relative humidity control by desiccant dehumidifier, 040607

Relative Humidity Control

Using

Desiccant Dehumidifier

By

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

Inge-95

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


Low surface vapor pressure when dry

High moisture capacity (upto 10000 percent of dry weight)

Low mass (low heat capacity)

Methods of dehumidification

Inge-95

40°C

30°C

20°C

10°C

0°C

-10°C

30%

50%70%

100%

5 g/kg 15 g/kg

tdry =

t wet

Sorption

Cooling

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

40°C

30°C

20°C

10°C

0°C

-10°C

30%

50%

70%

100%

5 g/kg 15 g/kg

t dry =Investment-expenses and

capacities are comparable!

Lower running costs

for condensation.

Inge-95

t våt

1 - 2: sorption

2 - 3: desorption, heating

of desiccant

3 - 1: cooling of desiccant

Desiccant Cycle

Temperature rise during dehumidification

process

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

process)

Comparison with dehumidification by cooling

coil

Desiccant Cycle

Desiccant Dehumidification System Alternatives

100 % Recirculation

X % Outdoor Air without Cooling


X % Outdoor Air with Pre-Cooling


X % Outdoor with After Cooling


X % Outdoor with After Cooling & Pre-Cooling


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)

Hospitals

Medical Facilities

Pharmaceutical

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 < ºC dp X >ºC dp

X < ºC dpX >ºC 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.

ϕϕϕϕ < 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.

ϕϕϕϕ > 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 doesn’t rust if the air over

the surface has a relative humidity below 50%RH.

ϕϕϕϕ < 50%RH ϕϕϕϕ > 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.

ϕϕϕϕ > 50%RHϕϕϕϕ < 50%RH

Electronics: The characteristics of electronic products are

changed at a high relative humidity.


Mould: Mould and fungus formation is prevented if

the surrounding air is kept below 70%RH.

ϕϕϕϕ < 70%RHϕϕϕϕ > 70%RH

Odours: Bad smell will be drastically reduced if the

relative humidity is kept below 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.

HONEYSAVEHONEYSAVE

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

Inge-95

unintentional

Intentional

Moisture load,

water surfaces

Moisture load

Moisture load

from people

Diffusion

Moisture load

Combustion

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

HOW MUCH 4 PROGRAM

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 process air inlet conditions to the unit

and what moisture content the dehumidifier has to have

in the dry air to meet the moisture loads given

Offer data room calculation:This page is to determine running cost, to present a pay-off time and a present value of the investment .

Climate data is automatically taken from "Room calculation" which has to be filled in to use this page.

You can compare different solutions, when comparing

different solutions. When doing such a comparison the

most important figure is the present value of cost, which

is summing up investment and running cost

Air mixture calculation:This page offers an overview of an installation with a dehumidifier, it is not linked to any other page.

Follow the step by step instructions on the page

Air mixture calculation by-pass:Same as previous page but with the dehumidifier in a by-

pass airflow

Air mix ambient by-pass:Same as previous page but with the inlet from ambient

DR air mixture calculation:Same as "Air mixture calculation" page but with a

dehumidifier with a common inlet for both process and

regeneration air

DR air mixture calc. by-pass:Same as "Air mixture calculation by-pass" page but with

a dehumidifier with a common inlet for both process and

regeneration air

Review of Program

Online Access

Example

Calculate the dehumidification capacity of a

Room with the following data:

• Room Design Conditions (20 ºCDB& 20% RH)

• Ambient Design Conditions (41 ºCDB& 32% RH)

• Room Dimensions (13.5m length* 7.7 m width * 5.4 m

height)

• Concrete Building Type.

• Surrounded from 2-sides

• Air Lock Dimensions(2.2 m Length * 3 m width* 2.5 m

Height ) with 4 min/hour openings.

• Wind Speed 8 m/s.

• 16-persons work light in the test room.

Program Execution

Online Access

relative humidity control by desiccant dehumidifier, 040607

Engineering