Kenda,
I'm curious as to whether you received any responses to the query below.
I have incorporated a practice of manually setting the site altitude in
my eQuest models to "0," because (1) as designers we do not specify
equipment with CFM's at sea level, it is already "corrected" for
altitude in this sense, and (2) those who review models tend to miss the
significance of the altitude factor in the SV-A reports and complain of
incorrectly entered airflow rates.
I don't know if this constitutes a "best practice," but I know it is
common to a number of the regular contributors to these lists besides
myself. As it stands, current standards (90.1-2007) don't have us
"de-rate" or otherwise adjust required minimum SEER/EER values for
altitude... so inversely it seem appropriate to remove the altitude
variable when modeling for a performance rating... thoughts?
NICK CATON, E.I.T.
Nick/Kendra,
The altitude factor effects the supply air and the outside air. The write up
below suggests that one calculate both supply and outside air at sea level
and then apply the altitude factor to it. In real life, however, the design
engineer is calculating and specifying supply and outside air cfm that is
already adjusted for the altitude and the altitude factor should simply be
set at 1.0 in eQUEST.
The excerpts below discuss the areas of eQUEST impacted by the altitude
factor:
Note: the quantities in this report (SV-A) have been adjusted for
altitudeeven though DOE-2 requires that any flows you enter in SYSTEMS
be at sea
level.
1. SUPPLY FLOW
is the calculated or user-specified supply flow for each zone. Only if you
have specified a value for the ASSIGNED-FLOW keyword in the ZONE command
will the value here correspond to your input. The ZONE keywords
AIR-CHANGES/HR and FLOW/AREA will be accepted by SYSTEMS only if they are
consistent with the user-supplied HEATING-CAPACITY and COOLING-CAPACITY and
are equivalent to a flow larger than that of the exhaust from or the
ventilation to the zone. The ALTITUDE FACTOR will be applied.
5. OUTSIDE AIR FLOW
reflects the user-specified outside air quantity entered at the zone level.
If OUTSIDE-AIR-FLOW is specified, its value is multiplied by the ALTITUDE
FACTOR and reported here. Otherwise the reported value is the maximum of
the flow-equivalent values of OA-CHANGES and OA-FLOW/PER, multiplied
by ALTITUDE
FACTOR. For the actual amount of outside air delivered to the zone for
central systems, see OUTSIDE AIR RATIO above.
Unfortunately I think it might be more complicated than that; I wish it
wasn't. I have an old printout of an eQuest/DOE2 document called
"Sizing Air-Side HVAC Systems" which steps through the calculation. I
haven't been able to figure out where I printed it from (help file?,
dictionary? engineers manual?).
My take is that eQuest sizes coils based on sea level airflow. At the
end of the calculation it simply multiples the needed sea level airflow
by the altitude factor to account for altitude effects. Per Carol's
email, eQuest assumes any user entered flows are at sea level. This is
frustrating because plans usually show airflows at site altitude, not
sea level. We've tried Nick's work-around which is to change the
altitude to 0 and enter airflows per the drawings. This works on the
airflow side but not the coil side. The coils are sized using the
entered airflow x 1.08 so eQuest creates coils that are larger than what
you actually have.
What seems to be more correct is to keep the altitude specified and
divide every airflow you enter into the model by the altitude factor.
If you're in Denver and want to model a 10,000 CFM fan, enter
10,000/1.21 into the supply CFM input. eQuest takes that number and
multiplies it by 1.21. Every input with a "CFM" must be adjusted
including exhaust, CFM/SF, etc.
Matt Dubrovich, PE, CEM, BEMP
What a pain. =(
Matt:
If the ?coil upsizing factor? for altitude (1.08 in your example below?) is not derived from the site altitude as entered under ?site properties,? a much easier solution might be to identify where else the altitude is being from and entering a zero there? The site properties dialog pulls it from the weather file, right?
DOE2 / eQuest developers:
Is there any feasible possibility of adding an option in a future release/patch to turn altitude adjustments on/off? This might logically be located adjacent to the altitude input in the Site Properties dialog (I?d vote for default = off).
NICK CATON, E.I.T.
Hi Matt,
That's the same approach we take. Then I double check to make sure the
airflow rates in the HVAC summary report in eQuest match what the actual
rates are.
*Aleka Pappas*
Nick,
I skipped over a few details in the last email. The ?1.08? is the typical sea level transfer factor in: Q=1.08*CFM*Delta_T. eQuest?s altitude factor is the sea level transfer factor over the site?s transfer factor which yields what I have seen called the CFM correction factor. For Denver that is: 1.08/0.89 = 1.21. There are lookups for what the appropriate factor is for your site?s elevation. From what I can tell, eQuest performs all of its calculations with a transfer factor of 1.08, and modifies the airflows at the end of process by multiplying by the site specific CFM correction factor.
Matt Dubrovich, PE, CEM, BEMP
Just to add my two cents: the 1.08 is the sensible heat correction factor, Cs. ASHRAE Fundamentals 18.13 starts the discussion on when to correct for elevation by stating that ?Because air passes through the equipment at a density close to standard for locations below about 1000 ft, the accuracy desired normally requires no correction.? Ironically, ASHRAE Fundamentals uses 1.1 instead of 1.08. 1.08 is the factor I remember from basic air conditioning coursework in college?
When I have applied this to eQuest, I usually zero out the elevation for altitudes less than 1000 ft above sea level and let eQuest correct elevations higher than that. As noted earlier in a somewhat indirect way, you don?t want to include this elevation correction twice, so if you are calculating CFM values outside of eQuest and fixing the supply air and outside air to those values then you don?t want eQuest subsequently applying the elevation correction a second time. If you are letting eQuest determine the air volumes then you should let the elevation factor stay, especially for elevations over 1000 ft.
The handbook briefly explains the reasoning behind this factor as being due to the mass-based thermodynamics equations having been converted to volume-based to make calculations easier. Since air is assumed to be an ideal gas, when you exceed 1000 ft in elevation you?ve changed atmospheric pressure enough that 1 CF of air no longer carries the same mass of air.
Jeremy R. Poling, PE, LEED AP+BDC