Chilled Water Plant Design and Control 65 ECWT

0.555 KW/ton

56°F

0.59 KW/ton 48°F

CH-1

95°F

40°F

KW/ton

0.52 KW/ton

CH-2

90°F

75 ECWT

85 ECWT

1.0

0.8

0.6

0.4

85°F

0.2

25

50

75

Percent Loaded

Kent W. Peterson, PE, FASHRAE P2S Engineering, Inc. Long Beach, CA [email protected]

March 2, 2012

100

Handouts  

A copy of the today’s slides in PDF format will be available from Trent Hunt

2

Agenda                    

CHW Distribution Systems CHW Distribution System Balancing CW Distribution Systems Break Selecting CHW Distribution Systems Selecting CHW ΔT Selecting CW ΔT Selecting Chillers Optimizing Control Sequences Questions 3

Optimizing Energy Usage    

     

Chillers

• 

Type, efficiency, size, VFD

Cooling Towers

• 

Fan type, efficiency, approach, range, speed control, flow turndown

Chilled Water Pumps

• 

Arrangement, flow rate (delta-T), pressure drop, VFD

Condenser Water Pumps

• 

Flow rate (delta-T), pressure drop, VFD

Air Handling Units

• 

Coil sizing, air-side pressure drop, water-side pressure drop 4

Pop Quiz 1   What

happens to component energy usage if we lower CWS setpoint?

•  Chiller •  Towers •  Pumps

Pop Quiz 2   What

happens to component energy usage if we lower CW flow?

•  Chiller •  Towers •  Pumps

Pop Quiz 3   What

happens to component energy usage if we lower CW flow AND the CWS setpoint?

•  Chiller •  Towers •  Pumps

Optimizing CHW Plant Design   Ideal:

Design a plant with lowest life cycle costs (first cost plus lifelong operating costs) accounting for all the complexities and interaction among plant components

  Practical:

Design plant subsystems to be near-life cycle cost optimum using techniques that are simple and practical enough to be used without a significant increase in design time 8

Chilled Water Distribution Systems

9

Chilled Water System Classes   Constant

Flow

•  No control valves •  3-way control valves

  Variable

Flow

•  Primary-Only •  Primary/Secondary (/ Tertiary) •  Primary/Distributed Secondary •  Primary/Variable Speed Coil Secondary

These are the major categories of distribution systems. The list is by no means complete. There are many permutations and combinations. 10

Constant Flow Single Chiller, Single Coil, No Control Valve

11

“Constant” Flow 3-Way Valves 3-Way Mixing Valve

DPft 20

Bypass Balance Valve

Item

Pressure Drop @ 100 GPM 100% to Coil 50% to Coil 0% to Coil

Pipe/Valves Coil and/or Bypass Globe Control Valve Total GPM @ 20’ ΔP*

2 8 10 20

2 2 7.5 11.5

2 6 12 20

100

132

100

*actual ΔP available may change 12

Constant Flow Single Chiller, Multiple Coils

13

Constant Flow Multiple Parallel Chillers, Multiple Coils

CH1 240 gpm

How many chillers do we need to run?

CH2 240 gpm

Ballroom A 240 gpm 100% Loaded

Ballroom B 240 gpm Unoccupied

14

Variable Flow   Vary

Flow Through Coil Circuit

•  Two-way valves •  Variable speed coil pump

  Configurations

•  Primary-secondary •  Primary-secondary variations •  Primary-secondary-tertiary •  Primary-only 15

Variable Flow Chilled Water Systems   Old

Paradigm

•  Controls respond to changes in CHW temperature •  Variable flow causes low temperature trips, locks out chiller, requires manual reset (may even freeze) •  Hence: Maintain constant flow through chillers 16

Variable Flow Primary/Secondary, Multiple Chillers and Coils

Hydraulic Independence

If there is no resistance in the common leg, then no flow is induced in the other circuit. 17

