Mechanical Engineering Department - Louisiana State University

HVAC Systems’ Primary Functions

Temperature ControlCooling

Heating

Humidity ControlHumidification

Dehumidification

Air Quality ControlVentilation

Cleaning

Mechanical Engineering Department

HVAC Systems –Energy Considerations

Typical Energy Use in a Commercial Building


HVAC Systems

Effi i t U f E R i tEfficient Use of Energy Requirements

Optimum Energy Designs

Well-Developed Energy Use PoliciesWell Developed Energy Use Policies

Dedicated Management backed up byProperly Trained and Motivated Operating StaffProperly Trained and Motivated Operating Staff


HVAC Systems

Mi i G id li i EMinimum Guidelines in Energy Conservation, Design and Operation

ASHRAE Standard 90.1-2004, “Energy Efficient Design of New Buildings Except Low-Rise Residential Buildings”

ASHRAE Standard 100-1995, “Energy Conservation in Existing Buildings”


HVAC Systems

T i l B ildi D i H t L G i


Typical Building Design Heat Losses or Gains

HVAC Systems

Some Relevant Energy/Emissions StatisticsBuildings account for:

39% of all the energy (36%)71 % of all the electricity (66%)

Emissions related to building energy use account for

12% of water consumptionused in the US

Emissions related to building energy use account for40% of non-industrial waste38% of CO2 emissions (35%)

>47 % of SO2 emissions>47 % of SO2 emissions>22% of NOx emissions


HVAC Systems

Annual Energy Use Per Unit Floor Area


HVAC Systems

10 years ago

Capital Cost Estimating Factors


BuildingBuilding Costs

Energy CostsEnergy Costs


Schematic of a Typical Commercial Air-Conditioning SystemHVAC Systems


Elementary Air Temperature Control System

HVAC SystemsElementary Air Temperature Control System


Air Handler and Associated Controls for Simple Constant-Volume, HVAC Systems

Single-Duct All-Air System


Schematic of a Blow-Through Air Handler With Hot and Cold Decks HVAC Systems

and Zone Dampers


Simplified Control Schematic for a Constant-Volume reheat SystemHVAC Systems

Simplified Control Schematic for a Constant-Volume reheat System


HVAC Systems

Si lifi d C t l S h ti f Si l D t VAV S t


Simplified Control Schematic of a Single-Duct VAV System

HVAC Systems


Simplified Control Schematic of a Dual-Duct System

HVAC Systems

Multi-Zone System With Hot and Cold Plenum Reset


HVAC SystemsAir-Water Induction Unit

Typically installed at perimeter wall under

From Central A/C Unit

perimeter wall under window or overhead

Hot or Chilled Water


HVAC SystemsT i l F C il U it

Chilled Water or Brine/ Hot Water or Steam or

Typical Fan-Coil Unit Electric HeaterAir-Conditioned Air


Recycles Room Air, Cheapest Perimeter System, Ventilation Provided Separately

HVAC Systems

Typical Air-Conditioning Ventilator with Separate Coils


HVAC SystemsSchematic View of a Room Air-Conditioner


Residential Cooling and Heating LoadsDistinguishing features from other buildingsDistinguishing features from other buildings

Smaller Internal Heat Gainsheat gain or loss through structural components g g pair leakage or ventilationsmall internal heat gains (occupants , lights)

Varied Use of SpacesVaried Use of Spacesflexible localized or temporary temperature excursions tolerable

Fewer ZonesFewer Zones.single or few zones - one thermostatCapacity cannot be redistributed as loads change over day

Greater Distribution LossesGreater Distribution Losses. ducts are installed in unconditioned buffer spacesrequire significant increase in unit capacitydi t ib ti i /l t b l t d


distribution gains/losses cannot be neglected

Residential Cooling and Heating LoadsDistinguishing features from other buildingsDistinguishing features from other buildings

Partial Loadssystems use units of small capacity

~ 12,000 to 60,000 Btu/h cooling~ 40,000 to 120,000 Btu/h heating

units mostly operate at partial loadoversized units are bad for system performance(especially for cooling in areas of high WBT)

Dehumidification Issuesdehumidification only when cooling unit operatesspace condition control is driven by room thermostats

(sensible heat actuated)excessive sensible capacity leads to short-cycling

and degraded dehumidification


Residential Cooling and Heating LoadsClassification based on load profilesClassification based on load profiles

Single-Family DetachedExposed Walls in four directionsSi l hSingle zone – one thermostatTwo-story houses may have separate cooling systems per floor

MultifamilyExposed Walls not in four directions (east/west exposure plays a role)

Other


Residential Cooling and Heating LoadsApproachesApproaches

HeatingNo solar or internal gains and no heat storage

l d iHeat losses assumed instantaneousCooling

Need to take account of temperature swing via empirical data and modelsNon-residential methods lead to unrealistically high loadsUse Residential Load Factor (RLF) method

From detailed residential heat balance (RHB) of prototyped buildings f liover a range of climates.


