Humidity Control


Humidity is defined as the amount of water vapor in the air. Absolute humidity is the actual water vapor in a given volume of air, usually expressed in grains per cubic foot. Relative humidity (RH) is the water vapor in a given volume of air compared to the amount the same volume of air will hold at saturation (100% RH) at a given temperature.

Example: One cubic foot of air at 0°F holds 0.48 grains at saturation. At 70°F it will hold 8.1 grains. When 0°F saturated air is heated to 70°F the absolute humidity remains at 0.48 grains per cubic foot. The RH of this 70°F air will be 6% RH (0.48 / 8.1) - the amount of water in the 70°F air relative to the amount it can hold at saturation.

The problems caused by too-dry air typically fall into four major categories:

1. Static electricity (shocks, jammed equipment, disrupted operations, sparks)
2. Health (dried out nasal and throat passages lead to more susceptibility to colds and virus infections, increased absenteeism)
3. Discomfort (urge to turn up the thermostat heating setting)
4. Poor moisture stability (deterioration of books, paintings, furniture)

"It isn't the heat, it's the humidity" is a truthful old saying as most people are comfortable at higher temperatures if there is a lower humidity. Likewise, there is an adverse effect in cold weather if the humidity is too low. All of this means humidity control is an important aspect in the HVAC system design with dehumidification required in hot weather and humidification required in cold weather.

In recent years there has been a trend to the use of dedicated ventilation air units that decouples this load from the space cooling and heating requirements. The results are better control of building humidity and pressurization, and a more consistent supply of the required amount of outdoor air (OA). Some designs go further and decouple both the OA load and the space latent load from the space HVAC system. Here the supply air dew point temperature is lowered enough below the design space dew point temperature to offset the space latent load.


Regardless of whether or not a dedicated OA unit is used, the dehumidification process is accomplished with a wide selection of techniques. The most commonly used is the lowering of the evaporator temperature on Direct Expansion (DX) systems or the entering chilled water temperature. However these actions are not energy efficient. More efficient strategies include:

  • Cooling and dehumidification coils with
    • Refrigerant hot-gas or condenser water reheat
    • Hot water, steam, or electric reheat
    • Sensible recovery from exhaust air (with or without evaporative precooking of the exhaust air)
  • Cooling and dehumidification coils with
    • Coil energy recovery (runaround) loops with water or glycol precooking and reheating coils
    • Heat pipe precooking and reheating of intake and exhaust air streams
    • Air-to-air heat exchanger precooking and reheating of intake and exhaust air streams
    • Rotary wheel heat exchangers for precooking and reheating of intake and exhaust air streams
    • Liquid refrigerant subcooling/ air reheating exchanger
  • Cooling coils with
    • Heat-powered desiccant with recuperative heat transfer
    • Rotary wheel heat exchangers


Heat Recovery

Heat recovery may add to the cost of a HVAC system but offers operating cost savings as the offsetting benefit. Heat recovery equipment may reduce the size of equipment such as boilers, chillers, and burners, and the size of piping and electrical services to them. Larger fans and fan motors (and hence fan energy) are generally required to overcome increased static pressure losses caused by the energy recovery devices. Auxiliary heaters may be required for frost control.

Selecting total energy recovery equipment to operate with humidifying or dehumidifying equipment results in the transfer of moisture from the airstream with the greater humidity ratio to the airstream with the lesser humidity ratio. In many situations this is desirable because humidification costs are reduced in cold weather and dehumidification loads are reduced in warm weather.


When the air intake and air exhaust is in close proximity, heat pipe, heat exchanger, or rotary wheel systems can be considered.

The rotary wheel system components are supply fan, exhaust fan, heat recovery wheel, cooling coil, and, as an alternative, a passive dehumidification wheel. The heat recovery wheel is used to precondition intake outdoor air using the exhausted building air.


The cooling coil (and if used, the passive dehumidification wheel) work in concert to further treat the intake air stream to produce room temperature air at a much reduced humidity level. The system must respond to various combinations of temperature and humidity by modulating the cooling coil (and passive dehumidification wheel). This approach can provide a constant stream of conditioned ventilation air while exhausting a building's stale air. It provides operating savings by eliminating the need for traditional over-cooling and reheat systems, and reduces the demand for overall heating and cooling capacity.

Where there are several air intake and air exhaust streams or they are not in close proximity, the coil run-around loop system is usually applied. The 1996 ASHRAE Handbook, HVAC Systems and Equipment, Chapter 42 describes this system as follows:

COIL ENERGY RECOVERY (RUNAROUND) LOOPS A typical coil energy recovery loop system places extended surface, finned tube water coils in the supply and exhaust air streams of a building or process. The coils are connected in a closed loop via counterflow piping through which an intermediate heat transfer fluid (typically water or a freeze-preventive solution) is pumped. This system operates for sensible heat recovery only. In comfort-to-comfort applications, energy transfer is seasonally reversible-the supply air is preheated when the outdoor air is cooler than the exhaust air and precooled when the outdoor air is warmer.

