Engineering Reference — EnergyPlus 8.2

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Engineering Reference

Air System Compound Component Groups[LINK]

Unitary Systems[LINK]

Overview[LINK]

The input object AirLoopHVAC:UnitarySystem provide a “virtual” component that collect and control a set of components: fan, heating coil, cooling coil, and/or reheat coil. Reheat coil is modeled for controlling high zone humidity levels. The unit may be configured to have either a blow through or draw through fan. In a blow through configuration, fan is generally the fist component upstream of heating ro cooling coil. In a draw through fan configuration, fan is placed directly after the heating coil and before reheat coil.

Schematic of the EnergyPlus Unitary System (Blow Through Configuration)

Schematic of the EnergyPlus Unitary System (Blow Through Configuration)

Model Description[LINK]

As described previously, the unitary system is a “virtual” component consisting of a fan, heating coil, cooling coil and reheat coil. The sole purpose of the unitary system model is to properly coordinate the operation of the various system components. The following sections describe the flow of information within the model, as well as the differences between cycling and continuous supply air fan operation.

Controls[LINK]

There are two types of control types allowed to be specified in the unitary system which are setpoint based and load based. Each control type is described in detail below.

Setpoint based control:

The unitary system calculates the current sensible load using the temperature of the inlet node and the System Node Setpoint Temp on the control node. If the control node is not the outlet node, the desired outlet node temperature is adjusted for the current temperature difference between the outlet node and the control node. Likewise, the current latent load is calculated using the humidity ratio of the inlet node and the System Node Humidity Ratio Max on the control node. The controls determine the required coil run-time fraction and dehumidification mode (if applicable) using the steps outlined below.

Step 1 - Meet Sensible Load Requirement[LINK]

The controls first attempt to meet the sensible requirement. The specified coil model is called with a part-load ratio (PLR) of 1.0 to determine the full-load output of the coil. This is compared with the desired outlet node temperature and a sensible PLR is calculated. If the PLR is <1.0, a Regula-Falsi iteration routine is called to determine the coil run-time fraction which results in the desired outlet node temperature. For a variable-speed DX cooling coil, if the load is smaller than the sensible capacity at the lowest speed, the coil run-time fraction is determined in the same way as a single-speed DX cooling coil. Otherwise, its speed number and speed ratio between two neighboring speeds are selected to match the load.

Step 2 - Meet Latent Load Requirement (if activated)[LINK]

If dehumidification controls are active, the leaving humidity ratio resulting from operation to meet the sensible load (Step 1 above) is compared with the desired outlet node humidity ratio. If the humidity requirement is already met, then no further control action is taken. If the humidity requirement has not been met, then the coil is re-simulated depending on the type of humidity control.

Step 2a - Humidity Control = MultiMode[LINK]

If the humidity control type is MultiMode, then the coil’s enhanced dehumidification mode is activated when the coil type is Coil:Cooling:DX:TwoStageWithHumidityControlMode and Step 1 above is repeated to meet the sensible load using the coil performance resulting from the enhanced dehumidificaiton mode. This is a semi-passive approach to dehumidification which may fall short or may exceed the dehumidification requirement.

Step 2b - Humidity Control = CoolReheat[LINK]

If the humidity control type is CoolReheat, the coil is re-simulated to achieve the desired outlet node humidity ratio. This option is valid for all cooling coil types. When the coil type is Coil:Cooling:DX:TwoStageWithHumidityControlMode, only the cooling performance mode is used for this step and enhanced dehumidification mode is not activated.

