System Design
Loads and Air Flow Rates[LINK]
The purpose of the system design calculation is to estimate
design heating and cooling loads and air flow rates for each
air loop in the simulation problem. The calculation sequence
for system level design loads and air flow rates resembles the
calculation sequence for zone loads and air flow rates. There
is an update subroutine UpdateSysSizing called at the
beginning, during, and end of a loop in the Sizing Manager
over all the design days. The major difference is that this
calculation is done at the zone time-step only. There is no
idealized component calculation triggered at the system
time-step as in the zone calculation. The system design
calculation operates at the zone time step using the design
environment weather data and the data stored in the zone
sizing arrays. The results of the system design calculation
are stored in the system sizing arrays described below.
System Design Data
Arrays[LINK]
The system design data arrays are:
SysSizInput(i) stores the input data from the Sizing:System
objects.
SysSizing(i,j) stores the results of the system
design calculations for all systems and all design days. The
index i is for air loops, j for design days.
CalcSysSizing(i) stores the results of the system
design calculations for the peak heating and cooling cases for
each air loop. The index i is for the air loops.
FinalSysSizing(i) corresponds to
CalcSysSizing but includes the effect of the user
specified sizing factor or user specified system design flow
rate.
The data stored in SysSizing,
CalcSysSizing and FinalSysSizing includes
the following data items.
System Sizing Data
| Name |
Description |
| All the data from
SysSizInput |
|
| CoinCoolMassFlow |
coincident peak cooling mass
flow rate [kg/s] |
| CoinHeatMassFlow |
coincident peak heating mass
flow rate [kg/s] |
| NonCoinCoolMassFlow |
noncoincident peak cooling mass
flow rate [kg/s] |
| NonCoinHeatMassFlow |
noncoincident peak heating mass
flow rate [kg/s] |
| DesMainVolFlow |
design main supply duct volume
flow [m /s] |
| DesHeatVolFlow |
design heat supply duct volume
flow [m /s] |
| DesCoolVolFlow |
design cool supply duct volume
flow [m /s] |
| SensCoolCap |
design sensible cooling capacity
[W] |
| TotCoolCap |
design total cooling capacity
[W] |
| HeatCap |
design heating capacity [W] |
| PreheatCap |
design preheat capacity [W] |
| MixTempAtCoolPeak |
mixed air temperature at the
time of the cooling peak [C] |
| MixHumRatAtCoolPeak |
mixed air humidity ratio at the
time of the cooling peak [kg water/kg dry air] |
| RetTempAtCoolPeak |
return air temperature at the
time of the cooling peak [C] |
| RetHumRatAtCoolPeak |
return air humidity ratio at the
time of the cooling peak [kg water/kg dry air] |
| OutTempAtCoolPeak |
outside air temperature at the
time of the cooling peak [C] |
| OutHumRatAtCoolPeak |
outside air humidity ratio at
the time of the cooling peak [kg water/kg dry air] |
| HeatMixTemp |
design mixed air temperature for
heating [C] |
| HeatMixHumRat |
design mixed air humidity ratio
for heating [kg water/kg dry air] |
| HeatRetTemp |
design return air temperature
for heating [C] |
| HeatRetHumRat |
design return air humidity ratio
for heating [kg water/kg dry air] |
| HeatOutTemp |
design outside air temperature
for heating [C] |
| HeatOutHumRat |
design outside air humidity
ratio for heating [kg water/kg dry air] |
| HeatFlowSeq(i) |
daily sequence of system heating
air mass flow rate (zone time step) [kg/s] |
| CoolFlowSeq(i) |
daily sequence of system cooling
air mass flow rate (zone time step) [kg/s] |
| SumZoneCoolLoadSeq(I) |
daily sequence of zones’ summed
cooling load (zone time step) [W] |
| CoolZoneAvgTempSeq(I) |
daily sequence of zones flow
weighted average temperature (zone time step) [W] |
| SensCoolCapSeq(I) |
daily sequence of system
sensible cooling capacity (zone time step) [W] |
| TotCoolCapSeq(I) |
daily sequence of system total
cooling capacity (zone time step) [W] |
| HeatCapSeq(i) |
daily sequence of system heating
capacity (zone time step) [W] |
| PreHeatCapSeq(i) |
daily sequence of system preheat
capacity (zone time step) [W] |
| SysCoolRetTempSeq(i) |
daily sequence of system cooling
return temperatures (zone time step) [C] |
| SysCoolRetHumRatSeq(I) |
daily sequence of system cooling
return humidity ratios (zone time step) [kg water/kg dry
air] |
| SysHeatRetTempSeq(i) |
daily sequence of system heating
return temperatures (zone time step) [C] |
| SysHeatRetHumRatSeq(I) |
daily sequence of system heating
return humidity ratios (zone time step) [kg water/kg dry
air] |
| SysCoolOutTempSeq |
daily sequence of system cooling
outside temperatures (zone time step) [C] |
| SysCoolOutHumRatSeq |
daily sequence of system cooling
outside humidity ratios (zone time step) [kg water/kg dry
air] |
| SysHeatOutTempSeq |
daily sequence of system heating
outside temperatures (zone time step) [C] |
| SysHeatOutHumRatSeq |
daily sequence of system heating
outside humidity ratios (zone time step) [kg water/kg dry
air] |
System
Design Flow Rate and Load Summation and Adjustment[LINK]
There is no system level subroutine corresponding to
SizeZoneEquipment. Instead the system design loads
and flow rates are calculated using the zone level results.
