Air Conditioners
Air conditioning is the process of mechanically cooling air to create appropriate temperatures within a room, building, or facility. Air conditioners can be small enough to fit on a window ledge, large enough to cool an entire skyscraper, or any portion thereof. They utilize a system of compressors, condensers, fans, refrigerant gases, thermostats, and, sometimes duct work to chill and transport air for human comfort, or to keep processing components and materials, such as electronics or volatile chemicals, at proper operating and storage temperatures.
Quick links to Air Conditioners Information
The History of Air Conditioners
- Early History of Air Conditioning
- Evaporative cooling was the first step toward air conditioning and was used by ancient Egyptians who hung curtains of wet reeds to cool the air as breezes evaporated the water. The Romans used cool water from aqueducts to fill channels within the walls of certain buildings. The Persians used a system of cisterns and wind towers for cooling.
- Chinese prisoners were employed to operate a series of rotating fans to cool the Imperial Palace during the Han Dynasty of the 2nd century. This system remained in effect, largely unchanged, through the Song Dynasty of the 13th century.
- Doctor John Gorrie was convinced that he could overcome what he called "the evils of high temperatures" in 1840's Florida. He believed that cooling the air in hospitals could help prevent the spread of disease and provide relief to malaria patients. His initial efforts required shipping in blocks of ice from frozen northern states. The air was cooled by fans placed near the ice blocks. This method was only mildly effective and very expensive, but was the first step toward cooler air.
- Gorrie, working with Michael Faraday's 1824 principles of refrigeration and discovery that liquefied ammonia will chill air as it evaporates, experimented with mechanical cooling and invented a rudimentary compressor that could make ice. He patented his invention in 1851, but when his primary financier died, so did his dream.
- Air Conditioners in the 1900s
- In 1902, Willis Carrier was challenged with solving problems associated with high humidity for a Brooklyn lithographer and printer. The ambient humidity was wrinkling magazine pages. His attempts eventually begat his patented "Apparatus for Treating Air" which utilized tubular coils filled with water to humidify or dehumidify a room. Further developments included an automatic control system for managing temperatures and humidity levels in textile mills.
- Carrier soon realized the implications of his inventions and broke into his own manufacturing business where he pursued the development of modern air conditioning on an industrial scale.
- Air conditioning got its first big break in 1920's theaters. Early systems used modified heating units equipped with refrigeration compressors to blow air through floor vents, leaving the movie goers with cold feet and hot heads.
- In 1922, the Metropolitan Theater in Los Angeles touted the first well designed vent system to distribute air more efficiently. That same year, a centrifugal chiller was introduced at the Rivoli Theater in New York. The centrifugal chiller had fewer moving parts, was more reliable, and less expensive to make and operate.
- Refrigerant gases originally used for air conditioning included ammonia, methyl chloride, and propane. These gases are toxic and/ or highly flammable, making them potentially deadly. In 1928, Thomas Midgely, Albert Henne, and Robert McNary were able to synthesize chlorofluorocarbon (CFC) refrigerants which were the first non-flammable refrigerant gases, including freon.
- The discovery and development of chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFC), and hydrofluorocarbons (HFC) allowed manufacturers to produce safer, more effective air cooling. These gases are alpha-numerically labelled by their molecular composition (e.g. R-11, R-12, R-13a, etc...) Many were discontinued or even prohibited from use, by various agencies and protocols, due to their contribution to climate change and ozone depletion, but not before they played an enormous role in creating comfort for billions of grateful citizens.
- A split system room cooler was created for home use in 1929, but it was heavy, expensive, and required a separate, remote condenser. Frank Faust worked with the design and by 1931 was able to offer a self contained room cooler.
- H. H. Schultz and J. Q. Sherman were the first to offer a window ledge air conditioner in 1932. It was a compact unit but very costly and was not met with great consumer enthusiasm.
- Henry Galson developed a more compact, lighter weight, and less expensive window cooler for home use, and by 1947, had sold 43,000 units.
- 1957 saw the first rotary compressor in an air conditioning unit. This made the equipment much smaller, lighter, and easier to handle.
- By the late 1960s, nearly all new construction boasted central air. This innovation caused a population boom in hot climates, like Arizona, Texas, and Florida.
- The energy crisis of the mid-1970s caused the Department of Energy to establish energy reduction laws and efficiency standards. 1992 introduced DOE conservation standards regarding the energy efficiency of home air conditioning units. These standards have been instrumental in creating A/C units today that use only half the energy of those made in 1990.
- Micro-processing began to develop in the 1990s and semiconductor technology lent itself to making high efficiency, low energy air conditioners that could be controlled manually or automatically. Programmable thermostats became available to help reduce energy consumption.
