Automotive Air Conditioning Systems

Today, as we drive our automobiles, a great many of us, can enjoy the same comfort levels that we are accustomed to at home and at work. With the push of a button or the slide of a lever, we make the seamless transition from heating to cooling and back again without ever wondering how this change occurs. That is, unless something goes awry.

Since the advent of the automotive air conditioning system in the 1940's, many things have undergone extensive change. Improvements, such as computerized automatic temperature control (which allow you to set the desired temperature and have the system adjust automatically) and improvements to overall durability, have added complexity to today's modern air conditioning system. Unfortunately, the days of "do-it-yourself" repair to these systems, is almost a thing of the past.

To add to the complications, we now have tough environmental regulations that govern the very simplest of tasks, such as recharging the system with refrigerant R12 commonly referred to as Freon® (Freon is the trade name for the refrigerant R-12, that was manufactured by DuPont). Extensive scientific studies have proven the damaging effects of this refrigerant to our ozone layer, and its manufacture has been banned by the U.S. and many other countries that have joined together to sign the Montreal Protocol, a landmark agreement that was introduced in the 1980's to limit the production and use of chemicals known to deplete the ozone layer.

Now more than ever, your auto mechanic is at the mercy of this new environmental legislation. Not only is he required to be certified to purchase refrigerant and repair your air conditioner, his shop must also incur the cost of purchasing expensive dedicated equipment that insures the capture of these ozone depleting chemicals, should the system be opened up for repair. Simply put, if your mechanic has to spend more to repair your vehicle - he will have to charge you more. Basic knowledge of your air conditioning system is important, as this will allow you to make a more informed decision on your repair options.

Should a major problem arise from your air conditioner, you may encounter new terminology. Words like "retrofit" and "alternative refrigerant" are now in your mechanics glossary. You may be given an option of "retrofitting", as opposed to merely repairing and recharging with Freon. Retrofitting involves making the necessary changes to your system, which will allow it to use the new industry accepted, "environmentally friendly" refrigerant, R-134a. This new refrigerant has a higher operating pressure, therefore, your system, dependant on age, may require larger or more robust parts to counter its inherent high pressure characteristics. This, in some cases, will add significantly to the final cost of the repair. And if not performed properly, may reduce cooling efficiency which equates to higher operating costs and reduced comfort.

From : www.aircondition.com

Air-Conditioning Project Development and System Design

The goal of an air-conditioning/HVAC&R system is to provide a healthy and comfortable indoor environment with acceptable indoor air quality, while being energy efficient and cost effective. ASHRAE Standard 62-1989 defines acceptable indoor air quality as “air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction.”

The basic steps in the development and use of an air-conditioning project are design, installation, commissioning, operation, and maintenance. There are two types of air-conditioning projects: designbid and design-build.

A design-bid project separates the design (engineering consultant) and installation (contractors) responsibilities. In a design-build project, the design is also done by the installation contractor. A design-build project is usually a small project or a project having insufficient time to go through normal bidding procedures. In the building construction industry, air-conditioning or HVAC&R is one of the mechanical services; these also include plumbing, fire protection, and escalators.


Air-conditioning design

is a process of selecting the optimum system, subsystem, equipment, and components from various alternatives and preparing the drawings and specifications. Haines (1994) summarized this process in four phases: gather information, develop alternatives, evaluate alternatives, and sell the best solution. Design determines the basic operating characteristics of a system. After an air-conditioning system is designed and constructed, it is difficult and expensive to change its basic characteristics.

The foundation of a successful project is teamwork and coordination between designer, contractor, and operator and between mechanical engineer, electrical engineer, facility operator, architect, and structural engineer. Field experience is helpful to the designer. Before beginning the design process it is advisable to visit similar projects that have operated for more than 2 years and talk with the operator to investigate actual performance.

Air Conditioning System Selection

When considering and selecting an air conditioning system, the designer must understand the
building and the client’s requirements and try to study and evaluate the following factors:
• Building location, surrounding environment and external climate
• Uses and functional requirements of the building
• Client’s budget, investment policy and expected quality of service
The designer should consider various system options and recommend one or several that will be likely to perform as desired. Some of the selection criteria include:
• Performance requirements – on comfort, noise, control options, flexibility and meeting
requirements of local regulations/codes
• Capacity requirements – range of capacity, multiple units, zoning, etc.
• Spatial requirement – plant room space, space for ducting and piping (vertical shafts), space for terminal equipment
• Costs – initial cost, operating cost and maintenance cost
• Energy consumption – for both economic and environment reasons
• System qualities – e.g. aesthetics, life, reliability and maintainability

How air conditioning works

The essential ingredients in an air conditioning system are a fan to blow air around, a cold surface to cool and dehumidify the air, a warm surface and a source of water vapour. In a large system there will also be a tangle of tubes to distribute the air and collect it again. Notice that the cold surface has two independent jobs to do: it is used to cool the air and it is also used to dehumidify, by condensing water from the air. This means that the air is cooled more than is necessary for temperature control, so that it must be heated again afterwards.
Air conditioning wastes energy. It also wastes space, because air has to be pumped around in quite large tubes, so that it doesn't make a rushing noise. It does, nevertheless, generate a disturbing background noise if the room is otherwise quiet. There is worse news to come: air conditioning is spreading to more and more buildings in climates that we once thought were so mild that they would be immune.

