Examining The Automatic Control System Application In Hvac Engineering Essay
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With the rapid development of the Computer-Based Control System, The Building Automation System (BAS) has come to control and manage the equipments in a building more scientifically, economically, rationally and reliably, which can not only enhance the function and the comfort level of the building occupancy, but also save energy and reduce the environmental impacts.
Economic development, civil, public and commercial building central air conditioner Popularizations bring a serious energy consumption problem. The more the amount of air conditioner in use, the more electricity expensed, it leads to the lack of electricity and limit the municipal use of electricity in summer. HVAC system consumes approximately about 50%~60% of the whole building consumption in the intelligent building. Especially the freezes unit, the cooling tower, the circulating water pump, the air conditioning unit and the fresh air unit, consume most of the energy. In order to control and manage the air-conditioning system effectively, and take full use of energy-saving technology, we apply the Automatic Control System. DDC (Direct digital control) is one of the ACS types, as the name implies, it involves direct digital communication between sensors, controllers, and actuators. DDC systems can be controlled by microprocessors or computers, which allow for more flexibility in control algorithms and also allow to be monitored remotely. The DDC has recently become an Open-System and adopted by the most international vendors, which mean that it can be an integral Sub-LAN of any international centralized control web. The DDC (other than only an HVAC control System), can now be integrated in any BAS system through a Native BACnet network (adopted by ASHRAE).In addition to the Automatic Control System application for energy saving and high HVAC system performance, a green HVAC system design will
achieve all aspects of comfort, energy saving, low initial and operational capital costs, and adds more efficient performance in conjunction with the DDC system, an example of such green HVAC system would be an Optimal Air System, and will be discussed in this report.
2. Automatic control System
An Automatic Control System is a preset closed-loop control system that requires no operator action. This assumes the process remains in the normal range for the control system.
An Automatic Control System has two process variables associated with it, a controlled variable and a manipulated variable.
a) A controlled variable is the process variable that is maintained at a specified value or within a specified range.
b) A manipulated variable is the process that is acted on by the control system to maintain the controlled variable at the specified value or within the specified range.
Automatic controls take the data produced by monitoring instruments and use these data to control everything from processing equipment to space heating and cooling systems. Many manufacturing processes that used to be manually controlled are now controlled automatically. This innovation has several benefits:
a) Immediate correction of unpre
b) Dictable changes;
c) Simultaneous adjustment of many functions;and
d) Highly consistent control.
The benefits realized from the use of automatic controls in process equipment are evident in quality and productivity improvements. When used for energy management applications, automatic controls can reduce energy costs by strictly controlling temperatures and flow rates and by adjusting lighting, motor speeds, and fluid and gas flows as required by the process.
Fig 1, Typical Automatic Control system, DCS LAN layout
2.2 Functions of Automatic Control
In any automatic control system, the four basic functions that occur are:
2.3 Elements of Automatic Control
The three functional elements needed to perform the functions of an automatic control system are:
a) A measurement element (Sensor, Transmitter, Transducer)
b) An error detection element (Digital/Analog/pneumatic Controller, PCU)
c) A final control element (Motor/Piston Actuator, VFD, VSD, Relay)
2.4 Remote location control and monitoring
The Automatic Control System' LAN and Sub-LANs can be accessed, controlled and monitored from remote locations via the Internet trough a developed centralized data management system which is capable of collecting data from multiple sites. This is accomplished by connecting with a gateway for collecting data from the lighting and air-conditioning control systems installed in each building or factory, and the center server for providing data collection, database and web server functions along with security measures applied to all transmitted data.
Based on the capability of real-time monitoring and analysis of actual energy consumption such as electricity and gas from a remote location by using a web browser, this system is able to achieve the maximum level of energy saving in buildings and factories which in turn, reduce the emissions and the environmental impacts by taking advantage of its cost effectiveness and by limiting the required energy for a specific application or function.
