hvac psychrometry and concepts
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PRESENTED BY:AISHWARYA DEOPUJARI
PRERANA DASNISHTHA DUGGAL
VASUNDHRA SINGHSRIDEVI
SECTION-B6TH SEMESTER
ContentsBasic ConceptsPsychrometryOutdoor Design ConditionsIndoor Design CriteriaCooling Load PrinciplesCooling Load ComponentsHeating Load
Basic ConceptsThermal load
The amount of heat that must be added or removed from the space to maintain the proper temperature in the space
When thermal loads push conditions outsider of the comfort range, HVAC systems are used to bring the thermal conditions back to comfort conditions
PSYCHROMETRY
What is PSYCHROMETRY
The field of engineering concerned with the determination of physical and
thermodynamic properties of gas-vapor mixtures.
Study of various properties of air, method of controlling its temperature and
moisture content or humidity and its effect on various materials and human
beings.
Helps in understanding different constituents of air and how they affect each
other.
Air (ordinary) = mixture of various gases + water vapor or moisture.
Air without any water vapor - dry air (ideal condition, not possible)
Composition of air:Nitrogen (78%), Oxygen (21%)Others (1%) – like carbon dioxide, hydrogen,
helium, neon, and argon along with water vapor.
State Point
Air Properties Dry-bulb temperature, which is usually referred to as simply air temperature, is the air property that is most familiar. Dry-bulb temperature, Tdb, can be measured using a standard thermometer or more sophisticated sensors. This temperature is an indicator of heat content and is shown along the bottom axis of the psychrometric chart. The vertical lines extending upward from this axis are constant-temperature lines.
Wet-bulb temperature, Twb, represents how much moisture the air can evaporate. This temperature is often measured with a common mercury thermometer that has the bulb covered with a water-moistened wick and with a known air velocity passing over the wick. On the chart, the wet-bulb lines slope a little upward to the left, and this temperature is read at the saturation line.
Relative humidity, RH, is the ratio of the actual water vapor pressure, Pv, to the vapor pressure of saturated air at the same temperature, Pvs, expressed as a percentage.
Relative humidity is a relative measure, because the moisture-holding capacity of air increases as air is warmed. In practice, relative humidity indicates the moisture level of the air compared to the airs moisture-holding capacity.
Relative humidity lines are shown on the chart as curved lines that move upward to the left in 10% increments. The line representing saturated air (RH = 100%) is the uppermost curved line on the chart.
Dewpoint, Tdp, is the temperature at which water vapor starts to condense out of air that is cooling. Above this temperature, the moisture stays in the air.
This temperature is read by following a horizontal line from the state-point (found earlier) to the saturation line.
Humidity ratio, w, is the dry-basis moisture content of air expressed as the weight of water vapor per unit weight of dry air.
Humidity ratio is indicated along the right-hand axis of a psychrometric chart.
Specific volume represents the space occupied by a unit weight of dry air, in ft3/lb, and is equal to 1/air density. Specific volume is shown along the bottom axis of a psychrometric chart, with constant-volume lines slanting upward to the left.
Enthalpy, h, is the measure of airs energy content per unit weight (Btu/lbda). Wet-bulb temperature and enthalpy are related intuitively. So, enthalpy is read from where the appropriate wet-bulb line crosses the diagonal scale above the saturation curve.
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Humidity in airRelative
HumidityA measure of of
much water is in the air relative to the maximum amount air can hol at that tmperature
EGEE 102 - Pisupati 17http://www.ae.iastate.edu/Ast473/Lectures/%285%29Psychrometric_Chart/sld024.htm
BASIC FACTORS THAT AFFECT HUMAN COMFORT IN THE INTERNAL ENVIRONMENT-
THERMAL COMFORT
Thermal and air qualityWhat affects the surroundings you live in?Air quality is affected by how hot it is outside or
inside your environmentWhat is humidity and what affects humidity?The amount of moisture that is present within
the air will have an effect on humidity, which is linked to the amount of ventilation entering
What is the normal temperature of a human being?
