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RE: Re: [EnergyPlus_Support] air volume flow rate per watt of total cooling capacity

Warning!  I started writing this as an answer to Rahul’s question, but it has evolved into some philosophical musing�.


Whether a real DX coil will cause freezing conditions and comfort problems is an issue for the mechanical engineer / designer and the controls associated with the equipment.  Whether EnergyPlus “sees” such a problem is something completely different.  EnergyPlus normally assumes a well-mixed zone and never assesses comfort associated with poor air mixing patterns; it’s always an artificial or pre-defined mixing / stratification regime.  It’s not a CFD program!  E+ does warn about potential freezing conditions on the coil outlet.  That is for the modeler’s benefit in order to remind him to size and control the coil properly.


Energy modeling and real-world operation are two very different concerns.  The modeler’s responsibility is to take into account significant energy-related aspects of operation, but certainly not all of the minute details needed for actual operation.  For all energy models used for new construction, I think it is naïve to believe that your model is more than 80% correct compared to what will occur in actual operation �� and that might be an overly optimistic estimate!  There are far too many details about which a modeler makes assumptions.  The people who build, install and commission the facility know little about these assumptions and care less!

I’d love to see a “God-level” understanding of all of the thousands of operational details that affect actual energy usage and how an energy model represents and is affected by them, but I suspect that God is the only one who will ever know them.  In the meantime, I am trying to focus on the details that are most significant and represent them well in my models.


Back a few years ago when I felt the need to choose an energy modeling program that was better than the bin-method tools I had been using up until that point, I chose EnergyPlus for its robust history,  strong development, and ongoing support by its developers.  I’m glad that I made that choice, even though I (figuratively) almost died trying to learn it!  Now and then I wonder why so many people are still using the “really old” programs like DOE2 J.  I think the reason, however, is that those programs had a very sound basis in thermodynamics and physics and covered all of the basics of energy calculations very well.  I think that today we might describe them as “no frills” energy modeling.  A real risk for the people who use E+ with its greater capabilities, especially the newer modelers who seem less inclined to dig into the details of its functions, is to imagine that it is substantially more than an energy modeling package.  For example, it can model displacement ventilation well enough for energy predictions, but it is definitely NOT a CFD program.  It can model daylighting controls, but it is NOT a lighting program.  There are probably other examples, but those are the ones that come to mind.


In short:  Know and understand the limitations of the tools that you use!  Use appropriate tools for the task at hand!  Don’t use inappropriate tools and fool yourself into thinking that they are appropriate!


p.s., I hope that was helpful.  My sole intention was to offer my insight in the hope that it benefits some of you good people on this forum. I continue to enjoy and regularly be amazed at the depth of knowledge held by many of you!


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


From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of rahula7@xxxxxxxxx
Sent: Wednesday, November 20, 2013 5:16 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: RE: Re: [EnergyPlus_Support] air volume flow rate per watt of total cooling capacity



Will you not run into coil freezing and comfort issues with very low cfm/ton?


There is another field in the CoilSystem:Cooling:DX object, "Outdoor Air DX Cooling Coil Leaving Minimum Air Temperature" that allows setting a lower limit on the leaving air temperature. This could be used to avoid freezing problems. I would still test these two fields to see what is happening before using them.




---In EnergyPlus_Support@xxxxxxxxxxxxxxx, <jim@...> wrote:


Jim Dirkes II 

Building Performance Team


On Nov 20, 2013, at 11:55 AM, "Solomon Yin" <solomonyp@...> wrote:




If I enable the "Use DOAS DX Cooling Coil" mode, I can simulate the A/C performance at reduced evaporator airflow condition (say 215 cfm/ton) without triggering the volume flow rate warning. It that correct?



To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: jeannieboef@...
Date: Wed, 20 Nov 2013 08:36:37 -0800
Subject: RE: RE: [EnergyPlus_Support] air volume flow rate per watt of total cooling capacity


excellent tip. thanks for sharing.

---In EnergyPlus_Support@xxxxxxxxxxxxxxx, <jim@...> wrote:


This warning is the bane of many of my E+ models that use DX coils.  My current “record” is over 3,000,000 such warnings for one model J.

The logic behind the warning is that most DX coils cannot or should not operate outside of the range defined by the warnings and E+ assumes that you have defined the coil incorrectly in some fashion.  When these limits are exceeded, there are often related psychrometric errors (as you saw).

