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Air conditioner and Refrigeration cycle
Refrigeration cycle
In the refrigeration cycle, a heat pump transfers heat from a lower
temperature heat source into a higher temperature heat sink. Heat would
naturally flow in the opposite direction. This is the most common type of air
conditioning. A refrigerator works in much the same way, as it pumps the heat
out of the interior into the room in which it stands.
This cycle takes advantage of the universal gas law PV = nRT, where P is
pressure, V is volume, R is the universal gas constant, T is temperature, and n
is the number of moles of gas (1 mole = 6.022×1023 molecules).
The most common refrigeration cycle uses an electric motor to drive a
compressor. In an automobile the compressor is driven by a pulley on the
engine's crankshaft, with both using electric motors for air circulation. Since
evaporation occurs when heat is absorbed, and condensation occurs when heat is
released, air conditioners are designed to use a compressor to cause pressure
changes between two compartments, and actively pump a refrigerant around. A
refrigerant is pumped into the cooled compartment (the evaporator coil), where
the low pressure and low temperature cause the refrigerant to evaporate into a
vapor, taking heat with it. In the other compartment (the condenser), the
refrigerant vapour is compressed and forced through another heat exchange coil,
condensing into a liquid, rejecting the heat previously absorbed from the cooled
space.
Humidity
Refrigeration air conditioning equipment usually reduces the humidity of
the air processed by the system. The relatively cold (below the dewpoint)
evaporator coil condenses water vapor from the processed air, (much like an
ice cold drink will condense water on the outside of a glass), sending the
water to a drain and removing water vapor from the cooled space and lowering
the relative humidity. Since humans perspire to provide natural cooling by
the evaporation of perspiration from the skin, drier air (up to a point)
improves the comfort provided. The comfort air conditioner is designed to
create a 40% to 60% relative humidity in the occupied space. In food
retailing establishments large open chiller cabinets act as highly effective
air dehumidifying units.
Some air conditioning units dry the air without cooling it. They work like a
normal air conditioner, except that a heat exchanger is placed between the
intake and exhaust. In combination with convection fans they achieve a
similar level of comfort as an air cooler in humid tropical climates, but
only consume about 1/3 of the electricity. They are also preferred by those
who find the draft created by air coolers uncomfortable.
Refrigerants
"Freon" is a trade name for a family of haloalkane refrigerants manufactured
by DuPont and other companies. These refrigerants were commonly used due to
their superior stability and safety properties. Unfortunately, evidence has
accumulated that these chlorine bearing refrigerants reach the upper
atmosphere when they escape. The chemistry is poorly understood but general
consensus seems to be that CFCs break up in the stratosphere due to
UV-radiation, releasing their chlorine atoms. These chlorine atoms act as
catalysts in the breakdown of ozone, which does severe damage to the ozone
layer that shields the Earth's surface from the strong UV radiation. The
chlorine will remain active as a catalyst until and unless it binds with
another particle forming a stable molecule. CFC refrigerants in common but
receding usage include R-11 and R-12. Newer and more environmentally-safe
refrigerants include HCFCs (R-22, used in most homes today) and HFCs
(R-134a, used in most cars) have replaced most CFC use. HCFCs in turn are
being phased out under the Montreal Protocol and replaced by
hydrofluorocarbons (HFCs), such as R-410A, which lack chlorine.
Types of air conditioner equipment
Window and through-wall units
Many traditional air conditioners in homes or other buildings are single
rectangular units used to cool an apartment, a house or part of it, or part
of a building. For an example, see the photos to the right. Air conditioner
units need to have access to the space they are cooling (the inside) and a
heat sink; normally outside air is used to cool the condenser section. For
this reason, single unit air conditioners are placed in windows or through
openings in a wall made for the air conditioner; the latter type includes
portable air conditioners.[1]
Window and through-wall units have vents on both the inside and outside, so
inside air to be cooled can be blown in and out by a fan in the unit, and
outside air can also be blown in and out by another fan to act as the heat
sink. The controls are on the inside.
A large house or building may have several such units. Should virtually
every room be cooled with its own air conditioning unit, most of the day, it
would be less expensive to use central air conditioning, though that may not
be physically possible.
Evaporation coolers
In very dry climates, so-called "swamp coolers" are popular for improving
comfort during hot weather. An evaporative cooler is a device that draws
outside air through a wet pad, such as a large sponge soaked with water. The
sensible heat of the incoming air, as measured by a dry bulb thermometer, is
reduced. The total heat (sensible heat plus latent heat) of the entering air
is unchanged. Some of the sensible heat of the entering air is converted to
latent heat by the evaporation of water in the wet cooler pads. If the
entering air is dry enough, the results can be quite comfortable. These
coolers cost less and are mechanically simple to understand and maintain.
An early type of cooler, using ice for a further effect, was patented by
John Gorrie of Apalachicola, Florida in 1842. He used the device to cool the
patients in his malaria hospital.