Human temperature maintain an average core temperature of 37º depending on the metabolic rate
Nature of heat• What is the measure of temperature• Temperature is measured in degrees celsius • The lower is 0 fixed at a melting point of ice at a
stand at atmospheric pressure of 101.32kN/m2• The upper point is 100 degrees – temperature of
steam above the boiling point• What is the acceptable value of temperature
taken at normal design?• Normal design temperature are taken at 21
degrees inside and -1 degrees outside on average
Thermodynamic temperature scale• This is another measure of temperature in
degrees Kelvin• 0 degree celsius= 273.16 Kelvin (K)• 100 degree celsius = 317.16 Kelvin• The unit of thermodynamic temperature is the
fraction of the thermodynamic temperature at the triple point water
• (equilibrium point of the temperature and pressure at which three known phases of substance can exist i.e. liquid, water vapour and pure ice)
Quantity of heat
How do we measure the quantity of heat?Heat is measured in joules (J) which is a
measure of work doneThe rate of expenditure of energy or
doing work or of heat loss is measured in watts (W)
1 watt is = 1 Joule per second1 W =1 J/s
Heat transferName three ways heat is transferred from
one mass to another, for instance a person sitting next to a radiator.
ConductionConvectionRadiation
Thermal comfortIn high activity the temperature rises and the
more heat you will give off. Several factors influences the level heat is generated (metabolic rate) including:
Your surface areaAgeGenderLevel of activitye.g. Sleeping heat output 70W. Lifting 440W.
Typical heat output of an adult male
Activity Example Heat output
Immobile Sleeping 70W
Seated Watching TV 115W
Light work Office 140W
Medium work Factory Work 265W
Heavy work Lifting 440W
ClothingThe amount of clothing that we wear
generally depends on the season and affects our thermal comfort
Clothing is measured in a scale called clo value
1 clo= 0.155m2 K/W of insulation to the bodyTypical values vary from 1-4 clo
Typical clothing values
Clo value Clothing Typical comfort temperature when sitting
0 clo Swimwear 29ºC
0.5 clo Light clothing 25ºC
1 clo Suit , jumper 22ºC
2 clo Coat, gloves, hat 14ºC
Heat losses from buildingsComfortable temperature for humans is
provided by balancing the heat lost through conduction and ventilation through the fabric with similar heat
Optimum temperature will depend on material used , type of construction, orientation of the building and degree of exposure to the rain and wind
Room temperaturesWhat would you consider in design to maintain
temperature in buildings?The resistance of a material to the passage of
heat and the thermal conductivity of the material in passing the heat along are the basics of understanding of maintaining a steady temperature and a comfortable thermal indoor environment
In order to maintain a comfortable room temperature the building must be provided with as much heat as is lost through ventilation
What will the loss of heat in buildings depend on?Materials usedType of constructionOrientation of the building in relation to the
sunDegree of exposure to rain and wind
Thermal conductivity (k)The amount of heat loss in one second
through 1m2 of material, whose thickness is 1 metre
The units are W/mK (watts per metre Kelvin)
K-Values
Material K Value (W/mK)
Brickwork (internal/exposed) (1700kg/m3) 0.84
Concrete, dense (2100kg/m3) 1.40
Concrete, lightweight (1200kg/m3) 0.38
Plaster, dense 0.50
Rendering 0.50
Concrete block, medium, weight (1400kg/m3)
0.51
Concrete block, lightweight (600kg/m3) 0.19
Thermal resistivity (r)Thermal resistivity is the reciprocal of
thermal conductivity:R=1/K
Air movementProperties are tested for airtightnessDraught seals are fitted to all openings
to restrict thermal lossesIf warmer air enter a room is not mixed
with cooler air the room becomes hotter near the ceiling and colder at floor level
Humidity & VentilationHumidity- the amount of water or moisture in
the air measured in grams per cubic metre(g/m3)
Relative Humidity or percentage saturation
This the percentage saturation Actual amount of water vapour/maximum
amount of water vapour that can be held X 100% of the temperature
RELATIVE HUMIDITYHumans are used to a relative humidity of
between 40 and 60%. Greater than this we start to describe air as being ‘Humid’.
HEAT LOSS DUE TO VENTILATIONNatural ventilation leads to the complete
volume of air in a room changing a certain number of times in one hour
Type of room Air changes in hrHalls 1.0Bedrooms /lounges 1.5WCs and bathrooms 2.0
HEAT LOSS DUE TO VENTILATIONThe fresh air entering the room will need to be
heated to the internal temperature of the room. This is calculated with the formula:
Volume of room x air change rate x volumetric specific heat for air x temperature difference
The volumetric specific heat for air is approximately 1300j/m3K and is considered a constant in this formula which will give an answer in joules per hour.
