3. 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
5. 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.
6. 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.
10. 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.
11. 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.
12. 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.
13. 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.
14. 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.
15. 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.
16. 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
18. BASIC FACTORS THAT AFFECT
HUMAN COMFORT IN THE
INTERNAL ENVIRONMENT-
THERMAL COMFORT
19. 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
20. 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
21. 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)
22. 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
23. 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
24. 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.
25. 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
26. 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
27. 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
28. 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
29. 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
30. 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
31. 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)
34. 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
35. 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
36. 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’.
37. 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
38. 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
39. 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
40. 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.
42. 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.
43. 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
45. 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
47. 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
48. 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
49. U-Values
A measurement of the rate of heat loss through a
structure
Thermal resistivity is the reciprocal of thermal
conductivity:
R=1/K
55. 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
EGEE 102 - Pisupati 55
57. 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
EGEE 102 - Pisupati 57
58. 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
EGEE 102 - Pisupati 58
59. 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.
EGEE 102 - Pisupati 59
61. 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.
EGEE 102 - Pisupati 61
62. 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
EGEE 102 - Pisupati 62
63. 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.
EGEE 102 - Pisupati 63
64. 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
EGEE 102 - Pisupati 64
65. 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
EGEE 102 - Pisupati 65
66. 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
EGEE 102 - Pisupati 66
67. 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
EGEE 102 - Pisupati 67
68. 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.
EGEE 102 - Pisupati 68
69. 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
70. Basic Concepts
Heat transfer mechanism
Conduction
Convection
Radiation
Thermal properties of building materials
Overall thermal transmittance (U-value)
Thermal conductivity
Thermal capacity (specific heat)
71. 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
72.
73. 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
74. 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
75. Outdoor Design Conditions
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
76.
77. 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
78. 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.
79. Outdoor Design Conditions
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
80. 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
85. 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
90. Cooling Load Principles
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
95. Cooling Load Principles
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
98. 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
99.
100. 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
101. 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?
103. 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?
106. 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
107. 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)
108.
109. 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
110. 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
Editor's Notes
Room air conditioners use the standard compressor cycle and are sized to cool just one room. To cool an entire house, several room units are necessary. Central air conditioning systems also operate on the compressor cycle principle and are designed to cool the entire house. The cooled air is distributed throughout the house using air ducts, which may be the same ducts that are used by the heating system.
Heat pumps, described in the heating section, use the compressor cycle, but it is reversible. In the summer, the heat pump transfers heat from indoors to outdoors. In the winter, the heat pump transfers heat from outdoors to indoors. Heat pumps may be powered by electricity or natural gas.
Evaporative coolers, also called &quot;swamp coolers&quot;, do not use the compressor cycle. Instead, they cool air by blowing it over a wet surface. You have experienced this phenomenon when you get out of a swimming pool while a breeze is blowing. As water evaporates, it absorbs heat from the air. Evaporative cooling systems depend on the ability of air to absorb moisture, and so they only work in dry climates such as the Southwest U.S.
That is all there is to the part of the system in the room, which is sketched on the left in figure 2. The bit that is more difficult to understand, or at least unfamiliar to most people, is how the cooling fluid is produced and controlled. That is the part on the right of the diagram.
The cooling fluid used to be a chlorofluorocarbon compound, and often still is, though they all more or less ravage the earth&apos;s ozone layer. The essential characteristics of these fluids is that they have quite a low boiling point at atmospheric pressure and that they can stay in the pipes for a long time without decomposing either themselves or the pipes. Finally they need to have some lubricating ability, or the ability to carry a lubricant, because the fluid has to be compressed and pumped round the system. This rare set of necessary properties has proved difficult to combine with friendliness to the earth&apos;s atmosphere.
