Chilled Water Design - NCHEA flow / high ∆T chilled water systems! ... Life Cycle Cost Analysis!Detailed load profile! ... decoupler (from return to

CHILLED WATER SYSTEMSFOR THE 21ST CENTURY

Ed Tinsley, PEAnnual ASHE Conference

Nashville, Tennessee

CHILLED WATER SYSTEMSFOR THE REAL WORLD

21st CENTURY

Hospitals In The 21st Century

! Downward revenue pressure� Negative Medicare / Medicaid Margins� Managed care discounts� More difficult to obtain capital� More indigent care

! Upward expense pressure� Higher labor costs� Higher drug costs� Higher technology costs� More stringent regulations� Higher and more volatile energy costs

Where Does This Leave You?

Hospital Engineer�s Challenge

! Stretch the capacity of existing equipment and distribution systems to serve new loads

! Increase infrastructure and utility system reliability

! Reduce maintenance costs, utility costs, and construction costs

�Do More With Less�

Session Goals

! Increase chilled water system reliability

! Increase chilled water system distribution capacity

! Increase equipment capacity! Increase chilled water system

efficiency

Hospital Energy Use

Other 60%

CHW System 40%

Chilled Water System Energy Use

1970�s Today

Pumps 18%

Water Chiller 73%

Pumps 26%

Tower 16%

Water Chiller 58%

Tower 9%

Centrifugal Chiller Performance History

0.40

0.50

0.60

0.70

0.80

0.90

1970 1975 1980 1985 1990 1995 2000

Year

kW/T

on

Chiller Efficiency

! Theoretical efficiency� Refrigerant (HCFC-123, HFC-134a, etc.)� Heat transfer surface area (approach

temperatures)! Compressor efficiency

� Refrigerant lift� Compressor speed

! Drive efficiency� Motor� Gear box

Refrigerant Lift

! Refrigerant lift = Refrigerant ∆P! Refrigerant lift is necessary for oil return and

hermetic motor cooling! Can be approximated by:

Condenser � Evaporator + Evaporator + CondenserLWT LWT Approach Approach

! Typically available at chiller control panel

Chiller Selection

Ref

riger

ant L

ift

Load

Elevated ECWT

Design Conditions

Constant ECWT Unloading

Surge Line

�Watch the surge line�

Improving CHW System Performance

! Variable speed water chillers! Primary/secondary chilled water systems! Variable primary chilled water systems! Variable volume tower water systems! Low flow tower water systems! Low flow / high ∆T chilled water systems! Series � series piping arrangements! District cooling systems

Variable Speed Water Chillers

! Better efficiency at part loads! Better efficiency at lower lifts! Higher kW / Ton at design conditions! Majority of operating hours at part load

and lower lifts

�Does this mean that variable speed is always better than constant speed?�

Variable Speed Case Study

! Average load is 1,000 tons! Average variable speed chiller efficiency is 0.5

kW / Ton! Average constant speed chiller efficiency is 0.6

kW / Ton! Average electricity unit cost is $ 0.10 / kWh! Variable speed savings:

1,000 x 8,760 x (0.6 � 0.5) x 0.10 = $87,600! What is wrong with this picture?

What Is Wrong With This Picture?! Demand costs may account for a

substantial portion of the �blended� unit cost

! There may be a ratchet clause! There may be a minimum demand charge! The chiller may not operate many hours

at lower loads and lower lifts! Evaluating chiller alternatives requires a

complete understanding of the electricity rate structure, load profile, & lift profile

Life Cycle Cost Analysis!Detailed load profile!Weather data!Cooling tower performance!Tower water pump performance!Chilled water pump performance!Detailed electricity rate analysis

The Big Picture

Energy Costs

Chiller Costs

Installation Costs

Maintenance Costs

UAMS Load Profile

UAF Southwest Plant

0

500

1,000

1,500

2,000

2,500

35 45 55 65 75 85 95 105

Outside Air Temperature

Chill

er P

lant

Dem

and

(kW

)

Variable Speed Chillers

Constant Speed Chillers

Evaluation Results

Water Chiller Cost ($) 650,000 646,962Water Chiller Capacity (Tons) 3,000 3,000

Water Chiller Unit Cost ($/Ton) 216.67 215.65Electricity Consumption (kWh) 11,853,001 12,115,651Monthly Peak Demand (kW) 1,865 1,818