There is a related, more complex process called absorptive refrigeration
which uses heat to produce cooling. In one instance, a three-stage
absorptive cooler first dehumidifies the air with a spray of salt-water or
brine. The brine osmotically absorbs water vapor from the air. The second
stage sprays water in the air, cooling the air by evaporation. Finally, to
control the humidity, the air passes through another brine spray. The brine
is reconcentrated by distillation. The system is used in some hospitals
because, with filtering, a sufficiently hot regenerative distillation
removes airborne organisms.
Absorptive chillers
Some buildings use gas turbines to generate electricity. The exhausts of
these are hot enough to drive an absorptive chiller that produces cold
water. The cold water is then run through radiators in air ducts for
hydronic cooling. The dual use of the energy, both to generate electricity
and cooling, makes this technology attractive when regional utility and fuel
prices are right. Producing heat, power, and cooling in one system is known
as trigeneration.
Central air conditioning
Central air conditioning, commonly referred to as central air (US) or
air-con (UK), is an air conditioning system which uses ducts to distribute
cooled and/or dehumidified air to more than one room, or uses pipes to
distribute chilled water to heat exchangers in more than one room, and which
is not plugged into a standard electrical outlet.
With a typical split system, the condenser and compressor are located in an
outdoor unit; the evaporator is mounted in the air handling unit (which is
often a forced air furnace). With a package system, all components are
located in a single outdoor unit that may be located on the ground or roof.
Central air conditioning performs like a regular air conditioner but has
several added benefits:
- When the air handling unit turns on, room air is drawn in from various
parts of the house through return-air ducts. This air is pulled through a
filter where airborne particles such as dust and lint are removed.
Sophisticated filters may remove microscopic pollutants as well. The
filtered air is routed to air supply ductwork that carries it back to rooms.
Whenever the air conditioner is running, this cycle repeats continually. - Because the central air conditioning unit is located outside the home, it
offers a lower level of noise indoors than a free-standing air conditioning
unit.
Thermostats
Thermostats control the operation of HVAC systems, turning on the heating or
cooling systems to bring the building to the set temperature. Typically the
heating and cooling systems have separate control systems (even though they
may share a thermostat) so that the temperature is only controlled
"one-way". That is, in winter, a building that is too hot will not be cooled
by the thermostat. Thermostats may also be incorporated into facility energy
management systems in which the power utility customer may control the
overall energy expenditure. In addition, a growing number of power utilities
have made available a device which, when professionally installed, will
control or limit the power to an HVAC system during peak use times in order
to avoid necessitating the use of rolling blackouts. The customer is given a
credit of some sort in exchange.
Equipment Capacity
Air conditioner equipment power in the U.S. is often described in terms of
"tons of refrigeration". A "ton of refrigeration" is defined as the cooling
power of one short ton (2000 pounds or 907 kilograms) of ice melting in a
24-hour period. This is equal to 12,000 BTU per hour, or 3517 watts (http://physics.nist.gov/Pubs/SP811/appenB9.html).
Residential "central air" systems are usually from 1 to 5 tons (3 to 20 kW)
in capacity.
The use of electric/compressive air conditioning puts a major demand on the
nation's electrical power grid in warm weather, when most units are
operating under heavy load. In the aftermath of the 2003 North America
blackout locals were asked to keep their air conditioning off. During peak
demand, additional power plants must often be brought online, usually
natural gas fired plants because of their rapid startup. A 1995 study of
various utility studies of residential air conditioning concluded that the
average air conditioner wasted 40% of the input energy. This energy is lost
in the form of heat, which must be pumped out. There is a huge opportunity
to reduce the need for new power plants and to conserve energy.
In an automobile the A/C system will use around 5 hp (4 kW) of the engine's
power.
The Association of Home Appliance Manufacturers (AHAM) offers a worksheet
that can help you estimate how powerful an air conditioner you need. The
worksheet guides you through the measurements needed to calculate the size
of the air conditioner, and then it automatically calculates the final
answer for you.
Seasonal Energy Efficiency Rating (SEER)
For residential homes, some countries set minimum requirements for energy
efficiency. The efficiency of air conditioners are often (but not always)
rated by the Seasonal Energy Efficiency Ratio (SEER). The higher the SEER
rating, the more energy efficient is the air conditioner. The SEER rating is
the Btu of cooling output during its normal annual usage divided by the
total electric energy input in watt-hours (W·h) during the same period. [2]
SEER = BTU ÷ W·h
For example, a 5000 Btu/h air-conditioning unit, with a SEER of 10,
operating for a total of 1000 hours during an annual cooling season (i.e., 8
hours per day for 125 days) would provide an annual total cooling output of:
5000 Btu/h × 1000 h = 5,000,000 Btu
which, for a SEER of 10, would be an annual electrical energy usage of:
5,000,000 Btu ÷ 10 = 500,000 W·h
and that is equivalent to an average power usage during the cooling season
of:
500,000 W·h ÷ 1000 h = 500 W
SEER is related to the coefficient of performance (COP) commonly used in
thermodynamics and also to the Energy Efficiency Ratio (EER). The EER is the
efficiency rating for the equipment at a particular pair of external and
internal temperatures, while SEER is calculated over a whole range of
external temperatures (i.e., the temperature distribution for the
geographical location of the SEER test). The COP is different in that it is
a unitless parameter. Formulas for the approximate conversion between SEER
and EER or COP are available from the Pacific Gas and Electric company in
California:[3]
- SEER = EER ÷ 0.9
- SEER = COP x 3.792
- EER = COP x 3.413
From equation (2) above, a SEER of 13 is equivalent to a COP of 3.43, which
means that 3.43 units of heat energy are pumped per unit of work energy.