Load based control:

While the unitary system may be configured to serve multiple zones, system operation is controlled by a thermostat located in a single “control” zone. One of the key parameters for the unitary system component is the fraction of the total system air flow that goes through the control zone. This fraction is calculated as the ratio of the maximum air mass flow rate for the air loop’s supply inlet node for the control zone (e.g., AirTerminal:SingleDuct:Uncontrolled, field = Maximum Air Flow Rate, converted to mass flow) to the sum of the maximum air mass flow rates for the air loop’s supply inlet nodes for all zones served by this air loop. The unitary system module scales the calculated load for the control zone upward based on this fraction to determine the total load to be met by the unitary system. The module then proceeds to calculate the required part-load ratio for the system coil and the supply air fan to meet this total load. The heating or cooling capacity delivered by the unitary system is distributed to all of the zones served by this system via the terminal units that supply air to each zone. The supply air fraction that goes though the control zone is calculated as follows:

where:

=maximum air mass flow rate for the air loop’s supply inlet node (terminal unit) for the control zone (kg/s)

=maximum air mass flow rate for the air loop’s supply inlet node for the jth zone (kg/s)

=number of zones, or number of air loop supply air inlet nodes for all zones served by the air loop (-)

The unitary system component is able to model supply air fan operation in two modes: cycling fan - cycling coil (i.e., AUTO fan) and continuous fan - cycling coil (i.e., fan ON). Fan:OnOff must be used to model AUTO fan, while Fan:OnOff or Fan:ConstantVolume can be used to model fan ON. The fan operation mode is specified using a supply air fan operating mode schedule where schedule values of 0 denote cycling fan operation and schedule values other than 0 (a 1 is usually used) denote continuous fan operation. Using this schedule, the unitary system fan may be cycled with cooling or heating coil operation or operated continuously based on time of day (e.g., cycling fan operation at night and continuous fan operation during the daytime). If the fan operating mode schedule name field is left blank in the unitary system object, the unitary system assumes cycling or AUTO fan mode operation throughout the simulation.

The unitary system operates based on the user-specified (or autosized) design supply air flow rate(s). The ‘design’ supply air mass flow rate may be different for cooling, heating, and when no cooling or heating is required and the fan operates continuously based on user-specified inputs.

Cooling Operation

If EnergyPlus determines that the unitary system must supply cooling to the control zone to meet the zone air temperature setpoint, then the model computes the total sensible cooling load to be met by the unitary system based on the control zone sensible cooling load and the fraction of the unitary system air flow that goes through the control zone.

If the supply air fan operating mode schedule requests cycling fan operation, the model first checks for the presence of an ecomomizer in the outside air system serving the unitary system’s air loop (Ref. AirLoopHVAC:OutdoorAirSystem). If an outside air system is not present or if an air-side economizer is not used, the unitary system’s compressor is used to meet the unitary system cooling load. If an air-side economizer is used and is active (i.e., economizer controls indicate that conditions are favorable to increase the outside air flow rate), the unitary system will try to meet the cooling load by operating only the supply air fan. If the fan is able to satisfy the unitary system cooling load, the compressor remains off for the entire simulation time step. If the operation of the fan alone is unable to meet the entire cooling load, then the compressor is enabled and additional calculations are performed to determine the compressor’s part-load ratio.

The model then calculates the unitary system’s sensible cooling energy rate delivered to the zones being served when the system runs at full-load conditions and when the cooling coil is OFF. If the supply air fan cycles with the compressor, then the sensible cooling energy rate is zero when the cooling coil is OFF. However if the fan is configured to run continuously regardless of coil operation, then the sensible cooling energy rate will probably not be zero when the cooling coil is OFF. Calculating the sensible cooling energy rate involves modeling the supply air fan (and associated fan heat), the cooling coil, and the heating and reheat coil (simply to pass the air properties and mass flow rate from its inlet node to its outlet node). For each of these cases (full load and cooling coil OFF), the sensible cooling energy rate delivered by the unitary system is calculated as follows:

where:

= air mass flow rate through unitary system at full-load conditions, kg/s

= enthalpy of air exiting the unitary system at full-load conditions, J/kg

= enthalpy of air in the control zone (where thermostat is located), J/kg

HRmin = enthalpies evaluated at a constant humidity ratio, the minimum humidity ratio of the unitary system exiting air or the air in the control zone

= air mass flow rate through the unitary system with the cooling coil OFF, kg/s

= enthalpy of air exiting the unitary system with the cooling coil OFF, J/kg

= Sensible load difference between the system output node and the zone inlet node at full-load conditions

where:

Frac = Control zone air fraction with respect to the system mass flow rate

= Sensible load difference between the system output node and the zone inlet node with the heating coil OFF conditions

With the calculated sensible cooling energy rates and the total sensible cooling load to be met by the system, the part-load ratio for the unitary system is estimated.

