Air mixer for fresh air loop in E+?

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Hey all,

I am trying to model a heat exchanger assisted cooling coil for the fresh
air system of a laboratory building in Singapore. Essentially, outdoor air
(32C DB/ 26C WB) passes through the supply side of the heat exchanger (24C
DB/ 24C WB), then passes through the cooling coil (13C DB/ 13C WB). Outlet
air from the cooling coil then passes through the secondary side of the
heat exchanger (28C DB/ 18C WB). In order to attain constant supply air DB
temperature of 22C, design has allowed for mixing of outlet air from the
cooling coil with outlet air from the secondary side of the heat exchanger.
Is it possible to model this in E+? Somehow "trick" it with the OA mixer

Aaron Boranian
Research Associate, Energy Research Institute @ Nanyang Technological
University (ERI at N)
M.S. Civil Engineering, Building Systems Program, University of Colorado at
B.S. Engineering - Civil Specialty, Colorado School of Mines
Cell: (+65) 8588-5961

Aaron Boranian's picture
Joined: 2014-05-24
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I don?t know enough about EnergyPlus to know if it can model the system you described, but the main programs that I use, Trane TRACE and eQUEST, cannot. I have dealt with it in the past by using the manufacturer?s software to model the DOAS, removing the ventilation from the main model, then combining the results in a spreadsheet. The spreadsheet for the SEMCO Pinnacle (which sounds similar to what you have) is available at The spreadsheet does not directly output the energy numbers that I needed, but I was able to get them indirectly by manipulating the utility costs. For example, to get kWh I entered $1/kWh for the kWh cost and $0 for the other utility costs so that the cost output equaled the kWh that I needed.

For a project that had a more complex DOAS, I had to create a custom spreadsheet to model it using basic psychrometrics. You might be able to access the presentation I did at last year?s ASHRAE Energy Modeling Conference at I was told afterwards by someone from IES VE that their software could model the system I had, so that may be worth looking into if EnergyPlus won?t do it for you.

Keith Swartz, PE, BEMP, LEED AP
Senior Energy Engineer | Energy Center of Wisconsin | Madison.Chicago.Minneapolis
608.210.7123 |

Keith Swartz's picture
Joined: 2011-09-30
Reputation: 0

Dear Aaron,
I think you are describing something like a wrap-around heat pipe which provides its own reheat. A schematic would be helpful to understand your actual intent!

The scenario below is something E+ does easily, and I think it would not be too hard to move the source for the HX secondary inlet (now #1002) to #1006.

[cid:image002.jpg at 01CF79AF.BA17E6C0]

James V Dirkes II, PE, BEMP, LEED AP
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

James V Dirkes II, PE's picture
Joined: 2011-10-02
Reputation: 203


A cooling coil with heat exchanger (heat wheel, enthalpy wheel, counter-flow heat exchanger, heat pipe, etc.) connected immediately upstream and/or downstream of the coil as you have described to assist in cooling and/or to provide ?free? reheat immediately following sub-cooling for dehumidification can readily be modeled in the ApacheHVAC module of the IES Virtual Environment. In the example below, I?ve added the damper set highlight in red to show how your mixing damper downstream of the cooling coil can be explicitly modeled; however, this is necessary only if you want to query air network node results for the inputs for this damper. A functionally equivalent bypass damper is built into the heat exchanger component. In this particular system network, there is a heating coil to provide backup source of reheat if/when the heat exchanger is insufficient to reheat the supply air to the desired temperature after sub-cooling for dehumidification. There is also an explicitly modeled bypass damper for the supply side of the heat exchanger and duplicate supply fan component (think of this as a meter with a performance curve) on the path through the supply side of the heat exchanger to account for the added pressure, and thus added fan power required, to move air through the heat exchanger when this component is not bypassed. I?m not sure whether this can be done in E+, but this ApacheHVAC example may give you some ideas:

[cid:image001.jpg at 01CF799C.D8F3F740]



Timothy Moore
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Timothy Moore2's picture
Joined: 2011-10-02
Reputation: 200

Thanks for the tips, everyone! I'll try to get a hold of someone on the
EnergyPlus forums. I'm trying to stick with E+, as the model as 100+ zones
and we have devoted lots of time already. We are thinking of possibly
modelling the unit with and without the mixing damper in TRNSYS to apply a
correction factor to E+ results.

In the meantime, I've attached a schematic of the dehumidification unit I
1 - Untreated outdoor air, inlet to primary side of HX
2 - Outlet of primary side of HX, inlet to CC
3 - Outlet of CC, inlet to secondary side of HX
4 - Outlet of secondary side of HX, mixed with air state 3 to achieve
desirable supply air temperature
5 - Supply air

[image: Inline image 2]

Thanks again,


Aaron Boranian's picture
Joined: 2014-05-24
Reputation: 0

In EnergyPlus, look for ?CoilSystem:Cooling:Water:HeatExchangerAssisted? and ?CoilSystem:Cooling:DX:HeatExchangerAssisted? in the documentation? I believe that these can model what you are looking for.

Don.Shirey at's picture
Joined: 2013-01-31
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