HVAC&R Control Systems
An HVAC&R control system has to conform with stringent health and safety laws, and react accordingly when a state of emergency occurs, such as fire, smoke or flood. It must also interface with other systems in the same building - boilers, chillers, etc. Tailor made electrical control panels available will provide the information needed at the required time and at the right place. Control panel information includes instrumentation and controls geared to individual requirements...
Heating, Ventilation, Air Conditioning & Refrigeration (HVAC&R) is the technology of indoor and vehicular environmental comfort. Its goal is to provide and acceptable indoor air quality. In present day of industrialization and modern living style, we are always in possession of any one of Heating, Ventilation, Air-conditioning or Refrigeration systems in our homes or in industry. HVAC& R is an important part of residential structures such as single family homes, apartment buildings, hotels and senior living facilities, medium to large industrial and office buildings such as skyscrapers and hospitals, onboard vessels, and in marine environments, where safe and healthy building conditions are regulated with respect to temperature and humidity, using fresh air from outdoors. HVAC&R system design is a sub discipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. The central functions of heating, ventilation, air conditioning and cooling are interrelated, especially with the need to provide thermal comfort and acceptable indoor air quality within reasonable installation, operation, and maintenance costs. Basic principles of equipment‘s working and performance along with its design and installation in a place (building), its control systems further play a great role in its working with respect to its efficiency and long life. Since last few decades, manufacturers of HVAC&R equipment have been making an effort to make the systems more efficient. This was originally driven by rising energy costs, and has more recently been driven by increased awareness of environmental issues. Additionally, improvements to the HVAC system efficiency can also help increase occupant health and productivity.
Basics of HVAC&R Control system
To keep customers satisfied, there is a need to supply robust machines that are energy-efficient and reliable all at a reduced cost and with short lead-times. Customers also expect the best service, anytime and anywhere in the world. Choice of control solutions is the key to distinguish at every stage of the process, from design and development, to implementation and machine maintenance. Electrical control system is an integral and important part of HVAC&R system. Any HVAC&R equipment needs a control system to regulate the operation of a heating and/or air conditioning system. Usually, a sensing device is used to compare the actual state (e.g. temperature) with a target state. Then the control system draws a conclusion what action has to be taken. An HVAC&R control system has to conform with stringent health and safety laws, and react accordingly when a state of emergency occurs, such as fire, smoke or flood. It must also interface with other systems in the same building - boilers, chillers, etc. Tailor made electrical control panels available will provide the information needed at the required time and at the right place. Control panel information includes instrumentation and controls geared to individual requirements. This would include, but not be limited to:
• Motor and process control, environmental control (HVAC&R)
• Comprehensive local or remote, relay or PLC automatic controls
With all the necessary cable entries and correct instrumentation to suit the application and systems will be designed to interface with fire alarm, boiler, and chiller systems as required. Other features or integrated options will include alarm systems and annunciators. Need is to design a cost-optimized HVAC&R control system with onboard energy-efficiency solutions. Improve machine performance with innovative automation technology and dedicated HVAC&R application functions, supplemented with advanced drive technology, to increase energy efficiency while reducing maintenance and improving reliability with energy-efficiency related functions such as energy management, floating high-pressure control, and compressor management. The benefits of HVAC&R Control Systems can be summarized as:
• Lower energy cost
• Lower operations cost
• Increase flexibility
• Ensure quality building environment
Elements of HVAC&R Control System
HVAC control system, from the simplest room thermostat to the most complicated computerized control, has four basic elements: i) Sensors, ii) Controllers, iii) Controlled Devices and iv) Source of energy.
1. Sensors: Sensor measures actual value of controlled variable such as temperature, humidity or flow and provides information to the controller.
Type of Sensors: Different types of sensors produce different types of signals as follows:
• Analog sensors are used to monitor continuously changing conditions. The analog sensor provides the controller with a varying signal such as 0 to 10V.
