Ever since the energy crisis, when digital controls (then called EMCS for energy management and control systems) were unceremoniously ushered into widespread use for HVAC control, the industry has tried to make them look and act like the pneumatic controls they have superseded.
Only occasionally are some of the profoundly expanded opportunities available with digital controls applied effectively. Furthermore, terms like reset schedule and direct acting, relevant only to pneumatic systems, are still commonly employed in what is now the digital controls era.
While the process of transition to digital control technologies tolerates this mixed bag, a multitude of new demands are requiring our industry to move ahead and realize the full potential of digital control technologies. Building occupants are demanding more comfortable and higher quality environments. Building owners continue to press for greater economies in construction, operation, and maintenance. Finally, a variety of pressures are upon us to provide more precise control and documentation that standards for temperature, ventilation, and indoor air quality are being met.
In this article, I will discuss how DDC technologies permit a new flexibility in the traditional rules concerning the need for linear signals and responses with input and output devices. When properly applied, this new flexibility can reduce the cost of DDC technologies. Next month, I will show how, by combining these fundamentals with emerging inter manufacturer controls integration, designers can achieve new horizons in performance and energy efficiency.
HVAC Control System
A HVAC control system is a computerized system for climate control in buildings. HVAC stands for humidity, ventilation , air-conditioning. Often, these integrate fire, security, and lighting controls into one system. These systems typically use one or more central controllers to command and monitor the remote terminal unit controllers, and they communicate with one or more personal computers that are used as the operator interface. These control systems are typically used on large commercial and industrial buildings to allow central control of many HVAC units around the building(s). The latest systems use the building ethernet for communications between central controllers, and allow operator access from a web browser.
Direct Digital Control
Central controllers and most terminal unit controllers are programmable, meaning the direct digital control program code may be customized for the intended use. The program features include time schedules, setpoints, controllers, logic, timers, trend logs, and alarms. The unit controllers typically have analog and digital inputs, that allow measurement of the variable (temperature, humidity, or pressure) and analog and digital outputs for control of the medium (hot/cold water and/or steam). Digital inputs are typically (dry) contacts from a control device, and analog inputs are typically a voltage or current measurement from a variable (temperature, humidity, velocity, or pressure) sensing device. Digital outputs are typically relay contacts used to start and stop equipment, and analog outputs are typically voltage or current signals to control the movement of the medium (air/water/steam) control devices.(Valves/dampers/motor speed)
It was only natural that the first HVAC controllers would be pneumatic, as the engineers understood fluid control. Thus mechanical engineers could use their experience with the properties of steam and air to control the flow of heated or cooled air. To this day, there is pneumatic HVAC equipment in operation, which can be a century old, in some buildings, such as schools and offices.
After the control of air flow and temperature was standardized, the use of electromechanical relays in ladder logic, to switch dampers became standardized. Eventually, the relays became electronic switches, as transistors eventually could handle greater current loads. By 1985, pneumatic control could no longer compete with this new technology.
By the year 2000, computerized controllers were common. Today, some of these controllers can even be accessed by web browsers, which need no longer be in the same building as the HVAC equipment. This allows some economies of scale, as a single operations center can easily monitor thousands of buildings.
Why Linear Devices?
When pneumatic controls dominated our industry, building owners paid a high price for modulati n g l o o p p e r for m a n c e and stability. One of the prices paid was the requirement that input and output devices be linear with respect to the system variable they sensed or controlled. This need for linear response was essential to match the limited control capabilities of pneumatic controllers.
A number of rules and conventions were established within our industry that made achieving this linear response requirement easier. Among these were the development of the equal percentage valve, which included the seemingly backwards rule of thumb that called for sizing control valves smaller than the pipe size. Similarly, mechanical sensing devices were constructed to provide linear change in control air pressure over their entire sensing range.
While these conventions and rules of thumb served the days of pneumatics, they now need to be rethought. Requiring what I call external linearization in digital control designs adds costs in two ways. Linear devices are often more expensive than nonlinear devices that may offer improved levels of performance in DDC applications.
Further, linear output conventions, such as designing a high pressure drop through valves or dampers, carry a substantial continuous operating energy penalty. By developing new rules and conventions, the knowledgeable designer can produce designs that have lower first and operating costs and may operate more reliably as well.
Linear Devices in the DDC Era
The need for linear response in modulating control loops has not been eliminated by the introduction of digital controls. While digital controls offer improved modulating control capabilities, including proportional/integral/derivative (PID) controllers, these control loops continue to be based on the principle of linear response, at least over certain ranges. However, in most typical applications, digital controls can easily internally linearize both input signals and output control functions.
