definitions - ballast 28 Flashcards
are a measure of the approximate average yearly temperature difference between the outside and the inside in a particular location.
degree days
is based on the transfer of heat during the liquefaction and evaporation of a refrigerant.
compressive refrigeration
produces chilled water and is accomplished by the loss of heat when water evaporates. This evaporation is produced in a closed system by a salt solution that draws water vapor from the evaporator.
absorption refrigeration
It is the cooling effect obtained when 1 ton of 32°F ice melts to water at 32°F in 24 hours. This is equivalent to 12,000 Btu/hr (3516 W).
ton of refrigeration
This is a self-contained unit that passes nonducted air, which is to be cooled, over the evaporator and back into the room.
direct expansion, or DX, system (DX), also known as an incremental unit.
cool or heat spaces by conditioned air alone. Heat is transported to the space with supply and return air ducts.
all air systems
This system provides two parallel ducts, one with hot air and one with cool air. These two streams of air are joined in a mixing box in proportions to suit the temperature require- ments of the conditioned space. A thermostat controls pneumatic valves in the mixing box to create the proper mixture
dual duct (high velocity system)
With this system, air is heated or cooled as required in a central plant and distributed to the building at a constant temperature through a single duct. At each zone, a thermo- stat controls a damper that varies the volume of conditioned air entering the space to respond to the user’s needs.
Variable air volume system VAV
takes return air and fresh outdoor air and cools and dehumidifies the mixture, which is then distributed in a constant volume at low temperature throughout the building. At or near the spaces to be conditioned, the air is reheated as required by the cooling load of the space.
Reheat (constantvolume) system
uses a fan coil unit in each conditioned space. The fan coils are connected to one or two water circuits. Ventilation is provided with openings through the wall where the fan coil unit is located, from interior zone air heating, or by simple infiltration.
all water system
rely on a central air system to provide humidity control and ventilation air to conditioned spaces. However, the majority of the heating and cooling is pro- vided by fan coil units in each space
air water systems
In air ducts, there is a loss of pressure due to the friction of the air moving through the ducts, fittings, registers, and other components. The pressure required to overcome this friction loss is called the
static head
reuses outdoor air when it is cool enough to mix with recirculated indoor air. This reduces the energy required for refrigeration and is useful when the outdoor air temperature is about 60°F
economizer cycle
heats water by passing hot gases directly through the water.
direct contact water heater
recovers the sensible and latent heat from the high heat of exhaust flue gases that would nor- mally be discharged to the atmosphere.
recuperative gas boiler
is an air distribution system in which supply air originates at floor level and rises to return air grilles in the ceiling as shown in Fig. 28.10.
Displacement ventilation
compose a heating and cooling system that uses a series of heat pumps (described earlier in this chapter) for different zones of a building, which are all connected to the same piping system of circulating water.
water loop heat pump
transfer heat between two air streams through the use of a heat exchanger wheel consisting of small openings through which the air passes.
Energy transfer wheels, also called enthalpy heat exchangers
is a self-contained device that transfers sensible heat energy from hot exhaust air to cool outdoor air. As the hot exhaust air passes over the heat pipe, it vaporizes a refrigerant inside the pipe, which passes to the area of cool incoming air.
heat pipe
use water or some other liquid transfer medium to exchange heat
Water-to-waterheat exchangers,sometimes called runaround coil
uses a double-paned insulated glass unit over which another pane of glass is placed on the inside of the building, Air is drawn up between the inside pane and the main window unit and is extracted into the return air system.
extract air system
heat or cool outside air by circulating it through pipes buried in the ground, In the summer the air can be used directly if the outdoor air is higher than the ground temperature. In the winter the system can preheat air for an energy recovery ventilator (to pre- vent frosting) or for a standard fan-coil heating unit.
ground coupled heat exchanger