Since the part-load performance of the cooling coil is frequently non-linear, and the supply air fan heat varies based on cooling coil operation for the case of cycling fan/cycling coil (AUTO fan), the final part-load ratio for the cooling coil compressor and fan are determined through iterative calculations (successive modeling of the cooling coil and fan) until the unitary system’s cooling output matches the cooling load to be met within the convergence tolerance. The convergence tolerance is fixed at 0.001 and is calculated based on the difference between the load to be met and the unitary system’s cooling output divided by the load to be met.

where:

= Unitary system delivered sensible capacity (W)

If the unitary system has been specified with cycling fan/cycling coil (AUTO fan), then the unitary system’s operating supply air mass flow rate is multiplied by PartLoadRatio to determine the average air mass flow rate for the system simulation time step. In this case, the air conditions at nodes downstream of the cooling coil represent the full-load (steady-state) values when the coil is operating.

If the fan operates continuously (i.e., when the supply air fan operating mode schedule values are NOT equal to 0), the operating air mass flow rate through the unitary system is calculated as the average of the user-specified air flow rate when the cooling coil is ON and the user-specified air flow rate when the cooling coil is OFF (user-specified supply air volumetric flow rates converted to dry air mass flow rates).

where:

= air mass flow rate through unitary system when the cooling coil is ON (kg/s)

= air mass flow rate through unitary system when no cooling or heating is needed (kg/s)

In this case, the air conditions at nodes downstream of the cooling coil are calculated as the average conditions over the simulation time step (i.e., the weighted average of full-load conditions when the coil is operating and inlet air conditions when the coil is OFF).

Cooling Operation (multi or variable speed coils)

After the unitary system cooling load is determined as described in Eq. above, the multi or variable speed cooling coil models calculations are described in this section.

The model calculates the unitary system’s sensible cooling energy rate delivered to the zones being served when the system runs at full-load conditions at the highest speed and when the DX cooling coil is OFF. If the supply air fan cycles with the compressor, then the sensible cooling energy rate is zero when the cooling coil is OFF. However if the fan is scheduled to run continuously regardless of coil operation, then the sensible cooling energy rate will not be zero when the cooling coil is OFF. Calculating the sensible cooling energy rate involves modeling the supply air fan (and associated fan heat) and the multi/variable speed DX cooling coil. The multi/variable speed DX heating coil and the supplemental heating coil are also modeled, but only to pass the air properties and mass flow rate from their inlet nodes to their outlet nodes. For each of these cases (full load at highest cooling speed and DX cooling coil OFF), the sensible cooling energy rate delivered by the unitary system is calculated as follows:

where:

= air mass flow rate through unitary system at the highest cooling speed [kg/s]

= enthalpy of air exiting the unitary system at full-load conditions [J/kg]

= enthalpy of air leaving the control zone (where thermostat is located) [J/kg]

HRmin = the minimum humidity ratio of the unitary system exiting air or the air leaving the control zone [kg/kg]

= air mass flow rate through the unitary system with the cooling coil OFF [kg/s]

= enthalpy of air exiting the unitary system with the cooling coil OFF [J/kg]

= Sensible load difference between the system output node and the zone inlet node at full-load conditions

where:

Frac = Control zone air fraction with respect to the system mass flow rate

= Sensible load difference between the system output node and the zone inlet node with the cooling coil OFF conditions

If the unitary system’s sensible cooling rate at the highest speed (full load, no cycling) is insufficient to meet the entire cooling load, the controlled zone conditions will not be met. The reported cycling rate and speed ratio are 1, and the speed number is set to the highest index number. If the total sensible cooling load to be met by the system is less than the sensible cooling rate at the highest speed, then the following steps are performed.

Calculate the sensible cooling energy rate at Speed 1

where

= air mass flow rate through unitary system at Speed 1 [kg/s]

= Sensible load difference between the system output node and the zone inlet node at full-load conditions at Speed 1

If the sensible cooling energy rate delivered by the unitary system at Speed 1 is greater or equal to the sensible load, the cycling ratio (part-load ratio) for the unitary system is estimated.

where

AddedFanHeat = generated supply air fan heat, which is a function of part load ratio and as internal component cooling load [W].

