[Equest-users] Packaged RTU w/ Precool Chiller

Thu Dec 22 07:14:49 PST 2016

Definitely a head-scratcher!

Taking a step back to pose some leading thoughts/questions:

1.       Does the packaged chiller have the capacity to accomplish all the cooling necessary to bring the OA stream down to the supply air temps, or can it only accomplish part of the job?

2.       Is it really necessary for the OA pre-cooling chiller "system" and the existing AHU "system" handling all other loads to be "connected" for a reasonable estimation of energy usage?

I too would be more keen on trying the OA-FROM-SYSTEM approach if they ultimately must be "connected" in the simulation.  You might also lump evaporative pre-cooling in the category of features that kinda approach the problem, but likely fall short for reflecting the actual sitation in some fundamental ways.

I think Neil is correct to assert the thermostat temperature of the "dummy zone" is what gets seen at the receiving system for the incoming OA airstream, so you might want to separately evaluate the temperature profile/schedule coming off the packaged chiller OA system, and feed that into your simulation as an hourly temperature schedule for the dummy zone (EMIT can help automate 8760 schedule generation, btw).

~Nick

[cid:image001.png at 01D25C31.6A1E76E0]
Nick Caton, P.E., BEMP
  Senior Energy Engineer
  Energy and Sustainability Services
  Schneider Electric

D  913.564.6361
M  785.410.3317
E  nicholas.caton at schneider-electric.com<mailto:nicholas.caton at schneider-electric.com>
F  913.564.6380

15200 Santa Fe Trail Drive
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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Neil Bulger via Equest-users
Sent: Thursday, December 22, 2016 8:34 AM
To: Tom Mickley <tmickley at thekcompany.com>; equest-users at onebuilding.org
Subject: Re: [Equest-users] Packaged RTU w/ Precool Chiller

Tom,

There are two ways you could go about this:

1.       You can use a combination of AHU systems, with one pulling outside air from the other. You would create a new System, serving a dummy zone (any zone, you can make a space without walls and just a floor). This will be your CHW system and you can hard-size the unit to move an explicit amount of fresh air, essentially capturing this energy-cost to dehumidify ventilation.

The other system your DX AHU, will pull outside air from the first system. In eQuest, any system that pulls air from another needs to be created after the outside air system is created. So in the navigational tree, the outside CHW AHU needs to be the first on the list essentially.

When you specify the DX AHU to "outside air from ___ system" I believe it supplys the second AHU with the equivalent of room air from the dummy zone. So if your thermostat was 75, your AHU DX system would be seeing 75. In reality, the CHW dehumidification process likely is providing colder air at a benefit to a cooling process, yet this detail likely cannot be captured in eQuest.

The DX AHU will need to same ventilation fraction or fixed amount and this will be the portion of air taken from the primary system. This is not a perfect solution so I anticipate you will hit some bumps along the way. The outside air from feature does some strange things. Sometimes you may need to size coils for instance, which actually you may know their size.

2.       There is a water-side economizing coil that can be put in upstream of a primary cooling object (in this case, your DX is your primary cooling object). The coil can only be directly attached to a cooling tower loop. This might be problematic since it would not capture as much thermal load if the eQuest condenser water was not cold enough year round. This would just be a way to capture some cooling that is occurring prior to the DX coil. Any BTU this coil provides would need to  be added up and some post processing of a COP applied. The more I think about this, the less I like it actually. But, it is a feature in eQuest.

Neil Bulger | PE | Principal
Building Performance Team

Integral Group | T 510.663.2070 x 2035 | D 510.457.0135 | M 707.363.3954
integralgroup.com<http://www.integralgroup.com/> | nbulger at integralgroup.com<mailto:nbulger at integralgroup.com>

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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Tom Mickley via Equest-users
Sent: Thursday, December 22, 2016 6:23 AM
To: equest-users at onebuilding.org<mailto:equest-users at onebuilding.org>
Subject: [Equest-users] Packaged RTU w/ Precool Chiller

Good morning,

I'm molding an existing outpatient facility and I've ran into an issue.

One of the HVAC units is a packaged DX unit supplying a few surgical rooms.  The HVAC was designed around a class A or less operating room way back at the time of the original install.  They've since changed to class C operating rooms which have more strict airflow requirements. The building owners decided to modify the existing system rather than replace with new.

That being said, the unit is still a DX cooled unit with in-duct steam humidification and HW reheat at the zone level.  There is electric backup heat in the unit if the boilers fail.

Here's the issue....

The surgeons requested the rooms maintain 60 degrees all year round.  The existing unit couldn't maintain temperature.  Therefore, there has been an air cooled packaged chiller added with a chilled water coil attached to the outdoor air intake to cool and dehumidify the outdoor air before entering the unit.

I cannot find a way to model DX and chilled water at the same time. Does anyone have any suggestions?

Thanks in advance!

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