Variable Flow Series Flow, Multiple Chillers

18

Variable Flow Primary/Distributed Secondary

19

Variable Flow Primary/Secondary/Tertiary

20

Variable Flow Chilled Water Systems   New

Paradigm

•  Modern controls are robust and very responsive to both flow and temperature variations •  Variable flow OK within range and rateof-change spec’d by chiller manufacturer

21

Variable Flow Primary-only, Multiple Chillers

22

Variable Flow Primary, Bypass Valve  

Location

• 

• 

Near chillers   Best for energy   Controls less expensive   Control more difficult to tune – fast response

Remote   Smaller pressure fluctuations (easier to control)   Keeps loop cold for fast response

 

Sizing

•  • 

Sizing critical when at chillers/pumps Different size if pump has VFD or not

 

Flow measurement

•  • 

Flow meter   Most accurate   Needed for Btu calc for staging

DP across chiller   Less expensive   Accuracy reduced as tubes foul   One required for each chiller 23

Primary CHW Pump Options

Headered Pumping Advantages: Dedicated Pumping Advantages: •  Better redundancy •  Less control complexity •  Custom pump heads w/ unmatched chillers •  Valves can “soft load” chillers with primary-only systems •  Usually less expensive if each pump is •  Easier to incorporate stand-by pump adjacent to chiller served •  Pump failure during operation does not cause multiple chiller trips 24

Balancing Variable Flow Systems

25

Balancing Issues  

Ensure “adequate” flow available at all coils to meet loads

• 

 

Ensure differential pressure across control valves is not so high as to cause erratic control

•  • 

 

Less than design flow may be “adequate” most of the time

“Two-positioning” Unstable control at low loads

Cost considerations

•  •  • 

First costs (installed costs and start-up costs) Pump energy costs (peak demand and annual) Rebalancing costs (if any) as coils are added to system 26

Balancing Options 1. 

2. 

3.  4.  5.  6.  7.  8. 

No balancing

• 

Relying on 2-way control valves to automatically provide balancing

Manual balance

•  • 

Using ball or butterfly valves and coil pressure drop Using calibrated balancing valves (CBVs)

Automatic flow limiting valves (AFLVs) Reverse-return Oversized main piping Undersized branch piping Undersized control valves Pressure independent control valves 27

Option 5: Oversized Main Piping   Advantages •  No balancing labor •  Coils may be added/ subtracted without rebalance •  Reduced overpressurization of control valves close to pumps •  Lowest pump head/ energy due to oversized piping, no balance valves •  Increased flexibility to add loads due to oversized piping

  Disadvantages •  Added cost of larger piping

28

Option 6: Undersized Branch Piping  

Advantages •  No balancing labor •  Reduced cost of smaller •  • 

piping Coils may be added/ subtracted without rebalance Reduced overpressurization of control valves close to pumps where piping has been undersized

 

Disadvantages •  Limited effectiveness and

•  •  •  • 

applicability due to limited available pipe sizes High design and analysis cost to determine correct pipe sizing Reduced flexibility to add coils where piping has been undersized Coils may be starved if variable speed drives are used without DP reset Slightly higher pump energy depending on flow variations and pump controls 29

Option 7: Undersized Control Valves  

Advantages •  No balancing labor •  Reduced cost of smaller

•  • 

• 

control valves Coils may be added/ subtracted without rebalance Reduced overpressurization of control valves close to pumps where control valves have been undersized Improved valve authority which could improve controllability where control valves have been undersized

 

Disadvantages •  Limited effectiveness and

•  •  • 

applicability due to limited available control valve sizes (Cv) High design and analysis cost to determine correct control valve sizing Coils may be starved if variable speed drives are without DP reset Slightly higher pump energy depending on flow variations and pump controls

30

Option 8: Pressure Independent Control Valves  

Advantages •  No balancing labor •  Coils may be added/

•  •  •  • 

subtracted without rebalance No over-pressurization of control valves close to pumps Easy valve selection – flow only not Cv Perfect valve authority will improve controllability Less actuator travel and start/stop may improve actuator longevity

 

Disadvantages •  Added cost of strainer •  •  •  • 

and pressure independent control valve Cost of labor to clean strainer at start-up Higher pump head and energy due to strainer and pressure independent control valve Valves have custom flow rates and must be installed in correct location Valves can clog or springs can fail over time 31

PICVs May Improve ΔT?