Internal Linearization of Inputs
One way to reduce the cost of some DDC configurations is to permit nonlinear input devices and use the DDC system for scaling to achieve the correct reading over the range required for the application.
I continue to see DDC specifications that limit the selection of input devices to those that provide a linear signal to the DDC system over a wide range of values.
Except in special cases, this is an unnecessary requirement that adds costs and may cause other problems. Consider temperature sensors. Fig. 1 shows a resistance curve for an inexpensive thermistor type temperature sensor that may be employed for room temperature sensing. Thermistors are excellent choices for HVAC applications. They are inexpensive, have excellent accuracy and very low hysteresis, and respond quickly to temperature changes. Furthermore, at temperatures normally involved in HVAC applications, thermistors have excellent long-term stability (some care should be taken in choosing thermistors when temperature may rise above 240 F).
Finally, because thermistors are typically high resistance (10,000 ohms is typical), they are not affected by variations in wiring distances.
However, some designers continue to exclude thermistors because the input signal is not linear with temperature over wide temperature ranges. Instead, low impedance RTD type sensors are often specified. This type of sensor typically requires an electric circuit at the sensor that linearizes and transmits the signal in a way that it will not be affected by wiring resistance (usually a current loop signal is used).
Employing low resistance RTD sensors with additional electronics presents a number of potential problems in DDC applications. First is the matter of accuracy.
While the RTD sensors themselves provide excellent accuracy, it is not uncommon to find end-toend accuracies (I use end-to-end as the comparison of the value read by a precision thermometer at the device compared with the actual reading at the DDC system operator’s terminal) out of tolerance.
Calibration of the current loop input may be more difficult than that of a simple resistance type thermistor.
Other potential problems with RTDs range from the additional electronics (usually located at the device) that may complicate reliability issues all the way to how the
Sensor curve 2 Nonlinear sensor resistance curve. The sensor performance curve is a smooth curve over the sensor’s operating pressure. The DDC linearized curve is a series of straight lines that closely approximates the sensor’s performance sensor and electronics are configured, which on occasion has been found to affect adversely the sensor signal.
Table functions that are now readily available with DDC products can be employed to scale thermistors and other nonlinear devices over a wide range of values.
Fig. 2 shows how a DDC system can linearize a continuous, nonlinear sensor input curve with a table function. A number of straight line curves are established in the table function to approximate closely the nonlinear function of the device. As long as simple, inexpensive devices can meet the repeatability, hysteresis, and stability requirements for an HVAC application, such devices should not be rejected because their signals are not linear.
Requirement of Linear Output
Once it is understood that input devices need not be linear, it is not a great leap to recognize that the response from output devices controlled by analog outputs similarly need not be linear. However, the issues here are more complex and more ingrained in the rules of thumb that engineers frequently apply automatically, so some indepth discussion is required.
Because of the pneumatic background, valve design manuals commonly stress the need to select coil/valve combinations for which equal increments in valve position will effect equal increments in heat transfer of a typical heating or cooling coil throughout the stroke of the valve actuator. Fig. 3 shows how traditional design practice seeks to linearize the overall performance of valve and cooling coil. Carefully selecting a coil and valve combination can provide nearly linear performance over the entire range of load possibilities.
Such selection is done because it is assumed that the valve will be operated by a controller with a fixed proportional gain.
Though this design principle is still widely employed, it is no longer applicable in many modern HVAC applications. In VAV cooling coil applications, the variations of air flow and air/chilled water temperature characteristics act to change dynamically the heat transfer characteristics of the valve/coil arrangement as these parameters change. This makes it very difficult to select a valve/coil combination that will be linear through the variety of conditions that may accompany its operation.
The higher performance of DDC systems permits designers much greater flexibility in the design of modulating controls without establishing static (and therefore unrealistic) design criteria. Fig. 4 shows a valve and coil combination that does not provide a linear response of valve position to coil capacity. However, modern DDC systems permit scaling tables to be applied to analog outputs as well as the inputs. Output scaling permits an inherently nonlinear device combination to respond in a linear fashion to signals from the DDC system. In this example, the valve and coil combination provides about 70 percent of the design cooling capacity at about 20 percent valve travel. The DDC output to the valve can be adjusted with the scaling table to position the valve at 20 percent travel at a 70 percent output signal from the DDC system. The scaling factor allows standard PID control to operate the valve effectively because of a software linearization of the valve/coil combination.
However, the chilled water flow and heat transfer performance assumed for Fig. 4 is valid only for constant load-side flows and inlet temperatures and for constant chilled water supply temperatures.
