Introduction
In the last chapter, we considered two types of single zone direct expansion systems: the packaged rooftop system and the split system. The direct-expansion- refrigeration rooftop unit contained all the necessary components to condition a single air supply for air-conditioning purposes.
These same components can be manufactured in a wide range of type and size. As an alternative to a rooftop unit, they can be installed indoors, in a mechanical room, with the different components connected by sheet-metal ducting.
Both the packaged rooftop unit and the inside, single-zone unit produce the same output: a supply of treated air at a particular temperature.
The heating or cooling effect of this treated airflow, when it enters a zone, is dependent upon two factors:
The flow rate, (measured in cubic feet per minute, cfm).
The temperature difference between the supply air and the zone temperature, (measured in degrees Fahrenheit, °F).
When the unit is supplying one space, or zone, the temperature in the zone can be controlled by Changing the air volume flow rate to the space. Changing the supply air temperature.
Changing both air volume flow and supply air temperature.
In many buildings, the unit must serve several zones, and each zone has its own varying load. To maintain temperature control, each zone has an individual thermostat that controls the volume and/or temperature of the air coming into the zone.
Air-conditioning systems that use just air for air conditioning are called “all-air systems”.
These all-air systems have a number of advantages:
Centrally located equipment—operation and maintenance can be consolidated in unoccupied areas, which facilitates containment of noise.
Least infringement on conditioned floor space—conditioned area is free of drains, electrical equipment, power wiring and filters (in most systems). Greatest potential for the use of an economizer cycle—as discussed in Chapter 2, this can reduce the mechanical refrigeration requirements by using outside air for cooling, and therefore reduce overall system operating costs.
Zoning flexibility and choice—simultaneous availability of heating or cooling during seasonal fluctuations, like spring and fall. The system is adaptable to automatic seasonal changeover.
Full design freedom—allows for optimum air distribution for air motion and draft control.
Generally good humidity control—for both humidification and dehumidification.
All-air systems generally have the following disadvantages:
Increased space requirements—significant additional duct space requirements for duct risers and ceiling distribution ducts.
Construction dust—due to problems with construction-dust, all-air systems are generally available for heating later in the construction schedule than systems that use water to convey heat.
Closer coordination required—all-air systems call for close cooperation between architectural, mechanical and structural designers.