NBCIP Test Lab (as reported by manufacturer)

32

Ranks Balancing Method

Controllability (all conditions)

Pump Energy Costs

First Costs

1

No balancing

7

3

3

2

Manual balance using calibrated balancing valves

4

6

6

3

Automatic flow limiting valves

4

Reverse-return

5

Oversized main piping

6

Undersized branch piping

7

Undersized control valves

7 2 3 6 5

7 2 1 4 4

7 5 4 2 1

8

Pressure independent control valve

1

78

8

33

Conclusions & Recommendations for Variable Flow Hydronic Systems  

Automatic flow-limiting valves and calibrated balancing valves are not recommended on any variable flow system

•     

 

Reverse-return and oversized mains may have reasonable pump energy savings payback on 24/7 chilled water systems Undersizing piping and valves near pumps improves balance and costs are reduced, but significant added engineering time required Pressure independent valves should be considered on very large systems for coils near pumps

•  •   

Few advantages and high first costs and energy costs

Cost is high but going down now with competition When costs are competitive, this may be best choice for all jobs

For other than very large distribution systems, option 1 (no balancing) appears to be a reasonable option to consider

• 

Low first costs with minimal or insignificant operational problems

34

Problems Caused by Degrading ∆T

Q= 500 X GPM X ΔT  

For a given load Q, when ΔT goes down, GPM goes up

 

Result:

•  •  • 

Increases pump energy Can require more chillers to run at low load, or coils will be starved of flow Can result in reduced plant effective capacity: chiller capacity without the capability of delivering it

35

ΔT Degradation in Large Chiller Plant (January through March)

Design ΔΤ=10oF

9.5°F-10.0°F

Evaporator Delta T (°F)

Coincident Wet Bulb Ranges 7.0°F-7.5°F

35°F-40°F 40°F-45°F 45°F-50°F 50°F-55°F 55°F-60°F

4.5°F-5.0°F

2.0°F-2.5°F

0

100

200

300

400 Approximate hrs/yr

500

600

700

800

∆T Conclusions   Design,

construction, and operation errors that cause low ΔT can and should be avoided   But other causes for low ΔT can never be eliminated   Conclusion: At least some ΔT degradation is inevitable   Therefore: Design the CHW Plant to allow for efficient chiller staging despite degrading ΔT 37

Some Solutions  

Design CHW distribution system so chillers can have increased flow so they can be more fully loaded at low ΔT

•  Primary-only pumping •  Unequal chiller and primary pump sizes, headered •  • 

pumps so large pump can serve small chiller Low design delta-T in primary loop  Insures low ΔT in secondary  Higher primary loop first costs & energy costs

Primary/secondary pumping with check valve in common leg

38

Check Valve in the Common Leg

CHECK VALVE IN COMMON LEG

39

Supposed Disadvantages Check Valve in Common Leg    

Circuits are not hydraulically independent

• 

So what?

• 

Seldom a real problem - pump capabilities usually fall off fast enough due to high chiller ΔP Maximum flow rates are usually arbitrary – occasional excursions should not be a problem Resolved by using high design ΔTs (or adding auto-flow limiting valves at chillers as last resort)

Flow rate may exceed maximum allowed by chiller manufacturer

•  •     

Pumps in series may force control valves open

• 

Not true with variable speed driven secondary pumps.