Today, it is rare to see systems rated below SEER 9 in the United States,
since older units are being replaced with higher efficiency units. The
United States now requires that residential systems manufactured in 2006
have a minimum SEER rating of 13 (although window-box systems are exempt
from this law, so their SEER is still around 10).[4] Substantial energy
savings can be obtained from more efficient systems. For example by
upgrading from SEER 9 to SEER 13, the power consumption is reduced by 30%
(equal to 1 - 9/13). It is claimed that this can result in an energy savings
valued at up to $US 300 per year (depending on the usage rate and the cost
of electricity). In many cases, the lifetime energy savings is likely to
surpass the higher initial cost of a high-efficiency unit.
As an example, the annual cost of electric power consumed by a 72,000 BTU/h
air conditioning unit operating for 1000 hours per year with a SEER rating
of 10 and a power cost of $0.08 per kilowatt-hour (kW·h) may be calculated
as follows:
unit size, BTU/h × hours per year, h × power cost, $/kW·h ÷ (SEER, BTU/W·h ×
1000 W/kW)
(72,000 BTU/h) × (1000 h) × ($0.08/kW·h) ÷ [(10 BTU/W·h) × (1000 W/kW)] =
$576.00 annual cost
Air conditioner sizes are often given as "tons" of cooling. Multiplying the
tons of cooling by 12,000 converts it to BTU/h.
A common misconception is that the SEER rating system also applies to
heating systems. However, SEER ratings only apply to air conditioning.
Air conditioners (for cooling) and heat pumps (for heating) both work
similarly in that heat is transferred or "pumped" from a cooler
"heat-source" to a warmer "heat-sink". Air conditioners and heat pumps
usually operate most effectively at temperatures around 50 to 55 degrees
Fahrenheit. Typically when the heat source temperature falls below 40
degrees Fahrenheit, the system begins to reach a point called the "balance
point", where the system is not able to "pull" any more heat out of the
heat-source (this point varies from heat pump to heat pump). Similarly, when
the heat-sink temperature rises to about 120 degrees Fahrenheit, the system
will operate less effectively, and will not be able to "push" out any more
heat. Ground-source (geothermal) heat pumps don't have this problem of
reaching a "balance point" because they use the ground as a heat source/heat
sink and the ground's thermal inertia prevents it from becoming too cold or
too warm when moving heat from or to it. The ground's temperature does not
vary nearly as much over a year as the air above it does.
Insulation
Insulation reduces the required power of the air conditioning system. Thick
walls, reflective roofing material, curtains, and trees next to buildings
also cut down on system and energy requirements.
Home air conditioning systems around the world
Domestic air conditioning is most prevalent and ubiquitous in developed
Asian nations such as Japan, Taiwan, South Korea, Singapore and Hong Kong,
especially in the latter two due to most of the population living in small
high-rise flats. In this area, with soaring summer temperatures and a high
standard of living, air conditioning is considered a necessity and not a
luxury. Japanese-made domestic air conditioners are usually window or split
types, the latter being more modern and expensive. It is also increasing in
popularity with the rising standard of living in tropical Asian nations such
as India, Malaysia and the Philippines.
In the United States, home air conditioning is more prevalent in the South
and on the East Coast, in most parts of which it has reached the ubiquity it
enjoys in East Asia. Central air systems are most common in the United
States, and are virtually standard in all new dwellings in most states.
In Europe, home air conditioning is less common in part due to higher energy
costs and more moderate summer temperatures. The lack of air conditioning in
homes, in residential care homes and in medical facilities was identified as
a contributing factor to the estimated 35,000 deaths left in the wake of the
2003 heat wave.
Health implications
Air conditioning has no greater influence on health than heating—that is to
say, very little—although poorly maintained air-conditioning systems
(especially large, centralized systems) can occasionally promote the growth
and spread of microorganisms, such as Legionella pneumophila, the infectious
agent responsible for Legionnaire's disease, or thermophilic actinomycetes.[5]
Conversely, air conditioning (including filtration, humidification, cooling,
disinfection, etc.) can be used to provide a clean, safe, hypoallergenic
atmosphere in hospital operating rooms and other environments where an
appropriate atmosphere is critical to patient safety and well-being. Air
conditioning can have a positive effect on sufferers of allergies and
asthma.[6]
In serious heat waves, air conditioning can save the lives of the elderly.
Some local authorities even set up public cooling centers for the benefit of
those without air conditioning at home.
Although many people superstitiously believe that air conditioning is
unconditionally dangerous for one's health, especially in areas where air
conditioning is not common, this belief is unsupported by fact; properly
maintained air-conditioning systems do not cause or promote illness. As with
heating systems, the advantages of air conditioning generally far outweigh
the disadvantages.