[Equest-users] Power induction unit/ reheat delta T
Bruce Easterbrook
bruce5 at bellnet.ca
Mon Sep 10 09:42:45 PDT 2012
It seems to me you have gone overboard on your zone air flows which will
generate unmet hours and also waste energy. This is all a balancing act
between several conflicting items, air flow just being one. I don't
think your delta T is the problem. Terminal units can heat and cool and
it sounds like yours are not cooling. I would also suspect your system
is fighting itself. You are going to have to troubleshoot what is going
on and figure out a solution. Your sim reports will tell you how much
heating and cooling each zone requires. From there you can figure out
how much air you need to to accomplish this. Compare that to how much
air you are putting in. I don't do a lot of these types of systems to
give you a quick answer with the information I have. Most of the time
there is no quick answer anyway. All HVAC systems follow a logical
pattern. You need to find where and why you have deviated. As you can
see eQuest will take you way out into left field if you let it.
Bruce Easterbrook P.Eng.
Abode Engineering
On 07/09/2012 06:05 PM, John Shen wrote:
>
> 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 <mailto:bruce5 at bellnet.ca>
> To: johnshen1 at hotmail.com <mailto:johnshen1 at hotmail.com>
> CC: equest-users at lists.onebuilding.org
> <mailto: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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