By D. Scott Herr, President, DGH Systems LLC (1990)

ABSTRACT

Proper humidity control is essential to maintaining a more healthful, comfortable environment which can have a dramatic effect on human productivity. A stable, humidity controlled, environment is critical to many advanced technologies such as are present in the micro chip and computer industries. Many industries and HVAC applications involve processes and machinery that generate heat in excess of the heating requirements of the working environment. Atomizing humidifiers provide evaporative cooling as well as humidification and can:

1. Achieve significant energy savings in both cooling and humidifying costs over the use of isothermal steam humidifiers,

2. Achieve a higher degree of humidification purity over other types of humidifiers,

3. Achieve a more stable control level than is possible with some other types of humidifiers.

INTRODUCTION

This paper identifies the industries where atomizing humidifiers are essential, categorizes and analyzes the design and application of atomizing humidifiers and presents the energy and purity issues with respect to Indoor Air Quality and energy concerns. Based on this paper and the information disclosed herein, situations are presented for the efficient, beneficial application and use of atomizing humidifiers.

BASICS OF ATOMIZATION

Atomizing humidifiers work by breaking up water into small droplets to dramatically increase the surface area presented to the air for evaporation. The speed of evaporation is directly related to the size of the droplet, the psychrometric properties and movement of the surrounding air, the surface tension of the water and, to a lesser extent, the mineral content of the water.

Any device or procedure that causes a break in the surface tension of the water will, in effect, cause atomization. Simply pouring water on the floor will create some atomization as it splatters. Methods of atomizing water include the following:

Pulverization - A process where water is subjected to violent action such as shaking or impact which overcomes the surface tension.

Shearing - A process where water is sheared or sliced into smaller droplets or sheets. Often, some form of pulverization is employed simultaneously.

Centrifugation - Water is spun or subjected to centrifugal action which overcomes the surface tension and literally tears the water apart.

Commercial mechanisms to effect atomization of water include, but are not limited to the following:

Centrifugal atomizers - These work by a combination of centrifugal action and pulverization. The water is dropped or pumped onto a spinning disk where centrifugal action spreads the water into sheets. As these sheets reach the edge of the disk, centrifugal action tears the sheets into droplets which fly off the edges and usually strike a screen or other mechanism which then pulverizes the droplets into smaller droplets. Air flow, forced through the unit by a fan, then discharges the droplets into the atmosphere for evaporation.

Air/steam activated ultrasonic atomizers - A stream of pressurized steam or compressed air is discharged through an orifice into a resonator cup. This resonator cup is positioned and designed so as to produce an ultrasonic field which, by means of sound wave pulverization, subjects the water introduced into the field, to violent shaking to overcome the surface tension and create droplets.

Piezoelectric ultrasonic atomizers - A piezoelectric device is employed to produce ultrasonic frequency vibration of a surface. Water is placed on the surface and is atomized by the vibrations by means of impact pulverization.

Air/water atomizers - Water is discharged at low velocity through an orifice into a much faster moving compressed air stream. The extremely higher speed of the air stream distorts or shears the water into droplets which are then discharged into the air for evaporation. The air may be moving at supersonic speed but no ultrasonic sound frequency is used in the process. Sometimes the water/air mixture exits the nozzle through an orifice which rapidly speeds up the discharge air flow. In this case, due to the difference in molecular weights of the air and the water droplets, the water droplets are further distorted and sheared into even finer droplets.

Pressurized water atomizers - In this mechanism, water is pressurized and flows through a nozzle that contains either a mechanism to pulverize the water by impact or a mechanism to spin the water thus effecting centrifugal atomization on discharge through the orifice into the atmosphere.

In all cases, the mechanism must overcome the surface tension of the water to produce a small droplet and increase the surface area of the water exposed to the air. The size of the droplet created by any mechanism is dependent on the surface tension of the water and the energy applied to disrupting it.

The evaporation of a droplet of water is dependent on the size of the droplet, the psychrometric properties and movement of the surrounding air and, the surface tension of the water. To a lesser extent, the mineral content of the water will improve evaporation in that, as the water droplet evaporates and shrinks, it becomes distorted by the porous and uneven surface of the contained mineral solids, thus increasing surface area and speeding evaporation.

PSYCHROMETRIC AND ENERGY ANALYSIS

Once a droplet is presented to the air, it begins the adiabatic process of evaporation, converting the sensible heat energy of the surrounding air into latent heat energy. This conversion of forms of energy decreases the dry bulb temperature but has no net effect on the total enthalpy. This can be seen more clearly on the ASHRAE psychrometric chart attached to this paper.