AddedFanHeatSpeed1 = generated supply air fan heat at Speed 1 (part load ratio=1) [W].

Since the part-load performance of the DX cooling coil is frequently non-linear,and the supply air fan heat varies based on cooling coil operation for the case of cycling fan/cycling coil (AUTO fan), the final part-load ratio for the cooling coil compressor and fan are determined through iterative calculations (successive modeling of the cooling coil and fan) until the unitary system’s cooling output matches the cooling load to be met within the convergence tolerance. The convergence tolerance is fixed at 0.001 and is calculated based on the difference between the load to be met and the unitary system’s cooling output divided by the load to be met.

where:

Unitary systemOutputCycling = unitary system delivered sensible capacity for Speed 1 operating at a specific cycling ratio (W)

where

= average air mass flow rate defined in the next section [kg/s]

hout, = enthalpy of air exiting the unitary system at part load conditions [J/kg]

Δcycling = average sensible load difference between the system output node and the zone inlet node

= Air mass flow rate in the supply inlet node in the controlled zone [kg/s]

For this case where speed 1 operation was able to meet the required cooling load, the speed ratio is set to zero and speed number is equal to 1.

If the unitary system’s cooling output at full load for Speed 1 is insufficient to meet the entire cooling load, the Cycling ratio is set equal to 1.0 (compressor and fan are not cycling). Then the cooling speed is increased and the delivered sensible capacity is calculated. If the full load sensible capacity at Speed n is greater than or equal to the sensible load, the speed ratio for the unitary system is estimated:

Although a linear relationship is assumed by applying the speed ratio to obtain the effective capacity and mass flow rate between speed n and n-1, the outlet air node conditions are dependent on the combined outputs and may not be linear. In addition, the supply air fan heat varies with the speed ratio due to different supply mass flow rates between speed n and n-1 . Therefore, the final speed ratio for the cooling coil compressor and fan are determined through iterative calculations (successive modeling of the cooling coil and fan) until the unitary system’s cooling output matches the cooling load to be met within the convergence tolerance. The convergence tolerance is fixed at 0.001 and is calculated based on the difference between the load to be met and the unitary system’s cooling output divided by the load to be met.

where:

Unitary systemOutputSpeed,n= unitary system delivered sensible capacity between two consecutive speeds at a specific speed ratio (W)

Where

AddedFanHeatSpeedRatio= generated supply air fan heat at a specific speed ratio [W]

In this case, the reported cycling ratio is 1 and speed number is equal to n.

Air Mass Flow Rate Calculation[LINK]

Speed 1 operation

If the unitary system has been specified with cycling fan/cycling coil (AUTO fan), then the unitary system’s operating supply air mass flow rate is determined by the cycling ratio (PartLoadRatio) for Speed 1. The supply air mass flow rate is multiplied by the cycling ratio to determine the average air mass flow rate for the system simulation time step. The air conditions at nodes downstream of the cooling coils represent the full-load (steady-state) values when the coil is operating.

If the fan operates continuously (i.e., when the supply air fan operating mode schedule values are NOT equal to 0), the operating air mass flow rate through the unitary system is calculated as the average of the user-specified air flow rate when the unitary system cooling coil is ON at Speed 1 and the user-specified air flow rate when the unitary system cooling coil is OFF (user-specified supply air volumetric flow rates converted to dry air mass flow rates).

where:

= average air mass flow rate through unitary system [kg/s]

= air mass flow rate through unitary system when cooling coil is ON at Speed 1 [kg/s]

= air mass flow rate through unitary system when no heating or cooling is needed [kg/s]

In this case, the air conditions at nodes downstream of the cooling coils are calculated as the average conditions over the simulation time step (i.e., the weighted average of full-load conditions when the coil is operating and inlet air conditions when the coil is OFF).