The zone design flow rates for the zones served by an air loop
are summed to obtain the system level design flow rates. These
air flows are mixed with the system level design minimum
outside air flow rate to obtain system design coil loads.
These activities are all performed within the
UpdateSysSizing subroutine in the
SimAirServingZones module. It is called at the start
of each design day (CallIndicator = BeginDay), at the
zone time-step (CallIndicator = DuringDay), at the
end of the design day (CallIndicator = EndDay) and at
the end of the zone design calculation (CallIndicator =
EndSysSizingCalc).
There is a logical flag SysSizingCalc
corresponding to ZoneSizingCalc. It is used to allow
the component routines to distinguish a normal simulation call
from a being called during a system sizing calculation.
The environment (in this case, a design day) name is
stored in the system sizing data structures.
Loop over the zones cooled by this air loop:
- NonCoinCoolMassFlow\(_{sys}\) =
DesCoolMassFlow\(_{zone}\)
- Loop over the zones heated by this air loop:
- NonCoinHeatMassFlow\(_{sys}\) =
DesHeatMassFlow\(_{zone}\)
- Loop over the zones cooled by this air loop:
CoolFlowSeq\(_{sys}\)(i) =
CoolFlowSeq \(_{zone}\)(i)
SysCoolRetTemp(i) = (CoolZoneRetTempSeq(i) *
CoolFlowSeq\(_{zone}\)(i))/
CoolFlowSeq\(_{sys}\)(i)
SysCoolRetHumRat(i) = (CoolZoneHumRatSeq(i) *
CoolFlowSeq\(_{zone}\)(i))/
CoolFlowSeq\(_{sys}\)(i)
FracOA = r\(_{air}\) *
DesOutAirVolFlow\(_{sys}\)/
CoolFlowSeq\(_{sys}\)(i)
T\(_{mix}\) =
T\(_{outside}\) *
FracOA + SysCoolRetTemp(i)(1 -
FracOA)
W\(_{mix}\) =
W\(_{outside}\) *
FracOA + SysCoolRetHumRat (i)(1 -
FracOA)
SysCoolOutTempSeq(i) = T\(_{outside}\)
SysCoolOutHumRatSeq(i) = W\(_{outside}\)
Get the current (zone time-step) system cooling
capacity:
SysSensCoolCap\(_{cur}\) = C\(_{p,air}\) *
CoolFlowSeq\(_{sys}\)(i) *( T\(_{mix}\)-T\(_{sup}\))
SensCoolCapSeq(I) = SysSensCoolCap\(_{cur}\)
If SysSensCoolCap\(_{cur}\) is the maximum for
the day so far then save SysSensCoolCap\(_{cur}\) as the design
value:
SensCoolCap(i )\(_{sys}\) =
SysSensCoolCap\(_{cur}\)
And save the corresponding mixed, return and outside
conditions:
CoolMixTemp\(_{sys}\) = T\(_{mix}\)
CoolMixHumRat\(_{sys}\) = W\(_{mix}\)
CoolRetTemp\(_{sys}\) =
SysCoolRetTemp(i)
CoolRetHumRat\(_{sys}\) =
SysCoolRetHumRat(I)
CoolOutTemp\(_{sys}\) = T\(_{outside}\)
CoolOutHumRat\(_{sys}\) = W\(_{outside}\)
Here r\(_{air}\)
is the density of dry air at 20C and standard elevation
corrected pressure, [kg/m\(^{3}\)];
*FracOA* is the outside air fraction;
C\(_{p,air}\)* is the
specific heat of dry air at 20C, [J/kg-K];
T\(_{sup}\)* is the
user specified design cooling supply temperature [C];
T\(_{mix}\)* is the
current mixed air temperature [C];
W\(_{mix}\)* is the
current mixed air humidity ratio [kg water / kg dry
air];
\(T_{outside}\) is the
current outside air temperature [C];
\(W_{outside}\) is the
current outside air humidity ratio [kg water / kg dry
air].
- Loop over the zones heated by this air loop.
HeatFlowSeq\(_{sys}\)(i) =
HeatFlowSeq \(_{zone}\)(i)
SysHeatRetTemp(i) = (HeatZoneRetTempSeq(i) *
HeatFlowSeq\(_{zone}\)(i))/
HeatFlowSeq\(_{sys}\)(i)
SysHeatRetHumRat(i) = (HeatZoneHumRatSeq(i) *
HeatFlowSeq\(_{zone}\)(i))/
- HeatFlowSeq\(_{sys}\)(i)*
FracOA = r\(_{air}\) *
DesOutAirVolFlow\(_{sys}\)/
HeatFlowSeq\(_{sys}\)(i)
T\(_{mix}\) =
T\(_{outside}\) *
FracOA + SysHeatRetTemp(i)(1 -
FracOA)
W\(_{mix}\) =
W\(_{outside}\) *
FracOA + SysHeatRetHumRat (i)(1 -
FracOA)
SysHeatOutTempSeq(i) = T\(_{outside}\)
SysHeatOutHumRatSeq(i) = W\(_{outside}\)
Get the current (zone time-step) system heating
capacity:
SysHeatCap\(_{cur}\) = C\(_{p,air}\) *
MinFlowRat\(_{sys}\)
*HeatFlowSeq\(_{sys}\)(i) *( T\(_{sup}\)-T\(_{mix}\))
HeatCapSeq(I) = SysHeatCap\(_{cur}\)
If SysHeatCap\(_{cur}\) is the maximum for
the day so far then save SysHeatCap\(_{cur}\) as the design
value:
HeatCap(i )\(_{sys}\) =
SysHeatCap\(_{cur}\)
And save the corresponding mixed, return and outside
conditions:
HeatMixTemp\(_{sys}\) = T\(_{mix}\)
HeatMixHumRat\(_{sys}\) = W\(_{mix}\)
HeatRetTemp\(_{sys}\) =
SysHeatRetTemp(i)
HeatRetHumRat\(_{sys}\) =
SysHeatRetHumRat(I)
HeatOutTemp\(_{sys}\) = T\(_{outside}\)
HeatOutHumRat\(_{sys}\) = W\(_{outside}\)
Here MinFlowRat\(_{sys}\) is the user
specified minimum supply flow ratio.