In the USA, we have a “rule of thumb” that typical DX coils operate at about 400 CFM / ton.  100% outdoor air coils operate in the range of about 200-250 CFM / ton � the E+ warning range roughly duplicates thesee values.

By the way, E+ does not allow an auto-sized coil to fall outside the range of the warnings � this causes some DX coils to be greatly undersized and forces you to raise the Cooling sizing factor in order to provide adequate cooling for those zones.

The CoilSystem:Cooling:DX object in E+ received a new field in v8, called “Use DOAS DX Cooling Coil” which extends the warning range downward very significantly.  If your DX coil is supposed to operate to high delta T (low airflow / watt), try it out.






Availability Schedule Name


DX Cooling Coil System Inlet Node Name


DX Cooling Coil System Outlet Node Name


DX Cooling Coil System Sensor Node Name


Cooling Coil Object Type


Cooling Coil Name


Dehumidification Control Type


Run on Sensible Load


Run on Latent Load


Use DOAS DX Cooling Coil


DOAS DX Cooling Coil Leaving Minimum Air Temperature




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


From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of fpasserini.tn@...
Sent: Wednesday, November 20, 2013 8:18 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] air volume flow rate per watt of total cooling capacity



For the object Coil:Cooling:DX:SingleSpeed “The rated air volume flow rate should be between .00004027 m3/s and .00006041 m3/s per watt of rated total cooling capacity”.
Since dQ/dt = rho * dV/dt * c * DT , a condition regarding the volume flow rate / cooling power ratio (dV/dt) / (dQ/dt) can be expressed as a condition regarding the temperature difference DT.
I consider rho * c = 1206 J / (m^3 * K)
If (dV/dt) / (dQ/dt) = 0.00004027 m3/s / W then DT = 21 K
If (dV/dt) / (dQ/dt) = 0.00006041 m3/s / W then DT = 13,7 K
Therefore the temperature difference between inlet temperature and outlet temperature must be between 13,7 K and 21 K.
Why such a condition?
I have to model a split with 0,13 m^3/s and 4000 W, therefore the ratio is 3,25*10^(-5) m^3/s / W and it isn't acceptable.
I get the following error:
** Severe ** Sizing: Coil:Cooling:DX:SingleSpeed "0TERRA:LOCMACCHINETTE PTHP DX COOLING COIL": Rated air volume flow rate per watt of rated total cooling capacity is out of range.
** ~~~ ** Min Rated Vol Flow Per Watt=[4.027E-005], Rated Vol Flow Per Watt=[3.250E-005], Max Rated Vol Flow Per Watt=[6.041E-005]. See Input Output Reference Manual for valid range.
** Severe ** For object = Coil:Cooling:DX:SingleSpeed, name = "0TERRA:LOCMACCHINETTE PTHP DX COOLING COIL"
** ~~~ ** Calculated outlet air relative humidity greater than 1. The combination of
** ~~~ ** rated air volume flow rate, total cooling capacity and sensible heat ratio yields coil exiting
** ~~~ ** air conditions above the saturation curve. Possible fixes are to reduce the rated total cooling
** ~~~ ** capacity, increase the rated air volume flow rate, or reduce the rated sensible heat ratio for this coil.
** ~~~ ** If autosizing, it is recommended that all three of these values be autosized.
** ~~~ ** ...Inputs used for calculating cooling coil bypass factor.
** ~~~ ** ...Inlet Air Temperature = 26.67 C
** ~~~ ** ...Outlet Air Temperature = 8.51 C
** ~~~ ** ...Inlet Air Humidity Ratio = 1.125000E-002 kgWater/kgDryAir
** ~~~ ** ...Outlet Air Humidity Ratio = 7.773262E-003 kgWater/kgDryAir
** ~~~ ** ...Total Cooling Capacity used in calculation = 4000.00 W
** ~~~ ** ...Air Mass Flow Rate used in calculation = 0.146115 kg/s
** ~~~ ** ...Air Volume Flow Rate used in calculation = 0.130000 m3/s
** ~~~ ** ...Air Volume Flow Rate per Watt of Rated Cooling Capacity is also out of bounds at = 3.2500000E-005 m3/s/W
** ~~~ ** During Warmup, Environment=BOLZANO ANN HTG 99.6% CONDNS DB, at Simulation time=01/21 00:00 - 00:10
** Fatal ** Check and revise the input data for this coil before rerunning the simulation.
What shall I do?



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