FC+DOAS

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hi equesters
Dose anybody have experiences for FC+DOAS, in my modle, outdoor air processed by independent air-condition unit, then supplied to room directly, do not go through FC inlet. as far as i kown, outdoor air and return air combined process in FC in some countries,like below picturebut On the upper right corner,i see an outdoor air fan, is it an DOAS? if so ,how to realize it in equest? and how to set in the tab "OA FROM SYSTEM"?
any words will be appreciated!
yours sincere

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Search DOAS in the user archives and you will find a bunch of discussions on
this topic.

First you need to be in detailed mode, for some reason the DOE engine (maybe
just eQuest), doesn't have the skills to make a dedicated outdoor air system
as a drop down selection.

Next, what is the supply air temperature entering the zones from the DOAS?
Cold? Room temperature? Hot?

What is the process that the DOAS goes through to get the leaving air
temperature at set-point?

Typically, you need to create a "dummy space" and assign it to a "dummy
zone" then assign your first system to this zone. If you already created
systems you can always open your .inp file and cut and paste the DOAS to the
top of the systems list so that it will become your first system. Go ahead
and assign all of the outside air to your DOAS "dummy zone" and then choose
outside air from system for all of the systems that this DOAS serves. It
will only be able to find OA from systems above it on the component tree, so
make sure your DOAS is at the top.

Joe Fleming, E.I., LEED AP BD+C, BEMP

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Search DOAS in the user archives and you will find abunch of discussions on this topic.

First you need to be in detailed mode, for some reason the DOE engine (maybe just eQuest), doesn?t have the skills to make a dedicated outdoor air system as a drop down selection.

Next, what is the supply air temperature entering the zones from the DOAS? Cold? Room temperature? Hot?

What is the process that the DOAS goes through to get the leaving air temperature at set-point?

Typically, you need to create a ?dummy space? and assign it to a ?dummy zone? then assign your first system to this zone. If you already created systems you can always open your .inp file and cut and paste the DOAS to the top of the systems list so that it will become your first system. Go ahead and assign all of the outside air to your DOAS ?dummy zone? and then choose outside air from system for all of the systems that this DOAS serves. It will only be able to find OA from systems above it on the component tree, so make sure your DOAS is at the top.

Joe Fleming, E.I., LEED AP BD+C, BEMP

From:equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org]On Behalf Oftiejun hu
Sent: Thursday, April 14, 2011 1:11 AM
To: equest-users
Subject: [Equest-users] FC+DOAS

hi equesters

Dose anybody have experiences for FC+DOAS, in my modle, outdoor air processed by independent air-condition unit, then supplied to room directly, do not go through FC inlet. as far as i kown, outdoor air and return air combined process in FC in some countries,like below picturebut On the upper right corner,i see an outdoor air fan, is it an DOAS? if so ,how to realize it in equest? and how to set in the tab "OA FROM SYSTEM"?

any words will be appreciated!

yours sincere

hi joe

thanks for your reply,this is my Processing,, independent outdoor air from DOAS are processed to S( isenthalpic with room N?H:90%), then mixed with supply air from FC outnet(L), outdoor air doesn't undertake indoor load. Note that: my outdoor air do not get through FC, delivered to room derectly.

i doubt that , as your method, Whether outdoor air from DOAS will be delivered to FC, flow through the fan in FC, then increase fan energy?

thanks again!

hutiejun's picture
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Regarding the fan power. If we assume that both fans and fan motors have
the same efficiency and the units are constant volume, then you will adjust
the static pressure of your fan coil units by the current flow over the new
flow when you add the outside air.

i.e. if you have two FCU?s current supplying 2000 cfm and you have a 1000
cfm outside air unit delivering 500 cfm to each FCU, then your new FCU
supply air rate will be 2500 cfm. You will build your DOAS as is designed
and you will bump up your FCU airflow quantity to the total of the design
and the DOAS supply to that space/zone/system. You will then reduce your
static pressure for the FCU by 2000/2500 or 80% of what it is designed at,
if the design total static is 4 inches then your new static will be 3.2
inches. This will eliminate any increase in fan power that you may see to
the increased flow. (If you have ten FCUs at 2000 cfm and a DOAS at 2000
cfm, your numbers with the DOAS would be, DOAS as designed, 2200 cfm for
each FCU, and a new total static pressure at 2000/2200 of design or 90.9% of
design static).

