dehumidiﬁcation - humidity · pdf file10 dehumidiﬁcation and controlling units/systems...

Basic Technical Information

DehumidificationControlling ambient humidity

CONTENTS

01 Air and its thermo-hygrometric parameters

05 Why is humidity important?

06 Humidity loads in systems

10 Dehumidification and controlling units/systems

DehumidificationBasic Technical Information

1Fisair Controlling ambient humidity

Ambient air contains a range of different gases in solution as well as particulate matter. These include oxygen, nitrogen, etc. Another of the gases is wa -ter vapour. The study of the thermal characteristics of air is called psychrometrics, and uses a Mollier diagram, also known as the ix or psychrometric chart. This chart quantifies and measures the work required to change these characteristics (Fig. 1). Its variables are defined below:

DRY BULB TEMPERATURE: This is shown on the horizontal axis and measured using a standard ther -mometer. It indicates the sensible heat transported by the air. (Fig. 2)

AIR and its thermo-hygrometric parameters1.

% 00

.

1=

X

t

Fig. 2: Sensible heating process.

Fig. 1

PSYCHROMETRIC CHARTNORMAL TEMPERATURESI Units SEA LEVELBarometric pressure: 101.325 KPa

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DRY BULB TEMPERATURE - °C

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Chart by: HANDS DOWN SOFTWARE, www.handsdownso

1.000.95

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0.350.300.250.20 SENSIBLE HEAT RATIO = Qs / Qt

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Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com

Dehumidification Basic technical information 2

RELATIVE HUMIDITY: Expressed as the percent -age humidity content of the mass of air in compari -son with the maximum quantity it could contain at the dry bulb temperature in question. The curves represented are the humidity percentages the air could hold at the given temperature. The top curve represents 100% (maximum content) and is referred as the saturation line. If this line is exceeded, the air cannot hold anymore water vapour so it changes phase into its liquid state; it condenses.

The general public is familiar with this variable and it is helpful in terms of communication, but must always be accompanied by the temperature, be -cause its expression is the relationship between these two values. (Fig. 3)

Most sensors work with this variable. It is formally defined as the relationship between the vapour pressure of the air in specific conditions (at a given temperature) and the vapour pressure of saturated air at the temperature in question.

SPECIFIC HUMIDITY: Total mass of water vapour contained in one kilogram of dry air in the given con -ditions. It is expressed as kg/kg or in gm/kg. (Fig. 4)

X (g/kg) = Weight of water vapour (g)/(Weight of air – Weight of water vapour) (kg). (Fig. 4)

ABSOLUTE HUMIDITY: The same concept but in the humid air. Its units are gm/m3 or kg/m 3.

DEW POINT TEMPERATURE: Temperature at which the humidity content in the mass of air starts to con -dense. On cooling –when the dry bulb temperature falls- the capacity to retain water vapour is reduced, until it reaches a RH of 100%. This is not reflected di -rectly in the ix; it is just necessary to go to the satura -tion line and move vertically downwards to the DBT line. It depends on the pressure. (Fig. 5)

This is essential to understanding the problems caused by humidity as a result of the cooling of the system and building (envelope). It clearly defines the humidity content in absolute terms, and is there -fore employed to control humidity contractually so as to avoid interference from the temperature. At a given pressure, a DPT is equivalent to a given ab -solute humidity.

.

.

.

.

% 001=

X

t

Fig. 3: Constant relative humidity lines.

t

% 001=

X

Fig. 4: Isothermal humidification process.

% 001=

X

t

Fig. 5: Process to obtain the dew point temperature of the air.

Air and its thermo-hygrometric parameters


When forecasting times, it is used to obtain the mini -mum temperatures that can be endured because when water vapour condenses it releases latent heat which keeps it very close to this value without descending.

WET BULB TEMPERATURE: Temperature at which evaporation occurs. It is measured using a wet thermometer (with muslin for example) and a draft of air moving across it at a speed of over 2.5m/s. When the water evaporates off the surface of the bulb, the temperature descends because the air absorbs the sensible heat required to turn the water into vapour. When the temperature stabilizes, we measure it to obtain the WBT. The constant WBT lines are diagonal. At the saturation line no more wa -ter can evaporate: WBT=DBT. Our bodies use this evaporative cooling system to regulate our body temperature through the evaporation of the sweat we generate. In the past, this was employed widely in the air conditioning industry since the sensors are relatively simple. Nowadays, because the price of relative humidity sensors has fallen, these are used more. (Fig. 6)

VAPOUR PRESSURE: Every particle in a gas ex -erts a pressure the sum of which is the pressure of the gas. Every molecule of water exerts a pres -sure and that is why the (partial) pressure of water vapour is directly related to the humidity content of the air. Very humid air has a very high vapour pres -sure, and vice versa. It is represented in a vertical column on the right of the chart and parallel to the absolute humidity column. It is the variable that has the greatest impact on the spread of water vapour in materials. It can for example cause humid wood, saturated in humidity content, to crack on the side in contact with dry air as a result of the difference in pressures in the material. It is measured in units of pressure (SI=PA). (Fig. 7)

ENTHALPY: This quantifies the total heat content of the air sample. It is the result of the sum of the sensible heat (measured by dry bulb thermometer) and the latent heat (required by the water contained in the air to have evaporated). The isenthalpic lines on the ix are almost parallel to those for the constant wet bulb temperature. (Fig. 8)

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.