If the user has specified coincident system sizing
then:
DesCoolVolFlow\(_{sys}\) = r\(_{air}\)
*CoinCoolMassFlow\(_{sys}\)
DesHeatVolFlow\(_{sys}\) = r\(_{air}\)
*CoinHeatMassFlow\(_{sys}\)
DesMainVolFlow\(_{sys}\) =
Max(DesCoolVolFlow\(_{sys}\),
DesHeatVolFlow\(_{sys}\))
If the user has specified noncoincidentsystem
sizing then:
DesCoolVolFlow\(_{sys}\) = r\(_{air}\)
*NonCoinCoolMassFlow\(_{sys}\)
DesHeatVolFlow\(_{sys}\) = r\(_{air}\)
*NonCoinHeatMassFlow\(_{sys}\)
DesMainVolFlow\(_{sys}\) =
Max(DesCoolVolFlow\(_{sys}\),
DesHeatVolFlow\(_{sys}\))
Based on the outdoor air method selected, the
DesCoolVolFlow\(_{sys}\) and
DesHeatVolFlow\(_{sys}\) are modified based
on the system ventilation effciency calculated based on the
maximum outdoor air fraction.
EndSysSizingCalc[LINK]
At this point all the calculations have been done in
SysSizing(i,j): we have results for each design day.
Now these results need to be processed to find the heating and
cooling design quantities for each system over all the design
days.
For coincident sizing the task is quite easy.
- Loop over all of the air loops.
Loop over all of the design days.
If the value of DesCoolVolFlow in
SysSizing for the current design day is greater than
the value stored in CalcSysSizing, then move
DesCoolVolFlow from SysSizing into
CalcSysSizing along with CoolDesDay,
CoinCoolMassFlow, SensCoolCap,
CoolFlowSeq(i), SensCoolCapSeq(i),
CoolMixTemp, CoolRetTemp,
CoolMixHumRat, CoolRetHumRat,
CoolOutTemp, CoolOutHumRat,
SysCoolRetTempSeq(i),
SysCoolRetHumRatSeq(i), SysCoolOutTempSeq(i)
and SysCoolOutHumRatSeq(i).
- If the value of DesHeatVolFlow in
SysSizing for the current design day is greater than
the value stored in CalcSysSizing, then move
DesHeatVolFlow from SysSizing into
CalcSysSizing along with HeatDesDay,
CoinHeatMassFlow, HeatCap,
PreHeatCap, HeatFlowSeq(i),
HeatCapSeq(i), PreHeatCapSeq(i),
HeatMixTemp, HeatRetTemp,
HeatMixHumRat, HeatRetHumRat,
HeatOutTemp, HeatOutHumRat,
SysHeatRetTempSeq(i),
SysHeatRetHumRatSeq(i), SysHeatOutTempSeq(i)
and SysHeatOutHumRatSeq(i).
At the end of each design day loop the peak cooling and the
peak heating data will be stored in CalcSysSizing. At
this point we set DesMainVolFlow in
CalcSysSizing equal to the maximum of
DesCoolVolFlow and DesHeatVolFlow.
For noncoincident sizing the task is harder since we don’t
have a single time-step during which all the zone peaks occur.
So there is no obvious value for outside air temperature at
the peak, return air temperature at the peak and so forth. We
must return to the zone sizing data and calculate average
values for return and outside conditions.
Loop over all of the zones cooled by this air
loop
In FinalZoneSizing replace the value in
DesCoolCoilInTemp with the user specified
CoolSupTemp\(_{sys}\). Do the same for
DesCoolCoilInHumRat and CoolSupHumRat. This
ensures that zone equipment connected to an air loop will use
the system design supply air conditions as coil entering
conditions.
- NonCoinCoolMassFlow\(_{sys}\) =
DesCoolMassFlow\(_{zone}\)
SysCoolRetTemp = (ZoneRetTempAtCoolPeak
*DesCoolMassFlow\(_{zone}\))
/NonCoinCoolMassFlow\(_{sys}\)
SysCoolRetHumRat = (ZoneHumRatAtCoolPeak
*
DesCoolMassFlow\(_{zone}\))/NonCoinCoolMassFlow\(_{sys}\)
SysCoolOutTemp = (T\(_{OA,zone\, peak}\)
*DesCoolMassFlow\(_{zone}\))/
- NonCoinCoolMassFlow\(_{sys}\)*
SysCoolOutHumRat = (W\(_{OA,zone\\ peak}\)
*DesCoolMassFlow\(_{zone}\))/
- NonCoinCoolMassFlow\(_{sys}\)*
At the end of the zone loop calculate mixed air conditions
and the system sensible cooling capacity.
FracOA = r\(_{air}\) *
DesOutAirVolFlow\(_{sys}\)/
NonCoinCoolMassFlow\(_{sys}\)
T\(_{mix}\) =
SysCoolOutTemp * FracOA +
SysCoolRetTemp * (1 - FracOA)
W\(_{mix}\) =
SysCoolOutHumRat * FracOA +
SysCoolRetHumRat *
(1 - FracOA)
SysSensCoolCap = C\(_{p,air}\) *
NonCoinCoolMassFlow *( T\(_{mix}\)-T\(_{sup}\))
Then (for noncoincident sizing) the variables calculated in
section (ii) are moved into the CalcSysSizing
Array.
