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Back to Basics – Part One
As I’d mentioned back in the April column, I thought I’d “cool my
engines” so to speak during the summer months, and present a “back to
basics” series to take us through to the fall, at which point I’ll
redirect my focus to current trends and new products.
In thinking how to put this series together, I went back to the book that I had published back in 2003, to see how relevant the concepts presented are, almost ten years later. To my delight, much (most) of what I wrote is still very timely today. I did make it a point to write the book with that in mind, such that it wouldn’t become obsolete a few short years after it was published. I believe that I’ve succeeded to the extent that the book was intended, and that was to illustrate the concepts of HVAC controls from a practical standpoint, hence the title: Practical Controls: A Guide to Mechanical Systems (sorry, I couldn’t resist).
This series will be presented in three parts. Part one will cover fundamental concepts and simple definitions. Part two will discuss the different methods of control, components, and control systems architecture. And part three will get into some miscellaneous stuff that I have not yet had the chance to think about, so I’ll be winging it by the time we get to August! I now present…Part One:
Mechanical System: a formal definition of the term might read as follows: “mechanical equipment and the means of connecting the equipment, to other equipment, and to the real world, in some meaningful, functional manner.” In the HVAC world, mechanical systems are typically designed to perform heating, cooling, and ventilation (H-V-A-C) of spaces requiring such types of environmental control.
Mechanical systems in HVAC are made up of equipment such as fans, pumps, dampers, valves, ductwork, and piping. Put ‘em all together and you have yourself a mechanical system! As well, mechanical systems can be further categorized as airside systems (fans/dampers/ductwork), waterside systems (pumps/valves/piping), and miscellaneous systems (systems and equipment that fall into neither of the aforementioned categories). In my own way of thinking, I like to define an HVAC System as a mechanical system plus the associated controls and control system required to properly and efficiently operate it.
Zone: in the Glossary of my book, I define the term zone as “an area of temperature control”. It’s a pretty simple definition, but what does that really mean? In HVAC, a zone is typically a single space, room, open area, or even several individual spaces served by a single piece of heating/cooling equipment. A sensor, thermostat, or similar device within the zone, in simple terms, controls the piece of equipment that serves the zone. There is only one device per zone, this being perhaps the most telltale evidence of a zone and its existence. Count up the sensors in the building, and you have your number of zones! Well, it’s a little more involved than that, but hopefully you get the idea.
Equipment Levels: expanding on the concept of the term “zone”, we can define types of mechanical systems and equipment as zone-level (or unitary), equipment level, and plant level. These terms are used to loosely describe the relative size and complexity of various systems. At the zone level are the types of equipment that would individually serve single zones. Equipment falling under this category include VAV boxes, fan-coils, and unit heaters. At the equipment level, we typically describe large packaged and built-up air handling equipment. And at the plant level, we’re talking systems that are made up of several pieces of equipment. For instance, a boiler plant would consist of boilers, pumps, valves, and piping that make up the system.
A word about the two terms used above: packaged and built-up. A packaged piece of equipment is one that is furnished with some amount of factory mounted and installed controls devices. These controls may partially or even completely allow the equipment to operate, without any other added controls. A packaged rooftop unit is a perfect example. The unit is ready to go right out of the crate. Just hook up the gas, electricity, and space thermostat, and fire it up! Conversely, an air handling unit that comes from the factory as nothing more than a fan, a hot water coil, and a mixing box, is an example of a piece of equipment that needs to be “built up”. Controls required to make this unit operate include, but are not limited to, a fan controller, a control valve for the hot water coil, a damper actuator for the mixing box, and some kind of a temperature controller.
Sensor & Controller: Contrary to popular belief, a sensor does not have the capability to control! Maybe it’s just a matter of terminology, and the improper use of it, but quite often the word sensor is referred to in terms of “controlling the operation” of something. No, in the truest sense of the word (no pun intended), the only thing a sensor can do is “sense”. A sensor’s main function is to measure a “controlled variable” in an accurate and continuous manner. In HVAC, we’re typically measuring and controlling temperature, pressure, and humidity.
We can describe a controller by listing its three distinct functions. The first is that the controller must be able to gather and process the information measured by the sensor. The second is that a preference, or setpoint, must be able to be established via the controller. And the third is that the controller must be able to act upon the controlled variable, in a manner beneficial to the controlled process. A simple example is a temperature controller reading temperature from a space temperature sensor. Through the controller a setpoint is set, say, a space temperature setpoint of 72 degrees. The controller in turn may be controlling the position of a hot water control valve. The controller acts upon the controlled variable, which in this case is space temperature, by allowing the proper amount of hot water to flow through a radiator coil, let’s say, to achieve and maintain space temperature setpoint.
Controller Levels: Before we get into the definition of Direct Digital Control (DDC), let us examine the three levels of “digital” controllers. First is the unit level controller, referring to a controller suited for unitary control (zone level equipment such as VAVs and fan-coils). Unit level controllers are often referred to as “application specific”, however this nomenclature is product dependent and doesn’t necessarily mean that a given unit level controller can’t be utilized for a range of applications. Equipment level controllers are suited for equipment level control (huh?). For instance, a large built-up air handler requiring a controller with enough inputs and outputs to accommodate equipment of this size. And at the plant level, you guessed it, the plant level controller! A controller of this size will have the required I/O to accommodate perhaps a boiler/chiller plant, and will typically consist of a “main brain” module plus the required number of satellite input/output modules.
DDC & BAS: The term Direct Digital Control is used to describe microprocessor-based control. Analog information (i.e. from sensors) is converted to digital information (0s and 1s), so that it can be processed and manipulated by a microprocessor. Likewise, digital results are converted to analog information, to control the operation of “real world” end devices (more on this whole discussion of analog and digital stuff later in the series). BAS stands for Building Automation System, and refers to when a multitude of digital controllers are networked together and interfaced to a front-end.
The two terms are not really interchangeable, even though a Building Automation System is sometimes referred to as a DDC system. In its purest form, a DDC system consists of a single digital controller operating a piece of equipment in a stand-alone mode, with no network to support it. Take it a step further, and consider a multitude of digital controllers, all operating stand-alone, with no communication to and from a higher entity. This in fact is still a DDC system, albeit a non-networked DDC system. Why you would want to do this is beside the point, the point being that you need to have the network and the front-end in order to call a system of DDC controllers a BAS. Get it? Got it? Good!
Points: last term to discuss before we conclude this part of the series, is the concept of a point. The term is used to describe a control operation, whether it be a sensing action or a controlling action. A point can be anything from a temperature sensor, to an output of a proportional controller operating a control valve. With digital controllers, a tally of the used inputs and outputs can provide an overall “point count” of a control system, and in this respect can serve as a quantitative evaluation of a given system.
Tip of the Month: Did you know??? Manufacturers often encode input/output information into the model numbers of their digital controllers. For instance, given the model number SRC424 of a fictitious controller, the letters could be an acronym for the type of controller (in this case, my initials!), and the numbers represent the types and quantities of the onboard I/O. The first number could represent inputs (analog or binary), the second representing analog (proportional) outputs, and the third representing binary (two-state) outputs. This cataloging scheme extends to different industries as well; one that comes to mind is the digital audio industry, whereby digital audio interface equipment is categorized in much the same way (for what it’s worth!).
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