Zone Design Loads and Air Flow Rates[LINK]

Overview[LINK]

There is no single best way to establish design HVAC flow rates and size HVAC equipment. Different building designs, climates, and HVAC systems will impose varying constraints on the designer. The method used to size an HVAC system in a hot, moist climate such as Miami will be different than the method used for a building in Albuquerque. The type of building is also relevant - a simple watts per square foot loads estimate could be adequate for a building containing a network server farm while a detailed, dynamic loads simulation would be necessary for a passive solar building. In the end the designer’s experience and engineering judgement will play an important role in any sizing calculation.

HVAC equipment sizing begins with the calculation of space heating and cooling sensible and/or latent loads. A space cooling (heating) sensible load is defined as the rate at which heat must be removed (added) to a space to maintain a constant zone temperature. A space cooling (heating) latent load is defined as the rate at which moisture must be removed (added) to a space to maintain a constant zone moisture/humidity ratio/relative humidity level. The current industry standard method for calculating space loads is the heat balance method [ASHRAE Fundamentals (2001), page 29.1; Pedersen et al., (1997); Pedersen (2001). Since EnergyPlus is a heat balance based simulation program it is straightforward for the program to use this method for calculating zone loads.

Zone Design Data Arrays[LINK]

The zone design data arrays are:

ZoneSizingInput(i) stores the input data from the Sizing:Zone objects.

CalcZoneSizing(i,j) stores the results of the zone design calculations for all zones and all design days. The index i is for the controlled zones, j for design days.

CalcFinalZoneSizing(i) stores the results of the zone design calculations for the peak heating and cooling cases for each zone. The index i is for the controlled zones.

ZoneSizing(i,j) corresponds to CalcZoneSizing but includes the effect of the user specified sizing factor or user specified zone design flow rate.

FinalZoneSizing(i) corresponds to CalcFinalZoneSizing but includes the effect of the user specified sizing factor or user specified zone design flow rate.

TermUnitFinalZoneSizing(k) corresponds to FinalZoneSizing but includes the effect of any air system terminal unit sizing factors input with DesignSpecification:AirTerminal:Sizing objects. The index k is for each air terminal unit. There may be more or less terminal units than there are controlled zones.

The data stored in CalcZoneSizing, CalcFinalZoneSizing, ZoneSizing, FinalZoneSizing and TermUnitFinalZoneSizing includes the following data items.

Zone Design Load Calculation[LINK]

As described in the preceding section, the Sizing Manager initiates the zone design calculation by looping over all of the design days and calling the Heat Balance Manager for each zone time-step in each design day. The Heat Balance manager then causes the HVAC Manager to be called in a manner identical to a normal simulation. The ZoneSizingCalc set to true signals the HVAC Manager to ignore the actual HVAC system and instead calculate the design sensible and latent zone loads and air flow rates using an ideal zonal system.

In module HVACManager, subroutine ManageHVAC calls SimHVAC. SimHVAC checks ZoneSizingCalc. If it is true, SimHVAC calls ManageZoneEquipment and returns, rather than simulating the actual system. In turn ManageZoneEquipment checks if ZoneSizingCalc is true; if it is it calls SizeZoneEquipment rather than SimZoneEquipment.

SizeZoneEquipment assumes that each controlled zone is served by an ideal air conditioning unit. This unit supplies sensible and latent (if latent sizing is requeted) heating or cooling air at a fixed, user input temperature and humidity (specified in the Sizing:Zone objects). The units have infinite capacity: the flow rate can be any amount. Zone humidity is controlled at a constant level when latent sizing is requested, otherwise zone humidity is allowed to float. The sensible and latent loads and air mass flow rates are saved and reported to eplusout.zsz.

Before the ideal zone load is calculated, the function checks whether the user wants to account for the heat gain or loss caused by the ventilation air from a Dedicated Outdoor Air System (DOAS). If the user has selected Account For Dedicated Outdoor Air = Yes the function performs an ideal DOAS calculation. The DOAS supply temperature is set according to the user’s choice of 1 of 3 possible control strategies: NeutralSupplyAir, NeutralDehumidifiedSupplyAir, or ColdSupplyAir. This value is saved and reported to eplusout.zsz. The different strategies are:

• DOAS Control Strategy = NeutralSupplyAir. The purpose of this strategy is to cool or heat the outdoor air (OA) to keep it between the T\(_{l}\) and T\(_{h}\) setpoints.

