The Moisture Problem

Hospitals are required to maintain specific humidity ranges in surgical suites, sterile processing departments, and other critical care areas. ASHRAE Standard 170 specifies a relative humidity range of 20 to 60 percent for operating rooms, with many facility infection control committees adopting a tighter 30 to 60 percent target. Below the lower limit, static electricity increases electrosurgical ignition risk and patient mucous membrane drying accelerates. Above the upper limit, moisture promotes microbial growth on surfaces and in HVAC ductwork, and condensation can form on cold surgical instruments and supply air diffusers.

The consequences of humidity excursions are clinical, regulatory, and financial. High humidity in an operating room increases the risk of surgical site infection, a complication that carries substantial treatment cost and extends patient length of stay. Accreditation organizations and state health departments survey operating room environmental conditions, and humidity readings outside the specified range during surgery generate findings that require corrective action. Some facilities have delayed or cancelled surgical cases when humidity could not be maintained within specification, directly affecting surgical revenue and patient scheduling.

The challenge is that hospital buildings are among the most complex HVAC environments in any building type. Operating rooms require 100 percent outside air or very high percentages of fresh air, positive pressurization relative to adjacent corridors, and precise temperature control simultaneously with humidity control. The outdoor air load alone during a humid summer day can overwhelm a cooling system's ability to hold the upper humidity limit, particularly when the operating room suite is running at full surgical schedule with high occupancy and continuous door traffic.

Why Regular Air Conditioning Is Not Enough

Hospital air handling systems are typically designed to meet the substantial ventilation requirements of ASHRAE Standard 170: operating rooms require a minimum of 20 air changes per hour, with significant outside air fractions for pressurization and indoor air quality. In cooling mode, these air handlers remove moisture as a secondary effect of chilling the supply air. In humid climates, the outside air entering the system can reach 78 degrees Fahrenheit and 70 percent relative humidity, carrying 101 grains of moisture per pound. Delivering supply air at 55 degrees Fahrenheit saturated removes a portion of that moisture, but in many configurations the residual moisture in the supply air still pushes room conditions above 60 percent relative humidity during peak outdoor humidity events.

The problem compounds during partial load conditions. Shoulder seasons, nighttime operation, and reduced surgical schedules lower the sensible cooling load. The air handler modulates cooling capacity to maintain discharge air temperature, and as it does, the dehumidification effect diminishes. The system holds temperature while humidity rises. An operating room suite may hold 68 degrees Fahrenheit perfectly while relative humidity climbs to 65 or 70 percent, producing a compliant temperature reading and a non-compliant humidity reading simultaneously. Standard chilled water air handling systems tie temperature and humidity control to the same coil, and when the sensible load is satisfied, latent removal is compromised.

How Desiccant Dehumidifiers Remove Moisture

A rotary desiccant wheel applied to hospital outdoor air treatment removes moisture independently of the cooling system's operating state. The wheel treats incoming ventilation air before it reaches the air handler, reducing its moisture content to a level where the air handler's cooling coil can maintain both temperature and humidity within specification under all load conditions. For an operating room suite processing 100 percent outside air at 20 air changes per hour, the desiccant system reduces the entering moisture load to a level the downstream cooling coil can manage without overcooling the process airstream to the point where reheating it back to delivery temperature becomes necessary.

The separation of latent and sensible control is the operational advantage. The air handler modulates chilled water to maintain discharge temperature. The desiccant system modulates reactivation energy to maintain discharge dew point. During full surgical schedule with peak outdoor humidity, both systems run at or near capacity. During low-load periods, the air handler reduces cooling while the desiccant system independently adjusts moisture removal to maintain the upper humidity limit. Neither system's performance depends on the other's load condition. The operating room suite holds 68 degrees Fahrenheit and 45 percent relative humidity regardless of whether it's a full surgical day in August or a light schedule in October.

ASHRAE Systems and Equipment (Chapter 24) identifies healthcare facilities as applications where independent humidity control through desiccant dehumidification provides the consistent atmospheric conditions that infection control and patient safety require, particularly in high-ventilation-rate spaces that process large volumes of outdoor air.

