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.
Multiple-Zone Variable Air Volume for heating
Perimeter convectors and radiant panels are often used as a source of heat in conjunction with VAV terminals for cooling.
Perimeter convectors use electric resistance elements or hot water piping equipped with fins to enhance heat transfer.
Multiple-Zone Variable Air Volume
Finned tube
installed in a baseboard or a sill line – take perimeter space.
Multiple-Zone Variable Air Volume
Radiant panels
installed at the ceiling – radiate infrared heat downwards – cannot reach areas under desks and tables.
Variable Air Volume Reheat Terminals
terminals are equipped with electric or hot-water heating coils. Airflow is variable
the cooling load is reduced, the airflow is reduced
heating, the heating coil warms the minimum quantity of chilled air to a temperature suitable for heating the space.
Variable Air Volume Dual Duct Terminals
Served from a warm-air duct and a chilled-air duct.
For cooling, chilled air is supplied through a damper from the chilled-air duct.
When heating is required, warm-air damper will start to open to increase the flow of warm air.
Variable Air Volume Multizone
Separate dampers are used for the warm and cold portions of the mixing
cold-air damper is almost closed before the warm-air damper is allowed to open.
Prevents energy waste due to mixing
Fan Terminal Units
designed to overcome the complaints about the performance of VAV terminals.
contains a VAV damper
Chilled air through VAV damper is mixed with return air from above the ceiling and delivered to the space via a small constant-volume fan within the unit.
Fan Coils
consists of a filter, a cooling and/or heating coil, a fan, and controls.
located above the ceiling, in wall cabinets, or in soffits.
Designed with two or four pipes. Return and extract if 4 warm and cool pairs
Water-source Heat Pumps
used for individual zone control in large buildings.
operate for either heating or cooling
incorporate heat exchangers for heating and cooling
