Ideal Loads Air System[LINK]
The input object ZoneHVAC:IdealLoadsAirSystem
provides a model for an ideal HVAC system. It occupies a place
in the program hierarchy corresponding to a zone HVAC unit. It
is not connected to a central air system - instead each ZoneHVAC:IdealLoadsAirSystem
object supplies cooling or heating air to a zone in sufficient
quantity to meet the zone load or up to its limits, if
specified. The supply air conditions are controlled based on
specifications in the ZoneHVAC:IdealLoadsAirSystem
input. The system has options for humidity control, outdoor
air, economizer, demand controlled ventilation, and heat
recovery.
The ZoneHVAC:IdealLoadsAirSystem
object is modeled as an ideal VAV terminal unit with variable
supply temperature and humidity. The supply air flow rate is
varied between zero and the maximum in order to satisfy the
zone heating or cooling load, zone humidity controls, outdoor
air requirements, and other constraints, if specified.
The user specifies some or all of the following data for
each ZoneHVAC:IdealLoadsAirSystem
object:
- name of unit availability schedule
- name of the zone inlet node;
- name of the zone exhaust node;
- maximum supply air temperature when in heating mode
Tmax,heating [C];
- minimum supply air temperature when in cooling mode
Tmin,cooling [C];
- maximum supply air humidity ratio when in heating mode
Wmax,humid [kg water/kg dry air];
- minimum supply air humidity ratio when in cooling mode
Wmin,dehum [kg water/kg dry air];
- heating limit type flag (LimitFlowRate, LimitCapacity,
LimitFlowRateAndCapacity or NoLimit)
HeatingLimit;
- maximum heating air flow rate [m3/s]
- maximum sensible heating capacity [W]
- cooling limit type flag (LimitFlowRate, LimitCapacity,
LimitFlowRateAndCapacity or NoLimit)
CoolingLimit
- maximum cooling air flow rate [m3/s]
- maximum total cooling capacity [W]
- name of heating availability schedule
- name of cooling availability schedule
- dehumidification control type flag
(ConstantSensibleHeatRatio, Humidistat, None, or
ConstantSupplyHumidityRatio)
DehumidCtrlType
- cooling sensible heat ratio
- humidification control type flag (Humidistat,
None, or ConstantSupplyHumidityRatio)
HumidCtrlType
- name of a DesignSpecification:OutdoorAir object
- outdoor air inlet node name
- demand controlled ventilation control type flag (None,
OccupancySchedule or CO2Setpoint)
- outdoor air economizer type flag (NoEconomizer,
DifferentialDryBulb, or DifferentialEnthalpy)
- heat recovery type flag (None, Sensible, or
Enthalpy)
- sensible heat recovery effectiveness
- latent heat recovery effectiveness
Set the unit on/off flag UnitOn.
The unit is off (UnitOn = False) if the
unit availability schedule value is <=0; otherwise the unit
is on (UnitOn = True). If the unit is on,
the calculation proceeds through the remaining steps. If the
unit is off, the zone inlet node conditions are set to the
zone node condition, the inlet node mass flow rate is set to
zero, and the unit outputs are set to zero.
Calculate the minimum outdoor air mass flow rate based
on the specifications in the DesignSpecification:OutdoorAir
object, if specified.