Electricity Cost ($/year) 449,500 447,962Electricity Unit Cost (cents/kWh) 3.79 3.70

Maintenance Cost ($/year) 23,125 16,927Life Cycle Cost ($) 13,890,835 13,816,154

Item Variable Speed Chillers

Constant Speed Chillers

Types of Chilled Water Systems

! Constant volume! Primary/secondary! Variable primary! Parallel arrangements! Series arrangements

Primary / Secondary CHW System

Primary / Secondary Advantages

! Primary and secondary circuits are �decoupled�� Relatively constant evaporator flow rates� Variable volume secondary

! Improved energy efficiency� Reduces direct pumping costs� Reduces indirect pumping costs

! Avoids mixing problems associated with parallel connected chillers in constant volume systems

Primary / Secondary Disadvantages

! Chiller operation depends on single CHW pump

! Increased CHW supply temps due to negative decoupler (from return to supply) mixing

! Decreased CHW chiller return temps due to positive decoupler (from supply to return) mixing

! Wasted energy caused by decoupler flow ! Maintenance for 2 sets of CHW pumps! Floor space for 2 sets of CHW pumps! Capital cost for 2 sets of CHW pumps

Decoupler Flows Waste Energy

! Direct Effect� BHP = GPM x Head = 1,000 x 60 = 20.2

3,960 x PE 3,960 x 75%� KW = BHP x 0.746 = 20.2 x 0.746 = 16.8

ME 90%

! Indirect Effect� Tons = BHP x 0.212 = 20.2 x 0.212 = 4.3� KW = Tons x CE = 4.3 x 0.75 = 3.2

! Total� KW = 16.8 + 3.2 = 20� Energy Cost = 20 KW x 8,760 Hrs x 0.06 = $10,512 / year

Variable Primary CHW System

Variable Primary Sequence of Operation

! Isolation valve opens when chiller is started


!Pumps sequenced based on flow and head


!Pump speeds are modulated to maintain plant ∆P


!Bypass valve prevents low evaporator flow

Typical Building

Building Sequence of Operation

! Control valve enables/disables cooling, limits ∆P, limits flow, and maintains ∆T

Building Sequence of Operation

! Pump operates at low ∆P only. Speed is modulated to maintain remote ∆P.

Variable Primary Advantages

! Automatic redundant CHW pump! No decoupler and no mixing! No wasted pumping energy! Maintenance for only 1 set of pumps! Floor space for only 1 set of pumps! Capital cost for only 1 set of pumps! Lower installation costs

Variable Primary Disadvantages

! Control sequence depends on flow meters

! More complicated with different size chillers

! Foreign to chiller service personnel! Bypass valve selection is critical

Typical Tower Water System!Most tower water systems are

constant volume!Each chiller is connected to a

pump and a tower!Bypass piping and manual valves!Some method of low temperature

control� Tower bypass valve� Tower fan control� Head pressure valve

Variable Volume Tower Water System

Variable Volume Tower Water System Sequence of Operation

! Chiller isolation valves modulate to maintain refrigerant lift with minimum and maximum flow overrides


!Pumps sequenced based upon flow and head requirements


! Pump speeds modulated to maintain ∆P. Setpoint is reset based on valve positions.


!Towers sequenced based on flow


! Tower fan speeds modulated to maintain tower supply temperature at setpoint. Setpoint is reset based on OSA WB and load.

Variable Volume TW System Advantages

! Automatic redundant pump! Flexibility (any chiller with any pump

with any tower)! Lower cost (less piping)! No tower bypass valve! Simultaneous chiller and HX operation! Lower energy costs

Low Flow Tower Water System

Tower Fan

Condenser Pump

Chiller

1.5 2.0 2.5 3.0

100,000

0

200,000

300,000

400,000

500,000

600,000

Condenser Flow Rate, GPM/Ton

Ener

gy C

onsu

mpt

ion,

kW

h

Low Flow / High ∆T CHW Systems

!Effect of ∆T on CHW capacity!CHW coil performance!Causes of low ∆T!Real system performance!Correcting low ∆T

Distribution System Capacity

02,0004,0006,0008,000

10,00012,000

6 8 10 12 14 16 18 20Nom inal Pipe Size (in)

Flow

Cap

acity

(gpm

)

Flow Rate @10 FPS

Flow Rate @12 FPS

Distribution System Capacity

01,0002,0003,0004,0005,0006,0007,000

2,000 4,000 6,000 8,000 10,000Flow (gpm )

