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Building Automation Systems for IAQ

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1 Building Automation Systems for IAQ
Chapter 13 Building Automation Systems for IAQ Building Automation Systems and Controllers • Operator Interface Methods • Building Automation System Input and Output Components • Direct Digital Control Strategies, Features, and Algorithms • IAQ Strategies and Energy Considerations

2 Building Automation Systems for IAQ
Chapter 13 Building Automation Systems for IAQ Building Automation Systems and Controllers • Operator Interface Methods • Building Automation System Input and Output Components • Direct Digital Control Strategies, Features, and Algorithms • IAQ Strategies and Energy Considerations

3 Distributed direct digital control systems have multiple CPUs at the controller level.
A distributed direct digital control (DDC) system is a building automation control system that has multiple central processing units (CPUs) at the controller level. See Figure In distributed DDC systems, each controller makes decisions. Distributed DDCs began replacing central-direct digital control systems in the 1980s.

4 Application-specific controllers are designed to control only one type or part of an HVAC system.
An application-specific controller (ASC) is a controller designed to control only one section or type of HVAC system. See Figure In most applications, application-specific controllers are more cost-effective than other types of controllers. Application-specific controllers are more cost effective because most of the software programming is done beforehand, reducing setup time.

5 Unitary controllers are designed for simple zone control using a standard wall-mounted temperature sensor. A unitary controller is a type of controller designed for basic zone control using a standard wall-mounted temperature sensor. See Figure Unitary controllers are designed to control packaged HVAC equipment such as rooftop units, heat pumps, and fan coil units. Unitary controllers are usually compact to allow field mounting at the packaged HVAC equipment. Rain-tight and heated enclosures are often used, which allow operation of multiple stages of heating and cooling, economizer dampers, heat pump reversing valves, and supply fans. Some unitary controllers provide supply-air sensing capabilities, airflow switches, and dirty-filter-condition switches.

6 Air-handling unit controllers are used when the sophisticated control of large central-station air-handling units is required. An air-handling unit (AHU) controller is a controller that contains input terminals and output terminals required to operate large central-station air-handling units. See Figure AHU controllers control humidification, static pressure, and indoor air quality. Many central-station air-handling units have complex control seq-uences.

7 Variable-air-volume (VAV) air terminal unit controllers are similar in appearance to unitary controllers. The difference is in the factory programming of the erasable programmable read-only memory (EPROM) chip. A variable-air-volume (VAV) air terminal unit controller is a controller that modulates the damper inside a VAV air terminal unit or rpm of the blower motor(s) with signals to variable frequency drives to maintain a specific building space temperature. VAV air terminal unit controllers are similar in appearance to unitary controllers. The differ-ence is in the factory programming of the erasable, programmable read-only memory (EPROM) chip. See Figure 13-5.

8 Variable-air-volume (VAV) air-handling unit controllers are used to control VAV air-handling units in order to maintain a minimum static pressure in supply ducts. A common AHU controller application is the control of a variable-air-volume (VAV), air-handling unit (AHU) to maintain a minimum static pressure in the supply duct. See Figure The VAV AHU heating and cooling controls maintain a constant discharge air temperature of 55°F. An output signal from the air-handling unit controller is provided to control (increase) the volume of air provided by the supply fan. The air volume is increased by controlling the speed of the blower motor using an electric motor drive or by opening dampers to admit a greater amount of air.

9 Variable-air-volume (VAV) air-handling controllers are used to control building space temperature and air volume in VAV air-handling systems. Variable-air-volume air-handling unit controllers are also used in pressure-independent VAV terminal boxes with reheat applications. In a pressure-independent appli-cation, building space temperature is controlled by the amount of airflow to the space. In addition to controlling building space temperature, a flow sensor is installed at the inlet to the VAV terminal box. See Figure The flow sensor (pitot tube or differential pressure sensor) is connected to the VAV air-handling unit controller to measure the amount of airflow. An output connection from the controller is provided to the coil of the reheat valve, so the reheat valve can be opened on a call for heat.

10 Network communication modules coordinate communication from controller to controller on a network and provide a location for operator interface. A network communication module (NCM) is a special controller that coordinates communication from controller to controller on a network and provides a location for operator interface. See Figure In most building automation systems, individual controllers do not communicate directly with each other. Network commu-nications modules usually have a number of ports (series and parallel) for connecting operator interface devices to the system.

