1. Lecture 22: Primary System Loops in EnergyPlus
Notes: __________________________________________________________________
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Material prepared by GARD Analytics, Inc. and University of Illinois
at Urbana-Champaign under contract to the National Renewable Energy
Laboratory. All material Copyright U.S.D.O.E. - All rights reserved

2. Purpose of this Lecture
Gain an understanding of:
Water (fluid) side loop structures in EnergyPlus
How to control fluid flow rates using the EnergyPlus pump strategies

3. Defining Loops
Loops define the movement of mass and energy within an HVAC system
Plant loop
Condenser loop
Heat recovery loop
Systems defined by the order of branches/components on the HVAC loop

4. Defining Loops (cont’d)
Main loops divided into “sub-loops” or “semi-loops” for organizational clarity & simulation logistics
Sub-loops are matched pairs that consist of half of the main loop.
For example, plant and condenser loops are broken into supply and demand sides.

5. Demand- & Supply-Side Sub-Loops
Plant demand-side sub-loop contains equipment that creates a load on the plant.
Plant supply-side sub-loop contains equipment that meets these loads.
Each supply-side sub-loop must be connected to a demand-side loop.  

6. Demand- & Supply-Side Loops (cont’d)
Heating Coil
HW Pump Outlet Node
HW Pump
HW Demand
Outlet Node
Boiler Inlet Node
HW Supply inlet Node
Heating Coil
Inlet Node
Exit Pipe
Boiler
Heating Coil Outlet Node
Inlet Pipe
HW Supply Outlet Node
HW Demand Inlet Node
Boiler Outlet Node
HW Supply Outlet Pipe
Demand-Side Loop
Supply-Side Loop

7. Branches for Water-Side Loops
Maximum Branch Flow Rate (Ignored)
Comp1 Type
Comp1 Name
Comp1 Inlet Node Name
Comp1 Outlet Node Name
Comp1 Branch Control Type

8. Branches for Water-Side Loops - Example
Heating Supply Inlet Branch, !- Branch Name
, !- Maximum Branch Flow Rate {m3/s}
PUMP:VARIABLE SPEED, !- Comp1 Type
HW Circ Pump, !- Comp1 Name
HW Supply Inlet Node, !- Comp1 Inlet Node Name
HW Pump Outlet Node, !- Comp1 Outlet Node Name
ACTIVE; !- Comp1 Branch Control Type

9. Layout for Individual HVAC Supply or Demand Sub-Loops
Elements can be defined in series, in parallel, or both with some restrictions
Branches are defined as individual legs within the loop structure
Segment between point A & B is defined as a branch, as is the section between points E and F

10. Layout for Individual HVAC Supply or Demand Sub-Loops
The first supply side component between A & B must be the loop pump, which controls the loop flow.
There may be multiple branches between the splitter and mixer (between points C1 & D1 to Cn & Dn).
Each sub-loop may only have one splitter and one mixer.

11. Layout for Individual HVAC Supply or Demand Sub-Loops
Equipment may be in parallel between the mixer and splitter,
Within any branch, there can only be elements in series.
Sub-loops do not require a splitter or mixer, if all equipment on the sub-loop is in series—a single branch defines the entire sub-loop.

12. Pumping Rules Pumps must be on the supply side
Splitter
Mixer
Components
Loop Pump
From Demand- Side Sub-Loop
To Demand- Side Sub-Loop
Supply-Side Sub-Loop
Pumps must be on the supply side
Pumps can operate as constant or variable flow
Pumps can run continuously or intermittently

13. Pumping Rules (cont’d)
Single boiler/chiller with NO bypass, PUMP:CONSTANT SPEED
Boiler/chiller should be constant flow
Pump should be intermittent
Single boiler/chiller with NO bypass, PUMP:VARIABLE SPEED
Boiler/chiller should be variable flow, regardless of whether pump is intermittent or continuous (runs at the minimum if demand is less than minimum, this includes zero.)

14. Pumping Rules (cont’d)
Single boiler/chiller with bypass, PUMP:CONSTANT SPEED
Boiler/chiller can be constant or variable flow
Pump may be intermittent or continuous as long as the bypass can handle the entire pump volume when the boiler is not operating

15. Pumping Rules (cont’d)
Multiple branches add more complexity, but it is nothing more than continuity.
If the pump is putting out flow then it has to have a branch to flow down whether it is a chiller or a bypass.

16. Pumping Rules (cont’d)
You are always safer adding the bypass for a simulation. If the active machines require the flow the bypass will be dry
Thermodynamically it does not make any difference if the flow goes through a machine that is OFF or it flows down the bypass.
There is no pressure simulation and flow losses are not accounted for.

