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Article - July 99
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Thomas Hartman, P.E. The Hartman Company

The Hartman Company is working with interested firms to define and modularize networking technology "packages" such as outlined in this article.

Thomas Hartman is an internationally recognized expert in the field of advanced high-performance building operation strategies.

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Although HVAC systems have had network based control capabilities for a number of years, this capability is seldom employed. Now, the widespread use of AC variable speed motor drives for fans, pumps and chillers is making the use of network based control imperative for the conscientious designer who is looking to achieve greener designs. This article discusses the shortcomings of conventional control in the variable speed era and shows how network based control can improve HVAC energy efficiency while also improving comfort performance. Additionally, a new approach to implementing advanced network based control technologies is presented in order to engage designers in an industry dialog on how to most effectively implement advanced control technologies.

A NEED FOR IMPROVED CONTROL

Except for electric heat, virtually all the electric consuming HVAC equipment in large commercial buildings operate in accordance with the fan and pump laws. These fan and pump laws dictate that a centrifugal fan, pump or compressor can supply 50% of design flow (or capacity) at 50% speed and require only 12.5% (0.5 cubed) of the full flow power. This means that as the capacity requirements of variable speed devices decrease to one-half, the operating efficiencies can increase by 400% (50% divided by 12.5%), but only if the pressure at which the fluid is supplied is permitted to fall to 25% (0.5 squared) of the full flow pressure. If the supply pressure does not fall with the square of the flow requirements, then the pump speed cannot be reduced as the law allows and substantial energy is lost due to a reduction in both flow and fan or pump efficiency. These relationships are shown in Figure 1.

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Figure 1 shows the zone of optimum efficiency for a variable speed centrifugal pump, fan or compressor. If the device can be made to operate within that zone as capacity requirements change, it will operate at highest efficiency. To remain in this zone, the head pressure requirements must fall with the square of the flow as shown in system curve "A." Such a system curve ensures the device operates at optimum efficiency through all flows, and that power required falls with the cube of the flow requirements. However, the system curve most typically selected in today's designs for variable speed pumps, fans, and chillers is more closely represented by curve "B."

CatNet SystemsIn curve "B," the pump, fan or compressor efficiency falls as flow requirements are reduced. But there is another costly energy penalty for systems that follow curve "B." This is the high operating pressure at decreased loads. Because the overwhelming majority of variable flow chiller, fan and pump systems are operated to meet a pressure (or temperature) setpoint that is based on satisfying peak conditions, a substantial energy penalty occurs as the load falls below full load. This energy penalty consists of two components: 1. Reduced operating efficiency because the device is not operating on its optimum efficiency (natural) curve. 2. Increased energy consumption due to the higher than necessary operating (head) pressure. 1. The combination of these two energy penalties limits power reduction opportunities at part loads for most variable speed centrifugal chillers, pumps and fans in operation today.

Because centrifugal fans, pumps and compressors are simple devices that are very efficient at their "sweet spots" (the high efficiency zone in Figure 1), it is reasonable to try to find ways to accommodate their limitations and develop control concepts that permit the pressure to fall as flow decreases in order to operate them at the highest possible efficiencies throughout all flow conditions.

PART LOAD OPERATION

The value of such an approach is illustrated in Figure 2 which shows the heating and cooling load profiles for HVAC systems that provide comfort conditioning in the Denver, Colorado climate. These profiles are representative of the load profiles in most North American climates. Figure 2 shows the percent of total operating hours the cooling and heating systems spend at various load capacities. Note that the overwhelming majority of heating and cooling HVAC system operating hours are spent at substantially less than full load. By employing a control strategy for chillers, fans and pumps that operates these units to meet capacity requirements independent of pressure, substantial annual energy savings can be achieved because HVAC systems spend such an overwhelming majority of their operating hours at part load conditions. Therefore, from an energy efficiency standpoint, it is desirable to switch from pressure control (for fans and pumps) or temperature control (for chillers) to a means of control that permits fans, pumps, and chillers to operate according to Curve "A." Network control in which the speed of the device is set based on actual requirements of all the loads served rather than to meet a static pressure or temperature setpoint is the solution The Hartman Company has developed to achieve this result.