The liquid is let into the cooling unit through a valve marked B on the diagram. It evaporates while it passes through the pipe, taking heat from the air just as water evaporating from a towel laid on your fevered brow cools you when on holiday in the Mediterranean. The temperature in the cooling coil depends partly on the amount of fluid let in by the valve, which is controlled by the thermostat or the humidistat. But now comes a crucial difference from your Mediterranean experience: the minimum temperature at the cold surface can be fixed by controlling the pressure in the cooling coil, with the valve marked A on the diagram. The boiling point of any liquid depends on the pressure. One could use water in the cooling coil, if the pressure is kept low enough. At 1000 Pa pressure, which seems a lot but is just 1% of atmospheric pressure, water boils at 7 degrees. It isn&apos;t used in cooling coils of this evaporative type because it has practical disadvantages.
The reason for wanting to limit the minimum temperature is to stop ice clogging the air passage. There are clever systems which notice when ice has formed and hold a melting pause, but that adds to the cost. The pressure controller is therefore set to make the cooling fluid boil at the lowest temperature that is likely to be needed to control the humidity, but always over zero degrees. The temperature needed for cooling is nearly always higher than that needed for dehumidification so it is the RH setting that is decisive.
This brings me to the first point that conservators need to understand: it is expensive to produce air at a dew point below about 4 degrees in this type of equipment. This dewpoint corresponds to 50% RH at 15°C. This sort of air conditioning is entirely suitable for keeping people comfortable but it is not good for specialised stores, for films or for furs, for example, where one needs a temperature below 15 degrees. Such equipment is, however, often used for such places. A better solution is to use an absorption dehumidifier, which will be described in a later article.
Now back to the main story: The vapour that emerges through the pressure controller is gathered up by a compressor. The compression also heats the gas, as will be understood by anyone who has pumped up a cycle tyre. The hot gas is then led away from the room, to be cooled down. This is often done on the roof or in a small enclosure which vibrates to the roar of the fan blowing air over the fins of a condenser. The cooled, now liquid coolant is piped back to the reservoir, ready for its next tour through the room air conditioner.
The entire process described above is inefficient and uses electricity, which is itself produced by inefficient conversion of heat energy. Such systems are therefore confined to small places where the inefficiency is compensated by the generally high reliability and freedom from maintenance.
In a split-system central air conditioner, an outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator. In many split-system air conditioners, this indoor cabinet also contains a furnace or the indoor part of a heat pump. The air conditioner&apos;s evaporator coil is installed in the cabinet or main supply duct of this furnace or heat pump. If your home already has a furnace but no air conditioner, a split-system is the most economical central air conditioner to install.
In a packaged central air conditioner, the evaporator, condenser, and compressor are all located in one cabinet, which usually is placed on a roof or on a concrete slab next to the house&apos;s foundation. This type of air conditioner also is used in small commercial buildings. Air supply and return ducts come from indoors through the home&apos;s exterior wall or roof to connect with the packaged air conditioner, which is usually located outdoors. Packaged air conditioners often include electric heating coils or a natural gas furnace. This combination of air conditioner and central heater eliminates the need for a separate furnace indoors.
The principle of operation is the same as that of the small system described above except that the cooling fluid is usually water, which has itself been cooled by the refrigeration system described above. The air is circulated through ducts, with a portion of fresh air added. There is therefore a pre-heater, because the outside air may be below zero and will therefore freeze the water in the cooling coil. A humidifier and various filters have also been added in figure
Some refinements to the basic system compensate for the different heat requirements of different rooms in the building. Figure 4 shows a complete system, with two details that have not been mentioned yet: the outer zone of the building, which loses more heat in winter, has radiators to supplement the heat supply through the air conditioning. The inner zone has, in this example, an archive room that is not much used and so is cooler, and drier, than the rooms with people, computers and coffee machines. To keep the climate uniform throughout the building there is a little local heater and humidifier placed just before the air reaches the room. The main air supply is kept a little too cold and a little too dry. Any one of these local humidifiers can give trouble, with rapid over-humidification of the room. Again, here is a dangerous detail that is provided by the engineer to protect himself against complaints that the equipment does not achieve the standard required.