17. Summary of Loop Limitations
Each sub-loop allowed one splitter and one mixer
One bypass on each sub-loop optional
No other components may be in series with a bypass
Equipment may be in parallel only between the splitter and mixer
Equipment may be in series in each branch

18. 3-Zone VAV System
We are going to look at this system input Loop by Loop

19. Cooling Coil & Chilled Water Demand Side

20. Cooling Coil & Chilled Water Demand Side
Objects required:
BRANCH LIST
CONNECTOR LIST
SPLITTER
MIXER
BRANCH, VAV Sys 1 ChW-Branch
COIL:Water:DetailedFlatCooling, VAV SYS 1 Cooling Coil

21. Cooling Coil & Chilled Water Demand Side (cont’d)
Objects required (cont’d):
BRANCH, Chilled Water Loop 1 CHW Inlet Branch
PIPE
BRANCH, Chilled Water Loop 1 CHW Outlet Branch
BRANCH, Chilled Water Loop 1 CHW Bypass Branch

22. Hot Water Demand Side

23. Hot Water Coils & Demand Side
Objects required:
BRANCH LIST
CONNECTOR LIST
SPLITTER
MIXER
BRANCH,Zone X Reheat Branch (3)
COIL:Water:SimpleHeating(3)
BRANCH, HW Outlet Branch
PIPE
BRANCH, HW Inlet Branch
BRANCH, HW Bypass Branch

24. Hot Water Loop Supply Side

25. Hot Water Loop Supply Side
Objects required:
PLANT LOOP
BRANCH LIST
CONNECTOR LIST
SPLITTER
MIXER
BRANCH, Supply Inlet Branch
PUMP:VARIABLE SPEED
Boiler
Plant Demand Side Heating Loop
Outlet Node
HW Pump
HW Pump Outlet Node
Bypass
Boiler Inlet Node
HW Supply Inlet Node
HW Supply Outlet Node

26. Hot Water Loop Supply Side
Hot Water Loop Supply Side (cont’d)
Hot Water Loop Supply Side
Objects required:
BRANCH, Boiler Branch
BOILER:SIMPLE
BRANCH, Supply Bypass Branch
PIPE
BRANCH, Supply Outlet Branch
PLANT OPERATION SCHEME
HEATING LOAD RANGE BASED OPERATION
HEATING LOAD RANGE EQUIPMENT LIST
Boiler
Plant Demand Side Heating Loop
Outlet Node
HW Pump
HW Pump Outlet Node
Bypass
Boiler Inlet Node
HW Supply Inlet Node
HW Supply Outlet Node

27. Chilled Water Supply Side

28. Chilled Water Supply Side
Plant Supply Side Cooling Loop
CW Pump
Chiller
Bypass
Plant Demand Side Cooling Loop
Objects required:
PLANT LOOP, Chiller Plant Chilled Water Loop
BRANCH LIST
CONNECTOR LIST
SPLITTER
MIXER

29. Chilled Water Supply Side (cont’d)
Plant Supply Side Cooling Loop
CW Pump
Chiller
Bypass
Plant Demand Side Cooling Loop
Objects required (cont’d):
BRANCH, Chiller Plant Cooling Supply Inlet Branch
PUMP:VARIABLE SPEED
BRANCH, Chiller Plant Chiller Branch
CHILLER:Electric
BRANCH, Chiller Plant Cooling Supply Bypass Branch
PIPE

30. Chilled Water Supply Side (cont’d)
Plant Supply Side Cooling Loop
CW Pump
Chiller
Bypass
Plant Demand Side Cooling Loop
Objects required (cont’d):
BRANCH, Chiller Plant Cooling Supply Outlet Branch
PIPE
PLANT OPERATION SCHEMES
COOLING LOAD RANGE BASED OPERATION
COOLING LOAD RANGE EQUIPMENT LIST

31. Condenser Loop Demand Side

32. Condenser Loop Demand Side
Objects required:
BRANCH LIST
CONNECTOR LIST
SPLITTER
MIXER
BRANCH, Condenser Demand Inlet Branch
PIPE
BRANCH, Chiller Condenser Branch
CHILLER:Electric
BRANCH, Cond. Demand Bypass Branch
BRANCH, Cond. Demand Outlet Branch
Plant Supply Side Cooling Loop
Chiller
Condenser Supply Side Loop
Condenser Bypass

33. Condenser Supply Side

34. Condenser Loop Supply Side
Condenser Supply Side
Condenser Loop Supply Side
Cooling Tower
Condenser Pump
Condenser Bypass
Condenser Demand Side Loop
Objects required:
CONDENSER LOOP
BRANCH LIST
CONNECTOR LIST
SPLITTER
MIXER
BRANCH, Supply Inlet Branch
PIPE
BRANCH, Condenser Branch
COOLING TOWER:Single Speed

35. Condenser Loop Supply Side (cont’d)
Condenser Supply Side
Condenser Loop Supply Side (cont’d)
Cooling Tower
Condenser Pump
Condenser Bypass
Condenser Demand Side Loop
Objects required (cont’d):
BRANCH, Cond. Supply Bypass Branch
PIPE
BRANCH, Supply Output Branch
CONDENSER OPERATION SCHEMES
COOLING LOAD RANGE BASED OPERATION
LOAD RANGE EQUIPMENT LIST

36. Summary Loops are backbone of HVAC simulation
all equipment attached to air or fluid loops
Loop structure permits assembly of any system through input—not hardwired
Primary system may consist of four different loops: plant supply and demand, condenser supply and demand
Pumps regulate flow while attempting to meet requests on both sides of each loop pair
Individual loops may have components in series and parallel with some limitations