Use of refrigeration and air conditioning (HVAC/R) systems in daily personal life as well as in various industrial processes have already become an integral part of present generation. The HVAC/R industry is changing. This change may not be rapid, but it’s visible. With the growing demand of these utilities, companies are coming out with latest developments making use of advanced electronic controls – in order to make these units more durable, economic and safe. Innovations like smart thermostats and geothermal heating systems show that homeowners are becoming more conscious about being environmentally friendly and using energy-efficient technology. Solid state electronic controls are commonly used in refrigeration and air conditioning applications viz., operation and troubleshooting of assembled components such as circuit boards, time delay relays, and programmable controllers. Today, companies all over the world supply electromechanical and electronic controls for the domestic refrigeration, HVACR, food service equipment and automotive climate markets. This aricle will highlight the development and use of such electronic control advances with respect to versatile use of refrigeration and air conditioning units by making them user friendly with other desired characteristics.
Electronic control
The process of controlling an HVACR system involves three steps. These steps include first measuring data, then processing the data with other information and finally causing a control action. These three functions make up what is known as a control loop. The control loop consists of three main components: a sensor, a controller and a controlled device. These three components or functions interact to control a medium. The sensor measures the data, the controller processes the data and the controlled device causes an action. The sensor measures the controlled medium or other control input in an accurate and repeatable manner. Common HVACR sensors are used to measure temperature, pressure, relative humidity, airflow state and carbon dioxide. Other variables may also be measured that impact the controller logic. Examples include other temperatures, time-of-day or the current demand condition. Additional input information (sensed data) that influences the control logic may include the status of other parameters (airflow, water flow, current) or safety (fire, smoke, high/low temperature limit or any number of other physical parameters). Sensors are an extremely important part of the control system and can be the first, as well as a major, weak link in the chain of control.
The controller processes data that is input from the sensor, applies the logic of control and causes an output action to be generated. This signal may be sent directly to the controlled device or to other logical control functions – and ultimately to the controlled device. The controllers function is to compare its input (from the sensor) with a set of instructions such as set point, throttling range and action, then produce an appropriate output signal. This is the logic of control. It usually consists of a control response along with other logical decisions that are unique to the specific control application. How the controller functions is referred to as the control response. A controlled device is a device that responds to the signal from the controller, or the control logic, and changes the condition of the controlled medium or the state of the end device. These devices include valve operators, damper operators, electric relays, fans, pumps, compressors and variable speed drives for fan and pump applications.
User-friendly controls
While smart, Wi-Fi-connected thermostats promise significantly increased energy efficiency, the real draw for homeowners seems to be how easy the interfaces are to operate. Sometimes thermostats are programmed by the user to remember his preferred temperature settings. If the user prefers the thermostat to kick in a faster, warmer temperature, on cold, wintery mornings, the thermostat will store that information and adjust the temperature accordingly. The thermostat also monitors temperature with humidity and activity sensors, and it can determine regional climate by the user punching in their zip code. Furthermore, it can be accessed remotely through the internet or using a smart phone app, giving the user the ability to control the heat even when out of town. Wireless-controlled thermostats present HVAC data in a language that contemporary consumers understand. Homeowners, especially the younger ones, are accustomed to easily digestible, visual representations of information and data. The reporting tools that smart systems include take the mystery – and obscurity out of an industry that has been thriving on overall complacency from its customers.
Application of electronic controls
Electronic controls must be applied and installed in line with appliance manufacturer instructions, local approvals and practices. Suitable earthing must be made by appliance manufacturer and installer. Here, some of the advances in electronics control techniques applied in HVAC systems are discussed.
Air conditioner controllers
Air conditioner controllers have evolved to become more advance in tandem with the advancement made in the field of microcomputer. These days, most window air conditioner and room air conditioner in homes or offices have electronic controllers in them that make the units more user friendly, intelligent and feature-rich. Some of the HAVCR systems with electronic control are being discussed in the subsequent paragraphs.
Control of on/off compressor: The compressor is the most important component of a vapour compression refrigeration system. The ON/OFF type of compressor used to be popular in the past – but DC Inverter type of compressor is the latest technology in the market though more costly.
The ON/OFF compressor will totally ON or totally OFF depending on the set temperature and the ambient temperature. Usually, there is a dead band of about 1.5 to 2.0OC to prevent ON-OFF cycling of compressor that will reduce its lifespan.
In cooling mode, the compressor will turn ON when the ambient temperature of the room is higher than the set temperature by 0.75OC (different manufacturer will have different value). It will only go off when the room temperature drops below 1.25OC (different manufacturer will have different value).
Control of ac/dc inverter compressor: The DC Inverter or AC Inverter type of control (obviously the compressor will also be of DC or AC Inverter type) has better control in that it does not have to turn off totally.
The frequency of the rotation of the compressor can vary depending on the load required. Hence the room temperature of the space being cooled can be very close to the set temperature most of the time with some slight variation.
This type of air conditioner controllers provide a more comfortable cooling to the occupants as the temperature is constant most of the time. It is also more energy efficient and environmentally friendly as most of the refrigerant used is ozone-friendly.