Simple air conditioning

A fan draws air from the room first through a cooling device, consisting of metal fins extending from a pipe through which cooling fluid circulates, at a rate determined by the thermostat or by the humidistat. The air next passes over a heater, usually electrical, which is energised on instructions from the room thermostat. Air conditioning has in the past been used where the climate is too hot for comfort. Cooling will increase the relative humidity of the air, so humidification is not usually built into these systems. If it is necessary, the usual method is to inject steam from electrically boiled water.

Large air conditioning systems

The principle of operation is the same as that of the small system described above except that the cooling fluid is usually water, which has itself been cooled by the refrigeration system. The air is circulated through ducts, with a portion of fresh air added. There is therefore a pre-heater, because the outside air may be below zero and will therefore freeze the water in the cooling coil.

Classification of Air Conditioning Systems

The purpose of classifying air conditioning systems is to distinguish one type from another and to provide a background for selecting the optimum air conditioning system based on building requirements. A classification of air conditioning systems should include the classification of air and refrigeration systems in order to define a more specific system.
Air conditioning systems can be classified into three categories corresponding to their related equipment as follows:

• Individual Systems – use a self-contained , factory-made air conditioner to serve one or two rooms (e.g. room/ window air conditioner and split-type units).
• Unitary Packaged Systems – similar in nature to individual systems but serve more rooms or even more than one floor, have an air system consisting of fans, coils, filters, ductwork and outlets (e.g. in small restaurants, small shops and small cold storage rooms).
• Central (Hydronic) Systems – basically consists of three major parts:
Air system – air handling units (AHU), air distribution (air duct) system and terminals.
Water system – chilled water system, hot water system, condenser water system.
Central plant – refrigeration (chiller) plant, boiler plant.

Individual Systems
An individual air-conditioning system normally employs either a single, self-contained, packaged room air conditioner (installed in a window or through a wall) or separate indoor and outdoor units to serve an individual room. “Selfcontained, packaged” means factory assembled in one package and ready for use.

Unitary Packaged Systems or Packaged Systems.
These systems are installed with either a single selfcontained, factory-assembled packaged unit (PU) or two split units: an indoor air handler, normally with ductwork, and an outdoor condensing unit with refrigeration compressor(s) and condenser . In a packaged system, air is cooled mainly by direct expansion of refrigerant in coils called DX coils and heated by gas furnace, electric heating, or a heat pump effect, which is the reverse of a refrigeration cycle.

Central Hydronic or Central Systems
A central system uses chilled water or hot water from a central plant to cool and heat the air at the coils in an air handling unit (AHU) . For energy transport, the heat capacity of water is about 3400 times greater than that of air. Central systems are built-up systems assembled and installed on the site.

An air-handling unit (AHU) is the basic piece of equipment used in an air system. It can be either a field-assembled built-up system or a factory-made unit. Because a central system always has a chilled water system, the type of coil installed in an AHU is a water cooling coil, which is different from an air handler in a packaged system, which uses a DX (direct expansion) coil.

Air Conditioning

is a process that simultaneously conditions air; distributes it combined with the outdoor air to the conditioned space; and at the same time controls and maintains the required space’s temperature, humidity, air movement, air cleanliness, sound level, and pressure differential within predetermined limits for the health and comfort of the occupants, for product processing, or both.

Comfort Air-conditioning – a process of controlling the air temperature, relative humidity, ventilation, air movement and air cleanliness of a given space in order to provide the occupants with a comfortable indoor temperature.

Air-conditioning System – consists of a group of components or equipment connected in series to control the environmental parameters.

An air-conditioning or HVAC&R system consists of components and equipment arranged in sequential order to heat or cool, humidify or dehumidify, clean and purify, attenuate objectionable equipment noise, transport the conditioned outdoor air and recirculate air to the conditioned space, and control and maintain an indoor or enclosed environment at optimum energy use.

The types of buildings which the air-conditioning system serves can be classified as:

• Institutional buildings, such as hospitals and nursing homes

• Commercial buildings, such as offices, stores, and shopping centers Air-Conditioning and Refrigeration

• Residential buildings, including single-family and multifamily low-rise buildings of three or fewer stories above grade

• Manufacturing buildings, which manufacture and store products