Chilled Water Piping Systems (VPF Focus)
Agenda – Chilled Water Distribution Systems
Chilled Water Distribution Systems
Primary (Constant) / Secondary (Variable – 2W Valves)
Low Delta T
Primary Only (Variable Flow - 2W Valves)
VPF Design/Control Considerations
Primary (Constant Flow) / Secondary (Variable Flow)
Primary/Secondary System
Primary Pumps
Secondary Pumps
Common Pipe
Typical load with two way valve
Primary (Constant Flow) / Secondary (Variable Flow)
2 Way Valves
Higher Capital Cost Installed (vs Constant Flow 3W Valve system)
Lower CHW Pumping Energy (vs Constant Flow 3W Valve system)
Well Understood & Easy to Control
Primary/Secondary System at Design
56.0 °F
56.0 °F
56.0 °F
44.0 °F
44.0 °F
44.0 °F
Primary Pumps
1000 GPM Each
3000 GPM @ 56.0 °F
Secondary Pumps
3000 GPM @ 44.0 °F
Typical Coil
No flow
44.0 °F
56.0 °F
500 ton chillers1000 GPM Each56.0-44.0°F
Primary/Secondary System at Part Load
53.0 °F
53.0 °F
53.0 °F
44.0 °F
44.0 °F
44.0 °F
Primary Pumps
1000 GPM Each
3000 GPM @ 53.0 °F
Secondary Pumps
2250 GPM @ 44.0 °F
Typical Coil
44.0 °F
56.0 °F
750 GPM @ 44.0 °F
2250 GPM @ 56.0 °F
75% System Load
Primary/Secondary System
53.0 °F
53.0 °F
44.0 °F
44.0 °F
Primary Pumps
1000 GPM Each
2000 GPM @ 53.0 °F
Secondary Pumps
1500 GPM @ 44.0 °F
Typical Coil
44.0 °F
56.0 °F
500 GPM @ 44.0 °F
1500 GPM @ 56.0 °F
OFF 50% System Load
Low Delta T Syndrome
10
Dirty Coils
Major Causes of Low Delta T
Chilled Water Coil
12
Dirty Coils
Controls Calibration
Leaky 2 Way Valves
3 Way Valves at end of Index circuit
Major Causes of Low Delta T
Primary/Secondary System
Primary Pumps
Secondary Pumps
Common Pipe
Primary/Secondary System
Primary Pumps
Secondary Pumps
Common Pipe
15
Dirty Coils
Controls Calibration
Leaky 2 Way Valves
3 Way Valves at end of Index circuit
Coils piped up backwards
Major Causes of Low Delta T
Chilled Water Coil
17
P Load = Flow X Delta T S Load = Flow X Delta T
Primary (Constant) / Secondary (Variable)
Secondary Pumps
Primary Pumps
Typical load with 2 way valve
Decoupler/Bypass
18
100% Load = 100% Sec Flow
Primary (Constant) / Secondary (Variable)Ideal Operation
Secondary Pumps
Primary PumpsDecoupler/Bypass
100% Flow = 3000 gpm
100% Flow = 3000 gpm
0 gpm
12
Secondary Pumps
Primary PumpsDecoupler/Bypass
67% Flow = 2000 gpm
67% Flow = 2000 gpm
0 gpm
19
67% Load = 67% Sec Flow
Primary (Constant) / Secondary (Variable)Ideal Operation
12
20
Primary / Secondary Rule of Flow
Primary flow must always be equal to or greater than Secondary flow.
Secondary Pumps
Primary PumpsDecoupler/Bypass
100% Flow = 3000 gpm
80% Flow = 2400 gpm (400 gpm over-pumped)
600 gpm
21
67% Load = 80% Sec Flow
Primary (Constant) / Secondary (Variable)Low Delta T Operation
10
Higher Secondary Pump Energy
Higher CHW Plant Chiller/Auxiliary Energy
Major Effects of Low Delta T
23
Solution to (or reduce effects of) Low Delta T
Address the causes
Clean Coils
Calibrate controls occasionally
Select proper 2W valves (dynamic/close-off ratings) and maintain them
no 3W valves in design
find and correct piping installation errors
Over pump chillers at ratio of Design Delta T / Actual Delta T
Increase Delta T across chillers with CHW Re-set (down).
Use Variable Speed Chillers & sequence to operate from 30 to 70% Load
Use VPF Systems (mitigates energy waste in plant)
Header pumps & operate more pumps than chillers
If dedicated pumping, over-size (design at 80% speed).
