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NEWS • 02.04.2020

The importance of HVAC systems in hospitals

HVAC systems are playing a very important role in hospitals, not only by maintaining comfortable climate conditions of temperature and humidity control, but also by maintaining a clean, germ-free environment to contribute to the well-being of patients and to prevent the spread of disease. These determining factors mean that the design of air conditioning systems for the hospital sector must take into account a series of characteristics that, although important in other sectors, must be given special attention in this sector.

 

There are different standards and application codes that are specific to the design, construction and maintenance of air conditioning installations in hospitals, the most relevant in Spain being UNE-100713: 2005, Air Conditioning Installations in Hospitals, UNE-EN ISO 14644, Clean Rooms and Controlled Premises, and at the international level the HVAC Design Manual for Hospitals and Clinics published by ASHRAE (American Society of Heating, Refrigeration and Air Conditioning Engineers), or the Mechanical Systems Handbook for Health Care Facilities published by ASHE (American Society for Hospitals Engineers).

More than just comfort for patients and employees

Healthcare facility air conditioning plays a much more important role than simply providing comfort to patients and employees. Medical equipment in hospitals and healthcare facilities is sensitive to temperature and humidity levels and requires perfect air control to function accurately. But also, the necessity of having rooms with very different use in hospitals, adds an additional complexity to the design of these systems.

 

It is necessary to organize very clearly the different spaces and their use. In a hospital, it is necessary to provide rooms where patients with various pathologies will be treated and rested, among them a group will be exposed to infectious-contagious diseases, which require a certain level of isolation, as well as rooms for patients with depressed or weak immune systems (such as intensive care rooms, or neonatal rooms, operating theaters), where it is necessary to prevent the arrival of pathogens that abound in hospitals. Hospitals are therefore places where a much higher concentration of pathogens accumulates than the average building, and most of these agents travel in the air currents, making air conditioning equipment very sensitive to accumulating large quantities of pathogens and even serving as areas for their cultivation, with the consequent risk to the health of users. All this must be avoided with a correct design of the facilities, so that patients, hospital workers and visitors are protected from exposure to these pathogens.

 

For this reason, in addition to providing for temperature control in air-conditioning systems through heat and cold, and humidity control, in these systems, zoning in areas of over-pressure and depression is particularly important, and therefore coherence when designing the zoning of each area to be air-conditioned, as well as designing with elements that maintain a correct degree of watertightness, but are at the same time accessible and built with clean materials, so that they can be easily dismounted and cleaned regularly. It is also very important to design with an adequate level of external air supply, often considerably higher than in other types of installations, as well as a more exhaustive level of filtration, which allows for the maximum retention of the micro-particles on which the bacteria, fungi and viruses causing the diseases settle.

 

The most important considerations in the design of this type of system are the following:

  • Filtration, classifying the premises and using HEPA filters in the most exposed premises. Combination with biocide systems (ultraviolet lamps, etc.) but never substitution the latter with the first one
  • Ventilation systems with a significant supply of outside air. Precise study of the location of the external air supply and air expulsion intakes to the outside, in order to avoid contamination outside the building
  • Study of the premises that must be in overpressure and under pressure according to use, and control of the direction of air flows according to specifications. Include odor control both in filtration and in study of air flow directions
  • Monitoring of working pressures to detect when a failure happens and to check the effectiveness of the maintenance service policy
  • Study of sound levels, installation of acoustic attenuators
  • Study of air flows and air speeds in critical rooms, taking into account the position of the patients, design of grilles. Recommended in some cases to use hot/cold wall systems to minimize draughts
  • Specifications of air handling units according to health requirements, including energy recovery elements
  • Design and construction of the duct network to facilitate disassembly and cleaning
  • Consideration of pressure losses of fire control elements, such as shut-off dampers and firewalls if they exist, in the calculation of air distribution

UTAS and Handlers

AHUs (air handling units) used in air-conditioning systems for health centers and hospitals require higher construction and quality characteristics than other types of installations. Generally these units are classified in Europe according to the UNE EN 1886 regulation, where the requirements of mechanical resistance, watertightness, thermal transmittance, thermal bridge factor and filter bypass leakage are defined. We will not go into the definition of these aspects because they are general for any installation. However, in the specific case of hospital units, it is also necessary to comply with the requirements defined in UNE 100713.

