optimizing chilled water plants - tech seminar

March 2, 2012

Kent W. Peterson, PE, FASHRAE

P2S Engineering, Inc. Long Beach, CA

[email protected]

Chilled Water Plant Design and Control

CH-1 CH-2

95F 85F90F

0.52 KW/ton 0.59 KW/ton

0.555 KW/ton

40F56F 48F

0.2

0.4

0.6

0.8

1.0

25 50 75 100

65 ECWT 75 ECWT 85 ECWT

Percent Loaded

KW

/ton

2

Handouts

A copy of the todays slides in PDF format will be available from Trent Hunt

3

Agenda

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

4

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

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

8

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

9

Chilled Water Distribution Systems

10

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.

11

Constant Flow Single Chiller, Single Coil, No Control Valve

12

Constant Flow 3-Way Valves

100% to Coil 50% to Coil 0% to CoilPipe/Valves 2 2 2Coil and/or Bypass 8 2 6Globe Control Valve 10 7.5 12Total 20 11.5 20

GPM @ 20 P* 100 132 100

Pressure Drop @ 100 GPM

*actual P available may change

Item

3-Way Mixing Valve

Bypass Balance Valve

DP 20 ft

13

Constant Flow Single Chiller, Multiple Coils

14

Constant Flow Multiple Parallel Chillers, Multiple Coils

CH2 240 gpm

CH1 240 gpm How many chillers do we need to run?

Ballroom A 240 gpm 100% Loaded

Ballroom B 240 gpm Unoccupied

15

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

16

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

17

Variable Flow Primary/Secondary, Multiple Chillers and Coils

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

Hydraulic Independence

18

Variable Flow Series Flow, Multiple Chillers

19

Variable Flow Primary/Distributed Secondary

20

Variable Flow Primary/Secondary/Tertiary

21

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 rate-of-change specd by chiller manufacturer

22

Variable Flow Primary-only, Multiple Chillers

23

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

24

Primary CHW 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 Pump failure during operation does not cause multiple chiller trips

Headered Pumping Advantages: Better redundancy Valves can soft load chillers with primary-only systems Easier to incorporate stand-by pump

25

Balancing Variable Flow Systems

26

Balancing Issues

Ensure adequate flow available at all coils to meet loads Less than design flow may be adequate most of the

time Ensure differential pressure across control valves

is not so high as to cause erratic control 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

27

Balancing Options

1. No balancing Relying on 2-way control valves to automatically provide

balancing 2. Manual balance

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

3. Automatic flow limiting valves (AFLVs) 4. Reverse-return 5. Oversized main piping 6. Undersized branch piping 7. Undersized control valves 8. Pressure independent control valves

28

Option 5: Oversized Main Piping

Advantages No balancing labor Coils may be added/

subtracted without rebalance

Reduced over-pressurization 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

29

Option 6: Undersized Branch Piping

Advantages No balancing labor Reduced cost of smaller

piping Coils may be added/


Reduced over-pressurization 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

30

Option 7: Undersized Control Valves

Advantages No balancing labor Reduced cost of smaller

control valves Coils may be added/


Reduced over-pressurization 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

31

Option 8: Pressure Independent Control Valves

Advantages No balancing labor Coils may be added/


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

32

PICVs May Improve T?

NBCIP Test Lab (as reported by manufacturer)

33

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 7 7 7 4 Reverse-return 2 2 5 5 Oversized main piping 3 1 4 6 Undersized branch piping 6 4 2 7 Undersized control valves 5 4 1 8 Pressure independent control

valve 1 8 8

7

34

Conclusions & Recommendations for Variable Flow Hydronic Systems

Automatic flow-limiting valves and calibrated balancing valves are not recommended on any variable flow system Few advantages and high first costs and energy costs

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 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

35

Problems Caused by Degrading T

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

Q= 500 X GPM X T

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

T Degradation in Large Chiller Plant (January through March)

0 100 200 300 400 500 600 700 800

Approximate hrs/yr

Eva

pora

tor

Del

ta T

(F)

35F-40F40F-45F45F-50F50F-55F55F-60F

2.0F-2.5F

4.5F-5.0F

7.0F-7.5F

9.5F-10.0F

Coincident Wet Bulb Ranges

Design =10oF

37

T Conclusions

Design, construction, and operation errors that cause low T can and should be avoided

But other causes for low T can never be eliminated

Conclusion: At least some T degradation is inevitable

Therefore: Design the CHW Plant to allow for efficient chiller staging despite degrading T

38

Some Solutions

Design CHW distribution system so chillers can have increased flow so they can be more fully loaded at low T 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 T in secondary Higher primary loop first costs & energy costs

Primary/secondary pumping with check valve in common leg

39

Check Valve in the Common Leg

CHECK VALVE IN COMMON

LEG

40

Supposed Disadvantages Check Valve in Common Leg

Circuits are not hydraulically independent So what?