11 Desktop personal computers (PCs), alarm printers, notebook PCs, portable operator terminals, keypad displays, and dumb terminals are used in large buildings to access and troubleshoot a building’s automation system. In medium- and large-sized buildings, such as hospitals, colleges, or office buildings, maintenance or manage-ment personnel commonly access and troubleshoot the automation system of a building as part of normal day-to-day responsibilities. On-site methods and tools used in large buildings to access and troubleshoot a building automation system include desktop personal computers (PCs), alarm printers, notebook PCs, portable operator terminals, keypad displays, and dumb terminals. See Figure 13-9.

12 A building automation system with web browser software packages allows a stationary engineer to communicate and change settings from anywhere in the world. The use of web browser interfaces is an important BAS development. It allows a stationary engineer to access and view the BAS over the Internet using a computer that is running web browser software. Web browser capability is available via a software package, which usually runs on a dedicated web server, or is built into the highest-level controllers provided with the BAS system. Stationary engineers can take advantage of this capability to monitor and control the BAS from any computer equipped with a web browser. Due to security concerns, stringent firewall and password protection considerations must be given to off-site operator interfaces. See Figure

13 Analog input devices send 4 mA to 20 mA or 0 V to 10 or 20 V signals to a building automation system’s controllers, causing a proportional electrical signal change to output components. An analog input device (AI) is a sensor that indicates a variable such as temperature, pressure, or humidity and causes a proportional electrical signal change at the building automation system controller. See Figure Analog input devices provide stationary engineers with an actual readout of the variable on a personal computer or other operator interface device. Analog input devices are usually more expensive than digital input devices.

14 Temperature sensors are available in a variety of packages and mounting configurations including wall-mounted, duct-mounted, immersion (well-mounted), and averaging. An analog temperature sensor is an analog input device that measures the temperature in a duct, pipe, or room and sends a signal to a controller. Temperature sensors are the most common analog input devices used in building automation systems. Temperature sensors are available in a variety of packages and mounting con-figurations. Common temperature sensor designs include wall-mounted, duct-mounted, immersion (well-mounted), and averaging. See Figure Averaging tem-perature sensors are the most common design used in large buildings.

15 Humidity sensors contain a hygroscopic element that changes physical characteristics as the humidity changes. Human comfort, product integrity, and corrosion prevention require that the humidity level in a commercial building be controlled. An analog humidity sensor is an analog input device that measures the amount of moisture in the air and sends a proportional signal to a controller. The most common humidity sensors measure the percent of relative humidity (%RH), while other humidity sensors measure dew point or absolute humidity. Most humidity sensors use a hygroscopic element. A hygroscopic element is a sensor mechanism that changes its characteristics as the humidity level in the air changes. See Figure

16 Piezoelectric elements (pressure-sensitive crystals) are used in duct pressure-sensing applications, while bellows elements are used in many piping applications. An analog pressure sensor is an analog input device that measures the pressure in a duct, pipe, or room and sends a signal to a controller. Pressure sensors are used in HVAC applications to provide an indication of the actual pressure inside a duct or pipe to a BAS controller. Piezoelectric elements (pressure-sensitive crystals) are used in some pressure-sensing applications. Bellows elements are used in many piping applications. See Figure Standard signal values of analog pressure sensors include 4 mA to 20 mA and 0 VDC to 10 VDC.

17 In IAQ applications, differential pressure sensors or pitot tubes and temperature sensors in a duct can act as airflow measuring stations (AMS). In IAQ applications, a differential pressure sensor in a duct can act as an airflow measuring station (AMS). An airflow measuring station consists of a number of sensors at regular intervals in a duct. The sensor location is the same as that used in performing a duct traverse. See Figure There are different types of AMSs available.

18 A digital input device signal is a binary (ON or OFF) signal.
A digital (binary) input device is a sensor that produces only an ON or OFF signal. See Figure Digital input devices differ from analog input devices in that analog input devices provide a varying readout of the actual value, while digital input devices indicate when a value is above or below a certain setpoint. For example, an analog input device may provide a reading of 74.5°F. A digital input device may indicate that a temperature is above or below 70°F.

