THE GREAT CHILLED WATER DEBATE

Michael Dagher Central Chiller Plant

• Direct-Primary, Variable Flow?

• Variable Primary / Variable Secondary (Decoupled)?

• Parallel Vs Series Counterflow?

• Pressure Control?

• What is right/wrong? Chilled Water Pumping Schemes

Way Before • Primary Only, Constant Flow • Constant flow variable return water temperature • Bypassed chilled water mixes with return water resulting in lower chilled water return temperature to the plant. This lower return water temperature reduces the temperature differential (∆T) across the chillers and decreases the overall performance and efficiency of the entire system This phenomenon is known as low ∆T syndrome

Load Load Load

Constant Primary Chilled Water Pumping Schemes

Little Later in 1950s • Decoupled Constant Primary, Variable Secondary • Maintained use of 3-way valves • Decoupler pipe located in plantroom • +ve or –ve flow possible through decoupler • The low ∆T syndrome reduced but still a problem

-ve

Load Load Load

+ve

Constant Primary Variable Secondary Chilled Water Pumping Schemes

Then in Late 1980s • Decoupled Constant Primary, Variable Secondary • Two way control valves introduced • Secondary pumps staged to match coil water flow demands • Some three-way control valves are used together with two-way valves to provide a means of maintaining a minimum secondary pump flow • The low ∆T syndrome further reduced but still a problem

Staged

-ve M M

Load Load Load

+ve

Constant Primary Variable Secondary Chilled Water Pumping Schemes

Early 1990s • Primary Only, Constant Flow • Constant flow variable return water temperature • Smaller less complex distribution with Bypass in plantroom with field predominantly 2 way control valves at coils. Constant flow in plantroom, variable flow in field. The configuration also suffers from the low ∆T syndrome.

M M M

Load Load Load

Constant Primary Chilled Water Pumping Schemes

Since late 1990s • Decoupled Variable Primary, Variable Secondary • Chiller manufacturers allow designers to vary flow through the chillers, provided evaporator tube velocities and rate of change of flow through the evaporator are managed. • With the introduction of variable primary flow, it was now possible to match the primary and secondary flows and the decoupler flow was minimised, addressing the low ∆T syndrome for majority of the time.

Variable Speed Pumps

-Ve M M

FM Load Load Load

+Ve

VSD VSD VSD

Variable Primary Variable Secondary Chilled Water Pumping Schemes

Since late 1990s • Variable Primary Only • Bypass located remotely in field with predominantly 2 way control valves at coils. Flow is modulated to maintain pressure in the field, while the by-pass modulates to maintain chiller minimum flow. The configuration relies on proper control loop tuning ensuring the chiller is not exposed to rapid flow changes. This configuration addresses the low ∆T syndrome issue for the majority of the time.

M M M

Load Load Load

Variable Primary Only Chiller Arrangement

Now • Variable Primary Only, Series Counterflow • This configuration improves overall chiller efficiency by reducing the lift on the compressor. Pumping energy can be higher than parallel arrangement. Pump selection crucial and avoid oversizing (as they need to modulate flow between 100 down to 40%)

M M M

Load Load Load

Series Counterflow Chiller Arrangement Variable Primary Only Chilled Water Distribution Schemes Pressure M Sensors & By-

M M M Pass Load Load Load

M M M Load Load Load

M M M Load Load Load

M M M

Building A Load Load Load Building B Building C

M M M Load Load Load

VSD Booster Pump

Pressure Sensors & By- Pass M

VSD VSD VSD

Variable Primary + Booster Pressure Sensors & Pressure Sensors & Pressure Sensors & Chilled Water By-Pass By-Pass By-Pass M M M

M M M Distribution Load Load Load

M Schemes M M Load Load Load

M M M Load Load Load

M Variable M M Load

Building A Load

Load Building C Secondary Building B

M M M Load Load Load

VSD VSD VSD

Decoupler

FM

VSD VSD VSD

Variable Primary Advantages & Disadvantages of Decoupled Primary/Secondary

Advantages •Simple to apply to large complex precinct systems •Stable Flow through Chillers

Disadvantages •Costly (≈ 5% higher than Primary Only) •Greater pumping energy (≈ 5% higher overall plant energy than Primary Only) Pressure Control – Design Load

A B

C D

E F G H I DP Index Coil 100% Design Flow Setpoint Pressure Drop J K L M Pressure Drop N Node

F G H I C E

M M M Pressure Sensors & By- Load Load Load Pass

B D A VSD VSD VSD

N M L K J Variable Primary Only Pressure Control – Part Load

A B

C D F E G H I DP Index Coil Part Load Flow Setpoint Pressure Drop J K N M L Pressure Drop Node

F G H I C E

M M M Pressure Sensors & By- Load Load Load Pass

B D A VSD VSD VSD

N M L K J Variable Primary Only Pressure Control – Part Load

A B D C F E G H I

DP DP Design Setpoint Index Coil Part Load Flow Operating DP Pressure Drop

J L K M N Pressure Drop Node

F G H I C E &

M M

- Pass Load Load Load By B D Pressure Sensors A VSD VSD VSD

N M L K J Variable Primary Only What is right or wrong?