.

% 001=

X

t

Fig. 6: Wet bulb temperature.

15ºC

% 001=

X

t

Fig. 8: Increase in the enthalpy of the air.

% 001=

X

Fig.7: Increase in vapour pressure.


There are several different versions of the psychrometric chart:

• Depending on the school (American – Carrier or European – Mollier) the layout of the lines changes. The DBT on the European chart is vertical and rising.

• Depending on the units employed: British/American system or international system.

• The chart changes according to height above sea level. As the height increases, the air becomes less heavy or less dense. That is why in high mountain areas climates tend to be drier because in general less humidity content is possible in the air. The lower the atmospheric pressure, the lower the compression capacity of the mix.

COMMENTS

American school European school

At sea level High above sea level

Air and its thermo-hygrometric parameters

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25 TEMPERATURA DE BULBO HÚMEDO - °C

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Our perception of the climatic conditions in an en -closed area depends on two main factors: tempera -ture and humidity. Human beings are especially sensitive to temperature, and to a much lesser ex -tent relative humidity.

The opposite could be said for almost all inanimate objects. In other words, relative humidity is in this case much more important than temperature.

Relative humidity plays a very important role for instance, in the storage of raw materials, semi-fin -ished goods and end products. Temperatures can vary without limit on one condition: the relative hu -midity is always kept steady at around 50%. In this way the effects of corrosion are kept at bay, and the energy required to keep humidity under control is very small.

Why is HUMIDITY important?2.

We could cite a long list of examples demonstrating the importance of maintaining suitable relative hu -midity levels. We are however sure the explanations already given have made one thing clear: relative humidity is the main climatic factor impacting on al -most all inanimate objects.

Dehumidifiers sold to clients in industrial and mili -tary sectors in different countries the world over, have clearly demonstrated the importance of main -taining suitable humidity levels in storage and pro -duction centres.

Values for relative humidity and temperature ap -plicable to a wide range of products and industrial processes are given on pages 14 and 15.


The concept of humidity control is different from simply maintaining moderate humidity levels. If the owner of a building tells the technicians installing the system to achieve a comfortable ambient hu -midity, this means it may vary within broad margins throughout the day, generally from 15 to 70% RH. Ambient humidity control means this cannot vary beyond strict margins. This is more expensive and involves a more in-depth study of the factors caus -ing variations. There are factors which the designer of the system is not a specialist in, such as those related to the materials used in the construction

HUMIDITY LOADS in systems3.

and their location. A joint project is therefore rec -ommended for humidity control drawn up between the owner, architect, builder, HVAC system designer and units/controls supplier.

Some of the main factors giving rise to variations in the humidity content of controlled environments are as follows, in a general order of importance:


VENTILATION: Exterior air may enter controlled areas for a number of reasons:

• At the demand of product designers. There are also the regulatory demands applicable to the type of activity undertaken in the space in question, and dependent on the number of people. The pollutants the occupants generate from their breath and internal sources are di -luted by a particular volume of fresh air..

• As a replacement for air that has to be extract -ed. This is known as “make-up air”. The sum of all the extraction must be carefully measured, to ensure the necessary inflow is provided so the pressure does not fall in the area involun -tarily sucking in untreated air. The infiltration of humid air in refrigerated zones can cause dam -age to the building from the potential conden -sation caused by contact with surface areas at dew point.

Once the flow of exterior air has been determined, the supply can be calculated in kg/h of flow as fol -lows:

Wv(kg/h)=Q (m3/h) x d (kg/m3) x (Xe – Xi) (kg/kg)

This is nearly always the largest entry of humidity into the system at wet times of year.

HUMIDITY EMITTED BY THE OCCUPANTS: People emit water vapour when they breathe and through the evaporation of sweat. The clothes we wear adsorb humidity in humid environments and emit this in the controlled environment

a) Respiration and transpiration: a) Every time we breathe out is one volume of our lungs at 37ºC and 40.4gm/kg (almost saturated). The quantity inputted by this means therefore de -

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pends on the number of people present and their activity related to the number of breaths taken. (Fig. 9)

Wrp= (n1 x rp1) + (n2 x rp2) +...

Where rpi is the factor quantified in tables accord -ing to the activity. Normally for non-arduous activi -ties related to modern industry, 100gm/h per person is expected.

b) Clothing: If during wet times of year or after rain the same clothing is brought into the con -trolled environment, the clothing will then emit a quantity of water vapour into this environ -ment in accordance with the duration of the visit, the type of clothing, and the humidity in the area in question.

Wr= n x t/60 x (Rr1-Rr2)/2 where t is the visit time and Rri the kg/h of water emitted by the clothing.