Whether inherent in the system design or for optimization reasons, rarely in real HVAC applications do these other variables remain constant as control loops operate. As previously discussed, the issue of linear output combinations has therefore been only weakly resolved in the past by attempting to linearize components at one set of system conditions.
Obtaining good control over wide ranges of system conditions can be resolved far more completely and effectively with the higher performance capabilities of DDC systems. The proportional, integral, and derivative gains can be tied to algorithms that adjust their values as the variables such as load-side flow, temperatures, and chilled water temperature change. Even more impressive is the emergence of self-tuning controllers.
These controllers continually re-establish the various gains associated with a control loop to provide continuously precise control without hunting. The benefits of self-tuning are especially important because variables beyond the immediate control loop can have profound and widely varying effects on each control loop. Self-tuning features are becoming widely available with DDC systems and are enormously effective in adjusting control loops to continue stable operation as other system variables change.
As previously discussed, selecting equipment for linear response should not be an overriding consideration for designers in this era of digital controls. However, this does not mean designers can be imprecise in their designs or in the selection of control loop components.
The issue of controllability is one that will continue to play a prominent role both in the design of systems and the selection of individual components.
Controllability remains largely a sizing issue. If a valve is oversized for given conditions such that the smallest increment possible from the control loop will substantially overshoot the desired control conditions, the loop has become uncontrollable.
This is a problem that typically emerges during periods of low load. Fully understanding the issue of controllability and applying DDC capabilities correctly allows designers to solve such problems and at the same time vastly improve the efficiency and performance of these systems.
Selecting a control valve with a lower pressure drop will reduce the pumping power required to meet the load conditions. Traditional practice strongly condemns the idea of employing large valves with lower pressure drops because of the nonlinear response and the lack of controllability at low loads.
Fig. 5 illustrates the dilemma. The valve/coil combination with Valve A may be selected according to traditional design practice because it is reasonably controllable at low loads. The vertical axis intercept represents the smallest incremental cooling transfer possible as the valve is cracked open. Note that it is small-only about 10 percent of the design maximum cooling rate.
The coil combination with Valve B has a much lower pressure drop because Valve B is a larger size valve. While valve/coil Combination B would require less pumping power, the Y-axis intercept is much higher than that for Combination A. Traditional design criteria typically declare Valve B unsuitable for the application because it is uncontrollable at lower loads and the valve position/ cooling capacity relationship is nonlinear. But when it is integrated with a high-performance control system that can adjust both the chilled water temperature and the loop head pressure, will linearity and controllability of Combination B really be a problem?
To see how this question can be answered, consider the graphs in Figs. 6 and 7. Fig. 6 shows the operation curves for valve/coil Combination B at a number of different approach (chilled water supply less air temperature leaving coil) temperature conditions.
It is clear that increasing the chilled water temperature relative to the leaving air temperature markedly improves the controllability at low loads. Similarly, Fig. 7 illustrates that the decrease in pressure across the valve/coil combination also improves the controllability at low loads.
Designers can use these relationships to reduce substantially the problem of controllability. At periods of uniform low loads, the DDC system can reduce the head pressure across a valve and increase the chilled water temperature to improve controllability. If all valves on a common chilled water loop experience similar decreases in load concurrently, as is typical in many HVAC applications, this parameter adjustment is a great help in improving controllability at low loads.
It is apparent from the two figures that larger rangeability and low load controllability are achieved by controlling the chilled water temperature for load adjustment.
Raising the chilled water temperature provides a bonus of chiller efficiency increases, but chilled water adjustment reduces pumping savings because a higher chilled water temperature increases the water flow necessary to meet loads. Additionally, under certain circumstances dehumidification requirements may limit the permissible chilled water adjustment.
Exploiting the integrated control capabilities of DDC systems and controlling chilled water temperature and hydronic loop pressure in coordination with the control valves allows valve/coil Combination B to perform very well in many HVAC applications.
Next month we will focus on the level of integration required to make valve/coil Configuration B operate effectively. We will discuss integrating the operation of the various equipment involved in providing comfort, possible now through the industry moves to provide communication bridges among manufacturers. By concentrating on selecting the most costeffective input/output devices and by utilizing the emerging communications pathways between equipment from various suppliers, we will see that new horizons of performance and energy efficiency can be attained with simple and economical controls configurations.
Designers must exploit the benefits of higher performing DDC systems to develop an understanding of the fundamentals of interfacing hardware points to DDC systems. In so doing, a more in-depth look into total system operation must be evaluated before solutions are selected. Simply following traditional rules of thumb regarding linear input and output devices is a poor design practice in this digital controls era.