Primary pumps may ride out their curves and overload

• 

Seldom a real problem - pump capabilities usually fall off fast enough due to high chiller ΔP, and motor may be selected to avoid this problem. 40

Real Disadvantages Check Valve in Common Leg

  Possible

dead-heading secondary pumps if primary pumps are off and chillers isolation valves are closed

•  Logically interlock secondary pumps to primary pumps

flow through inactive chillers with dedicated pumps

  “Ghost”

•  Use isolation valves rather than dedicated pumps 41

Check Valve in the Common Leg  

Recommendation

•  For fixed speed chillers with ∆T problems, a check

• 

valve in the common leg can help. Make sure pump design/controls address secondary pump deadheading and ghost-flow issues. Select a check valve with low pressure drop (i.e. swing check, not spring) For variable speed chillers, do not put check valve in common leg. It has little value (unless ΔT degradation is severe) since chiller plant will not be inefficient by staging chillers on before they are fully loaded 42

Condenser Water Distribution Systems

43

Condenser Water Systems   Old

paradigm: constant flow & speed   New paradigm: variable flow & speed

•  Control logic to maximize efficiency?

44

Variable Speed CW Pumps

VSCW

CSCW 45

Condenser Water Pump Options

Dedicated Pumping Advantages: •  Less control complexity •  Custom pump heads w/ unmatched chillers •  Usually less expensive if each pump is adjacent to chiller served and head pressure control not required

Headered Pumping Advantages: •  Better redundancy •  Valves can double as head pressure control •  Easier to incorporate stand-by pump •  Can operate fewer CW pumps than chillers for fixed speed pumps 46

Tower Isolation Options

1. 

Select tower weir dams & nozzles to allow one pump to serve all towers

•  •  •  2.  3. 

Always most efficient Almost always least expensive Usually possible with 2 or 3 cells

Install isolation valves on supply lines only

• 

Need to oversize equalizers

Install isolation valves on both supply & return

• 

Usually most expensive but fail safe 47

Non-integrated water-side economizer (WSE) Try to avoid this!

Twb 36F Twb 41F

44F

You have to shut off the economizer to satisfy the load!

44F

44F 49F

44F >46F

60F

41F 46F

Heat Exchanger in parallel with chillers

Integrated water-side economizer Twb 41F

You can use either a control valve or pump

44F

44F

46F

100 gpm)

Any

None

Variable

Small (< 100 gpm)

Low (< 40 feet)

3-way

Constant or Staged

Primary-only

Small (< 100 gpm)

High(> 40 feet) Variable

Primary-only Or Primary-Secondary

Few (2 to 5) serving similar loads or system has only one chiller Few (2 to 5) serving similar loads Many (more than 5) or few serving dissimilar loads

Small (< 100 gpm)

2-way Any

Primary/ distributed secondary Primary/coil secondary

55

Primary/Secondary Secondary Pump w/ VFD at Chiller Plant

2-Way Control Valves at AHUs

56

Primary/Distributed Secondary Distributed Secondary Pump w/ VFD Typical at each Building No Secondary Pumps at Plant

Central Plant 57

Advantages of Distributed P/S versus Conventional P/S or P/S/T   Reduced

pump HP - each pump sized for head from building to plant   Self-balancing   No over-pressurized valves at buildings near plant   Reduced pump energy, particularly when one or more buildings are off line   No expensive, complex bridge connections used in P/S/T systems   Similar or lower first costs 58

Primary/Coil Secondary Distributed Secondary Pump w/ VFD Typical at each AHU No Secondary Pumps at Plant

No Control Valves at AHUs Large AHU-1

Large AHU-2

59

Hybrid Systems

Advantages of VFD Coil Pumps versus Conventional P/S system  

 

 

 

 

Reduced pump HP

•  • 

Each pump sized for head from coil to plant Eliminated 10 feet or so for control valves

Self-balancing

• 

No need for or advantages to balancing valves, reverse return

Lower pump energy

•  • 

No minimum DP setpoint Pump efficiency constant

Better control

•  • 

Smoother flow control - no valve hysteresis No valve over-pressurization problems

