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This report will investigate how energy efficient design, installation and operation of Heating Ventilation and Air Conditioning (HVAC) systems can be implemented in operating theatres without compromising air quality, comfort of occupants, security and health and safety.
The main objective of HVAC systems is to create proper combination of temperature, humidity and air motion in the occupied zone of the air conditioned room. The objectives along with the along with the design parameters of the particular room must be met by proposed design. The requirements of an operating theater are generally completely different than that of residential health care due to the nature of the applications and requirements. Medical professionals and designers of equipment enforce many restrictions on the comfort requirements to form health criteria. These criteria are accumulated and compiled to healthcare standards such as ASHRAE standard for the residential and commercial applications and the National Health Service (NHS) standard for healthcare applications. (Kameel, 2003)
This report shall look at the requirements set out in various relevant standards, examine examples of current systems in operation and suggest alternatives as an energy efficient solution.
HVAC REQUIREMENTS & STANDARDS FOR OPERATING THEATRES
Energy efficient design is not one of the main concerns when designing HVAC systems for operating theatres. The main target is the optimization of airflow regimes and heat transfer for comfort and hygiene. The majority of operating theatres are run on 100% fresh air principles, typically requiring excessive fresh air cooling loads especially in humid climates.
Table 1 summarizes the criteria set out by ASHRAE, The American Institute of Architects and the Canadian Standards Association for the design of HVAC systems for various operating rooms.
Standards for design of HVAC systems for Hospitals
ASHRAE systems require a minimum of 5 air changes of outside air, 25 air changes of recirculated air for a total of 30 air changes in the operating room. A minimum filtration efficiency of 90 to 99.97% is required depending on the type of operating room. Filtration requirements for medical facilities are not a new concept. In the US the original requirements were published in 1947 under the Hill-Burton Act.
One of the main reasons the standards are as severe is to improvement infection control. A recent systematic review found that out of 40 studies (only 10 of which were considered conclusive) there is a link between ventilation and transmission of infection (e.g. measles, T.B., influenza, small pox, chicken pox). (Li, 2007)
The HVAC design standards for Operating Rooms are well defined and specific. The energy requirements for such a system is quite high when compared to other commercial/industrial buildings of a similar floor space. The primary concern has not been to conserve energy, but to create an environment suitable for tasks carried out, however there are still opportunities for energy conservation without sacrificing comfort, and the overall quality of patient care or services.
TYPICAL LAYOUT FOR OPERATING ROOM HVAC SYSTEM
Operating theater ventilation systems are expensive to operate in terms of energy costs. This is mainly due to the high volumetric flow rates caused by the operating theater ventilation systems. Minimum air volumes are usually fixed by the room loads or fresh air requirements. (Nilson, 2004) Figure 1 shows a schematic for a HVAC system for a general Operating Room. A Constant Air Volume (CAV) system is generally used. Both the exhaust and intake fans are provided with Variable Speed Drives (VSD). The VSD allows for set back for unoccupied hours and also helps to maintain the flow against static. The return air is collected through extraction vents located near floor level in the OR. This is not very effective as the majority of the air will exfiltrate to the surrounding rooms to maintain the pressure differential. There are generally two different air filter types incorporated into the HAVC system. The return air passes through the primary filter which are generally 80-90% efficient, before passing through 99.997% efficient terminal HEPA filters and discharge vertically to provide a laminar downflow pattern.
Operating Theater Schematic (Mills, 2008)
There are a number of operations or considerations within the HVAC system within the Operating Theater which influence the energy consumption, however the vast proportion of energy consumed is in relation to the air flow rates. The following simplified equation can be used to determine the cooling load:
Cooling Load (kW) = (V& x ρair) x (cp x ΔT) 3a
V& = Air volume flow rate
ρair = Air density, kg/m3
cp = Specific heat, kJ/kgK
ΔT = Temperature Difference
A number of assumptions are taken into consideration when using this equation:
All other heat gains and heat losses are neglected
Air density and specific heat will be constant (Mills, 2008)
ENERGY EFFICIENT HVAC OPTIONS FOR OPERATING ROOMS
Energy recovery is of considerable importance for a hospitals financial well being. Energy efficiency is a broad term and covers a myriad of different approaches. The most direct approach is to ensure that each component of the HVAC system is not being overused. This can be achieved by fitting VSD for pumps and AHUs. The use of natural ventilation should also be investigated. Probably the most effective way to implement an energy efficient system is to recover the waste energy or to reduce the required air flow rates. On a broader context the efficiency of the HVAC system can be improved by investigating:
The air handling system,
The air distribution system, and
The filter system type.
