With much of a health-care facility's profits impacted by the type and quantity of surgeries performed, hospital administrators are prioritizing indoor environmental conditions not just for the safety of patients, but to improve the efficiency and quality of work life of surgeons.
The multiple layers of clothing, face masks, hair covers, and protective gloves surgeons wear trap body heat and exacerbate perspiration. A sweaty brow is counterproductive for a concentrating surgeon. Thus, surgeons are demanding cooler, drier operating rooms.
General comfort is significantly greater and perspiration issues are minimized when relative humidity is kept below 60 percent. Additionally, lower relative humidity decreases the likelihood that condensation and fogging will occur on magnifying lenses and polished-metal working apparatus.
Common areas of hospitals generally are maintained at 75°F and 50-percent RH, which equates to a 55°F dew point, while operating rooms often are maintained at 60°F and 50-percent RH, which equates to a 41°F dew point (Figure 1). While the relative humidity is the same, operating rooms require substantially drier air.
Requirements for drier air magnify the challenges of moisture-management design. Unwanted moisture is most likely to be introduced into a space via ventilation air. For example, in Greensboro, N.C., each 1,000 cfm of fresh air represents 7.1 gph of water relative to a space set point of 60°F/50-percent RH. Conventional equipment that heats and cools air can remove some of this moisture, if the cooling load is great enough. Heating and cooling equipment controlled by a thermostat provides dehumidification by chance, not by design.
Typical-meteorological-year weather-bin data indicate the ambient dew point in Greensboro is above 55°F 3,623 hr. The ambient temperature is below 75°F 2,418 of those hours, meaning 66.7 percent of the dehumidification season is cool and wet simultaneously. Those cool and wet conditions demand a means of positive dehumidification.
Following are general guidelines for outside-air preconditioning:
Positively dehumidify all makeup air to the required dew point. Supply dew point must be depressed to at least space-dew-point set point at all times, regardless of space-temperature needs.
Account for internal latent load. People load, open water, and infiltration represent internal sources of moisture. Dehumidification equipment must supply air with a dew point lower than the desired space dew point to support internal loads.
Provide reheat to prevent overcooling of a space. This is especially critical with operating-room applications (considering the conventional method of achieving 40°F dew point is to overcool air to 40°F).
There are two common approaches to maintaining cool and dry conditions in operating rooms: low-temperature chiller (Figure 2) and desiccant (Figure 3). Both positively dehumidify ventilation air, offset internal loads, and reheat air so as to provide temperature control for multiple zones.
The low-temperature-chiller approach to dehumidifying operating rooms entails mixing ventilation air and return air and then cooling the result to 40°F. Because 40°F supply air cannot be achieved using typical chilled water, this generally is a two-stage process: During the first stage, normal chilled water is used; during the second, a lower-temperature chilled-water/brine mixture is used. Because 40°F air is too cold, reheat is applied at each zone to maintain the temperature set point. A simpler one-stage approach by which the entire dehumidification process is accomplished using one large coil and a 35°F water/brine mixture also can be used.
Challenges associated with the low-temperature-chiller approach include:
A low-temperature chiller has a 20-percent first-cost premium relative to an equivalent-tonnage standard chiller.
Kilowatts per ton is approximately 22-percent higher with a low-temperature chiller than it is with an equivalent-tonnage standard chiller.
Forty degrees Fahrenheit is the lowest dew point that can be expected without the risk of a chilled-water coil freezing. All air must be cooled to this dew point for desired space humidity to be achieved, and substantial sensible cooling must be consumed before any moisture is condensed.
Substantial reheat energy is required to prevent overcooling.
Either a separate chiller is required or the whole chilled-water system needs to be of the low-temperature variety.
Conveying cold saturated air from a main air handler to reheat boxes challenges insulation to prevent condensation on duct exteriors.
Any apparatus, such as filters, will create condensate downstream of a chilled-water coil. Saturated air moving across any pressure-loss device will condense.
A 35°F water/brine mixture requires better piping insulation to prevent condensation.
A 35°F water/brine mixture has a higher probability of leaking at connection areas.
An air-handler system in a low-temperature-chilled-water application should be constructed with limited through metal from inside to outside. Whether an air handler is mounted inside or outside, the surrounding dew point often will be higher than 40°F, and any through-metal area will sweat profusely. Air-handler door frames, wall panels, and through connections should include a thermal break. This construction is not conventional and will cost more.
Desiccant dehumidifiers utilize a desiccant wheel to dehumidify supply air (Figure 4). A desiccant wheel is split into two sectors: supply air and reactivation air, which rotate continuously. Once a wheel absorbs moisture, the moisture is driven off of the wheel during a “reactivation process.” The wheel rotates into a secondary air stream (reactivation air), where water vapor is transferred to heated air and exhausted.
Reactivation temperature can range from 110°F to over 300°F, depending on the application. Generally, the drier the air requirements, the warmer reactivation air must be. As supply air passes through a desiccant, it simultaneously is dried and heated. For example, saturated air entering at 55°F (64 grains per pound) will leave at 97°F (29 grains per pound). Contrary to popular belief, that heat is not carryover from reactivation. The desiccant wheel absorbs moisture at a linear rate for more than 10 min before capacity is reduced measurably. Therefore, dwell time — the time during which a given point is in contact with supply or reactivation air — is approximately 8 min. Rotor speed generally is eight revolutions per hour. Slow rotation speed results in little heat carryover from reactivation air. Most of the heat associated with the drying process is converted latent energy.
FIGURE 3. Desiccant-dehumidification approach.
Note the following:
The cost of operating a desiccant dehumidifier is approximately 40-percent less than the cost of operating a low-temperature chiller ($9,881 vs. $17,141) (Table 1).
Desiccant pre-cooling and final post-cooling requirements are met using standard-temperature chilled water.
Because a desiccant dehumidifier easily can achieve dew points lower than 40°F, only a portion of the total supply air needs to be dehumidified. This allows greater system-design flexibility. In the example of Greensboro, N.C., only outside air is dehumidified. All of the work associated with sensibly cooling supply air to dew point is removed from the equation (Table 1).
Cooling fluid is warmer, reducing the chance of pipe condensation.
Cooling fluid does not require brine mixture, avoiding the potential for leaks and removing the cost of maintaining correct fluid chemistry.
Supply air is not overcooled, dramatically reducing the reheat energy required at each zone (Table 1).
Conveyed air is not cold and saturated, which reduces the chance of duct condensation and condensation across pressure-loss apparatus within a supply duct.
The desiccant approach does not overcool supply air. Humidity and temperature control are decoupled. Therefore, post-cooling is applied only as needed to satisfy zones with the greatest cooling requirements. Note that for both the desiccant and low-temperature-chiller approaches, multiple-zone service was assumed. For single-zone service, the desiccant approach would not require reheat, while the low-temperature-chiller approach might in the main air handler.
The health-care industry is advancing at a quick pace, as are dehumidification technologies. With surgical suites requiring significantly drier air, design challenges are magnified. Equipment must positively dry supply air to room set point, account for internal loads, and reheat air. Engineers must evaluate the advantages and disadvantages of different dehumidification approaches and provide the most reliable and energy-efficient solution.
A sales manager for Munters Des Champs, Mike Herwald has spent the last 14 years designing custom energy-recovery solutions across the United States and Canada. He is an expert in the utilization of enthalpy wheels, plates, heat pipes, and desiccants to reduce the energy consumption of commercial and industrial HVAC systems.