19 Limit thermostats maintain a temperature above or below an adjustable setpoint.
A thermostat is a digital input device and controller. Limit thermostats indicate whether a temperature is above or below a certain value. See Figure Limit thermostats are usually used to indicate an improper temperature level and are not used as primary temperature controllers. Limit thermostats are usually inserted into a duct or strapped to a pipe. Limit thermostats also have a setpoint that determines the temperature at which the thermostat changes state (ON or OFF).

20 Humidistats are digital input devices used in ducts and indicate when a humidity setpoint has been reached. A humidistat is a digital input device that indicates that a humidity setpoint has been reached. Humidistats are often used to indicate the humidity level in a duct or building space. Like digital thermostats, digital humid-istats have a setpoint with normally open and normally closed contacts. Digital humidistats are usually less expensive than analog humidity sensors. Care must be taken when installing a humidistat and selecting the setpoint. See Figure

21 Differential pressure switches are often used to indicate a difference in pressure across a fan or pump or to indicate the condition of an air-handling unit filter. Differential pressure switches usually have a section of tubing located on the suction (inlet) side of a fan or pump and another section of tubing located on the discharge side. See Figure Differential pressure switch tubing must be installed to avoid close bending, kinking, or long piping runs. Any air leaks must be located and corrected before differential pressure switches are placed in service.

22 Electric/pneumatic transducers have an analog input signal, such as 4 mA to 20 mA, from the building automation system and an output air pressure signal of 0 psi to 20 psi. Electric/pneumatic transducers usually have an analog input signal, such as 4 mA to 20 mA, from the building automation system controller and output a proportional air pressure signal of 0 psi to 20 psi. See Figure The transducer output air pressure is often adjustable to match the actual spring range of the actuator. Pneumatic transducers are used in applications that have existing pneumatic valve and damper actuators. Pneumatic transducers enable pneumatic actuators to be used with modern building automation systems, simplifying instal-lation and reducing job costs.

23 Triac-switched controller output terminals require an external power supply and are used to switch alternating current (AC). Most digital output terminals of a building automation system controller are triac controlled. Triac-switched controller output terminals require an external power supply that is 12 V or 24 V. A triac is a solid-state switching device used to switch alternating current (AC). See Figure Digital output components include indicator lights, relays, incremental output components (stepper motors), and pulse-width-modulated output components (electric motor drives).

24 Relays are used to allow low-voltage-rated triacs of building automation system controllers to switch line-voltage components ON and OFF. The majority of controlled components, such as motors, are line-voltage components that operate a line voltage of 120 VAC and higher. BAS controller output terminals are generally rated at low voltage. Therefore, a relay must be used to allow the low-voltage-rated triacs in the BAS controller output terminals to switch line-voltage components. See Figure Triac output terminals are usually not powered, so an external power supply such as one from a 24 VAC transformer must be used. The application-specific relay can be supplied by the manufacturer or purchased separately.

25 In a direct digital control system, a building automation system controller is wired directly to controlled components. A control strategy is a BAS software method used to control the energy-using equipment in a building. A direct digital control (DDC) strategy is a control strategy in which a building automation system performs closed-loop temperature, humidity, or pressure control. A direct digital control (DDC) system is a building automation system in which controllers are wired directly to controlled components to turn them ON or OFF. See Figure

26 In closed-loop control, feedback occurs among the controller, sensor, and controlled component(s).
Closed-loop control is a type of control system where feedback occurs among the controller, sensor, and controlled component. Feedback is the measurement of the results of a controller action by a sensor or switch. See Figure

27 In open-loop control, no feedback is provided from sensors to a building automation system controller. Open-loop control is a type of control system where no feedback occurs among the controller, sensor, and controlled components. See Figure For example, a controller cycles a chilled water pump ON when the outdoor-air temperature is above 65°F. In an open-loop control system, the controller has no feedback regarding the status of the pump, such as whether or not the pump started.

28 Setpoint is the desired value to be maintained by a system with the desired accuracy to the setpoint being programmed into a BAS controller. The setpoint and the desired accuracy can be program-med into a BAS controller. For example, a building automation system in the summer is required to maintain a temperature of 72°F in a commercial building. The 72°F temperature is the setpoint of the DDC system. See Figure

29 Setup and setback setpoints are values that are active during the unoccupied mode of a building automation system. Another common direct digital control feature is setup and setback setpoint control. Setup and setback setpoints are values that are active during the unoccupied mode of a building automation system. Setup is the unoccupied cooling setpoint. For example, a cooling setpoint is raised from 74°F during the day to 85°F at night. See Figure The setup setpoint is 85°F. Setback is the unoccupied heating setpoint. For instance, if a heating setpoint is lowered from 70°F during the day to 55°F at night, the setback setpoint is 55°F.