3. Types of Automatic Control System
3.1 Programmable logic controllers
The automatic devices most commonly used in process control are programmable logic controllers (PLCs). A PLC system has three main components:
a) Input module: devices such as limit switches,push buttons,pressure switches, sensors,electrodes and even other PLCs provide incoming control signals (digital or analogue) to the input module.The module converts the signal to a level that can be used by the central processing unit (CPU) of the controller. It then electrically isolates it and sends it to the CPU.
b) Controller: a programmable memory in the controller that stores instructions for implementing specific functions and converts the inputs into signals that go out of the PLC to the output module.
c) Output system: this system takes the CPU's control signal (programmed instructions), isolates it electrically and energizes (or de-energizes) the module's switching device to turn on or off the output field devices (e.g. motor starters, pilot lights and solenoids).
An example of a simple PLC used for energy management is one for an air-supply system. The PLC system controls variables such as temperature, airflow to various zones, humidity, filtering, shut-off of airflow to unoccupied areas and exhaust volume. Larger, more complex applications require more sophisticated PLCs, including ancillary data-entry equipment with trouble-shooting capabilities.
To control critical variables more closely, closed-loop PLCs of various degrees of complexity and configurations use a feedback from field devices. This provides more accurate and more adaptive control.
A PLC can be programmed through a hand-held device or downloaded from a personal computer (PC). Conveniently, a programmer can often be used for developing documentation that describes the system's configuration and operation. Sometimes this step is neglected. Documentation should be added to a user program for many reasons. Especially in energy management situations, the documentation will assist in the following ways:
Operators will receive system information to understand how the system operates.
Maintenance personnel will be able to troubleshoot and maintain the system.
Upgrading decisions will be simplified.
It will help answer questions, diagnose problems and make system modifications if requirements change.
It will allow easy reproduction of the system if another installation is needed.
3.2 Industrial Automation Controllers
These devices are a new breed of controllers that do not fit neatly into the PLC or PC classification. They are often used for special application controls such as motion and process control, particularly in complex closed-loop servo controls, such as those in robotic systems.
3.3 PC Process Control
Individual PLCs can be replaced by fully integrated PC process control packages. The energy manager profits from consistent, repeatable process control that integrates operations. Various packages are available, and their application can assist energy-saving efforts in, for example, the boiler house, refrigeration and packaging. They can be complemented by simulations packaged to test various "if" scenarios. In the integration of the system, various means of electronic signal transmission are employed and may include, for example, radio frequency (RF).
Investment in a process optimizer will reduce the specific energy consumption in a plant through the use of a sampling system and a control computer so that the factory operates with the minimum amount of energy.
3.4 Direct Digital Controls
Direct digital control (DDC) systems are generally used in large, complex operations where the operations of many devices must be coordinated. Like PLCs, DDC systems include sensors and output devices. A DDC system, however, has a computer instead of a logic controller. The computer makes DDC systems flexible and capable of managing complex processes because the setpoints can be changed dynamically and remotely by the principle of IP addresses assigned for every device within the network. They also permit operators to start and stop specific equipment remotely. Another strength of DDC systems is that they can store, analyse and display data.
4. HVAC Automatic Control System
HVAC Controls, building automation (BAS), direct digital control (DDC Controls) are at the heart of many energy management systems for energy savings. DDC Control Systems are mainly used in commercial HVAC control and energy management system applications in building automation.
DDC & building automation itself is an energy management system which saves management companies and building owners hundreds of thousands if not millions of dollars every year by efficiently controlling air DDC/building
automation system (BAS) should have trim and respond capabilities.
DDC is where mechanical and electrical systems and equipment are joined with microprocessors that communicate with each other and possibly to a central computer. This computer and controllers in the building automation system can be networked to the internet or serve as a stand alone system for the local peer-to-peer controller network only. Additionally, the controllers themselves do not need a computer to operate efficiently as many of these controllers are designed to operate as stand-alone controllers and control the specific equipment they are assigned to control. With a few exceptions, each DDC or building automation controller holds their own programs and has the ability to communicate to other DDC building automation controllers. It is important for the DDC or building automation controllers to communicate to each other. If the network fails for whatever reason then the system may still function (because the DDC controllers in building automation systems are stand-alone) but it will not function as efficiently as designed. Building Automation & DDC Control Systems grows more and more complex as time passes but it will save in energy and maintenance costs if installed and programmed properly. Energy Management Systems, DDC Controls Systems and Building Automation Systems (can be one in the same) are definitely the way of the future and are replacing older less efficient systems.
