CENTRAL AIR-CONDITIONING SYSTEM
A comprehensive report on:
Central Air-Conditioning
System
Presented to: Dr. Atef Khalil
Presented by:
Alaa Saif Elnasr Ibrahim Sec:1 BN:11
Aya Mohamed Abdulazzaq Sec:1 BN:20
Rania Atef Omar Sec: 1 BN:31
Radwa Said Mohamady Sec: 1 BN:33
Table of Contents
INTRODUCTION .................................................................................................................................... 2
CLASSIFICATIONS OF CENTRAL AIR CONDITIONING SYSTEMS ........................... 2
DIRECT EXPANSION (DX) SYSTEMS .................................................................................................... 2
CHILLED WATER SYSTEMS ................................................................................................................. 3
THE PARTS OF A CENTRAL AIR CONDITIONING UNIT ................................................ 5
CHOOSING OR UPGRADING YOUR CENTRAL AIR CONDITIONER ....................... 6
OVERVIEW OF HEALTH CARE HVAC SYSTEMS................................................................ 7
INTRODUCTION .................................................................................................................................... 7
VENTILATION AND ENVIRONMENTAL CONTROL FOR SPECIAL FUNCTIONS ...................................... 7
ROLE OF THE BUILDING ENVELOPE ................................................................................................... 7
DESIGN CRITERIA ............................................................................................................................... 7
HVAC SYSTEM HYGIENE ................................................................................................................... 8
INFECTION CONTROL ...................................................................................................................... 9
HOSPITAL ROOM DESIGN ............................................................................................................ 10
POSITIVE AND NEGATIVE PRESSURE ROOMS .................................................................................... 10
CONDITIONS IN EACH ROOM ............................................................................................................. 11
ISOLATION ROOMS .......................................................................................................................... 12
AII ROOMS ........................................................................................................................................ 12
PE ROOMS ......................................................................................................................................... 13
COVID-19 AND HVAC SYSTEMS ................................................................................................ 13
DISADVANTAGES ............................................................................................................................... 14
CONCLUSION AND RECOMMENDATIONS ......................................................................... 15
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1-Introduction
-In central air conditioning, air, water, or both are used as working fluids to
produce the required heating and/or cooling, and therefore based on working
fluids, central air conditioning systems can be classified into three groups:1-All-air systems: in these systems air is used as working fluid for heating
and/or cooling purposes.
2-All-water (hydronic) systems: in these systems water is used as working fluid
for heating and/or cooling purposes.
3-Air-water systems: in these systems both air and water are used as working
fluids for producing heating and cooling purposes.
-It is may be of interest to note that each type of the central air conditioning
systems has several systems of sometimes different configurations, and the use
of 1 any system depends on its advantages and disadvantages.
-The following sections will provide descriptions of all types of central air
conditioning systems.
2. Classifications of central air conditioning systems
Direct Expansion (DX) systems:
-In DX systems, the air is cooled with refrigerant passing through the tubes of
the finned cooling coil. The Figure-1 below provides a conceptual view of a DX
air-conditioning system. In this figure, the heat is extracted from the space and
expelled to the outdoors (left to right) through 3 loops of heat transfer.
• In the leftmost loop, a supply air fan drives the indoor air across the
evaporator, where it transfers its heat to the liquid refrigerant. The resultant
cooled air is thrown back to the indoor space. The liquid refrigerant is vaporized
in the tubes of the evaporator.
• In the middle loop, a refrigeration compressor drives the vapor refrigerant
from evaporator to the condenser and back to the evaporator as a liquid
refrigerant. The cycle continues in a closed loop copper tubing.• In the
rightmost loop, a condenser air fan drives the ambient air across the condenser,
where it transfers heat of refrigerant to the outdoors. The refrigerant is cooled
and liquefied after expanding it through an expansion valve located between
condenser and the evaporator.
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Window air-conditioners, package units and split systems are typical examples
of DX systems. These comprise of a hermetic sealed compressor/s, evaporator
(cooling coil fabricated out of copper tubes and aluminum fins), a supply air
fan, filter and a condensing unit. DX systems are essentially the factory
assembled self-contained units and are also known as local systems.