30 Low-limit controls stop outside-air dampers from closing 100% and still allow ventilation air into a building when ventilation-air temperatures are excessively low outside. An excessively low outdoor-air temperature causes a low ventilation-air temperature. For example, when the ventilation-air temperature of a commercial building drops below 45°F, the controller wants to close the outside (ventilation)- air damper 100%. However, because of the low-limit control, the controller overrides the normal control logic and forces the outside (ventilation)- air damper not to close more than is required for a 45°F temperature. See Figure

31 Proportional control algorithms position a controlled component in response to the amount of offset experienced in a building automation system. A proportional control algorithm is a control algorithm that positions the controlled component in direct response to the amount of offset in a building automation system. Proportional control algorithms are the most common algorithms used in DDC systems. See Figure Proportional control algorithms are also known as P control or P only control.

32 Derivative control algorithms determine the instantaneous rate of change of a controlled variable.
A derivative control algorithm is a control algorithm that determines the instantaneous rate of change of a variable. See Figure Derivative control algorithms provide real-time data to a BAS controller. A derivative control algorithm acts against the integration control algorithm. The derivative control algorithm is usually used to increase the speed at which a controlled component eliminates an offset.

33 Adaptive control algorithms are self-tuning algorithms that are often used to correct an overcycling control system. The most sophisticated control algorithms used today are adaptive control algorithms. An adaptive control algorithm is a control algorithm that automatically adjusts its response time based on environmental conditions. Adaptive control algorithms are a self-tuning form of PID control but there is no substitute for manual loop tuning. See Figure

34 Acceptable indoor air quality numbers for ventilation are detailed in ANSI/ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality. Many BAS and energy control strategies are generally compatible with indoor air quality (IAQ) strategies, provided that the strategies are instituted with certain IAQ protections. Energy retrofits and control strategies must include provisions to protect IAQ and provide additional outdoor air to meet the ventilation requirements of ANSI/ASHRAE Standard , Ventilation for Accept-able Indoor Air Quality. Implementing IAQ strategies usually results in energy savings. See Figure

35 CO2 levels in building spaces or zones are used as an indicator of occupancy.
Carbon-dioxide-controlled ventilation changes the outdoor-air supply in response to CO2 levels in building spaces or zones, which is used as an indicator of occupancy. See Figure CO2 controls may be useful for reducing energy use for places where occupancy is highly variable and irregular such as general meeting rooms, studios, theaters, and educational facilities.

36 A typical HVAC system increases ventilation when CO2 levels rise to 600 parts per million (PPM) so that levels do not exceed 1000 PPM. A typical HVAC system increases ventilation when CO2 levels rise to 600 parts per million (PPM) to 800 PPM so that levels do not exceed 1000 PPM. See Figure The system must incorporate a minimum outdoor-air setting to dilute building-related contaminants during low-occupancy periods. Carbon dioxide sensors must be calibrated periodically and setpoints adjusted based on outdoor CO2 levels around the building.

37 Economizers use outdoor air to provide free cooling and are not practical or advisable in hot-humid climates. Air-side economizers use outdoor air to provide free cooling. The operation of these economizers potentially improves IAQ by helping to ensure that the outdoor-air ventilation rate meets IAQ requirements. See Figure With the exception of use in dry climates, moisture control must also be incorporated. Air-side economizers are not practical or advisable in hot-humid climates. An air-side economizer is disengaged when a problem occurs involving outdoor air pollution. These economizers usually reduce annual HVAC energy costs when used in cold or temperate climates.

38 In night precooling, cool outdoor night air is used to cool a building while simultaneously exhausting accumulated pollutants. Cool outdoor air at night is used to cool a building while simultaneously exhausting accumulated pollutants. This process is known as a building flush. However, the cool outdoor air may also have a high moisture content and could humidify the building at night, so caution must be used. See Figure In addition to preventing microbiological growth, controls must stop precooling operations when the dew point of the outdoor air is high enough to cause condensation on building surfaces and equipment.


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