DDC and Small and Medium Size Buildings
The control of heating, ventilating and air-conditioning (HVAC) systems is changing as a result of applying direct digital control (DDC) techniques to HVAC control. This report outlines the main features of DDC compared with conventional pneumatic control and shows that, for small-to-medium-size buildings, the DDC system can pay for itself within two years, after which it affects net savings over pneumatic systems.
Comparison between Pneumatic Control and DDC
Direct digital control of HVAC systems is the direct monitoring of every system input (temperature, flow, pressure) and direct control of every system output (position, onlaff) from a central controller which is a single computer or combination of computers. DDC is a simple concept, but its significance is not obvious until it is compared with traditional forms of HVAC control.
Traditionally, the control of HVAC systems was based on independent pneumatic controllers, which used compressed air t o operate the dampers and valve actuators t o control space condition such as temperature, humidity and fresh-air circulation. One building would have several such systems, which were controlled independently. For example, an air-handling system composed of two fans, three dampers and three valves (Figure 1) would be controlled by local pneumatic controllers which operated as independent units. Each controller had a simple task: to maintain a constant set point (for example, supply air temperature) by monitoring and controlling a very l i m i t e d number of variables connected to it by means of compressed air lines whose pressures represented the values of the variables. The control was adjusted mechanically by a technician in the field, and, as calibration of the pneumatic components was rarely carried out, these systems often did not control the building efficiently. Because the pneumatic controllers were purely electromechanical devices, their sophistication and accuracy of control were extremely limited.
A later variant (of pneumatic control) also employed pneumatic centrals, but w i t h the addition of a computer system. This computes system monitored some additional points (for example, space temperatures) and either calculated new set points for each pneumatic controller or allowed an operator at a computer terminal to transmit manual set points to the pneumatic controllers. Although this newer variant aided the building manager by providing more information about building conditions and performance, overall effective control of the building was still compromised by the local pneumatic controllers.
Each controlled point was still operated by a pneumatic controller with very limited sophistication and virtually no flexibility. These limitations became more important as ways to manage energy became more sophisticated, Some WAC system, such as variable air volume (VAV) systems, required an accuracy of control not attainable in most cases by pneumatic controllers. As a result, building energy managers were frustrated by their inability to improve the control strategies without rebuilding the pneumatic control system for each change.
DDC has solved both problems;. Instead of independent local pneumatic controllers, DDC uses control or monitoring points, each connected to a computer (or interconnected computers) which reads the value of each input and transmits commands to each output (Figure 2). The control strategies are implemented by computer programs, which can be changed by the operator at will. Also, each strategy has available to it the value of every system input instead of a very l i m i t e d local set. In short, under the DDC concept, the entire building operates as one integrated system rather than as independent srrrall systems.
Four main results accrue:
- Control can be as simple or sophisticated as desired, and can be changed easily;
- The system is more reliable because fewer electromechanical components are needed;
- Control is more accurate because of the inherent greater accuracy of DDC electronic components; and
- Energy is saved because an overall strategy eliminates energy waste resulting from simultaneous heating and cooling, which usually occurs in pneumatic systems.
The ability of DDC to accommodate virtually any control strategy has had a dramatic impact on mechanical design. Some new mechanical systems can operate in many different modes, depending on external conditions, space temperatures, season, condition of storage tanks, and utility-pricing structures. DDC allows such systems to be operated continuously in their optimum modes, a standard which simply cannot be attained by ordinary pneumatic systems or even pneumatic systems with computer monitoring. Consequently, mechanical designers are now free to d e s i g n the best energy system for a particular building with the assurance that whatever control strategies they specify can be carried out.
Each loop at the remote processors can activate itself independent of the others; however, the most efficient use of energy is achieved by controlling all the loops through the central processor. Scheduling air-conditioning and heating loads and selectively dropping electrical loads if the total building power approaches the demand limit are two common energy optimization features available.
Other features, such as optimal stop/start, which calculates the optimum starting and stopping times of heating/cooling units to prepare spaces for occupancy without wasting energy, are also used as part of an over-all strategy. Most of these optimization routines do not require any additional hardware since they are implemented by simply adding programs that sense existing inputs and change the strategy for controlling existing output actuators.
The building owner or manager who uses DDC effectively needs feedback to evaluate his strategies for optimizing building performance. DDC simplifies this process because it continually monitors each input directly and has storage capacity to keep files of the historical data thus obtained. These historical data can be plotted in color on a TV screen or summarized and printed in report format for management review. The most advanced DDC systems (Figure 3) include a generalized report generator which can produce nee types of reports at any time rather than limit the user to the reports engaged when the system was procured* This feature of DDC i s particularly important since the owner’s power to change his energy strategy generally creates a need for new reports on energy-sensitive areas identified by continued use of the system.