• Depends on the particular application • Look at all options don’t jump to conclusion too soon • Its about the system nothing but the whole system • Optimise each element, chillers, pumping, flow/pressure control, cooling tower fan energy • Staging strategy • Temperature controlled • Reset T and DP • Avoid the temptation of oversizing, can hurt you at part load (+70% of time). Don’t optimise the plant for that one hour of the year. • Work with the chiller manufacturer don’t do it alone THE GREAT CHILLED WATER DEBATE

Stefan Sadokierski Primary – Secondary Arrangement

• Primary circuit: • Fixed or variable flow, • Pump controlled to bypass flow or system thermal load • Secondary circuit – variable flow controlled to pressure • Positives • Primary / secondary hydraulically decoupled, • Simple, robust, well understood, easy to operate • Issues • Excess pumping • Mixing of excess primary and secondary return Primary Only Arrangement

• Single pump set controlled to pressure • Bypass only opens below lowest turndown of smallest chiller • dP (as shown) • Flow • Positives – cost, plant space, efficiency • Issues • Complex controls • Loss of LWT set point on staging • Different chiller sizes means dissimilar pumps in parallel Primary Only – Pros and Cons

• PROS • Lower first cost • Less plant space • Improved efficiency (typically 3-8%) • Fewer components (possibly improved reliability) • CONS • Likely loss of LWT set point when staging on / off • Increased controls complexity • Best performance with chillers all same capacity • Additional commissioning

• RECOMMENDATION • Significant potential benefits – should be considered • Application – not recommended if stable LWT is needed for critical cooling or dehumidification processes • End User – must be informed and trained Series Counter-Flow Arrangement

• Variable primary arrangement • Chiller pairs in series • Counterflow – condenser water flows in opposite direction (not shown) • Compressor lift minimised Series Counter-Flow – Pros and Cons

• PROS • Improved compressor efficiency (~5% for 3+3 = 6°C dT CHW) • Transient chilled water flow issues on staging mitigated • CONS • Increased pumping power (out weighted by compressor savings) • Increased “N” chiller capacity – impact on redundancy • Increased controls complexity

• RECOMMENDATIONS • Standard approach for district cooling applications: • Well suited to high dT applications (dT > 7°C) – maintain compressor efficiency and reduce distribution costs • Well suited to large load applications – larger N capacity ok • Can be applied elsewhere, more suited to larger loads • Consider planned and unplanned maintenance activities Other ways to make similar savings

• Compressor power ~ refrigerant mass flow x lift… • Increase evaporating temperature • Reduce condensing temperature

• Chilled water leaving temperature • Often at 6/12 or 7/14 °C • 10 / 17 °C common practice in UK (won’t work for LT VAV) • Condenser water temperature • Typically WB approach of 5.5 (24 °C wb + 5.5 = 29.5 °C LWT) • Larger tower can be reduced to 2.5 (26.5 °C LWT) • Compare extremes: • CHW at 6/12 and CDW at 29.5 = 23.5 °C lift • CHW at 10/17 and CDW at 26.5 = 16.5 C lift

• Optimised control of CDW temperature to minimise compressor and fan power (ASHRAE sequencing strategy) THE GREAT CHILLED WATER DEBATE

Barry Abboud Overview • The three basic piping systems • Low DeltaT Syndrome – causes, effects, and solutions • Design & Control Considerations (VPF) • Series Counter Flow Chilled Water Piping System Types (typical)

Load Configuration Valves Installed Cost Pumping Cost

Constant Primary Flow 3-way Lowest Highest

Primary / Secondary 2-way Highest Medium

Variable Primary Flow 2-way Medium Lowest Constant Primary Flow Load = Flow X DeltaT With Dedicated Pumping

Secondary Pumps

4 Per Chiller System

Constant Primary Flow Load 125 Tons (440kW) 375Tons (1320kW) With Dedicated Pumping Primary

Flow 3000gpm (189 l/s)

Delta T 3oF (1.7oC) 47 ºF 25% (8.3 ºC)

Secondary Pumps

47 ºF (8.3 ºC) (189 l/s) @ 6.7 ºC)

