Oil Fired Boiler Flue / Exhaust Gas

[aquatica]

Hi,

Can anyone reply please!,

1. what is the effect of inlet air temperature on the flue gas temprature and flow in an oil fired boiler? any info? tips? or a method to calculate it...?

the question relates to heavy oil fired boilers.

Cheers!

 

[RobertACookPE]

It should be roughly linear: Assuming a constant air flow and constant fuel flow with no change in fuel chemistry or burning adjustments = same heat gain by the furnace air.

Cooler inlet combustion air, assuming the induction fan flow doesn't change much with the cooler air, and assuming the cooler combustion air doesn't change burner characteristics, means cooler outlet exhaust air.

Slight changes in inlet air temp are implied.

As always check my assumptions, check my math.

 

[itdepends]

It's a boiler though so there's heat transfer to the water in the fire tubes. Chances are the outlet temperature will increase- but only slightly -depending on the setup of the tubes in the firebox.

 

[aquatica]

Thanks for the reply.

I was thinking that mass flow is not constant in boiler, for instance; a load change would definately change the flue gas temp. also, backpressure changes in stack, the blower operating point moves along the Q-H curve..

what are your thoughts?

Cheers

 

[RobertACookPE]

Well load changes will drive everything: You have got to fix something before analyzing the effect of temperature on flue gas temperature.

That why I assumed you were at some specific load (implied 100%) at your highest reasonable summertime temperature: That means (perhaps) 95-98 degree (inlet suction temperature) of the combustion air, 90 degree temperature of the fuel (unless it is stored underground in a constant temperature tank), and all boiler feed pumps and boiler piping and fuel pumps/bloweers and combustion air fans at their nominal rated conditions: not dirty, or corroded or worn, but at nominal conditions.

This gives you some reasonable (not exactly worst-case) combustion efficiency and heat gain.

Then change to your worst-expected winter conditions: Cold (35 degrees? 15-20 degree? -15 degree? You know your conditions at your site.) Recalculate your fan efficiencies at that temperature, fan flow rates and heat gains for that lower temperature of combustion air, reheat air temperature after the economizer, and actual energy gain by burning the (colder) fuel.

I expect you'll find more changes from the lower air temperature affecting fan flows than anything else. And that the changes while controlling combustion air flow from 10% load to 100% load will be much, much more important than the minor changes at 100% load from -15 degees outside air to 95 degree outside air.