 

In accordance with this standard, the units must have a hygienic finish, which hinders the proliferation of microorganisms, contributes to the control of air quality, as well as facilitating cleanliness. A uniform interior finish is required, with no gaps or folds that could lead to the deposition of water condensate inside. In this way, interior surfaces must all be coated with epoxy or polyester paint, or be made of stainless steel, completely smooth, and must be constructed in order that the different elements (filters, dampers, heat exchanger batteries) can be removed independently for total cleaning, being accessible to all interiors. Condensate trays must be removable, made of stainless steel, covered with sufficient insulation so that they do not produce additional condensate and with a slope so that the condensate water is easily removed. All panels must have inspection windows, with lighting so that the situation inside the unit can be checked from the outside, and a differential pressure detection system to check that dirt is not building up, causing increased pressure drop. The panels must be sandwich, totally watertight and completely painted with M0 fireproof insulation. They must be able to be completely removed from the structure to allow total access to the interiors.

 

The heat exchange coils, which must be constructed in independent uprights so that they can be completely removed for cleaning, must have no more than four rows or ranges of tubes, with a minimum fin spacing of 3.2 mm (equivalent to 8 fpi fins per inch), with fin thicknesses of no less than 0.12 mm, so that they are sufficiently rigid to be damaged during cleaning. The flow velocity should not exceed 2 m/s (4000 feet per minute) [1] and they should be far enough away from other elements to avoid dirt accumulation between them.

 

Filters shall be two or three stages, depending on the type of room the unit is intended for, easily accessible also for maintenance and cleaning, resistant to humidity and rigid enough to avoid by-pass. It is advisable to place a filtering stage before the battery and another after it, as well as always placing a filtering stage behind each mixture, to prevent unfiltered air from being blown into the room.

 

Fans should be plug-fan, with direct coupling. Couplings with belts and pulleys should always be avoided because they release particles. Fans must also be mounted on a structure, with anti-vibration plugs and with a flexible joint to the sheet metal to reduce vibrations and noise. They must be dimensioned with a certain safety coefficient to be able to and with variable speed system to ensure a constant drive flow, adapted to the filter contamination. It is important to control this flow because its supervision will allow us to adapt the maintenance policy of the unit.

 

Finally, the high level of filtration and ventilation means that the energy recovery aspects must also be considered with special care.

 

The units incorporating heat recovery must have, in addition to the minimum efficiency required by the standards, in the sense of avoiding the exchange of air flows, so that depending on the country or region, only heat-pipe or dual coil type recuperators are accepted as the only systems that avoid the crossing of air flows, and in other countries or regions the use of cross-flow plate recovery exchangers is also allowed as long as the gas transmission rate is verified to be less than 1 per thousand.

Filtration

Filtration as a basis for maintaining air quality control at the appropriate standards is one of the fundamental aspects to take into account in this type of installation. In general, according to most regulations (e.g. Spanish standard UNE 100713), a distinction must be made between two types of premises, class I premises with very high aseptic requirements, which require a minimum of three levels of filtration, and class II premises, with the usual requirements, which require two levels of filtration. In class I premises, the incorporation of the three levels of filtration meets the objective of achieving maximum particle retention in the filter, the last level being a HEPA (High Efficiency Particulate Air) type filter with a minimum filtration level of H13 (99.5%) or H14 (99.95%) depending on the type of operating theatre. The recommendation of the standard is to install F5 + F9 + H13/H14 for this type of premises, locating the HEPA filter in the impulse if possible, or as close as possible to the impulse. In class II premises, the recommendation is to install F5 + F9 filters.

