Efficient Heating, Ventilation and Air Conditioning (hvac) systems
Heating, ventilation and air conditioning (HVAC) systems include heating and cooling, filtration, and dehumidification and humidification with respect to the climate (Levine, Urge-Vorsatz, Blok, Geng, Harvey, Lang, … &Yoshino, 2007, pp. 399). HVAC systems are in most cases used in commercial buildings where mechanical ventilation is inevitable, more so in climatic regions where seasonal heating is almost airtight. HVAC systems combine both heating and cooling systems which may be coordinated to function simultaneously. In order to maintain the desired room temperature, a simple HVAC system would circulate a fixed amount of air at the required cold or warm temperature by heating or cooling the air where necessary (Levineet al., 2007, pp. 399).
The circulation of air by the system is in most cases much higher than the actual circulation required for ventilation to eliminate the contaminants within the indoor environment. Air is supplied at very low temperatures than needed in any zone during the cold/winter season requiring the system to first heat the air before releasing it into the indoor environment (Levineet al., 2007, pp. 399). As a result, HVAC systems consume a lot of energy which called for need for innovative measures to scale down the energy consumption of the system. The innovation brought about a number of changes in the system including the use of variable air volume systems which are capable of reducing simultaneous cooling and heating of air (Levineet al., 2007, pp. 399).
The changes have also brought about reduction in the energy used by the fun and pump of the system through regulation of the speed of rotation, recovery of coldness or heat coming from exhaust air from ventilation by use of heat exchangers, use of hot or chilled water for separation of cooling and heating functions from ventilation and circulation of air has been limited to just the amount necessary for ventilation, seasonal variation of the HVAC system maintained temperature with respect to exterior conditions, use of desiccant dehumidification to separate functions of dehumidification from those of cooling, correct sizing of the HVAC system components, automatic variation of the system with respect to changes in the number of a building’s occupants (demand controlled HVAC system) which has helped in saving the amount of energy wastage by the system by an approximate margin of 20 to 30 percent (Levineet al., 2007, pp. 399) as cited in (Brandemuehl & Braun, 1999, pp. 15).
The main purpose of HVAC systems has always been to increase the level of operator comfort within the building. Demand for such systems have been on the rise over the past few years due to the increased importance accorded to operator comfort (Jensen, 2012, pp. 26). The demand led to innovation of new technology in the HVAC system components which was accompanied by new methods of modernization, system control, industrial revolution, and increased efficiency which were introduced constantly by worldwide investors and companies. The quest for providing acceptable indoor air quality and thermal comfort that are within reasonable maintenance, installation, and operation costs brought about the interrelation of the three major functions of air conditioning, heating, and ventilating in HVAC systems. Through room air distribution, HVAC systems are enabled to maintain relationships of indoor pressure, reduce infiltration, and provide ventilation in between spaces (ASHRAE, 2002, pp. 10).
Intelligent HVAC systems
An intelligent HVAC system’ is a broad term meaning many things. More discussion is needed in the community to reach a common definition. Reijula et al. (2011, pp. 227) have defined an intelligent work environment to be able to: sense the interaction between users and space, process this information and understand the context data, react in a way that adjusts to users’ needs and enhances their endeavors in the environment, be active and autonomous, omnipresent and enhance the worker’s flow of work and perception of their physical and psychological well-being. The same is also required from HVAC systems in an intelligent hospital environment.
Most modern HVAC systems have the ability to control IAQ by detecting certain parameters of indoor air such as air temperature, carbon oxide (CO2) concentration, humidity or air flow rate and adjusting it to match a predefined, ‘optimal’ value. This makes indoor air pleasant for the hospital staff and patients and also creates energy savings as excessive heating/cooling can be avoided. However, many modern HVAC systems only respond to a single IAQ parameter (i.e. air temperature or CO2) while disregarding others. This may be partly due to a lack of feasible HVAC optimization techniques and advanced HVAC sensors. Rapidly advancing technologies have, however, enabled new systems capable of detecting several air quality parameters simultaneously like the human nose or body – and optimizing them to suit the needs of hospital users. HVAC systems should be developed towards more adaptive and user-centric ones; to take users’ needs and preferences into account and adjust the room temperature, humidity and airflow rate to meet these preferences (Wong & Li, 2010, pp. 262).