An ancillary benefit is the ability of the DDG system to include facilities other than WAC. With little increase in cost, factors such as control of security and lighting can be added to the system, thereby enabling greater energy savings and eliminating the need to purchase separate systems for badge reading and door-lock control. There is no doubt that DDC offers more effective energy management than conventional controls but, until very recently, its application to HVAC installations has been limited to large building complexes. Many small- and medium size building installations do not use DDC mainly because of its high cost. In the following sections a typical small building is analyzed and DDC is compared with pneumatic control on a cost and payback basis.
Small Building Systems
The cost of an HVAC controls Installation is generally related to the number of “points” t o be monitored or controlled, where each point is defined as an analog or digital input (e.g., temperature sensor, fan status switch) or analog or digital output (e.g. damper position or pump on/off control. Each building system, such as air handling, domestic hot water, or chilled water, includes a certain number of points. A recent study which included detailed analysis of a series of building HVAC system, showed that a small- to medium-size building of about 37,175 m2 (400,000 sq. ft .) would contain about 180 points, of which 35% would be analog inputs, 19% analog outputs, 25% digital inputs and 21% digital outputs. Although different building configurations and mechanical designs would affect the distribution of point types, the total number of points for a building of this size would usually be close to 180.
Designing a DDC System
Given the building layout and the number of points in HVAC equipment, the single greatest design trade-off is that between centralization and distribution of computer power. At the fully centralized extreme a single central computer controls all functions directly and all points are wired to it. At the other extreme (fully distributed), a smaller central computer is connected t o a myriad of other small computers, each of which is wired to 10 to 20 nearby points, In this second instance the central machine presides aver the whole system and controls the points through the intermediary of the remote processors. Each remote processor can control a single HVAC system (e.g. air-handling unit, chiller) independently. A median approach is to employ a moderate number of remote units each of which is wired to 50 t o 120 points.
Although all these approaches utilize the benefits of DDC, the three levels of centralization/distribution involve three factors that must be weighed against one another. The first factor is the cost of computer hardware. The fully-centralized approach employs a single processor, which is the least expensive since it combines all the computing power in one place w i t h one enclosure and no duplication of functions. The fully-distributed approach requires the heaviest capital cost for computer hardware.
The second factor is electrical installation cost. The fully distributed arrangement yields the lowest installation cost because each remote processor can be located very close to its points and thus wiring runs are short. The fully-centralized arrangement may be quite expensive unless all points are in one mechanical room. The median arrangement (Figure 4) may be the most economical over-all because four remote processors can be used, one in a penthouse, one in some other logical location such as a basement mechanical room, and others on various floors of the building.
The third factor is reliability. The fully-centralized scheme is most sensitive to failure since failure of the single computer causes the entire system to fail. Although the system can be made to fail safely, a system failure is inconvenient. The fully-distributed scheme is least sensitive since any component computer can fail while still leaving all the others running, but, as previously mentioned, the cost of the computing equipment is highest.
A median approach for small buildings makes good sense; a compromise on all factors is established by designing a system consisting of a central computer and four remote units.
Cost Analysis: DDC versus Pneumatic Control
The installed cost of DDC systems has traditionally been higher than for pneumatic sys tens, especially in small installations, where the cost of the DDC control processor is spread over fewer points. The cost of a pneumatic system tends to rise linearly with the number of points, as a large system requires more independent local controllers, whereas with DDC a central processor is required even for system with very few points. However, the rapidly falling cost of computing hardware has eroded the historical price difference between DDC and pneumatic installations. For a specific building of 37,175 m2 (400,000 sq. ft.), the installed cost of a pneumatic system is about 75% of the cost of a DDC system Although the initial cost of a DDC system is higher than for a pneumatic system, it can be recovered in a surprisingly short time . It is realistic to assume that a DDC system will yield a 10X% energy saving over and above conventional pneumatic control, due simply to its more accurate and sophisticated control, and t o its ability to provide the building owner with information about building performance and areas where energy should be better controlled. Features such as load shed and flexible scheduling alone will produce large energy savings, and these savings will increase as the owner becomes more familiar with the operation of the building. If we assume yearly maintenance costs of $12,000 and $10,000 for the DDC and pneumatic systems respectively, and an energy usage of 322 equivalent kWh/m2/yr. (30 kWh/sq.ft./yr.) at $0.0275 per kWh for both systems, it will take 1.4 years more for the DDC to pay for itself than it will for the pneumatic system when used in the building under consideration. After that time the DDC system will save money compared with the pneumatic controls. Another simple calculation shows that for a three-year payback the DDC energy saving need be only 5.7%, an e
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