This then has to be converted into watts in order to find the rate of heat loss which is achieved by dividing the number of joules by the number of seconds in one hour
Heat loss to ventilationThis then has to be converted into watts in order to
find the rate of heat loss which is achieved by dividing the number of joules by the number of seconds in one hour
Volume of room/building x air changes hr x 1300J x Temperature difference / 3600s = Watts
It is convenient when carrying out heat loss calculations to assume an average internal temperature of 19°C minus average of -1°C in winter which gives 20°C difference between inside and outside temperatures
Theory into practiceCalculate the rate of heat loss due to
ventilation for the building measuring 4.5m x 3.25 in plan and has a ceiling height of 2.6m. The number of air changes in one hour is 1.35. The outside temperature is 6°C and the inside temperature is 19°C.
Calculation{(4.5x3.25x2.6)m3 x 1.35 x 1300J x (19-6)°}/
3600s
240.983 Watts
Theory into practiceA domestic semi-detached dwelling is subject
to 1.5 changes per hour. Calculate the total heat loss due to ventilation. In this example we have removed the circulation space which is uninhabited.
Room DimensionsLounge is 3.5m x 3.5mKitchen/diner is 4.0m x 2.5mBedroom 1 is 3.0m x 3.0mBedroom 2 is 2.75m x 2.75mBathroom 3 is 2.5m x 2mStorey height is 2.4mAir changes for all rooms 1.5 per hourTemperature difference -1°C outside, 19°C
inside.
CalculationLounge 3.5 x 3.5 x 2.4 =29.4Kitchen 4.0 x 2.5 x 2.4 =24.0Bedroom One 3.0 x 3.0 x 2.4 =21.6Bedroom Two 2.75 x 2.75 x 2.4 =18.15Bathroom Three 2.5 x 2.0 x 2.4 =12.0
Total volume = 105.91m3
Calculation
condensationThis is formed when hot , humid air meets a cold
surface, it condenses onto this surface forming droplets of water vapour.
What are the effects of condensation in the internal environment?
Cause timber rotEncourage mould growthProduce cold spotsProduce high humidityCause corrosion to steelworkDampen insulation, reducung its effectiveness
Heat flow through a structure
Acceptable values The acceptable values of heat loss or U-values
is a complicated topic and you will need to refer to the Building regulations Part L Conservation of fuel and power for guidance on the acceptable U- values.
Ventilation is linked to the Building Regulation Part L that it restricts air tightness of modern structure. Forced ventilation has to be provided in form of fans in bathrooms and cooking areas
Thermal conductivity (k)The amount of heat loss in one second through
1m2 of material, whose thickness is 1 metreThe units are W/mK (watts per metre Kelvin)
P= kA (T1-T2)/ x
A= AreaX= thickness in m² and m respectivelyT1-T2= temperature difference in °C or K Which can be written as follows
W=k x m² x °C/m ; k = W x m/(m² x °C) = W/m°C or W/mK
U-ValuesA measurement of the rate of heat loss
through a structureThermal resistivity is the reciprocal of
thermal conductivity:R=1/K
PRINCIPLES OF AIR COOLING
Principle
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A. Expansion ValveB. Compressor
Arrangement
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TYPES OF AIR CONDITIONERS Room air conditioners Central air conditioning systems Heat pumps Evaporative coolers
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Air Conditioning
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Room air conditionerRoom air conditioners cool rooms rather
than the entire home. Less expensive to operate than central
unitsTheir efficiency is generally lower than
that of central air conditioners.Can be plugged into any 15- or 20-amp,
115-volt household circuit that is not shared with any other major appliances
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Central Air conditioningCirculate cool air through a system of
supply and return ducts. Supply ducts and registers (i.e., openings in the walls, floors, or ceilings covered by grills) carry cooled air from the air conditioner to the home.
This cooled air becomes warmer as it circulates through the home; then it flows back to the central air conditioner through return ducts and registers
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Types of Central ACsplit-system
an outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator
Packagedthe evaporator, condenser, and
compressor are all located in one cabinet
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Large air conditioning systems Outside air is drawn in,
filtered and heated before it passes through the main air conditioning devices. The colored lines in the lower part of the diagram show the changes of temperature and of water vapor concentration (not RH) as the air flows through the system.
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Total Air Conditioning
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Variable fresh air mixer and dust and pollutant filtration.
Supplementary heating with radiators in the outer rooms and individual mini heater and
Humidifier in the air stream to each room.