• DOAS Control Strategy = Neutral Dehumidified Supply Air. The purpose of this strategy is to cool and dehumidify the outdoor air, then reheat it to a “neutral” temperature so that no sensible load is imposed on the space or AHU unit. The DOAS will with this strategy handle some or all of the latent load. If the outdoor air temperature is greater than T\(_{l}\) the outdoor air is cooled to T\(_{l}\) and reheated to T\(_{h}\). If the outdoor air temperaure is below T\(_{l}\) it is heated to T\(_{h}\).

• DOAS Control Strategy = ColdSupplyAir. The purpose of this strategy is to provide cool, dehumidified ventilation air to the zone. In this case the DOAS can handle part of the sensible zone cooling load as well as meet part or all of the latent load. If the outdoor air temperature is below T\(_{l}\) it is heated to T\(_{h}\). If it is above T\(_{l}\), it is cooled to T\(_{l}\).

With the DOAS supply temperature set and the air mass flow rate set to the minimum design ventilation flow rate the heat addition rate is just

\[{\dot Q_{doa}} = {c_{p,air}}{\dot m_{vent,min}}({T_{sup}} - {T_z})\]

UpdateSystemOutputRequired is then invoked to adjust the load to be met by the ideal zone system.

When latent sizing is requested sensible and latent loads provided by the DOAS are updated. Latent load units are kgw per second.

\[{\dot Q_{doa, moisture}} = {\dot m_{vent,min}}({w_{sup}} - {w_z})\]

The ideal loads calculation steps are as follows.

1. Loop over all the controlled zones.

2. If the system is active (zone temperature not in the deadband and zone load greater than 1 watt) the sign of the zone load is used to determine whether heating or cooling is required and T\(_{in}\) and W\(_{in}\) are set to the appropriate values from the Sizing:Zone input. When the SupplyTemperature method is specified in the Sizing:Zone object, T\(_{in}\) is fixed at the cooling or heating supply temperature. When the TemperatureDifference method is selected, T\(_{in}\) is calculated at each time step using the current zone air temperature. The system output Q\(_{sys}\) is simply set equal to the zone demand - it is assumed that the ideal system can always meet the zone load. The air flow rate corresponding to the load is just

\[{\dot m_{sys}} = {Q_{sys}}/({C_{p,air}} \cdot ({T_{in}} - {T_z}))\]

Similar calculations are performed when latent sizing is requested where a zone latent mass flow rate is calculated.

\[{\dot m_{sys, latent}} = {Q_{sys, latent}}/({w_{in}} - {w_z})\]

If the system is not active, the mass flow rate is set to zero and the system output is left at zero.

1. The results for each zone are stored in the zone sizing data arrays. See comma separated variable file named eplusout.zsz.

Updating and Adjusting the Zone Results[LINK]

The results from SizeZoneEquipment are at the system time-step and are for all design days. These results then need to be summed or averaged over the zone time-step, peak values calculated for each design day, a sensible and latent (when latent sizing is requested) heating & a cooling load sequence chosen for each zone from all the design day results, possible further smoothing of results done, zone coil loads calculated, and user sizing multipliers or user specified design flows taken into account. These tasks are accomplished by the subroutine UpdateZoneSizing. 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 = EndZoneSizingCalc).

BeginDay[LINK]

The environment (in this case, a design day) name and number are stored in the zone sizing data structures

DuringDay[LINK]

The calculated and stored sequences are summed or averaged over the zone time-step.

EndDay[LINK]

1. Smooth the design sequences by applying a moving, fixed-width averaging window to the sequences. The width of the window is user specified in the Sizing:Parameters input object. The sequences that are smoothed are:

1. The peak sensible and latent (when latent sizing is requested) heating and cooling loads and mass & volume flow rates are extracted from each set of design sequences.

2. Using the time of the peak and the design outside air fraction the design zone heating and cooling coil inlet temperatures and humidity ratios are calculated.

3. For each zone, looking at the results for all of the design days, the design days that cause the peak heating and peak cooling for that zone are chosen and the corresponding design sequences and peak loads and flow rates are saved in the CalcFinalZoneSizing array. This finishes the calculated - unmodified by the user - portion of the zone design calculation.