As for the process, you say isenthalpic, so to me that means that the air is
delivered off of the evaporator coil after the refrigerant goes through an
expansion valve?

Or could you please tell us what N, L, O, S, and W stand for, and what the
axes of this chart are, (temperature and entropy?) It doesn?t look like a
psychrometric chart.

Joe Fleming, E.I., LEED AP BD+C, BEMP

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this is a phychrometric chart, y axes is "enthalpy" ?x axes has 145 ?angle with the y axis ?stand for "humidity". N stand for indoor Indoor State point; W stand for outdoor state point; S stand for state point which belong to outdoor air processed by DOAS. L stand for state point processed by FCU.; O stand for mix point between S and L.
S' enthalpy value = N' enthalpy value

Regarding the fan power. If we assume that both fans and fan motors have the same efficiency and the units are constant volume, then you will adjust the static pressure of your fan coil units by the current flow over the new flow when you add the outside air.

i.e. if you have two FCU?s current supplying 2000 cfm and you have a 1000 cfm outside air unit delivering 500 cfm to each FCU, then your new FCU supply air rate will be 2500 cfm. You will build your DOAS as is designed and you will bump up your FCU airflow quantity to the total of the design and the DOAS supply to that space/zone/system. You will then reduce your static pressure for the FCU by 2000/2500 or 80% of what it is designed at, if the design total static is 4 inches then your new static will be 3.2 inches. This will eliminate any increase in fan power that you may see to the increased flow. (If you have ten FCUs at 2000 cfm and a DOAS at 2000 cfm, your numbers with the DOAS would be, DOAS as designed, 2200 cfm for each FCU, and a new total static pressure at 2000/2200 of design or 90.9% of design static).

As for the process, you say isenthalpic, so to me that means that the air is delivered off of the evaporator coil after the refrigerant goes through an expansion valve?

Or could you please tell us what N, L, O, S, and W stand for, and what the axes of this chart are, (temperature and entropy?) It doesn?t look like a psychrometric chart.

Joe Fleming, E.I., LEED AP BD+C, BEMP

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One implied detail in Joe's description is setting up a dummy space and
assigning it to the dummy zone.

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As Arpan stated, we are assuming you know how to set-up the ?dummy? space
and zone. Also, if you aren?t aware, you cannot assign two (2) systems to
a single (1) zone.

With the information you have provided below it appears that the DOAS is
adding some load to the FCU and handling some load itself. Point ?S? is
warmer than the room set-point ?N?. If this is the case then the leaving
air conditions of your DOAS in eQuest should match the leaving air
conditions of your design DOAS. If the DOAS is delivering 78 degF air at
50% rh in the design, then you should set your DOAS up in eQuest with those
leaving air conditions. Your FCUs will receive air at those conditions at
the quantity specified in your OA input.

You are losing mixed air at the outdoor air rate to exfiltration or adjacent
spaces. I don?t think you can model this aspect with eQuest, eQuest will
do what it does here.

The difference in airflows and leaving air conditions though the FCUs in the
design and in the eQuest model will be the outdoor air rate. i.e. an FCU in
the engineered design has 2000 cfm delivered at point ?L? and the zone it
serves has 500 CFM delivered to it from the DOAS at point ?S?, your FCU in
eQuest will have 2500 cfm delivered to the zone at mixed air conditions or
point ?O?.

One more important item is, what is the process your DOAS goes through? Is
it simply using an ERV to remove some enthalpy (db and wb) from the outdoor
air? Or is it bringing the air to its dewpoint and then reheating it before
delivery to the space? Sorry that I am not able to gather this from your
chart.

Joe Fleming, E.I., LEED AP BD+C, BEMP

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i have known setting "dummy" space. and can you explain this "

You are losing mixed air at the outdoor air rate to exfiltration or adjacent spaces. I don?t think you can model this aspect with eQuest, eQuest will do what it does here.