Higher Speed Operation[LINK]

When the unitary system operates at higher speeds to meet the required cooling load, the supply air mass flow rate is linearly interpolated between two consecutive speeds:

where:

= average air mass flow rate through the unitary system for the time step [kg/s]

= air mass flow rate through unitary system when cooling coil is ON at Speed n [kg/s]

= air mass flow rate through unitary system when cooling coil is ON at Speed n-1 [kg/s]

For this case of higher speed operation, the air conditions at nodes downstream of the cooling coils are determined by the delivered cooling capacity and supply air mass flow rates between two consecutive speeds.

Although the above sections present the capacity and air mass flow rate calculation separately, they are dependent and change every iteration until convergence is reached for the time step being simulated.

Heating Operation

Calculations for heating operation are similar to those for cooling operation in most respects. However, due to the inclusion of a supplemental heating coil, additional calculations are necessary to properly meet the total heating load for the zones being served.

If EnergyPlus determines that the unitary system must supply heating to the control zone to meet the zone air temperature setpoint, then the unitary system model computes the total sensible heating load to be delivered to the zones being served based on the control zone sensible heating load and the control zone airflow fraction.

The model then calculates the unitary system’s sensible heating energy rate delivered to the zones being served when the system runs at full-load conditions and when the heating coil is OFF (without supplemental heater operation in either case). If the supply air fan cycles with the compressor, then the sensible heating energy rate is zero when the compressor is OFF. However if the fan is scheduled to run continuously regardless of coil operation, then the sensible heating energy rate will not be zero when the compressor is OFF. Calculating the sensible heating energy rate involves modeling the supply air fan (and associated fan heat), the cooling coil (simply to pass the air properties and mass flow rate from its inlet node to its outlet node), the heating coil, and the supplemental heating coil (simply to pass the air properties and mass flow rate from its inlet node to its outlet node). For each of these cases (full load and heating coil OFF, without supplemental heater operation in either case), the sensible heating energy rate delivered by the unitary system is calculated as follows:

where:

= air mass flow rate through unitary system at full-load conditions, kg/s

= enthalpy of air exiting the unitary system at full-load conditions, J/kg

= enthalpy of air leaving the control zone (where thermostat is located), J/kg

HRmin = enthalpies evaluated at a constant humidity ratio, the minimum humidity ratio of the unitary system exiting air or the air leaving the control zone

= air mass flow rate through the unitary system with the heating coil OFF, kg/s

= enthalpy of air exiting the unitary system with the heating coil OFF, J/kg

= Sensible load difference between the system output node and the zone inlet node at full-load conditions

where:

Frac = Control zone air fraction with respect to the system mass flow rate

= Sensible load difference between the system output node and the zone inlet node with the heating coil OFF conditions

With the calculated sensible heating energy rates and the total sensible heating load to be met by the system, the part-load ratio for the unitary system is estimated.

Since the part-load performance of the heating coil is frequently non-linear, and the supply air fan heat varies based on heating coil operation for the case of cycling fan/cycling coil (AUTO fan), the final part-load ratio for the heating coil compressor and fan are determined through iterative calculations (successive modeling of the heating coil and fan) until the unitary system’s heating output matches the heating load to be met within the convergence tolerance. The convergence tolerance is fixed at 0.001 and is calculated based on the difference between the load to be met and the unitary system’s heating output divided by the load to be met.

where:

= Unitary system delivered sensible capacity (W)

If the unitary system’s heating coil output at full load is insufficient to meet the entire heating load, PartLoadRatio is set equal to 1.0 (compressor and fan are not cycling) and the remaining heating load is passed to the supplemental heating coil. If the unitary system model determines that the outdoor air temperature is below the minimum outdoor air temperature for compressor operation, the compressor is turned off and the entire heating load is passed to the supplemental gas or electric heating coil. The unitary system exiting air conditions and energy consumption are calculated and reported by the individual component models (fan, heating coil, and supplemental gas or electric heating coil).

