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CATEGORIES (articles) > Interior > Components > Air conditioning explained

Air conditioning explained

The external section of a typical single-room air conditioning unit. For ease of installation, these are frequently placed in a window. This one was installed through a hole cut in the wall.
The internal section of the same unit. The front panel swings down to reveal the controls.

Note: in the broadest sense, "air conditioning" can refer to any form of "heating, ventilation, and air-conditioning." This article is specifically about the use of refrigeration for this purpose.

An air conditioner is an appliance, system, or mechanism designed to extract heat from an area using a refrigeration cycle. In construction, a complete system of heating, ventilation, and air conditioning is referred to as HVAC. Some refer to air conditioner or air conditioning as AC or A/C for short.


In 1820 British scientist and inventor Michael Faraday discovered that compressing and liquefying ammonia could chill air when the liquefied ammonia was allowed to evaporate.

In 1842, Florida physician Dr. John Gorrie used compressor technology to create ice, which he used to cool air for his patients in his hospital in Apalachicola, Florida. ([1] bg.htm) He hoped eventually to use his ice-making machine to regulate the temperature of buildings. He even envisioned centralized air conditioning that could cool entire cities. ([2] AirCo.htm) Though his prototype leaked and performed irregularly, Gorrie was granted a patent in 1851 for his ice-making machine. His hopes for its success vanished soon afterwards when his chief financial backer died. Gorrie did not get the money he needed to develop the machine. According to his biographer Vivian M. Sherlock, he blamed the "Ice King," Frederic Tudor, for his failure, suspecting that Tudor has launched a smear campaign against his invention. Dr Gorrie died impoverished in 1855 and the idea of air conditioning faded away for 50 years.

Early commercial applications of air conditioning were manufactured to cool air for industrial processing rather than personal comfort. In 1902 the first modern electrical air conditioning was invented by Willis Haviland Carrier. Designed to improve manufacturing process control in a printing plant, his invention controlled not only temperature but also humidity. The low heat and humidity were to help maintain consistent paper dimensions and ink alignment. Later Carrier's technology was applied to increase productivity in the workplace, and The Carrier Air Conditioning Company of America was formed to meet rising demand. Over time air conditioning came to be used to improve comfort in homes and automobiles. Residential sales expanded dramatically in the 1950s.

In 1906, Stuart W. Cramer of Charlotte, North Carolina, USA, was exploring ways to add moisture to the air in his textile mill. Cramer coined the term "air conditioning," using it in a patent claim he filed that year as an analogue to "water conditioning", then a well-known process for making textiles easier to process. He combined moisture with ventilation to "condition" and change the air in the factories, controlling the humidity so necessary in textile plants. Willis Carrier adopted the term and incorporated it into the name of his company.

The first air conditioners and refrigerators employed toxic gases like ammonia and methyl chloride, which could result in fatal accidents when they leaked. Thomas Midgley, Jr. created the first chlorofluorocarbon gas, Freon, in 1928. The refrigerant was much safer for humans but was later found to be harmful to the atmosphere's ozone layer. "Freon" is a trade name of Dupont for any CFC, HCFC, or HFC refrigerant, the name of each including a number indicating molecular composition (R-11, R-12, R-22, R-134). The blend most used in direct-expansion comfort cooling is an HCFC known as R-22. It is to be phased out for use in new equipment by 2010 and completely discontinued by 2020. R-11 and R-12 are no longer manufactured in the US, the only source for purchase being the cleaned and purified gas recovered from other air conditioner systems. Several ozone-friendly refrigerants have been developed as alternatives, including R-410A, known by the brand name "Puron".

Latest air conditioners usually have air sterilization effects, such as the recent air conditioners that have germicidal and neutralization benefits.

Air conditioning applications

Air conditioning engineers broadly divide air conditioning applications into comfort and process.

Comfort applications aim to provide an indoor environment that remains relatively constant in a range preferred by humans despite changes in external weather conditions or in internal heat loads. Some have claimed that comfort air conditioning increases worker productivity but this claim is disputed, one counter argument being that apparent increases in productivity can be explained as resulting from workers perceiving that their employer shows an interest in their welfare. (See Hawthorne effect). What is certain is that comfort air conditioning makes deep plan buildings feasible. Without air conditioning, buildings must be built narrower or with light wells so that inner spaces receive sufficient fresh air. Air conditioning also allows buildings to be taller since wind speed increases significantly with altitude making natural ventilation impractical for very tall buildings. Comfort applications for various buiding types is quite different and may be categorized as:

  • Residential Buildings including single family houses and hi-rise buildings.
  • Institutional Buildings includes Hi-Rise offices, large complex buildings, hospitals, and so on.
  • Commercial Buildings which are built for profit and commerce, including malls, shopping centers, apartment housings, etc.
In addition to buildings, air conditioning can be used for comfort in a wide variety of transportation including land vehicles, trains, ships, and aircraft.

