Civil Engineering Reference
In-Depth Information
the last time at set point (integrated), and (3) how fast (steep) you are approaching
or going away from the set point (derivative).
If you imagine your brain is similar with the direct digital controller of the HVAC
system, the combination of your foot and the gas pedal as the actuator, the odometer
as the sensor, and the car speed with any one of HVAC system characteristics, such
as temperature or pressure, you can relate to and understand how the HVAC control
logic works. For example as due to additional load in the space, the room tempera-
ture deviates from the set point of 75 °F sensed by the temperature sensor, the direct
digital controller issues a command that modulates the chilled water valve on the
cooling coil of the air handling unit. This action generates some cooling that after a
few times repeating of this routine of measuring the temperature and modulating the
cooling valve position, brings the space air temperature back to 75 °F again. Here
the discrepancies at each instance are the sum of (1) how much the real temperature
deviates from the set point (proportional), (2) a weighted average of the undesired
temperatures and the time spent in these temperatures form the last time at set point
(integrated), and (3) how fast (steep) the system is approaching or going away from
the set point (derivative).
In HVAC industry, most of the controls that are utilized are capable of perform-
ing all three algorithms, but due to nature of the systems and lack of need for very
fast response, usually the derivate logic is not going to be used (ASHRAE
2011
).
9.4
Direct Digital Control
The basic and yet the most important benefi t of a Direct Digital Control (DDC)
system in a building is its capability to optimize the operation of the HVAC system
by conducting the system elements to perform in harmony and near their highest
possible effi ciency levels and also directing the different system components to
operate only when they are needed to do so. This has a major impact on the level of
energy effi ciency of the building and therefore prevents energy wasting during the
system operations. In addition since the DDC system is a programmable system, it
not only provides the capability of recording the environment and system data, and
changing the components and set points of the system if the recorded trend shows
the presence of abnormality in the past, but also provides programming change
capability if the desired condition of the targeted venue changes later on. A sample
of the fi rst condition is when an owner by referring to and reviewing the recorded
temperature log in a specifi c room fi gures out that specifi c space is continuously
slightly hotter than the expected temperature set-point and after investigation fi nds
out the terminal unit damper serving that space is not functioning properly, and
therefore attempts to fi x the problem. On the other hand a sample of the second
condition could be seen when the owner of a museum decides to change the func-
tion of a space from a function requiring little to no humidity control to a space that
requires tight humidity control such as changing the function of an offi ce space in
the museum to a storage space for storing artwork.