If the unitary system has been specified with cycling fan/cycling coil (AUTO fan), then the unitary system’s operating supply air mass flow rate is multiplied by PartLoadRatio to determine the average air mass flow rate for the system simulation time step. The air conditions at nodes downstream of the heating coils represent the full-load (steady-state) values when the coils are operating. If the fan operates continuously (i.e., when the supply air fan operating mode schedule values are NOT equal to 0), the operating air mass flow rate through the unitary system is calculated as the average of the user-specified air flow rate when the unitary system heating coil is ON and the user-specified air flow rate when the unitary system heating coil is OFF (user-specified supply air volumetric flow rates converted to dry air mass flow rates).

where:

= air mass flow rate through unitary system when the heating coil is ON (kg/s)

= air mass flow rate through unitary system when no heating or cooling is needed (kg/s)

In this case, the air conditions at nodes downstream of the heating coils are calculated as the average conditions over the simulation time step (i.e., the weighted average of full-load conditions when the coils are operating and inlet air conditions when the coils are OFF).

Heating Operation (multi or variable speed coils )[LINK]

After the unitary system heating load is determined as described in Eq. above, the multi or variable speed heating coil models calculation are described in this section.

The model calculates the unitary system’s sensible heating energy rate delivered to the zones being served when the system runs at full-load conditions at the highest speed and when the DX heating coil is OFF (without supplemental heater operation in either case). If the supply air fan cycles with the compressor, then the sensible heating energy rate is zero when the compressor is OFF. However if the fan is scheduled to run continuously regardless of coil operation, then the sensible heating energy rate will not be zero when the compressor is OFF. Calculating the sensible heating energy rate involves modeling the supply air fan (and associated fan heat), the DX cooling coil (simply to pass the air properties and mass flow rate from its inlet node to its outlet node), the DX heating coil, and the supplemental heating coil (simply to pass the air properties and mass flow rate from its inlet node to its outlet node). For each of these cases (full load and DX heating coil OFF, without supplemental heater operation in either case), the sensible heating energy rate delivered by the unitary system is calculated as follows:

where:

= air mass flow rate through unitary system at the highest heating speed [kg/s]

= enthalpy of air exiting the unitary system at full-load conditions [J/kg]

= enthalpy of air leaving the control zone (where thermostat is located) [J/kg]

HRmin = enthalpies evaluated at a constant humidity ratio, the minimum humidity ratio of the unitary system exiting air or the air leaving the control zone

= air mass flow rate through the unitary system with the heating coil OFF [kg/s]

= enthalpy of air exiting the unitary system with the heating coil OFF [J/kg]

= Sensible load difference between the system output node and the zone inlet node at full-load conditions

where:

Frac = Control zone air fraction with respect to the system mass flow rate

= Sensible load difference between the system output node and the zone inlet node with the heating coil OFF conditions

If the unitary system’s DX heating coil output full load at the highest speed is insufficient to meet the entire heating load, the remaining heating load is passed to the supplemental heating coil. If the unitary system model determines that the outdoor air temperature is below the minimum outdoor air temperature for compressor operation (specified by the user), the compressor is turned off and the entire heating load is passed to the supplemental gas or electric heating coil. The unitary system exiting air conditions and energy consumption are calculated and reported by the individual component models (fan, DX heating coil, and supplemental gas or electric heating coil).

If the total heating load to be met by the system is less than the sensible heating rate at the highest speed, then the following steps are performed.

Calculate the sensible heating energy rate at Speed 1

where:

= air mass flow rate through unitary system at Speed 1 [kg/s]

= Sensible load difference between the system output node and the zone inlet node at full-load conditions at Speed 1

If the sensible heating energy rate delivered by the unitary system at Speed 1 is greater or equal to the sensible load, the cycling ratio (part-load ratio) for the unitary system is estimated.

where

AddedFanHeat= generated supply air fan heat, which is a function of part load ratio and as internal component heating load [W].

AddedFanHeatSpeed1= generated supply air fan heat at Speed 1 (part load ratio=1) [W].