The application and unit capacity ranges are as follows:
• Room air conditioner (capacity range of 0.5 to 3 TR per unit, suitable for an
area of not more than 1000 square feet)
• Packaged unit integral air-cooled condenser (capacity range of 3 to 50 TR,
suitable for a maximum an area of 1000 – 10000 square feet)
• Split system with outdoor air-cooled condenser (capacity range of 0.5 to 50
TR, suitable for an area of 100 – 10000 square feet)
-Note: Each building is different and the design conditions differ greatly
between regions to region. On hotter & humid regions the cooling requirement
may be as high as 150 sqft/TR and in cooler places it could be as low as 500
sqft/TR. For comfort applications, it is reasonable to assume a figure of 200
sqft/TR as a rule of thumb for a preliminary estimation in absence of heat load
calculations.
Chilled Water Systems:
-In chilled water system the air is cooled with chilled water passing through the
tubes of a finned coil (cooling coil). Chilled water systems are further
categorized as air-cooled or water cooled system depending on how the heat is
rejected out of the system. The Figure-2 below provides a conceptual view of
chilled water air-conditioning system with air-cooled condenser. The figure
below depicts that heat is extracted from the space and expelled to the outdoors
(left to right) through 4 loops of heat transfer. The chilled water is produced in
the evaporator of the refrigeration cycle and is pumped to a single or multiple
air-handling units containing cooling coils. The heat is rejected through an aircooled condensing unit in the rightmost loop.
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The Figure-3 below provides a conceptual view of chilled water airconditioning system with water-cooled condenser.
Here the heat is extracted from the space and expelled to the outdoors (left to
right) through 5 loops of heat transfer. The chilled water is produced in the
evaporator of the refrigeration cycle and is passed through a single or multiple
cooling coils. The heat is rejected through a water-cooled condenser and the
condenser water pump sends it to the cooling tower. The cooling tower’s fan
drives air across an open flow of hot condenser water, transferring the heat to
the outdoors.
The main equipment used in the chilled water system is a chiller package
that includes
• A refrigeration compressor (reciprocating, scroll, screw or centrifugal type)
• Shell and tube heat exchanger (evaporator) for chilled water production
• Shell and tube heat exchanger (condenser) for heat rejection in water cooled
configuration
• Copper tube/Aluminum finned condenser coil and fan (condensing unit) for air
cooled configuration
• An expansion valve between condenser and the evaporator
-The middle refrigerant loop is connected through a copper piping forming a
closed loop. The water circuit on the chilled waterside is connected through an
insulated carbon steel pipe and is a closed loop. The condenser water connected
through a carbon steel piping is an open loop and requires 2 to 3 % make up
water as a result of evaporation, drift and blow down losses from the cooling
tower. The chilled water system is also called central air conditioning system.
This is because the chilled water system can be networked to have multiple
cooling coils distributed through out a large or distributed buildings with the
refrigeration equipment (chiller) placed at one base central location.
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-Chilled water systems are typically applied to the large and/or distributed areas.
(Capacity range of 20- 2000 TR, suitable for an area of 3000 square feet and
above)
3-The Parts of a Central Air Conditioning Unit
-the main parts of your air conditioning system:Condenser: is a very essential part of your system and the outside unit of your
air conditioning system, It sits outside on the ground and is typically round or
square-shaped, it’s responsible for releasing the heat that your refrigerant has
absorbed into the atmosphere, Refrigerant enters the condenser as a vapor but
turns into a liquid as it cools while flowing through the condenser and
condenser coils.
Compressor: The compressor is a large AC component located inside of the
condenser and isn’t visible, it helps the condenser by increasing the refrigerant’s
pressure enough for the condenser to change the high pressure vapor into a
liquid.
Lineset: Lineset is the term that refers to the copper lines that run between the
outdoor and indoor portions of your unit, these linesets are used to transport
either liquid or gas refrigerant from one component to the next in the cycle.
Evaporator: it serves essentially the opposite purpose of the condenser and is
typically located inside the top of your air conditioning system.