"clealy for me . i am sorry i can't understand well.
the process of DOAS is From W to S, which handled by AHU. and i don' t want outdoor air flow through FCU. becase outdoor air needn't be processed , and it will lead to increse of fan energy. if equest do not do it as my thought ,then any good ways can reduce the increase of fan energy?

As Arpan stated, we are assuming you know how to set-up the ?dummy? space and zone. Also, if you aren?t aware, you cannot assign two (2) systems to a single (1) zone.

With the information you have provided below it appears that the DOAS is adding some load to the FCU and handling some load itself. Point ?S? is warmer than the room set-point ?N?. If this is the case then the leaving air conditions of your DOAS in eQuest should match the leaving air conditions of your design DOAS. If the DOAS is delivering 78 degF air at 50% rh in the design, then you should set your DOAS up in eQuest with those leaving air conditions. Your FCUs will receive air at those conditions at the quantity specified in your OA input.

You are losing mixed air at the outdoor air rate to exfiltration or adjacent spaces. I don?t think you can model this aspect with eQuest, eQuest will do what it does here.

The difference in airflows and leaving air conditions though the FCUs in the design and in the eQuest model will be the outdoor air rate. i.e. an FCU in the engineered design has 2000 cfm delivered at point ?L? and the zone it serves has 500 CFM delivered to it from the DOAS at point ?S?, your FCU in eQuest will have 2500 cfm delivered to the zone at mixed air conditions or point ?O?.

One more important item is, what is the process your DOAS goes through? Is it simply using an ERV to remove some enthalpy (db and wb) from the outdoor air? Or is it bringing the air to its dewpoint and then reheating it before delivery to the space? Sorry that I am not able to gather this from your chart.

Joe Fleming, E.I., LEED AP BD+C, BEMP

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?and can you explain this "

You are losing mixed air at the outdoor air rate to exfiltration or adjacent
spaces. I don?t think you can model this aspect with eQuest, eQuest will
do what it does here.

"clealy for me . i am sorry i can't understand well.?

Just ignore what I said about exfiltration due to pressurization here, I was
talking too much.

?and i don' t want outdoor air flow through FCU. becase outdoor air
needn't be processed , and it will lead to increse of fan energy. if equest
do not do it as my thought ,then any good ways can reduce the increase of
fan energy??

that both fans and fan motors have the same efficiency and the units are
constant volume, then you will adjust the static pressure of your fan coil
units by the current flow over the new flow when you add the outside air.

i.e. if you have two FCU?s current supplying 2000 cfm and you have a 1000
cfm outside air unit delivering 500 cfm to each FCU, then your new FCU
supply air rate will be 2500 cfm. You will build your DOAS as is designed
and you will bump up your FCU airflow quantity to the total of the design
and the DOAS supply to that space/zone/system. You will then reduce your
static pressure for the FCU by 2000/2500 or 80% of what it is designed at,
if the design total static is 4 inches then your new static will be 3.2
inches. This will eliminate any increase in fan power that you may see to
the increased flow. (If you have ten FCUs at 2000 cfm and a DOAS at 2000
cfm, your numbers with the DOAS would be, DOAS as designed, 2200 cfm for
each FCU, and a new total static pressure at 2000/2200 of design or 90.9% of
design static).?

The statement above is so you won?t see an increase in fan energy when
bringing the OA through the fan coil units. You can?t assign 2 systems to
1 zone but you should still model the total airflow (FCU plus OA in the
zone) into the zone to better model the proper behavior of the space thermal
conditions.

And, setting the FCU leaving air conditions to point ?O? at the higher
airflow quantity will give a close approximation of the coil load, (instead
of using point ?L? with the higher airflow quantity which will
overestimate coil load and cooling/heating energy)

There is another way to separate the two systems, FCU and DOAS, but involves
a different kind of ?dummy zone? with the DOAS assigned to it and you will
have to schedule your DOAS operation. I personally don?t like this method
because it does not properly simulate the space conditions, so I will not
explain it here.

Joe Fleming, E.I., LEED AP BD+C, BEMP

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