47 ºF (8.3ºC)

(1760 kW)

(63 l/s) 56 ºF (13.3 ºC) 47 ºF 47 ºF (189 l/s) @ 8.3 ºC) (8.3 ºC)

5 Constant Primary Flow

Advantages Lowest installed cost Less plant space than P/S Easy to Control & Operate Easy to Commission Disadvantages Highest Plant Energy Cost (must run all, even at low loads)

6 Primary (const.) / Secondary (Variable)

SLoad = Flow X DeltaT

Secondary Pumps

PLoad = Flow X DeltaT

7 PRIMARY (VARIABLE) / SECONDARY (VARIABLE) HEADERED PUMPING

Secondary Pumps

8 Per Chiller System PRIMARY/SECONDARY AT 25% LOAD Load 375 Tons (1320kW) 375 Tons (1320kW) Primary Secondary Bypass

Flow 1000gpm (126 l/s) 750gpm (47 l/s) 250gpm (16 l/s)

Delta T 9oF (5oC) 12oF (6.7oC) ----

25% Load25% = 25% Sec Flow Secondary Pumps 750 GPM @ 44 ºF 47 l/s @ 6.7 ºC

53 ºF 44.0 °F (11.7 ºC) (6.7 °C)

(1760 kW) 250 GPM @ 44 ºF 16 l/s @ 6.7 ºC (63 l/s)

1000 GPM @ 53 ºF 750 GPM @ 56 ºF 56 ºF (63 l/s) @ 11.7 ºC) (47l/s) @ 13.3 ºC) (13.3 ºC)

9 Set Point kPa) (76 ft P=25 ) P

PRESS URE DIFFER ENTIAL SENSO PD+Safety R CONTR OLS SECON

DARY PD+Piping PUMP SPEED WPD+Valve

controls speed (coil Point Set to end at located efficiency best for Index of Circuit . . . Differential Pressure sensor down stream down sensor Differential Pressure 10 Variable Primary Flow Load = Flow X DeltaT

Variable Primary Flow at 100% System Load Two-way valves control capacity By varying flow of water in coils

Primary Pumps Chillers Closed

11 Per Chiller System Variable Primary Flow at 25% Load Load 375 Tons (1320kW) 375 Tons (1320 kW)

Primary Bypass

Flow 750 gpm (95 l/s) 0 gpm (0 l/s)

Delta T 12oF (6.7oC) ---- Variable Primary Flow 25%at Load25% System = 25% Flow Load Two-way valves control capacity By varying flow of water in coils

750 GPM @ 44 ºF 47 l/s @ 6.7 ºC

56 ºF 44.0 °F (13.3 ºC) (6.7 °C)

0 GPM @ 44 ºF Primary Pumps 0 l/s @ 6.7 ºC 750 GPM Closed (47 l/s)

750 GPM @ 56 ºF 750 GPM @ 56 ºF 56 ºF (47 l/s) @ 13.3 ºC) (47 l/s) @ 13.3 ºC) (13.3 ºC)

12 Issues with Varying Flow through Chillers V ARYING

 Issue During Normal Operation F LOW  Chiller Type (centrifugal fast, absorbers slow)  Chiller Load (min load - no variance, full load - max variance) T

 System Water Volume (more water, more thermal capacitance, faster variance allowed) HROUGH  Active Loads (near or far from plant)  Typical VSD pump ramp rate setting of 10%/minute (guide for stable temp control)

 Issue Adding Chillers C HILLERS

 Modulating isolation valves on chillers - I SSUES

13 13 Variable Primary Flow (VPF) System Arrangement

 Advantages  Lower Installed Cost (approx. 5% compared P/S)  No secondary Pumps or piping, valves, electrical, installation, etc.  Offset somewhat by added 2W Bypass Valve and more complex controls  Less Plant Space Needed  Best Chilled Water Pump Energy Consumption (most optimised configuration)  VSD energy savings  Lower Pump Design Head  Higher Pump Efficiency  Lower potential impact from Low Delta T (can over pump chillers if needed)

14 PUMP CURVES - PUMP EFFICIENCY

With VPF you will need larger pumps compared to P/S, but they will be operating at a more efficient point, yielding energy savings 15 Variable Primary Flow (VPF) System Arrangement

 Disadvantages Requires more robust (complex and properly calibrated) control system Requires coordinated control of chillers, isolation valves, and pumps Potentially longer commissioning times to tune the system Need experienced facility manager to operate/maintain properly

16 MAJOR CAUSES OF LOW DELTA T

 Dirty Coils  Controls Calibration  Leaky 2-Way Valves  Coils Piped-Up Backwards  Mixing 2-Way with 3-Way Valves in the same system