Limitations of new technology
In order to examine the limitation of the new technology in HVAC systems, we decided to examine one of the environments that most makes use of and/or depends on HVAC systems to a greater extent. We found that other than offices, the hospital environment also made great use of HVAC systems and as a result, we examine the limitations of the system basing most of our argument on the hospital environment.
Present situation in hospitals
The facilities management commonly has the final say on HVAC system implementation. They value the financial aspect, which may mean saving money by choosing cheaper alternatives for hospital HVAC solutions. However, needs of the facilities management often contradict needs of the actual users of the hospital – the patients and physicians. They value a comfortable environment to be treated or treat patients (Fransson, Västfjall, & Skoog, 2007, pp. 1887). The focus of value should thus be shifted from financial considerations towards those of user centricity
The age and condition of hospital buildings often closely correlates with the quality of the hospital’s HVAC systems. Twentieth century saw quick development in medical technology and after the Second World War, a period of intense hospital construction and renovation began. Many of today’s hospitals and their HVAC systems have been built during that era and are now approaching the end of their life-cycles and are in dire need of a major repair or renovation (Bartley, Olmsted, & Haas, 2010, pp. S5; Hellgren, Palomaki, Lahtinen, Riuttala, & Reijula, 2011, pp. 47). As a consequence, the quality of HVAC systems is not as high as it should be and the prevalence of indoor air-related problems (temperature, humidity, infection control, etc.) has been high worldwide (Nordstrom, Norback, & Akselsson, 1995, pp. 172; Hellgren et al., 2008, pp. 59).
Limitations of theintelligent HVAC systems
Although a growing amount of research is put into development and implementation of intelligent HVAC systems, several challenges have been encountered along the way. Some of the most relevant challenges facing the system are illustrated herein.
Poor hospital design
Despite extensive knowledge on the mechanisms of infection spread in hospitals, little has been done to radically redesign hospitals with an emphasis on contamination control (Clark & Calcina-Goff, 2009, pp. S775). New hospitals are often designed by architects who lack knowledge of the function of contamination control (Clark & de Calcina-Goff, 2009, pp. S775). Designers may also be hampered by being unable to implement radical features; those that do not appear in national guide- lines or are outside nationally accepted norms (Clark &Calcina-Goff, 2009, pp. S776). Furthermore, once a hospital has been built on traditional lines, a retro-fit for greater patient protection is seldom a viable option (Clark &Calcina-Goff, 2009, pp. S776).
Lack of adequate sensors
Many IAQ sensors have not achieved commercial success in the past years (BCS Partners, 2002, pp.9). Ihasalo argues that some HVAC systems are installed with inadequate sensors which hinders their sensitivity to the indoor air quality (Ihasalo, 2012, pp. 210). Although a variety of sensors are becoming commercially available, more work could be carried out in sensor development as well as establishing common interfaces for them. The developed HVAC sensors and devices must be simple, feasible and functional with a common interface or they will not be adopted by hospitals.
Poor data collecting, archiving and visualization by building automation systems
Current building automation systems used to manage and control HVAC equipment – have a limited ability to collect, archive and visualize data (Brambley et al., 2005, pp. 156; Haves & Hitchcock, 2008, pp. 25). They are usually capable of real-time monitoring of only temperature and humidity, while several important parameters of air quality are left unmonitored. The visualization of building automation system programs has been mainly created for adjusting system parameters and for system monitoring. Each program developer has their own program, which causes challenges for the user. Thus the functionality and feasibility of building automation system programs should be improved. Although it is possible to simulate building automation systems in order to improve their feasibility, it is currently only performed rarely (because this requires special expertise in building automation systems). It is likely that future generations will capitalize on building automation systems much better.
Complexity of airborne infection spread prevention
The generation of infectious aerosols from infectious human pathogens can occur in many ways, and in many settings in a hospital environment (Tang et al. 2006, pp. 111). For instance, droplets generated by talking, laughing, coughing and sneezing may lead to the generation of an infectious aerosol (Tang et al. 2006, pp. 112). The environmental conditions (i.e. temperature and humidity) determine the survival of these pathogens (Tang et al. 2006). However, they vary with the season and the indoor building environment. The aerosols can also be transmitted over short distances (large droplet aerosols) or long distances (small droplets) (Tang et al. 2011, pp. 216). This implies that it would be very difficult for one to make preparations in readiness for an outbreak of airborne infection.