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IMPROVING ENERGY PERFORMANCE AND COMFORT WITH NETWORK CONTROL

To achieve the full potential performance capabilities of variable speed equipment, designers must consider more closely the actual objectives of an HVAC system. Building occupants seldom call their building operator to complain about the chilled water temperature, or air or water supply pressure. Rather, most complain when their comfort or environmental quality expectations are not being met. Controls employed for most variable speed equipment today not only fail to achieve optimum efficiencies of variable speed equipment, but also only indirectly affect the ability of an HVAC system to meet occupants' environmental requirements. HVAC systems are designed to maintain certain space conditions at one or more calculated peak load conditions. Equipment is then selected to perform with sufficient capacity to meet these conditions while operating at certain established setpoints. So, those setpoints are implemented into the controls, and the system is turned on when the building schedule declares it to be occupied. No information concerning the actual space conditions is employed to adjust the operation of the plant and distribution systems. If a number of spaces in the building begin to overheat, the central systems does not self-adjust to try to provide more cooling to those spaces. Nor when all spaces are satisfied, does the systems self-adjust to meet the reduced load with greater efficiency. To do these things requires a "network" based control scheme that employs the control network to communicate with the loads being served. Network based control is very cost effective because in most circumstances it requires no additional equipment or controls. In fact, when properly applied, network control requires less equipment, and often employs simpler configurations than the conventional systems and controls it replaces.

WHAT IS "NETWORK" CONTROL?

While the purpose of network control is to tie the operation of all HVAC equipment to actual space conditions, this does not mean that chillers and towers operate directly from space temperature sensors. Rather, network control is based on an understanding that a building HVAC system is a single system whose energy efficiency and comfort performance are optimized when the operation of all components are coordinated together to meet the actual needs in the spaces served. This is not a radical idea, but HVAC designers are so bombarded with out of date technical information that entreats them to isolate systems from one another, that truly integrated control is almost never accomplished in practice. For example, while low delta T is a serious problem in chiller plants, designers still regularly employ "decoupling lines" that permit direct mixing of supply and return chilled water (and a reduced delta T) under the outdated notion that system isolation and independent control of plant equipment is the correct approach.

Actually, the opposite is true. It is intuitive that coordinating the operation of a chiller plant and the chilled water distribution system is a requirement for achieving the highest overall system efficiency, but the industry's approach continues to be to isolate the two as separate systems and operate each by nonintegrated stand alone controls. The chiller plant is operated to maintain a specific chilled water setpoint and the distribution system to maintain a specific pressure setpoint. Does this make sense? Obviously not. What does make sense is to integrate these together and operate all equipment as a single system to most efficiently meet all cooling loads at all times. That is what network based HVAC controls have been developed to do.

HOW A "NETWORK" CONTROL HVAC SYSTEM WORKS

The question is, "How does one accomplish network based control effectively?" To help answer this question, consider the building air conditioning system diagram in Figure 3 which is typical of many commercial buildings today. Conventional control would operate each VAV box damper to maintain a space setpoint. The air handlers, distribution chilled water pumps, chillers, and cooling towers would each operate to maintain temperature and/or pressure setpoints with little or no feedback from the loads served. In this way, the plant and distribution equipment is effectively decoupled from the loads they serve.

Operating under a network control strategy, however, the zone temperature sensor still operates the box damper, but the air handlers coordinate supply air flow, outside air dampers and cooling valve to ensure the desired cooling level in each zone is maintained most efficiently while also ensuring adequate ventilation and air flow. Under network control, air handlers are not operated to maintain static supply temperature and pressure setpoints, but to satisfy the thermal loads and comfort requirements of the zones they serve. For typical VAV systems this means that both total fan air flow and outside air are maintained within certain ranges at all times. The fan speed, mixed air dampers and cooling valve are adjusted within the ranges established by basic system requirements as the need for cooling in the zones served changes.

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Under the network control regimen The Hartman Company has developed for air systems (called TRAV for terminal regulated air volume), the fan digital controller employs the network to determine two separate real time characteristics of the loads served; 1) the average load, and 2) the deviation (or extreme loads). The value of the average load served indicates generally how much cooling resource needs to be expended. The deviation is employed to determine the relationship between the air flow and cooling effect of the air delivered to meet these loads. When the deviation is small, the supply air flow and cooling effect are coordinated to provide optimal overall energy efficiency through simple flow relationships. However, when the deviation increases, the cooling ratio of the supply air is increased away from optimal as the most effective means to meet exceptional localized space conditions.

