Some gas fired heat exchanger questions

realolman

I have recently been working on  two similar gas fired industrial heat exchangers that sit side by side.  They are not from the same manufacturer, but are similar... both are  boxes with two burners and tube bundles that contain circulated oil. 

They are both about 14 million BTU, and are both set to a similar oxygen level... about 3 %.  They have impulse air modulated  North American regulators and burners... they are similar.

1.  Why would the stack temp be 400 deg. F less in one than the other ?.( 900+ in one 500+ in the other)... is the one with the lower stack that much more efficient?  Over the years that one has also had more failures of "inconel" tube bundle hangers than the other 



2.  Why does the Oxygen vary between low fire and high fire?  We have newer models where the gas is modulated directly by a valve instead of impulse air to a regulator; and the valve has several set screws that can be adjusted all through the range.  Is it just the nature of the beast that the impulse air to the regulator cannot maintain the same ratio the whole way through the range.?



3. Would the adjustment of the regulating orifice (which is down stream of the regulator) have any effect on the burner at low fire, or is it all regulator at that point,  ... with the regulating orifice doing it's thing at high(er) fire ? Gas pressure @ low fire approx 0.7 " wc and approx 15 osi @  high fire...



Thanks

Mark Eatherton

What's the excess air doing....

It sounds as if excess air is carrying heat away on the higher flue stack temperature.



What is impulse air? New term to me.



ME

realolman

Impulse air modulates main gas regulator

Thanks for your reply.  maybe impulse air is some sort of jargon... I don't know... I figure you'd know all the jargon anyway : )  More like an old term... this stuff is pretty old

The process controller modulates the combustion blower air through the use of a wafer valve... the modulated combustion air is tapped and plumbed to one side of the main gas regulator, which modulates the main gas.... supposedly keeping the ratio throughout the entire burner range. 

The farther the process temp is below setpoint, the more the controller opens the wafer valve, increasing combustion air... the more combustion air, the more the piping from the combustion air to the main regulator (the impulse air) increases the gas pressure from the main gas regulator.

I think I'm not understanding what you meant about the excess air... if the stack Oxygen content is similar, shouldn't the excess air be similar?  That's kinda at the heart of one of my questions.

Mark Eatherton

Excess air is different than residual O2...

A good combustion analyzer will show what this is.



It sounds as if the blower is what controls the gas input with positive air pressure.



The new mod con boilers are controlled with negative air pressure, which I believe would be more reliable and dependable than would positive air pressure. Positive air pressure could cause issues should the heat exchanger become partially plugged with the byproducts of combustion.



The system with adjustable cams is probably much more accurate and reliable as it pertains to excess air being blown through the combustion process, but this is really all just a guess, because I've never seen the boilers with which you are referencing. Are they Fultons or some commercial brand like that?



ME

realolman

Same Oxygen different Excess air?

I guess one of the things I'm wondering about is how two similar burners could have similar stack oxygen content , but different excess air.



Isn't the analyzer reading the amount of Oxygen remaining in the flue gas and calculating the excess air from that?



I could see it if one of them had 10% Oxygen and the other 2%..

Wouldn't similar ratio flames be the same temperature, no matter they're size?  Just a whole lot more of 3000° in a large natural gas fire of 2% than in a small 2% gas flame?  ... or am I not understanding correctly?



Wouldn't 2 similarly tuned burners in two similar heat exchangers have similar stack temps, unless the one was sucking more heat out of the products of combustion than the other?

Mark Eatherton

My understanding of excess air...

There is primary air, mixed with the gas and shot into the burner, and secondary air, used to move the heat through the exchange process. Excess air comes from the secondary air.



Air contains O2 and more. I have seen a mod con boiler with a low residual 02 content, but as much as 100% excess air. The draft over fire was significant due to stack action because of stack height, hence excess air. Low residual O2 is an indication of the efficiency of the burner to convert the gas through oxidation into thermal energy. There are certain theoretical physical limits as to what it can do (Stoichiometric conditions).



In an atmospheric boiler, I have seen the excess air caused by either an improperly adjusted baro damper, or improper air/fuel ratio. The two are related, but not proportionally.



Excessively high stack temperatures can be a cause of either fouled heat exchange surface on the fire or water side (visual verification on the fire side, pressure drop verification on the fluid side), loss of baffling in the flue gas stream (again, visually verified) or improper flow rate (based on delta T) across the process, including load.



If there is no load, the heat source should be bouncing off its' high limits. If there is a good load, and the DT across the source is high, then the flow rate is low. If the flow rate is correct, then it indicates a loss of baffles to slow the flow of flue gas and allow a good transfer of the thermal energy to the fluid.



I am somewhat familiar with the heat transfer characteristics of oil, particularly Braco 888, formerly used in solar thermal systems, and I know that it has the specific heat of ground up rocks :-) Not real good compared to water. I also know that when exposed to continuously high heat that it will break down into a tar based acid.



I assume that the designers of your system took all of this into consideration, but who knows for sure. Unfortunately, it is people like yourself who discover that they didn't, or that the initial operators were told about it, but it didn't get documented, and consequently, maintenance procedures not followed, and you get stuck with the consequences.



Keep us posted on your findings, and good luck in your endeavors.



ME