Air flow movement in rooms
Examining IAQ in hospitals
PROFESSOR DR ESSAM E KHALIL* puts the spotlight on indoor air quality (IAQ) in health facilities and discusses how to achieve a balance between thermal comfort and air quality through air-conditioning design.
01 July 2005
The design of an HVAC’s (heating, ventilation and air-conditioning) airside system plays an important role in attaining the optimum air quality and comfort level in air-conditioned spaces. However, achieving the right design with efficient energy consumption is a great challenge1-10.
Research into how the different designs of the HVAC systems affect energy consumption should focus on the optimisation of airside design as the means to enhance the indoor environment.
Mathematical modelling techniques can be used to adequately predict the air flow, thermal behavior and relative humidity in surgical operating theatres. Using computational techniques and energy assessments, this article highlights the importance of a proper airside design on the IAQ and introduces some recommendations for airside designs to facilitate the development of an ideal HVAC system.
Health considerations and hygiene requirements necessitate the following:
• Restricting air movement in and between the various departments;
• Using appropriate ventilation and filtration to dilute and reduce contamination in the form of odour, air-borne micro-organisms, viruses and, hazardous chemicals;
• Regulation of the different temperature and humidity requirements for various medical areas; and
• Maintaining an accurate control of environmental conditions.
Temperature/humidity control
Codes and guidelines specify the temperature range criteria for some hospital areas as a measure for infection control as well as comfort. Local temperature variations greatly affect the occupant’s comfort and perception of the environment. Temperature should be controlled by change of supply temperature without any airflow, and the temperature difference between warm and cold regions should be minimised to decrease airflow drift.
Efficient air distribution is needed to create a homogenous domain without large differences in the temperature distribution. The laminar airflow concept developed for industrial clean room applications has attracted the interest of some medical authorities. There are advocates of both vertical and horizontal laminar airflow systems.
For highly-contaminated areas, the local velocity should be greater than or at least equal to 0.2 m/s. For hospital wards, 0.1 m/s is sufficient in the occupied area. The unidirectional laminar airflow pattern is commonly attained at a velocity of 0.45 ± 0.10 m/s.
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Different configurations |
Air change & filtration
Three basic filtration stages are usually incorporated, namely: Primary filter, second-stage filter (the high-efficiency particulate bag filter) and a third-stage filter, which is the high-efficiency particulate filter located at the air supply outlets.
Air change per hour (ACH) plays an important role in providing a contamination-free space. Hospital wards are usually served by 2 to 6 ACH. Some critical rooms could be served with up to 12 ACH, while others, such as the surgical operating theatres, are usually supplied with 15 to 25 ACH. However, there are some guidelines that advise a value of 60 ACH for the critical areas11, 12.
Negative pressure is obtained by supplying less air to the area than is exhausted from it. This induces a flow of air into the area around the perimeters of doors and prevents an outward airflow.
The operating room offers an example of the opposite condition. This room, which requires air that is free of contamination, must be positively pressurised relative to adjoining rooms or corridors to prevent any airflow from these relatively highly-contaminated areas.
In general, outlets supplying air to sensitive ultraclean areas and highly-contaminated areas should be located on the ceiling or on sidewalls closing to ceiling (Figure 1), with perimeter or several exhaust inlets near the floor. The bottoms of return or exhaust openings should be at least 0.075 m above the floor.
Design specifications
As a perfect air-conditioning system is helpful in the prevention and treatment of disease, the construction of such a system for health facilities presents many challenges not encountered in the standard air-conditioning systems.
In the isolation rooms for infectious patients, the patient’s bed should be located close to the extract ports. The infectious isolation rooms should be maintained at negative pressure. The immunosuppressed patient’s bed should be located on the side of the supplied air, or close to the supply outlets (Figure 2). Previous calculations of local velocity profiles, air temperatures, relative humidity distributions were provided by Kameel and Khalil5,6 using a finite difference computer programme that solves the governing equations for mass, momentum, energy, relative humidity and age equations in three-dimensional configuration of rooms as indicated by Khalil10.
Immunosuppressed patients are highly susceptible to diseases, hence an air distribution of 15 ACH supplied through a nonaspirating diffuser is recommended for these areas. When the patient is immunosuppressed but not contagious, a positive pressure should be maintained between the patient’s room and the adjacent area. A study of the velocity, air temperatures and relative humidity contours in an immunosuppressed patient room shows that the air flow passes over the bed at a relatively low velocity and turbulence to ensure comfort. High velocity fields can be observed near the extract ports.
