Test 2 Flashcards
HVAC = Heating, Ventilation, Air Conditioning
These systems are designed to provide:
● Thermal conditions
● Adequate amounts of outdoor air
A typical system consist of
● a minimum of one fan to move the air
● a provision for introducing outdoor air
● a filter medium to reduce the concentration of particulate
matter in the air
● coils for heating and cooling the air
● a distribution system
HVAC SYSTEMS
The distribution system typically consists of ductwork connected to supply registers and a pathway for air leaving the occupied space to return to the air-handling unit.
Outdoor Air Intake, Outdoor Air Dampers, Mixing Plenum, Air Filters, Cooling Coils, Condensate Drain Pans, Supply-Air Fans, Distribution (Ductwork), Return-Air Fan, Boiler, Chiller
Outdoor Air Intake:
- Air intakes are located 1/3 of the way up the side of the building
- Very often air intakes are found next to loading docks, at ground level near
roadways, or on roofs near exhausts.
Outdoor Air Dampers:
They open and close to control the volume of air passing through an air inlet.
Mixing Plenum:
It is the locations where the return and outdoor airstreams meet and are combined.
Since rain or snow may be sucked in along with the outdoor air, there should be
a floor drain to prevent water accumulation.
If this floor drain is connected to a sewer line, it must have a functional water trap,
with water present, so that sewer odors will not be drawn into the air stream.
Purpose of cooling coils:
- To lower the temperature of the air
- To provide dehumidification as water condenses from
the airstreams.
Requirements
* They must be accessible
* They must be clean
* They must have sufficient capacity to provide thermal comfort.
* They can be located either centrally or distributed throughout the building.
Why clean the reaheat coils
- Cleaning is required because coils can become clogged and thus reduce
the quantity of supply air passing through them. - Calibration is required because improperly operating coils can cause
comfort problems and wasted energy.
Condensate Drain Pans:
They collect moisture below the cooling coils and then transfer this water out
of the air-handling unit.
When the air passing through the cooling coils is humid that the temperature
to which it is cooled is below its dew point, dehumidification of this airstreams
will occur and liquid water will form on the cooling coils.
Supply-Air Fans:
They provide the driving force to move the air through the distribution
system to the occupied spaces in the building.
Distribution System:
It conveys the conditioned air from the conditioning equipment
and delivers it to the occupied spaces in the building.
It consists of
* Ductwork
* Mixing boxes
* Terminal units
* Connectors
Return-Air Fan:
It draws air out of the occupied spaces and delivers it back to the
air-handling unit.
Boilers:
- They generate the heated water for either thermal conditioning or
domestic hot-water needs. Additives are added to control corrosion in the piping system.
Chillers:
They provide sufficient quantities of chilled water to meet cooling loads
necessary for providing thermally comfortable conditions.
HVAC systems are made up of subsystems that produce
- Heating
- Cooling
- Move heat transfer fluids
- Control delivery to a space to maintain stable conditions
Heating, cooling are performed by equipment such as
* Refrigeration devices
* Furnaces
* Boilers
Movement of heat, transfer fluids are performed by air-handling equipment such as
- Ductwork
- Grillers
- Diffusers for air
- Pumps
- Piping systems for water
Air Systems control the amount of heating or cooling by the following methods:
- Varying the temperature of the air supplied, while holding the flow constant.
- Varying the flow of warm or cold air supplied, while holding the temperature
constant - Varying both the temperature and the flow of air supplied.
Zoning
Zone: is one area for which temperature is controlled by a single thermostat.
Single-zone system: a house with one furnace controlled by a single thermostat.
Two zones system: a house with two furnaces each controlled by a
separate thermostat.
Automatic controls
The control systems control:
* Temperature with sensors set
* Pressure with sensors set
* Flow rate with sensors set
* Humidity with sensors set
* Speed with sensors set
* time with a clock or a program
Basic Control Devices:
- sensors
- controllers
- actuators
Sensors:
measure the monitored variable.
The signal from the sensors is input to a controller.
Controllers:
provide the decision making function of the control system.
It determines if a signal should be sent to a monitoring station or to
an actuator.
Actuators:
provide the physical control of the equipment by valves or dampers.
Thermostats:
are devices that sense and respond to temperature, combining the
functions of the sensor and controller.
Humidistats:
are devices that sense and respond to humidity – either relative or
absolute.
Constant Temperature, Variable Volume (on-off)
This is a simple residential system. As the space temperature falls below
or rises above the thermostat set point, the system is activated, and hot or
cold air is supplied to the space at a constant rate until the space warms or
cools.
When the proper amount of warm or cold air has been supplied, the
system deactivates.
Single-Zone Constant Air Volume:
It is a simple system and is used in commercial buildings.
Fans run continuously and provide ventilation.
Air is supplied to the space at a constant rate: (constant volume)
This system includes a cooling coil and/or a heating coil that varies
the temperature of the air in response to a space thermostat.
Cooling Coil = is constructed of tubes surrounded by fins
A chilled fluid passes through the tubes, and cooling is conveyed to the airstream, which flows over the fins.
The air-handling system consists of the following components.
- Fan sections, for supply air and return air fans
- Cooling section, for chilled water or refrigerant cooling coils
- Heating section, for hot-water or steam coils
- Humidification section for extra humidity
- Filter sections, for filtering
- Air-mixing section, for outdoor air to mix with recirculated air
- Discharge air plenum
- Other components, for electrical power, controls, operating a
motor, drainage, etc.
Fans:
A fan is a mechanical device that moves air used in HVAC systems to
ventilate or transport heat or cooling.
Ductwork includes:
- supply duct
- return duct
- outside air
- relief air
- exhaust air ducts
Duct Shapes and Insulation Methods
- Rectangular cross section:
Insulation is applied to the interior of the duct and provides thermal and acoustical insulation
It reduces heating or cooling loss from the duct.
They are used for low-velocity applications.
- Round or flat oval ducts:
They are used for medium- and high-velocity ductwork.
The insulation is applied externally in the form of a fiberglas
blanket wrap.
They have an external vapor barrier to prevent the migration
of humid air though the insulation, which will result in
condensation on the surface of a cold duct.
External insulation is more expensive because it requires
labor for installation.
Air devices include:
- diffusers
- grilles
- registers
Coanda effect:
Allows the airstream to fall gradually and mix with the room air
over a large area.
If this effect is not obtained the chilled air will fall downward and
create a cold spot beneath.
This is called dumping and results in an uncomfortable cold draft.
Good coverage proper throw
