[Equest-users] Power induction unit/ reheat delta T
John Shen
johnshen1 at hotmail.com
Fri Sep 7 15:05:46 PDT 2012
I am working on a building with air handling units on each floor. Interior zones are served by the AHU while exterior zones are served by the AHU as well as FC unit which provides furthur heating and cooling. I modeled the system as a Power Induction Unit with series PIU to represent the FC units. I used a reheat delta T determined from AHU and FC's spec'd heating capacity and air flow rates and allowed eQUEST to autosize and distribute airflow. Thanks to the advice given from members on this list I was able to reduce my unmet hours to a reasonable range.
I am working on the baseline model and am again having issues with unmet hours. As mentioned the heat capacity of VAV systems are determined by the design flow rate and the reheat delta T. In order to reduce unmet hours I would increase flow rates to particular zones by manually increasing Min design flow rates with my reheat delta T set at 20F which is what is required by ASHRAE. I've been at this for a while and my building is basically a wind tunnel, I have floors with supply cfm upwards of 100,000 CFM and I still have thousands of unmet hours. (my throttling range are at 6)
Am I allowed to change reheat_delta_T to a larger value?
For a power induction unit with series PIU boxes, is there any way for the boxes to provide zone cooling? Looking at hourly reports for "zone coil cooling" I have 0's.
Much of this might be brought about by the fact that my building heating and cooling load has doubled in the baseline model.
Any advice would be much appreciated.
Date: Mon, 27 Aug 2012 09:40:41 -0400
From: bruce5 at bellnet.ca
To: johnshen1 at hotmail.com
CC: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Power induction unit/ reheat delta T
For reference
on the sim reports go to help/Tutorials and Reference/Detailed
Simulation Reports Summary. It will explain all the reports. I
don't use the SS-G report much. I also don't do much chilled
water cooling. I looked at a DX design I had done and the SS-G
shows no cooling.
Cooling is the most difficult to get right because the delta-T is
almost always lower than the heating side and on the heating side
you can always cheat a little with baseboards to do a final trim
even if you don't have them in your system. You can get a quick
idea of the magnitude you are short. Cooling has no final trim
and the eQuest focus is on cooling. You have also bumped into
another limit on cooling capacity. You are correct that the
cooling capacity is dependent on the delta-T and the air flow.
There are 2 delta-T's, the one on your water and the one on your
air. You can play with the water one all day but if you don't
have enough air you won't get anywhere. Equest has a default on
the airflow to a zone that is 0.6 cfm/SF. Your main AHU maximum
flow will be based on this value for the sum of all the zones it
is supplying. Most of the time this value is too small for a sun
side perimeter zone.
I would start with the zone with the worst unmet hours. You want
to use zone reports, start with LS-A which will give you your peak
heating and cooling loads for all your zones. LS-B will give you
the components of that load except OA. SS-R is handy. Keep an
eye on SV-A, it will list the zones and their airflow's as
sub-components of the main AHU.
You want to go to the air-side HVAC tab and select the zone you
want to work on. Right click and select "Properties". In the
"Basic Specifications" tab you will see on the right side half way
down "Zone Design Flow Rates", the first value is "Min Design
Flow" and is probably set at 0.6 cfm/SF, start increasing this
value. You may need to be in the 1.2 cfm/SF range, it depends on
the zone and the cooling load. You can calculate what you need
too. As you bring the number up you will see the AHU airflow
increase and also the cooling/heating capacity of the AHU increase
in SV-A. Remember that you just want enough air. Excess air
moving wastes energy. Your unmet hours should be decreasing for
that zone. Some of the other zones will begin to decrease as well
but your main problem zones should still have unmet hours. Deal
with each one the same way.
Remember this, it is probably THE most important setting in eQuest
on the air side.
Bruce Easterbrook P.Eng.
Abode Engineering
On 24/08/2012 05:16 PM, John Shen
wrote:
Thank you for the response, couldn't have asked for a better
explanation.
I was able to get the heat unmet hours down to a
reasonable range. However, I am having difficulties with the
Cool unmet hours. It appears I am unable to acquire any
cooling capacity which is resulting in a few unmet hours
(~150). As mentioned the heating capacity is dependent on the
reheat delta t and airflow. I can't seem to find a place to
enter delta T for cooling, I have coil delta T set along with
an appropriate CHW loop. Yet I don't get any cooling in any of
my zones in the SS-G section of the report. Any further help
would be appreciated.
John
Date: Fri, 24 Aug 2012 09:57:49 -0400
From: bruce5 at bellnet.ca
To: johnshen1 at hotmail.com
CC: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Power induction unit/ reheat
delta T
You
can try changing your interior control zone as this will
effect the base capacity that the AHU will supply to the
floor. As for the exterior zones your heat/cool
capacity is based on the delta-T and the airflow. You
said you specified the air flow. Let eQuest auto-size
the flow. By specifying the air flow you have removed
most of eQuest's ability to adjust capacity. One other
thing to remember is eQuest is working on the maximum
flow as well as all the intermediate conditions as
well. It is these intermediate loads which will trigger
most of your unmet hours. As the sun moves across your
exterior zones through the day the peak load will be
shifting through the exterior zones as well. So a zone
which was the peak at 10 am won't be the peak at 3 pm.
eQuest will count an unmet hour if you are high or low
on a zones temperature. By specifying the cfm to the
zone I would guess you may be over cooling some of the
zones and triggering unmet hours. eQuest, if allowed,
would reduce the airflow to the 10 am zone later in the
day and shift the CFM to the 3 pm zone. This is mostly
under the hood stuff and you have to drill pretty deep
into your reports to have an idea of what is going on
and you won't find the direct answer as to the lower cfm
to the zone. You can get an idea of the heat/cool
capacity eQuest is dealing with in each and follow it
across the building.
Bruce Easterbrook P.Eng.
Abode Engineering.
On 23/08/2012 01:56 PM, John
Shen wrote:
Working on a building with an
air handling unit which serves the entire floor. In
the exterior rooms there are Fan coils to supply
further heating and cooling to the floor. I have split
the floor up into interior zones and exterior zones
(each containing a fan coil unit). The AHU I have
modeled as a power induction unit which serves all the
zones on the floor, the interior zones have terminal
type std VAV and the exterior zones have series PIU.
With in the exterior zones I have specified flow
rates, heat/cool capacity reflecting the FC. However I
am getting lots of unmet hours from these exterior
zones (~500 hrs each). I inputted a REHEAT-DELTA-T in
of 57F (based on fan flow rates and heat capacity)
into each of the exterior zones. This greatly reduced
the unmet hours; however I find it strange because the
unmet hours are completely dependent on REHEAT-DELTA-T
and completely independent of the zone heat capacity.
I appear to have a poor understanding of how the PIU
system works, if anyone could provide further insight
it would be much appreciated.
John
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