Residential Heating Load Considerations

No solar, internal gains, heat storage (highest load during early am)during early am)

Heat losses assumed instantaneousCalculated for a “normal worst case” conditionCalculated for a normal worst case condition

(indoor/outdoor design conditions, ventilation/infiltration)Estimate maximum probable heat loss per room

Transmission Losses (walls, floor, roof/ceiling, fenestration/doors)

Infiltration & Ventilation

If night thermostat set-back is used, may need excess capacity .


Residential Heating Load ProcedureOutdoor design condition (temp., wind speed and dir.)Indoor design condition (temp., humidity level)T f dj t diti d G d t ifTemps. of adjacent unconditioned spaces; Ground temp. if below gradeEstimate overall heat transfer coefficients for every b d l tboundary elementEstimate area of each boundary elementCompute heat transmission losses (Table 6-17)Estimate infiltration and compute associated energyEstimate required ventilation and compute associated energyCalculate the total heating loadEstimate “pickup” loads for intermittently heated buildings or thermostat set-back.


Residential Heating Load Equations

Heating Load Factor


Heating Load Factor

Residential Heating LoadBelow-Grade SurfacesBelow Grade Surfaces

Mechanical Engineering Department Heating Load Factor

Residential Heating LoadBelow-Grade Surfaces – Basement Walls


Residential Heating LoadBelow-Grade Surfaces – Basement FloorsBelow-Grade Surfaces – Basement Floors


Residential Heating LoadOn-Grade Surfaces

Concrete Slab FloorsUnheatedHeated

Mechanical Engineering Department Heating Load Factor

Residential Heating LoadOn-Grade Surfaces – Heat Loss CoefficientOn-Grade Surfaces – Heat Loss Coefficient


Residential Heating LoadInfiltration Heat LossesInfiltration Heat Losses

Sensible

L t tLatent


On InfiltrationEstimation Methods

Air Change MethodSimpleHighly Empirical (performance of similar construction)


On InfiltrationEstimation MethodsEstimation Methods

Crack MethodCrack MethodRequires estimation of indoor-outdoor pressure differences

Wind EffectWind EffectStack EffectPressurization

Requires estimation of building envelope permeability and associated crack characteristics.


On InfiltrationEstimation MethodsEstimation Methods

Estimation based on Effective Leakage Area


Design ConditionsOutdoor – Weather DataOutdoor – Weather Data

Figure 4-4 – Climatic Design InformationTable 4-7 – Design Conditions by locationTable 4 7 Design Conditions by locationInformation is provided on two levels

Annual seasonal meansMonthly means (to include seasonal variation)

Often data is given in association with percentilesWarm Season – 0.4, 1, 2 annual percentilesCold Season – 99.6, 99 annual percentiles0 4 2 5 10 thl til0.4, 2, 5, 10 monthly percentiles“Variable value at n%” means that the value is

equaled or exceeded n% of the time.


equa ed o e ceeded % o t e t e.

Design ConditionsOutdoorOutdoor

Annual Heating and Humidification Design Conditions

Annual Cooling, Dehumidification, and Enthalpy Design Conditions

Extreme Annual Design Conditions

Monthly Design Conditions

Temperatures, Degree-Days, and Degree-Hours

Monthly Design Dry-Bulb, Wet-Bulb, and Mean Coincident Temperatures

Mean Daily Temperature Range

Cl Sk S l I diClear-Sky Solar Irradiance


Design ConditionsAnnual Heating and HumidificationAnnual Heating and Humidification

Coldest Month (1=January) Maximum i d

( y)Heating, 99.6% and 99%: Dry-Bulb Temperature (DB)Humidification, 99.6% and 99%:

Dew Point (DP)

Heating LoadTo Size

EquipmentFor HumidificationDew Point (DP)

Humidity Ratio (HR)Mean Coincident Dry Bulb Temperature (MCDB)

Coldest Month 0 4% 1%:

For Humidification Decisions

Coldest Month, 0.4%, 1%:Wind Speed (WS) - mphMean Coincident Dry Bulb Temperature (MCDB)

F th 99 6% DB l

Peak Loads accounting for

InfiltrationFor the 99.6% DB value Mean Coincident Wind Speed (MCWS) - mphPrevailing Coincident Wind Direction (PCWD)

Infiltration


Design ConditionsAnnual Cooling, Dehumidification, and Enthalpyg, , py

Hottest Month (1=January)DB Range

Time that Maximum Sensible Cooling Load occursg

Cooling, 0.4%, 1%, 2%:DB and Mean Coincident Wet Bulb Temperature (MCDB)

For Cooling Load

DB and Mean Coincident Wet Bulb Temperature (MCDB)

Evaporation, 0.4%, 1%, 2%:Wet Bulb Temperature (WB) and MCDB

Sizing Chillers &Air-Conditioners

Wet Bulb Temperature (WB) and MCDB

For the 0.4 % DB value MCWS d PCWD

Design of Cooling Towers, Evaporative Coolers, Fresh-Air

MCWS and PCWD Ventilation Systems

Estimates of Peak Loads accounting for Infiltration


Design ConditionsAnnual Cooling, Dehumidification, and Enthalpyg, , py

H idit C t l A li ti

Dehumidification, 0.4%, 1%, 2%:

Humidity Control ApplicationsDesiccant Dehumidification,

Cooling-based Dehumidification

DP, HR, MCDB

Enthalpy, 0.4%, 1%, 2%:

Fresh-Air Ventilation SystemsSystem Analysis at Partial-Load Conditions

Useful for Cooling Load relatedpyEnthalpy (Btu/lb) and MCDB

Number of Hours between 8am and 4pm with 55F<DB<69F

Useful for Cooling Load related to Infiltration and/or Ventilation

Number of Hours between 8am and 4pm with 55F DB 69F


Design ConditionsExtreme Annual Design ConditionsExtreme Annual Design Conditions

Used for Smoke Management S stemsExtreme Annual WS, 1%, 2.5%, 5%

Extreme Maximum WBDB and Mean Coincident Wet Bulb Temperature (MCDB)

Management Systems

Extreme Annual DBMean & Standard Deviation

Minima & Maximan-Year Return Period Values of Extreme DB

n=5, 10, 20, 50 yearsMinima & MaximaMinima & Maxima


Design ConditionsMonthly Design Conditionsy g

Annual and Monthly Data

Average Temperatures (Tavg) and associated Standard Deviations (Sd)

Heating Degree Days, for 50F (HDD50) and 65 F (HDD65) bases

Cooling

Degree Days , for 50F (CDD50) and 65 F (CDD65) bases

Degree Hours, for 74F (CDH74) and 80F (CDD80) bases

Used for EnergyUsed for Energy Estimation


Design ConditionsMonthly Design Conditionsy g

Annual and Monthly Data

DB and MCWB; at 0.4%, 2%, 5%, 10%

WB and MCDB; at 0.4%, 2%, 5%, 10%

Mean Daily Temperature Range:

Mean Dry Bulb Range (MDBR)

Mean Coincident Dry Bulb Range (MCDBR) and Wet Bulb Range (MCWBR) for 5% DB and 5% WBRange (MCWBR) for 5% DB and 5% WB

Clear Sky Solar Irradiance

Beam (taub) & Diffuse (taud) Irradiance Optical DepthsBeam (taub) & Diffuse (taud) Irradiance Optical Depths

Beam Normal (Ebn, noon) & Diffuse Horizontal (Edh, noon) Irradiances at Solar Noon


Design ConditionsCommentsComments

Design values based on DB temperature relate to peak ibl dsensible outdoor component

Design values based on WB temperature relate to enthalpy of outdoor air

Conditions based on DP relate to peaks of humidity ratio

Designer must decide which set(s) of conditions and probability of occurrence (expressed by the percentiles) apply to the design situation in hand.


Design ConditionsComments - HeatingComments - Heating

Minimum Temperatures usually occur between solar 6:00am-8:00am

For continuous occupancy the recommendedFor continuous occupancy the recommended DB design temperatures should be used

For occupancy predominantly during the middle of the day may use DB temperatures above the recommended minimum


Design ConditionsComments - Coolingg

Maximum Temperatures usually occur between solar 2:00pm-4:00pm

Design DB and MCWB temperatures should be used for building cooling loads

For contin o s occ panc the recommended designFor continuous occupancy the recommended design temperatures should be used

For occupancy predominantly during the middle of the day may use temperatures below the recommended maximum

Peak occupancy load may occur before the effect of the maximum temperature is feltp

Peak occupancy load may occur during months other than the ones during which the maximum temperature is expected.


Indoor Design ConditionsComfort and HealthComfort and Health

Physiological Principles

Perception of the Environment – often subjective

Sick Building Syndrome (SBS)

O i i t l b iOrigin not always obvious

Irritation of mucus membranes, fatigue, headache, lower resp. symptoms, nausea, nosebleeds, chest tightness, fever

Building-Related Illness (BRI)

K i iKnown origins

Bioaerosols – humidifier fever, asthma, allergies


Indoor Design ConditionsComfort and Health

C i Ai Q li

Comfort and HealthPhysiological Principles

Contaminants – Air Quality

Body Temperature

Internal 98 6+/ 1FInternal 98.6+/-1F

Skin Temperature varies 88F to 96.8F under normal conditions –91.5F +/-2.5F typically for comfort.

Moisture/Humidity level

Static Electricity

Prevention and Treatment of Disease (50% highest mortality of some organisms)

Mold and bacteria growth due to Visible and Concealed condensation


Mold and bacteria growth due to Visible and Concealed condensation

Indoor Design ConditionsBody’s Interaction with the Environmenty


Indoor Design ConditionsEnvironmental Indices


Indoor Design ConditionsMean Radiant Temperaturep

Definition

Simplification


Indoor Design ConditionsOperative Temperaturep p

Definition

Simplification

t0 is the operative temperature

ta is the operative temperature


a p p

Indoor Design ConditionsComfort Chart


Indoor Design ConditionsOther Factors and Conditions

Metabolic Rate

Clothing Level


Documents

Mechanical Engineering Department - Louisiana State University