Coo

ling

Cap

acity

(ton

s)

1833 Tons

4400 Tons8 º∆T

10 º∆T

12 º∆T

16 º∆T

CHW Coil Performance

!Converting to low flow / high ∆T by reducing the CHW supply temperature

!Reduced face velocity (VAV)!Higher leaving air temperature

(single zone)

Standard Coil Performance

35

40

45

50

55

60

65

70

75

80

85

1 2

Location

Tem

pera

ture

(ºF) EAT

Air Side

Water Side

dq

LWT1

EWT1

LATQ

High ∆T Coil Performance

35

40

45

50

55

60

65

70

75

80

85

1 2

Location

Tem

pera

ture

(ºF)

EAT

Air Side

Water Side

LWT2

EWT1

LAT

EWT2

LWT1

Reduced CHW Supply Temperature

53

54

55

56

57

45 44 43 42 41 40Entering Water Temperature (F)

Leav

ing

Wat

er T

emp

(F)

Trane LWTCarrier LWTYork LWT

Reduced Face Velocity! When we lower the CFM & face velocity in a fixed coil, the LWT

goes up in order to cool the now decreased amount of air to the same temperature.

52

54

56

58

60

62

12,000 10,000 8,000 6,000 5,000Cubic Feet Per Minute (CFM)

Leav

ing

Wat

er T

emp

(F)


Higher Leaving Air Temperature! When we increase the leaving air dry bulb temperature in a

fixed coil, the LWT goes up in order to cool the same volume of air to a higher temperature.

53545556575859606162

54 55 56 57 58Leaving Air Dry Bulb (F)

Leav

ing

Wat

er T

emp

(F)


Coil Theory vs. Real Life

!Coil theory tells us reducing the load on a CHW coil should increase the CHW return temperature and the ∆T

!What happens in real life?

Real Life (UAMS) ∆T Profile

Load (Tons)

∆T (º

F)14

12

10

8

6

4

2

0

0 1000 2000 3000 4000 5000 6000 7000 8000

Wild Coils!�Wild coils� are the reason why flow

and ∆T do not react to lower supply water temperatures and reduced loads as expected

!A �wild coil� is a coil where flow is not controlled

!Flow in a �wild coil� depends on the available ∆P only and is not a function of load

Causes of Wild Coils And Low ∆T

!No control valve!LAT setpoint too low!Chilled water ∆P is higher than

the control valve close-off rating

!Defective control valve!2 position control valve!Control valve is hunting!Control valve is too large

Causes of Wild Coils And Low ∆T

!Chilled water supply temperature is too high

!Coil is too small!Coil is piped backwards!2-pipe system!Process cooling!Air-side economizer cycles

Coil Performance at Low LAT Setpoints

Based on a six-row 100 fpf coil, 78ºF entering dry-bulb 63ºF entering wet-bulb

Cannot be attained49

409%4.332750

260%6.520851

179%8.514352

130%1110453

100%138054

% of Design GPM

CHW ∆T (ºF)

Flow Rate (GPM)

Leaving Air Temperature Setpoint (ºF)

Control Valve Hunting

0

100

50

60

Time

Valv

e Po

sitio

n (%

)

Leav

ing

Air

Tem

pera

ture

(ºF)

Changed Proportional Gain

Valve Position

Leaving Air Temperature

�Real� System � CHR Temperatures

5051525354555657585960

38 40 42 44 46 48 50CHS (ºF)

CHR

(ºF)

0% Wild

30% Wild

100% Wild

38 40 42 44 46 48 50

�Real� System - Load

30% Wild

0% Wild

100% Wild

130%

100%

115%

Chilled Water Supply Temperature (ºF)

Coo

ling

Load

(% o

f des

ign)

Theoretical Equipment Performance

Chilled Water Temperature (ºF)

Pow

er (%

)

CHW Pumps

Chiller

Total

38 39 40 41 42 43 44 45

100

40

60

20

80

Real Equipment Performance

30% Wild

0% Wild

100% Wild

Chilled Water Supply Temperature (ºF)

Pow

er (%

)

38 39 40 41 42 43 44 45

115%

85%

0%

100%

Identifying and Correcting Low ∆T! Install temperature sensors in major

returns! Reduce chilled water supply temperature! Reduce system ∆P setpoints! Trend return temperatures! Establish return temperature alarms! Identify and correct problem buildings

and coils! Perseverance and commitment

Converting To Low Flow / High ∆T

! Can increase CHW distribution system capacity by as much as 100%

! Can reduce CHW system energy use by 15%

! Cannot be accomplished by simply lowering the CHW supply temperature

! Identifying and correcting wild coils (low ∆T) requires resources and commitment

