ASHRAE Winter MeetingASHRAE Winter Meeting

January 24, 1999January 24, 1999

Hugh Henderson, Jr., P.E. Hugh Henderson, Jr., P.E. Mukesh K. Khattar, PhD., PEMukesh K. Khattar, PhD., PE

CDH Energy Corp.CDH Energy Corp. EPRIEPRI

www.cdhenergy.comwww.cdhenergy.com www.epri.comwww.epri.com

The Impact of HVAC ControlThe Impact of HVAC ControlImprovements on SupermarketImprovements on SupermarketHumidity LevelsHumidity Levels

PremisePremise

QQ Minor HVAC system control changes canMinor HVAC system control changes cangreatly improve dehumidificationgreatly improve dehumidificationperformance and system efficiencyperformance and system efficiency

QQ Key HVAC operating issues:Key HVAC operating issues:–– supply air flow (cfm/ton & cfm/ftsupply air flow (cfm/ton & cfm/ft22))

–– evaporator temperature controlevaporator temperature control

–– staging and part load controlstaging and part load control

–– supply fan controlsupply fan control

Two Test SupermarketsTwo Test Supermarkets

QQ Full direct digital control (DDC) systemsFull direct digital control (DDC) systemsinstalled to monitor performance andinstalled to monitor performance andimplement control improvementsimplement control improvements–– instrumentation installed to measure HVACinstrumentation installed to measure HVAC

and refrigeration system performanceand refrigeration system performance

QQ Both stores typicalBoth stores typical–– packaged unitary cooling equipmentpackaged unitary cooling equipment

–– refrigeration heat reclaim for space heatingrefrigeration heat reclaim for space heating

Test Store CharacteristicsTest Store Characteristics

Characteristics Store A Store BLocation Minneapolis, MN Indianapolis, INGross Floor Area 33,400 ft2 50,000 ft2

Sales Area(% of gross area)

77% 73%

Operating Schedule 24 hours per day 24 hours per day

HVAC SystemsHVAC Systems

Characteristics Store A Store BType of System Spilt system

50 ton condensing unit, 40 ton air handler

Three Rooftop units60 ton with heat pipes,

30 ton, & 8 ton, two-speed fans

Total Installed Capacity 50 tons(668 ft2/ton)

98 tons(510 ft2/ton)

Supply Air Flow–High Speed

- Low Speed

16,000 cfm(0.6 cfm/ft2)

na

41,000 cfm(0.8 cfm/ft2)24,000 cfm(0.6 cfm/ft2)

Nominal Ventilation Air 3,000 cfm(17 cfm/person)

5,000 cfm(20 cfm/person)

Heating Heat Reclaimw/ Natural Gas Backup

Heat Reclaimw/ Electric Resistance

Backup

Original HVAC ControlsOriginal HVAC Controls

Store A Store BOverall Controls Two stages of cooling, 5

stages of heating.Two stages of cooling, 5stages of heating.

Cooling Controls Thermostat activates 2cooling coil sections (bottom& top section of face splitcoil) with LLSV. Bothcompressors controlled tomaintain suction pressure.

1st stage thermostatactivates one compressor inRTU#1 (60 ton) and RTU#2(30 ton). 2nd stagethermostat activatesremaining compressors.Inter-twined coils.

DehumidificationControls

Humidistat activates 1st

stage cooling.Humidistat activates 1st

stage of cooling (on 60 and30 ton units).

Fan Control na Supply fan jumps to highspeed when any cooling orheating is required

HVAC System - Store AHVAC System - Store A

C

C

C

C

Stage 2Coil

Stage 1Coil

SupplyFans

HeatReclaimCoil

H

R

Gas-FiredDuctHeaters

Fresh Air(3,000 cfm)

Air Handler (40 ton)

MixedAir

Return Air: from Ceiling and Under-CaseReturns (13,000 cfm)

Supply Air

(16,000 cfm)

CondensingUnit (50 ton)