Usually lower first costs due to eliminated control valves, reduced pump and VFD HP 61

Disadvantages of VFD Coil Pumps Versus Conventional P/S system   Cannot

tap into distribution system without pump

•  May be problem with small coils (low flow, high head pump)

  Possible

reduced redundancy/reliability unless duplex coil pumps are added   Possible low load temperature fluctuations

•  Minimum speed on pump motor •  May need to cycle pump at very low loads

62

Primary-only System Headered Pumps & Auto Isolation Valves Preferred to Dedicated Pumps: •  Allows slow staging •  Allows 1 pump/2 chiller operation •  Allows 2 pump/1 chiller operation if there is low ΔT

BYPASS VALVE

Flow Meter or DP Sensor Across Chiller

63

Advantages of Primary-only Versus Primary/Secondary System   Lower

first costs   Less plant space required   Reduced pump HP  

 

Reduced pressure drop due to fewer pump connections, less piping Higher efficiency pumps (unless more expensive reduced speed pumps used on primary side)

  Lower pump energy   Reduced connected HP   “Cube Law” savings due to VFD and variable flow through both primary and secondary circuit 64

Pump Energy Primary vs. Primary/Secondary (3-chiller plant) 40.00 35.00 30.00

Pump kW

25.00

Primarysecondary

20.00 15.00 Primary-only 10.00 5.00

%

0.00 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% % GPM

65

Disadvantages of Primary-only Versus Primary/Secondary System  

Failure of bypass control

•  Not as fail-safe - what if valve or controls fail? •  Must avoid abrupt flow shut-off (e.g. valves interlocked • 

 

with AHUs all timed to stop at same time) Must be well tuned to avoid chiller short-cycling

Flow fluctuation when staging chillers on

•  Flow drops through operating chillers •  Possible chiller trips, even evaporator freeze-up •  Must first reduce demand on operating chillers and/or slowly increase flow through starting chiller; causes temporary high CHWS temperatures

 

(Problems above are seldom an issue with very large plants, e.g. more than 3 chillers)

66

Primary-only System Staging 0 GPM

0 GPM

1000 GPM

67

Primary-only System Staging 0 GPM

500 GPM

500 GPM

68

Variable Flow Primary/Secondary with CHW Storage

 

Advantages

•  •  •  •  • 

 

Peak shaving Simplifies chiller staging Provides back-up for chiller failure Secondary water source for fire department Secondary water source for cooling towers

Disadvantages

•  • 

Installed cost Space

69

Primary-only vs. Primary/Secondary

 

Use primary-only systems for:

•  Plants with many chillers (more than three) and with

•   

fairly high base loads where the need for bypass is minimal or nil and flow fluctuations during staging are small due to the large number of chillers; and Plants where design engineers and future on-site operators understand the complexity of the controls and the need to maintain them.

Otherwise use primary-secondary

•  Also for plants with CHW storage

70

Pipe Sizing

Pipe Sizing   Need

to balance

•  Cost of pipe and its installation •  Cost of pump energy •  Longevity of piping (erosion) •  Noise •  Sometimes space limitations

72

Accurately sizing pump head

 

Guessing at pump heads

•  • 

   

Wastes money in oversized pumps, motors and (sometimes) VFDs and (sometimes) need for impeller trimming Wastes energy (minor impact w/VFD or if impeller is trimmed)

Calculating pump heads

• 

Takes about 20 minutes of engineering time

Guessing cannot possibly be cost effective!

73

Optimum ΔT

Flow Rate and ΔT

Q = 500 GPM ΔT Load from Load Calc’s (Btu/hr)

Flow rate (GPM)

Conversion “constant” =8.33 lb/gal * 60 minutes/hr

Temperature Rise or Fall (ºF)

75

CHW ΔT Tradeoffs ΔT Low

High

Typical Range

8°F

25°F

First Cost Impact

smaller condenser

smaller pipe smaller pump smaller pump motor

Energy Cost Impact

lower fan energy

lower pump energy

76

Coil Performance with ΔT Chilled Water ΔT Coil water pressure drop, feet H2 O Coil airside pressure drop, inches H 2 O

11 28

13 20

15 15

18 10

20 8.1

0.46 0.48 0.49 0.52 0.54

43°F chilled water supply temperature, 78°F/62°F entering air and 53°F leaving air temperature.