In this example, we assume a starting condition of 70 ^oF and 20 %RH (point A) which equals .003 lb water/lb dry air and 20.3 BTU/lb dry air. If we raise the humidity of the air to 60 %RH using an isothermic type humidifier, such as steam, we will arrive at a final condition of 70 ^oF, 60 %RH (point B), .0094 lb water/lb dry air and 27.2 BTU/lb dry air enthalpy. The result is an increase in enthalpy of 6.9 BTU/lb of dry air and an increase in the humidity ratio of .0064 lb water/lb dry air. If the area being humidified is under a cooling load, the additional energy will have to be removed by the air conditioning system. If the area is under a heating load, the net result to the heating system is negligible.

If we raise the humidity of the air to 60 %RH using an adiabatic type humidifier, such as atomizing, we will arrive at a final condition of 57 ^oF, 60 %RH (point C), .006 lb water/lb dry air and 20.3 BTU/lb dry air enthalpy. The result is no net change in enthalpy, an increase in the humidity ratio of .003 lb water/lb dry air and a decrease in dry bulb temperature of 13 ^oF. If the area being humidified is under a cooling load, the resulting reduction in dry bulb temperature will reduce the air conditioning load correspondingly. If we are using an economizer system which normally operates at a 55 ^oF mixed air temperature, we will be able to reduce the amount of outside air required and thus reduce the actual humidification load, saving energy. If the area is under a heating load, additional heating may be needed to compensate for the evaporative cooling effect.

To put this in perspective, we use an example of a 2000 lbs/hr humidification system.

Given: Evaporation of 1 lb of water requires 1075 BTU.
Producing 1 lb of steam at 15 psi requires 1150 BTUs.
Typical atomizing system requires .12 CFM per lb of water per hour. Fuel oil contains 140,000 BTUs per gallon.
Natural gas contains 100,000 BTUs per therm.
A Ton of air conditioning is equal to 12,000 BTUs of cooling.
A typical compressor produces 4 CFM per hp.
Steam boilers operate at 60% efficiency with piping losses.
Electricity costs $.08/kW, oil $.80/gal, gas $.40/therm.
Normal humidification season is 1500 full load hours per year.
750 hours of yearly humidification are under cooling load.

Atomizing system operating cost:

2000 lbs/hr x .12 CFM/lbs/hr
---------------------------- = 60 hp compressor
4 CFM/hp

60 hp x .7457 kW/hp x $.08/kW = $3.58/hr operating cost
$3.58/hr x 1500 hrs/yr = $5,370/yr operating cost

Atomizing system evaporative cooling savings:

Atomizing system reheat costs:

Oil fired steam humidification costs:

2000 lbs/hr x 1150 BTUs
----------------------- ) .60 efficiency x $.80/gal = $21.90/hr
140,000 BTU/gal

$21.90/hr x 1500 hrs/yr = $32,857/yr

Gas fired Humidification:

2000 lbs/hr x 1150 BTUs
----------------------- ) .60 efficiency x $.40/therm = $15.33/hr
100,000 BTU/therm

$15.33/hr x 1500 hrs/yr = $23,000/yr

Cooling cost to eliminate excess heat:

2000 lbs/hr x 1150 BTUs = 2,300,000

- 2000 lbs/hr x 1075 BTUs = 2,150,000
--------------------------------------
150,000 BTU/hr excess heat

150,000 BTUs/hr ÷ 12,000 BTUs/Ton = 12.5 Tons cooling
12.5 Tons x 3.5 kW/Ton x $.08/kW = $3.50/hr cooling cost
$3.50/hr x 750 hrs/yr = $2,625/yr cooling cost

Total annual operating cost for a 2000 lbs/hr atomizing humidification system using oil reheat:

Operating cost =   $5,370
Reheat cost =      15,360
Cooling Savings = (37,627)
-------------------------
                 ($16,897)

Total annual operating cost for a 2000 lbs/hr atomizing humidification system using gas reheat:

Operating cost =   $5,370
Reheat cost =      10,747
Cooling Savings = (37,627)
-------------------------
                 ($21,510)

Total annual operating cost for a 2000 lbs/hr atomizing humidification system where year round cooling exists with no reheat:

Operating cost =   $5,370
Cooling Savings = (37,627)
-------------------------
                 ($32,257)

The negative cost figures shown above are due to the savings provided by evaporative cooling over mechanical cooling and therefore, total operating costs are reduced.