In general minimal consideration has been given to energy efficient design of air handling system. Specific areas of improvement include duct design, air distribution, selection of energy efficient motors and filter systems and our of hours operation energy saving methods. (Jaisinghani, 2003) We must remember that the purpose of ventilation in the operating theater is to minimize the risks to patients and wounds of contaminants from human and other unsterile sources. Air ventilation also removes any anesthesia gases that escape into the air. (Ekono, 1990)
AIRFLOW DESIGN METHODS
In general the current design methods employed in HVAC design for Operating Theaters or Clean Rooms are based on experience and little attention has been paid to energy efficient design of air handling systems. Specific areas of improvement include air distribution, selection of energy efficient motors and filter systems. In an attempt to develop a rational basis for designing cleanroom airflow Jaisinghani developed a simplified dilution model based on the primary purposes of Cleanroom/OR HVAC systems, which are:
Dilute the contaminants produced in the room, and
To transport and carry away such particles.
The net result of refining the design, reduces the number of air changes per hour and thus reduces the amount of energy consumed. The results of such models my in effect
AIR HANDLING AND DISTRIBUTION SYSTEM
The airflow in OR is often incorrectly referred to as laminar. Usually the air flow is unidirectional and as the flow rate increases the turbulence increases, resulting in the increase of eddies and currents within the flow. It is essential to avoid turbulent flow within the operating environment. The laminar flow ventilation system was designed to provide a system which prevents air contaminants, or in the case of Operating Theaters pathogens, from being diffused through out the process room. The system works by eliminating turbulent flows with high eddies and providing a stable, uniform low velocity air flow. If we were to ignore financial constraints a fully engineered ceiling consisting entirely of laminar flow diffusers would be installed.
The critical area in the Operating Theater is the Operating Table and the adjacent area. A system can be installed where by laminar diffusers are installed in the critical zone only. However, the distance between the laminar flow diffusers and the area of concern must be kept to a minimum. A certain amount of entrainment occurs as result of limiting the laminar diffusers to the critical zone.
It is also possible to surround the critical zone with an air curtain. This can be achieved by installing slot diffusers around the critical zone. The supply air is discharged from the slot diffusers
In many cases, the ongoing energy costs associated with a full ceiling laminar flow ventilation system can be reduced by reducing the size of the area requiring laminar airflow. Essentially, creating a clean zone around the operating table within the operating room. This is achieved by surrounding the operating table with an air curtain. The air curtain is formed by the interaction between the vertical air diffused by the ceiling diffusers and the horizontal air.
Laminar flow diffusers installed in the ceiling inside the air curtain provide low velocity, laminar flow of clean air over the surgical staff, patient and operating table.
The supply air for this type of system is typically filtered using HEPA or VVC air sterilizers filters.
FILTER SYSTEM PERFORMANCE
Air filters are basic components of air handling systems. The life-cycle pressure drop of the filter significantly impacts the energy consumed by the HVAC system in an Operating Room. Air filtration consumes energy in the way that conditioning coils do. The filter functions in such a way that its resistance to air flow decreases over time and hence overtime the energy consumed by the filter increases. To reduce the energy consumed, a filter with a lower pressure drop must be specified.