Since the part-load performance of the DX heating coil is frequently non-linear, and the supply air fan heat varies based on heating coil operation for the case of cycling fan/cycling coil (AUTO fan), the final part-load ratio for the heating coil compressor and fan are determined through iterative calculations (successive modeling of the heating coil and fan) until the unitary system’s heating output matches the heating load to be met within the convergence tolerance. The convergence tolerance is fixed at 0.001 and is calculated based on the difference between the load to be met and the unitary system’s heating output divided by the load to be met.

where:

Unitary systemOutputCycling= unitary system delivered sensible capacity for Speed 1 operating at a specific cycling ratio (W)

where

= average air mass flow rate defined in the next section [kg/s]

hout, = enthalpy of air exiting the unitary system at part load conditions [J/kg]

Δcycling = average sensible load difference between the system output node and the zone inlet node

= Air mass flow rate in the supply inlet node in the controlled zone [kg/s]

For this case where speed 1 operation was able to meet the required heating load, the speed ratio is set to zero and speed number is equal to 1.

If the unitary system’s heating output at full load for Speed 1 is insufficient to meet the entire heatling load, the Cycling ratio (PartLoadRatio) is set equal to 1.0 (compressor and fan are not cycling). Then the heating speed is increased and the delivered sensible capacity is calculated. If the full load sensible capacity at Speed n is greater than or equal to the sensible load, the speed ratio for the unitary system is estimated:

Although a linear relationship is assumed by applying the speed ratio to obtain the effective capacity and air mass flow rate between speed n and n-1, the outlet node conditions are dependent on the combined outputs and may not be linear. In addition, the supply air fan heat varies based on heating coil operation for the case of cycling fan/cycling coil (AUTO fan). Therefore, the final speed ratio for the heating coil compressor and fan are determined through iterative calculations (successive modeling of the heating coil and fan) until the unitary system’s heating output matches the heating load to be met within the convergence tolerance. The convergence tolerance is fixed at 0.001 and is calculated based on the difference between the load to be met and the unitary system’s heating output divided by the load to be met.

where:

UnitarySystemOutputSpeedRatio= unitary system delivered sensible capacity between two consecutive speeds at a specific ratio [W]

Where

AddedFanHeatSpeedRatio= generated supply air fan heat at a specific speed ratio [W]

In this case, the reported cycling ratio is 1 and speed number is equal to n.

Air Mass Flow Rate Calculation[LINK]

The air mass flow rate calculations during heating operation are the same as those described above for cooling operation for multi/variable speed.

High Humidity Control[LINK]

The specific configuration of the unitary system with supplemental heating coil is shown above (see Figure 227). This figure shows the fan placement when a blow through fan is specified. If a draw through fan is specified, the fan is located between the heating coil and the supplemental heating coil. The system is controlled to keep the high relative humidity in the control zone from exceeding the setpoint specified in the object ZoneControl:Humidistat. This option is available when the supply air fan operates continuously (i.e., the supply air fan operating mode schedule values are never equal to 0) or the supply air fan cycles with the compressor. In addition, when high humidity control is specified and the compressor operates, the unitary system operates at the cooling air flow rate when a zone heating load is present as determined by the zone thermostat. High humidity control is specified as either None, MultiMode, or CoolReheat in the Dehumidification Control Type input field. MultiMode is specified when a heat exchanger is used to improve the dehumidification performance of the cooling coil. The heat exchanger will be activated when the sensible part-load ratio is insufficient to meet the zone latent load. CoolReheat is specified when a cooling coil is used to over-cool the supply air stream in order to meet the zone latent load. In this case, a supplemental heating coil will ensure the zone temperature does not fall below the zone heating temperature set point. When a heat exchanger is used in conjunction with a cooling coil and CoolReheat is specified as the Dehumidification Control Type, the heat exchanger is “locked on” to meet either the sensible or latent cooling load. If the dehumidification control type is selected as None and a heat exchanger assisted cooling coil is used, the heat exchanger is “locked on” and the air conditioner runs only to meet the sensible cooling load. A supplemental heating coil is required for all dehumidification control types.

