Humidification and Dehumidification

1.3.1.16 Introduction

Humidification and dehumidification (drying) of air are required in many commercial and industrial applications for the following reasons:

• The control of air moisture content within the occupied space to ensure the well-being of human, animal, or plant life

• The control of air moisture content within a space for process control or to protect products in store

Table 9.4 indicates some of the factors that have to be considered when dealing with the moisture content in air. In the case of inert gases used in vari­ous industrial processes, other factors have to be considered.

The process of air humidification is achieved by means of a device called a humidifier.

1.3.1.17 Humidifier Types

Many different types of humidifiers are in common use; the humidification process is simply achieved by adding moisture into the air to be conditioned.

Humidification and Dehumidification

Factors

High humidity

Low humidity

111 health and associated

Encourages the growth of

Lowers the body’s resistance to

Allergies

Spores and fungi.

Infection and respiratory disease.

Electrostatic shocks

Eliminated at high moisture content.

Increased at low moisture content, causing discomfort to occupants and damage to electronic compo­nents. High explosion risk.

Eye conditions

Possible bacteria buildup and health effects.

A high rate of evaporation of the eyeball oil film results in dry, itchy eyes with an increase m eve damage in dusty environments,

Respiratory tract (nose and throat) problems

Increase in bacterial infection.

Resistance to throat, nose, and lung infection is increased, due to the breakdown of the protective mechanisms in the body.

Thermal stress

Reduction in evaporative body cooling. In hot industries, this results in body overheating.

Increase in body cooling with an increase in air movement.

Skin effects

Bacteria growth in skinfolds. Clothing type has to be con­sidered.

As moisture is required to keep the skin supple, low humidity causes cracking of the skin, increasing the risk of chemicals and bacteria entering the body

Materials

Pollution

Rusting of steel, breakdown of timber and fabrics. Damage of stored goods by insects and mold growth.

Rapid drying out of paper, timber, and fabrics. This is an important factor for storing antiques.

The dust concentration is higher in a dry atmosphere, causing health and discomfort problems. Odors, irritant gases, and vapors are more noticeable at low humidiry.

Vegetation effects

Various diseases, depending on produce.

Various diseases, depending on produce.

The selection of a particular method depends on the size of the installation, availability of water or steam, the degree, of control required, and the methods used for conditioning and distributing the air.

Humidification is achieved by one of the following methods:

1. By producing a fine mist or spray

2. By evaporation from a pad of absorbent material within the airsrream

3. By vaporizing water

4. By steam injection

Figure 9.14 shows the processes of humidification and dehumidification in a skeleton psychrometric chart, while Table 9.5 explains the cycle.

Note: The actual final air conditions given in Fig. 9.14 and Table 9.5 must always be related to the slope of the line, as in some cases the specified condi­tions may not be met.

Humidifiers Complete with Water Storage

Humidification and Dehumidification

-> 3

подпись: -> 3In this type of humidifier, the water required for the humidification pro­cess is stored within the unit and is normally fed automatically from the water mains. However, smaller portable units do not have this automatic facility and have to be manually filled with water as required.

Moisture content, kg kg*1 dry air

Dry-bulb temperature, ”C FIGURE 9.14 Humidification and dehumidification processes.

State line: Center point to

Process

Final air conditions

Equipment used

1

Humidification

Only

Increase in moisture content, specific volume, dry and wet bulb temperature, spe­cific enthalpy, and % satu­ration

Air washer with beared water

9

Heating and humidification

Increase in moisture content, specific volume, dry — and vvet-bulb temperature, and specific enthalpy: decreased % saturation

Steam humidifier or recircu­lated hor-water spray

3

Sensible heating only

Constant moisture content; increase in specific volume, dry — and wet-bulb tempera­ture, and specific enthalpy; decreased % saturation

Steam, hot-water coils, or electric heating

4

Dehumidification and heating

Decreased moisture content: increase in specific volume and dry — and wet-bulb tem­perature; decreased specific enthalpy and % saturation

Chemical dehumidification

5

Dehumidification

Only

Decreased moisture content and specific volume; con­stant dry-bulb temperature: decreased wet-bulb tempera­ture, specific enthalpy, and % saturation

Not practical

6

Cooling and dehumidifying

Decreased moisture content and specific volume, dry — and wet-bulb temperature, specific enthalpy, and % sat­uration

Chilled-water washer

7

Sensible cooling only

Constant moisture content; decreased specific volume, dry — and wet-bulb tempera­ture, and specific enthalpy; increased % saturation. When point 7 reaches the 100% saturation line, the air is saturated; further cooling will result in moisture being removed from the air.