Loop over all of the zones heated by this air
loop.
In FinalZoneSizing replace the value in
DesHeatCoilInTemp with the user specified
HeatSupTemp\(_{sys}\). Do the same for
DesHeatCoilInHumRat and HeatSupHumRat. This
ensures that zone equipment connected to an air loop will use
the system design supply air conditions as coil entering
conditions.
- NonCoinHeatMassFlow\(_{sys}\) =
DesHeatMassFlow\(_{zone}\)
SysHeatRetTemp = (ZoneRetTempAtHeatPeak
*DesHeatMassFlow\(_{zone}\))
/NonCoinHeatMassFlow\(_{sys}\)
SysHeatRetHumRat = (ZoneHumRatAtHeatPeak
*
DesHeatMassFlow\(_{zone}\))/NonCoinHeatMassFlow\(_{sys}\)
SysHeatOutTemp = (T\(_{OA,zone\\ peak}\)
*DesHeatMassFlow\(_{zone}\))/
- NonCoinHeatMassFlow\(_{sys}\)*
SysHeatOutHumRat = (W\(_{OA,zone\\ peak}\)
*DesHeatMassFlow\(_{zone}\))/
- NonCoinHeatMassFlow\(_{sys}\)*
At the end of the zone loop calculate mixed air conditions
and the system sensible cooling capacity.
FracOA = r\(_{air}\) *
DesOutAirVolFlow\(_{sys}\)/
NonCoinHeatMassFlow\(_{sys}\)
T\(_{mix}\) =
SysHeatOutTemp * FracOA +
SysHeatRetTemp * (1 - FracOA)
W\(_{mix}\) =
SysHeatOutHumRat * FracOA +
SysHeatRetHumRat *
(1 - FracOA)
SysHeatlCap = C\(_{p,air}\) *
NonCoinHeatlMassFlow *( T\(_{sup}\)-T\(_{mix}\))
Then (for noncoincident sizing) the variables calculated in
section (ii) are moved into the CalcSysSizing
Array.
- We now have the calculated system sizing data. This data
needs to be altered to take into account the user input system
design flow rates (if any), or the fact that the user may have
requested that the system flow rate be sized on the
ventilation requirement. Note that user specified sizing
ratios have already been applied to the zone sizing data which
have been used in out preceding system sizing calculation.
Thus the user specified sizing ratios do not have to be
explicitly taken into account at the system level.
First we move the calculated system sizing data from
CalcSysSizing array into the FinalSysSizing
array. FinalSysSizing will contain the user modified
system design data when we are all done.
Loop over the air loops.
- As in the zone case, the user specified system design flow
rates are turned into sizing ratios by dividing the user input
value by the calculated value. The same strategy is employed
for sizing on the ventilation requirement: the design
ventilation flow rate is divided by the calculated design flow
rate value. For each air loop this gives us a SizRat\(_{cool}\) and
SizRat\(_{heat}\).
CoinCoolMassFlow = SizRat\(_{cool}\) *
CoinCoolMassFlow\(_{calc}\)
NonCoinCoolMassFlow = SizRat\(_{cool}\) *
NonCoinCoolMassFlow\(_{calc}\)
DesCoolVolFlow = SizRat\(_{cool}\) *
DesCoolVolFlow\(_{calc}\)
Since the flow rates have been altered the outside air
fraction will change. This will alter the design mixed air
conditions and lead to an altered value for the cooling
capacity. This must be done for the time-step sequence and for
the peak value.
- Loop over the zone timesteps (index = i).
CoolFlowSeq\(_{sys}\)(i) =
SizRat\(_{cool}\) *
CoolFlowSeq\(_{sys,calc}\)(i)
FracOA = r\(_{air}\) *
DesOutAirVolFlow\(_{sys}\)/
CoolFlowSeq\(_{sys}\)(i)
T\(_{mix}\) =
SysCoolOutTempSeq(i) *FracOA +
SysCoolRetTempSeq(i) *(1-FracOA)
SensCoolCapSeq(i) = C\(_{p,air}\) *
CoolFlowSeq\(_{sys}\)(i) *( T\(_{mix}\)-T\(_{sup}\))
- Do the same calculation for peak cooling.
FracOA = r\(_{air}\) *
DesOutAirVolFlow\(_{sys}\)/
DesCoolVolFlow
T\(_{mix}\) =
CoolOutTemp\(_{sys}\)
*FracOA + CoolRetTemp\(_{sys}\) *(1-FracOA)
W\(_{mix}\) =
CoolOutHumRat\(_{sys}\)
*FracOA + CoolRetHumRat\(_{sys}\) *
(1-FracOA)
SensCoolCap\(_{sys}\) = C\(_{p,air}\) *
DesCoolVolFlow\(_{sys}\) *( T\(_{mix}\)-T\(_{sup}\))
T\(_{mix}\) and
W\(_{mix}\) are
saved in FinalSysSizing .
- Do the same calculation for the heating case.
CoinHeatMassFlow = SizRat\(_{heat}\) *
CoinHeatMassFlow\(_{calc}\)
NonCoinHeatMassFlow = SizRat\(_{heat}\) *
NonCoinHeatMassFlow\(_{calc}\)
DesHeatVolFlow = SizRat\(_{heat}\) *
DesHeatVolFlow\(_{calc}\)
- Loop over the zone timesteps (index = i).