When latent sizing is requested the zone latent loads are either used (when latent only sizing is requested) or compared to the zone sensible loads. If the zone latent load is greater than the zone sensible load the latent zone sizing information is used for the simulation.

EndZoneSizingCalc[LINK]

1. Write out onto a comma-separated file the calculated design sequences for each zone: HeatLoadSeq, CoolLoadSeq, HeatFlowSeq, CoolFlowSeq, LatentHeatLoadSeq, LatentCoolLoadSeq, LatentHeatFlowSeq, LatentCoolFlowSeq, HeatLoadNoDOASSeq, CoolLoadNoDOASSeq, HeatLatentLoadNoDOASSeq, CoolLatentLoadSeq, HeatZoneTempSeq, HeatZoneRHSeq, CoolZoneTempSeq, CoolZoneRHSeq and the corresponding peaks and volumetric flow peaks.

2. The data in CalcZoneSizing and CalcFinalZoneSizing is moved to ZoneSizing and FinalZoneSizing. The user modifications to the calculated sizing will be applied to and stored in ZoneSizing and FinalZoneSizing.

3. The user can modify the calculated zone design results by specifying heating and cooling sizing factors at the global or zone level or by specifying and actual design heating or cooling zone design volumetric flow rate. All of this input is treated as a sizing factor. If the user inputs a cooling design volumetric flow rate for a zone it is divided by the calculated cooling design volumetric flow rate for the zone to give a zone cooling sizing factor. Note that the user can input a zone sizing factor or a zone design flow rate - not both - so there is never a conflict.

4. Once the zone heating and cooling sizing factors are established, the design flow and load sequences as well as peak loads and flows are multiplied by the appropriate sizing factor and stored in ZoneSizing and FinalZoneSizing. This is the data that will be used for sizing zone HVAC equipment and in the system sizing calculation.

5. The outside air fractions are recalculated using the new user-modified design flow rates and new design zone coil inlet conditions calculated and stored. At this point the condition that the design flow rates are never allowed to be less than the minimum outside air flow rate is imposed.

If outside air method is flow/zone, the input outside air flow per zone value will be used, even if it is zero or blank. If outside air method is sum, the sum of the outside air flow per person * DesignNumberOfPeople + outside air flow per area * ZoneArea will be used. If outside air method is maximum, the maximum of the outside air flow per person * DesignNumberOfPeople and outside air flow per area * ZoneArea will be used. If outside air method is flow/person, outside air flow per person will be used to calculate the design minimum outside airflow rate.

If cooling design air flow method is flow/zone, then cooling design air flow rate will be used for the design max cooling air flow rate. If cooling design air flow method is design day, then the design day calculation will set the design max cooling air flow rate. If cooling design air flow method is design day with limit, then the maximum from cooling min flow per area and cooling min flow will set a lower limit on the design max cooling air flow rate. In all cases, the maximum from cooling min flow per area, cooling min flow, and cooling min flow fraction will set a minimum zone cooling air flow rate. In all cases the maximum design cooling air flow rate must be > = to the ventilation requirement.

If heating design air flow method is flow/zone, then heating design air flow rate will be used for the design max heating air flow rate. If heating design air flow method is design day, then the design day calculation will set the design max heating air flow rate. If heating design air flow method is design day with limit, then the maximum from heating max flow per area, heating max flow and heating max flow fraction will set an upper limit on the design max heating air flow rate. The design max heating air flow rate must always be > = the ventilation requirement. In each case, the outside airflow will be modified based on zone ventilation effectiveness specified in the zone sizing object.

This concludes the calculation of the zone design flow rates and loads.

