e uml - rses · understanding the role of psychrometric • in e uml • psychrometrics is an...

DEHUMIDIFICATION

Understanding the Role of Psychrometric

• • In e uml Psychrometrics is an essential

foundation for understanding how

to change air from one condition to

another. Here's how to apply it to

dehumidification

18 RSES Joumal - May 1998

The desiccant dehumidification

unit contained In the large cylinder at the

right of the photo provides dry air to a

fluid drying bed In the manufacturing

. facility at Knoll Pharmaceutical Co.

By Mario A. Ranieri

Dehumidification is a requisite for many manufacturing and processing applications. Dry air can make the difference between operating with a profit or loss, or even as dramatic as being able to run production lines or being forced to

close an entire plant. Dehumidification applications range from providing dry

air to positive displacement blowers in pneumatic conveying applications to prevent sugar from clumping, dehumidifying fertilizer storage facilities to prevent caking and clumping, and preventing corrosion in lithium battery manufacturing operations .

Dehumidification can decrease the size of fluidized bed dryers by providing dry air or can increase capacity of existing fluid beds. Dehumidification can also provide an effective means to dry temperature-sensitive products without the use of heat.

The use of psychrometries The rudimentary basis for understanding any air-conditioning process is psychrometrics, the study of the properties of air-water mixtures. Psychrometrics is an essential foundation for understanding how to change air from one condition to another. Some of the key variables and their definitions found in air-conditioning processes are: • Dry bulb temperature. This is the temperature of air measured by a thermometer with a dry sensing bulb. • Wet bulb temperature. This is the temperature of air measured by a thermometer with its sensing bulb covered by a wet wick. Wet bulb temperature is useful in air conditioning because, when combined with the dry bulb, the wet bulb temperature allows an inexpensive method to determine all the properties of the air by putting the dry bulb and wet bulb on the psychrometric chart (see Figure O. Useful for industrial drying processes, the difference between the dry bulb and wet bulb temperatures is a measure of the drying capacity of the air. • Dew point temperature. This is the temperature at which water in the sample will condense from the air, forming dew on the nearest surface. The dew point is expressed in degrees and, like the humidity ratio, it represents an absolute measure of the moisture in the air at a constant pressure. • Humidity ratio. This represents the amount of moisture in the air as compared to the weight of dry air, which is usually expressed in grains of water vapor per pound of dry air. It may also be expressed in units of pounds of water

May 1998 - RSES Joumal 19

DEHUMIDIFICATION

Understanding the Role of Psychrometric

• • In e uml Psychrometrics is an essential

foundation for understanding how

to change air from one condition to

another. Here's how to apply it to

dehumidification

The desiccant dehumidification

unit contained In the large cylinder at the

right of the photo provides dry air to a

fluid drying bed In the manufacturing

. facility at Knoll Pharmaceutical Co.

By Mario A. Ranieri

Dehumidification is a requisite for many manufacturing and processing applications. Dry air can make the difference between operating with a profit or loss, or even as dramatic as being able to run production lines or being forced to

close an entire plant. Dehumidification applications range from providing dry

air to positive displacement blowers in pneumatic conveying applications to prevent sugar from clumping, dehumidifying fertilizer storage facilities to prevent caking and clumping, and preventing corrosion in lithium battery manufacturing operations .

Dehumidification can decrease the size of fluidized bed dryers by providing dry air or can increase capacity of existing fluid beds. Dehumidification can also provide an effective means to dry temperature-sensitive products without the use of heat.

The use of psychrometries The rudimentary basis for understanding any air-conditioning process is psychrometrics, the study of the properties of air-water mixtures. Psychrometrics is an essential foundation for understanding how to change air from one condition to another. Some of the key variables and their definitions found in air-conditioning processes are: • Dry bulb temperature. This is the temperature of air measured by a thermometer with a dry sensing bulb. • Wet bulb temperature. This is the temperature of air measured by a thermometer with its sensing bulb covered by a wet wick. Wet bulb temperature is useful in air conditioning because, when combined with the dry bulb, the wet bulb temperature allows an inexpensive method to determine all the properties of the air by putting the dry bulb and wet bulb on the psychrometric chart (see Figure O. Useful for industrial drying processes, the difference between the dry bulb and wet bulb temperatures is a measure of the drying capacity of the air. • Dew point temperature. This is the temperature at which water in the sample will condense from the air, forming dew on the nearest surface. The dew point is expressed in degrees and, like the humidity ratio, it represents an absolute measure of the moisture in the air at a constant pressure. • Humidity ratio. This represents the amount of moisture in the air as compared to the weight of dry air, which is usually expressed in grains of water vapor per pound of dry air. It may also be expressed in units of pounds of water

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Rgure 1. The psychrometric chart shows the relationship between the various properties of air.

vapor per pound of dry air or grams of water vapor per kilogram of dry air.