Fig 2, typical VAV DDC layout
4.1 DDC/BAS features and functions
A set-up in a multi-story automated building would have many DDC building automation controllers serving different types of air conditioning and heating equipment (DDC is not limited to just HVAC applications). Every building is different and it is important to select the proper control system and programs to control the various types of systems in a particular automated building. For building automation systems to be effective, it is important that the system is installed and tuned properly. Some important functions of an efficient HVAC DDC building automation system are:
a) DDC/building automation system (BAS) allow the owner to set up schedules of operation for the equipment and lighting systems so that energy savings can be realized when the building or spaces in the building are unoccupied;
b) Allow the equipment optimal start with adaptive learning. Optimal start is allowing the equipment to be brought on in an ordered and sequential manner automatically on a schedule before the building is reoccupied so that space set points can be realized before occupation. Adaptive learning allows the system to compare space temperature, outside air conditions, and equipment capabilities so that the equipment can be turned on at an appropriate time to ensure space set points are achieved before occupation.
c) Have trim and respond capabilities. Based on zone demand the set point for various heating and cooling sources will change according to demand from the zones. In a VAV system all the VAV boxes are served from a central air handling unit. If all the zones are at set point then the supply air temperature set point of the air handler is automatically changed to prevent mechanical cooling from occurring when it is unnecessary. When the zones grow warmer the supply air temperature set point is automatically lowered to allow mechanical cooling to satisfy demand. Older systems have a single supply air temperature set point of 55° Fahrenheit which requires the compressors to cycle even when it is not necessary;
d) Have the ability to monitor energy usage including the ability to meter electric, gas, water, steam, hot water, chilled water, and fuel oil services;
e) In conjunction with the appropriate mechanical system set-up, offer economizing based on enthalpy calculations and/or CO2 set point control;
f) Have algorithms as reset schedules for heating plants, static pressure control, and other systems where energy savings can be realized through these predictive programs;
g) Offer load shedding when power companies are at peak demand and need business and industry to cut back on power usage to prevent black outs. Building automation systems allow the owner to cycle various things off like water heaters or drinking fountains where use of these things will not be noticed even though they are off;
h) Offer the ability to send alarms via email, pager, or telephone to alert building managers and/or technicians of developing problems and system failures;
i) Management companies who acquire a good DDC/building automation system (BAS) can have DDC set up to bill tenants for energy usage;
j) Have the communications abilities to be integrated with other building automation control systems and TCP/IP family of protocols. It is BACnet compatible and other open source communication protocol.
4.2 LAN-WAN Configuration
The DDC can be configured as independent (localized) closed-system, or DDC open-system based on accessibility options required by a group of buildings managed by a single company or property management firm (centralized), or a single property to be monitored and controlled by its own (localized).
A closed protocol is a proprietary protocol used by a specific equipment manufacturer. An open protocol system uses a protocol available to anyone, but not published by a standards organization. A standard protocol system uses a protocol available to anyone. It is created by a standards organization.
An open system is defined as a system that allows components from different manufacturers to co-exist on the same network. These components
would not need a gateway to communicate with one another and would not require a manufacturer specific workstation to visualize data. This would allow more than one vendor's product to meet a specific application requirement.
Fig 3, Integrated BACnet based WEB Browser BAS Control System Layout
5. High-performance Low-energy HVAC design
Recall the Introduction, In addition to the Automatic Control System application for energy saving and high HVAC system performance, a green HVAC system design will achieve all aspects of comfort, energy saving, low initial and operational capital costs, and adds more efficient performance in conjunction with the DDC system, an example of such green HVAC system would be an Optimal Air System.
Optimal Air systems uses less energy than conventional systems on an annual basis, for example, In a conventional system, supply air temperatures run between 54°F -57°F from the air handling unit. With duct heat gain, the supply air ranges from approximately 56°F-59°F out of the air diffuser.
In Optimal Air System, supply air temperature run between 45-52°F from the air handling unit to optimize energy consumption, reduce first capital cost and improve humidity control. Optimal Air has for years been extensively used in grocery stores and is gaining increasing popularity in comfort cooling applications such as offices and schools.
