Desiccant dehumidification for surgical rooms

Some surgical rooms can tolerate no more than +/- 2% change in relative humidity; desiccant dehumidifiers can be used as standalone solutions.


A newly constructed medical facility planned to offer patients corneal refractive (Lasik) surgery by reshaping the cornea with state-of-the-art equipment. But shortly after the facility was completed, surgical procedures were suspended abruptly. The existing HVAC system serving the surgical procedure room couldn’t deliver the required temperature and humidity necessary for optimal surgical outcomes.

Studies have shown that successful refractive surgeries demand exacting environments. That means no more than a +/- 1 F swing in temperature and a +/- 2% change in relative humidity (rh) over a 10-min period. It was a non-negotiable issue for surgeons who monitor digital readings of room temperature and humidity during every procedure. If the room doesn’t hold conditions, surgery cannot be performed.  As a result, the new surgical suite was temporarily unusable, and surgical procedures had to be performed offsite.

“Everyone was, to say the least, very disappointed,” the medical center’s facility manager said. “This was meant to be a showcase piece among surrounding hospitals.”

Obviously, the priority was to get the surgical facility up and running as soon as possible. Kroeschell Inc., a Chicago-based mechanical engineering firm, was chosen to evaluate and make recommendations to correct the problem.

According to Steve Cooper of Kroeschell, an investigation found that the existing chilled-water coil temperature did not allow for the discharge air temperature to be any lower than 50 F. This would not allow the space to be much below 50% rh, which was unacceptable because the required optimal conditions are 68 to 72 F and 40 to 45% rh.  There was an existing supplemental fan coil installed above the ceiling in the room, but even this unit could not effectively lower the relative humidity to acceptable levels.

“It was determined that mechanical dehumidification was needed to achieve the temperature and humidity criteria requested,” Cooper said. “The original equipment did not include dehumidification. In addition, highly accurate sensing instruments had to be included in the design.”

Stand-alone dehumidification

Kroeschell recommended a Munters HCD Series desiccant dehumidifier as a standalone solution to the problem. This unit incorporates Munters’ HoneyCombe wheel that is capable of collecting and releasing water six times every hour, 24/7. In addition to the desiccant wheel, the unit includes filtration, a supply fan, and a reactivation heater and discharge fan for wheel.

Fresh outdoor air comes into the dehumidification unit, which is controlled to maintain 43% rh discharge air humidity, and then enters the air handling unit serving the room. Room temperature and humidity sensors control the chilled-water coil valve, the electric duct heater, and the humidifier to maintain the design parameters. Air temperature and humidity sensors for air leaving the unit are incorporated into the programming logic of the existing direct digital control (DDC) system.

Though not ideal, the original air handler above the ceiling was kept in place because its removal would create dust in the surgical room, thereby causing problems with sensitive laser equipment in the room. Constant volume (CV) boxes were added to ensure a positive pressure in the room.

Achieving accuracy

The project was a success, with surgical procedures commencing immediately and continuing on schedule without interruption. Medical personnel report the system is maintaining humidity with a deviation of only +/- 0.6% rh, a tolerance they believe is virtually unprecedented in other Lasik surgery facilities in the country.

“We’re very proud of the outcome of this high-profile project,” Cooper said. “The Munters dehumidifier provided a straightforward, affordable solution for a facility with extraordinary requirements. This tells us the product has application possibilities wherever original chilled water design misses the mark in terms of latent load.”

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