Sinopsis
We need controls and control systems because, in our modern age of technology, they make our lives more convenient, comfortable, efficient, an effective. A control enables equipment to operate effectively and sometimes gives the ability to change their actions as time goes on and conditions or occupancies change. Controls can be devices used to monitor the inputs and regulate the output of systems and equipment. You use controls every day. For example, when you shower in the morning you sense the water temperature and manually modulate the hot and cold water valves to produce the desired temperature. When you drive to work, you monitor your speed using the speedometer and manually control the accelerator of your car to maintain the desired speed. When you get to your office, you sense a shortage of light so you manually switch on the overhead lighting.
These are all examples of closed-loop manual controls. The term manual means that you (a person, rather than a device) are acting as the controller; you are making the decisions about what control actions to take. The term closed-loop means that you have feedback from the actions you have taken. In these examples, the feedback comes from your senses of touch and sight: as you open the hot water valve in your shower, you can sense the temperature of the water increase; when you depress the accelerator, you can see that your speed is increasing by viewing the speedometer; and when you turn on the light, you can see that the brightness in the space has increased.
Your car may also be equipped with cruise control, to automatically maintain speed on a clear road, which is an example of an automatic control. An automatic control is simply a device that imitates the actions you would take during manual control. In this case, when you press the set-button on the cruise control panel, you are telling the controller the speed you desire, or the setpoint. The controller measures your speed and adjusts the position of the accelerator to attempt to maintain the car’s speed at setpoint – the desired speed – just as you do when you manually control the speed. You may notice that your cruise control system is able to maintain your car’s speed at a given setpoint more precisely than you can manually. This is generally because you are not paying strict attention to controlling your speed; you must also steer, watch for traffic and perform all of the other functions required for safe driving. This is one reason why we use automatic controls: we do not have the time or desire, or perhaps the ability, to constantly monitor a process to maintain the desired result.
Controls of heating, ventilating and air-conditioning, and refrigerating (HVAC&R) systems are analogous in many ways to the controls we use to drive our cars. Just as we use speed as an indicator of safe driving, we generally use dry bulb temperature (the temperature that a common thermometer measures) as an indicator of comfortable thermal conditions. Just as speed is not the only factor that affects driving safety, temperature is not the only factor that affects our perception of thermal comfort. But like speed is the major factor in driving, temperature is the major factor in comfort and is readily measured and controlled. Your car’s engine was designed to bring the car up to speed quickly, to drive it up a hill, or to carry a heavy load. But because we do not need this peak power output all of the time, we need a control device (the accelerator) that can regulate the engine’s power output.
The same can be said of HVAC systems. They are generally designed to handle peak cooling or heating loads that seldom, if ever, take place, so we must provide controls that can regulate the system’s output to meet the actual cooling or heating load at a given time.
We use automatic controls for HVAC systems in place of manual controls, just as we might use cruise control to control the speed of our car. Automatic controls eliminate the need for constant human monitoring of a process, and, therefore, they reduce labor costs and provide more consistent, and often
improved, performance.
The ultimate aim of every HVAC system and its controls is to provide a comfortable environment suitable for the process that is occurring in the facility. In most cases, the HVAC system’s purpose is to provide thermal comfort for a building’s occupants to create a more productive atmosphere (such as in an office) or to make a space more inviting to customers (such as in a retail store). The process may also be manufacturing with special requirements to ensure a quality product, or it may be a laboratory or hospital operating suite where, in addition to precise temperature and humidity control, the HVAC system must maintain room pressures at precise relationships relative to other rooms. With all of these systems, the HVAC system and its controls must regulate the movement of air and water, and the staging of heating, cooling, and humidification sources to regulate the environment. Another capability that is expected of modern control systems is energy management. This means that while the control systems are providing the essential HVAC functions, they should do so in the most energy efficient manner possible.
Safety is another important function of automatic controls. Safety controls are those designed to protect the health and welfare of people in or around HVAC equipment, or in the spaces they serve, and to prevent inadvertent damage to the HVAC equipment itself. Examples of some safety control functions are: limits on high and low temperatures (overheating, freezing); limits on high and low pressures; freezestats; over current protection (e.g. fuses); and fire and smoke detection.
Content
- Introduction to HVAC Control Systems
- Basics of Electricity
- Control Valves and Dampers
- Sensors and Auxiliary Devices
- Self- and System-powered Controls
- Electric Controls
- Pneumatic Controls
- Analog Electronic Controls
- Control Diagrams and Sequences
- DDC Introduction to Hardware and Software
- DDC Networks and Controls Protocols
- Digital Controls Specification
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