There are several benefits of Optimal Air that make it an attractive system for use in a wide variety of applications.
It Saves Space and Reduces Energy and Construction Costs, increases the amount of sensible heat that each CFM delivered to a zone can absorb. While 50°F air may not seem much colder than 55°F air, the delta T rises from 20°F to 25°F. That is an increase of 25%.
This affects the sizing of the ducts, air handling units and fan motors, all of which will be smaller and results in a system that requires less space and uses less power. In many applications, fans can use more power annually than refrigeration (chillers, condensing units, pumps, and compressors).
An example of annual 10-story building energy usage of 200,000 square-feet of HVAC components, the fan energy use is high because the fans operate every hour the building is occupied providing minimum air movement, ventilation air, heating, etc. In this case, an Optimal Air system would have a very real impact on overall energy costs.
Fig 4, Annual HVAC Energy Usage
5.2 Less Humidity, more comfort
Optimal Air systems take more moisture out of the return and ventilation air mixture as it passes over the cooling coil. The lower moisture content in the supply air reduces the "Psychrometric balance point" humidity level in the conditioned space. This allows the space temperature to be set higher while achieving the same comfort level for occupants and further reduces the supply air quantity and fan power requirement.
5.3 Quieter and Improve Indoor Air Quality (IAQ)
The lower air volume required for Optimal Air systems makes them quieter than conventional systems. Fan sound generation is a function of fan type, static pressure and air volume. By reducing air volume (and often the total fan static pressure) Optimal Air systems generate lower fan sound which can result in more desirable space conditions. This reduced sound generation can also be used to reduce the cost of any required noise attenuation in critical applications.
The lower required air volume can also be used to reduce filter face velocities, allowing more efficient filters to be used without high energy cost penalties. The lower air temperature and resultant humidity levels also reduce the chance of mold growth in the air handling units, ducts or the occupied space.
The example of the building above requires a supply air of 26,667 CFM. The HVAC system is floor by floor VAV air handling units with a two chiller primary secondary system, Optimal air works equally well with applied rooftop units or indoor vertical self-contained units.
Table 1, HVAC system performance with optimal air system
Table 1 shows the HVAC system performance as the supply air temperature, to the duct, is lowered. It is important to differentiate between supply air temperature off the cooling coil and supply air temperature into the duct.
To accommodate the lower supply air temperature, the chilled water supply temperature (CWST) was gradually lowered, the air handling unit coils deepened to allow for closer approaches, and chiller performance was adjusted to deal will the increased lift. Because of their basic operating differences, DX rooftop and self-contained systems may have a different Optimal Air temperature than a chilled water system. When considering multiple system options, it is important to use Energy Analyzer for each in order to identify the best option.
5.4 Optimal Air Balance Point
Reduced fan energy must be "traded off" against increased refrigeration energy. This trade off varies with the type of building, the type temperature control system, the type air conditioning system and geographic locale. Therefore, the "optimal" supply air temperature is different for every job. When only energy costs are a factor and no thermal storage is involved, this optimal supply air temperature generally falls in the 47°F -52°F range. It can be determined by comparing total system energy consumption with varying supply air temperatures using an energy analysis program.
5.5 Space Design Temperature and Related Comfort
Temperature, humidity, air velocity and mean radiant temperature directly influence occupant comfort. Conventional designs are usually based on maintaining 75°F and 50% RH (Relative Humidity) in the occupied space. Figure 5 shows the ASHRAE comfort zone where 80% of the people engaged in light office work are satisfied. As the relative humidity is lowered, the space air temperature can be raised and still provide occupant comfort.
The leaving air condition from the air handling unit is the primarily control of the relative humidity in the occupied space.
The internal moisture gains from people, kitchens, etc, as well as infiltration also play a part.
Fig 5, Equivalent comfort chart
In most climates, the lower the supply air temperature, the lower the humidity ratio and the drier the space. Figure 5 shows sensible heat ratio lines for conventional, Optimal and low supply air temperatures. As the space relative humidity is lowered, the space temperature set-point rises from 74°F to 78°F.