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Sizing Air Conditioners
how large your home is and how many windows it has;
how much shade is on your home's windows, walls, and roof;
how much insulation is in your home's ceiling and walls;
how much air leaks into your home from the outside; and
how much heat the occupants and appliances in your home generate
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Energy ConsumptionAir conditioners are rated by the number
of British Thermal Units (Btu) of heat they can remove per hour. Another common rating term for air conditioning size is the "ton," which is 12,000 Btu per hour.
Room air conditioners range from 5,500 Btu per hour to 14,000 Btu per hour.
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Energy EfficiencyToday's best air conditioners use 30% to 50%
less energy than 1970sEven if your air conditioner is only 10 years
old, you may save 20% to 40% of your cooling energy costs by replacing it with a newer, more efficient model
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Energy EfficiencyRating is based on how many Btu per hour
are removed for each watt of power it draws
For room air conditioners, this efficiency rating is the Energy Efficiency Ratio, or EER
For central air conditioners, it is the Seasonal Energy Efficiency Ratio, or SEER
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Room Air ConditionersBuilt after January 1, 1990, need have an
EER of 8.0 or greater EER of at least 9.0 if you live in a mild climate EER over 10 for warmer climates
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Central ACNational minimum standards for central air
conditioners require a SEER of 9.7 for single-package and 10.0 for split-systemsUnits are available with SEERs reaching nearly
17
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Energy Saving MethodsLocate the air conditioner in a window or
wall area near the center of the room and on the shadiest side of the house.
Minimize air leakage by fitting the room air conditioner snugly into its opening and sealing gaps with a foam weather stripping material.
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Basic ConceptsPurpose of HVAC load estimation
Calculate peak design loads (cooling/heating)Estimate likely plant/equipment capacity or
sizeProvide info for HVAC design e.g. load profilesForm the basis for building energy analysis
Cooling load is our main targetImportant for warm climates & summer designAffect building performance & its first cost
Basic ConceptsHeat transfer mechanism
ConductionConvectionRadiation
Thermal properties of building materialsOverall thermal transmittance (U-value)Thermal conductivityThermal capacity (specific heat)
Basic ConceptsA building survey will help us achieve a
realistic estimate of thermal loadsOrientation of the buildingUse of spacesPhysical dimensions of spacesCeiling heightColumns and beamsConstruction materialsSurrounding conditionsWindows, doors, stairways
Basic ConceptsBuilding survey (cont’d)
People (number or density, duration of occupancy, nature of activity)
Lighting (W/m2, type)Appliances (wattage, location, usage)Ventilation (criteria, requirements)Thermal storage (if any)Continuous or intermittent operation
Outdoor Design Conditions
They are used to calculate design space loads
Climatic design informationGeneral info: e.g. latitude, longitude, altitude,
atm. pressureOutdoor design conditions
Derived from statistical analysis of weather data Typical data can be found in handbooks/databooks,
such as ASHRAE Fundamentals Handbooks
Outdoor Design Conditions
Climatic design conditions from ASHRAEPrevious data & method (before 1997)
For Summer (Jun. to Sep.) & Winter (Dec, Jan, Feb) Based on 1%, 2.5% & 5% nos. hours of occurrence
New method (ASHRAE Fundamentals 2001): Based on annual percentiles and cumulative
frequency of occurrence, e.g. 0.4%, 1%, 2% More info on coincident conditions Findings obtained from ASHRAE research projects
Data can be found on a relevant CD-ROM
Outdoor Design Conditions
Climatic design conditions (ASHRAE 2001):Heating and wind design conditions
Heating dry-bulb (DB) temp. Extreme wind speed Coldest month wind speed (WS) & mean coincident
dry-bulb temp. (MDB) Mean wind speed (MWS) & prevailing wind
direction (PWD) to DB Average of annual extreme max. & min. DB temp. &
standard deviations
Outdoor Design Conditions
Climatic design conditions (ASHRAE):Cooling and dehumidification design conditions
Cooling DB/MWB: Dry-bulb temp. (DB) + Mean coincident wet-bulb temp. (MWB)
Evaporation WB/MDB: Web-bulb temp. (WB) + Mean coincident dry-bulb temp. (MDB)
Dehumidification DP/MDB and HR: Dew-point temp. (DP) + MDB + Humidity ratio (HR)
Mean daily (diurnal) range of dry-bulb temp.
