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Relative Humidity control by desiccant dehumidifier, 040607

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  • Relative Humidity Control


    Desiccant Dehumidifier


    Prof. Moustafa M. Elsayed

    (consultant)[email protected]

  • Dehumidification by Cooling

    Chilled water

    Chilled brine

    Direct expansion (DX)

    Sensible and latent loads

  • Cooling dehumidifier (with Reheat)

    Functional principle


    Dry airair

    Condensed water

    CondenserAir heater

    Expansion valveWater side

    Cooling compressorGas side

    EvaporatorAir cooler

  • Dehumidification By Desiccant

  • Dehumidification by Dessicant

    Solid desiccant: silica gel, activated carbon, synthetic polymer, etc

    Liquid desiccant: triethylene glycol, lithium chloride solution, calcium chloride solution, etc

    Desiccant material

    Examples of desiccant material

    Adsorption: collect water as a sponge, example silica gel

    Absorption: collection of water causes a chemical or physical change in the desiccant, example liquid desiccants

  • Characteristics of desiccant materials

    Dehumidification by Dessicant

    Low surface vapor pressure when dry

    High moisture capacity (upto 10000 percent of dry weight)

    Low mass (low heat capacity)

  • Dehumidification by Dessicant

  • Methods of dehumidification











    5 g/kg 15 g/kg

    tdry =

    t wet



    Dew-point temp.

    Condenser Evaporator

    Evaporating temp.

    Cooling versus Sorption

  • Cooling and desiccant-based dehumidification systems are most economical when used together

    The difference in the cost of electrical power and thermal energy will determine the optimum mix of desiccant to cooling-based dehumidification in a given situation

    Cooling-based dehumidification systems are more economical than desiccants at high air temperatures and moisture levels

    Desiccants are especially efficient when drying air to create low relative humidities, and cooling-based dehumidification is very efficient when drying air tosaturated air conditions

    Cooling Versus Desiccant

  • Expense Comparison

    Condensation versus Sorption











    5 g/kg 15 g/kg

    t dry =Investment-expenses and

    capacities are comparable!

    Lower running costs

    for condensation.


    t vt

  • 1 - 2: sorption

    2 - 3: desorption, heating

    of desiccant

    3 - 1: cooling of desiccant

    Desiccant Cycle

  • Temperature rise during dehumidification


    Temperature rise is directly proportional to the

    amount of moisture removed from the air

    The drier the air leaves the humidifier the

    warmer it will be (Constant enthalpy


    Comparison with dehumidification by cooling


    Desiccant Cycle

  • Desiccant Dehumidification System Alternatives

    100 % Recirculation

  • X % Outdoor Air without Cooling

    Desiccant Dehumidification System Alternatives

  • X % Outdoor Air with Pre-Cooling

    Desiccant Dehumidification System Alternatives

  • X % Outdoor with After Cooling

    Desiccant Dehumidification System Alternatives

  • X % Outdoor with After Cooling & Pre-Cooling

    Desiccant Dehumidification System Alternatives

  • Methods of Desiccant Dehumidification

  • Methods of Desiccant Dehumidification

    Pre-cooling + dehumidification:AC+CB

    Pre-cooling + dehumidification + after cooling: AC+CE+EB

    Dehumidification + after cooling:AD+DE

  • Applications

    Removal of water vapor

    Removal of volatile organic compound (VOC) molecules

    Preventing the growth of mold, mildew, and bacteria by keeping the building dry

    Applications (where airborn microorganisms can cause costly problems)


    Medical Facilities


    Biomedical manufacturing facilities

  • Food & Confectionery

  • Pharmaceutical

  • Defense Systems

  • Condensation: A surface will not have condensation on it if the air in contact with it has a dewpoint lower

    than the surface temperature.

    X < C dp X >C dp

  • X < C dpX >C dp

    Ice formation: A surface will not have ice formation on it if the air in contact with it has a dewpoint lower

    than the surface temperature.

  • < 50%RH

    Bacteria: Bacteria needs humidity to survive and multiply.

    Most bacteria will not find a suitable environment if the

    Relative humidity is kept below 50%RH.

    > 50%RH

  • Dehumidifying Heating

    Drying out buildings: Heating will only move the moisture to another part of the building.

