Francesco,
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 these 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.
| CoilSystem:Cooling:DX |
Name | AirLoop_UnitCool1_AdminGroundFlr:NortheastCrnrOffice |
Availability Schedule Name | Sched_AlwaysOn |
DX Cooling Coil System Inlet Node Name | NodeOut_znSFbt_Base4-1 |
DX Cooling Coil System Outlet Node Name | NodeOut_znClgCoilAir_Base4-1 |
DX Cooling Coil System Sensor Node Name | NodeOut_znClgCoilAir_Base4-1 |
Cooling Coil Object Type | Coil:Cooling:DX:SingleSpeed |
Cooling Coil Name | Coil-ERVclg_AdminGroundFlr:NortheastCrnrOffice |
Dehumidification Control Type | CoolReheat |
Run on Sensible Load | MT |
Run on Latent Load | MT |
Use DOAS DX Cooling Coil | Yes |
DOAS DX Cooling Coil Leaving Minimum Air Temperature | 7.2 |
James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
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@xxxxxxxxx
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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