HeatFlowSeq\(_{sys}\)(i) =
SizRat\(_{Heat}\) *
HeatFlowSeq\(_{sys,calc}\)(i)
FracOA = r\(_{air}\) *
DesOutAirVolFlow\(_{sys}\)/
HeatFlowSeq\(_{sys}\)(i)
T\(_{mix}\) =
SysHeatOutTempSeq(i) * FracOA +
- SysHeatRetTempSeq(i) ** (1-FracOA)
HeatCapSeq(i) = C\(_{p,air}\) *
HeatFlowSeq\(_{sys}\)(i) *(T\(_{sup}\)-T\(_{mix}\))
- Do the same calculation for peak heating.
FracOA = r\(_{air}\) *
DesOutAirVolFlow\(_{sys}\)/
DesHeatVolFlow
T\(_{mix}\) =
HeatOutTemp\(_{sys}\)
*FracOA + HeatRetTemp\(_{sys}\) *
(1-FracOA)
W\(_{mix}\) =
HeatOutHumRat\(_{sys}\)
*FracOA + HeatRetHumRat\(_{sys}\) *
(1-FracOA)
HeatCap\(_{sys}\)
= C\(_{p,air}\) *
DesHeatVolFlow\(_{sys}\) *( T\(_{sup}\)-T\(_{mix}\))
T\(_{mix}\) and
W\(_{mix}\) are
saved in FinalSysSizing .
(vii)DesMainVolFlow\(_{sys}\) =
MAX(DesCoolVolFlow\(_{sys}\),DesHeatVolFlow\(_{sys}\))
This concludes the system design
calculation.
Scalable System HVAC Sizing
The scalable system sizing applies to system supply air
flow rates and sysyem capacity in coolin and heating
modes.
Scalable System Air Flow Sizing
The scalable sizing methods for supply air flow rate
allowed are either, FlowPerFloorArea,
FractionOfAutosizedCoolingAirflow, or
FlowPerCoolingCapacity. The scalable system air flow
sizing methods are defined as follows:
FlowPerFloorArea means the program calculates the
cooling supply air volume flow rate from zone floor area
served by the airloop and user specified Flow Per Floor
Area value.
FractionOfAutosizedCoolingAirflow means the
program calculates the cooling supply air volume flow rate
from user specified fraction and the autosized design cooling
supply air volume flow rate value determined by the
simulation.
FractionOfAutosizedHeatingAirflow means the
program calculates the heating supply air volume flow rate
from user specified fraction and the autosized design heating
supply air volume flow rate value determined by the
simulation.
FlowPerCoolingCapacity means the supply air volume
is calculated from user specified flow per cooling capacity
and design cooling capacity determined by the simulation.
FlowPerHeatingCapacity means the supply air volume
is calculated from user specified flow per heating capacity
and design heating capacity determined by the simulation.
Scalable System Capacity Sizing
The scalable sizing methods for system capacity available
are: CapacityPerFloorArea,
FractionOfAutosizedCoolingCapacity and
FractionOfAutosizedHeatingCapacity. The scalable
system capacity sizing methods are defined as follows:
CapacityPerFloorArea means the program calculates
the design capacity from user specified capacity per floor
area and floor area of the zones served by the airloop.
FractionOfAutosizedCoolingCapacity means the
program calculates the design cooling capacity from user
specified fraction and the auto-sized design cooling
capacity.
FractionOfAutosizedHeatingCapacity means the
program calculates the design heating capacity from user
specified fraction and the auto-sized design heating
capacity.
System Design Loads and Air Flow Rates[LINK]
Overview[LINK]
The purpose of the system design calculation is to estimate design heating and cooling loads and air flow rates for each air loop in the simulation problem. The calculation sequence for system level design loads and air flow rates resembles the calculation sequence for zone loads and air flow rates. There is an update subroutine UpdateSysSizing called at the beginning, during, and end of a loop in the Sizing Manager over all the design days. The major difference is that this calculation is done at the zone time-step only. There is no idealized component calculation triggered at the system time-step as in the zone calculation. The system design calculation operates at the zone time step using the design environment weather data and the data stored in the zone sizing arrays. The results of the system design calculation are stored in the system sizing arrays described below.
System Design Data Arrays[LINK]
The system design data arrays are:
SysSizInput(i) stores the input data from the Sizing:System objects.
SysSizing(i,j) stores the results of the system design calculations for all systems and all design days. The index i is for air loops, j for design days.
CalcSysSizing(i) stores the results of the system design calculations for the peak heating and cooling cases for each air loop. The index i is for the air loops.
FinalSysSizing(i) corresponds to CalcSysSizing but includes the effect of the user specified sizing factor or user specified system design flow rate.
The data stored in SysSizing, CalcSysSizing and FinalSysSizing includes the following data items.
System Design Flow Rate and Load Summation and Adjustment[LINK]
There is no system level subroutine corresponding to SizeZoneEquipment. Instead the system design loads and flow rates are calculated using the zone level results. The zone design flow rates for the zones served by an air loop are summed to obtain the system level design flow rates. These air flows are mixed with the system level design minimum outside air flow rate to obtain system design coil loads. These activities are all performed within the UpdateSysSizing subroutine in the SimAirServingZones module. It is called at the start of each design day (CallIndicator = BeginDay), at the zone time-step (CallIndicator = DuringDay), at the end of the design day (CallIndicator = EndDay) and at the end of the zone design calculation (CallIndicator = EndSysSizingCalc).
