Civil Engineering Reference
In-Depth Information
parameters at each location, each space, and at each time. Each set point should be
presented by the acceptable parameter range. A typical sequence of control for a
space temperature can be written as: “The control system shall maintain tempera-
ture of the space 'xyz' at 75 ± 2 °F at all time” and a typical sequence of control for
a space carbon dioxide level can be written as: “The control system shall maintain
carbon dioxide level in space 'abc' at a maximum value of 1000 ± 100 parts per mil-
lion (ppm) at all time.”
A generic, simple, and step-by-step sequence of operations for a typical variable
volume air handling unit with water cooling and heating coils can be written as
follows:
• Typical sequence of operation for starting and stopping a variable volume air
handling unit system: The direct digital control system shall automatically start
and stop the air handling unit whenever the designated operation switch is in the
automatic position, and allows for manually starting and stopping the air han-
dling unit whenever this switch is in hand position (this allows for both auto-
matic and manual start and stop of the system).
• Typical sequence of operation for opening and closing dampers: The direct digi-
tal control system shall open the return, minimum outside air and smoke damp-
ers before the air handling unit fan is started. (This control allows for a safe
operation of the system so the fan does not operate against dampers that are
closed, and cause any damage to the ductwork network or fan.)
• Typical sequence of operation for morning warm-up: When the system is set for
morning warm-up mode, the direct digital control system shall run the air han-
dling unit and close the minimum and maximum outside air and relief dampers
and the air handling unit chilled water valve. The direct digital control system
shall modulate the terminal units air valves to fully open position, and modulate
the heating coil valves to provide hot supply air until space temperature rises
above a warm-up mode set point. (This control allows the space temperature to
reach an acceptable level before the scheduled time for the occupant's arrival.
Therefore when the early crowed enter the building do not feel cold and
uncomfortable.)
• Typical sequence of operation for night low limit mode (NLL): On a request for
night low limit operation from minimum fi ve VAV zones (adjustable), the direct
digital control system shall enable the supply fan variable frequency drive (VFD)
and supply air temperature controls. During NLL mode the direct digital control
system shall set the supply air temperature set-point to the warm-up supply air
temperature set-point (e.g. 75 °F, adjustable). When the number of VAV zones
requesting NLL drops below the minimum required VAV zones, or occupied
mode becomes active, the direct digital control system shall disable NLL. (This
control prevents the building temperature during the off hours drops below a
minimum allowable setting.)
•
Typical sequence of operation for night high limit mode (NHL): On a request for
night high limit operation from a minimum fi ve VAV zones (adjustable), the
direct digital control system shall enable the supply fan VFD and supply air tem-
perature controls. When the number of VAV zones requesting NHL drops below