77

System Performance With ΔT Varying Airside Pressure

1200 CHP Energy kWh/year Chiller Energy kWh/year

1000

Fan Energy kWh/year

kWh/ton/year

800

600

400

200

0 11

13

15

18

20 CHW Delta-T

CHWST = 44F 78

System Performance and ΔT Constant Airside Pressure

1400

CHP Energy kWh/year Chiller Energy kWh/year

1200

Fan Energy kWh/year

kWh/ton/year

1000 800 600 400 200 0 41/16

42/14

43/12

44/10 CHWST/Delta-T 79

Choosing the “Right” CHW ΔT  

 

Both energy and first costs are almost always minimized by picking a very high ΔT (>18°F to 25°F) Savings even greater with systems that have

•  Large distribution piping network •  Water-side economizers •  CHW thermal energy storage

80

Condenser Water (Tower) Range at Constant CWST ΔT Low

High

Typical Range

8°F

18°F

First Cost Impact

smaller condenser

Energy Cost impact

lower chiller energy

smaller pipe smaller pump smaller pump motor smaller cooling tower smaller cooling tower motor lower pump energy lower cooling tower energy

81

Condenser Water Range at Constant Tower Fan Energy

600 Tower Fan CW pump

kWh/ton/year

500

Chiller

400 300 200 100 0 73/16

73.5/14

74.5/12

75.5/10 CWST/Delta-T 82

COOLING TOWER SELECTION DOE 2 Curve: Percent rated capacity at 70.0°F wet bulb

2 X design capacity Design capacity 210%

% Design Capacity

200%capacity 1/2 of design 190% 180% 170% 160% 150% 140% 130% 120% 110% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%

19 17 15 1

13 2

3

4

11 5 Range (°F)

6

9 7

8

Approach (°F)

200%-210% 190%-200% 180%-190% 170%-180% 160%-170% 150%-160% 140%-150% 130%-140% 120%-130% 110%-120% 100%-110% 90%-100% 80%-90% 70%-80% 60%-70% 50%-60% 40%-50% 30%-40% 20%-30% 10%-20% 0%-10%

7 9

10

5 11

83

Cooling Tower Selection   Fan

Control   Efficiency   Approach

84

Tower Fan Control One Cell Tower

Single Speed Fan

% Power

Free Cooling ~ 15% of Capacity

Two-Speed or Variable-Speed Fan

% Capacity

85

Tower Fan Control Two Ce ll Towe r 100%

Two Cell Tower 90%

Two 1-Speed Fans

80%

70%

One 1-Speed Fan and One 2-Speed Fan

% Power

60%

50%

40% Two 2-Speed Fans 30% Two Variable Speed

Free Cooling Below 15% Capacity

20%

10%

0% 0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

55%

% Capacity

60%

65%

70%

75%

80%

85%

90%

95%

100 %

86

Tower Fan Control    

 

One-speed control is almost never the optimum strategy regardless of size, weather, or application VFD fan speed control is best choice now

•  •  •  •  • 

Costs now comparable to two speed motors & starters Soft start reduces belt wear Lower noise Control savings for DDC systems (network card options) More precise control

Multiple cell towers should have speed modulation on at least 2/3 of cells (required by ASHRAE 90.1). For redundancy, use VFDs on all cells

87

Tower Efficiency LCC

1000 ton Oakland Office

90 GPM/HP

70 GPM/HP

50 GPM/HP 88

Tower Efficiency Guidelines  

Use Propeller Fans

•  Avoid centrifugal except where high static needed or • 

where low-profile is needed and no prop-fan options available Consider low-noise propeller blade option and high efficiency tower where low sound power is required