Under this network control regimen, the chillers and chilled water distribution equipment are operated in a fashion similar to the air side equipment, except the loads served are the cooling coils, not VAV boxes. This network control regimen for chiller plants is called the Hartman LOOP. Like TRAV for networked air side control, chillers, towers and pumps operating under LOOP control continuously collect real time data concerning average load and deviation. This data is employed to operate the plant such that each load it serves is met effectively, but also with the highest possible efficiency.

By communicating and acting on actual zone conditions in the operation of plant and distribution equipment, these network control strategies improve building comfort and environmental quality as well as improve the energy performance in buildings. While enhancement to comfort and environmental quality is often best determined subjectively, the energy savings attributable to network based control can be accurately determined by hourly simulation. The simulation results for the electric portion of the HVAC system in a Denver high rise office building is shown in Figure 4. This building was designed as a low energy building, using less than 50,000 BTU/SF (550 MJ/SM) total energy annually. Note that despite the initial low energy design, the network based control further reduces the electric energy use of the chiller plant and HVAC distribution system by 50%! At the same time this networked control results in a more comfortable building and a higher level of indoor environmental quality.

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A TECHNOLOGY WORTH PURSUING

By employing network based control, equipment can operate at improved efficiency and also operate to meet the loads served more effectively. Network control is possible because most DDC systems now have the ability to "network" (or share the value and status of points) among controllers. If you are a designer who wants to improve the comfort and environmental quality of your projects and at the same time improve their energy efficiency, it's time to consider network based control. Imagine a building in which control is provided by actual load requirements sent over the system's communications network. Fans, pumps and chillers operate at all times to meet flow and capacity requirements as efficiently as possible. Properly designed, such a building requires no time schedule, since the entire system reacts to current occupancy conditions in the building. The most exciting features of network control strategies is that they can provide savings in both first costs and annual energy/operating costs, and still result in more comfortable buildings!

THE PROCESS IS A PROBLEM

This advance in technology is an exciting prospect, but the question that begs an answer is "How can I as a designer specify such technologies and be certain they will be implemented effectively and smoothly?" It is not too difficult to envision the basic changes in controls and operation required to capture these substantial energy savings, but designers cringe at the prospect of trying to describe unusual network based solutions in a sequence of operations, and most will dismiss entirely the chances that a controls contractor will provide a trouble free implementation of such a network based sequence however well it is described.

The unhappy truth is that the process by which HVAC systems are implemented is an enormous part of the problem in raising the level of controls technology applied in the HVAC industry today. Most designers realize that modern DDC systems have far greater capacity than is utilized in their designs, but they are reluctant to change a design approach they know can be implemented without troublesome startup or ongoing operations problems.

FORMULATING A SOLUTION

To try to develop a solution to this enigma, consider what it would mean if the applications technologies outlined in this article were a product that designers could specify, and with it came design, implementation and operations support sufficient to ensure success. In this technology product, the supplier and not the designer assumes ultimate responsibility for the performance of the product. Software "products" are widely used today, but not as elements in HVAC system design documents. However, by designating certain more complex network based sequences as products whose basic features are generally understood by a wide segment of our industry, an implementation path is being developed that is similar to that of software products in other industries. This promising model for successfully implementing new network based application controls is the approach our firm has taken to work with HVAC designers to improve the comfort and energy efficiency of their commercial building designs.

The Hartman Company is working with interested firms to define and modularize networking technology "packages" such as outlined in this article. The improvement in performance and reduction in energy available from implementing such networked controls are substantial. We invite all interested industry members who wish to see advanced technologies applied more effectively to work together with us to determine how specific networked technologies can best be "packaged" in order that they can be employed efficiently and effectively as applications arise. To get the benefit of all voices in our industry, I urge interested readers to visit our web site to get a better understanding of the both the technologies and process we are developing to support them, and to send us comments on this developing vision concerning how networking technologies can be more effectively implemented in buildings today.

SUMMARY AND CONCLUSION

While the industry employs modern HVAC equipment, the energy efficiency and comfort enhancing capabilities of this equipment can be substantially improved by applying "network" based control strategies. However, the full performance capabilities of network controls technology is rarely achieved. Current design and implementation practices act to limit the potential effectiveness of these advanced technologies. A promising prospect that could change this situation is the development of network based control technology "products." To undertake this challenge, the industry needs to hear all interested voices to determine if, and then how such packaged products can be developed and implemented in HVAC systems to achieve higher performance.

Additional information on technologies discussed in this article is available at www.hartmanco.com. Comments and questions about the article may be addressed to Mr. Hartman at tomh@hartmanco.com.


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