Primary/Secondary System
Primary Pumps
Secondary Pumps
Common Pipe
P
Primary Only (Variable Flow)
Primary/Secondary System
Variable Primary System
Secondary Pumps
Primary Pumps
Primary Pumps
Flow Meter
BypassValve
Automatic Isolation Valve
Typical load with 2 way valve
Typical load with 2 way valve
Primary Only (Variable Flow)
2 Way Valves
Lower Capital Cost Installed (vs Primary/Secondary)
No secondary pumps/piping/valves/electrical to buy and install
No large Common pipe, but smaller Bypass pipe/valve/flow meter/controls
Lower CHW Pumping Energy
Smaller Footprint (vs Primary/Secondary)
Relatively New & More Complex Controls
Reduces Negative Impacts from Low Delta T
Chillers are not staged on by flow requirements
Chillers can load up and are staged on load
Primary Only (Variable Flow)
Disadvantages
Higher (potentially) PSID rated 2-Way valves in system
Requires more robust (complex and calibrated) control system
Requires coordinated control of chillers, isolation valves, and pumps in sequencing
Longer (potentially) Commissioning time
Requires greater operator sophistication
Variable-Primary-Flow System
Primary Pumps
Flow Meter
Typical load with two way valve
Automatic Isolation Valve
Bypass
Variable Primary System at Design
56.0 °F
56.0 °F
56.0 °F
44.0 °F
44.0 °F
44.0 °F
Primary Pumps
1000 GPM Each
3000 GPM @ 56.0 °F
3000 GPM @ 44.0 °F
Automatic Isolation Valve
Bypass Closed
500 ton chillers1000 GPM Each56.0-44.0°F
Typical load with two way valve
Variable Primary System – Part Load
56.0 °F
56.0 °F
56.0 °F
44.0 °F
44.0 °F
44.0 °F
Primary Pumps
750 GPM Each
2250 GPM @ 56.0 °F
2250 GPM @ 44.0 °F
Automatic Isolation Valve
Bypass Closed
Typical load with two way valve
75% System Load
Variable Primary System – Part Load
56.0 °F
56.0 °F
44.0 °F
44.0 °F
Primary Pumps
750 GPM Each
1500 GPM @ 56.0 °F
1500 GPM @ 44.0 °F
Automatic Isolation Valve
Bypass Closed
Typical load with two way valve
50% System LoadChiller off
Pump off
Variable Primary System – Part Load
52.0 °F
52.0 °F
44.0 °F
44.0 °F
Primary Pumps
750 GPM Each
2250 GPM @ 52.0 °F
2250 GPM @ 44.0 °F
Automatic Isolation Valve
Bypass Closed
Typical load with two way valve
50% System LoadLow Δ T
Chiller off
Pump on
Variable Primary System – Min Flow (400 gpm each)
50.0 °F
44.0 °F
Primary Pumps
400 GPM (one operating)
400 GPM @ 50.0 °F
200 GPM @ 44.0 °F
Automatic Isolation Valve
Bypass Open
Typical load with two way valve
System flow below chiller minimum flow
Chiller off
Chiller off
Closed
Closed
200 GPM @ 44.0
200 GPM @ 56.0 °FFlowmeter
Pumps off
Chiller Design Considerations
Flow rate changes – Staging on additional chillers
Variable Primary System (1 chiller running)
56.0 °F
44.0 °F
Primary Pumps
333 GPM Each
1000 GPM @ 56.0 °F
1000 GPM @ 44.0 °F
Automatic Isolation Valve
Bypass Closed
Typical load with two way valve1000 GPM
Variable Primary System (Staging on second chiller)
57.0 °F
45.0 °F
Primary Pumps
333 GPM Each
1100 GPM @ 57.0 °F
1100 GPM @ 45.0 °F
Automatic Isolation Valve
Bypass Closed
Typical load with two way valve1100 GPM
Need to add chiller
Variable Primary System (Open isolation valve)
57.0 °F
45.0 °F
Primary Pumps
333 GPM Each
1100 GPM @ 57.0 °F
1100 GPM @ 45.0 °F
Automatic Isolation Valve
Bypass Closed
Typical load with two way valve550 GPM
550 GPM
Load = F X DT DT = 12 = 57- 45 24
Load = 1/2F X 2DT DT = 24 24 LCHWT = 35!
