Is it "Raining" in Your Surgery Suite?

Mechanical Technology (Conventional Air Handling Systems)
As can be recognized, a conventional HVAC system that includes cooling coils with 38 degrees Farenheit entering chilled water temperatures will result in a supply dewpoint temperature of something in the 42 degrees Farenheit range.
Old School - Cool, Re-Heat
Even with the 38 degrees Farenheit entering chilled water temperatures it's difficult to get to 41 degrees dewpoint. But let's say you can.

Figure 1. Psychometric Chart.
Click to zoom in.

Figure 1 is an example for a 2 room suite that uses a total of 25.1 tons of energy. You always have to cool to get the moisture out of the air, even on a cold and rainy day. Where as the desiccant based system could dry with or without cooling, only using the energy it needs to remove the moisture irrespective of temperature.

Desiccant Technology

A more effective technology to consider for these lower dewpoint supply conditions would be a desiccant-based air handler. By incorporating a desiccant-based DOAS dehumidification system into the mechanical design for the hospital's operating suite, the overall system can be designed for optimal and active humidity control. The outside air is all that goes through the active desiccant, this higher air pressure drop portion of the system is limited to the 5 air changes. The existing or new primary AHU's move the 25 air changes, only doing sensible cooling at much lower static pressures and HP.

Figure 2. Psychometric Chart.
Click to zoom in.

In Figure 2, the example is for a 2 room suite that uses a total of 21.7 tons of energy, including the active reactivation, to deliver 25 air changes at 41 degrees dewpoint.

How about PASSIVE Desiccant Technology?

There is a lot of talk about Type III desiccants, using the Cromer Cycle for the hospital application. Even though they are called passive, the active energy of the cooling coil is still required to get to the resultant dewpoint. It is the cooling coil that condenses the moisture out of the airstream.Thepassive system only works if the return air is what is expected; any more moisture than expected and this passive system can't make the supplycondition. If the supply condition cannot be achieved then the return condition is poorer still and the system goes out of control.

Because we consider the critical nature of the Hospital Operating Suite imperative to make conditions we do not recommend the use of passive desiccants to meet the design criteria. A case can be made that they use less energy, but they have no pull down capacity, meaning if the operating suite is humid the type III desiccant cannot reactivateunless you add heat to the reactivation airstream, thereby making it active. At that point you have anactive desiccant system and shoulduse the DOAS solution that lowers air pressure drop and HP required to move the air.

Figure 3.
Psychometric Chart. Normal temperature at sea level.
In I-P Units. Baraometric Pressure 20.021 in HG.
Click to zoom in.

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