karim 2 Flashcards
Constant-Temperature, Variable Volume (On-Off)
When set temp is reached air is pumped into get the space air to desired temp
Single-Zone Constant Air Volume
single zone. Fan constantly run constantly run. has heating and coiling coil. Designed for only small buildings.
Single-Zone Reheat
constant-volume system used for air-conditioning when humidity control is especially important.
uses a cooling coil to cool and dehumidify air, along with a reheat coil for temperature control.
Humidifiers may also be used
why are single zone reheat unquie
Single-zone reheat systems are characterized by high energy usage.
Air may be overcooled to maintain humidity and reheated to compensate for the overcooling.
These systems are used only for special applications, such as hospital operating rooms and computer rooms.
Constant-Volume Terminal Reheat, Multiple Zones
The air-handling unit contains a cooling coil that chills all the air supplied to the various zones
One or more main trunk ducts are used to distribute air
The cooling coil chills air to the same temperature for all zones. The reheat coils respond to their respective thermostats to maintain temperature control in each zone.
benefits of a Constant-Volume Terminal Reheat, Multiple Zones
flexible enough that one may add or rearrange reheat boxes on their main trunk as space is reconfigured
Excellent humidity
why did Constant-Volume Terminal Reheat, Multiple Zones lost favor
concern for energy conservation and operating cost in 1970s
Constant-Volume Dual Duct
send warm and chilled air through a pair of main trunk ducts to the areas being served.
a single fan blows air through a heating coil and a cooling coil into a warm-air trunk duct and a chilled-air trunk duct.
Constant-Volume Dual Duct mixing boxes
controlled with dampers to vary the quantity of warm and chilled air in response to the zone thermostat.
dampers will be positioned to deliver more warm air and less chilled air if a space needs heating or less cooling.
total airflow, consisting of warm and chilled air, stays fairly constant, since each damper closes as the other opens.
advantage of constant -volume dual duct
Unlike the constant-volume terminal reheatsystem it has no coils installed over occupied spaces; thus it reduces maintenance and avoids the possibility of water leaks.
Multizone
similar in operation to dual-duct systems.
Warm and cold air are mixed to produce the right air temperature for conditioning
unlike dual duct systems, the mixing is done by dampers located at the air handling unit rather than at terminals located near the spaces served
One multizone unit can serve up to approximately
12 zones
multi zone advantage and disadvantage
Multizone systems have the disadvantage of being inflexible with regard to change. Rearranging or adding zones will involve considerable ductwork and modification of the unit.
Multizone have the advantage of centralizing controls within the equipment room for easier maintenance and less intrusion into occupied spaces. This is important where security is an issue or where ceiling terminals may be difficult to access.
. Single-Zone Variable Air Volume
used for cooling vary thequantity of air supplied to the space
the mid-1970s as an energy-efficient
Airflow is increased or decreased by adjusting dampers or the speed of the fan.
Multiple-Zone Variable Air Volume
multiple VAV terminals that in turn serve individual zones of control.
terminals have dampers that vary the airflow to individual zones
VAV terminals are designed to provide only cooling. For buildings with heating loads, other devices should be added to provide heat.
