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If a fridge as an example has a temperature deviation of half a degree with a controller scanning the temperature every halfsecond to make system adjustments or execute a set action , this would likely result in a problem as the compressor and fan would continually be switched on an off causing premature wear .
Conversely , looking into an automation system in the built environment , a one-second response time to a deviation is too long because the various valves that are opening and closing require a real-time scan and output response . This essentially would mean a controller with features capable of managing 100 or 200 milliseconds should be selected .
If you then take that same controller into an industrial process , even 100-millisecond control is just too long , and you would need to respond a lot quicker to changes given the possible implications that deviation could spoil an entire production run or batch . Naturally , this all means that the complexity of the equipment involved in any system increases as you need to delivery faster and more accurate cycles , and with the advanced algorithms , the price of the unit increases . System designers therefore need to make sure that the specified controller does the required job , and for the best possible price both up front and over the plant lifespan .
Controllers , in the context of this article , should also not be confused with building management systems ( BMS ) as these advanced devices extend to , and fulfil , a different function and include many more elements or sub systems within a building that can be monitored and controlled to create overall building efficiency . This topic will be covered in a future feature in the RACA Journal .
As these devices were developed by various suppliers over time , they each had their own logic to clearly separate brands from one another through their unique coding concept . This naturally became a problem for the industry as what resulted is known as ‘ supplier lock-in ’ – meaning that system owners could not incorporate different brands together ( for possibly pricing and practicality reasons ), therefore having to stick to one supplier throughout their system .
This proved an unfavourable condition for the clients who were not free to choose , and so in the interest of transparency , and because of the facts around the way the world was progressing in terms of technology development , the American Society of Heating , Refrigerating and Air-Conditioning Engineers ( ASHRAE ) intervened , and through various research and collaborations introduced open protocol .
The idea behind open protocol was that the communication language would not be owned by any particular company or manufacturer . It would be an industry standard and be based on the overall interest of that industry . In simple terms this open protocol was the establishment of a common communication language that different brand ’ s devices would use to talk to each other , and so was developed into what is known as BACnet ( abbreviated from building automation and control network ).
HVAC SYSTEM CONTROL AND COMMUNICATION Focussing on the HVAC systems in a building , these are categorised into two functional areas , namely the primary plant operation and secondary plant operation . The primary plant is what provides pre-conditioned bulk supply and secondary plant is what provides fine control for the particular space and is needed to match occupant requirements through specific set points .
Plant controllers overall may have many inputs and outputs and can perform significant processing functions . Because the primary plant has a lot more important elements under control , these devices tend to be more costly than secondary plant controllers that require fewer functions to be delivered and are thus simpler and smaller .
Now , when it comes to controller communication , it is useful to understand a little about the history and lead-up to the technology one has access to today , and to be aware of factors such as compatibility - essentially the ‘ language ’ of these devices that enable them to be incorporated into the larger scope that is building management .
www . hvacronline . co . za RACA Journal I September 2021 29