Freeze Protect

Freeze Protection Moisture must not freeze in the exhaust coil air passage. A dual-purpose, three-way temperature control valve prevents the exhaust coil from freezing. The valve is controlled to maintain the temperature of the solution entering the exhaust coil at 30°F or above. This condition is maintained by bypassing some of the warmer solution around the supply air coil. The valve can also ensure that a prescribed air temperature from the supply air coil is not exceeded.


The coil energy recovery loop cannot transfer moisture from one air stream to another; however, indirect evaporative cooling can reduce the exhaust air temperature, which significantly reduces cooling loads. For the most cost-effective operation, with equal airflow rates and no condensation, typical effectiveness values range from 45 to 65%. Highest effectiveness does not necessarily give the greatest net cost savings.

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A heat pipe heat exchanger is a passive energy recovery device. It has the outward appearance of an ordinary plate-finned water or steam coil, except that the tubes are not interconnected and the pipe heat exchanger is divided into evaporator and condenser sections by a partition plate. Hot air passes through the evaporator side of the exchanger, and cold air passes through the condenser side. Heat pipe heat exchangers are sensible heat transfer devices, but condensation on the fins does allow latent heat transfer, resulting in improved recovery performance.


Heat pipe tubes are fabricated with an integral capillary wick structure, evacuated, filled with a suitable working fluid, and permanently sealed. The working fluid is normally a Class I refrigerant, but other fluorocarbons, water, and other compounds are used for applications with special temperature requirements. Fin designs include continuous corrugated plate fin, continuous plain fin, and spiral fins. Fin design and tube spacing cause variations in pressure drop at a given face velocity.


Principle of Operation

Hot air flowing over the evaporator end of the heat pipe vaporizes the working fluid. A vapor pressure gradient drives the vapor to the condenser end of the heat pipe tube, where the vapor condenses, releasing the latent energy of vaporization. The condensed fluid is wicked or flows back to the evaporator, where it is revaporized, thus completing the cycle. Thus the heat pipe's working fluid operates in a closed-loop evaporation/condensation cycle that continues as long as there is a temperature difference to drive the process. Heat pipes have a finite heat transfer capacity that is affected by such factors as wick design, tube diameter, working fluid, and tube (heat pipe) orientation relative to horizontal.


Changing the slope, or tilt, of a heat pipe offers a way to control the amount of heat that it transfers. Operating the heat pipe on a slope with the hot end below (above) the horizontal improves (retards) the condensate flow back to the evaporator end of the heat pipe. This feature can be used to regulate the effectiveness of the heat pipe heat exchanger. In practice, pivoting the exchanger about the center of its base and attaching a temperature-controlled actuator to one end of the exchanger affect tilt control. Pleated flexible connectors attached to the ductwork allow freedom for the small tilting movement (6° maximum).



Fixed surface plate exchangers have no moving parts. The heat exchanger core consists of alternate layers of plates, separated and sealed, form the exhaust and supply airstream passages. Heat is transferred in counterflow patterns directly from the warm air streams through the separating plates into the cool air streams. Energy is transferred but moisture is not. While some crossflow heat transfer affects heat transfer effectiveness, recovering upward of 80% of the available waste exhaust heat is not uncommon. Fixed-plate heat exchangers can economically achieve high sensible heat recovery because there is only a primary heat transfer surface area separating the air streams and there is no other resistance (i.e., pumping liquid, condensing and vaporizing gases, or transporting a heat transfer medium) inherent in other exchanger types. These exchangers offer simplicity and lack of moving parts, which adds to the reliability, longevity, low auxiliary energy consumption, and safety performance. Most plate exchangers are equipped with condensate drains, which remove both the condensate and wastewater when a water-wash system is used for periodic cleaning.


Humidification equipment must be installed where the air can absorb the vapor and the temperature of the air being humidified must exceed the dew point of the space being humidified. When fresh or mixed air is humidified, the air may need to be preheated to allow absorption to take place. Some typical humidifiers are:

Heated Pan Humidifiers

These units offer a broad range of capacities and may be heated by a heat exchanger supplied with either steam or hot water. They may be installed directly under the duct or they may be installed remotely and feed vapor through a hose. Steam heat exchangers are commonly used in heated pan humidifiers, with steam pressures ranging from 5 to 15 psig. All pan-type humidifiers should have water regulation and some form of drain or flush system.

When water is evaporated some mineral residue is left which requires periodic cleaning to remove the buildup of minerals. Use of softened or demineralized water can greatly extend time between cleanings. All water should be drained off when the system is not in use to avoid the possibility of bacterial growth in the stagnant water.

