Heating, Ventilating, and Air Conditioning (HVAC) equipment perform heating and/or cooling for residential, commercial or industrial buildings. The HVAC system may also be responsible for providing fresh outdoor air to dilute interior airborne contaminants such as odors from occupants, volatile organic compounds (VOC’s) emitted from interior furnishings, chemicals used for cleaning, etc. A properly designed system will provide a comfortable indoor environment year-round when properly maintained.
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How does my AC work?
An air conditioner cools and dehumidifies the air as is passed over a cold coil surface. The indoor coil is an air-to-liquid heat exchanger with rows of tubes that pass the liquid through the coil. Finned surfaces connected to these tubes increase the overall surface area of the cold surface thereby increasing the heat transfer characteristics between the air passing over the coil and liquid passing through the coil. The type of liquid used depends on the system selected. Direct-expansion (DX) equipment uses a refrigerant as the liquid medium. Chilled-water (CW) can also be used as a liquid medium.
When the required temperature of a chilled water system is near the freezing point of water, freeze protection is added in the form of glycols or salts. Regardless of the liquid medium used, the liquid is delivered to the cooling coil at a cold temperature.
As this liquid passes through the indoor cooling coil on the inside of the heat exchanger, two things happen to the air that passes over the coil’s surface on the outside of the heat exchanger. The air’s temperature is lowered (sensible cooling) and moisture in the air is removed (latent cooling) if the indoor air dew point is higher than the temperature of the coil’s surface.
What type of AC systems are available?
A split system is a combination of an indoor air handling unit and an outdoor condensing unit. The indoor air handling unit contains a supply air fan and an air-to-refrigerant heat exchanger (or cooling coil), and the expansion device. The outdoor condensing unit consists of a compressor and a condenser coil. Split-systems are typically found in residential or small commercial buildings. These systems have the highest energy efficiency rating (EER) of all the available AC systems.
Manufacturers are required to take the EER rating a step further and provide a seasonal energy efficiency rating (SEER) for use by consumers. SEER ratings vary widely and range from 10 to 20. The higher the SEER rating, the more efficient the AC system operates. If heating is required, an alternate method of heating the interior of the building must be used, usually in the form of electric or gas heating.
Cooling Only Packaged-System
A packaged system is a single unit combining all the components described in the split system. Since the unit is a package, it must be placed outside the building and indoor air is “ducted” from the building to the packaged system and back through an air distribution system. These units typically have SEER rating from 10 to 18. If heating is required, an alternate method of heating the interior of the building must be used, usually in the form of electric or gas heating.
Heat pumps are similar to cooling only systems with one exception. A special valve in the refrigeration piping allows the refrigeration cycle to be operated in reverse. A cooling only system cools the indoor air and rejects heat to the outdoors. A heat pump can also cool the indoor air, but when the valve is reversed, the indoor air is heated. A supplementary electric resistance heater may also be used to assist the heat pump at lower outdoor temperatures. In colder climates, heat pumps require a defrost period. During defrost, times the electric heater is the only means of heating the interior of the building. These units are manufactured as either split or packaged systems.
Chilled Water System
In a chilled water system, liquid water is pumped throughout the building to “chilled water coils”. Since the liquid water needs to be at a cold temperature, a “cooling plant” is required. The plant is typically referred to as a chiller plant. Vapor compression equipment in the plant, similar to that described in “How does my AC work”, cool water to a cold temperature and pump the cold water to air-to-water heat exchangers where needed.
Window Air Conditioners
As the name implies, a window air conditioner is typically installed in a window or custom opening in a wall. The Window AC can only cool small areas and are not intended to provide cooling to multiple rooms or zones. These air conditioners are manufactured as cool only or can provide both cooling and heating. An optional damper in the unit can provide fresh outdoor air if necessary.
Packaged Terminal Heat Pump
Packaged terminal heat pumps (PTHP) are similar to a window-mounted air conditioner. These units are typically installed in a sleeve passing through the outdoor wall of an apartment, hotel, school classroom, etc. PTHPs are completely self-contained and require only an electrical connection in addition to the opening in the building shell. They use the outdoor air as the heat source in winter and as a heat sink in summer. They also can provide ventilation air. Flexibility and lower installed cost are the primary advantages of the PTHP. Disadvantages include in-room maintenance, higher operating cost, relatively short life, imprecise “on-off” temperature control, and they can be rather noisy.