Direct fired MUA
Direct fired MUA
(OP)
Our customer has purchased four direct fired MUAs for a bakery building, constant speed each 50,000 CFM. There are many exhaust fans in the building mostly integrated with process equipment. B149.1 requires these MUAs to be interlocked with exhaust fans. Each MUA cannot be interlocked with more than one fan. In BC, MUA can be interlock with a fan that represents the 1/20 of total exhaust rate, is there a similar regulation in Ontario?
How can I operate these units and meet code requirements.
How can I operate these units and meet code requirements.





RE: Direct fired MUA
why does a customer just buy 4 MUA before talking to an an engeineer and thinking about code?
RE: Direct fired MUA
RE: Direct fired MUA
I'm reviewing two options:
1- Installing static pressure controller for zone served by each MUA and control air pressure at almost neutral level
2- Adding airflow switches on each exhaust fan and get the signals from these switches and control MUAs.
Code doesn't clearly mention that an electric interlock is required and these are kind of mechanical interlocks.
Please let me know your thoughts,
Thanks,
RE: Direct fired MUA
But, 50,000 CFM at constant speed? That's a lot of 'all or nothing' air.
1. Trying to control with zone pressure controller is going to be problematic, I would expect the pressure to be all over the place as the controller cycles the MUAs on and off.
2. Not sure I see how this would be a better option; surely simple interlocks are cheaper/easier to install.
Where is the 'requirement' that "Each MUA cannot be interlocked with more than one fan." coming from? I've seen examples of where exhaust fans out number supply fans and vice versa.
I'm guessing your bakery has a lot of small exhaust fans for hoods and direct vented appliances, and there is a large diversity. If this is the case, I think your best solution would be to go with your option 1 PLUS add VFDs on the MUA units. This assumes the burners in the MUA units can be turned down.
RE: Direct fired MUA
Code asks to interlock EFs to MUAs, although we need an intermediate controller to make sure all fans are on to turn on MUA which doesn't provide us the best operating conditions. As you're saying there are many EFs, for instance in each oven and proofer there are between six to ten fans.
Since I have one big room with four MUAs, the system can already be controlled in 25%, 50%, 75% and 100% of full capacity. Maybe I should stick to SP measurements and find proper spots for placing these sensors. The most effective way both in operation and cost.
Again, what's your understanding from this clause in B149.1, do you think I'll be fine?
7.20.3
Except as specified in Clauses 7.20.4, 7.20.12, and 7.20.13, a DFMAH shall
(a) have an air discharge capacity that does not exceed the total discharge capacity of the exhaust
system by more than 10%; and
(b) be interlocked in such a way as to prevent its operation unless the exhaust system is functioning.
Thank you all and let me know if you have any other thoughts.
RE: Direct fired MUA
RE: Direct fired MUA
RE: Direct fired MUA
To provide interlock between make up air and exhaust system, it should be all done thru control system. When even one fan out of a group of six or eight is not running, one MUA shuts down. Basically, I'll tie each MUA with a series of exhaust fans. This is not the way I'd like to do it, but apparently the only way.
Even though, I'm trying to avoid this and get a more accurate control by static pressure measurements.
Still considering various possible options!
Ideas are welcome,
Thanks,
RE: Direct fired MUA
RE: Direct fired MUA