There is a logical flag SysSizingCalc corresponding to ZoneSizingCalc. It is used to allow the component routines to distinguish a normal simulation call from a being called during a system sizing calculation.
BeginDay[LINK]
The environment (in this case, a design day) name is stored in the system sizing data structures.
Loop over the zones cooled by this air loop:
DuringDay[LINK]
CoolFlowSeq\(_{sys}\)(i) = CoolFlowSeq \(_{zone}\)(i)
SysCoolRetTemp(i) = (CoolZoneRetTempSeq(i) * CoolFlowSeq\(_{zone}\)(i))/ CoolFlowSeq\(_{sys}\)(i)
SysCoolRetHumRat(i) = (CoolZoneHumRatSeq(i) * CoolFlowSeq\(_{zone}\)(i))/ CoolFlowSeq\(_{sys}\)(i)
FracOA = r\(_{air}\) * DesOutAirVolFlow\(_{sys}\)/ CoolFlowSeq\(_{sys}\)(i)
T\(_{mix}\) = T\(_{outside}\) * FracOA + SysCoolRetTemp(i)(1 - FracOA)
W\(_{mix}\) = W\(_{outside}\) * FracOA + SysCoolRetHumRat (i)(1 - FracOA)
SysCoolOutTempSeq(i) = T\(_{outside}\)
SysCoolOutHumRatSeq(i) = W\(_{outside}\)
Get the current (zone time-step) system cooling capacity:
SysSensCoolCap\(_{cur}\) = C\(_{p,air}\) * CoolFlowSeq\(_{sys}\)(i) *( T\(_{mix}\)-T\(_{sup}\))
SensCoolCapSeq(I) = SysSensCoolCap\(_{cur}\)
If SysSensCoolCap\(_{cur}\) is the maximum for the day so far then save SysSensCoolCap\(_{cur}\) as the design value:
SensCoolCap(i )\(_{sys}\) = SysSensCoolCap\(_{cur}\)
And save the corresponding mixed, return and outside conditions:
CoolMixTemp\(_{sys}\) = T\(_{mix}\)
CoolMixHumRat\(_{sys}\) = W\(_{mix}\)
CoolRetTemp\(_{sys}\) = SysCoolRetTemp(i)
CoolRetHumRat\(_{sys}\) = SysCoolRetHumRat(I)
CoolOutTemp\(_{sys}\) = T\(_{outside}\)
CoolOutHumRat\(_{sys}\) = W\(_{outside}\)
Here r\(_{air}\) is the density of dry air at 20C and standard elevation corrected pressure, [kg/m\(^{3}\)];
C\(_{p,air}\)* is the specific heat of dry air at 20C, [J/kg-K];
T\(_{sup}\)* is the user specified design cooling supply temperature [C];
T\(_{mix}\)* is the current mixed air temperature [C];
W\(_{mix}\)* is the current mixed air humidity ratio [kg water / kg dry air];
\(T_{outside}\) is the current outside air temperature [C];
\(W_{outside}\) is the current outside air humidity ratio [kg water / kg dry air].
HeatFlowSeq\(_{sys}\)(i) = HeatFlowSeq \(_{zone}\)(i)
SysHeatRetTemp(i) = (HeatZoneRetTempSeq(i) * HeatFlowSeq\(_{zone}\)(i))/
HeatFlowSeq\(_{sys}\)(i)
SysHeatRetHumRat(i) = (HeatZoneHumRatSeq(i) * HeatFlowSeq\(_{zone}\)(i))/
FracOA = r\(_{air}\) * DesOutAirVolFlow\(_{sys}\)/ HeatFlowSeq\(_{sys}\)(i)
T\(_{mix}\) = T\(_{outside}\) * FracOA + SysHeatRetTemp(i)(1 - FracOA)
W\(_{mix}\) = W\(_{outside}\) * FracOA + SysHeatRetHumRat (i)(1 - FracOA)
SysHeatOutTempSeq(i) = T\(_{outside}\)
SysHeatOutHumRatSeq(i) = W\(_{outside}\)
Get the current (zone time-step) system heating capacity:
SysHeatCap\(_{cur}\) = C\(_{p,air}\) * MinFlowRat\(_{sys}\) *HeatFlowSeq\(_{sys}\)(i) *( T\(_{sup}\)-T\(_{mix}\))
HeatCapSeq(I) = SysHeatCap\(_{cur}\)
If SysHeatCap\(_{cur}\) is the maximum for the day so far then save SysHeatCap\(_{cur}\) as the design value:
HeatCap(i )\(_{sys}\) = SysHeatCap\(_{cur}\)
And save the corresponding mixed, return and outside conditions:
HeatMixTemp\(_{sys}\) = T\(_{mix}\)
HeatMixHumRat\(_{sys}\) = W\(_{mix}\)
HeatRetTemp\(_{sys}\) = SysHeatRetTemp(i)
HeatRetHumRat\(_{sys}\) = SysHeatRetHumRat(I)
HeatOutTemp\(_{sys}\) = T\(_{outside}\)
HeatOutHumRat\(_{sys}\) = W\(_{outside}\)
Here MinFlowRat\(_{sys}\) is the user specified minimum supply flow ratio.