Conflicting indoor air preferences between patients and staff
According to Skoog et al. the hypothesis that staff and patients can be treated as one coherent group of hospital users with the same needs and preferences is incorrect (Skoog, Fransson, & Jagemar, 2005, pp. 873). For instance, the patients accept the thermal climate in a higher degree than the staff – possibly because the patients will leave the hospital in foreseeable time (Skoog, Fransson, & Jagemar, 2005, pp. 873). Also, the patients’ and physicians’ preferences for indoor temperature and humidity differ from summer to winter seasons (Skoog, Fransson, & Jagemar, 2005, pp. 873). Good mental and physical health speeds up the recovery of the patients and enhances the work efficiency of physicians (Wells-Thorpe, 2000, pp. 13). It is thus important to pay attention to the indoor air needs of each hospital user individually.
Insufficient knowledge on HVAC systems
Lack of good instructions for ventilation design and maintenance and improper HVAC system use have resulted in infections inside the hospital (Hellgren et al., 2011, pp. 61). In many cases, the provisions for contamination control, particularly in relation to HVAC systems, do not operate as designed (Clark & de Calcina-Goff, 2009, pp. S769). The hospital staff has usually not been trained to monitor such systems and as a result often report faults and are confused with the over-complex and unreliable engineering controls (Clark & de Galcina-Goff, 2009, pp. S769).
Technical comparison of HVAC systems versus conventional systems
Conventional systems operate on the principals of diffusion. Colder air from the external environment enters the room via vents normally placed close to the ceiling at approximately 500 to 700 feet per second. After the air gets into the room, it descends naturally as a result of convectional currents (cold air goes down as warm air rises). The even distribution of air in each room is determined by the placement of return and supply vents. Conventional controls, such as receiver-controllers or electric balanced bridgecontrollers, are also known as single-loop controls because they control only one sequence or loop in an entire control system. Direct digital controllers are superior to traditional controllers because they follow program instructions and can manage many control loops simultaneously. In addition, a digital controller can be reprogrammed as needed, without changes in hardware (Reeves, 1996, pp. 28).
HVAC systems provide very accurate control of set points. The accuracy of mechanical controllers can drift over time, which decreases the performance of the control system. The operating functions of a digital controller can be fine-tuned through software modifications to allow for shifts in floor plan layouts and changing tenants, while a change in mechanical control functions requires the replacement or modification of costly hardware. Furthermore, using software with adaptive control capabilities, a digital controller can self-adjust program variables in response to changing environmental conditions(Reeves, 1996, pp. 28).
Recent generations of microprocessors are more reliable and inexpensive than their predecessors, making modern digital systems more affordable to price sensitive builders and developers. Innovations in the design of HVAC systems have scaled down their cost to the point of competing on a first-cost basis with conventional pneumatic control systems. Eighty percent of respondents to a recent survey thought that only certain pneumatic products would survive the digital controls revolution, such as pneumatic actuators for dampers and valves, due to their cost effectiveness in large applications. In addition, building developers are quick to recognize that an integrated building automation system can add value to property and provide property managers with excellent tools for managing building energy costs(Reeves, 1996, pp. 29).
Direct digital control (DDC) is an extremely fast and accurate process in which a microprocessor controller constantly monitors information from the environment, performs calculations based on internal programming and database information, and provides corrective action. It is essentially a three-step signal conditioning process: input signals are converted from analog to digital format so the microprocessor controller can read the incoming datacontrol computations are performed by the microprocessor that incorporate the incoming data; and the corrective output response of the controller to the device it is controlling is reconverted from digital data into an analog output signal that is recognized by the controlling device(Reeves, 1996, pp. 29).
Control system elements
The DDC system is characterized by four functions: sense, decision, memory, and action. Sensors sense environmental changes (i.e., temperature, pressure, and humidity) and transmit signals to the controller for interpretation and response. These input signals may be either analog (continuously variable) or digital (on/off switch). The decisions are computations and logical operations that take into account the inputs and information stored in memory. The memory function is comprised of the rules for making decisions and controlling the system, as well as step-by-step sequencing of events. The microprocessor carries out the decision made by controlling something external to the system or by communicating information to people or other machines. The action may be turning on a supply fan, closing a valve, energizing a heating coil, or turning on a warning light(Reeves, 1996, pp. 29).
