Is it "Raining" in Your Surgery Suite?

General Design and Application Considerations:

There are a number of reasons why controlling humidity in the operating rooms has been such a struggle for the hospital facilities management personnel. Three of the fundamental reasons are:

1) A lack of understanding of basic psychrometrics.
2) The HVAC systems are being designed without consideration of ASHRAE's more recently published weather design data and "real world" space condition design guidelines.
3) Inappropriate HVAC systems are being designed and installed for the task (technology and capacity).

Whether considering the retrofit of an existing HVAC installation or designing a new system, the fundamentals outlined in this article will ensure that the wide ranges of temperatures desired by the surgical staff can be readily achieved. With proper design considerations, HVAC systems can be designed without the threat of higher than acceptable humidity levels or condensate dripping from the ceiling and suspended fixtures.

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Basic Understanding of Psychrometrics

Psychrometrics is simply the study of the physical and thermodynamic properties of air and water vapor mixtures. If the hospital facilities engineer and design team have an adequate understanding of the basics of psychrometrics, then they are well on their way to designing and operating an HVAC system for the hospital and its surgical suite that will actively control the humidity in the space.

Design Considerations for Sizing the HVAC Equipment

Two important factors must be considered when determining the capacity requirements of the HVAC systems. First, it must be determined what conditions are expected to be maintained in the surgical suites. It is recommended that the design team (architect and engineer) and representatives of the hospital staff (facilities personnel as well as surgical staff) meet to determine the actual space conditions to be achieved and maintained by the HVAC system. Simply sizing equipment to achieve "normal" or "typical" conditions isn't sufficient. We find that most facilities are requiring a temperature of 62 degrees with 50% RH, or 41 grain control level.

Secondly, the quantity of ventilation air, or outdoor air, is significant (5 air changes) for a surgical suite. Therefore care must be taken when calculating the cooling and dehumidification requirements (as well as humidification requirements for winter months) that accurate ambient conditions are considered.

Selecting the Right HVAC System for the Project

Before the proper HVAC technology can be selected, it is imperative that the appropriate design conditions be determined for the space. Rather than only considering the applicable design standards available (e.g., AIA, ASHRAE, etc.), the design team should also seek input from the surgical staff. The obvious next step to take would be to calculate the expected moisture gain.

The HVAC equipment designed into the project must be capable of delivering air with the appropriate supply air dewpoint. It is important to understand that moisture can be removed from an airstream in only one of two ways. It must either be condensed out of the air (which would employ a mechanical-based system), or it must be adsorbed out of the airstream (using a desiccant-based system).

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For more information contact Concepts and Designs, Inc. by email sales@cdihvac.com or phone 507.451.2198.