Calculate the sensible and latent impact of the outdoor
air flow relative to the zone conditions
Determine if the unit needs to heat or cool
- If outdoor air sensible impact is >= load to zone
cooling setpoint and the current thermostat type is not
SingleHeatingSetPoint, then unit is in cooling mode
- If outdoor air sensible impact is < load to zone
heating setpoint then unit is in heating mode
- Else if neither condition is true, then unit is in
deadband mode (provides outdoor air but shuts off economizer
and heat recovery and all humidity control options except
Humidistat option)
If in cooling mode, simulate outdoor air economizer and
adjust outdoor air mass flow rate
Calculate supply air mass flow rate
- If outdoor air flow rate exceeds applicable maximum flow
rate (heating or cooling) then reduce outdoor air mass flow
rate, issue warning, and set supply air mass flow rate equal
to outdoor air mass flow rate
Else
- Calculate supply air mass flow rate required to meet zone
sensible load at the applicable (heating or cooling) supply
temperature limit (Tmax,heatingor
Tmin,cooling)
\({\dot m_s} = {\dot
Q_z}/({c_{p,air}}({T_s} - {T_z}))\)
If DehumidCtrlType= Humidistat (and other
conditions are met, see below), then calculate the supply air
mass flow rate required to meet the humidistat
dehumidification setpoint at
Wmin,dehum
If HumidCtrlType= Humidistat (and other
conditions are met, see below), then calculate the supply air
mass flow rate required to meet the humidistat humidification
setpoint at Wmax,humid
Set the supply air mass flow rate to the greatest of
these, but limit to the applicable (heating or cooling)
maximum flow rate
Calculate the mixed air conditions, modeling heat
recovery, if applicable
The recirculation air conditions are set equal to the
zone return air node conditions; if there is no return air
node the recirculation air conditions are set equal to the
conditions at the zone node.
The unit entering conditions are then:
If \({\dot m_s}\) >
\({\dot m_{oa}}\) then
\({h_{ma}} = ({\dot m_{oa}} \cdot
{h_{oa}} + ({\dot m_s} - {\dot m_{oa}}) \cdot
{h_{recirc}})/{\dot m_s}\)
\({W_{ma}} = ({\dot m_{oa}} \cdot
{W_{oa}} + ({\dot m_s} - {\dot m_{oa}}) \cdot
{W_{recirc}})/{\dot m_s}\)
\({T_{ma}} = {\mathop{\rm
PsyHFnTdbW}\nolimits} ({h_{ma}},{W_{ma}})\)
Otherwise the entering air conditions are set equal to the
outside air conditions.
Calculate the supply air temperature required to meet
the zone sensible load at the supply air mass flow rate, but
limit to the applicable (heating or cooling) supply
temperature limit (Tmax,heatingor
Tmin,cooling)
\({T_s} = {T_{ma}} + {\dot
Q_z}/({c_{p,air}}{\dot m_s})\)
Calculate the supply humidity ratio based on the
specified humidity control types, but limit to the applicable
(heating or cooling) supply humidity ratio limit
- DehumidCtrlType = None sets the supply air
humidity ratio equal to the mixed air humidity ratio.
- DehumidCtrlType = Humidistat, this will actively
dehumidify to the humidistat dehumidification setpoint during
cooling and deadband operation, and during heating operation
if HumidCtrlType = Humidistat
- DehumidCtrlType = ConstantSensibleHeatRatio sets
the supply air humidity ratio using the cooling sensible heat
ratio.
- DehumidCtrlType = ConstantSupplyHumidityRatio
sets the supply air humidity ratio =
Wmin,dehum.
- HumidCtrlType = None sets the supply air humidity
ratio equal to the mixed air humidity ratio.
- HumidCtrlType = Humidistat, this will actively
humidify to the humidistat humidifying setpoint during heating
and deadband operation, and during cooling operation if
DehumidCtrlType = Humidistat
- HumidCtrlType = ConstantSupplyHumidityRatio sets
the supply air humidity ratio =
Wmax,humid.
Limit supply humidity ratio to saturation at the supply
temperature
Check the applicable capacity limits (sensible heating
and total cooling) and adjust supply air temperature and
humidity if needed.
Set the zone inlet node conditions to the supply air
mass flow rate, temperature, and humidity ratio.
Calculate the unit output and load components.
No specific references.
Ideal Loads Air System[LINK]
Overview[LINK]
The input object ZoneHVAC:IdealLoadsAirSystem provides a model for an ideal HVAC system. It occupies a place in the program hierarchy corresponding to a zone HVAC unit. It is not connected to a central air system - instead each ZoneHVAC:IdealLoadsAirSystem object supplies cooling or heating air to a zone in sufficient quantity to meet the zone load or up to its limits, if specified. The supply air conditions are controlled based on specifications in the ZoneHVAC:IdealLoadsAirSystem input. The system has options for humidity control, outdoor air, economizer, demand controlled ventilation, and heat recovery.