The humidity ratio is very useful in dehumidification because at normal pressures this ratio represents an actual measure of water mass relative to air mass. Hence, the humidity ratio does not change with changes in air temperature. • Relative humidity. This is the amount of moisture in the air compared to the maximum amount the air can hold at that dry bulb temperature. Relative humidity is expressed as a percent of the maximum water content at the specified temperature.

As the term relative humidity implies, it is a relative measure as opposed to an absolute amount because maximum humidity varies with temperature. Warm air can hold more moisture than cold air. • Vapor pressure. This is the total pressure exerted by the water molecules in air. Sometimes expressed in psia or in Hg, it is useful in dehumidification because water vapor moves through air to equalize difference in vapor pressure. The operating principle of desiccant dehumidifiers is to move water vapor to and from a desiccant through differences in vapor pressure. • Enthalpy. This is the total energy content of the air, consisting of the sensible heat plus the heat originally used to evaporate the water in the air, expressed in Btullb. of dry air.

Cooling-based dehumidification Cooling-based dehumidification is the most common method of removing moisture from air. Most air-cQnditioning units dehumidify as they cool.

The basic principle of dehumidification is that cold air cannot hold moisture as well as warm air. Therefore, air is cooled to force out moisture by condensation on a cold surface, such as cooling coils. The direct expansion cooling system, which is the most widely applied method, moves heat from one air stream to another by heating a gas and moving it with a compressor.

A specific conditioning unit is the "cool-reheat" dehumidifier, which chills the air to dehumidify it', then uses the rejected heat to reheat the air after it has been dehumidified. This is the most energy-efficient dehumidifier because all Btu are used effectively.

An advantage of refrigeration-based systems is that they are energy efficient at high temperatures and at dew points about 50· F. A disadvantage is that the use of cooling dehumidification below 40· F is not practical because of the risk of freezing and the high energy cost. Also, if low relative humidity is required, additional energy must be used to reheat after cooling in order to artificially decrease the relative humidity.

Desiccant-based dehumidification Dry desiccant acts on each water vapor molecule directly and attracts them by differences in vapor pressure. Virtually all materials are desiccants, meaning that they absorb moisture in humid air and desorb in dry air (see Figure 2).

The difference in absorbing moisture between human air and industrial desiccants, such as silica gel and lithium chloride, is capacity. Most materials absorb 3 percent to 15 percent of dry weight, whereas desiccants absorb 20 percent to 1,200 percent. The

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Rgure 2. The moisture chart shows the capacity of common materials to absorb moisture based on the surrounding relative humidity.

desiccant process automatically creates low relative humidity, where cooling automatically creates high relative humidity.

Desiccants have historically been used for industrial processes because of the economic benefits of low humidity for productivity gain. More recently, low relative humidity is being recognized as useful in commercial applications.

The fundamental process of desiccant dehumidification is to absorb and release moisture, which is based on the differences between water's vapor pressure at the desiccant surface and vapor pressure in the air around its surfaces. Desiccant dehumidification changes the vapor pressure of the water in the desiccant by changing the desiccant's temperature and moisture content.

Low temperature and moisture content in the desiccant provide for low vapor pressure at the desiccant's surface (attracts moisture from air). High temperature and moisture content in the desiccant provide for high vapor pressure of water in the desiccant (gives off moisture to air).

Desiccants follow four major steps in removing moisture from the air (see Figure 3). Step one is sorption, where moisture is removed from the air by the desiccant and the surface vapor pressure rises as the desiccant collects moisture. The desiccant temperature also rises as latent heat in the air is converted to sensible heat.

In step two, the moisture content in the desiccant reaches equilibrium. The surface vapor pressure is equal to the vapor pressure of the surrounding air. Without a vapor pressure differential, there is no further moisture movement to the desiccant.

The reactivation of the desiccant occurs in step three, when the surface vapor pressure rises as the desiccant is heated. Because the vapor pressure of the moisture in the desiccant is higher than that in the surrounding air, moisture leaves the desiccant. The moisture content is reduced, but vapor pressure remains high because of the higher temperature.

Step four is the cooling of the desiccant. The vapor pressure at the desiccant's surface reduces rapidly with cooler temperatures. When the desiccant is cool, the vapor pressure once again is lower than the surrounding air, so it can collect moisture again.

The following are key points in the desiccant cycle: • Energy to drive the cycle is proportional to the mass that is heated and cooled. Hence, the more desiccant and water that is heated, the more energy is used. • All desic~ants move through the same cycle because there is no escaping thermodynamics. All desiccants must be heated and all must be cooled. • The cost of operation depends primarily on the cost of the reactivation energy plus cooling energy.

The Honeycombe wheel One popular and effective technique in desiccant dehumidification is to have the process air (the air being dehumidified) dried in the rotating wheel of a lightweight, corrugated ceramic, which resembles a honeycomb. Air can pass through the flutes, which are impregnated with a desiccant.

The wheel rotates at 6 to 10 revolutions per hour,

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Rgure 3. The chart shows the desiccant cycle.