Compressors

Condenser

RefrigerantPiping

Roof

FlowStation

FlowStation

Bypass Damper

Damper

WU

TAORHO

FAO

TAM

TAS

PSUC

FC

TAS2

TAS1

TAS3

SC1SC2

SR1SR2SR3

SG1SG2SG3

TARRHR

HVAC System - Store BHVAC System - Store B

Heat ReclaimCoil

CondenserSectionSupply

Fan

DXCoils

HeatPipes

ReturnAir

Compressors

Outdoor

Air

Filter s

TAORHO

TARFC

WUWF

SF1SF2

TAS1

SR1SR2SR3

TAS

FAS

FAR

2-Speed

HeatPipe

Detail

RTU#1 - 60 ton unit w/ heat pipes

TDXE

TDXL

TAMRHM

Store A - Test ScheduleStore A - Test Schedule

QQ Store monitored “as is” from June 1994 toStore monitored “as is” from June 1994 toMay 1995May 1995

QQ Control improvements implemented in JuneControl improvements implemented in June1995, monitoring continued through1995, monitoring continued throughSeptember 1995September 1995

QQ Compared performance of two periodsCompared performance of two periods

Control Improvements - Store AControl Improvements - Store A

QQ Closer control of evaporator temperature throughCloser control of evaporator temperature throughDDC control (instead of mechanical suctionDDC control (instead of mechanical suctionpressure control).pressure control).

QQ Reduction of coil temperatures inReduction of coil temperatures indehumidification mode.dehumidification mode.

QQ Lower supply air flow rates at night.Lower supply air flow rates at night.

QQ Tighter control of store temperatures in theTighter control of store temperatures in thedehumidification mode (by raising the heating setdehumidification mode (by raising the heating setpoint when reheat is required to limitpoint when reheat is required to limitovercooling).overcooling).

Store A (June 94 - May 95)

10 20 30 40 50 60

Store Relative Humidity (%)

0

200

400

Num

ber

of H

ours

Space Humidity - Store ASpace Humidity - Store A

Space Humidityexceeded 50%RH for only 12hours per year

Impact on Conditions - Store AImpact on Conditions - Store A

0 20 40 60 80 100 120 140

Outdoor Humidity (gr/lb)

10

20

30

40

50

Indo

or R

elat

ive

Hum

dity

(%

)

Orignal Controls (6/94 to 5/95)Improved Controls (6/95 to 9/95)

Also lowerventilation rates inthis period

Impact of Lower Airflow at NightImpact of Lower Airflow at Night

28 30 32 34 36

Store Relative Humidity (%)

0.5

1.0

1.5

2.0

2.5

Late

nt C

apac

ity (

tons

)

Supply Fan @ 60 hzSupply Fan @ 45 hz

Lower flow from midnightto 6 am (low occupancyhours)

Store A - ResultsStore A - Results

QQ Control improvements had little impact onControl improvements had little impact onstore humidity levels because originalstore humidity levels because originalsystem already provided good control:system already provided good control:–– low supply air flow rate: 0.6 cfm per ftlow supply air flow rate: 0.6 cfm per ft22

–– independent suction pressure control and coilindependent suction pressure control and coilstaging consistently maintained cold suctionstaging consistently maintained cold suctiontemperaturestemperatures

QQ Several “problems” with the original systemSeveral “problems” with the original systemcontrols actually improved dehumidificationcontrols actually improved dehumidification

Store B - Test ScheduleStore B - Test Schedule

QQ Monitored original system starting in MayMonitored original system starting in May19951995

QQ Implemented improved controls in mid-Implemented improved controls in mid-August 1995August 1995

QQ Continued monitoring into 1996Continued monitoring into 1996

Control Improvements - Store BControl Improvements - Store B

QQ Operate supply fans at low speed (instead of highOperate supply fans at low speed (instead of highspeed) for first stage cooling.speed) for first stage cooling.

QQ Decouple operation of the two main RTUs…..Decouple operation of the two main RTUs…..Operate heat pipe assisted RTU for first stage ofOperate heat pipe assisted RTU for first stage ofcooling and for dehumidification.cooling and for dehumidification.

QQ Tighter control of store temperatures in theTighter control of store temperatures in thedehumidification mode (by raising the heating setdehumidification mode (by raising the heating setpoint when reheat is required to limitpoint when reheat is required to limitovercooling).overcooling).