Model[LINK]
The ZoneHVAC:IdealLoadsAirSystem object is modeled as an ideal VAV terminal unit with variable supply temperature and humidity. The supply air flow rate is varied between zero and the maximum in order to satisfy the zone heating or cooling load, zone humidity controls, outdoor air requirements, and other constraints, if specified.
Inputs and Data[LINK]
The user specifies some or all of the following data for each ZoneHVAC:IdealLoadsAirSystem object:
All input data for the ZoneHVAC:IdealLoadsAirSystem is stored in the array PurchAir. The model and data are encapsulated in the module PurchasedAirManager.Calculation[LINK]
Set the unit on/off flag UnitOn.
The unit is off (UnitOn = False) if the unit availability schedule value is <=0; otherwise the unit is on (UnitOn = True). If the unit is on, the calculation proceeds through the remaining steps. If the unit is off, the zone inlet node conditions are set to the zone node condition, the inlet node mass flow rate is set to zero, and the unit outputs are set to zero.
Calculate the minimum outdoor air mass flow rate based on the specifications in the DesignSpecification:OutdoorAir object, if specified.
Calculate the sensible and latent impact of the outdoor air flow relative to the zone conditions
Determine if the unit needs to heat or cool
If in cooling mode, simulate outdoor air economizer and adjust outdoor air mass flow rate
Calculate supply air mass flow rate
Else
\({\dot m_s} = {\dot Q_z}/({c_{p,air}}({T_s} - {T_z}))\)
If DehumidCtrlType= Humidistat (and other conditions are met, see below), then calculate the supply air mass flow rate required to meet the humidistat dehumidification setpoint at Wmin,dehum
If HumidCtrlType= Humidistat (and other conditions are met, see below), then calculate the supply air mass flow rate required to meet the humidistat humidification setpoint at Wmax,humid
Set the supply air mass flow rate to the greatest of these, but limit to the applicable (heating or cooling) maximum flow rate
Calculate the mixed air conditions, modeling heat recovery, if applicable
The recirculation air conditions are set equal to the zone return air node conditions; if there is no return air node the recirculation air conditions are set equal to the conditions at the zone node.
The unit entering conditions are then:
If \({\dot m_s}\) > \({\dot m_{oa}}\) then
\({h_{ma}} = ({\dot m_{oa}} \cdot {h_{oa}} + ({\dot m_s} - {\dot m_{oa}}) \cdot {h_{recirc}})/{\dot m_s}\)
\({W_{ma}} = ({\dot m_{oa}} \cdot {W_{oa}} + ({\dot m_s} - {\dot m_{oa}}) \cdot {W_{recirc}})/{\dot m_s}\)
\({T_{ma}} = {\mathop{\rm PsyHFnTdbW}\nolimits} ({h_{ma}},{W_{ma}})\)
Otherwise the entering air conditions are set equal to the outside air conditions.
Calculate the supply air temperature required to meet the zone sensible load at the supply air mass flow rate, but limit to the applicable (heating or cooling) supply temperature limit (Tmax,heatingor Tmin,cooling)
\({T_s} = {T_{ma}} + {\dot Q_z}/({c_{p,air}}{\dot m_s})\)
Calculate the supply humidity ratio based on the specified humidity control types, but limit to the applicable (heating or cooling) supply humidity ratio limit
Limit supply humidity ratio to saturation at the supply temperature
Check the applicable capacity limits (sensible heating and total cooling) and adjust supply air temperature and humidity if needed.
Set the zone inlet node conditions to the supply air mass flow rate, temperature, and humidity ratio.
Calculate the unit output and load components.
References[LINK]
No specific references.
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This documentation is made available under the EnergyPlus Open Source License v1.0.