Hvac psychrometry and concepts

PRESENTED BY:
AISHWARYA DEOPUJARI
PRERANA DAS
NISHTHA DUGGAL
VASUNDHRA SINGH
SRIDEVI
SECTION-B
6TH
SEMESTER

Contents
Basic Concepts
Psychrometry
Outdoor Design Conditions
Indoor Design Criteria
Cooling Load Principles
Cooling Load Components
Heating Load

Basic Concepts
Thermal 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

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.

Hvac psychrometry and concepts

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.

EGEE 102 - Pisupati 16
Humidity in air
Relative Humidity
A measure of of
much water is in the
air relative to the
maximum amount
air can hol at that
tmperature

http://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 quality
What affects the surroundings you live in?
Air quality is affected by how hot it is outside or
inside your environment
What 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 done
The rate of expenditure of energy or doing
work or of heat loss is measured in watts (W)
1 watt is = 1 Joule per second
1 W =1 J/s

Heat transfer
Name three ways heat is transferred from one mass to
another, for instance a person sitting next to a
radiator.
Conduction
Convection
Radiation

Thermal comfort
In 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 area
Age
Gender
Level of activity
e.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

Clothing
The 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 body
Typical 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 buildings
Comfortable 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 temperatures
What 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 used
Type of construction
Orientation of the building in relation to the sun
Degree 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 movement
Properties are tested for airtightness
Draught seals are fitted to all openings to
restrict thermal losses
If warmer air enter a room is not mixed with
cooler air the room becomes hotter near the
ceiling and colder at floor level

Humidity & Ventilation
Humidity- 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 HUMIDITY
Humans 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 VENTILATION
Natural 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 hr
Halls 1.0
Bedrooms /lounges 1.5
WCs and bathrooms 2.0

HEAT LOSS DUE TO VENTILATION
The 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 ventilation
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
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 practice
Calculate 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 practice
A 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 Dimensions
Lounge is 3.5m x 3.5m
Kitchen/diner is 4.0m x 2.5m
Bedroom 1 is 3.0m x 3.0m
Bedroom 2 is 2.75m x 2.75m
Bathroom 3 is 2.5m x 2m
Storey height is 2.4m
Air changes for all rooms 1.5 per hour
Temperature difference -1°C outside, 19°C inside.

Calculation
Lounge 3.5 x 3.5 x 2.4 =29.4
Kitchen 4.0 x 2.5 x 2.4 =24.0
Bedroom One 3.0 x 3.0 x 2.4 =21.6
Bedroom Two 2.75 x 2.75 x 2.4 =18.15
Bathroom Three 2.5 x 2.0 x 2.4 =12.0
Total volume = 105.91m3

condensation
This 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 rot
Encourage mould growth
Produce cold spots
Produce high humidity
Cause corrosion to steelwork
Dampen insulation, reducung its effectiveness

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 metre
The units are W/mK (watts per metre Kelvin)
P= kA (T1-T2)/ x
A= Area
X= thickness in m² and m respectively
T1-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-Values
A measurement of the rate of heat loss through a
structure
Thermal resistivity is the reciprocal of thermal
conductivity:
R=1/K

Principle
A. Expansion Valve
B. Compressor

Arrangement

TYPES OF AIR CONDITIONERS
Room air conditioners
Central air conditioning systems
Heat pumps
Evaporative coolers

Air Conditioning

Room air conditioner
Room air conditioners cool rooms rather than the
entire home.
Less expensive to operate than central units
Their 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

Central Air conditioning
Circulate 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

Types of Central AC
split-system
an outdoor metal cabinet contains the condenser
and compressor, and an indoor cabinet contains
the evaporator
Packaged
the evaporator, condenser, and compressor are all
located in one cabinet

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.

Total Air Conditioning

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.

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

Energy Consumption
Air 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.

Energy Efficiency
Today's best air conditioners use 30% to 50% less
energy than 1970s
Even 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

Energy Efficiency
Rating 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

Room Air Conditioners
Built 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

Central AC
National minimum standards for central air
conditioners require a SEER of
9.7 for single-package and
10.0 for split-systems
Units are available with SEERs reaching nearly 17

Energy Saving Methods
Locate 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.

Basic Concepts
Purpose of HVAC load estimation
Calculate peak design loads (cooling/heating)
Estimate likely plant/equipment capacity or size
Provide info for HVAC design e.g. load profiles
Form the basis for building energy analysis
Cooling load is our main target
Important for warm climates & summer design
Affect building performance & its first cost

Basic Concepts
Heat transfer mechanism
Conduction
Convection
Radiation
Thermal properties of building materials
Overall thermal transmittance (U-value)
Thermal conductivity
Thermal capacity (specific heat)

Basic Concepts
A building survey will help us achieve a realistic
estimate of thermal loads
Orientation of the building
Use of spaces
Physical dimensions of spaces
Ceiling height
Columns and beams
Construction materials
Surrounding conditions
Windows, doors, stairways

Basic Concepts
Building 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 information
General info: e.g. latitude, longitude, altitude, atm.
pressure
Outdoor design conditions
 Derived from statistical analysis of weather data
 Typical data can be found in handbooks/databooks, such as
ASHRAE Fundamentals Handbooks

Climatic design conditions from ASHRAE
Previous 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

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

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.

Other climatic info:
Joint frequency of temp. and humidity
 Annual, monthly and hourly data
Degree-days (cooling/heating) & climatic normals
 To classify climate characteristics
Typical year data sets (1 year: 8,760 hours)
 For energy calculations & analysis

Indoor Design Criteria
Basic design parameters: (for thermal comfort)
Air temp. & air movement
 Typical: summer 24-26 o
C; winter 21-23 o
C
 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 Criteria
Indoor air quality:
Air contaminants
 e.g. particulates, VOC, radon, bioeffluents
Outdoor ventilation rate provided
 ASHRAE Standard 62-2001
Air cleanliness (e.g. for processing)
Other design parameters:
Sound level
Pressure differential between the space & surroundings
(e.g. +ve to prevent infiltration)

Cooling Load Principles
Terminology:
Space – a volume w/o a partition, or a partitioned
room, or group of rooms
Room – 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

Space and equipment loads
Space heat gain (sensible, latent, total)
Space cooling load / space heating load
Space heat extraction rate
Cooling coil load / heating coil load
Refrigeration load
Instantaneous heat gain
Convective heat
Radiative heat (heat absorption)

Convective and radiative heat in a conditioned space

Conversion of heat gain into cooling load

Instantaneous heat gain vs space cooling loads
They are NOT the same
Effect of heat storage
Night shutdown period
 HVAC is switched off. What happens to the space?
Cool-down or warm-up period
 When HVAC system begins to operate
Conditioning period
 Space air temperature within the limits

Thermal Storage Effect in Cooling Load from Lights

Load profile
Shows the variation of space load
Such as 24-hr cycle
What factors will affect load profile?
Useful for operation & energy analysis
Peak load and block load
Peak load = max. cooling load
Block load = sum of zone loads at a specific time

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

• 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

• 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 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

• 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)

References
ASHRAE Handbook Fundamentals 2001
Chapter 26 – Ventilation and Infiltration
Chapter 27 – Climatic Design Information
Chapter 28 – Residential Cooling and Heating Load
Calculations
Chapter 29 – Nonresidential Cooling and Heating Load
Calculations
Chapter 30 – Fenestration
Chapter 31 – Energy Estimation and Modeling Methods

References
Air 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

Hvac psychrometry and concepts

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