-Cooler, liquid refrigerant enters the evaporator from the condenser (via the
lineset) and is met with lower pressure when this happens, the liquid refrigerant
turns back into a gas and absorbs heat from the air around it, cooling the inside
of your home, After this takes place, the gas refrigerant makes its way back to
the condenser and the cycle repeats itself until your home is at a comfortable
temperature.
Expansion Valve: Just as the compressor assists the condenser by adjusting the
pressure of the refrigerant, assists the evaporator and serves to lessen the
pressure of the liquid refrigerant so that it can turn back into a gas inside of the
evaporator.
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Other Components: Besides the parts mentioned above, there are a few other
components of your air conditioning system worth mentioning: thermostat that monitors the temperature in your home and either
sustains from or sets the cooling process in motion according to your
settings.
When your air conditioning system pulls in air for cooling, the air filter
helps trap any pollen, pet dander, dust or other particles from being recirculated.
4- Choosing or Upgrading Your Central Air Conditioner
-Central air conditioners are more efficient than room air conditioners, quiet and
convenient to operate.
- When buying an air conditioner, look for a model with a high efficiency.
Central air conditioners are rated according to their seasonal energy efficiency
ratio (SEER), SEER indicates the relative amount of energy needed to provide a
specific cooling output. Many older systems have SEER ratings of 6 or less.
-Proper sizing and installation are key elements in determining air conditioner
efficiency. Too large a unit will not adequately remove humidity. Too small a
unit will not be able to attain a comfortable temperature on the hottest days.
Improper unit location, lack of insulation, and improper duct installation can
greatly diminish efficiency.
-The standards do not require you to change your existing central air
conditioning units, and replacement parts and services should still be available
for your home's systems.
-Other features to look for when buying an air conditioner include:
A thermal expansion valve and a high-temperature rating (EER) greater than
11.6, for high-efficiency operation when the weather is at its hottest
A variable speed air handler for new ventilation systems
A unit that operates quietly
A fan-only switch, so you can use the unit for nighttime ventilation to
substantially reduce air-conditioning costs
A filter check light to remind you to check the filter after a predetermined
number of operating hours
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An automatic-delay fan switch to turn off the fan a few minutes after the
compressor turns off.
5- Overview of health care HVAC systems
5.1-introduction
-In support of the health care process, HVAC systems are called upon to
perform several vital functions that affect environmental conditions, infection
and hazard control building life safety to Staff and patient comfort, the
provision of therapeutic space conditions and facilitate optimum patient
treatment outcomes.
-Certain medical functions, treatments or healing processes demand controlled
environmental temperature and/or relative humidity conditions that deviate from
the requirements for personal comfort (i.e Operating rooms and nursery units)
often require a range of room temperatures spanning several degrees, regardless
of the season, to best facilitate a given procedure or patient condition, Burnpatient treatment rooms and bedrooms may require elevated temperature and
relative humidity (RH) conditions.
5.1-Ventilation and Environmental Control for Special Functions
-That include Many medical facilities functions or processes in which chemical
fumes, aerosols, or harmful gases are stored and generated, posing health or
safety hazards. Examples include laboratories where aerosolizing chemicals are
used to fix slide specimens, preserve tissues or perform other processes
orthopedic appliance and artificial limb shops involving adhesives and other
aerosolizing agents, In such applications, HVAC equipment operates in
conjunction with primary containment equipment.
5.2-Role of the Building Envelope
-That include the integrity of the building envelope is essential to minimize the
introduction of unconditioned, unfiltered air into a building, as well as to
effectively exclude moisture.
5.3-Design Criteria
-Typical criteria for HVAC design include indoor and outdoor environmental
design conditions:1-outdoor air and total air change requirements.
2- economic considerations for equipment selection.
3-requirements for redundancy or backup equipment capacity.
4-room pressure relationships.
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5- required filtration, and other benchmarks for systems and equipment
selection and sizing.
-Other criteria that influence the HVAC design may involve:1-envelope configuration and thermal performance.
2-environmental requirements for special equipment and processes.
3- operation and maintenance considerations.
4-clearance and conditioning requirements for electrical and electronic
equipment.
5-equipment Redundancy and Service Continuity.
