Hello everyone,
I was recently asked to model the impact of Cambridge 'direct' fried air handling units for a warehouse building compared to a conventional MUA with supply and exhaust. Has anyone had experience with these units?
They boast a very low fan power consumption (5 hp for 8565 cfm of air), 92% thermal efficiency, and a temperature rise and max discharge temp of 160 F.
Most of the inputs are straightforward however I am not sure I am modeling the 160 F temperature rise properly. As of now I have Packaged Single Zone systems and have entered 160 F as both the 'zone entering max supply temp' and 'hot deck max leaving temp'. Would this fully capture that temperature rise? I ask as I am getting about 150-200 unmet heating hours in these zones, even though all other inputs are as per the mechanical engineer. Is this significant, or likely just the difference between how eQuest and the mechanical engineer size their loads? The building is a cold climate (southern Ontario, Canada) LEED building, so I want to make sure I am modeling as much benefit as possible.
Adam Barker, C.E.T., LEED AP BD+C
You have to be careful where you use these types of heaters due to IAQ.
Direct fired units dump the products of combustion into the building as
well as fresh air. So you basically have CO2, CO, and water, minimum
going into the building with the air. There can be other nasties as
well depending on how efficient the burn is. In Canada their
application is very limited by law.
Bruce Easterbrook P.Eng.
Adam,
I haven't had any experience with the Cambridge unit, nor am I able to answer your question about the unmet heating hours. Have you tried auto-sizing just for a cfm and Btuh capacity comparison with the mech engineer's load calc? Also, is the weather file representative of the warehouse location?
Because the building is a warehouse, it triggered a question in my mind that somebody else might be able to answer. That is if eQUEST models thermal stratification in high bay areas such as warehouses or hangars? If so, it may calculate unmet hours on the coldest days near floor level and a nice warm temperature at ceiling/roof level. Just a wild thought.
Mike Busman
Michael R. Busman, CEM
You raise a good point, Bruce. I forgot to add in my other email if consideration had been given to closed-combustion infrared heating as I was pondering the difficulties in heating high bay areas.
Mike
Michael,
The weather file is representative of the area, and I did run an auto sized load, which calculated heating and airflow capacities much higher than what was specified (about 3x higher on average off the top of my head).
You raise a good point regarding de-stratification - reduced stratification is another claim the company boasts, as the building is pressurized. The claim to cut ACH due to infiltration in half. I believe the team is looking into reducing the # of de-stratification fans in response to this. I searched the forums for how to model de-stratification about 2 weeks ago and came up with a few threads basically hinting that it can't be accurately done. Does anyone have a different opinion, or know if a way where this can at least be approximated? I want to avoid playing with the infiltration rates as I do not believe that is acceptable for a LEED model (at least here in Canada it isn't). As of now I have not modeled this effect.
The combustion problem is somehow managed through some limited venting, though I can't remember exactly what is going on, which is why I called them 'direct' fired units.
Adam Barker, C.E.T., LEED AP BD+C
I have used infrared heaters in a high bay application but they were
wall mounted closer to the floor and were used to protect the footings
from freezing in a mothballed building. They can be a hazard with
forklifts running around. Most high bays use large fans to drive the
warm air back down and prevent or reduce stratification. Much depends
on the warehouse and how it is used. Many don't have much of an
occupant load and don't need much fresh air. Saving heating costs means
minimizing fresh air. Mostly you are protecting the products stored in
the warehouse. ASHRAE has a 30% guideline on their website for
warehouses which gives some guidance on good design to reduce energy
consumption.
http://www.ashrae.com/standards-research--technology/advanced-energy-design-guides
Bruce
ASHRAE fundamentals has design information on stratification and on
supply air throws etc. You are getting into some pretty advanced
modelling which eQuest won't do and your customer won't pay for. Also
remember the Cambridge sheet is done by marketers and advertisers not
engineers. I'm sure the unit will do much of what it claims but I don't
think this is what you want to do if you are designing an energy
efficient building. You are going to be able to control the buildings
air leakage with good design. What you need initially is an idea of how
much heat the building needs and how much fresh air. A MUA unit is only
used to temper the incoming fresh air and can be used for
pressurization. They are not used for heating. Too much pressurization
is a problem as well. It doesn't necessarily reduce ACH, maybe a little
if it is set up right and there is no wind. A supply air fan can be
configured with diffusers to give you a high velocity discharge which
will mix the air in the building. It will, if the air is released high
enough also entrain some of the hot stratified air at the ceiling and
bring it down with the main air flow. This flow will drive down into
the cooler air near the floor and mix. This warmer air will also want
to rise as it is less dense than the cooler air in the building and a
circulation flow can be established. I say can because generally a
warehouse is full of racks floor to ceiling which will prevent this
circulation pattern from happening. You are going to have many dead
zones which will need stratification fans. You may not want all of
these fans pushing air down. You will need another form of heat
independent from the MUA unit to provide the rest of the heat the
building requires because heating a building with 100% OA is not
efficient nor would regular building require this much fresh air.
Consult ASHRAE 62.1 to determine how much OA that you need. Consult
SB-10 January 2012 of the OBC. You will find it will reference ASHRAE
92.1 with exceptions. You must follow these codes. They will also
dictate your allowable lighting loads. If you are dumping a lot of air
then heat recovery becomes an option. A warehouse can also have a lower
heating set point which will reduce heat loss through the envelope.
They typically don't have to be cooled. But that brings up another
problem, summer heat. You need ventilation to remove heat. Economizers
work very well in Canada. If you use a AHU with an economizer you don't
need a MUA unit. A warehouse is a building with tons of mass, over cool
it at night with an economizer set at 100% exhaust, let the air stratify
and exhaust the hot air off the bottom of the roof. Go to minimum fresh
air in the heat of the day and let the mass provide cooling.
Stratification fans are not bad in the heating season as all the heat
they make and energy they use stays inside the building.
This problem comes down to visualization. You can calculate with
reasonable accuracy if your supply fan discharge will get down to the
floor. If you do it at an isle crossing you will get 4 circulations in
the isles. Put a up-blast stratification fan between the supply air
diffusers to assist. Outboard areas require you to see the air flows
and decide if an up or a down flow fan gives you the most benefit. You
can't rip the sheets off the order pickers clip board either.
With your auto size load at 300% you will need to determine why. Check
your people count, outside air and check infiltration. Compare OA to
62.1. Check your envelope losses, compare them to the engineers heat
loss. When it comes to heat loss OA is the elephant. Check all the
eQuest defaults, many will have to be changed.
Bruce Easterbrook P.Eng.
Actually ASHRAE 90.1 is the reference standard not 92.1
Bruce