DDC Controls Systems Quickpage Jump Links
DDC Controls Overview
The Basic DDC Controls System Loop for HVAC Controls
DDC Controls System Responses for HVAC Controls
DDC Controls Page 2: The Technical
Aspects of DDC
DDC Controls Page 3: Variable Air Volume (VAV) Systems
DDC Controls Page 4: DDC and Economizers
DDC Controls Page 5: DDC Programming and Logic
DDC Controls Related Links
DDC Systems Overview
DDC Panel being Terminated
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Many people wonder what Building Automation Systems do. Building Automation Systems are mainly used in commercial HVAC control systems and energy management system applications. Building Automation itself is an energy management system which saves management companies and building owners by efficiently controlling air conditioning and heating comfort systems. BAS is where mechanical and electrical systems and equipment are joined with microprocessors that communicate with each other and possibly to a computer. This computer and controllers in the building automation system can be networked to the
internet or serve as a stand alone system for the local peer to peer controller
network only. Additionally, the BAS controllers themselves do not need a computer to
operate efficiently as many of these controllers are designed to operate as
stand-alone controllers and control the specific equipment they are assigned
to control. With a few exceptions, each building automation controller has their own program and has the ability to communicate to other DDC building automation controllers. It is important for the building automation system controllers to communicate to each other. If the network fails for whatever reason then the system may still function (because the BAS controllers in building automation systems are stand-alone) but it will not function as efficiently as designed. Building Automation Systems grows more and more complex as time passes but it will save in energy and maintenance costs if installed and programmed properly. Energy Management Systems and Building Automation Systems (can be one in the same) are definitely the way of the future and are replacing older less efficient systems everyday.
A set-up in a multi-story automated building would have many building automation controllers serving different types of air conditioning and heating equipment (BAS is not limited to just hvac applications). Every building is different and it is important for the BAS engineer to select the proper HVAC BAS control system and programs to control the various types of HVAC systems in a particular automated building. For building automation systems to be effective, it is important that the BAS system is installed and tuned properly. Some advantages of a good HVAC BAS building automation system are:
- Building Automation System (BAS) should allow the owner to set up schedules of
operation for the equipment and lighting systems so that energy
savings can be realized when the building or spaces in the building
are unoccupied.
- Building Automation System (BAS) should allow the equipment optimal start with
adaptive learning. Optimal start is allowing the equipment to be
brought on in an ordered and sequential manner automatically on a
schedule before the building is reoccupied so that space set points
can be realized before occupation. Adaptive learning allows the
system to compare space temperature, outside air conditions, and
equipment capabilities so that the equipment can be turned on at
an appropriate time to ensure space set points are achieved before
occupation.
- Building Automation System (BAS) should have trim and respond capabilities.
Based on zone demand the set point for various heating and cooling sources will change according to demand from the zones. In a VAV system all the VAV boxes are served from a
BAS Controlled Actuators Controlling a Hot Water Bypass Valve
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central air handling unit. If all the zones are at set point then
the supply air temperature set point of the air handler is automatically
changed to prevent mechanical cooling from occurring when it is
unnecessary. When the zones grow warmer the supply air temperature
set point is automatically lowered to allow mechanical cooling to
satisfy demand. Older systems have a single supply air temperature
set point of 55° Fahrenheit which requires the compressors to
cycle even when it is not necessary.
- Building Automation System (BAS) should have the ability to monitor energy usage including the ability to meter electric, gas, water, steam, hot water, chilled water, and fuel oil services.
- Building Automation System (BAS) in conjunction with the appropriate mechanical system set-up should offer economizing based on enthalpy calculations and/or CO2 set point control.
- Building Automation System (BAS) should have such BAS control algorithms as
reset schedules for heating plants, static pressure control, and
other systems where energy savings can be realized through these
predictive programs.
- Building Automation System (BAS) should offer load shedding when power companies are at peak demand and
need business and industry to cut back on power usage to prevent
brown outs. Building Automation Systems allow the owner to cycle various things off like water heaters or drinking fountains where use of these things will not be noticed even though they are off.
- Building Automation System (BAS) should offer the ability to send alarms via email, pager, or telephone to alert building managers and/or technicians of developing problems and system failures.
- Management companies who acquire a good building automation system (BAS) can have BAS set up to bill tenants for energy usage.
- Building Automation Systems (BAS) should have the communications abilities to be integrated with other building automation control
systems and TCP/IP. BACnet compatible or other open source communication protocol is a plus.
The Basic DDC Control Loop
To understand Direct Digital Control (DDC) we must understand the basic
control loop. Even the most adept HVAC controls technicians benefit in their work from
going back to the basic control loop to solve problems or break down and
understand complex DDC control algorithms. For a basic DDC Controls or building automation control loop
we need three things:
1. Input from a sensor or device. This can be analog or
digital. (See DDC/Building Automation page two for descriptions
of analog and digital). In this step we are Measuring temperature
or C.F.M.'s (any variables) and collecting data. DDC building automation inputs basically measure
a medium or monitor the HVAC systems such as smoke detectors and high/low
limit switches. DDC inputs measure temperature, humidity, pressure, current,
wattage, and air and water flow among other things.
2. A DDC or building automation controller to process information and which holds the
logic or programming. In this step the DDC or building automation controller is processing
the information from the input device(s) and based on the algorithm,
possibly sending an output signal to a device to take appropiate action if necessary. The input
device(s) does not need to be hard wired to the local equipment
controller nor does an output response from the local equipment
DDC building automation controller going to effect the DDC building automation controllers local equipment. Over
a communication trunk the DDC or building automation controller can receive input signals
from distant automation controllers and issue output commands to those same
or other distant building automation or DDC automation controllers. It really depends on the program
and set-up of the entire system in the algorithms of the building
automation system as a whole.
3. The actual device being controlled based on what the input
is feeding to the DDC controls controller. In this step the controlled
device is taking action to maintain the program based on program
variables. Output devices can be damper actuators, valve
actuators, relays (electrical and or pneumatics (p-e or e-p),
variable frequency or speed drives, compressors, blowers, and
pumps.
Basic DDC Control Loop
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Example of a DDC Controls Loop: A VAV box is reading
600 C.F.M.'s and the zone temperature
is 68 degrees F. The controller takes this input information and
processes it based on pre-programmed set points. In this situation
the zone is too cold so the DDC controller sends an output signal
to the damper actuator to close it (some to maintain a minimum
heating C.F.M. set point), energize a heating relay for heat and
(with a parallel box) turn the fan on. (For more on VAV boxes see
DDC/Building Automation (BAS) page 3). Because the output device is taking
an action it is reasonable to assume that our input sensor variables
are going to change. The process starts all over again until all set
points in the DDC building automation algorithm are satisfied.
Solid State PCBs with Processors that have the capability to communicate with other DDC controllers throughout the building
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DDC Control Responses
DDC building automation algorithms contain what is called control responses. This is
built into the logic and is responsible for the output to the
controlled device. Some of these control responses prevent overshooting
and/or undershooting. In these control responses we have five different
responses:
- two-position control - either open or closed (can utilize upper and lower limits).
- floating control - example: an actuator controlled vortex damper that moves within a setpoint range to control static pressure. Floating control commonly uses deadbands. In the example above, when the static pressure is in the deadband, the damper does not move.
- proportional control (p)*
- proportional plus integral (pi)*
- proportional plus integral plus derivative (pid) *
*Proportional (P), proportional plus integral (PI), and proportional plus integral plus derivative (PID) control responses are very complex algorithms. To read a detailed report on these algorithms and how they work click here
DDC Start-up Tech's Office with a View
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