Civil Engineering Reference
In-Depth Information
still is not satisfi ed, the direct digital control system shall modulate the chilled
water valve open to satisfy the space demand. Upon receiving a command from
a low limit control with averaging element which senses temperature of the air
entering the coils, the direct digital control system shall override other damper
controls, and modulate the maximum outside air, and relief dampers to limit
mixed air temperature to (45 °F adjustable) minimum (this control allows the air
handling unit takes advantage of cold enough outdoor air when it is available and
provides a cooling air into the space without need for utilizing cooling coil).
• Typical sequence of operation for maintaining air entering cooling coil: The
direct digital control system shall utilize a separate low limit safety which senses
the temperature of the air entering the cooling coil and is set at 40 °F (adjustable)
to stop the fan (this is another control to protect the cooling coil from freezing).
• Typical sequence of operation on power interruption: When the power interrupts
or fan shuts down, the direct digital control system shall close the minimum and
maximum outside air, relief, smoke and return dampers, and chilled water and
hot water valves. The direct digital control system also shall stop the terminal
units and close their associated heating coils valves (this control protects the
system against unexpected conditions where the power supply to the system is
interrupted or the system is shut down for maintenance).
• Typical sequence of operation for variable volume terminal units: The direct
digital control system shall modulate the primary air valve of each variable vol-
ume air terminal unit between its designated minimum and maximum quantities
to maintain space temperature (this control allows proper air delivery and also
required minimum outdoor air through the primary air valve minimum setting to
each individual space).
• Typical sequence of operation for variable volume-reheat terminal units: The
direct digital control system shall modulate the primary air valve of each variable
volume air terminal between its designated minimum and maximum quantities
to maintain space temperature. When the space temperature drops below a set
point the direct digital control system shall modulate the primary air valve to its
minimum setting and shall modulate the hot water control valve to maintain
space temperature (72 °F adjustable) (this control allows proper air delivery and
also required minimum outdoor air through the primary air valve minimum set-
ting to each individual space).
•
Typical sequence of operation for parallel fan powered terminal units: The direct
digital control system shall modulate the primary air valve of each parallel fan
powered variable volume air terminal between its designated minimum and max-
imum quantities to maintain space temperature. When the space temperature
drops below a set-point the direct digital control system shall modulate the pri-
mary air valve down to its minimum setting, start the fan, and modulate the hot
water control valve to maintain space temperature (72 °F adjustable) (this control
allows a variable fl ow capacity with proper air temperature delivery to each
individual space. This airfl ow will be between the maximum design airfl ow and
minimum required outdoor airfl ow which is usually called primary air).