When HEPA or ULPA filters are specified with only small reductions in pressure drop, significant energy savings can result from their long-term use. Filter media manufacturers have made great progress in developing low-pressure-drop media. Typically, specifying deeper filters reduces pressure drop and increases energy savings. For example, the average pressure drop at a face velocity of 100 feet per minute is between 0.35 and 0.40 inches w.g. for two-inch HEPA filters. A four-inch HEPA filter, which increases active media surface by 50 percent over a two-inch filter, lowers the pressure drop to between 0.25 and 0.35 in. w.g. A six-inch HEPA filter reduces the pressure drop to between 0.15 and 0.20 in. (McIlvaine, 1992)
An effectively controlled and managed maintenance schedule will ensure that the HVAC system operates to the designed standards. Many maintenance systems are operated based on a reactive rather than a preventative approach. Keeping HVAC systems running properly and at peak efficiency is the first step in managing energy use. A benefit of planned HVAC maintenance is its affects on energy efficiency. Facilities in which proper maintenance is completed will use at least 15/20% less energy than those systems that are allowed to deteriorate over time. For example, consider the operation of a central building air conditioning package unit. These type of unit are typically are the single largest user of electricity in a building. To keep them operating as efficiently as possible, maintenance tasks must be performed on a daily, weekly, monthly or annual basis. Let them lapse, and efficiency will decrease, increasing energy use.
ENHANCED AUTOMATION AND CONTROLS
An enhanced automation and control system will result in the more efficient management of the HVAC system within the Operating Theater. The strict air quality standards that set out in the various design guides for Operating Theaters do not have to be maintained during out of hours use. During non use it should be possible to reduce the number of air changes per hour in the Operating Theater and also for Theater to remain at a higher (or lower temperature depending on climate) temperature. This would involve the installation of an ‘economy switch' which would be operated during non-use hours.
CONCLUSIONS & RECOMMENDATIONS
The design and installation of HVAC systems in hospital operating rooms is controlled by strict requirements. The energy efficiency of HVAC systems within the health care environment has not been of a primary concern. The main focus has been to develop a system that can provide healthy environment and can control the spread of infections. This report investigated possible methods by which the energy costs of HVAC systems in Operating Theaters can be reduced. The Tan Tock Seng Hospital in Singapore
The Tan Tock Seng Hospital in Singapore was retrofitted with an energy efficient system to reduce the energy costs. The approach taken by the designers was to:
Install a heat recovery wheel,
Include a variable volume fan control,
Reduce the number of air changes/hour and higher temperatures during non usage, and
Install HEPA and UVC air sterilizers.
To retrofit the HVAC system cost a total of $350,000 and provided savings of $242,502 in energy costs per year. (Lui, 2006)
To summarize, energy efficiency is becoming a major component of design, installation and operation of HVAC facilities. The energy efficiency of a system for a hospital Operating Theater can be improved through a number of approaches. At the design stage consideration must be given to the most appropriate technologies that meet the design requirements. It is also becoming apparent that the standards used in the design of HVAC systems for clean rooms and ORs are generally not based on scientific or mathematical fact but rater on experience, more emphasis has to be placed on the development of accurate and appropriate standards based on scientific foundations. The efficiency of the HVAC system can be greatly improved by the choice of air handling and distribution system and also the filtration system incorporated in the system. Appropriate maintenance and control of the system can also lead to increased efficiency of the system.
Ekono O (1990). Energy Savings and Hospital Hygiene, Helsinki 1990
Jaisinghani, R (2003). Energy Efficient Low Cost Cleanroom Airflow Design
Kameel, R (2003). Airflow Regimes in Operating Theatres for Energy Efficient Performance.
Lui, WS (2006). Sustainable Energy Management through Controls and Automation.
McIlvaine, R W (1992). Cleanrooms — 1992-2000, Rooms and Components Vol. Three. McIlvaine, Robert W., Sally Halderman, Alpa Bagga, and Joseph Schwartz, eds. Northbrook, IL: The McIlvaine Co.
Mills, F (2008). HVAC in Health Care. Available: www.cibseashrae.org/presentations/Mills1105.pdf. Last assessed May 2009.
Nilson, A (2002). A Brief Note on the Energy Consumption for Operating Theatre Ventilation Systems. Available: www.johnsonmedical.com/energy_consumption.htm Last accessed May 2009.