Cooling coil and washer at dew point

8

Evaporative cool­ing only

Increased moisture content and specific volume; decreased dry — and wet-bulb tempera­ture; increased specific enthalpy and % saturation

Washer

To prevent contamination of the water mains by back-siphonage, the wa­ter inlet to the appliance should incorporate either

• an air gap,

• an approved type of pipe interrupter, or

• a combined check and antivacuum valve.

The national and local water bylaws and codes should be consulted to ensure that the device fitted is of an approved type.

Spray-Type Humidifier

Water is pumped from a storage tank located at the base of the unit to one or more spray nozzles that inject a fine water spray into the airstream. In large units, the air movement through the ductwork system ensures efficient mixing of the moisture and the air. Units used in small enclosures or rooms use a fan with the pump on the same shaft.

Pan-Type Humidifier

In this type, an absorbent material is partially immersed in a water pan store at the base of the unit. The evaporation of the water into the airstream takes place from the wetted surface of the absorbent material.

Mechanical Pan

In this case, an absorbent material is fixed to a drum or disk. Part of this drum is immersed in the water. When the drum is rotated in the water pan, it absorbs moisture, which is then transferred into the airstream by evaporation.

Steam-Generating Pan

This type consists of an enclosed water tank, which is connected to the main air duct.

High-temperature water coils or direct-fired gas or electrical elements heat the water in the tank. The tank water vaporizes, and the moisture is entrained into the airstream as it passes over the tank.

Humidifiers without Water Storage

Water need not be stored within the unit; this is often the case where the equipment used for humidification forms part of a central station plant or where a suitable source of steam is available.

Spinning-Disk Humidifier

In this case, a controlled quantity of water is discharged against a rapidly rotating disk where centrifugal force spins the droplets radially against cir­cumferentially placed baffles, producing a fine water mist which is discharged into the airstream.

Steam Jet

Steam generated from an external source positioned close to the unit is in­jected into the airstream. It is essential that the steam supply is uncontami­nated and odorless. In order to stop scale buildup, it is essential that some means of water treatment be provided.

Full use can be made of ultrasonic frequencies in producing a fine, atom­ized jet, which is liberated into the airstream.

Air Washer

A device called an air washer is also used for humidification and dehu — midification. It consists of a chamber incorporating a water spray system, a collection tank, and an eliminator section. The eliminator plates are necessary to reduce the incidence of water droplets that are carried out of the plant into the duct run.

The air, in passing through the chamber at velocities in the range 1.5 to

3.5 m s-1, comes into intimate contact with the water, and depending on the conditions required, mass transfer of moisture into the airstream occurs. This transfer produces either addition or removal of moisture; hot or chilled water is used in this process.

The spray bank consists of a series of standpipes with nozzles connected to a horizontal header. The nozzles are arranged to ensure that the spray gives good coverage of the spray chamber without causing any interference with the adjacent nozzles. The pressure through these nozzles is normally between 140 kPa and 280 kPa.

The spray water requirements vary from 0.3 L s_1 for a single bank to

K. 8 L s_1 per 0.5 m3.

Depending on spray pressure and the strainer, pipeline, and valve resis­tance, the pumping head is normally in the 16 to 24 m range.

By having more than one spray bank discharging into the direction of air­flow, the mixing efficiency is improved. The nozzles provide water atomiza­tion by means of forcing the water under pressure through a small orifice, producing a wide-angle rotating spray.

In the past, air washers were used for air cleaning; today, however, more efficient methods are available for the cleaning process.

The continuous recirculation and spraying results in dirty water building up in the sump. In order to reduce the incidence of infection to occupants and fouling of the nozzle, water treatment with biocides and softening of the water supply are required. The sump is complete with strainers in a position which allows easy access for cleaning. See Fig. 9.15.

Another type of air washer is the capillary air washer; see Fig. 9.16. It consists of many thousands of small glass or plastic filaments (cells) providing a large, wetted surface area. These are placed to allow a parallel path of air through them. The surface of each of these strands is covered with the water that is discharged from sprays.

1.3.1.18 Selection Factors

Humidification can be achieved by placing separate humidifiers directly in the conditioned space. In order to maintain the design conditions, however, humidity control should be incorporated into the system. When positioning humidifiers, care has to be taken to ensure that the leaving moisture does not impinge on adjacent surfaces, forming lime or algae deposits.

