Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations IDS on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Direct Fired Make-up AHU Blower shutoff 1

Status
Not open for further replies.
overdesigned

overdesigned

Structural
Jan 26, 2024
4
Hi All,

I will preface this by saying that I am a building envelope engineer. So while I deal with mechanical equipment, it is not my specialty. I am working on a project where the flame on a direct fired make-up AHU went out, but the unit continued to blow cold air into the building. The building is in Montana and this happened when the temperature dropped to -46 F. It is a Mercury make up AHU that connects to a Honeywell burner controller. I am trying to figure out if the make up air handler should automatically turn off the blower when the flame goes out since this likely would have prevented the pipes to freeze inside the building.

If anyone has any insight into where I may be able to find this information, that would be much appreciated. I have included a picture of the product label for reference.

MicrosoftTeams-image_qujihu.jpg
 
Replies continue below

Recommended for you

Sort by date Sort by votes
Its a process safety requirement. Blower keeps running to purge out flammable gas that may have accumulated in the firebox from the time the burner flame went out.

Keep it that way. If you have a double block and bleed arrangement on gas supply to each burner, you could recommend auto stopping the blower say 5minutes after the flame is detected to have gone out, AND you have positive confirmation at the Honeywell BMS of the close status of both block valves and open status of the intermediate bleed on each burner gas supply. Am assuming the piping takeoff to the pilot gas burner is downstream of this double block and bleed.

Any change to this current safety measure will not be possible if you have only a single block and bleed on each burner gas supply.
 
Upvote 0 Downvote
This should all be in the manual.

There will be a shutdown sequence with timings to purge gases out of the burner although it is often part of the pre start sequence.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
Upvote 0 Downvote
I don't think the OP is referring to the combustion air fan, which would not be blowing into the building.

The fan blowing air into the building is kept on until a thermostat detects that the heater has cooled below about 130F. And it also does not turn on the fan until the heater is hot enough to supply hot air. If this thermostat fails the fan stays on off all the time. The consequences of failure is what determines what fail-safes are required. It seems that in this case there is a missing fail-safe to keep the building from freezing. I just replaced one of these thermo-switches. The part cost less than $10.

This failure would not have caused any significant problem, If the air was only drawn from inside the building
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

Ahmed Elarbagi
  • Locked
  • Question
Horizontal Direct Fired Heater Minimum Stack Height Requirements
Replies
3
Views
238
shvet
shvet
Z
  • Question
Pipe Bowing in a fired heater 2
Replies
10
Views
3K
dvd
dvd
L
  • Question
Help setting up a tolerance stack up analysis for a door latch assembly
Replies
1
Views
165
3DDave
3DDave
Mizzy07
  • Question
Redesigning an elevation mechanism, does this make sense? 2
Replies
8
Views
301
BEMPE16524
BEMPE16524
Bradley5
  • Locked
  • Question
Help selecting a low pressure, higher volume air blower.
Replies
5
Views
205
EdStainless
EdStainless

Part and Inventory Search

Sponsor

Back
Top Bottom