Heat exchanger contractual point meaning

[maxch93]

Hello everyone,

I am new on this forum, so I hope this topic does not already exist.

1) First, I would have confirmation that the sizing point of a heat exchanger is the operating point (let's say the triplet : Power to be evacuated P, T_Cold_Inlet and T_Hot_Outlet) that maximises the required heat transfer coefficient (KS_required in W/K).

2) Then, I would have confirmation (that correctly specified ) contractual point of a heat exchanger must be the sizing point.

3) This is my main issue. I feel that the common practice is to specify to the supplier only one point (= the contractual point). Does this practice ensure that if we deviate from this contractual point (P_CP, T_Cold_Inlet_CP, T_Hot_Outlet_CP), we will still have KS_fouled_supplier(P_new, T_Cold_Inlet_new, T_Hot_Outlet_new)> KS_required(P_new, T_Cold_Inlet_new, T_Hot_Outlet_new) ? Even if KS_required(P_new, T_Cold_Inlet_new, T_Hot_Outlet_new) < KS_required(P_CP, T_Cold_Inlet_CP, T_Hot_Outlet_CP), maybe KS_fouled_supplier(P_new, T_Cold_Inlet_new, T_Hot_Outlet_new) will stall, I do not know the behaviour of the supplier function KS_fouled_supplier(P, T_Cold_Inlet, T_Hot_Outlet) which depends on the supplier heat exchanger design. Indeed, can you confirm that one point specification is sufficient in order to ensure correct sizing anywhere else (same question as for centrifugal pump specification, even if even more complex I guess).

4) Does the supplier ensure that its KS_fouled_supplier(P, T_Cold_Inlet, T_Hot_Outlet) > (KS_required(P_CP, T_Cold_Inlet_CP, T_Hot_Outlet_CP) - [X %]) ? Is there such a common criterion used by all heat exchangers manufacturers ?

Thank you for your help.

 

[MortenA]

Selling heat exchanger must be like selling rubber band by the meter.

There are many parameters that can vary and all will impact performance.

I think you can rest assured that the vendor has learned all the loop hole the hard way and will not deviate when you hit one of the soft spots.

With reagrds to your fouling margin, if you specify this then the vendor will oversize to ensure that you still have the required performance at the fouled conditions as you say yourself.Fouling factor is normally specified as a resistance in e.g. ft2*ºF*hr/BTU or m³*K/W or whatever unit suits the vendor.


Best regards, Morten

--- Best regards, Morten Andersen

 

[EdStainless]

People do use fairly standard fouling factors, unless they have very accurate ones from experience.
The reason is that you don't really know, and in many services operators are not measuring enough parameters in real time to calculate the actual fouling factor. I have only ever seen this done in power plant condensers.
One risk is that oversizing the HX may by fouling margin but it will hurt effectiveness and maybe pressure drops.
Rather than asking for more test points, having some insight into the modeling may be more helpful.
If there is one parameter that might vary more than the others let the designers know, They can facto this in.

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P.E. Metallurgy, consulting work welcomed