[Equest-users] Dehumidification issue - REPOST

Wed Jun 7 13:51:12 PDT 2017

Oops, meant to write that there is NOT direct control of humidity in supply air. That is, you can’t specify a coil leaving max RH of 80%, or something like that, which someone might put in a sequence, in lieu of some other measurement/control point.
Also in a VAV system the decrease in SAT called for by exceeding the system max RH setpoint can only happen if the current hour’s SAT > MIN-SUPPLY-T.

Also, I assume the “VAV” minimum flows are set to 100% everywhere in your modeled system. In a VAV system, what the ZONE cooling setpoint schedule really controls is the airflow modulation. As the zone temp increases through the throttling range around the hour’s cooling setpoint, the zone airflow increases from minimum to 100%. W/o a cooling setpoint, this increase never happens. If the minimum airflows are at 100%, then the cooling setpoint SHOULD do nothing, unless it is being referenced by a WARMEST reset control.

Fred
MIN-SUPPLY-T
For systems that can provide cooling, this is a required keyword that gives the minimum temperature of the air delivered to the zone. MIN-SUPPLY-T and COOL-DESIGN-T are used to size the capacity of the cooling coil and supply air flow rate. The supply air flow rates needed to satisfy the heating and cooling requirements are compared and the greater of the two quantities is used for the system air flow rate. Note that MIN-SUPPLY-T also controls the amount of moisture that can be removed by the cooling coils.
Note, that for those systems that are to maintain a constant cooling air discharge temperature (see keyword COOL-CONTROL), the control set point is determined by the value entered for COOL-SET-T rather than MIN-SUPPLY-T. In this case, the program uses MIN-SUPPLY-T to limit subcooling for dehumidification purposes (and to calculate the design air flow rate for cooling).
Note that MIN-SUPPLY-T is the design supply temperature at the zone, downstream of duct losses.  COOL-SET-T, COOL-SET-SCH, COOL-RESET-SCH and the heating counterparts are all defined as entering the duct, upstream of duct losses. As such, they should be adjusted for the expected duct losses so that the hourly supply temperature at the zone is the desired temperature.
COOL-SET-T

The cooling air supply temperature setpoint when COOL-CONTROL = CONSTANT. Note that even though COOL-SET-T determines the cooling air supply temperature, the program uses MIN-SUPPLY-T to calculate the design supply air flow rate.

Note that MIN-SUPPLY-T is the design supply temperature at the zone, downstream of duct losses.  COOL-SET-T, COOL-SET-SCH, COOL-RESET-SCH and the heating counterparts are all defined as entering the duct, upstream of duct losses. As such, they should be adjusted for the expected duct losses so that the hourly supply temperature at the zone is the desired temperature. They should also be reduced by one-half of the COOL-CTRL-RANGE.

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Porter, Frederick NOR via Equest-users
Sent: Wednesday, June 07, 2017 2:29 PM
To: David Eldridge; equest-users at onebuilding.org
Subject: [External] Re: [Equest-users] Dehumidification issue - REPOST

To go back to Jason’s original specific questions:

1.       How does eQUEST control the maximum humidity allowed in the supply airstream?
There is direct control of humidity in the supply airstream. Humidity is controlled by the “system” RH, which is really a return air RH.

2.       How could one possibly explain a cooling load/consumption without any cooling set point schedules in any of the zones supplied by my system and zero moisture removed from the air?
In a SINGLE ZONE system, the supply air temperature is controlled to meet zone temperature setpoint schedules. In a VAV/PIU, PVAV, CVRH system, i.e. the type of system you say you are modeling, the supply air is controlled in a variety of ways, but the simplest, and eQUEST default, is a constant 55F AHU “SAT”, i.e., discharge air temperature. If the MAT > SAT – fan dT, there will be a cooling load on the coil. If the MA WBT < SAT, which might be the case in your location, there can be sensible cooling w/o any condensation.

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of David Eldridge via Equest-users
Sent: Wednesday, June 07, 2017 1:03 PM
To: equest-users at onebuilding.org
Subject: [External] Re: [Equest-users] Dehumidification issue - REPOST

Jason, I think one mix-up may be the lingo – if the zones or supply air have heat removed, then it is being cooled even if the cooling is provided for dehumidification purposes, or if the economizer is operating and the zones are cooled as a byproduct of dehumidification.

Where is the 60% RH setpoint entered? The system could still supply lower humidity air to the spaces in order to maintain RH in the zones. I don’t see why this would require the coil to operate though, unless the packaged system requires one control zone that has a temperature control schedule. If none was provided is one made by default?