• Digital sensors are used to provide two position open or closed signal such as a pump that is on or off. The digital sensor provides the controller with a discrete signal such as open or closed contacts.
Classification of Sensors
Typical sensors used in electronic control systems are:
Resistance sensors are ‘Resistance Temperature Devices (RTD’s)’ and are used in measuring temperature. Examples are BALCO elements, Copper, Platinum, 10K Thermistors, and 30K Thermistors.
Voltage sensors could be used for temperature, humidity and pressure. Typical voltage input ranges are 0 to 5 Vdc (Volts direct current), 1 to 11 Vdc, and 0 to 10 Vdc.
Current sensors could be used for temperature, humidity, and pressure. The typical current range is 4 to 20 mA (milliamps).
• Bi-Metallic Strip
• Sealed Bellows
• Bulb & Capillary Sensors
• Resistance Temperature Devices (RTD)
Relative Humidity Sensors
• Resistance Relative Humidity Sensor
• Capacitance Relative Humidity Sensor
• Temperature Condensation
• Condensation & Wetting
• Quartz Crystal Relative Humidity Sensor
• Variable Resistance
• Venturi Tube
• Flow Nozzles
• Vortex Shedding Sensors
• Positive Displacement Flow Sensors
• Turbine Based Flow Sensors
• Magnetic Flow Sensors
• Ultrasonic Flow Sensors
Air Flow Measurements
• Hot Wire Anemometers
• Pitot – Static Tube
Liquid Level Measurements
• Hydrostatic Sensors
• Ultrasonic Sensors
• Capacitance Sensors
2. Controllers: Controller receives input from sensor, processes the input and then produces intelligent output signal for controlled device.
• Temperature Controllers
• Relative Humidity Controllers
• Enthalpy Controllers
• Universal Controllers
Controlled devices: Controlled device acts to modify controlled variable as directed by controller.
Controlled Devices Types
• Control Valves
• Heating and Cooling Coils
3. Source of energy: Source of energy is needed to power the control system. Control systems use either a pneumatic or electric power supply.
Supervisory Control System
The role of supervisory control is to control the scheduling and interaction of all the subsystems inside a building to meet building needs with appropriate operator input. Supervisory control systems have many names; each used for a particular emphasis. Among the names and their acronyms are the following:
BMS: Building management system
EMCS: Energy monitoring and control system
FMS: Facility management system
EMS: Energy management system
BAS: Building automation system (The most generic of these terms)
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 standalone system for the local peer to peer controller network only. Additionally, the BAS controllers themselves do not need a computer to process the control functions as the controllers have their own internal processors.
Type of HVAC&R Control Systems
There are five different types of HVAC&R Control Systems as follows:
Direct Acting Systems: The simplest form of controller is direct-acting, comprising a sensing element which transmits power to a valve through a capillary, bellows and diaphragm. The measuring system derives its energy from the process under control without amplification by any auxiliary source of power which makes it simple and easy to use. The most common example is the thermostatic radiator valve which adjusts the valve by liquid expansion or vapor pressure.
Electric / Electronic Systems: Electric controlled devices provide ON / OFF or two-position control. In residential and small commercial applications, low voltage electrical controls are most common. A transformer is used to reduce the 115 volt alternating current (AC) to a nominal 24 volts. This voltage signal is controlled by thermostats, and can open gas solenoid valves, energize oil burners or solenoid valves on the DX cooling, control electric heat, operate two position valves and damper or turn on-off fans and pumps. A relay or contactor is used to switch line voltage equipment with the low voltage control signal. An electronic control system can be enhanced with visual displays that show system status and operation.