The model first calculates the PartLoadRatio required to meet the sensible cooling load. The unitary system’s sensible cooling load is determined from the control zone sensible cooling load to the cooling setpoint and the control zone air flow fraction to maintain the dry-bulb temperature setpoint in the control zone.:

The unitary system’s sensible cooling load to be met and the full load cooling output are used to calculate the sensible the part-load ratio iteratively based on user specified convergence criterion.

hen the unitary system’s sensible cooling capacity meets the system sensible cooling load at a given sensible part load ratio, then the Unitary system meets the controlled zone cooling setpoint temperature. If a moisture (latent) load exists because the control zone humidity has exceeded the setpoint, the total moisture load to be met by the unitary systems (Unitary systemMoistureLoad) is calculated based on the control zone moisture load and the control zone air flow fraction.

Then the LatentPartLoadRatio required to meet the high humidity setpoint is calculated as follows:

The model uses the greater of the two part-load ratios, PartLoadRatio or LatentPartLoadRatio, to determine the operating part-load ratio of the Unitary system’s DX cooling coil.

As previously described, iterations are performed to converge on the solution within the convergence tolerance.

Where,

= the control zone sensible cooling load to the cooling setpoint, (W).

= the control zone moisture load to the dehumidifying relative humidity setpoint, (W).

= the supply air fraction that goes though the control zone, (-).

= the Unitary system’s latent cooling energy rate at full-load conditions, W

= the Unitary system’s latent cooling energy rate with cooling coil OFF, W

= the unitary system’s part-load-ratio required to meet system sensible load, (-).

= the unitary system’s part-load-ratio required to meet system moisture load, (-).

= the minimum part-load ratio, which is usually 0.0. For the case when the latent capacity degradation model is used (Ref: DX Cooling Coil Model), this value is the minimum part-load ratio at which the cooling coil will dehumidify the air.

When the predicted zone air temperature is above the heating setpoint and if there is a dehumidification load, the supplemental heating coil load is required to offset the excess cooling as shown in Figure 228. If the model determines that the LatentPartLoadRatio is to be used as the operating part-load ratio of the unitary system’s cooling coil, the supplemental heating coil is used to offset the excess sensible capacity provided by the unitary system cooling coil. The model first checks the sensible load that exists for the current simulation time step (predicted zone temperature with no HVAC operation compared to the thermostat setpoint temperatures). If a sensible cooling load or no sensible cooling or heating load exists, the model calculates the difference between the sensible heating load required to reach or maintain the heating dry-bulb temperature setpoint and the actual sensible cooling energy rate delivered by the unit (with LatentPartLoadRatio). In this case, the supplemental heating coil is used to offset the excess sensible cooling energy provided by the cooling coil (if any) that could have caused an overshoot of the heating dry-bulb temperature setpoint. Note that when a humidistat is used and high humidity control is required, the zone dry-bulb temperature will typically move toward the heating temperature setpoint when a high moisture (latent) load exists.

Supplemental heating coil load when predicted zone air temperature is above the heating Setpoint

Supplemental heating coil load when predicted zone air temperature is above the heating Setpoint

If a heating load exists (Figure 229), the supplemental heating coil is used to meet the heating coil load and at the same time offset the entire sensible cooling energy rate of the cooling coil (to meet the humidistat setpoint). Note that when a heating load exists and high humidity control is required, the unitary system operates at the user-specified cooling air flow rate for the entire simulation time step. As with the fan, and cooling coil, report variables associated with supplemental heating coil performance (e.g., heating coil energy, heating coil rate, heating coil gas or electric energy, heating coil runtime fraction, etc.) are managed in the supplemental (heating) coil object.

Supplemental heating coil load when predicted zone air temperature is below the heating setpoint

Supplemental heating coil load when predicted zone air temperature is below the heating setpoint

Waste Heat Calculation[LINK]

Waste heat calculations are done when the multi speed cooling and heating coils are specified in the unitary system and the heat recovery is active (the value of the Design Heat Recovery Water Flow Rate field is greater than 0), the outlet node temperature of heat recovery is calculated based on the recoverable waste heat generated by the child objects Coil:Cooling:DX:MultiSpeed and Coil:Heating:DX:MultiSpeed:

where

Toutlet = outlet node temperature of heat recovery, C

Tinlet = inlet node temperature of heat recovery, C

QWasteHeat = recoverable waste heat generated by its child objects, W

Cp = inlet node temperature of heat recovery, C