- major HVAC service does not jeopardize life or health, it may lead to inability
to continue medical functions and unacceptable economic impact to the building
owner. Designers should also recognize that routine maintenance requirements
will, at least on an annual or seasonal basis
5.4- HVAC system hygiene
-In addition to the general topic of infection cause and control discussed
previously, the designer must be aware of the potential for infection risks that
can arise through poor design or maintenance of HVAC equipment, infections
acquired within the health care facility Any location where moisture and
nutrient matter come together can become a reservoir for growth of harmful
microorganisms.
Generally, hard surfaces (such as sheet metal) require the presence of liquid
water to support microbe growth, whereas growth in porous materials may
require only high relative humidity.
-The task of the HVAC designers to minimize the opportunity for moisture and
nutrients to collect in the system, through proper design of equipment, including
adequate provisions for inspection and maintenance.
- Potential high-risk conditions in an HVAC system include the following:1-outdoor air intakes located too close to collected organic debris, such as wet
leaves, animal nests, trash, wet soil, grass clippings, or low areas where dust and
moisture collect; this is a particular concern with low-level intakes and a
primary reason for code-mandated separation requirements between intake and
ground, or intake and roof.
2-Outdoor air intakes not properly designed to exclude precipitation; examples
are intakes without intake louvers (or with improperly designed louvers) and
intakes located where snow can form drifts or splashing rain can enter.
3-Improperly designed or installed outdoor air intake opening ledges where the
collected droppings of roosting birds carry or support the growth of
microorganisms.
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4-Improperly designed or installed cooling-coil drain pans or drainage traps that
prevent adequate condensate drainage.
5-Air-handling unit or duct-mounted humidifiers not properly designed or
installed to provide complete evaporation before impingement on downstream
equipment or fitting.
6-Filters and permeable linings, which collect dust, located too close to a
moisture source, such as a cooling coil or humidifier.
7-Improper attention to maintenance during design, resulting in air-handling
components that cannot be adequately accessed for inspection or cleaning.
6-Infection control
Protection population
-Positive pressure environment (PPE) rooms are designed for patients with
severely compromised immune systems, such as bone marrow transplant and
HIV patients. The HVAC systems for PPE rooms are designed to preclude
airborne pathogens through the use of high-quality filtration, high air exchange
rates, anterooms, and pressurization.
-The emergency waiting room is an area of particular risk because both
immune-compromised and undiagnosed contagious patient populations often
coexist there, this type of space has high air exchange rates and all air is
exhausted directly outside.
Filtration
-The efficacy of air filters is determined primarily by particle size but can be
affected by the relative electrical charges of particles and filters.
Bacteria typically are quite small, requiring filters that remove particles below 1
μm in size. ANSI/ASHRAE Standard) specifies a test procedure for evaluating
the performance of air-cleaning devices as a function of particle size, resulting
in a minimum efficiency reporting value (MERV) for a given device.
-from ANSI/ASHRAE/ASHE Standard 170-2008, gives requirements for prefiltration and final filtration in different areas as indicated in Table 2-4
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Effect of infection control on HVAC design
-the figure below shows how the infection control measures affect the design of
HVAC systems by influencing the following HVAC design parameters:•Outdoor air quantity, including natural ventilation.
• Type and location of filters.
• Humidification.
• UV radiation.
• Chilled-water temperature.
• Supply air conditions.
• Supply air change rates in individual rooms.
• Air distribution and velocity.
• Locations of return air grilles.
• Balance of supply and return/exhaust air for
pressurization.
7-Hospital Room Design
-Many rooms in hospitals require special design considerations
because of heightened infection concerns, high internal loads, special
equipment, unique processes, and unique patients.
-Many spaces in hospitals require maintenance of a differential
pressure relative to adjacent spaces. For example, ORs, protective
isolation, and sterile supply require positive pressure, whereas airborne
infectious isolation, toilet, soiled, bronchoscopy, and decontamination rooms
require negative pressure.
7.1-Positive and Negative Pressure Rooms
-Positive pressure rooms maintain a higher pressure inside the treated area
than that of the surrounding environment, this means air can leave the room
without circulating back in, In this way, any airborne particle that originates in
the room will be filtered out.