Outdoor Design Conditions
Other climatic info:Joint frequency of temp. and humidity
Annual, monthly and hourly dataDegree-days (cooling/heating) & climatic
normals To classify climate characteristics
Typical year data sets (1 year: 8,760 hours) For energy calculations & analysis
Indoor Design CriteriaIndoor Design Criteria
Basic design parameters: (for thermal comfort)Air temp. & air movement
Typical: summer 24-26 oC; winter 21-23 oC Air velocity: summer < 0.25 m/s; winter < 0.15 m/s
Relative humidity Summer: 40-50% (preferred), 30-65 (tolerable) Winter: 25-30% (with humidifier); not specified (w/o
humidifier)See also ASHRAE Standard 55-2004
ASHRAE comfort zone
(*Source: ASHRAE Standard 55-2004)
Indoor Design CriteriaIndoor Design Criteria
Indoor air quality:Air contaminants
e.g. particulates, VOC, radon, bioeffluentsOutdoor ventilation rate provided
ASHRAE Standard 62-2001Air cleanliness (e.g. for processing)
Other design parameters:Sound levelPressure differential between the space &
surroundings (e.g. +ve to prevent infiltration)
COOLING LOAD PRINCIPLES
Cooling Load Principles
Terminology:Space – a volume w/o a partition, or a
partitioned room, or group of roomsRoom – an enclosed space (a single load)Zone – a space, or several rooms, or units of
space having some sort of coincident loads or similar operating characteristics Thermal zoning
Cooling Load Principles
Space and equipment loadsSpace heat gain (sensible, latent, total)Space cooling load / space heating loadSpace heat extraction rateCooling coil load / heating coil loadRefrigeration load
Instantaneous heat gainConvective heatRadiative heat (heat absorption)
Convective and radiative heat in a conditioned space
Conversion of heat gain into cooling load
Cooling Load Principles
Instantaneous heat gain vs space cooling loadsThey are NOT the same
Effect of heat storageNight shutdown period
HVAC is switched off. What happens to the space?Cool-down or warm-up period
When HVAC system begins to operateConditioning period
Space air temperature within the limits
Thermal Storage Effect in Cooling Load from Lights
Cooling Load Principles
Load profileShows the variation of space loadSuch as 24-hr cycleWhat factors will affect load profile?Useful for operation & energy analysis
Peak load and block loadPeak load = max. cooling loadBlock load = sum of zone loads at a specific
time
Block load and thermal zoning
Cooling Load Components
• Cooling load calculations• To determine volume flow rate of air system• To size the coil and HVAC&R equipment• To provide info for energy calculations/analysis
• Two categories:• External loads• Internal loads
Cooling Load Components
• External loads• Heat gain through exterior walls and roofs• Solar heat gain through fenestrations (windows)• Conductive heat gain through fenestrations• Heat gain through partitions & interior doors• Infiltration of outdoor air
Cooling Load Components
• Internal loads• People• Electric lights• Equipment and appliances
• Sensible & latent cooling loads• Convert instantaneous heat gain into cooling load• Which components have only sensible loads?
[Source: ASHRAE Fundamentals Handbook 2001]
Cooling Load Components
• Cooling coil load consists of:• Space cooling load (sensible & latent)• Supply system heat gain (fan + air duct)• Return system heat gain (plenum + fan + air duct)• Load due to outdoor ventilation rates (or
ventilation load)
• How to construct a summer air conditioning cycle on a psychrometric chart?
Cooling load
Cooling coil load
Schematic diagram of typical return air plenum
Cooling Load Components
• Space cooling load• To determine supply air flow rate & size of air
system, ducts, terminals, diffusers• It is a component of cooling coil load• Infiltration heat gain is an instant. cooling load
• Cooling coil load• To determine the size of cooling coil &
refrigeration system• Ventilation load is a coil load
Heating Load
• Design heating load• Max. heat energy required to maintain winter
indoor design temp.• Usually occurs before sunrise on the coldest days
• Include transmission losses & infiltration/ventilation
• Assumptions:• All heating losses are instantaneous heating loads
• Solar heat gains & internal loads usually not considered
• Latent heat often not considered (unless w/ humidifier)
ReferencesASHRAE Handbook Fundamentals 2001
Chapter 26 – Ventilation and InfiltrationChapter 27 – Climatic Design InformationChapter 28 – Residential Cooling and Heating
Load CalculationsChapter 29 – Nonresidential Cooling and
Heating Load CalculationsChapter 30 – FenestrationChapter 31 – Energy Estimation and Modeling
Methods
ReferencesAir Conditioning and Refrigeration
Engineering (Wang and Norton, 2000)Chapter 6 – Load Calculations
Handbook of Air Conditioning and Refrigeration, 2nd ed. (Wang, 2001)Chapter 6 – Load Calculations
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