all the suppliers to use the same fouling resistance, so the comparison will be fair.

HEI "It is recommended that a minimum fouling resistance of 0.0002 units be applied to the tube surface and corrected to the outside effective surface. An additional fouling resistance of 0.0003 units should be applied for the outer surfaces in the desuperheating and drains subcooling zones"

 

[zekeman]

Excuse me, but since when does the purchaser have to get down to working out
the details of a heat exchanger.
That should be left to the vendor who must warranty its performance based on the application. So what happened in the umteen years since I last bought one?

 

[rmw]

You'd be surprised how many specific details users dictate to Hx suppliers when the topic is power plant FWH's. And to their credit, there are (or were) some very astute engineers out there amongst the users whose diligence prevented the suppliers from furnishing junk. Where they weren't that discriminating, well some real less than first class stuff got shipped.

rmw

 

[EdStainless]

If ghesa has give us the direct wording then I don't think that either of you are right.
You shouldn't need to apply the 0.0002 to both sides, only the side that is limiting the heat transfer. And the DSH and DSC zones get an additional amount bringing them to 0.0005.
But either way, what are the contract penalties for not meeting the design point? Buying the smaller (less expensive) heater may cost you efficiency for the next 20 years.

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

 

[muld0020]

I assume you have the thermal rating (thermal gaurantee see HEI) the vendor gave you. Then what you can do to check is that the sum of the resistances (including fouling) equal 1/U that is given for the zone. The vendor's rating should balance. As noted in the standard there is no shell side fouling in the Condensing Zone. The resistance should be a minimum of 0.0002 in the tube side and 0.0003 shell side (DSH/DC) as you correctly noted in the standard. If it is a competive bid then they would lose with a 0.0005 zone fouling factor (unless it was in the specification), but they technically meet the standard. A full discussion about fouling factors could be very lengthy.

If you are concerned about getting the correct design be sure to find a vendor who specializes in Feedwater heaters, not just HX.

The required resistances are tube side fouling, tube side film, metal resistance, shell side film, and shell side fouling.

Here is an example of DSH resistances for a recent HP FWH I designed.
BTU/HR-SQ FT ºF 100.4
TUBE SIDE FOULING 1/U 0.000244
TUBE SIDE FILM 1/U 0.000406
TUBE WALL 1/U 0.000516
SHELL SIDE FILM 1/U 0.008499
SHELL SIDE FOUL 1/U 0.000300