Total annual operating cost for an oil fired steam humidification system:

Operating cost = $32,857.14
Cooling costs =    2,625.00
---------------------------
                 $35,482.14

Total annual operating cost for a gas fired steam humidification system:

Operating cost = $23,000.00
Cooling costs =    2,625.00
---------------------------
                 $25,625.00

These calculations deal with a normal air handling system. When an economizer is involved, the amount of outside air is adjusted by the outdoor temperature to achieve some amount of "free cooling". In most cases, the mixed air temperature being maintained is 55^oF. If we assume that the indoor design is 70^oF, 40%RH, 24 BTUs/lb dry air and 100% outside air intake occurs at a design condition of 55^oF, 35%RH, 16.6 BTUs/lb dry air, the following chart would apply. Note that as the amount of outside air increases, the amount of evaporative cooling versus economizer cooling increases almost to 50%.

Indoor Design Conditions: 70^oF 40%RH (8.10 x .40 = 3.24 gr/CF)
Outside Design Temperature: -10^oF
Mixed Air Temperature: 55^oF
System CFM: 30,000

It is not hard to see where even a small amount of humidification requirement during the cooling cycle of an air handling system can translate to tremendous energy savings of an atomizing system over a steam system on an annual basis. Additionally, there are many industries which require year round cooling whether due to the geographical location or internal heat production characteristics as detailed later on.

MINERAL DUSTING

Raw water usually contains some small amount of soluble and insoluble minerals and impurities. When an atomized water droplet evaporates in the air, it leaves behind a particle made up of these impurities. According to some studies done by the University of Eindhoven, a 10 micron droplet, evaporated in still air, will leave behind a dust particle of approximately .3 to .7 micron in size. This particle size is generally smaller than the normal room dust, but more visible when it accumulates, because of its usually white color. This size dust particle will only be arrested by a HEPA filter, but natural agglomeration will increase the size of the dust particles allowing return air filters to catch and remove them from the air stream.

Regarding mineral particulate in the air, there are no conclusive studies concerning this mineral dust and Indoor Air Quality. In fact, according to the ASHRAE Fundamentals Handbook, "particles .1 micron or less in diameter make up 80% of the number of particles in the atmosphere". Since most water used in humidifiers is from a potable water source, the human beings in the humidified area are exposed to no more than the same water and constituents that they ingest from the drinking fountain.

To put the quantity of dust produced in perspective, the following example:

Therefore, for a 100 lbs/hr atomizing system on this water, the amount of mineral dust created per hour would be as follows:

     10 grain/gallon Total Dissolved Solids = 0.001429 lbs/gallon TDS

     0.001429 lbs/gallon = 0.000171 lbs/lb TDS
       8.34 lbs/gallon

     100 lbs/hr water X 0.000171 lbs/lb TDS = 0.017129 lbs of mineral
     OR 0.274066 ozs of mineral per hour

     0.274066 ozs/hr  = 0.000015038 ozs/sq ft/hr
       18225 sq ft

OR

     0.000015038 ozs/sq ft/hr x 1500 hours = 0.02255691 ozs/sq ft/yr

OR

     0.0000001044 ozs/sq in/hr. x 1500 hrs = 0.000156645 ozs/sq in/yr

As you can see, the actual amount of mineral dust deposited in the room is infinitesimally small and quite a bit less than the amount of normal dust deposited or present in the air. As mentioned above, mineral dust is white and simply becomes more visible than the black dust it deposits on. In actual practice, levels of humidity above 35% have been shown to actually reduce the dust count in the air by causing agglomeration and falling out of dust particles, and by reducing sources of dust, such as fibers from the carpets and furnishings.

When the mineral content in the water is too high to be acceptable or if the environment to be humidified cannot tolerate the impurities (such as a high technology clean room), various methods are used to achieve the purity of humidification required. The most obvious method is to remove the mineral before it is used in a humidifier, either by reverse osmosis, deionization or distillation. All these processes result in pure water of extremely low mineral content that can then be used for pure humidification. Such systems are commonly found in clean rooms and sterile areas.

Another method of removing some of the mineral is the use of eliminator pads. Wherever a water droplet evaporates, it will leave its mineral content. If evaporated in the air, the mineral dust will continue to float in the air, eventually settling out. If the droplet evaporates on some surface, such as the fibers of a mist eliminator pad, it will leave its mineral content adhered to that surface. Some fine mist (approximately 5% to 7 %) will get through the pad and evaporate in the air, so that, generally speaking, the purity of atomizing humidification using mist eliminators is comparable to that of some steam systems, except that the boiler treatment chemicals are not present. Caution must be exercised to insure that the water used in the system is potable and free of algae or bacteria. Atomizing systems should be of a sealed design without open tanks or reservoirs.