Same asStore A

Heat Pipe PerformanceHeat Pipe Performance

Top Coil

35 40 45 50 55 60 65 70 75 80 85

Dry Bulb Temperature (F)

0

25

50

75

100

Hu

mid

ity

Rat

io (

gr/

lb)

Heat P

ipe

In

DX C

oil In

DX C

o il Out

Heat P

ipe O

ut

Heat Pipe delta-T: 10.9 FSens ible Heat Ratio: 0.653

Top C oil Flow (c fm): 6859

With low-speedfan operation at 1st

stage!

Impact of Improvements -Store BImpact of Improvements -Store B

Store B - Indianapolis

1 8 15 22 29 5 12 19 26 3 10 17 24 31 7 14 21 28 4 11 18 25

May June July August September

1995

20

30

40

50

60

70

Sto

re R

elat

ive

Hum

dity

(%

)

Dehumidification Set Point

ControlImprovementsImplemented

Store B - ResultsStore B - Results

QQ Low speed fan operation with first stageLow speed fan operation with first stagecooling dramatically improvedcooling dramatically improveddehumidification performancedehumidification performance–– allowed heat pipes to work effectively onallowed heat pipes to work effectively on

intertwined DX coilsintertwined DX coils

QQ Putting cooling priority on heat pipe RTUPutting cooling priority on heat pipe RTUincreased “passive” dehumidificationincreased “passive” dehumidification

QQ Active dehumidification improvements (i.e.,Active dehumidification improvements (i.e.,reheat control) were not ever requiredreheat control) were not ever required

Comparing PerformanceComparing Performance

0 50 100

Daily Avg Outdoor Humid ity (gr/lb)

10

20

30

40

50

60

Dai

ly A

vg I

ndoo

r R

elat

ive

Hum

idity

(%

)

Base Case HVAC (hypothetical)Orig inal Contro lsImproved Contro ls

Net Impact of Heat Pipe ACNet Impact of Heat Pipe AC

QQ Heat Pipes (with Improved Controls)Heat Pipes (with Improved Controls)reduced space humidity….. And decreasedreduced space humidity….. And decreasedrefrigeration energy use by 33,000 kWh/yrrefrigeration energy use by 33,000 kWh/yr

QQ Penalty of 3,000 kWh/yr due to extra latentPenalty of 3,000 kWh/yr due to extra latentload and increased fan powerload and increased fan power

QQ NET IMPACT: 30,000 kWh/yr, or $1,000NET IMPACT: 30,000 kWh/yr, or $1,000per yearper year

Key ImprovementsKey Improvements

QQ Low Supply Air Flow (cfm/ftLow Supply Air Flow (cfm/ft2 2 & cfm/ton)& cfm/ton)–– lowering first stage air flow (from 700 to 420lowering first stage air flow (from 700 to 420

cfm per ton) greatly improved dehumidificationcfm per ton) greatly improved dehumidificationperformance at Store Bperformance at Store B

–– low air flow in Store A (0.6 cfm/ftlow air flow in Store A (0.6 cfm/ft22) helped to) helped toprovide good dehumidification performanceprovide good dehumidification performance

QQ Continuously maintain low suctionContinuously maintain low suctiontemperaturestemperatures–– independently control suction pressure and coilindependently control suction pressure and coil

stagingstaging

Key Improvements (cont.)Key Improvements (cont.)

QQ Put priority on enhanced dehumidificationPut priority on enhanced dehumidificationequipmentequipment–– used heat-pipe-assisted unit as base cooling unitused heat-pipe-assisted unit as base cooling unit

as fist stage cooling in Store Bas fist stage cooling in Store B

QQ “Face-Split” cooling coil ensured that full“Face-Split” cooling coil ensured that fulldehumidification was provided at part loaddehumidification was provided at part loadconditionsconditions

ConclusionsConclusions

QQ Seemly minor control improvements canSeemly minor control improvements canhave a big impacthave a big impact

QQ Controls “tuning” can make a “simple”Controls “tuning” can make a “simple”HVAC system very effectiveHVAC system very effective

QQ Enhanced technologies (such as heat pipeEnhanced technologies (such as heat pipeAC) can also benefit from controls tuningAC) can also benefit from controls tuning

QQ Proper supply air flow is the single mostProper supply air flow is the single mostimportant factorimportant factor