Stencil: Zone HVAC Stencil: Grp-VAV Series PIU
Type: Air Terminal
Sub Type: ParallelPIU_Reheat
Note: Requires a connection to a hot water loop
EnergyPlus Object - AirTerminal:SingleDuct:Parallel
The parallel powered induction unit is an air system terminal unit that mixes varying amounts of secondary (recirculated) air and primary (conditioned supply) air to produce a variable total flow of air to a zone. The unit contains a small fan that acts to induce the secondary air and a heating coil for heating the mixed secondary and primary air. The secondary and primary air streams enter the unit in parallel. The fan sits in the secondary air stream and runs only when the primary air flow is below the Fan On Flow Fraction and the fan’s availability schedule is on or it is activated by an availability manager. The primary air inlet contains a damper that can move from fully open (maximum primary air) to a minimum stop (minimum primary air).
At full cooling load the primary air damper is fully open and the fan is off. The primary air flow is at maximum and there is little or no secondary air flow. As the cooling load decreases, the primary air damper gradually closes and the secondary air flow remains close to zero. At some point, usually when the primary air flow has reached the minimum, the fan switches on and secondary air is induced. The heating coil will switch on as needed to meet any heating demand.
The EnergyPlus model of the parallel PIU terminal unit is composed of three components: a constant volume fan, a zone mixer, and a heating coil (hot water, electric, or gas).
The table shows the properties that are displayed when the component is selected while in diagram mode. The second column shows the selection options available that are dictated by EnergyPlus or it shows the source for the library entries that are displayed in the drop down list.
| Property Name | Value Sources/Options |
|
Name |
Simergy provides unique name (editable) |
|
Availability Schedule Name |
|
|
Maximum Primary Air Flow Rate |
value, Autosize |
|
Maximum Secondary Air Flow Rate |
value, Autosize |
|
Minimum Primary Air Flow Fraction |
value, Autosize |
|
Fan On Flow Fraction |
0, Autosize |
|
Fan Name |
Libraries/Plant and Equip/Fans & Pumps/Supply Fan |
|
Reheat Coil Name |
|
|
Maximum Hot Water or Steam Flow Rate |
0, Autosize |
|
Minimum Hot Water or Steam Flow Rate |
0-Min; 0 default |
|
Convergence Tolerance |
0-Min; 0.001 Default |
Simergy automatically defines a unique name for each component. This can be changed by the user if desired.
The name of the schedule (ref: Schedule) that denotes whether the unit can run during a given hour. A schedule value greater than 0 (usually 1 is used) indicates that the unit can be on during the hour. A value less than or equal to 0 (usually 0 is used) denotes that the unit must be off for the hour.
The maximum volumetric air flow rate of primary air through the unit in cubic meters per second. This is the primary air flow rate at full cooling load when the primary air damper is fully open.
The maximum volumetric air flow rate of secondary air through the unit in cubic meters per second. This flow rate can be any amount but is commonly less than the maximum primary air flow rate.
The minimum volumetric air flow rate of primary air through the unit expressed as a fraction of the maximum volumetric air flow rate of primary air. This input can be 0.0.
The fraction of the primary air flow at which fan turns on. In the parallel PIU the fan operation is intermittent. If the primary air flow is above this fraction of the maximum, the fan is off. Otherwise the secondary fan will run according to the Availability Schedule specified in the Fan:ConstantVolume object (see Fan Name below), unless it is overridden by an availability manager (ref. AvailabilityManager:NightCycle and others).
The name of an zone mixer component (object: AirLoopHVAC:ZoneMixer) which composes part of the unit. Note that some of the input for the mixer will duplicate input fields of the powered induction unit. One of the zone mixer inlet nodes should be the same as the supply air inlet node of the PIU; the other inlet node of the zone mixer should be the same as the air outlet node of the fan. The outlet node of the zone mixer should be the same as the inlet node of the heating coil.
The name of a fan component which composes part of the unit. Note that the fan’s maximum flow rate should be the same as the maximum secondary air flow rate of the PIU and the type of fan object must be Fan:ConstantVolume. The fan’s inlet node should be the same as the PIU secondary air inlet node. The fan’s outlet node should be the same as one of the zone mixer’s inlet nodes. When the primary air flow is below the Fan On Flow Fraction (above), the secondary fan will run according to the Availability Schedule specified in the Fan:ConstantVolume object, unless it is overridden by an availability manager (ref. AvailabilityManager:NightCycle and others).
The name of the heating coil component which composes part of the unit. Note that the heating coil’s air inlet node is the same as the fan outlet node and the heating coil’s air outlet node is the same as the PIU outlet node.
The maximum hot water volumetric flow rate in m3/sec through the unit’s heating coil. If the heating coil is gas or electric this field should be blank.
The minimum hot water volumetric flow rate in m3/sec through the unit’s heating coil. If the heating coil is gas or electric this field should be blank.
The name of the HVAC system node which regulates the flow of hot water through the unit. This should be the same node as the water inlet node of the unit’s hot water coil. For gas or electric heating coils, this input should be blank.
The control tolerance for the unit heating output. The unit is controlled by matching the unit output to the zone demand. For units with water coils, the model must be numerically inverted to obtain a specified output. The convergence tolerance is the error tolerance used to terminate the numerical inversion procedure. Basically this is the fraction:
For gas or electric heating coils, this input should be left blank. The default is 0.001.
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