 

In rooms or operating rooms, sick people have their immune system at its weakest, making your body vulnerable to bacteria, viruses and other airborne infections. HVAC systems that use HEPA filtration can capture infectious particles, control bacteria and prevent the spread of airborne diseases to your most sensitive patients. However, without continuous monitoring and maintenance, including filter replacement and cleaning of heat exchangers, pockets of infection can occur. For this reason, the requirement to maintain the installation in perfect condition and extend the life of these elements must also be taken into account. Such precise systems also require adequate and continuous maintenance. Keeping the filters clean and in good condition, verifying the watertightness from time to time, and checking the differential pressure measurement elements between the different points, can be vital, because it would avoid that areas that must be protected keep the heat exchange batteries clean and the implementation of rigorous maintenance programs of cooling towers if they exist or of the ducts are only some of the ways in which HVAC systems prevent the appearance of risks.

 

HEPA filters are very important elements of the system, so keeping them monitored by monitoring pressure tapping before and after the filter elements is necessary to ensure proper operation at all times. Detecting when they are no longer in the operating conditions recommended by the manufacturer and must be replaced is vital to avoid taking unnecessary risks. However, HEPA filters are expensive elements, so maintaining them correctly and extending their life to the maximum is very important. Studies show that a low efficiency pre-filter prior to a HEPA filter extends the life of the filter by up to 25%, while the insertion of a medium efficiency element such as an F8 or F9 can extend the life of a HEPA filter by up to 900% [1]. By using this concept, called progressive filtration, HEPA filters can work under optimal conditions for several years.

Ventilation

Ventilation is another fundamental element to consider in the design of air conditioning installations in hospitals, mainly in the design of operating theatres [11], both from the point of view of air quality control and from the energy point of view, since ventilation represents one of the greatest energy costs in this type of system. It should be noted that operating rooms generally have a considerably lower set point temperature than other rooms, so the energy cost of ventilation in certain outdoor conditions is very high.

 

From a preventive point of view against infections in operating rooms, and depending on the type of operating room, a type of ventilation is recommended, unidirectional or turbulent, a minimum number of renewals per hour, between 20 and 35 depending on the type of operating room, and the possibility or not of recirculating air. These recommendations are included in the UNE-EN ISO 14644 standard. 3] The classification of operating theatres is type A, high technology operating theatres, for complex operations such as cardiovascular, organ transplants, etc. Conventional operating rooms, type B, for conventional and emergency surgery, and type C, for outpatient surgery and childbirth. In type A operating rooms, the unidirectional flow ventilation system is recommended, and air recirculation is allowed. The air must be from the same operating room and must be treated the same as the outside air, with a minimum of 35 renewals per hour. In Class B and C operating rooms, turbulent ventilation is allowed, and 20 renewals per hour for Class B and 15 for Class C. In both cases, it must be 100% outside air.

 

To complete the above considerations, the UNE 100713 standard indicates that although a good air quality is obtained with three stages of filtration, a minimum external air flow of 1200 mc/h must be promoted to maintain the concentration of anaesthetic and disinfectant gases within an acceptable environmental level of less than 0.4 ppm. 2] It should also be noted that in operating theatres with high requirements in terms of germs, a minimum of 2400 mc/h should be used when the rooms are equipped with a mixed air diffusion system, with a minimum of 20 renewals per hour. These considerations are interesting specially when air handling units system is designed for this kind of spaces.

Conclusion

This article has tried to list in a non-exhaustive way the different aspects to be considered in an air conditioning installation for hospitals and in the equipment necessary to carry out the installation in these singular buildings where some aspects which in other installations are not so relevant, here obviously take a greater importance for safety reasons both of the people who work there, as well as of the patients and the technical equipment and machinery that are handled. The aspects related to UTAS machines/handlers, maintenance in general, filtration and ventilation have been described in more detail, although it is necessary to remember how other aspects such as acoustics, vibrations, odor control and energy efficiency are also essential and should not be forgotten.

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