Flow rate may exceed maximum allowed by chiller manufacturer Seldom a real problem - pump capabilities usually fall off fast

enough due to high chiller P Maximum flow rates are usually arbitrary occasional

excursions should not be a problem Resolved by using high design Ts (or adding auto-flow limiting

valves at chillers as last resort) 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 P, and motor may be selected to avoid this problem.

41

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 Ghost flow through inactive

chillers with dedicated pumps Use isolation valves rather than dedicated

pumps

42

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 dead-heading 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 T degradation is severe) since chiller plant will not be inefficient by staging chillers on before they are fully loaded

43

Condenser Water Distribution Systems

44

Condenser Water Systems

Old paradigm: constant flow & speed

New paradigm: variable flow & speed Control logic to maximize efficiency?

45

Variable Speed CW Pumps

VSCW CSCW

46

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

47

Tower Isolation Options

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

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

2. Install isolation valves on supply lines only Need to oversize equalizers

3. Install isolation valves on both supply & return Usually most expensive but fail safe

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

Heat Exchanger in parallel with chillers

44F 60F

44F

41F

Twb 36F

Twb 41F

46F

49F

44F

44F

>46F

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

Heat Exchanger in series with chillers on CHW side

Integrated water-side economizer

You can use either a control valve or pump

44F 60F

Twb 41F

46F

49F

44F

44F

50

Example WSE savings building description

200,000 ft2 office building with ~ 110 tons of data center load

Location Pleasanton CA (ASHRAE Climate 3B)

(2) 315 ton chillers (630 tons total) Building has air-side economizer Data center has CRAH units Water-side economizer on central plant

with HX (integrated, see previous slide)

51

Example WSE Savings

~30% ~24%

~48%

~2%

Design Procedure

54

Design Procedure

Select Chilled Water Distribution System Select Temperatures, Flow Rate and

Primary Pipe Sizes Select Cooling Tower Design Criteria Select Chillers Finalize Piping System Design, Select

Pumps Develop Optimum Control System and

Control Sequences

55

Recommended Chilled Water Distribution Arrangement

Number of coils/loads served

Size of coils/ loads served

Distribution losses (excluding chiller)

Control Valves Flow

Recommended Distribution Type

One Any Any None Constant or Staged Primary-only

More than 1 Large Campus Any 2-way Variable Primary/ distributed secondary

More than 1 Large coils (> 100 gpm) Any None Variable Primary/coil secondary

Few (2 to 5) serving similar loads or system

has only one chiller

Small (< 100 gpm) Low (< 40 feet) 3-way

Constant or Staged Primary-only

Few (2 to 5) serving similar loads

Small (< 100 gpm) High(> 40 feet)

Many (more than 5) or few serving dissimilar

loads

Small (< 100 gpm) Any

2-way Variable Primary-only

Or Primary-Secondary

56

Primary/Secondary

Secondary Pump w/ VFD

at Chiller Plant

2-Way Control Valves at AHUs

Secondary Pump w/ VFD

at Chiller Plant

2-Way Control Valves at AHUs

57

Primary/Distributed Secondary

Central Plant

Distributed Secondary

Pump w/ VFD - Typical at each

Building

No Secondary Pumps at

Plant

58

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

59

Primary/Coil Secondary

Large AHU-1

Distributed Secondary

Pump w/ VFD - Typical at each

AHU

No Secondary Pumps at Plant

Large AHU-2

No Control Valves at AHUs

Hybrid Systems

61

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

62

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

63

Primary-only System

BYPASS VALVE

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 T

Flow Meter or DP Sensor Across Chiller

64

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

65

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

%

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

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

% GPM

Pum

p kW

Primary-only

Primary-secondary

66

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)

67

Primary-only System Staging

1000 GPM

0 GPM

0 GPM

68

Primary-only System Staging

500 GPM

500 GPM

0 GPM

69

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

70

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

Pipe Sizing

72

Pipe Sizing

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

73

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!

