[EnergyPlus_Support] DX coil psychrometric problems

["Jim Dirkes jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx>]

Dear Forum, 

(excuse cross-posting with UnMetHours)

Although I have had occasional problems with EnergyPlus giving a fatal error due to DX cooling coil sizing problems, most have been associated with autosized coils. I have a project now which uses 11 different rooftop units, all with DX cooling coils. Three of them are giving me fits (problems). So far, I have:

• disabled the coil below a specified outdoor temp (<= 65F) using an EMS script
• raised the airflow 20% (for improved flow/watt ratio)
• reduced the coil rated capacity by up to 50% (for improved flow/watt ratio)
• disabled the coil completely (availability = always OFF) 

The only strategy which works is to disable the coil - and that isn't a very good representation of actual performance :)

The current problem coil has these characteristics:

• 11,400 cfm VAV fan with minimum flow fraction = 0.7 
• 385 MBH / 113 kw rated capacity 
• single stage coil
  

The Fatal error looks like: * Severe * PsyRhoAirFnPbTdbW: RhoAir (Density of Air) is calculated <= 0 [-2.69960]. * ~~~ * pb =[98900.00], tdb=[-400.80], w=[9.9866244E-006]. * ~~~ * Routine=Unknown, Environment=STORE 291 - STURGIS (01-01:31-12), at Simulation time=05/16 01:40 - 01:50 * Fatal *Program terminates due to preceding condition.

Can you suggest ways to correct this? Thanks in advance. The rooftop unit uses a digital compressor, which unloads very flexibly. Perhaps I should make the coil multi-stage?

--

James V Dirkes II, CEO / President
The Building Performance Team Inc.
1631 Acacia Dr, Grand Rapids, MI 49504

(alphabet soup of efforts for greater understanding: PE, BEMP, BCxP, LEED AP)

Direct / Mobile: 616.450.8653
jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx

Website l  LinkedIn

Coffee conversation:
You are hereby authorized to make two big mistakes each year, otherwise you will never learn anything.

__._,_.___

---

Posted by: Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx>

---

EnergyPlus support is found at:
http://energyplus.helpserve.com or send a message to energyplus-support@xxxxxxxx

The EnergyPlus web site is found at:
http://www.energyplus.net/

The group web site is:
http://groups..yahoo.com/group/EnergyPlus_Support/

Attachments are currently allowed but be mindful that not everyone has a high speed connection.  Limit attachments to small files.

EnergyPlus Documentation is searchable.  Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.

Visit Your Group

• New Members 4

• Privacy • Unsubscribe • Terms of Use

__,_._,___