Examples of Buildings with Single-zone Package Air-Conditioning Units
Figure 6.1 shows four identical single story buildings, A, B, C, and D. Each has a single-zone package air-conditioning unit (marked “AHU”) located on the roof.
Building A: This unit has only an electrical supply. This single electrical supply provides all the power for heating, cooling, humidifying, and for driving the fans.
Building B: This unit has the electrical supply for cooling, humidifying, and for driving the fans, while the gas line, shown as “gas supply,” provides heating.
These first two arrangements are commonly available as factory engineered, off the shelf, rooftop packages. Among these packaged units, there is a great range in size, quality, and features. The most basic provide few, if any, options. They are relatively difficult to service and have a relatively short life. At the other end of the spectrum, there are large units with walk-in service access and numerous energy-conserving options. These are designed to last as long as any indoor equipment.
As well as the total pre-packaged units, there are units, typically in larger buildings or complexes of buildings, where the heating is provided from a central service. For example, a boiler room can produce hot water that is piped around the building or buildings to provide heat. Each air-handling unit that needs heating has hot water piped to it.
Building C: This unit has the electrical supply for cooling, humidifying, and for driving the fans. It also has supply and return hot-water pipes coming from a boiler room in another building. The unit contains a hot-water heating coil and control valve, which together take as much heat as needed from the hot water supply system.
Building D: In the same way, there may be a central chiller plant that produces cold water at 42°F – 48°F, called chilled water. This chilled water is piped around the building, or buildings, to provide the air-handling units with cooling. Like the heating coil and control valve in Building C, there will be a cooling coil and control valve in each unit, to provide the cooling and dehumidification.
To recap, a packaged unit can require just an electrical source of power, or it may get heating in the form of a gas or hot water supply, and may get cooling from a source of chilled water. The basic operation of the unit stays the same; it is just the source of heating and cooling energy that may change.