Germs, particles, and other potential contaminants in the surrounding
environment will not enter the room. In medical settings, a positive pressure
room allows staff to keep vulnerable patients safe from infections and disease.
-negative pressure room uses lower air pressure to allow outside air into the
segregated environment, these traps and keeps potentially harmful particles
within the negative pressure room by preventing internal air from leaving the
space, Negative pressure rooms in medical facilities isolate patients with
infectious conditions and protect people outside the room from exposure.
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7.2-conditions in each room
Operating rooms
- The purposes of the HVAC system in an operating room (OR) are to minimize
infection, maintain staff comfort, and maintain patient comfort.
- According to ASHRAE standard 170-2008 ORs requirements are:•air change per hour (ACH) value has been 25 ach supply air including 5 ach of
outdoor air.
• designed for a positive pressure differential is 2.5 Pa, 35-47 L/s.
• most standard operating rooms require MERV 7/8- 14/15-17 filtration.
• air velocity is 0.25-0.45 m/s.
• relative humidity 30-60%.
• temp. 18-26 °C.
• air distribution is laminar.
• Outdoor Air intakes: The location of outdoor air intake for an AHU must not
be located near potential contaminated sources like DG exhaust hoods, lab
exhaust vents, and vehicle parking area.
• Preventive Maintenance of the system: It is recommended that periodic
preventive maintenance be carried out in terms of cleaning of pre filters, micro
vee filters at the interval of 30 days
ICU
- The level of care and electronic monitoring of patients in these rooms are
greatly increased compared to conventional patient rooms.
- According to ASHRAE Standard 170-2008 requirements are:• variable-range temperature capability 21.1-23.9°C.
• relative humidity of 30- 60%.
• ACH value has been 6 ach as minimum, of which a minimum of 2 ach must be
outdoor air.
• air velocity is 1.5 to 2.5 m/s
• It is recommended that all air should be filtered to 99% efficiency down to 5
microns.
• Central HVAC with HIFA filter
Emergency department
-The emergency department is generally the point of entry to a hospital for
undiagnosed patients, some of whom may be carriers of dangerous infectious
diseases so it need good air conditioning system to avoid them.
- According to ASHRAE Standard 170-2008 requirements are:• relative humidity is max 65%
• variable-range temperature capability 21-24°C.
• ACH value has been 12 ach as minimum, of which a minimum of 2 ach must
be outdoor air.
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• there are basically two filtration methods: particulate and ultraviolet
germicidal irradiation (UVGI) Because each contaminant has a different
level of urgency, filtration methods can vary.
Burn unit (WIC)
-This room type requires careful humidity control and, normally, the ability to
achieve elevated temperature, Regimens of patient treatment vary between
providers, and some institutions use radiant-heating equipment to supplement
ambient heating.
- According to ASHRAE Standard 170-2008 requirements are:• variable-range temperature capability 21-24°C.
• with HEPA-filtered (MERV 17).
• air velocity is 0.25 m/s.
• relative humidity is 40- 60%.
• ACH value has been 12 ach as minimum, of which a minimum of 2 ach must
be outdoor air.
• maintain the room under positive pressurization at all times.
Cardiac and Interventional Lab
-In “cath” and interventional labs, stents, balloons, and valves are
placed in the body, the device placed inside the artery is then guided using
fluoroscopy.
-Because there is continuous X-ray exposure for considerable time, good
shielding is a necessity and requires:• ACH value has been 20 ach as minimum, of which a minimum of 4ach must
be outdoor air.
• laminar flow ceiling with laminar flow diffusers
•maintain the room under positive pressurization at all times.
• relative humidity heating 30-60%, relative humidity cooling 20-60%.
•noise criteria 35-45
• variable-range temperature capability 20-24°C.
8-Isolation rooms
-Airborne infectious isolation (AII) rooms are for patients having an airbornecommunicable disease. For an AII room, the HVAC system functions as one of
multiple levels of infection control designed to contain patient-generated
infectious microbials within the room, to prevent the spread of infection to other
patients and staff.
.9.1-AII Rooms
-AII rooms have two major ventilation design criteria:1- negative air pressure relative to all adjoining spaces
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2- an air distribution pattern within the room designed to reduce airborne
infection.