 

Efficiency

 

Approach

•  Minimum 80 gpm/hp for commercial occupancies •  Minimum 100 gpm/hp for 24/7 plants (data centers) •  Maximum 10°F for large central plants •  3°F for 24/7 plants (data centers) 89

Break

CHILLER SELECTION

Part-Load Ratio

91

Chiller Procurement Approaches   Most

Common Approach

•  Pick number of chillers, usually arbitrarily or as limited by program or space constraints •  Take plant load and divide by number of chillers to get chiller size (all equal) •  Pick favorite vendor •  Have vendor suggest one or two chiller options •  Pick option based on minimal or no analysis •  Bid the chillers along with the rest of the job and let market forces determine which chillers you actually end up installing

92

Chiller Procurement Approaches   Better

Approach

•  Estimate plant annual load profile •  Understand chiller efficiency curves •  Request chiller options from multiple

manufacturers based on a performance desired. Multiple options encouraged. •  Estimate energy usage of options with a detailed computer model of the building/plant

93

Sample Load Profile 800

700

Hours per year

600

500

Percent Load

400

300

200

100

0

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

94

Chiller Procurement Approaches   Another

Option

•  Pick a short list of vendors based on past

experience, local representation, etc. •  Request chiller bids based on a performance specification. Multiple options encouraged. •  Adjust bids for other first cost impacts •  Estimate energy usage of options with a detailed computer model of the building/plant •  Select chillers based on lowest life cycle cost •  Bid the chillers at end of design development phase 95

Chiller Bid Specification  

Don’t Specify:

•  •  •  •  • 

Number of chillers Chiller size Chiller efficiency Chiller unloading mechanism As much as possible – keep the spec flexible!

 

Do Specify:

•  Total design load •  Anticipated load profile •  Minimum number of •  •  •  •  • 

chillers and redundancy requirements Design CHW/CW entering and leaving temperatures and/or flows (or tables of conditions) Available energy sources Physical, electrical or other limitations Acoustical constraints Acceptable refrigerants 96

Zero Tolerance Data   Do

NOT allow tolerance to be taken in accordance with ARI 550/590   Why insist on zero tolerance?

•  Levels playing field – tolerances applied

inconsistently among manufacturers •  Modeled energy costs will be more accurate •  High tolerance at low loads makes chillers appear to be more efficient than they will be, affecting comparison with unequally sized, VFDdriven, or multiple chiller options 97

Zero Tolerance Data ARI 550/590 Tolerance Curve

45% 40% 35%

% Tolerance

30%

10F Delta-T 15F Delta-T

25%

20F Delta-T

20% 15% 10% 5% 0% 0%

20%

40%

60%

80%

100%

120%

% of Full Load

98

Factory Tests   Certified

Factory Tests

•  Need to verify performance to ensure accurate claims by chiller vendors in performance bids •  Field tests are difficult or impossible and less accurate •  Last chance to reject equipment

99

Chiller Bid Evaluation   Adjust

for First Cost Impacts   Estimate Maintenance Costs   Calculate Energy Costs

•  Energy model of building(s) and plant

  Calculate

Life Cycle Costs   Temper Analysis with Consideration for “Soft” Factors   Final Selection

100

Advantages & Disadvantages OF RECOMMENDED CHILLER SELECTION APPROACH

  Disadvantages

•  Extra work for both engineer and vendor •  Difficult to include maintenance impact •  Assumes energy rate schedules will remain as they are now with simplistic adjustments for escalation

  Advantages

•  Allows manufacturers to each find their own

“sweet” spots, both for cost and efficiency •  Usually higher energy efficiency •  More rational than typical selection approaches 101

OPTIMIZING CONTROLS

102

Optimizing Control Sequences   Cookbook

Solution

  Relational

Control Approach

•  Staging Chillers •  Controlling Pumps •  Chilled Water Reset •  Condenser Water Reset