EndDay[LINK]
If the user has specified coincident system sizing then:
DesCoolVolFlow\(_{sys}\) = r\(_{air}\) *CoinCoolMassFlow\(_{sys}\)
DesHeatVolFlow\(_{sys}\) = r\(_{air}\) *CoinHeatMassFlow\(_{sys}\)
DesMainVolFlow\(_{sys}\) = Max(DesCoolVolFlow\(_{sys}\), DesHeatVolFlow\(_{sys}\))
If the user has specified noncoincidentsystem sizing then:
DesCoolVolFlow\(_{sys}\) = r\(_{air}\) *NonCoinCoolMassFlow\(_{sys}\)
DesHeatVolFlow\(_{sys}\) = r\(_{air}\) *NonCoinHeatMassFlow\(_{sys}\)
DesMainVolFlow\(_{sys}\) = Max(DesCoolVolFlow\(_{sys}\), DesHeatVolFlow\(_{sys}\))
Based on the outdoor air method selected, the DesCoolVolFlow\(_{sys}\) and DesHeatVolFlow\(_{sys}\) are modified based on the system ventilation effciency calculated based on the maximum outdoor air fraction.
EndSysSizingCalc[LINK]
At this point all the calculations have been done in SysSizing(i,j): we have results for each design day. Now these results need to be processed to find the heating and cooling design quantities for each system over all the design days.
For coincident sizing the task is quite easy.
Loop over all of the design days.
If the value of DesCoolVolFlow in SysSizing for the current design day is greater than the value stored in CalcSysSizing, then move DesCoolVolFlow from SysSizing into CalcSysSizing along with CoolDesDay, CoinCoolMassFlow, SensCoolCap, CoolFlowSeq(i), SensCoolCapSeq(i), CoolMixTemp, CoolRetTemp, CoolMixHumRat, CoolRetHumRat, CoolOutTemp, CoolOutHumRat, SysCoolRetTempSeq(i), SysCoolRetHumRatSeq(i), SysCoolOutTempSeq(i) and SysCoolOutHumRatSeq(i).
At the end of each design day loop the peak cooling and the peak heating data will be stored in CalcSysSizing. At this point we set DesMainVolFlow in CalcSysSizing equal to the maximum of DesCoolVolFlow and DesHeatVolFlow.
For noncoincident sizing the task is harder since we don’t have a single time-step during which all the zone peaks occur. So there is no obvious value for outside air temperature at the peak, return air temperature at the peak and so forth. We must return to the zone sizing data and calculate average values for return and outside conditions.
Loop over all of the zones cooled by this air loop
In FinalZoneSizing replace the value in DesCoolCoilInTemp with the user specified CoolSupTemp\(_{sys}\). Do the same for DesCoolCoilInHumRat and CoolSupHumRat. This ensures that zone equipment connected to an air loop will use the system design supply air conditions as coil entering conditions.
SysCoolRetTemp = (ZoneRetTempAtCoolPeak *DesCoolMassFlow\(_{zone}\))
/NonCoinCoolMassFlow\(_{sys}\)
SysCoolRetHumRat = (ZoneHumRatAtCoolPeak *
DesCoolMassFlow\(_{zone}\))/NonCoinCoolMassFlow\(_{sys}\)
SysCoolOutTemp = (T\(_{OA,zone\, peak}\) *DesCoolMassFlow\(_{zone}\))/
SysCoolOutHumRat = (W\(_{OA,zone\\ peak}\) *DesCoolMassFlow\(_{zone}\))/
At the end of the zone loop calculate mixed air conditions and the system sensible cooling capacity.
FracOA = r\(_{air}\) * DesOutAirVolFlow\(_{sys}\)/ NonCoinCoolMassFlow\(_{sys}\)
T\(_{mix}\) = SysCoolOutTemp * FracOA + SysCoolRetTemp * (1 - FracOA)
W\(_{mix}\) = SysCoolOutHumRat * FracOA + SysCoolRetHumRat *
(1 - FracOA)
SysSensCoolCap = C\(_{p,air}\) * NonCoinCoolMassFlow *( T\(_{mix}\)-T\(_{sup}\))
Then (for noncoincident sizing) the variables calculated in section (ii) are moved into the CalcSysSizing Array.
Loop over all of the zones heated by this air loop.
In FinalZoneSizing replace the value in DesHeatCoilInTemp with the user specified HeatSupTemp\(_{sys}\). Do the same for DesHeatCoilInHumRat and HeatSupHumRat. This ensures that zone equipment connected to an air loop will use the system design supply air conditions as coil entering conditions.
SysHeatRetTemp = (ZoneRetTempAtHeatPeak *DesHeatMassFlow\(_{zone}\))
/NonCoinHeatMassFlow\(_{sys}\)
SysHeatRetHumRat = (ZoneHumRatAtHeatPeak *
DesHeatMassFlow\(_{zone}\))/NonCoinHeatMassFlow\(_{sys}\)
SysHeatOutTemp = (T\(_{OA,zone\\ peak}\) *DesHeatMassFlow\(_{zone}\))/
SysHeatOutHumRat = (W\(_{OA,zone\\ peak}\) *DesHeatMassFlow\(_{zone}\))/
At the end of the zone loop calculate mixed air conditions and the system sensible cooling capacity.
FracOA = r\(_{air}\) * DesOutAirVolFlow\(_{sys}\)/ NonCoinHeatMassFlow\(_{sys}\)
T\(_{mix}\) = SysHeatOutTemp * FracOA + SysHeatRetTemp * (1 - FracOA)
W\(_{mix}\) = SysHeatOutHumRat * FracOA + SysHeatRetHumRat *
(1 - FracOA)
SysHeatlCap = C\(_{p,air}\) * NonCoinHeatlMassFlow *( T\(_{sup}\)-T\(_{mix}\))
Then (for noncoincident sizing) the variables calculated in section (ii) are moved into the CalcSysSizing Array.
First we move the calculated system sizing data from CalcSysSizing array into the FinalSysSizing array. FinalSysSizing will contain the user modified system design data when we are all done.
