2 replies to this topic

[kybele39]

Hello All,

I am going through my notes and looking to summarize the differences in Natural versus Forced draft operation in water bath heater operating.

The differences are:

1. In size of Fire tube: Natural draft operation leads larger diameter size of the fire tube and this makes heater shell larger.

2. Flux Rate: Natural draft operation allows to design for 10,000btu/hr-ft2 flux rate on the fire tube but with forced draft you can use higher flux rate, such as 20,000btu/hr-ft2. So this makes fire tube smaller on forced draft operation

3. Natural draft operation cannot have flexible turndown but forced draft burners can have 100:1 turndown capacity

4. Natural draft operation has higher emission due to higher excess air

5. Forced draft needs combustion air fan and purges flue gases easily from the tube but natural draft does not require any fan. It uses large amount of oxygen and for purging they need a purge fan for the fire tube to purge

6. Natural draft operation needs less capital. But forced draft has high capital cost due to the electric demand. I cannot elaborate on operating cost because it is really different for each company operation.(?)

7. Natural draft operation start up is difficult then forced draft.

Anything I missed?

Please let me know your thoughts. Thank you.

Edited by kybele39, 08 March 2011 - 04:10 PM.

[Art Montemayor]

kybele39:

You state that the heat flux in a water bath heat is different for the two types of flue gas systems:

Natural draft = 10,000 Btu/hr-ft²
Forced draft = 20,000 Btu/hr-ft²

I don’t think that the maximum allowable design heat flux should be different due to the type of draft employed on the fired gas side. I have always used 8 to 10,000 btu/hr-ft² on submerged coils in reboilers or water heaters. The main reason I use a maximum design flux on such heat transfer equipment is avoid the vapor binding ON THE OUTER, LIQUID SIDE, that can take place when one concentrates too much heat on a unit surface. This reduces the heat transfer due to vapor binding and also causes hot spots which promote localized corrosion. The type of flue gas flow should not affect the results on the outside surface of the heating tube.

Where did you obtain your information? Are we talking about the same heat flux effect? Can you furnish a reference for your figures?

[kybele39]

Dear Art,

API 12K states the Natural draft operation should be based on 10,000 btu/hr-ft2 flux rate.

Meaning the duty x efficiency/ surface area of fire tube should be equal to flux rate.

On the other hand some burner companies mentioned they can provide forced draft operation for 20,000 btu/hr-ft2 flux rate. So fire tube surface area relatively smaller to natural draft operation.

Actually for forced draft operation I really do not have a literature beside a burner company input. It might be flux density... I have to check with them.

Thanks

Kybele

Edited by kybele39, 08 March 2011 - 04:09 PM.