5.6 ASHRAE Compliance
The 1999 and 2001 version of ASHRAE Standard 90.1, Energy Standard for Buildings except Low Rise Residential Buildings , has mandatory requirements for refrigeration equipment and prescriptive requirements for fan work. The Standard recognizes that Optimal Air systems improve fan work significantly and provides credits to account for improved fan performance. In addition, refrigeration system performance is rated at conventional conditions or special tables are provided to account for non-standard operating conditions (as is the case with centrifugal chillers). In either case, ASHRAE Standard 90.1 does not penalize Optimal Air systems.
5.7 Design Considerations
Design of refrigeration and air handling equipment for an Optimal Air system is similar to the design of a conventional air temperature system. Attention must be paid, however, to air distribution, controls and duct design. Conventional diffusers, when properly applied, will work with Optimal Air.
Controls also require only minor changes from conventional systems. In particular, programming of economizer controls and supply air temperature reset. Finally, the ducting system must be sized for the reduced air volume to take full advantage of the potential capital savings. Duct insulation and sweating should also be reviewed to provide a trouble free system.
Not every building type is a good candidate for Optimal Air. When air volumes are dictated by air turnover rates, such as some health care applications, Optimal Air offers no advantage. In fact, there would be increased reheat costs. Office buildings are a strong candidate for Optimal Air. They have high sensible heat ratios and typically less than 20% ventilation loads. Schools can also be a possibility. Generally speaking, as the percentage ventilation load increases, Optimal Air becomes less attractive.
Location and climate also impact whether or not Optimal Air is a good candidate. Locations where weather provides significant economizer hours between 45 and 55°F will limit the savings. Ultimately, each project must be checked by performing the applicable specific calculations. The following should be considered:
Load and Balance Point calculations, Space Temperature Set-point evaluation, Design Load Calculation, Primary and Secondary System Selection, Parallel, mixing or series VAV-Fan powered boxes, Perimeter Heating, Air Distribution, Diffusers (based on air flow and the throw distance calculation), Duct design (considering duct heat gain, sweating and insulation).
5.8 System Life-Cycle Analysis
Evaluating different engineering solutions is always part of a good proposal. Optimal Air systems are no different.
In the case of Optimal Air, there may be no need to do any calculations because Optimal Air systems cost less to build (lower capital cost) and have the same operating cost as conventional systems (assuming the balance point was used for the design).
Duct sizing will decrease almost linearly with reduction in air volume. The installed cost will not change linearly because of the labor portion. A 20% reduction in air volume can result in 80% savings of the 20% reduction or 16% overall savings in sheet metal cost.
On the plus side, there are less pounds of steel and fewer man-hours to install it. On the minus side there is more insulation. Terminal boxes and diffusers will be a wash since there are fewer of them but the equipment cost will be higher than conventional equipment.
HVAC equipment will cost about the same. This is conservative because the air handling equipment will cost less and refrigeration equipment will be slightly more. There is typically more capital invested in air handling than refrigeration.
Building envelope should be the same for new construction. In the case of retrofit applications, it will depend on the quality of the existing building.
The cost of space may also need to be evaluated. Not accounting for space savings is conservative. There will be space savings but they may be difficult to realize. If enough plenum height savings can be realized to add another floor within the same building envelope, then that rentable space should be accounted for.
Simple payback calculations do not take into account the cost of money, taxes and depreciation, inflation, maintenance or increases in the cost of energy. A more complete analysis should include Internal Rate of Return (IRR) and net present value (NPV). In the HVAC industry, many projects fail simple back (they are in the 5-year range) while passing IRR (they offer a 25% rate of return).
Software analysis tools can be used to perform both energy and life-cycle analysis that include simple payback, IRR and NPV.
Building owners and designers faced with increased concerns for energy saving and environmental stewardship search for cost effective system options for their projects.
The Automatic Control system, integrated with a high-performance low-energy HVAC system as the Optimal Air system can deliver both low first costs and reduced energy costs in a new construction and retrofit applications. This integrated system will not only meet the efficiency and sustainability of its performance at the desired set-parameters, but when designed with advanced selection tools, installed with the most advanced DDC, and supported by trained operators, allow building owners to compare predicted energy use to actual performance and environmental impacts reduction.
In today's challenging energy efficiency, building owners need proven system that delivers the necessary performance to meet their integrated environmental sustainability and business goals .
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