David

David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
Grumman/Butkus Associates

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Ng, Jason via Equest-users
Sent: Wednesday, June 07, 2017 10:00 AM
To: Porter, Frederick NOR <fporter at noresco.com<mailto:fporter at noresco.com>>; equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: Re: [Equest-users] Dehumidification issue - REPOST

Hi Frederick,

To clarify, my zones are not cooled (no cooling schedules, i.e. no set point for cooling to be met, my zone cooling loads are ignored). The system has a cooling coil that is sized to take the dehumidification load only. The system does not attempt to maintain a maximum temperature in the zone.

[Bouthillette Parizeau]<https://urldefense.proofpoint.com/v2/url?u=http-3A__www.bpa.ca_&d=DwMFAw&c=ilBQI1lupc9Y65XwNblLtw&r=3xdcdPfNRiIRbdcNOFLTvgBFEa2F6BAz_UMLqgqXn6k&m=w1NEVziC4VmLmVJepE8gLOcGVr7oG7LrS3R7mSinOsY&s=PwSyRZlcGKiJYVKIO66R0vfqogVvHe4TAV9h75ODiX4&e=>

Jason Ng_ing. M.Sc.A.
jng at bpa.ca<mailto:jng at bpa.ca> | t: 5143833747x2813

De : Porter, Frederick NOR [mailto:fporter at noresco.com]
Envoyé : 7 juin 2017 10:46
À : Ng, Jason <jng at bpa.ca<mailto:jng at bpa.ca>>; equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Objet : RE: Dehumidification issue - REPOST

Jason,
You state that this “system” will “dehumidify air only (no cooling…” Exactly HOW does it dehumidify, if not by cooling and condensation?
Fred

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Ng, Jason via Equest-users
Sent: Wednesday, May 24, 2017 7:14 AM
To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [External] [Equest-users] Dehumidification issue - REPOST

Hello all,

I posted this awhile back and received no response. Testing my luck again and hoping someone will have some insight!

Hello list,

I thought the dehumidification calculation would be a relatively straightforward calculation for QUEST, however, I have some results that are a bit puzzling.

I have a simple all-electric packaged VAV system that serves living quarters of an in-patient clinic. The system has 50% OA and uses an economizer. The system is designed to heat and dehumidify air only (no cooling, none of my zones have cooling set point schedules). My dehumidification set point is 60% RH. The building is located in Chibougamau, Quebec (quite north) where cooling and dehumidification requirements are relatively small, but still exist.  According to the SS-N report, the maximum RH is never surpassed for my system.

This is all fine and great until you start digging a bit deeper into the hourly results of the system in question. In my hourly data report I have the following variables : Total central heating/cooling coil outputs, dehumidification reheat heat output, return air humidity ratio, mix air humidity ratio, humidity ratio of air leaving cool coil, Moisture added or removed from air for (de)humidification, and maximum humidity set point. The strange thing I noticed is that my return air humidity ratio and mix air humidity ratio NEVER surpass my maximum humidity set point, yet I still have loads on my cooling coil during the summer, and moisture removed from the airstream. Furthermore, I noticed that there are 815 hours during which there is a load on the cooling coil, but no humidity is removed from the airstream whatsoever (mix air humidity ratio = humidity ratio of air leaving cool coil). My team here has experienced these results in more than 1 simulation that have a system with only dehumidification requirements on the cooling side.

So my questions are:

1.       How does eQUEST control the maximum humidity allowed in the supply airstream?

2.       How could one possibly explain a cooling load/consumption without any cooling set point schedules in any of the zones supplied by my system and zero moisture removed from the air?

Any insights would be much appreciated!

Many thanks.

  [Bouthillette Parizeau] <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.bpa.ca_&d=DwMFAw&c=ilBQI1lupc9Y65XwNblLtw&r=3xdcdPfNRiIRbdcNOFLTvgBFEa2F6BAz_UMLqgqXn6k&m=QRUn_fz_LOs_iOHt_0-lov2KxZe7ZclvRy6XedxQqHM&s=Xl2kZgukQwhwIiwsR-cVKoQgFE-OJwy5oJyK0Zr8kzQ&e=>

Jason Ng_ing. M.Sc.A.
jng at bpa.ca<mailto:jng at bpa.ca>   |  www.bpa.ca<https://urldefense.proofpoint.com/v2/url?u=http-3A__www.bpa.ca&d=DwMFAw&c=ilBQI1lupc9Y65XwNblLtw&r=3xdcdPfNRiIRbdcNOFLTvgBFEa2F6BAz_UMLqgqXn6k&m=QRUn_fz_LOs_iOHt_0-lov2KxZe7ZclvRy6XedxQqHM&s=ltbl3nk22R3GJG3At72nnGWr5THYZpEEzk6UvPZ2Cj8&e=>  |  t: 5143833747x2813

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