Pneumatic Systems: The most popular control system for large buildings historically has been pneumatics which can provide both On-Off and modulating control. Pneumatic actuators are described in terms of their spring range. Compressed air with an input pressure can be regulated by thermostats and humidistat. By varying the discharge air pressure from these devices, the signal can be used directly to open valves, close dampers, and energize other equipment. The copper or plastic tubing carry the control signals around the building, which is relatively inexpensive. The pneumatic system is very durable, is safe in hazardous areas where electrical sparks must be avoided, and most importantly, is capable of modulation, or operation at part load condition. While the 24-volt electrical control system could only energize a damper fully open or fully closed, a pneumatic control system can hold that damper at 25%, 40% or 80% open. This allows more accurate matching of the supply with the load. Pneumatic controls use clean, dry & oil free compressed air, both as the control signal medium and to drive the valve stem with the use of diaphragms.
Microprocessor Systems: Direct Digital Control (DDC) is the most common deployed control system today. The sensors and output devices (e.g., actuators, relays) used for electronic control systems are usually the same ones used on microprocessor-based systems. The distinction between electronic control systems and microprocessor-based systems is in the handling of the input signals. In an electronic control system, the analog sensor signal is amplified, and then compared to a set point or override signal through voltage or current comparison and control circuits. In a microprocessor-based system, the sensor input is converted to a digital form, where discrete instructions (algorithms) perform the process of comparison and control. Most subsystems, from VAV boxes to boilers and chillers, now have an onboard DDC system to optimize the performance of that unit. A communication protocol known as BACNet is a standard protocol that allows control units from different manufacturers to pass data to each other.
Mixed Systems: Combinations of controlled devices are possible. For example, electronic controllers can modulate a pneumatic actuator. Also, proportional electronic signals can be sent to a device called transducer, which converts these signals into proportional air pressure signals used by the pneumatic actuators. A sensor-transducer assembly is called a transmitter.
Future of HVAC&R Control System
The future of smart HVAC&R control systems is critical towards the comforts of life and the economy with the cost of natural resources, especially fossil fuels, undoubtedly set to rise over the coming years. Two futures are possible for HVAC&R controls. One is exciting; the other not so much so. In the exciting scenario, controls rapidly evolve so that, in just a few years, building controls have extensive self-commissioning, self-tuning, self-diagnostic and correction, and even self-configuring features. HVAC&R systems simply require components to be connected together with a short list of parameters set, and the system takes off from there—notifying the commissioning agent, contractor, engineer and/or operator if it is meeting its specified high-performance criteria, or, if it is not, what corrective steps are necessary. Multi-variable relational control can greatly improve performance, energy efficiency, and system stability. But relational control offers much more. This multivariable method of control provides an ideal platform for extension into a type of artificial intelligence called neural net control, which will begin a new era in building control. Relational control allows the software designer to select a wide variety of system variables that may influence the optimal operation of a system. The multivariable relationships may be very basic, such as fluid mixing laws, or much more complex, such overall energy optimization via the equal marginal performance principle. The logical next step in HVAC&R control software development is software modules that will automatically discover other variables (and/or combination of variables) that will assist further in tuning, optimization, self-configuration, self-setup, and fault detection with prescribed corrective actions. It’s becoming universally clear that such widespread implementation of advanced building control could cut total energy consumed by our buildings by about half, while at the same time improving occupant comfort.
Innovations That Will Change HVAC&R Forever
Innovative technologies are taking the world by storm. As high-tech gadgets and the latest smart phone innovations continue to improve our lives, people have something else to look forward to in terms of revolutionary HVAC&R technologies that could change how we heat and cool our comfort zones to industry. Many of these HVAC technologies are still on the drawing board, but there are some we can take advantage of now to boost HVAC&R comfort levels. These technologies are:
• Movement-Activated Air Conditioning
• Thermally Driven Air Conditioning
• On-Demand Hot Water Recirculator
• Ice-Powered Air Conditioning
• Sensor-Enhanced Ventilation
• Dual-Fuel Heat Pumps
• Geothermal Heat Pumps
• Smart Homes
• Fully Automated Homes
• 3-D Printed Air Conditioners
• Harnessing Heat from a Computer
AUTHORS CREDIT & PHOTOGRAPH
Dr S S Verma
Department of Physics,