Packaged RTU w/ Precool Chiller

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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!

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

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 | nbulger at integralgroup.com

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via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

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

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Nick Caton, P.E., BEMP
Senior Energy Engineer
Energy and Sustainability Services
Schneider Electric

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M 785.410.3317
E nicholas.caton at schneider-electric.com
F 913.564.6380

15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States

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via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Tom,
Stepping even further back - what is the purpose of your energy modeling? This is an existing facility with an existing system. Will you be modifying or replacing this system? Are you creating a calibrated energy model for some other purpose? Is the model primarily for design purposes?
For example, if you are proposing to replace the unit with something new, the configuration of the existing system isn't as important as understanding the loads. The energy of the existing system is important if you are evaluating it as an ECM.
Regards,
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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T: (585) 698-1956 F: (585) 325-6005

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via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Hi Tom,

You can model the aircooled chiller system as a separate system serving the
treated outside air to various zones or system.

This can be done by making a system supplying to a dummy zone then the
system serving the OA to various systems.

The supposed to be linked system comes in the outside air tab in air-side
system.

To maintain the set-point you need to have design set-point below 60 degree.

In the model you need to have a design cooling set-point less than 60
degree for all the systems and then take help of SS-D report to have a look
how the zone temperature is meeting the set-point.

For having the general system you can take packaged single zone and take
hot water loop at the zone level (or system level).

*Thanks,*

Sharad.Kumar

------------------------------ Forwarded message
------------------------------------

From: Tom Mickley via Equest-users
To: "equest-users at onebuilding.org"
Cc:
Date: Thu, 22 Dec 2016 14:22:57 +0000
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!

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Thanks for the replies Bill/Nick/Neil and Cole.

Bill - I am modelling it for a PACE energy project to completely replace all HVAC systems and controls in the building so the existing system energy is important.

Nick - No. The chiller was only sized to handle the excessive load produced by added ventilation air. I guess they technically don't have to be connected to the same load if I can create the chiller load to just handle outside air.

I used Neil's approach with the dummy zone. It seems to be working so far. I still have some tinkering to do with scheduling, but it's coming together.

Thank you all. You guys do wonders on here.

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Good to hear your precooling coil model seems to be working. I may be too late for any help w/your project, but thought I would contribute some advice.

This is an arrangement that the OA-FROM-SYSTEM should handle fairly well. In this case the dummy zone is really representing the preconditioned OA duct or plenum. The way I do this, it should not have any heating or cooling setpoint schedules, even though it is declared as ?conditioned.? Its temperature is determined by the SAT control and capacities of the virtual ?system? supplying the OA. The system serving it should be 100% OA to properly represent incoming air conditions.

IF you are doing any ?scheduling,? the main drawback with the OA-FROM-SYSTEM feature is that it passes/couples air conditions, but not air flows. Any flow variations need to be synchronized carefully for the two systems via multiple schedules. Check all hours to make sure the flow through the CHW coil OA ?system,? which should be 100% OA, is the same as the OA flow simulated as entering the recirculating OR AHU system. If the OA flow is ever reduced, the air FLOW in the OA system is reduced, typically by scheduling the ?dummy? zone minimum flow, while the OA fraction in the recirc system should be reduced during those hours, typically via the system OA MIN-AIR-SCH. Note that DOESIM may spit out an ?HVAC System? WARNING indicating the program has fixed some coordination issue, but my experience is that it really does not.

Following is a quick snippet which seemed to work pretty well in Chicago. It is a constant volume, constant OA model. Even though the systems are VAVS or PVAVS, the min air flows are set to 100%. I tried to allow the OA system to stay off until the OAT>70F, which might be a possible real control sequence, but saw some 85F OA duct/plenum ?zone? temps in its SS-O report, so the OA cooling is enabled whenever the OAT > OA system SAT setpoint ? OA fan dT.

Fred
Fred Porter, BEMP, LEED? AP
Principal Engineer
Sustainability Services
NORESCO
www.noresco.com

"OA w/CHW Precool" = SYSTEM
TYPE = VAVS
HEAT-SOURCE = NONE
ZONE-HEAT-SOURCE = NONE
BASEBOARD-SOURCE = NONE
SIZING-RATIO = 1
COOL-SIZING-RATI = 1.5
MIN-SUPPLY-T = 50
COOL-SET-T = 50
COOL-CONTROL = CONSTANT
COOL-MIN-RESET-T = 50
MIN-OUTSIDE-AIR = 1
OA-CONTROL = FIXED
SUPPLY-KW/FLOW = 0.0005
MIN-FLOW-RATIO = 1
CHW-LOOP = "Chilled Water Loop"
COOL-CTRL-RANGE = 1
..
"OA Duct Zn" = ZONE
TYPE = CONDITIONED
OUTSIDE-AIR-FLOW = 2000
ASSIGNED-FLOW = 2000
DESIGN-COOL-T = 55
SPACE = "OA Duct Spc"
..
"OR Recirc Sys" = SYSTEM
TYPE = PVAVS
HEAT-SOURCE = HOT-WATER
HEAT-SIZING-RATI = 2
HUMIDIFIER-TYPE = ELECTRIC
MAX-SUPPLY-T = 75
HEAT-SET-T = 50
MIN-SUPPLY-T = 45
COOL-SET-T = 45
MIN-HUMIDITY = 40
OA-CONTROL = FIXED
SUPPLY-KW/FLOW = 0.0012
RETURN-KW/FLOW = 0.0005
REHEAT-DELTA-T = 40
MIN-FLOW-RATIO = 1
HW-VALVE-TYPE = TWO-WAY
HW-LOOP = "Hot Water Loop"
COOL-CTRL-RANGE = 1
OA-FROM-SYSTEM = "OA w/CHW Precool"
..
"OR 1 Zn (G.NE1)" = ZONE
TYPE = CONDITIONED
OUTSIDE-AIR-FLOW = 1000
ASSIGNED-FLOW = 3000
DESIGN-HEAT-T = 70
HEAT-TEMP-SCH = "60F Heat Sch"
DESIGN-COOL-T = 62
COOL-TEMP-SCH = "62F Cool Sch"
SIZING-OPTION = ADJUST-LOADS
TERMINAL-TYPE = SVAV
SPACE = "EL1 NE Perim Spc (G.NE1)"
..
"OR 2 Zn (G.SW2)" = ZONE
TYPE = CONDITIONED
OUTSIDE-AIR-FLOW = 1000
ASSIGNED-FLOW = 3000
DESIGN-HEAT-T = 70
HEAT-TEMP-SCH = "60F Heat Sch"
DESIGN-COOL-T = 62
COOL-TEMP-SCH = "62F Cool Sch"
SIZING-OPTION = ADJUST-LOADS
TERMINAL-TYPE = SVAV
SPACE = "EL1 SW Perim Spc (G.SW2)"
..

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