3-The recommended design approach favors creating maximized air mixing,
pressurization, and airflow, thereby minimizing exposure.
9.2-PE Rooms
-Protective environment (PE) rooms include those for bone marrow transplant,
oncology, hematology, and similar rooms for any condition that leaves a patient
immunocompromised.
-A PE room seeks to protect the patient from all potential airborne infectious
organisms, some of which may be benign to those with normal immune
systems.
Combined AII/PE rooms are for patients suffering from a weakened immune
system who also have an airborne communicable disease. In this type of room,
HVAC issues involve a combination of both AII room and PE room
considerations.
- According to ASHRAE Standard 170-2008 requirements for isolation room
are:• variable-range temperature capability 21-24°C.
• most standard isolation rooms require MERV 8-17 filtration.
• air velocity 0.5 m/s.
• air distribution is laminar.
• pressure difference is 2.5 pa.
• ACH value has been 12 ach supply air including 2 ach of outdoor air.
9- Coronavirus covid-19 and HVAC systems
-According to the CDC (Centers for Disease Control and Prevention) and
ASHRAE (The American Society of Heating, Refrigerating and AirConditioning Engineers), hospital HVAC systems play a critical role in
mitigating the spread of diseases like COVID-19, MERS, SARS, and
tuberculosis ,When an infected person coughs or sneezes, respiratory droplets
can travel through the air and be inhaled by people nearby.
Room pressurization, air change rates, humidity, and temperature all play an
integral role in mitigating airborne contaminants to provide a healing space for
patients while protecting healthcare providers.
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- AIIRs constantly provide clean air to patients and help
protect staff from infection. AIIRs contain, dilute, and
exhaust contaminated air through a high-efficiency
particulate air (HEPA) filter, and then vent it outdoors.
- The negative pressure prevents contaminated droplets
from traveling to other areas of the hospital when the
doors are opened.
Room doors should be kept closed except when personnel are entering and
exiting. Entry and exits should be minimized as air currents from personnel
traffic can disrupt proper airflow, so AIIR HVAC equipment must be fastacting, adjusting immediately to changing conditions. Facilities should monitor
and document AIIR negative pressure function, and there should be a constant
visual indication from the room pressure controller or monitor.
Droplet suspension: illustration of the aerobiology of droplets and small airborne
particles produced by an infected patient.
10-Disadvantages of air conditioners
-Air conditioners use a lot of electricity.
- financial disadvantage.
-spending too much time in an air-conditioned environment can contribute to
health problems such as asthma, tightness in the chest and other respiratory
ailments.
-Low temperatures will lead to a sharp contraction of blood vessels, poor blood
flow and joint pain.
-the temperature difference between indoor and outdoor, people often will feel
hot and cold, this will result in the body conditioning system disorders.
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11-CONCLUSIONS AND RECOMMENDATIONS
-The air is not just a medium but it can be regarded as a guard in the critical
health applications.
-The proper direction of the airflow increases the possibilities of successful
pollutant scavenging from healthcare applications.
-The numerical tool, used here, was found to be so effective to predict the
airflow pattern in the healthcare facilities at reasonable costs and acceptable
accuracy.
-Good architectural design allows the HVAC system designers to properly
locate the supply outlets and extraction ports in the optimum locations.
-The hospitals/medical institution deals with life of the clientele, the lives saved
by providing appropriate therapeutic, diagnostic and treatment facilities justify
that the investment for air conditioning is a small cost for better quality care to
the patients.
References
- ANSI/ASHRAE (American Society of Heating, Refrigerating and Air
Conditioning Engineers) /ASHE standards 170-2017
-HVAC design manual for hospitals and clinics book
- https://www.health.ny.gov/facilities/cons/docs/ashrae_table_7-1.pdf
- https://www.energy.gov/energysaver/central-air-conditioning
- https://www.hometips.com/how-it-works/central-air-conditioners.html
- https://www.intechopen.com/online-first/central-air-conditioning-systems-and-applications
- https://www.richmondsair.com/hvac-guide/equipment-101/parts-central-air-conditioning-unit
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