Optimum T

75

Flow Rate and T

TGPM 500 =QLoad from Load Calcs (Btu/hr)

Conversion constant =8.33 lb/gal * 60

minutes/hr

Flow rate (GPM)

Temperature Rise or Fall (F)

76

CHW T Tradeoffs

T

Low

High

Typical Range

8F

25F

First Cost Impact

smaller condenser

smaller pipe smaller pump

smaller pump motor

Energy Cost Impact

lower fan energy

lower pump energy

77

Coil Performance with T

Chilled Water T 11 13 15 18 20Coil water pressure drop, feet H2O

28 20 15 10 8.1

Coil airside pressure drop, inches H2O

0.46 0.48 0.49 0.52 0.54

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

78

0

200

400

600

800

1000

1200

11 13 15 18 20CHW Delta-T

kWh/

ton/

year

CHP Energy kWh/yearChiller Energy kWh/yearFan Energy kWh/year

System Performance With T Varying Airside Pressure

CHWST = 44F

79

System Performance and T Constant Airside Pressure

0

200

400

600

800

1000

1200

1400

41/16 42/14 43/12 44/10CHWST/Delta-T

kWh/ton/year

CHP Energy kWh/yearChiller Energy kWh/yearFan Energy kWh/year

80

Choosing the Right CHW T

Both energy and first costs are almost always minimized by picking a very high T (>18F to 25F)

Savings even greater with systems that have Large distribution piping network Water-side economizers CHW thermal energy storage

81

Condenser Water (Tower) Range at Constant CWST

T

Low

High

Typical Range

8F

18F

First Cost Impact

smaller condenser

smaller pipe smaller pump

smaller pump motor smaller cooling tower

smaller cooling tower motor Energy Cost

impact

lower chiller energy

lower pump energy lower cooling tower energy

82

Condenser Water Range at Constant Tower Fan Energy

0

100

200

300

400

500

600

73/16 73.5/14 74.5/12 75.5/10

CWST/Delta-T

kWh/ton/year

Tower Fan CW pumpChiller

83

COOLING TOWER SELECTION

5

7

9

11

13

151719

1110

98

76

54

32

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

% D

esig

n C

apac

ity

Approach (F)

Range (F)

DOE 2 Curve: Percent rated capacity at 70.0F wet bulb

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%

Design capacity

2 X design capacity

1/2 of design capacity

84

Cooling Tower Selection

Fan Control Efficiency Approach

85

Tower Fan Control

Free Cooling ~ 15% of Capacity

Single Speed Fan

Two-Speed or Variable-Speed Fan

% Capacity

% Power

One Cell Tower

86

Tower Fan Control Two Cell Tower

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 85% 90% 95% 100%

% Capacity

% P

ower

Two 1-Speed Fans

Two 2-Speed Fans

One 1-Speed Fan andOne 2-Speed Fan

Free Cooling Below 15% Capacity

Two Variable Speed

Two Cell Tower

87

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

88

Tower Efficiency LCC

90 GPM/HP 70 GPM/HP 50 GPM/HP

1000 ton Oakland Office

89

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 Minimum 80 gpm/hp for commercial occupancies Minimum 100 gpm/hp for 24/7 plants (data centers)

Approach Maximum 10F for large central plants 3F for 24/7 plants (data centers)

91

CHILLER SELECTION

Part-Load Ratio

92

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

93


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

94

Sample Load Profile

0

100

200

300

400

500

600

700

800

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

Percent Load

Hour

s pe

r yea

r

95


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

96

Chiller Bid Specification Dont 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

97

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, VFD-driven, or multiple chiller options

98

Zero Tolerance Data

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

0% 20% 40% 60% 80% 100% 120%

% of Full Load

% T

oler

ance

10F Delta-T15F Delta-T20F Delta-T

ARI 550/590 Tolerance Curve

99

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

100

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

101

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

102

OPTIMIZING CONTROLS

103

Optimizing Control Sequences

Cookbook Solution Staging Chillers Controlling Pumps Chilled Water Reset Condenser Water Reset

Relational Control Approach

104

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, T), 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

105

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

106

Part Load Chiller Performance w/ Zero ARI Tolerance

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

% Load (with Condenser Relief)

%kW

Fixed SpeedVariable Speed

107

Controlling CHW Pumps

Primary-only and Secondary CHW Pumps 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

108

VFD Pump Power vs. Setpoint

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Percent GPM

Perc

ent P

ump

kW

DP setpoint = Design HeadDP setpoint = Head*.75DP setpoint =Head/2DP setpoint =Head/3DP setpoint = 0 (reset)

109

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

110

CHWST/DP Setpoint Reset for VSD CHW System

Back off on CHWST first Then back off on DP setpoint

Tmin+ 15F

Tmin

DPmax

5 psi

CHW setpoint

CHW setpoint

DP setpoint

DP setpoint

CHW Plant Reset 0 100% 50%

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CHW vs. DP Setpoint Reset

Plant with 150 ft CHW pump head

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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

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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?

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Summary

In this course, you have learned techniques to design and control chiller plants for near-minimum 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

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Questions

optimizing chilled water plants - tech seminar

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