By comparison steam created from raw water in a boiler will entrain up to 5% of the mineral content of the water and carry it out to the humidifiers where it will be discharged into the air. This is true also of electrode boilers. Generally, that small amount of carryover is not a problem. Additionally, if boiler treatment chemicals are used, 5% to 7% of these volatile chemicals may be entrained and discharged into the atmosphere through the humidifier. These chemicals, if they contain amines, are generally carcinogenic and as a result, various manufacturers of these chemicals specifically warn against their use in steam systems where the steam will be discharged into the air that people will breathe. Caution must therefore be exercised when using boiler treatment chemicals. Steam humidification systems are inherently free from algae and bacteria due to the sterilizing effect of the steam itself.

BIOLOGICAL CONSIDERATIONS

The 1988 ASHRAE Equipment Handbook calls for humidity to be maintained around 55% for human comfort and safety. It also states that at 50% RH, the mortality rate of certain organisms is highest, and the influenza virus loses much of its virulence. Increased humidity results in increased natural agglomeration of dust particulate in the air, causing them to fall to the ground or be more easily arrested by the filters in the air handling system, which can result in a decreased dust count in the air. Since dust in the air provides the surface on which airborne microorganisms can travel and grow, it follows naturally that anything that decreases the air borne particulate level will also reduce the level of airborne microbiological contaminants.

Of the various types of humidifiers, atomizers provide not only an increase in humidity, but also provide greater potential for collision of water droplets directly with airborne particulate and the subsequent increase in mass drops the particulate to the floor. Of course, care must be taken to insure that the particulate contained in the atomized water, if any, is not in itself a source of significant airborne contamination, exceeding the benefit of particle agglomeration.

Additionally, the design of an atomizer should allow for the unit to either seal on shut off or drain the reservoir empty to reduce the potential for microbiological contamination and buildup. The best and safest designs would be designs that incorporate the sealing of a potable water feed line without the use of reservoirs, as any wet surface exposed to the air has some potential for the buildup of slime molds.

According to studies by Delperin (1973), Green (1975, 1979, 1982), Ritzel (1966), Sale (1972) and Serati and Wuthrich (1969), there is a statistically significant reduction in absenteeism among occupants of humidified buildings. Although the cause of this reduction in absenteeism is not known, it is probably due to an increase in the settling rate of aerosols at higher humidity and/or a decrease in the survival of bacteria and viruses. The increase in the settling rate of aerosols may very well negate any mineral dust added by an atomization mechanism.

Of course, one of the greatest indoor air hazards is standing water. Standing water can be created by any type of humidification system, including steam, if the system is poorly designed, installed or controlled. Droplets are created by both steam and atomizing systems, although the droplets created by steam are generally in the range of .3 to 2 micron, while the droplets created by atomizers usually range from 10 to 100 micron in size. Due to their larger size and evaporative nature, atomized droplets require more time and hence, more straight distance of duct to evaporate. It is important to note that any place live droplets collect can eventually form a pool of water and, if undrained can become a breeding ground for bacterial and algal growth.

Most in duct atomizing systems now use control as well as high limit sensors to prevent condensation and thus prevent standing water. Recently, control techniques have come into use that allow atomizers to vary their output according to the ability of the air to absorb the moisture. One air/water atomizing system even changes the air/water ratio to produce smaller droplets (as small as .3 micron) during periods of low output, thus enhancing evaporation. Other recent developments include the use of drained, mist eliminator pads which help to control water absorption and prevent carryover into areas of the duct where condensation might occur. Of course, these special mist eliminators must be nonhygroscopic so as not to present a moist surface for bacterial growth.

ENVIRONMENTAL APPLICATIONS

In applications involving cold storage of fruits, vegetables and meats (34-55^oF, 85-95% RH), the required temperatures are too low and the required relative humidity too high for the use of any type of humidifier other than an atomizer. Steam, since it is isothermal will raise the dry bulb temperature while raising the wet bulb temperature resulting in increased run time of the cooling equipment to reduce the dry bulb to design levels. Increased run time of the cooling equipment results in reduction of the wet bulb temperature due to condensation on the coils and thus, a vicious cycle between the steam humidifier and the cooling equipment. Experience has shown that, under these conditions, the maximum achievable relative humidity is 65-70%. Evaporative type humidifiers reach a point of diminishing return at around the same level.