The sensor is the most critical element of the HVAC system because it measures the current value of the controlled variable. There are four categories of sensing devices used in automatic control systems: sensors, transmitters, transducers, and sensor-transmitters.
Most modern DDC systems include stand-alone panels and unit controllers. These controllers are the heart of the HVAC system.Stand-alone panels are used in areas of high point density, such as mechanical rooms, because these panels have large input/output point capacity. A typical stand-alone panel has the capacity to connect directly to about 50 input/output points and most have the capacity for expander boards that can increase the point capacity almost indefinitely. In nearly all DDC systems, the stand-alone panels are also required to tie the system together. A stand-alone panel can generally support from 50 to 200 unit controllers and one or two operator-interface devices; they typically cost between $2,000 and $5,000, uninstalled, for 50-point configuration(Reeves, 1996, pp. 30).
Unit controllers offer 16 to 32 input/output point capacity and usually have no expansion capacity. They are usually configured on a separate communications trunk that is supported by a stand-alone panel and do not employ peer-to-peer communications. Many manufacturers offer application-specific controllers to handle the variety of different control functions. Uninstalled, a unit controller typically sells for $100 to $300.
Controlled devices, or field devices, are controlled by electronic or mechanical means. Common controlled devices are control valves and control dampers. Control valves used in HVAC applications include valves that regulate flow (proportional), valves that stop flow (two position), and those that limit the direction of flow (check). To operate the control valve, the controller positions a valve actuator. A control damper is basically a valve for air.Damper actuators are similar to valve actuators and are obtainable in either pneumatic or electric form. Positive positioners provide finite adjustment and repeatable position changes to damper and valve actuators to improve their accuracy of position(Reeves, 1996, pp. 31).
Peripheral Control Devices Many conventional control devices, both electric and pneumatic, are used with HVAC systems to perform specific functions. Electric auxiliary control devices include transformers that provide proper voltage for electric devices, relays for start and stop devices, potentiometers for the manual positioning of controlled devices, electric switches for two-position manual control and digital input switching, and step controllers that use multiple switches activated in sequence to control equipment in several stages(Reeves, 1996, pp. 31).
Pneumatic auxiliary control devices include electric-to-pneumatic relays and switches that are electrically-actuated air valves used to operate pneumatic equipment; pneumatic-to-electric relays and switches actuated by air pressure from a receiver-controller; switching relays that are pneumatic air valves; and pneumatic switches that are two-position devices used to operate pneumatic devices(Reeves, 1996, pp. 31).Peripheral Control Equipment Air compressors are the source of air used to power pneumatic systems and usually include filters and air dryers to condition the air. Transducers are found in systems composed of both electronicand pneumatic controls. Square root extractors are commonly used in pressure control systems to provide linear input signals to the controller(Reeves, 1996, pp. 32).
Building automation systems
The computer is the heart of any DDC system; most operate in the same basic way and are similar in design to personal computers. When personal computer technology became available, a costeffective way to achieve highly-integrated control was created. Control blocks can now be linked in a hierarchy so that any controller in the system can know what every other controller is doing. Building automation systems are categorized by the functions they perform. They vary from systems that monitorcontrol equipment, environments, and apply artificial intelligence to execute complex control routines in anticipation of future events to systems that simply monitor building conditions(Reeves, 1996, pp. 32).
An energy management system comprises of systems ranging from simple two-position timer controllers to sophisticated digital computers and control devices. These systems are commonly used for monitoring energy demand, indicating status, and producing alarm responses to status changes.Energy management and control systems perform control routines, monitor building conditions, measure energy consumption, and execute control actions to manage the variables of an HVAC system. They are supervisory in nature and meant to assist building operators by collecting and evaluating information. Control systems and facilities management emanated from the integration of HVAC control, fire and life-safety control,security control, and lighting controlsystems. They provide monitoring and control functions and support communication between the HVAC control and other systems within a building(Reeves, 1996, pp. 33).
Host computers are used to provide centralized supervision and control of a distributed control system; they are usually personal computers with special-purpose software that captures data from the control system in real time to give system operators pertinent information.