Process applications aim to provide a suitable environment for a process being carried out, regardless of internal heat loads and external weather conditions. Although often in the comfort range, it is the needs of the process that determine conditions, not human preference. Process applications include:

  • Hospital operating theatres in which air is filtered to high levels to reduce infection risk and the humidity controlled to limit patient dehydration. Although temperatures are often in the comfort range, some specialist procedures such as open heart surgery require low temperatures (about 18 °C, 64 °F) and others such as neonatal relatively high temperatures (about 28 °C, 82 °F).
  • Cleanrooms for the production of integrated circuits, pharmaceuticals and the like in which very high levels of air cleanliness and control of temperature and humidity are required for the success of the process.
  • Facilities for breeding laboratory animals. Since many animals normally only reproduce in spring, holding them in rooms at which conditions mirror spring all year can cause them to reproduce year round.
  • Aircraft air conditioning. Although nominally aimed at providing comfort for passengers and cooling of equipment, aircraft air conditioning presents a special process because of the low air pressure outside the aircraft.
  • Data Processing Centers
  • Textile Factories
  • Physical Testing Facilities
  • Plants and Farm Growing Areas
  • Nuclear Facilities
  • Mines
  • Industrial Environments
  • Food Cooking and Processing Areas
In both comfort and process applications not only is the objective to control temperature (although in some comfort applications this is all that is controlled) but other factors including humidity, air movement and air quality.

Air Conditioning System Basics and Theories

Refrigeration cycle

A diagram of the refrigeration cycle: 1) condensing coil, 2) expansion valve, 3) evaporator coil, 4) compressor.

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 due to the second law of thermodynamics. 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 usually driven by a belt connected to 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 low pressure compartment (the evaporator coil), where, despite the low temperature, the low pressure causes 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. The heat exchanger in the condenser section (the heat sink mentioned above) is cooled most often by a fan blowing outside air through it, but in some cases can be cooled by other means such as water, especially on some ships.


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 discomforting.


"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.

Air conditioning system equipment

Window and through-wall units

Many traditional air conditioners in homes or other buildings are single rectangular units. 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. There are vents on both the inside and outside parts of the unit, so inside air to be cooled can be blown in and out by a fan in the unit, and so 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.

Evaporation coolers

Main article: Swamp cooler
In very dry climates, so-called "swamp coolers" are popular for improving comfort during hot weather. The evaporative cooler is a device that draws outside air through a wet pad. 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, FL in 1842, who used the device to cool the patients of his malaria hospital.

There is a 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.

Portable air conditioners

A portable air conditioner or portable A/C is an air conditioner on wheels that can be easily transported inside a home or office. They are currently commercially produced and sold in the U.S. with capacities of 6000 to 14000 BTU/h (1800 to 4100 watts output), with and without heaters, by companies like Gree, GE, EdgeStar, DeLonghi, Maytag, LG, Sunpentown, Fujitronic and others.

Portable A/C comes in two forms, split and monoblock. Monoblock systems have either one or two 4” or 6” air ducts which are vented to the outside. The single duct monoblock unit is often ineffective because air is pumped out of the room. This air is then replaced by hot air from outside, thus reducing efficiency. One simple method to counteract this undesirable effect is to attach a box to the back of the unit where the air is drawn over the condenser, and add some additional ducting to essentially convert the unit to twin pipe. This hack is reputed to be able to build up a much greater temperature difference than a single pipe unit.

Cooling power figures should be regarded suspiciously. It is best to look at both the input power and the heat transfer figures. For example, a 1350W unit will generally give a 3 kW heat transfer. Twin pipe mode will reduce the initial efficiency (see Carnot cycle, efficiency = TL/(TH-TL)) as TH has increased (assuming outdoors is hotter than indoors) but the obvious benefit of greater temperature difference will mean the unit doesn’t need to run continually so efficiency will increase. Avoid units using R-22 as it is ozone depleting and the less efficient ones using R407c. The best choice would be a unit using R-410a gas, also known as Puron that is ozone-friendly and allows a higher efficiency.

When selecting a new portable air conditioner it is very important to select one with the proper cooling capacity for the room you are trying to cool. Use this simple air conditioner sizing calculator to help with your window or portable air conditioner selection.

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 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 ([3] 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. Definition of SEER (scroll down to "Seasonal energy efficiency ratio")

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:SEER conversion formulas from Pacific Gas and Electric

(1)     SEER = EER ÷ 0.9

(2)     SEER = COP x 3.792

(3)     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 residental 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).Minimum SEER ratings required in the US 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 $0.12 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) × ($0.12/kW·h) ÷ (10 BTU/W·h) ÷ (1000 W/kW) = $864.00 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 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, South Korea, Taiwan, 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 North America, home air conditioning is more prevalent in the South, Midwest, East Coast, the Great Lakes States, and Ontario Canada 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. Older houses and buildings not retro-fitted with central air often still use window or through-wall units.

In Europe, home air conditioning is less common in part due to higher energy costs. 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. Due to the 2003 and the 2006 heatwaves, portable air conditioners have become more popular in France, with prices starting around 300 - 400 € and in the UK, with the prices falling below £200 for a 9000 BTU unit.

In many countries in the Arab Gulf, air conditioning is ubiquitous. This is due the very harsh atmoshpere, and the relatively high living standards.

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.Sick building syndrome 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.Home Control of Asthma & Allergies

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.

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