Direct Steam

Direct Steam Injection Humidifiers

Steam is water vapor under pressure and can be directly injected into the air. This is an isothermal process because the temperature of the air remains almost constant as the moisture is added. The steam source is usually a low-pressure central steam boiler. When steam is supplied at a constant supply pressure, humidification responds quickly to system demand. A control valve may be modulating or two-position in response to a humidity sensor/controller. Steam can be introduced into the airstream through one of the following devices:

  • Single or multiple steam-jacketed manifolds depending on the size of the duct or plenum. The steam jacket is designed to re-evaporate any condensate droplets before they are discharged from the manifold.
  • Non-jacketed manifold or panel-type distribution systems with or without injection nozzles for evenly distributing steam.
  • Electrically Heated, Self-Contained Steam Humidifiers convert ordinary city tap water to steam by electrical energy using either electrodes or resistance heater elements. The steam is generated at atmospheric pressure and discharged into the duct system through dispersion manifolds. Some units allow the use of softened or demineralized water, to extend the time between cleanings.
  • Electrode-type humidifiers operate by passing an electric current directly into ordinary tap water, thereby creating heat energy to boil the water. The humidifier usually contains a polypropylene plastic bottle, either throwaway or cleanable, that is supplied with water through a solenoid valve. Water is drained off periodically to maintain a desirable solids concentration and the correct electrical flow.
  • Resistance-type humidifiers utilize one or more electrical elements that heat the water directly to produce steam. The water can be contained in a stainless steel or coated steel shell, which should be accessible for cleaning out mineral deposits. The high and low water levels are controlled with either probes or float devices, and a blowdown drain system should be incorporated, particularly for off-operation periods.
  • Atomizing Humidifiers
    Water treatment should be considered if mineral fallout from hard water is a problem. Optional filters may be required to remove the mineral dust from the humidified air. Depending on the application and the water condition, atomizing humidifiers may require a reverse osmosis (RO) or a deionized (DI) water treatment system to remove the minerals. There are three main categories of atomizing humidifiers:
  • Ultrasonic humidifiers utilize a transducer submerged in demineralized water. The transducer converts a high-frequency mechanical electric signal into a high- frequency oscillation. A momentary vacuum is created during the oscillation, causing the water to cavitate into vapor at low pressure. The opposite oscillation produces a high-compression wave that drives the water particle from the surface to be quickly absorbed into the airstream. Because these types use demineralized water, no filter medium is required downstream.
  • Centrifugal humidifiers use a high-speed disk, which slings the water to its rim, where it is thrown onto plates or a comb to produce a fine mist. The mist is introduced to the airstream, where it is evaporated.
  • Compressed-air nozzle humidifiers can operate in two ways:

1. Compressed air and water are combined inside the nozzle and discharged onto a resonator to create a fine fog at the nozzle tip.
2. Compressed air is passed through an annular orifice at the nozzle tip, and water is passed through a center orifice. The air creates a slight vortex at the tip, where the water breaks up into a fine fog on contact with the high-velocity compressed air.

Adiabatic (adiabatic vs. isothermal) systems have a lower operating cost than any other technology. These humidifiers consist of compressed air atomizing nozzles, high-pressure water nozzles, and the hybrid combination of medium water pressure and evaporative ceramic media for short absorption ducted/AHU requirements.

  • Wetted Media Humidifiers
  • Rigid media humidifiers utilize a porous core. Water is circulated over the media while air is blown through the openings. These humidifiers are cooling the air as it is humidified. Rigid media cores are often used for the dual purpose of winter humidification and summer cooling. They depend on airflow for evaporation: the rate of evaporation varies with air temperature, humidity, and velocity. The rigid media should be located downstream of any heating or cooling coils. For close humidity control, the element can be broken down into several (usually two to four) banks having separate water supplies. Solenoids controlling water flow to each bank are activated as humidification is required.
  • Compressed Rigid media humidifiers have inherent filtration and scrubbing properties due to the water washing effect in the filter-like channels. Only pure water is evaporated, therefore, all contaminants introduced by the air and the water must be flushed from the system. A continuous bleed or regular pan flushing is recommended to prevent accumulation of debris in the pan and on the media.

Humid Unit

Humidifier units must be installed where the air can absorb the discharged steam before it comes into contact with coils, dampers, turning vanes, or other such items. Otherwise, condensation can occur in the duct. Absorption distance varies according to the design of the humidifier distribution device and the air conditions within the duct. In larger systems the humidifier can be included as a part of the air handling unit.

Mechanical Controls

Mechanical sensors depend on a change in the length or size of the sensor as a function of relative humidity. The most commonly used sensors are synthetic polymers or human hair attached to a mechanical linkage to control the mechanical, electrical, or pneumatic switching element of a valve or motor. This design is suitable for most human comfort applications, but it may lack the necessary accuracy for industrial applications.

Electronic Controllers

Electrical sensors change electrical resistance as the humidity changes. They typically consist of two conductive materials separated by a humidity-sensitive, hygroscopic insulating material (polyvinyl acetate, polyvinyl alcohol, or a solution of certain salts). Small changes are detected as air passes over the sensing surface. Electronic control is common in laboratory or process applications requiring precise humidity control.

Links to Related Topics

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Sick Building Syndrome