- Added Air Conditioner Requirements in the 2000s
- Since 2006, Seasonal Energy Efficiency Ratios (SEER) of air conditioning units have been required to meet minimum energy efficiency standards. The rating is based on the normal annual quantity of BTU of cooling output divided by the energy input, stated in watt hours for the same time period. [SEER= {BTU / (W x h)}] In the United States, the minimum SEER is 13.
Workings and Design of Air Conditioning
Rather than continuously cooling warm air from outside, modern air conditioners recirculate the air from within a space to make it increasingly cooler. Many air conditioning units are also equipped with a heat pump, allowing them to provide heat as well as cooling.
Air conditioners work on the base principle of evaporative cooling in a vacuum, a principle first discovered by Benjamin Franklin and John Hadley at Cambridge University in 1758. They noted that evaporation of volatile liquids, such as ether or alcohol, could be used to chill objects to below the freezing point of water.
Normally, cold moves toward heat. This is evident when you place your hand near an ice cold beverage on a hot day. You can feel the cold emanating from the glass and see the moisture from the air condensing on the outside.
In the refrigeration cycle, "work" is required to remove the warm air from inside the space to keep it cool. This is accomplished through the use of refrigerant gases that pass through a closed loop system of pipes or tubes. Separate fans are required for the circulation of air in the cooling/ evaporative cycle and the heating/ condensing cycle.
The gas enters a compressor where the temperature rises as it is compressed. The hot gas vapor moves into the condenser where air flowing over the coils causes the gas to liquefy as it cools. Still under pressure, the condensed gas passes through an expansion valve, which may be a simple hole in the piping or a pressure activated valve. The gas expands rapidly, resulting in flash evaporation, which drops the temperature drastically.
The chilled refrigerant flows into an evaporator where a fan blows air to be cooled across the evaporator coils. The refrigerant evaporates as it loses its cool, and the gas is pumped back into the compressor to continue the cycle. The most popular refrigerant found in home AC units is R-22, chlorofluoromethane. The HFC R-134a is most commonly used in cars.
Heating and air conditioning output are calculated in terms of British thermal units (btu). One btu is equal to the amount of energy required to raise the temperature of one pound of water one degree F. In heating, the btu is measured as the amount of energy required to add heat to a space. In air conditioning, they are measured as the amount of energy required to remove heat. A 15,000 btu air conditioner removes 15,000 units of heat.
Air Conditioner Location and Sizing
- Where to Find an Air Conditioner
- Air conditioners are found in cars, airplanes, trains, and cruise- and cargo- ships. They are staples of comfort for office buildings, restaurants, shopping centers, apartment buildings, sports stadiums, hospitals and schools. Air conditioning is imperative in industrial settings to maintain the comfort of workers and safety of equipment that might otherwise overheat.
- Air conditioners are widely used in residential, commercial and industrial settings, and air conditioner manufacturers design a variety of models to accommodate many different environments and needs. Air conditioning units are usually placed in buildings where fans or natural breezes are inadequate because of temperature, air disturbance, or pollution. Air conditioning systems often use air filters to maintain air quality and avoid spreading germs, pollutants or other unwanted irritants.
- Air Conditioners Sizes
- Variations in size, power and air delivery help consumers choose the right kind of unit for their needs. Examples include cabinet air conditioners, ductless air conditioners, central air conditioners and split air conditioners, to name just a few. The size and power of the unit is the most important specification. Portable air conditioners are easily moved from one area of a facility to another but can only provide limited cooling. They are also known as spot coolers and are frequently positioned in places where the main air conditioning system does not reach or where additional cooling is needed.
- Next in size are cabinet air conditioners, which are contained in a boxlike enclosure. The cabinet enclosure helps protect the internal components from dust, water, liquid spray, accidental contact, and other possible interferences. The unit is generally attached to a system of air ducts or air handling equipment that delivers the cool air throughout a building. In some cases, however, ductwork cannot be installed in a safe or cost effective way. Ductless air conditioners blow the air into a space directly from the unit. Ductless air conditioners are able to cool larger areas than portable or window mounted air conditioners because multiple evaporators can be connected to a single compressor. Central air conditioners are powerful enough to cool an entire building. For residential buildings, the unit is usually placed beside a structure; for industrial or commercial systems the units may be on top of the roof. Split air conditioners separate the hot condensing unit from the cool air handling unit. The hot components are kept outside in order to maximize the efficiency of the air conditioning. There may be one large condensing unit or a number of small ones to cool various areas of the building differently. Split air conditioners are ineffective for multi-level structures; large buildings require more power. Commercial air conditioners are used to cool commercial buildings such as offices and retail stores. They generally use central air conditioning systems to keep the entire building at one consistent, comfortable temperature. Similarly, industrial air conditioners are used for cooling in a variety of industrial applications such as manufacturing, shipping and warehouse facilities.