Humidification and Dehumidification

Regardless of the type of unit used, full consideration should be given to provide a suitable method of water treatment on the following counts:

• To prevent solids from being carried over into the airstream

• To stop deposits from fouling the unit and the associated heat exchanger

• To stop a fine dust of the salts from being discharged into the conditioned space. This is particularly important in the case of spinning-disk or spray-type humidifiers.

For all humidifiers, full consideration should be given to the possibility of the growth of fungi, algae, bacteria, and in particular Legionella pneumo­phila, the microbiological contamination that causes humidifier fever, etc.

Dehumidification

The reverse of the humidification process is that of dehumidification. In this process the water content of air, gases, or fluids is reduced.

In many industrial applications, the moisture extraction takes place at at­mospheric pressure; however, certain applications may require a reduction in the atmospheric pressure in order to achieve the maximum efficiency.

The process may be required for the following reasons:

1. Reducing the moisture content of a gas to aid the manufacturing and handling of hygroscopic materials

2. For comfort or process air conditioning in combination with cooling to reduce the latent load

3. Providing protective atmospheres to reduce the oxidization of metals

4. Controlling set humidity conditions in warehouses

Dehumidification can be achieved by one or more the following methods:

1. Compression

2. Refrigeration

3. Liquid sorption

4. Solid sorption

5. A combination of the above

Figure 9.14 and Table 9,5 cover the dehumidification processes.

Compression

If a gas is compressed, its absolute moisture content is reduced, generally resulting in a saturated gas at the elevated pressure. Due to the high cost of providing compression, it is used on its own only in limited applications. It is, however, used as the first stage with one of the other three methods. If a high — pressure gas is allowed to expand, the increase in volume and reduction in pressure result in a lower dew point.

Refrigeration

By allowing moisture-laden, relatively warm gas to come into intimate contact with a cold surface which is below the dew point of the gas, moisture is condensed from the gas.

The term refrigeration refers to the gas coming into contact with evapora­tor coils on a dx vapor-compression cycle, coils on an absorption cycle, vortex

Cube, thermoelectric cycle, chilled water coils, or even water mains passing through coils.

The required refrigeration capacity is <t>r = t7m(6] — hi) kW.

Chemical Dehumidiflcation

Certain chemicals (sorbents) have the ability to absorb moisture from a gas; they may be either solid or liquid. Performance of a chemical dehumidifi — cation device depends on the sorbent used. The sorbent must be able to attract and remove the sorbate, such as water, from the gas stream. Sorbents absorb water on the surface of the material by adsorption or by chemically combining with water (absorption). If the unit is regenerative, the process is reversible, al­lowing water to be removed. This is achieved by a sorbent such as silica gel, alumina gel, activated alumina, lithium chloride salt, lithium chloride solution, glycol solution, or molecular sieves. In the case of nonregenerative equipment, hygroscopic salts such as calcium chloride, urea, or sodium chloride are used.

An absorbent material is one which changes either chemically, physically, or both during the sorption process. Certain chemicals, in absorbing moisture during this process, will dissolve into the water from the initial crystalline structure. Further added water results in a phase change from solid to liquid. An adsorbent is another material in which there are no chemical, phase, or physical changes during the sorption process.

Liquid Sorption. If a moist gas is passed through sprays of a liquid sor­bent, such as lithium chloride or an ethylene glycol solution, moisture is re­moved from the air at a rate depending on the vapor pressure difference. This is a function of the absorbent concentration and is maintained at the required level by a regeneration cycle. The regeneration process is continuous and is achieved by allowing a percentage of the chemical into the exhaust-heated air.

If the vapor pressure of the sorbent in its active state is below that of the gas being dehumidified, moisture is absorbed from the gas stream. As the process con­tinues, the sorbent becomes diluted due to the moisture increase. See Fig. 9.17.

Air to waste

A

подпись: air to waste
a
Dehumidified air

A

Spray header

L

Humidification and Dehumidification

Ductwork

 

—— Ductwork

 

Heating coil

 

Humidification and Dehumidification

R

 

0 ~~ .I

 

T’—-—. Strong liquid

 

Weak

Humidification and Dehumidification

 

Sorbent tank

 

Humidification and Dehumidification Humidification and Dehumidification Humidification and Dehumidification Humidification and Dehumidification

Humidification and Dehumidification

Humidification and Dehumidification

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