Mode setting: The mode setting button at the remote control usually enables you to set the mode of operation. It can be set to COOL mode, HEAT mode (if your unit is a heat pump unit), DRY mode, AUTO mode or FAN mode.
Look at the manual provided to see the symbols of each mode. COOL mode is the default setting that you will use if you want to cool the room upon entering it. The air conditioner will regulate the temperature to achieve the setting temperature of your remote control. A temperature setting of 24OC is a good setting to save energy. HEAT mode is the mode that you will use when you need to heat up the room during winter. In this mode, the evaporator effectively becomes a condenser and vice versa by using a reversing valve to reverse the flow of the refrigerant.
A temperature setting of 28OC is a good setting to save energy. In heat pump model, AUTO mode is a mode that will automatically change the mode between COOL and HEAT by sensing the temperature in the room.
If the room temperature is low, HEAT mode will operate and if it goes up to a certain temperature, it will switch to COOL mode.
In air conditioning model and depending on the maker, the AUTO mode is actually operating in COOL mode but with a lower set temperature to cool down the room quickly before reverting to the set temperature of the user.
DRY mode is a variation of COOL mode where the compressor will cycle on and off (or low speed – if inverter model) for a few minutes each depending on the temperature of the room.
By this cycling, the moisture will be removed from the room, hence the humidity of the room will drop. This is basically a dehumidifying function.
FAN mode exists in some model. This mode is hardly used as there is no cooling effect to the room. Only the indoor fan is on.
Economical Mode: Most air conditioner controllers have the built-in economy or sleep mode function. This function is used when one is going to sleep.
When activated using the remote control, the air conditioner unit will raise the set temperature in steps after a period of time. This is because the required cooling will reduce when one sleeps.
Depending on manufacturer, the set temperature could increase by 1OC after 1 hour of operation, 1.5OC after 1.5 hours of operation and 2.0OC after 2 hours of operation. Use this mode to save energy.
Fan Speed: Usually the fan speed can be changed to Low, Medium, High or Auto. The Auto mode has a built-in intelligence that determines the speed of the fan depending on the room temperature and set temperature. Higher speed is used if the difference between these two temperatures is high and lower
if low.
Timers: Most unit will also have timers that you can use to ON or OFF the unit based on the clock of the remote control. Some simpler units do not have clocks – but has delay timers that allow you to set the number of hours elapsed before the unit is turn ON or OFF.
Auto Random Restart: The auto random restart is a feature that will retain the settings of the controls in the event of a power failure. Once power is restored, the air conditioner will restart to its last settings with the compressor being powered up randomly. This is a good feature to look out for – before you purchase the unit.
Ionizer: Ionizer is now a basic feature in most room air conditioner unit. It is located at the indoor unit and has a high voltage electronic module that ionized the surrounding air with negative ions.
These ions will attract dust and other contaminants from the return air to ensure that only fresh and clean air is discharged from the fan coil unit. Take note that the module will have to be changed after a certain hours of operation to ensure its continuous efficiency.
Air Louver: Air louver function helps in directing the air flow in the direction of your choice. Most controls have a least up and down control of the air. Some units may have left and right louver control as well.
Refrigeration controllers
Defrosting: Defrost timer used in domestic and commercial application is a single cam control that activates a SPDT switch. Its cycle consists of transferring the switch of a preset duration once every 4, 5, 6, 8, 10, 12, 16 or 24 hours. Both the operating cycle and switch transfer duration are pre-set at the factory and are not adjustable.
The cycle will repeat as long as the motor is energized. An internal clutch mechanism permits initial adjustment of the cycle starting time. It is a solution for the needs of an intelligent flexible product in a compact size.
Disadvantages
The disadvantage of this type of compressor is that the temperature range of the space being cooled is too big causing discomfort to the occupants during the time the compressor is either ON or OFF.
The other disadvantage is the starting current of easily few times of the steady current every time the compressor starts.
The disadvantage with electronically controlled inverter compressor unit is higher cost compared to the ON/OFF type.
Future
Efforts of scientists and engineers as well as manufacturers of HVAC/R systems are always directed to make use of advanced electronic controls in order to make these systems more reliable, durable, user friendly, economical and eco-friendly. Some of the possible breakthrough in near future are summarized inthe following paragraphs.
Zero-energy Buildings: Buildings that produce energy instead of just using large quantities of energy will gain traction with companies that target eco-friendly employees and consumers.
Many businesses are already headed in this direction with geothermal heating and cooling, solar-powered systems, and white roofing.
If HVAC manufacturers can work to design structures with advanced electronic controls that are both energy efficient and comfortable for their residents, it is sure to see some fresh companies take on this challenge.
Economical and efficient fan and pump control: Electronic control techniques (packaged products) have features to control and automatically transfer to bypass on Variable-Frequency Drives (VFDs) fault to provide the flexibility to meet the most demanding installations without adding options.
These techniques are highly reliable in case of fan and pump control packages that feature a three-contactor electronic bypass control and an affinity for easy-to-use HVACR control – so that the system has a graphical status display, dedicated alarm display, and Hand/Off/Automatic controls.
Energy Management System: An Energy Management System (EMS) is defined as a fully functional control system. This includes controllers, various communications devices and the full complement of operational
software necessary to have a fully functioning control system.