    With sorption dehumidifying , the moisture is removed.

  • Corrosion: Iron and steel doesnt rust if the air over the surface has a relative humidity below 50%RH.

    < 50%RH > 50%RH

  • Handling of hygroscopic material:The quality of dry drugs, dry food, hard candy and

    other hygroscopic material can only be maintained if

    the relative humidity is kept below a certain level.

    > 50%RH < 50%RH

  • Electronics: The characteristics of electronic products are

    changed at a high relative humidity.

    < 50%RH > 50%RH

  • Mould: Mould and fungus formation is prevented if the surrounding air is kept below 70%RH.

    < 70%RH > 70%RH

  • Odours: Bad smell will be drastically reduced if the relative humidity is kept below 50%RH.

    < 50%RH > 50%RH

  • No dehumidifyingDehumidifying

    Product drying: When drying products a low relative humidity is essential for a fast process.

  • Freezers problems with moisture

    Frost build up on evaporators

    Short periods between stops for defrosting

    Conveyors can lock up by ice

    Increasing temperature due to ice built up

    Snow or frost on the product

  • Freezer solution with overpressure

  • Freezer alternative solution with dry air to the airlocks

  • Merits of honeycomb structure

    Extremely low pressure loss

    There are many small openings in the section of honeycomb. Therefore, air

    can pass through very smoothly without getting excess load and, thus,

    unnecessary energy does not consume.

    Extremely large surface area

    With the special structure of laminated layers consisting of flat & corrugated

    materials, the surface area to be directly contacted with the air is greatly

    enlarged. Therefore, the capacity can be enhanced up to the maximum level

    with the minimum space required.

    Light, yet strong

    The combined structure with the corrugated material being inserted between

    the flat materials is very light but physically very strong and highly durable.


  • Moisture ratio

    H2O Dry material

    The weight of H2O

    The weight of dry material

    1 kg 10 kg

    = Moisture ratio

    ex. 1/10=0,1 i.e. 10% Moisture ratio

  • Moisture loads




    Moisture load,

    water surfaces

    Moisture load

    Moisture load

    from people


    Moisture load


    1 Unintended ventilation

    2 Intentional ventilation

    2 water surfaces

    4 Moisture load from people

    5 Diffusion from material

    6 Moisture load from material

    7 Moisture load combustion

  • Selecting Design Conditions

    Outside conditions, set 1: conditions immediately

    surrounding the controlled space

    Outside conditions, set 2: conditions of fresh air for

    ventilation of the controlled space

    Note: dew point temperature is more important than

    dbt & wbt for moisture load

  • Relative humidity for storage and production

    Type of Product RH Type of Product RH

    Sugar Storage 20-35% Laboratory electronics 45-60%

    Breweries 35-45% Plastic Pallets 5-30%

    Coffee Powder 30-40% Computer peripherals 50-60%

    Milk Powder Storage 20-35% Rust Resistance Below 40%

    Seed Storage 35-45% Medical Syrups 30-40%

    Unpacked Medicine 20-35% Capsule Storage 30-45%

    Anti-Fungus 45-55% Powder Storage 30-45%

    Camera Lenses 40-55% Wood Drying 25-35%

    HT Switch Room 45-55% Explosive *35-50%

    Transformer winding 15-30% Hospital Electronics 45-65%

    Semiconductors 30-50% Normal Storage 50-55%

    Archive 40-55% Musical Instrument 45-55%

    Paper Storage 35-45% Leather Product 40-55%

    Rust Preventionbelow 55%,

    >40% for zero rustCable Wrapping 15-25%

    Library 50-55% Chemical Lab 30-45%

    Spray Paint 30-50% Harddisk Production 40-50%

    Lithium Battery Below 2 % Magnetic Tapes 40-55%

    Note: * Explosive storage must not be less than 30% RH as static electricity may build up and

    cause sparks in the air to cause an explosion.

  • Using

    Design Of Dehumidification System


  • Introduction to How Much 4

    Room calculation:On this page you can determine moisture load and what dry air condition the dehumidifier has to perform.

    The page is divided in two parts. The first part is to sum

    up the different moisture loads. After that it make a

    approximate choice of a dehumidifier and type of installation .

    The final answer is proc

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