Air Terminal Unit Sizing[LINK]

Each controlled zone may have one or more air terminal units (AirTerminal:*) served by one or more central air handlers (AirLoopHVAC). Each air terminal unit may reference a DesignSpecification:AirTerminal:Sizing object which is used to adjust the FinalZoneSizing data for each terminal unit with these user inputs:

\(DesSensCoolingFrac\)	= Fraction of Design Sensible Cooling Load
\(DesCoolSATRatio\)	= Cooling Design Supply Air Temperature Difference Ratio
\(DesSensHeatingFrac\)	= Fraction of Design Sensible Heating Load
\(DesHeatSATRatio\)	= Heating Design Supply Air Temperature Difference Ratio
\(MinOAFrac\)	= Fraction of Minimum Outdoor Air Flow

All of these factors default to 1.0. Function UpdateTermUnitFinalZoneSizing first fills each instance of TermUnitFinalZoneSizing with a copy of FinaZoneSizing for the controlled zone served by the terminal unit. Then, if there is an applicable DesignSpecification:AirTerminal:Sizing input, the following adjustments are made:

\[{CoolLoad_{TU}} = {CoolLoad_{FZ}} \cdot {DesSensCoolingFrac}\]

\[{minOAFlow_{TU}} = {minOAFlow_{FZ}} \cdot {MinOAFrac}\]

\[{CoolFlowRatio} = {DesSensCoolingFrac} / {DesCoolSATRatio}\]

\[{CoolFlow_{TU}} = {CoolFlowNoOA_{FZ}} \cdot CoolFlowRatio + {MinOAFrac} \cdot ( {CoolFlowWithOA_{FZ}} - {CoolFlowNoOA_{FZ}} )\]

\[{CoolFlow_{TU}} = max({CoolFlow_{TU}},{minOAFlow_{TU}})\]

Where:

\(CoolLoad\)	= Cooling load variables: \(DesCoolLoad, CoolLoadSeq, NonAirSysDesCoolLoad\)
\(CoolFlow\)	= Cooling flow variables: \(DesCoolVolFlow, CoolFlowSeq\)
\(CoolFlowNoOA\)	= Cooling flow variables not adjusted for minimum outdoor air: \(DesCoolVolFlowNoOA, CoolFlowSeqNoOA\)
\(minOAFlow\)	= minimum outdoor air flow rate
Subscript \(FZ\)	= Value from \(FinalZoneSizing\)
Subscript \(TU\)	= Value for \(TermUnitFinalZoneSizing\)

Likewise for heating.

Zone HVAC Scalable Sizing[LINK]

For zone HVAC equipment, scalable sizing applies to supply air flow rate and capacity for both cooling and heating. The scalable sizing method allowed for supply air flow rates include: FractionOfAutosizedCoolingAirflow, sizing method allowed for supply air flow rates include: FractionOfAutosizedCoolingAirflow, FractionOfAutosizedHeatingAirflow, FlowPerFloorArea, FlowPerCoolingCapacity, and FlowPerHeatingCapacity. The supply air flow rate scalable sizing methods are defined as follows:

FlowPerFloorArea: the simulation engine determine the supply air flow rates from the user specified supply air flow rates per unit floor area and the zone floor area of the zone served by the zone HVAC equipment.

FractionOfAutosizedCoolingAirflow: the simulation engine determines the supply air flow rates from the user specified flow fraction and autosized cooling design supply air flow rate.

FractionOfAutosizedHeatingAirflow: the simulation engine determines the supply air flow rates from the user specified flow fraction and autosized heating design supply air flow rate.

FlowPerCoolingCapacity: he simulation engine determines the supply air flow rates from the user specified supply air flow per cooling capacity value and autosized cooling design capacity.

FlowPerHeatingCapacity: the simulation engine determines the supply air flow rates from the user specified supply air flow per heating capacity value and autosized heating design capacity.

The scalable capacity sizing may be indirectly impacted by the scalable supply air flow rates sizing values. Moreover, the autosized cold water, hot water and steam flow rates in the parent zone HVAC objects and capacity in child components are determined using the scalable sizing method. Scalable capacity sizing methods allowed for cooling and heating include: CapacityPerFloorArea, FractionOfAutosizedCoolingCapacity, FractionOfAutosizedHeatingCapacity. The scalable sizing capacity methods are defined as follows:

CapacityPerFloorArea: the simulation engine determines the cooling or heating capacity from user specified capacity per floor area value and the floor area of the zone served by the zone HVAC equipment.

FractionOfAutosizedCoolingCapacity: the simulation engine sizes the cooling capacity from the user specified capacity fraction and autosized cooling design capacity value.

FractionOfAutosizedHeatingCapacity: the simulation engine sizes the heating capacity from the user specified capacity fraction and autosized heating design capacity value.