Max. Temperature Limit (NFC-6 or Table 1B)

[Skorp1on]

I am not a full-time B&PV engineer but I am now faced with a question from someone reviewing a fin-and-tube heat exchanger design that needs to be commissioned quickly.

Reviewer is having concern with tube with material SB-111 C12XXX with Design Temperature of 400F exceeding those allowed by external pressure table NFC-6 (Sec. III, Part D), in this case 300F.

However, in reading UG-20(c) of SEC VII, Div I(2007), it stipulates "Design temperatures that exceed the temperature limit in the applicability column show in Sec II, par D, Subpart 1, Table 1A, 1B, and 2 are not permitted." SB-111 C12XXX falls under table 1B, where 400F is allowed when applied with SEC VIII, Div. I.

In actual operation, the metal temperature of the tube is not expected to not go higher than 350F. To be even more conservative, tubes are threaded with a collar that is 0.007" thickness with aluminum fins spaced very close to each other. Therefore, the bare tube is not expected to see the full 400F metal temperature.

Can ASME code board member advice whether design temperature of 400F is permitted for SB-111 C12XXX?

Thanks.
ASME

 

[bernoullies123]

If you are looking for a response from ASME can I suggest that you refer to ASME Section VIII MANDATORY APPENDIX 16.

 

[Skorp1on]

how about from non-ASME member?

 

[TomBarsh]

ASME UG-20(c) specifies that "design temperatures for vessels under external pressure shall not exceed the maximum temperatures given on the external pressure charts."

Thus if your tube is subjected to external pressure at a temperature greater than that listed in the vacuum table then the material likely isn't permitted for that application.

The "Guidebook for the Design of ASME Section VIII Pressure Vessels" (by Farr/Jawad) gives a method for determining allowable longitudinal compressive stress (as for weight, etc) when the temperature exceeds that on the external pressure charts (the "B" factors are also used to determine allowable longitudinal compressive stress). See equation 2.25 in Section 2.3.

As other poster stated, you can contact ASME directly.

 

[MrBTU]

Skorp1on,
Please see UG-19(a)(2) - mean metal temp design, and UHX-19.2.1(b). State to the reviewer that "tubes are designed for mean metal temp of XXX (350?)". Supply supporting thermal rating calcs that show that in operation, tube metal temp is XXX. Stamp on nameplate and show as a note on drawing "tubes designed for mean metal temp of XXX, cooling water must be flowing whenever hot fluid is admitted to shellside (or tubeside)"
This should get you your approval.

 

[Skorp1on]

Thanks MrBTU.

However, I am still concerned with what TomBarsh who pointed out that ASME UG-20(c) specifies that "design temperatures for vessels under external pressure shall not exceed the maximum temperatures given on the external pressure charts."

I suspect the tube's mean metal temp may still be greater than 300F (as stated in NFC-6), despite cooling water flow during actual operation.

Anyways, the HX designer is looking to supply supporting thermal rating calcs to show the expected actual tube metal temp in operation to make further evaluation to get approval.

Thanks again for your feedback.

 

[MrBTU]

Yes, if your mean metal temp is still greater than allowed by UG-20(c), then you're out of luck....

 

[curtis2004]

Skorp1on:
Can you give us more details about process and temperatures of mediums?

 

[Skorp1on]

coolant = water in tube
cooled medium = AIR in the shell
air tempearture is in the first post.
water temp is 30-40 C.

 

[MrBTU]

Skorp1on,
I find it hard to believe that a tube wall with water on one side at 40 deg C max, and air on the other side, can exceed 300 deg F... Please take a look at the respective heat transfer rates of the air and the water and calculate your mean metal temperature. It will be much closer to the water temp than to the air temp.

 

[unclesyd]

Are you concerned with the fins or the tube proper?

The fins will approach the max air temperature while as stated above the tube wall temperature will approach the water temperature in operation, water flowing.

 

[curtis2004]

Skorp1on,
Water inlet is 30-40C. Can you provide expected water outlet, air inlet and air outlet temperatures? This is the key to answer your question.

 

[Skorp1on]

MrBTU - thermal analysis now seems to show mean metal temperature to be less than 300F. yes, it is difficult to imagine tube temperature to exceed 300F but think we have to consider design temperature of shell that is limited by the hot air.

unclesyd - tube proper is point contention.

Any how, in reading further the note (G9) of Table 1B of Sect II, Part D, it gave a "Maximum temperature for external pressure not to exceed 400F" for the material in question - hopefully, this is enough to get the necessary approval.

thanks for everyone's input and contribution.