In operation theatres, an air distribution system that delivers air from the ceiling, with a downward movement to several exhaust inlets located on opposite walls, is probably the most effective air movement pattern. The air flow pattern forms a ‘curtain’ over the bed zones as can be better visualized in Figure 3 by the thermal zone barriers.
Based on the above analyses, the following design conditions are recommended for operating, catheterisation, cystoscopic, and fracture rooms (Figure 4):
• There should be a variable range of temperature – from 20 to 24 deg C;
• Relative humidity should be kept bet-ween 50 and 60 per cent;
• Positive air pressure should be maintai-ned by supplying about 15 per cent excess air;
• Devices indicating differential pressure should be installed;
• Humidity indicator and thermometers should be located for easy observation;
Filter efficiencies should be in accordance with codes;
• The entire installation should conform to NFPA (National Fire Protection Agency) Standard 99, Health Care facilities;
• All air should be supplied at the ceiling and exhausted from at least two locations near the floor; and
• Control centres that monitor and permit an adjustment of temperature, humidity, and air pressure may be located at the surgical supervisor’s desk.
The surgical operating suite should cover a complete floor in the hospital, separated from the other suites and wards. The above design features are strongly supported by the predicted air flow pattern, temperature contours and relative humidity as obtained in different operating theatres as discussed by Kameel and Khalil6.
Conclusion
The air is not just a medium but it can be regarded as a guard in critical health applications. The proper direction of the airflow increases the possibilities of successfully removing pollutants from healthcare applications. The numerical tool, used here, was found to be highly effective in predicting the airflow pattern in healthcare facilities at reasonable costs and with acceptable accuracy.
Good architectural design allows the HVAC system designer to properly locate the supply outlets and extraction ports in the optimum locations.
References
1. ASHRAE Standards 55-1966, published by ASHRAE, Atlanta.
2. ASHRAE Applications, 1999, published by ASHRAE, Atlanta.
3. Kameel, R, and Khalil, E E, 2000, Computer-aided design of flow regimes in air-conditioned spaces, Proc ESDA2000 ASME, Montreaux 2000.
4. Kameel, R, and Khalil, E E, 2001, Operating parameters affecting air quality in operating theatres: a numerical approach, CLIMA 2000 – Napoli, (I), September 2001.
5. Kameel, R, and Khalil, E E, 2002, Predictions of flow, turbulence, heat transfer and humidity patterns in operating theatres, 2002-120, ROOMVENT 2002.
6. Kameel, R, and Khalil, E E, 2003, Thermal Comfort Vs Air Quality in Air-Conditioned Healthcare Applications, proceedings AIAA 36TH thermo physics conference, paper AIAA-2003-40199.
7. Kameel, R, Khalil, E E, and Medhat, A A, 2002, Assessment of a 3-D numerical predictions of airflow regimes in air-conditioned spaces using an experimental reduced scale model, 40th Aerospace Sciences Meeting & Exhibition, Reno, Nevada, AIAA-2002-653,
8. Khalil, E E, 1994, Three-dimensional flow pattern in enclosures, Egyptalum, Egypt.
9. Khalil, E E, 2000, Computer-aided design for comfort in healthy air-conditioned spaces, Proceedings of Healthy Buildings 2000, Finland, Vol. 2, Page 461-466.
10. Khalil, E E, 2004, Requirements of Air-Conditioning Systems’ Development in Hospitals and Critical Healthcare Facilities Comfort, Air Quality, and Energy Utilisation, 2nd BSME-ASME International Conference on Thermal Engineering, Dhaka. See also “Proceedings of ESDA04, 7th Biennial Conference on Engineering Systems Design and Analysis - Manchester, United Kingdom, Paper ESDA-58112, July 2004.
11. NHS, HTM, 1994, Ventilation in Healthcare Premises, Health Technical Memorandum 2025, NHS Estates 1994
12. SSI, 1999, Guideline for prevention of surgical site infection, Infection control and hospital epidemiology, Vol. 20, No. 4, 247-278, Jan 1999, US.
* Professor Dr Essam E Khalil is deputy director (International) of AIAA (American Institute of Aeronautics and Astronautics), Convenor ISO TC205 WG2, chairman National HVAC Code Committee-Egypt, president of ASHRAE Cairo Chapter and professor of mechanical engineering at the Cairo University in Egypt.