Variable Primary System (Open isolation valve)
57.0 °F
45.0 °F
Primary Pumps
333 GPM Each
1100 GPM @ 57.0 °F
1100 GPM @ 45.0 °F
Automatic Isolation Valve
Bypass Closed
Typical load with two way valve550 GPM
550 GPM
LCHWT approaches 35
LWT Cutout at 4 deg below44 set-point or 40
Off goes chiller 1
Variable Primary System (Open isolation valve slowly)
57.0 °F
45.0 °F
Primary Pumps
333 GPM Each
1100 GPM @ 57.0 °F
1100 GPM @ 45.0 °F
Automatic Isolation Valve
Bypass Closed
Typical load with two way valve1100 GPM
Open over 1.5 to 2 min
42
VPF Systems Design/Control Considerations Summary
Chillers
Equal Sized Chillers preferred, but not required
Maintain Min flow rates with Bypass control (1.5 fps)
Maintain Max flow rates (11.0 to 12.0 fps)
Isolation Valves (Modulating or Stroke-able to 1.5 to 2 min)
Don’t vary flow too quickly through chillers (VSD Ramp function – typical setting of 10%/min)
Chiller Type
System Water Volume
Chiller Load
Active Loads
Sequence
If Constant Speed – run chiller to max load (Supply Temp rise). Do not run more chillers than needed (water-cooled)
If Variable Speed – run chillers between 30% and 70% load (depending on ECWT). Run more chillers than load requires.
Add Chiller - CHW Supply Temp or Load (Adjusted* Flow X Delta T) or amps (if CSD)
Subtract Chiller - Load (Adjusted* Flow X Delta T) or Amps (if CSD)
43
VPF Systems Design/Control Considerations Summary
Pumps
Variable Speed Driven
Headered arrangement preferred
Sequence
with chillers (run more pumps than chillers for over-pumping capability)
on flow (add pump when existing inadequate, subtract when can)
optimized algorithm (total kW of more pumps, lower than less pumps)
Stay within pump/motor limits (25% to 100% speed)
Subtract a Pump at 25 to 30% speed
Add a pump back when speed of operating pumps high enough
Speed controlled by pressure sensors at end of index circuit
44
VPF Systems Design/Control Considerations Summary
Bypass Valve
Maintain a minimum chilled water flow rate through the chillers
Differential pressure measurement across each chiller evaporator
Flow meter preferred
Modulates open to maintain the minimum flow through operating chiller(s).
Bypass valve is normally open, but closed unless Min flow breeched
Pipe and valve sized for Min flow of operating chillers
High Rangeability (100:1 preferred)
PSID Ratings for Static, Dynamic, And Close Off = Shut Off Head of Pumps
Linear Proportion (Flow to Valve Position) Characteristic preferred
Fast Acting Actuator
Locate in Plant around chillers/pumps (preferrred)
Energy
Avoid Network traffic
45
VPF Systems Design/Control Considerations Summary
Load Valves
High Rangeability (200:1 preferred)
PSID Ratings for Static, Dynamic, And Close Off = Shut Off Head of Pumps
Equal Percentage (Flow to Load) Characteristic
Slow Acting Actuator
Staging Loads
Sequence AHUs On/Off in 10 to 15 min intervals
46
Summary on VPF Design
Chillers Size equally with same WPDs (best) Respect Min/Max Flows through chillers Set Pump VSD Ramp function to about 10%/min (600 sec 0 to Max Speed) Use Modulating or Strokeable Valves (preferred) on chiller evaps, headered pumping Use 2 Position Valves (1 min stroke) on chiller evaps, dedicated pumpingPumps VSD Controllers Headered Pumping Arrangement (preferred) Dedicated Pumping OK (over-size pumps)2 Way Valves Select for Static, Dynamic, Close-off ratings (PSID) equal to pump SOH (plus fill pressure) Range-ability 100 to 200:1 If Bypass – fast acting, linear proportion If Coils – slow acting, equal percentage, “On-Off” stagger air units (10-15 min intervals)Controls Set-point far out in index circuit (lower the value, the better the pump energy) Set Ramp function in VSD Controller (10%/min average) Run 1 more pump than chillers (when headered) Chillers On by common Supply Temp, Load, Amps, Adj Flow (Adj for Low Delta T) Chillers Off by Amps, Load, Adj Flow (Adj for Low Delta T) Over-pump Chillers to combat Low Delta T and get Max Cap out of chillers Bypass controlled by Min flow (preferred) or Min WPD of largest chiller (locate in plant for best energy, but can go anywhere in system)
Chilled Water Piping Systems (VPF Focus)
Questions?
Su
pply
Terminal
Balanceand
Service Valve
2-WayControlValve
Re
turnService Valve
Air
2 Way Valve/Coil Detail
Electric Energy Cost Equations
ChillerEnergy Cost
Lbs Refrig/hr X Head
33,015 X Comp EffX Hours X Cost/Unit Energy=
PumpEnergy Cost
GPM X Head
3960 X Pump EffX Hours X Cost/Unit Energy=
FanEnergy Cost
CFM X TSP
6356 X Fan EffX Hours X Cost/Unit Energy=
0.7459
Mot Eff
0.7459
Mot Eff
0.7459
Mot Eff
X
X
X
Energy Cost Mass Flow/t X Lift
33,015 X EfficiencyX Hours X Cost/Unit Energy=
0.7459
Mot EffX