Types of Air Conditioning Systems
- Split Air Conditioner System
- In a split air conditioner system, the evaporator, or inside heat exchanger, is physically separated, by some distance, from the condenser, or outside heat exchanger. The mini-split system is typically a ductless air conditioner system that can be used to heat a single zone, or may be equipped with multiple evaporator units to heat up to eight zones from a single condenser.
- Multi-split Air Conditioner System
- Utilize a single, large capacity, outside condenser, equipped with ports in a manifold, to direct the flow through multiple evaporators to cool large buildings with multiple zones or rooms.
- Multi-Zoned Conditioner Systems
- Offer the advantage of flexibility due to their compact evaporator size and variety of installation configurations. The installation costs are higher than other systems but lower operation costs for split air conditioning systems can make them a worthwhile investment.
- Central Air Conditioner
- Or ducted air conditioners are found in most modern construction. Central air systems use forced air to move cooled air from a single unit through ductwork with louvered openings.
Industrial Air Conditioners Uses and Proper Care
- Air Conditioners and Avoiding Overheating
- Industrial air conditioners are the first line of defense against flagging personnel and loss of equipment. Studies have shown that workers are less productive in an overly warm environment, and, conversely, more productive when room temperatures are lowered even a few degrees. Electronic equipment emits a significant amount of heat which must be removed, to prevent overheating and the domino effect that loss of power or service creates.
- Industrial air conditioning is used to accurately control humidity levels to reduce static electricity, over-drying of textiles, warping of paper, and ensuring proper flow of powdered materials. Proper filtration to facilitate removal of contaminants for clean rooms and manufacturing environments must be incorporated into an industrial air system.
- Proper Care for Industrial Conditioners
- Mass air cooling and delivery is a carefully calculated equation, specifically suited to each application. The size, shape, purpose, and integrity of each space must be considered to provide adequate cooling capacities. Thought must be given to the quality of the air being delivered, as well.
- In an industrial setting, the ductwork for air conditioning may be extensive, spanning hundreds, if not thousands of feet. Dust, fumes, mold, and other contaminants can enter the system, spreading unhealthy air throughout an entire facility. In some cases, just entering the building can make people ill. This is known as "Sick Building Syndrome".
- Proper filtration, regular cleaning, and inspection of equipment, particularly ductwork, can eliminate the spread of disease or illness.
The right heating, ventilation, and air conditioning (HVAC) system will vary with each individual application. Air conditioners can be purchased at many department stores and most home improvement stores, but the best choice may not be available from off the shelf. A professional assessment of your particular needs should be the first step in the air conditioning process.
Evaluation and calculation for quantity of air, temperatures, humidity levels, and cost factors, with regard to installation and operation, are all critical to overall design. Your HVAC specialist can direct you in creating a system that meets your standards and budget.
Air Conditioner Terms
- Accumulator
- A shell-shaped device that is incorporated into an HVAC (heating, ventilation, air conditioning) system within the suction line in order to protect the compressor from liquids.
- Active Cooling
- Air-conditioning driven by a compressor.
- Adsorption
- Drying air or gas by holding moisture vapor on a desiccant surface without mixing with its molecular structure.
- Air Handler
- The indoor unit of an air conditioning system that provides conditioned air into a space. An air handler consists of a heat exchange coil, filters and a fan.
- BTU (British Thermal Unit)
- A unit of measurement of heat or energy. One BTU is the amount of heat needed to raise or reduce the temperature of one pound of water by one degree Fahrenheit.
- Capacity
- The maximum amount of heat energy that can be removed from or added to a medium by an HVAC system.
- Compressor
- Also referred to as the "heart" of a refrigerating or a/c system, a compressor is a pump that uses pressure to move refrigerant through pipes between an outdoor condensing unit and an indoor evaporating unit.
- Condensation
- The process of changing air into liquid.
- Desiccant
- A substance suitable for absorbing and absorbing moisture.
- District Heating and Cooling
- The utilization of a central utility system to heat or cool large residential and industrial areas.
- Duct
- A device used to distribute air.
- Filter
- A device used for the removal of solid and liquid particles.
- Gravity Cooling
- Cooling with an evaporator that does not have a fan to circulate the air.
- Heat Exchanger
- A device capable of transferring heat from one place or medium to another.
- Absolute Humidity
- Mass of water vapor present per unit volume of air, usually measured as grains/ft3, lbs/ft3 or grams/ft3.
- Inlet Pressure
- The total pressure at the inlet flange of the compressor.
- Load
- The required rate of heat removal.
- Moisture Separator
- A device that separates condensate from an air stream.
- Precooler
- A heat exchanger that lowers the temperature of the inlet air with the help of the outgoing cold air. In the process, the outgoing air is reheated by the incoming air.
- Refrigerant
- The medium of heat transfer in a refrigeration system that picks up heat by evaporating at a low temperature and rejects heat by condensing at a high temperature.
- Silica Gel
- A regenerative desiccant.
- Vapor Pressure
- The pressure exerted on a liquid and vapor surface.