Firetube design according to API 12K

[fdomin]

I´m designing a couple of indirect type oil heaters to API 12K and I have some questions about firetube design.

Firetube heat density is defined as the Firetube rating divided by Cross Sectional Area and Efficiency.
Is not clear to me which efficiency API is referring to.
Burner efficiency? Overall heater efficiency?

API also states a maximum heat density for natural draft burners. What about venturi type burners?

Any help will be appreciated.

 

[Hookem]

Saw your note about API 12K and heaters. Do you have excel equipment datasheet? That would be most useful.
chesney@uei-houston.com

Thanks.