I/O Termination Boards
The physical point of connection between the HVAC system and the sensors and control devices are IO termination boards, which are paired screw terminals that provide termination of field wiring and a point of connection for the communications bus of the computer to sensors and control devices.An I/O card is a personal computer board that translates incoming signals from the termination board into a signal that the controller can understand. There are several types of I/O cards. Analog inputs are proportional or variable input signals, such as temperature or pressure(Reeves, 1996, pp. 32).
Analog outputs are proportional output signals sent from the direct digital controller to modulate the operation of controlled devices. Digital inputs are normally contact closures, such as pressure switches. Digital outputs are contact closures that are sent from the DDC system to hvo-position field devices. While some digital controllers use individual I/O cards for each point type, others use a universal point concept, which means that any combination of inputs and outputs can be brought into and out of the controller system.
Selection of most efficient technology
Decisions on the best and most efficient kind of heating, ventilation and air conditioning system was based on a number of factors with some of the factors being illustrated hereunder. Efficiency was argued in terms of efficiency and these factors mainly look at efficiency issues of the system.
Energy management control strategies
HVAC system programs execute specific energy conservation and supervisory control routines to improve the performance of HVAC systems. There are no universally-applicable control routines that satisfy every building condition, but there are proven strategies that reduce energy consumption. Quite a number of the common software routines are discussed hereunder.
Duty Cycle Program
The duty cycle program reduces the amount of electrical energy consumed by cycling fans/pumps on and off. When space temperature is at the midpoint of a comfort range, the fan can be turned off for an extended time period without affecting comfort.
Power Demand Limiting Program
Power demand limiting programs monitor electrical consumption and shed (turn off) assigned loads to reduce demand. Loads are prioritized according to their importance to environment and comfort conditions. The program monitors consumption from the data input of a pulse transmitter located on the building’s electric meter.
Unoccupied Period Program (Night Cycle Program)
The unoccupied period program is primarily a heating season function. Office buildings are typically unoccupied about 72 percent of the time, so energy is wasted if these buildings are conditioned at the same level at all times. A space temperature and/or humidity sensor maintains temperature or humidity at pre-set levels during unoccupied times by cycling the heating or cooling source.
Optimum Start-Stop Program
The optimum start program monitors space and outdoor air temperatures several hours before the programmed occupancy time of the building. If the space is within the comfort limits, the heating or cooling equipment will be started at exactly the occupancy time. If the space is not within the comfort limits, the program will calculate the correct start-up time to achieve comfortable limits at occupancy. The optimum stop program calculates the earliest time during the occupancy period when the heating or cooling equipment can be shut down to keep the building within the comfort limits(Reeves, 1996, pp. 33).
Unoccupied Night Purge Program
Frequently during early morning hours in the summer, the outside air temperature is cooler than the building-space temperature. This cool air can be used to cool the building, eliminating mechanical cooling during the early morning occupancy hours. Humidity and space and outdoor temperatures are monitored by the unoccupied night purge program. The night purge program would be initiated in the case of an indication for the need for cooling in the space conditions, and if outdoor air conditions are suitable.
Traditional economizer control systems use outside air for cooling by sensing dry-bulb temperatures (ambient temperature indicated by a thermometer with a dry bulb). The economizer system, since it measures dry-bulb temperature only, keeps the system on return air throughout the cooling system. The enthalpy control system, however, selects the air source (return or outside air) that has the lowest total heat (enthalpy) based on a combination of temperature and relative humidity.
Load reset program
HVAC systems are sized for design conditions, or peak loads, which occur infrequently during the year. The load reset program controls heating and/or cooling to maintain comfort in the building while consuming a minimal amount of energy. Zero-Energy Band Program The space comfort range is divided into three bands: heating, cooling, and zero-energy. In the zero-energy band, both heating and cooling are disabled, and the program can reset discharge air temperature by modulating the mixing dampers (outside air and return air) to maintain space comfort conditions(Reeves, 1996, pp. 33).
