return air = through return ducts or space above ceiling called a plenum
Supply ducts usually run above ceiling to ceiling or wall registers
Occupied space can be a return plenum too, undercut doors, door registers and cold air registers between two floors are sometimes used to allow air to return to the unit
Air into the plant usually mixed with interior return air and fresh air from exterior
fresh air intake = at exterior of building, ducted connection to plant, away from cooling tower and other exhausts
outside air = requires warming or cooling
return air = must be cleand or filtered
air changes = number is required by code or good practice, must be provided despite differing thermal load requirements
Supply fan = pushes air from the plant through ductwork, must have sufficient flow volume to give sufficient pressure to overcome friction in the duct.
Pressure = expressed in terms of "static head of water", which is the height of a column of water which could be lifted by the pressure.
infiltration elimination = run supply fan at greater rate than sum of the return air fan rate plus the leakage rate of the building
deck temperature = also know as equipment tempurature, temp. of air as it leaves the equipment room
Insulate ducts to prevent cold or hot temp. loss (especially cold)
Exterior insulation = for temperature control
Interior insulation = for sound control
decrease noise transmission of fans and plants:
isolate on springs, rubber pads and concrete pad to mount to.
duct connection = rubber or fabric, pressure and flow maintained but connection is flexible
Simple forced air system:
Single duct system = mostly residential, constant air flow (constant volume), runs until temp. reaches thermostat setting
Can't heat or cool rooms separately, heats & cools simultaneously
damper = metal flap on diffuser used to shut off or reduce airflow.
system is problematic if greater need for ventilation than heat
Electric reheat system = variant of previous system
Air is cooled, electric resistance heater is placed in duct upstream of diffuser that needs heat,
very inefficient, only good in hot climate where heat is rarely needed
Double duct / dual duct system:
Combination or two single duct systems, one carries heated air, the other, cooled air
requires twice as much duct space
can heat one room but cool another
airflow rate is constant
but
amount pulled from each room is controlled by dampers and mixed in a mixing box which is controlled by the thermostat
Ideal for linear buildings requiring different thermal conditions
Multizone system:
reduces amount of ducting, mixing boxes are inside the mech. equipment room
pre-mixed air is sent to each zone
efficient for square building plan or a smaller few zone system
Fan coil system:
MOST EFFICIENT, high first cost (due to plumbing)
can heat and cool simultaneously
Constant volune of clean and conditioned air is supplied by a single duct.
Chilled water pipe & Hot water pipe
Each zone has a unit with a fan and two coils, one hot and one cold
Variable air volume (VAV) system:
MOST COMMON EFFICIENT SYSTEM
single duct system, may be three or four separate single duct systems serving the number of zones required
Zone makes up a group of rooms
flow rate may be varied instead of constant
Air to a zone is at a given same temp. amount of heating or cooling is determined by air volume delivered.
No comments:
Post a Comment