How We Combine Cooling and Desiccant in One System

The standalone desiccant approach adds a separately powered reactivation system, typically electric resistance heating, sized to handle the full outdoor air moisture load independently.

Desiccant Air Solutions offers two integration paths depending on the facility's infrastructure. Where the hospital has available chilled water capacity, a chilled water pre-cooling coil upstream of the desiccant wheel reduces the moisture content of outdoor air before the wheel processes it, removing a significant fraction of the total moisture load at the chiller's coefficient of performance rather than at the cost of desiccant reactivation alone. Reactivation energy comes primarily from the condenser side of the refrigeration cycle. When additional reactivation capacity is needed beyond recovered heat, the system can also draw from electricity, natural gas, steam, or hot water. Alternatively, when chilled water isn't available or practical, we include DX refrigeration in the package. The self-contained DX coil pre-cools the air, and an internal desuperheater recovers condenser heat from the unit's own refrigeration circuit and routes it directly to the reactivation airstream. Either approach reduces net reactivation energy compared to a standalone desiccant system with separate electric or steam heating.

Unlike catalog equipment designed for general-purpose dehumidification, Desiccant Air Solutions engineers each system for the specific process conditions and moisture loads of the application. Wheel media selection, pre-cooling capacity, reactivation temperature, and control logic are all configured for the target environment rather than selected from a standard product line.

System controls use PID logic with dew point sensor feedback to modulate moisture removal continuously. Standard configurations include BMS integration for remote monitoring, alarm management, and setpoint adjustment.

What to Think About When Sizing the System

Hospital dehumidification sizing centers on the outdoor air treatment load because operating rooms and surgical suites process very large volumes of fresh air per hour. A four-operating-room suite with 20 air changes per hour per room and 600 square feet per operating room at 10-foot ceiling height processes approximately 8,000 cubic feet per minute of supply air, a substantial portion of which originates as outdoor air for pressurization and ventilation requirements. At design conditions of 78 degrees Fahrenheit and 70 percent relative humidity, that outdoor air carries 101 grains per pound. To deliver supply air at a 55 degrees Fahrenheit dew point (64 grains per pound), the system must remove approximately 37 grains per pound across the full outdoor airflow.

Calculate the outdoor air fraction from the pressurization and ventilation requirements per ASHRAE 170, multiply by the outdoor air moisture content at design conditions, and subtract the target supply air moisture content. A practical starting point: at 78 degrees Fahrenheit and 70 percent relative humidity outdoor conditions, a system treating 5,000 cubic feet per minute of outdoor air for a surgical suite must remove approximately 119 pounds of moisture per hour to maintain 55 degrees Fahrenheit dew point supply air.

Hospital Space	Min Air Changes	Target RH Range	Key Sizing Factors
Operating rooms	20 ACH	20–60% (often 30–60%)	100% outdoor air fraction common, high personnel load
Sterile processing	10 ACH	30–60%	Steam sterilizer moisture, outdoor air
Isolation rooms	12 ACH (negative pressure)	30–60%	Exhaust air volume, outdoor air makeup
Recovery / post-anesthesia care	6 ACH	30–60%	Moderate outdoor air, personnel
General patient areas	6 ACH	30–60% max	Outdoor air per ASHRAE 170, infiltration

Redundancy deserves consideration in hospital applications. Surgical schedules are difficult to adjust around dehumidifier maintenance windows, and humidity excursions during procedures can generate compliance findings. Where the surgical schedule and facility risk tolerance warrant it, N+1 capacity or units with redundant wheel and reactivation sections can allow maintenance without taking the system offline.

Why It Matters

Operating room humidity control is a patient safety requirement with direct clinical, regulatory, and financial consequences when it fails. A desiccant dehumidification system treating outdoor air upstream of the surgical suite air handler separates humidity control from temperature control, holds conditions within ASHRAE 170 specification across all outdoor conditions and surgical schedules, and eliminates the humidity excursions that generate accreditation findings and threaten surgical case scheduling. The investment in dedicated humidity control competes directly against the combined cost of compliance remediation, surgical case delays, and the increased infection risk that uncontrolled humidity represents. Contact Desiccant Air Solutions at [email protected] to discuss sizing, system configuration, and chiller plant integration for your facility.