17 NEGATIVE EFFECTS OF LOW DELTA T IN P/S SYSTEMS

Consequences:  Higher secondary pump energy  pumps run faster

 Higher chilled water plant energy  Ancillary equipment

 Can’t load up chillers  more than ratio Act DT / Des DT  10/12 = 83%

18 NEGATIVE EFFECTS OF LOW DELTA T IN VPF SYSTEMS

Consequences:  Higher secondary pump energy  pumps run faster

 Higher chilled water plant energy  Ancillary equipment

 Can’t load up chillers  more than ratio Act DT / Des DT  10/12 = 83% or 417 tons

19 SOLUTION TO (OR REDUCE EFFECTS OF) LOW DELTA T

 Address the causes  Clean Coils  Calibrate controls periodically  Select proper 2W valves (dynamic/close-off ratings) and maintain them  No 3W valves in design  Find and correct piping installation errors  Over deltaT chillers by resetting supply water down (P/S)  Over pump chillers at ratio of Design Delta T / Actual Delta T (VPF)  Use VSD Chillers & Energy-based sequencing (from 30 to 80% Load)

Solve at Load, Mitigate at Plant

20 VPF SYSTEMS DESIGN/CONTROL CONSIDERATIONS

Chillers  Equal Sized Chillers preferred, but not required  Maintain Min flow rates with Bypass control (manufacturer)  Maintain Max flow rates (3 m/s) and max WPDs (manufacturer)  Modulating Isolation Valves (or 2-position stroke-able) set to open in 1.5 to 2 min  Don’t vary flow too quickly through chillers (VSD pump Ramp rate – typical setting of 10%/min)  Sequence  If CSD Chillers – Load-based sequencing…run chillers to max load (Supply Temp rise). Do not run more chillers than needed (water-cooled, single compressor assumed)  If VSD Chillers – Energy-based sequencing…run chillers between 30% and 80% load (depending on ECWT and actual off-design performance curves). Run more chillers than load requires.  Add Chiller - CHW Supply Temp or Load (Flow X Delta T) or amps (if CSD)  Subtract Chiller - Load (Flow X Delta T) or Amps (if CSD)

21 21 VPF SYSTEMS DESIGN/CONTROL CONSIDERATIONS

Pumps  Variable Speed Driven  Headered arrangement preferred  Sequence  with chillers (but run an extra pump than # chillers for over-pumping in low delta T situations)  Flow-based sequencing  Energy-based sequencing (most efficient combination of pumps)  Speed controlled by pressure sensors at end of index circuit (fast response important)  Direct wired  Piggyback control for large distances  Optimized - Reset pressure sensor by valve position of coils

22 VPF SYSTEMS DESIGN/CONTROL CONSIDERATIONS

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 (total min)  High Range-ability (100:1 or better 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  Control setpoint higher than absolute chiller minimum  Locate some distance from chillers/pumps (preferred)  Energy Storage / inertia

23 ENHANCED EFFICIENCY THROUGH SERIES COUNTER FLOW

Pressure Pressure

Condenser Condenser 2

Compressor 2 Lift 2 Condenser 1 Compressor Evaporator 2 Lift 1 Compressor 1 Evaporator 1 Evaporator

Enthalpy Enthalpy

140 C 100 C LCHWT 60 C ECHWT Evaporator Evaporator

LCWT

Condenser Condenser 0 35 C 0 32 C ECWT 290 C ENHANCED EFFICIENCY THROUGH SERIES COUNTER FLOW

Parallel Chillers SCF Chillers Total Capacity (kWr) 2 x 1500 2 x 1500 Evap Flow Total (L/s) 44.7 x 2 = 89.4 89.4 Evap DP (kPa) 82.4 78.9 Cond Flow Total (L/s) 69.8 x 2 = 139.6 138.7 Cond DP (kPa) 76.9 54.2 R134a Charge (kg) 2 x 603 = 1206 2 x 438 = 876 Cost ($) BASE Less than BASE

VPF Evap min (L/s) 13 22

Load (kWr) Parallel (kWe) SCF (kWe) Saving (kWe) % 3000 471.0 446.5 24.5 5.2% 2700 378.0 355.5 22.5 6.0% 2400 297.8 276.3 21.5 7.2% 2100 229.4 210.0 19.4 8.5% 1800 171.5 154.4 17.1 10.0% 1500 122.7 108.6 14.1 11.5% 1200 100.2 87.5 12.7 12.7% 900 80.9 69.5 11.4 14.1% 600 65.2 56.9 8.3 12.7% 300 75.4 66.0 9.3 12.4%