Series � Series Arrangement

CHR

TWRTWS

CHS

!Evaporates and condenses in series

!Single pass tube bundles

!CHW and TW flows in opposite directions

Low ∆T Parallel � Parallel Water Chiller

85

54 °F

96.5 °F

42.5 °F

44

5295 Condenser

Tem

pera

ture

Evaporator

High ∆T Parallel � Parallel Water Chiller

85

60 °F

96.5 °F

36.5 °F

38

5495 Condenser

Evaporator

Tem

pera

ture

High ∆T Series � Series Water Chillers

Tem

pera

ture

102.5ºF

48.5ºF

54ºF

98.5ºF

44.5ºF

54ºF

94.5ºF

40.5ºF

54ºF

90.5ºF

36.5ºF

54ºF

101

5485

38

97

50

89

42

Condenser

Evaporator46

93

Chiller Power Requirements

Pow

er (k

W/T

on)

0.55

54ºF

Low ∆T Parallel - Parallel

0.61

60ºF

High ∆T Parallel - Parallel

0.55

54ºF

High ∆T Series - Series

Comparison of Parallel � Parallel & Series � Series

0.625

0.650

0.675

0.700

0.725

0.750

0.775

0.800

0.825

8 10 12 14 16 18 20

CHW Temperature Difference (º F)

Syst

em E

ffica

cy (k

W/T

on)

Parallel - Parallel

Series - Series

District Cooling Systems

! Multiple plants serving a campus grid! Primary connected! Secondary connected! Combination of primary/secondary and

variable primary plants! Loop vs. radial distribution! Piping and insulation alternatives

Southern Arkansas University (Phase 1)








SAU District Cooling System

315,900415,440Annual Electricity Cost ($/Year)

1,3501,920Average Peak Demand (kW)

5,508,0006,480,000Annual Electricity Use (kWh)

1,6752,400Chiller Capacity (Tons)

622Number of Chillers

AfterBeforeItem

District Cooling Systems

! Flexible! Infinite distribution capacity ! Shared redundancy! Lower cost than a new central plant! Can be implemented in phases! Reduced peak demands due to diversity! Existing excess chiller capacity can be

used to serve new buildings

Suggested Reading! �The Search for Chiller Efficiency� by William

Landman! �All Variable Speed Centrifugal Chiller Plants� by

Thomas Hartman! �Primary Only vs. Primary-Secondary Variable Flow

Systems� by Steven Taylor! �Achieving High Chilled Water ∆T�s� by Don Fiorino! �Improving the Efficiency of Chilled Water Plants� by

Gil Avery! �Chilled Water System Forensics� by Kenneth Luther! �Degrading Chilled Water Plant ∆T: Causes and

Mitigation� by Steven Taylor! �Series � Series Counterflow for Central Chilled Water

Plants� by Steve Groenke

Other Related Topics

! Interoperable Control Systems! Hybrid (gas and electric) plants! Thermal storage! Cooling tower selection! Refrigerant selection! ARI standard 550 / 590! ASHRAE standard 15! Cooling tower water treatment

Review! The continued search for improved CHW system

performance must look beyond the chiller! Watch the surge line when selecting chillers! Consider demand costs when evaluating

variable speed chillers! Converting to low flow / high ∆T cannot be

achieved by simply lowering the CHS temperature

! 3.0 gpm per ton condenser flow is an outdated paradigm

! Remember the big picture when making decisions

21st Century CHW System

! Variable primary! Variable volume tower water system! Low flow / high ∆T (54 to 38)! Series � Series piping arrangement! Variable speed compressors! 2.0 � 2.5 gpm per ton condenser flow! District cooling system! Hybrid plants and / or thermal storage! Interoperable control systems! 0.625 kW per Ton

For Additional Information or Questions Please Contact:

Ed Tinsley, PE5800 Evergreen Drive, Suite �A�

Little Rock, AR 72205Toll Free: 866.863.2337Phone: 501.666.6776

Fax: 501.663.8888

Documents

Chilled Water Design - NCHEA flow / high ∆T chilled water systems! ... Life Cycle Cost Analysis!Detailed load profile! ... decoupler (from return to