With each new generation of micro chip technology, the specifications for humidity, temperature and cleanliness of the fabrication clean rooms become more stringent. Very often, the fabrication machinery generates enough heat from process to more than equal the heating load of the area, thus requiring year round cooling of the space. The increasing requirement for cleanliness in clean rooms means even raw steam humidifiers with their boiler treatment chemicals and minimal mineral carryover are not pure enough for direct injection into the primary cooling loop downstream of the HEPA filters. Additionally, isothermal humidification methods increase the enthalpy of the envelope and increase the cooling requirements. A sealed atomizer (that is one without tanks or reservoirs which may become contaminated) is increasingly the method of choice when fed with ultra pure demineralized water of about 18 meg ohm purity.

Just as in clean rooms, the internal heat load of computer rooms can very often provide a set of circumstances whereby an atomizing system can provide significant energy savings over isothermal methods. Again, as in other applications, the water used must be treated to remove the minerals and the atomizer should be of a sealed, reservoirless design.

In applications where dust is created by the process, such as in printing, textiles and asbestos manufacture, live mist spray in the air is helpful to achieve a reduction of the dust count in the air by causing agglomeration of the airborne dust particulate both through collision of the water droplets with the dust particles, and also through the increased humidity level produced.

With the increasing use of economizer cycle systems for environmental comfort cooling, the efficiency of atomization versus steam is becoming more apparent. When steam humidification is used with an economizer system, the increased enthalpy provided by the steam can very often cause the enthalpy controller to increase the amount of outside air brought in. Since the amount of outside air is the primary component of any humidification load calculation, even a slight increase in outside air for cooling purposes will result in an increased demand for humidification. The increased demand for humidification will call for more steam, which will increase the enthalpy even further and so on and so forth with one system fighting the other. In the case of an atomizing system, the enthalpy remains the same and eventually the two systems will achieve a balance.

In many applications where the atomizing system is installed at the ceiling level, the energy for evaporation comes from the waste heat which normally stratifies at the ceiling level. Since the heating system thermostats are usually at floor level, the only effect of a ceiling mounted atomizer, other than to increase the humidity in the area, is a reduction of the dry bulb temperature at the ceiling level. The reduction in dry bulb temperature at the ceiling level also reduces the differential between inside and outside temperatures and thus reduces the heat loss through the roof as well.

CONCLUSIONS

With America's thrust into high technology production of micro chips and memory storage media, the increasing use of computers, and the use of electronics in increasing numbers of products, the need for humidification/evaporative cooling systems which can cool the air, as well as for ultra pure humidification systems is moving in duct atomizers to the forefront of the humidification industry as one of the fastest areas of growth. Atomizing systems capable of modulating their output, and using demineralized water, offer the best answer to this requirement.

Atomizing systems can be applied to direct space humidification with the result of great energy savings where waste heat is allowed to stratify and collect at the ceiling level. Applied to air handlers, the direct evaporative cooling can be used to greatly stabilize the temperature and humidity control of an economizer system and to reduce the amount of mechanical cooling required. The use of evaporative cooling over mechanical cooling can result in a reduction of national energy use in the billions of dollars.

Mist eliminator pads are increasingly used for mineral elimination through droplet impaction to eliminate the traditional atomizer problems of duct wetting and mineral dusting. The mineral and chemical makeup of the water used in atomizers can be controlled or altered through the use of demineralizers and/or ion exchange units. By using demineralization, atomizers can be among the most pure forms of humidification.

In recent years, in order to set standards, some engineering groups have chosen the "easy way out" by recommending the exclusive use of steam whenever humidification is required. As is already known, and as pointed out in this paper, steam is not the purest form of humidification, has its own dangers in the use of carcinogenic boiler treatment chemicals, and is not always the most efficient form of humidification for every application. To pursue a policy of exclusively using steam for every humidification application, could result in the loss of billions of dollars of national energy savings per year, unnecessary exposure of the public to carcinogenic materials, and industry efficiency losses in the billions of dollars, and the hampering of key high technology industries in their bid for our national future.

The new wave of atomizing systems is upon us.

REFERENCES

Chigier, Norman and Lefebvre, Authur and Simmons, Harold "Atomization and Sprays", Carnegie Mellon (1988)

Brown, W.K. "Humidification by Evaporation for Control Simplicity and Energy Savings" (1989)

ASHRAE "1989 ASHRAE Handbook, Fundamentals", ASHRAE (1989)

ASHRAE "ASHRAE Standard, Ventilation for Acceptable Indoor Air Quality", ASHRAE (1989)