103

Staging Chillers  

Fixed Speed Chillers

•  Operate no more chillers than required to meet the •  •  • 

load Stage on when operating chillers maxed out as indicated by measured load (GPM, ΔT), CHWST, flow, or other load indicator For primary-secondary systems w/o check valve in the decoupler, start chiller to ensure primary-flow > secondary-flow Stage off when measured load/flow indicates load is less than operating capacity less one chiller – be conservative to prevent short cycling 104

Staging Chillers, continued   Variable

Speed Chillers

•  Operate as many chillers as possible provided load on each exceeds 30% to 40% load •  Energy impact small regardless of staging logic •  You MUST use condenser water reset to get the savings

105

Part Load Chiller Performance w/ Zero ARI Tolerance

100% 90% Fixed Speed

80%

Variable Speed 70%

%kW

60% 50% 40% 30% 20% 10% 0% 0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

% Load (with Condenser Relief) 106

Controlling CHW Pumps   Primary-only

Pumps

and Secondary CHW

•  Control speed by differential pressure measured as far out in system as possible and/or reset setpoint by valve demand •  Stage pumps by differential pressure PID loop speed signal:

 Start lag pump at 90% speed  Stop lag pump at 40% speed  For large HP pumps, determine flow and speed setpoints with detailed energy analysis

107

VFD Pump Power vs. Setpoint 100% 90%

DP setpoint = Design Head DP setpoint = Head*.75

80%

DP setpoint =Head/2 DP setpoint =Head/3

Percent Pump kW

70%

DP setpoint = 0 (reset) 60% 50% 40% 30% 20% 10% 0% 0

10%

20%

30%

40%

50%

60%

Percent GPM

70%

80%

90%

100%

108

Chilled Water Setpoint Reset  

Reset Impacts

•  • 

 

Resetting CHWST upwards reduces chiller energy but will increase pump energy in VFD variable flow systems Dehumidification   Reset with “open” or indirect control loops (e.g. OAT) can starve coils and reduce dehumidification   Reset by control valve position will never hurt dehumidification − humidity of supply determined almost entirely by supply air temperature setpoint, not CHWST

Recommendations

•  • 

Reset from control valve position using Trim & Respond logic For variable flow systems with VFDs   Reset of CHWST and VFD differential pressure setpoint should be sequenced − not independent like VAV systems since control valves are pressure-dependent   Sequence reset of CHWST and DP − next slide…

109

CHWST/DP Setpoint Reset for VSD CHW System DPmax

Tmin+ 15ºF

DP setpoint

CHW setpoint DP setpoint

CHW setpoint

Tmin

5 psi 0

   

50% CHW Plant Reset

100%

Back off on CHWST first Then back off on DP setpoint 110

CHW vs. DP Setpoint Reset

Plant with 150 ft CHW pump head 111

Condenser Water Setpoint Reset  

 

Optimum Strategy Cannot Easily Be Generalized

•  • 

Depends on efficiency/sizing of tower and type of chiller Relational control by monitoring plant efficiency

Recommendations

•  • 

CWS reset by plant load from [as low as manufacturer recommends] at 30% plant load up to [design CWST] at 80% load Reset based on wetbulb temperature not effective given inaccuracy of sensors

112

Optimum Sequences   All

plants are different

•  Tower efficiency, approach •  Chiller efficiency, unloading control •  Pump efficiency, head, unloading control •  Number of chillers, pumps, towers

  A

generalized sequence can be developed but it will not be optimum   Solution? 113

Summary  

In this course, you have learned techniques to design and control chiller plants for nearminimum life cycle costs, including:

•  •  •  •  • 

Selecting optimum chilled water distribution system Selecting optimum CHW supply & return temperatures Selecting optimum CW and tower range and approach temperatures, tower efficiency, and fan speed controls Selecting optimum chillers using a performance bid and LCC analysis Optimizing control sequences and setpoints

114

Questions

115