I have a high bay manufacturing facility with two100% OSA, CV, low cfm but high
air velocity heating only units. These are intended for comfort heating.
The building also has one 100% OSA, CV make-up air (MUA) unit, with a gas
furnace. The MUA is only used when one or more roll-up doors are open with
trucks inside and unloading. This blows the diesel fumes out of the building.
The MUA has almost 3x the air volume capacity as the two comfort heating units
combined.
When I model the proposed design, with just the two units intended for comfort
heating, I end up with almost 700 unmet heating hours. I add the capacity for
the MUA unit and schedule it for the purpose of comfort heating, and my unmet
hours go down to less than 10.
Also in 90.1, App G, if unmet load hours exceed 300, heating capacity can be
added incrementally until unmet load hours are reduced to below 300.
So would it be fair to not include the MUA unit for heating, but simply add one
or two more of the comfort heating units to get the unmet hours down? It seems
like it may be over-kill to include the MUA unit for the purpose of comfort
heating.
Any guidance would be appreciated.
Thank you,
Pete
Pete,
Before increasing the proposed design heating capacity, I would consider other changes you can make to the model:
* Increase the process/plug loads (after all, it is a manufacturing facility)
* Decrease the heating thermostat setpoint (70F or less)
* If you have nighttime setback, create a morning warm-up period
* Increase the thermostat throttling range a few degrees (reflecting less "comfort" but resulting in fewer unmet hours; IMO a last resort before adding heating capacity)
Note that you would make the same changes in the baseline for any/all of these suggestions.
Regards,
Bill
[Senior Energy Engineer 28Jun2012]
I would not include the large unit. It is a specific
purpose/intermittent unit. I would however investigate how it is going
to be used. Normally you try to segregate the truck loading areas from
the rest of the plant to limit the area exposed to the exhaust fumes and
to make it easier to remove them. Sometimes though, making large items,
this is not possible. Typically these units are connected to CO/CO2
sensors and also have an exhaust fan. If you use the MUA unit to blow
the exhaust fumes out it won't work when the doors are closed. This is
why an exhaust fan works better with the MUA unit. The way the loading
normally happens is the trailer is backed in first and the tractor comes
in last, the door is closed and the truck is shut off for the loading
period. That puts all the truck exhaust on the ceiling by the loading
doors. That is where you put the exhaust fans. The MUA unit is placed
across from the exhaust to push the truck exhaust to the exhaust fans.
The unit only runs until the sensor is satisfied. As Bill mentioned you
don't have to bring that MUA up to plant temperature either, that would
depend on where you are dumping it and how much air there is. You may
not have to heat it at all. I have used systems which bring in air
unheated and use fabric tubes to spread it at ceiling level in the hot
stratified air next to the ceiling.
I don't consider these events in the general system for heating the
building but you do have to consider the event and how to mitigate it's
impact on the plant and meet all the safety rules. You generally have
unit heaters near large roll up doors as they are leaky and not as well
insulated as the rest of the building. This comes in handy when you open
the doors or when you have 10 tons of cold steel sitting in the loading
bay. Don't forget floor drains.
Bruce Easterbrook P.Eng.