Loop over the air loops.
CoinCoolMassFlow = SizRat\(_{cool}\) * CoinCoolMassFlow\(_{calc}\)
NonCoinCoolMassFlow = SizRat\(_{cool}\) * NonCoinCoolMassFlow\(_{calc}\)
DesCoolVolFlow = SizRat\(_{cool}\) * DesCoolVolFlow\(_{calc}\)
Since the flow rates have been altered the outside air fraction will change. This will alter the design mixed air conditions and lead to an altered value for the cooling capacity. This must be done for the time-step sequence and for the peak value.
CoolFlowSeq\(_{sys}\)(i) = SizRat\(_{cool}\) * CoolFlowSeq\(_{sys,calc}\)(i)
FracOA = r\(_{air}\) * DesOutAirVolFlow\(_{sys}\)/ CoolFlowSeq\(_{sys}\)(i)
T\(_{mix}\) = SysCoolOutTempSeq(i) *FracOA +
SysCoolRetTempSeq(i) *(1-FracOA)
SensCoolCapSeq(i) = C\(_{p,air}\) * CoolFlowSeq\(_{sys}\)(i) *( T\(_{mix}\)-T\(_{sup}\))
FracOA = r\(_{air}\) * DesOutAirVolFlow\(_{sys}\)/ DesCoolVolFlow
T\(_{mix}\) = CoolOutTemp\(_{sys}\) *FracOA + CoolRetTemp\(_{sys}\) *(1-FracOA)
W\(_{mix}\) = CoolOutHumRat\(_{sys}\) *FracOA + CoolRetHumRat\(_{sys}\) *
(1-FracOA)
SensCoolCap\(_{sys}\) = C\(_{p,air}\) * DesCoolVolFlow\(_{sys}\) *( T\(_{mix}\)-T\(_{sup}\))
T\(_{mix}\) and W\(_{mix}\) are saved in FinalSysSizing .
CoinHeatMassFlow = SizRat\(_{heat}\) * CoinHeatMassFlow\(_{calc}\)
NonCoinHeatMassFlow = SizRat\(_{heat}\) * NonCoinHeatMassFlow\(_{calc}\)
DesHeatVolFlow = SizRat\(_{heat}\) * DesHeatVolFlow\(_{calc}\)
HeatFlowSeq\(_{sys}\)(i) = SizRat\(_{Heat}\) * HeatFlowSeq\(_{sys,calc}\)(i)
FracOA = r\(_{air}\) * DesOutAirVolFlow\(_{sys}\)/ HeatFlowSeq\(_{sys}\)(i)
T\(_{mix}\) = SysHeatOutTempSeq(i) * FracOA +
HeatCapSeq(i) = C\(_{p,air}\) * HeatFlowSeq\(_{sys}\)(i) *(T\(_{sup}\)-T\(_{mix}\))
FracOA = r\(_{air}\) * DesOutAirVolFlow\(_{sys}\)/ DesHeatVolFlow
T\(_{mix}\) = HeatOutTemp\(_{sys}\) *FracOA + HeatRetTemp\(_{sys}\) * (1-FracOA)
W\(_{mix}\) = HeatOutHumRat\(_{sys}\) *FracOA + HeatRetHumRat\(_{sys}\) *
(1-FracOA)
HeatCap\(_{sys}\) = C\(_{p,air}\) * DesHeatVolFlow\(_{sys}\) *( T\(_{sup}\)-T\(_{mix}\))
T\(_{mix}\) and W\(_{mix}\) are saved in FinalSysSizing .
(vii)DesMainVolFlow\(_{sys}\) = MAX(DesCoolVolFlow\(_{sys}\),DesHeatVolFlow\(_{sys}\))
This concludes the system design calculation.
Scalable System HVAC Sizing
The scalable system sizing applies to system supply air flow rates and sysyem capacity in coolin and heating modes.
Scalable System Air Flow Sizing
The scalable sizing methods for supply air flow rate allowed are either, FlowPerFloorArea, FractionOfAutosizedCoolingAirflow, or FlowPerCoolingCapacity. The scalable system air flow sizing methods are defined as follows:
FlowPerFloorArea means the program calculates the cooling supply air volume flow rate from zone floor area served by the airloop and user specified Flow Per Floor Area value.
FractionOfAutosizedCoolingAirflow means the program calculates the cooling supply air volume flow rate from user specified fraction and the autosized design cooling supply air volume flow rate value determined by the simulation.
FractionOfAutosizedHeatingAirflow means the program calculates the heating supply air volume flow rate from user specified fraction and the autosized design heating supply air volume flow rate value determined by the simulation.
FlowPerCoolingCapacity means the supply air volume is calculated from user specified flow per cooling capacity and design cooling capacity determined by the simulation.
FlowPerHeatingCapacity means the supply air volume is calculated from user specified flow per heating capacity and design heating capacity determined by the simulation.
Scalable System Capacity Sizing
The scalable sizing methods for system capacity available are: CapacityPerFloorArea, FractionOfAutosizedCoolingCapacity and FractionOfAutosizedHeatingCapacity. The scalable system capacity sizing methods are defined as follows:
CapacityPerFloorArea means the program calculates the design capacity from user specified capacity per floor area and floor area of the zones served by the airloop.
FractionOfAutosizedCoolingCapacity means the program calculates the design cooling capacity from user specified fraction and the auto-sized design cooling capacity.
FractionOfAutosizedHeatingCapacity means the program calculates the design heating capacity from user specified fraction and the auto-sized design heating capacity.
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This documentation is made available under the EnergyPlus Open Source License v1.0.