A newly constructed medical facility intended to provide corneal refractive (Lasik) surgery to patients to reduce their dependency on glasses or contact lens by reshaping the cornea with state-of-the-art equipment. But shortly after the facility was completed in 2008, surgical procedures had to be abruptly suspended. The existing HVAC system serving the surgical procedure room could not deliver the required temperature and humidity necessary for optimal surgical outcomes.
It wasn’t exactly a large target. Studies have shown that successful refractive surgeries demand exacting environments. That means no more than a +/- 1 degree Fahrenheit swing in temperature and a +/- 2% change in relative humidity over a ten-minute 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 brand new surgical suite was temporarily unusable, and surgical procedures had to be performed offsite.
“Everyone was, to say the least, very disappointed,” remarked the medical center's facility manager. “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, PE, of Kroeschell, an investigation determined that the existing chilled water coil temperature did not allow for the discharge air temperature to be any lower than 50 degrees. This would not allow the space to be much below 50% RH, which was unacceptable since the required optimal conditions are 68 to 72 degrees 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.
“The conclusion was drawn that mechanical dehumidification would be needed to achieve the temperature and humidity criteria requested,” said Mr. Cooper. “The original installed equipment did not include dehumidification. In addition, highly accurate sensing instruments would need to be included in the design.”
A Stand-alone Dehumidification Solution
Kroeschell, Inc. recommended a Munters HCD Series Desiccant Dehumidifier as a standalone solution to the problem. This unit incorporates Munters’ HoneyCombe® wheel, which 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. Leaving air temperature and humidity sensors 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 abandoned in place because its removal would create dust in the surgical room, which could cause problems with sensitive laser equipment already in the room. Constant Volume (CV) boxes were added to assure a positive pressure in the room.
Unprecedented Accuracy
The project was a resounding success, with surgical procedures commencing immediately and continuing on schedule without interruption since system start up in the first quarter of 2010. Medical personnel report that the system is maintaining humidity with a deviation of only +/- 0.6% RH most of the time – 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,” said Steve Cooper. “The Munters dehumidifier provided a straight forward, 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.”
Surgical procedures, which take less than 10 minutes to perform, take place one day a week at the Lasik surgery center. At present, the facility performs approximately twelve surgeries per week.