As cost estimators, we are asked to prepare cost estimates from preprogram order-of-magnitude to construction document definitive estimates. My experience suggests that mechanical designers either do not think about controls design until the design is nearly complete or they rely on controls manufacturers to tell them what they need. It is an estimator’s job to be able to anticipate appropriate controls costs based on the designer’s comments, design completion, the complexity of the HVAC systems, and how the building will be used.The cost of digital controls on new construction projects can range from negligible (i.e., the controls are supplied by the manufacturer of the equipment) to as much as 16 percent of the total mechanical costs. When definitive design documents are not available, many estimators determine controls costs based on a square-foot basis. I suggest that it is more realistic to base cost on the type and quantity of the HVAC “systems” because the cost of controls is a function of the equipment being controlled, not necessarily of the size of the building(Reeves, 1996, pp. 34).
Improvement expected from use of new technology
Centralized Control Systems
A centralized control system starts with a computer capable of handling a large quantity of data at very high speeds. All of the programs that operate the HVAC-controlled devices reside in the central computer. All control decisions occur at the central computer. Local data acquisition panels only collect and distribute information to the devices being controlled. The primary benefits of central control systems are their ability to handle many control loops simultaneously and to provide individual customized programming for each loop. In addition, operating costs are lower than for conventional control systems, control performance is improved, and monitoring and control functions can be integrated with life-safety and security management(Reeves, 1996, pp. 35).
Some disadvantages of central control are that computers are difficult for some building operators to understand and training is costly. Another serious concern is the possibility of central computer failure. Since the operation of the entire system depends on a single machine, if the central computer crashes, control of the HVAC systems is lost. Control system engineers usually incorporate redundant, hard-wired conventional controls into their designs to avoid failure catastrophe, which increases the first-cost of the system. Distributed Control Systems In distributed control systems, microprocessor control panels are located near the equipment being controlled and contain all of the intelligence needed to fully control their assigned equipment without intervention by a central computer. Communication takes place between the panels and a central computer through a network. Although the local panels act independently, they benefit from being able to access information from other control panels in the system(Reeves, 1996, pp. 35).
A distributed control system can operate during loss of communication with the central computer without any degradation of performance. No redundant electronic control is required as a backup to the DDC system. Distributed system architectures are considered the most reliable form of computerized control for HVAC systems. The most significant disadvantage of distributed control systems is the lack of compatibility between systems from different manufacturers. Stand-Alone Control Systems Stand-alone controllers have the intelligence to perform control routines without intervention by another controller. Local-loop, stand-alone controllers were designed to optimize the operation of environmental control equipment. While stand-alone controllers can act independently, they will accept commands to override current operating routines from the central computer(Reeves, 1996, pp. 35).
Improved emission control
Environmental concern for sustainable development has prompted for procedures for controlling greenhouse gases using HVAC systems. Commercial solutions are available for, that is, catalyzing nitrous oxides in hospitals. Local exhaust ventilation is often used to catalyze greenhouse gases before they are released into outdoor air. Hospital emission control is largely dependent on the hospital, and much work remains to be done to improve this issue worldwide.
Wireless sensor networks
Modern hospitals have an interface challenge with various brands of sensors measuring different IAQ parameters. There is a need to develop both sensor hardware and sensor information technology software in order to develop a functional and feasible system for hospital HVAC systems. Wireless sensor networks reduce the need for cabling and enable placed sensors where cabling is not appropriate, enable better indoor conditions and energy savings through improved sensor location, and can be quickly and effortlessly reconfigured and extended (Arens et al. 2005; Reinisch et al. 2007; österlind et al. 2007). Below, a couple of wireless sensor network protocols are presented for wireless sensor networks that could prove to be useful in hospital HVAC systems.
Breath is a new protocol for control applications, where sensor nodes transmit information via multi-hop routing to a sink node (Park 2011). The protocol is based on the modelling of randomized routing, medium access control (MAC), and duty-cycling (Park 2011). Basing his argument on experimental and analytical results, Park argues that Breath is reliable, has low delay, and exhibits virtually uniform distribution of the work load (Park 2011).
SERAN is a two-layer semi-random protocol that specifies a routing algorithm and a MAC layer for clustered wireless sensor networks (Bonivento et al. 2005). It combines a randomized and a deterministic approach: The former provides robustness over unreliable channels and the latter reduces the packet collisions (Bonivento et al. 2005). Excellent performance has been depicted by SERAN for low data rate transmissions with low average node duty cycle, which yields a long network lifetime.
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