INFILTRATION: Also known as involuntary venti -lation. This is the quantity of air entering the sys -tem without having been controlled by the piping. It comes from exterior or treated adjoining areas. It will depend on the quality of the construction of the building, the different pressures in different ar -eas and the air-tightness of the pipes (especially their longitudinal and transverse joins). In well in -sulated spaces this will amount to 0.33 vol/h up to 2 changes/h of the volume of the spaces with a higher tendency towards infiltration.

Having determined involuntary flow through the practical experience of the designer or by making tests, such as trace gas tests or blower door tests, and in full knowledge of the climatic conditions on the exterior, we arrive at:

Wi(kg/h) = Q (m3/h) x d (kg/m3) x (Xe-Xi) (kg/kg)


Humidity loads in systems

OPENING DOORS: The entry of humidity depends on the regularity with which doors are opened (as described by the owner), their size, the air speed across the surface, the density of the air, the time the door is open, and the difference between abso -lute humidity in each space: Wp = N (Aperturas/h) x A (m2) x V (m/s) x d (kg/m3)x t/120 (s) x {X1-X2} (kg/kg)

There a number of ways of reducing humidity loads entering through doors:

- Quick and automatic closing mechanisms.

- A double door system, where the humidity in the small intermediate space is controlled, and the second door cannot be opened until air in the dividing room is dried using a small dehumidi -fier.

- By using air curtains –where their effectiveness depends on the movement of the air (mass x speed)- or the strips of plastic typically found in warehouses.

- With overpressure in the room: This must be measured using specific calculations, the force of the exterior wind is nearly always greater than the normal overpressures.

WET MATERIALS AND PRODUCTS: Materials used for the packaging of products must be taken into consideration because they are made mainly of wood and cardboard, hygroscopic materials that absorb humidity in uncontrolled environments and emit vapour in dry areas with low vapour pressure. To measure this effect, the materials used must be tested, and the regularity of entries and environ -mental conditions must be explained by the owner.

The humidity given off by walls, construction mate -rials, furnishings, papers and books must be thor -oughly studied. riales de construcción, mobiliario, papeles y libros.

WET SURFACES: The evaporation of water from the surfaces of tanks, puddles or swimming pools increases in the following circumstances:

- Quick air flow over the surface.

- Hot water, because the vapour pressure of the surface is greater.

- Dry exterior environment, because this reduces its vapour pressure.

- A larger surface area.

This has been studied in detail as follows: Wsm = 0,1 x A (m2) x (pw - pa) x Fa

Where: A: Value of the surface under study.

pw: Saturation vapour pressure at the water temperature.

pa: Vapour pressure of the air at its tem -perature.

Fa: Activity coefficient. This varies from competition swimming pools, to private pools, to much lower values for tanks and ponds.

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.Evaporation resulting from washing floors is also considered as the volume of water used, over the time it takes to dry. Other types of cleaning are ob -served in the same way.

DISPERSION OF VAPOUR: The dispersion of wa -ter vapour molecules through solid materials is main -ly the result of the difference in vapour pressures on both sides and the porosity of the materials. That is why bread kept in a plastic bag takes more time to dry than in a paper bag. The permeability of every material and the thickness used, provides us with an idea of the loads to be taken into account,

We should only take the least porous material in the wall (the air passes through) into account.

Dispersion accounts for a load from 10 to 100 times less important than infiltration.

As explained above, the really important loads are caused by ventilation, and experience is the key to understanding the others in the order of importance given. When making their first rough calculations, some designers assume a 10% or 30% value for the ventilation load, dependent on the final inspection made of the installation.


From the point of view of the units, dehumidification is simple. It is just a question of injecting in enough dry air, which acts like a sponge to absorb the hu -midity in the environment.

It is unusual to find dehumidifying units (solely for this purpose) on commercial premises because hu -midity control is not required, just comfortable hu -midity levels. To this end, air handlers are employed to alleviate the humidity content. This is however problematic because they use sensible tempera -ture signals. To control humidity, units specifically designed for this purpose are needed. The follow -ing options are available:

A) CONDENSATION OR MECHANICAL DEHUMIDIFIERS:

Also known as swimming pool dehumidifiers be -cause this is their main application (Fig. 9).

They are widely used in commercial buildings. They make use of the cooling principle. Air is taken in be -low the dew point and reheated using the conden -sation coil. They are more efficient from an energy point of view, but can only be used in applications in which the dew point of the air cannot cause the water condensed by the evaporator coil to freeze. In buildings they are normally designed to fit the specific needs of projects. Let’s take a look in more detail (Fig. 10):

Air is cooled, first the dry bulb temperature falls, and then its relative humidity is progressively in -creased. When this reaches 100%, the saturation line, the air can no longer retain its original humid -ity content and precipitates out off the mix and condenses on the flaps and tubes of the evapo -rator coil. During condensation, the water vapour frees the heat absorbed during evaporation (latent heat). This heat is absorbed by the coolant gas in -side the evaporator. The gas is then compressed and sent hot and at high pressure to the conden -sation coil where heat is transferred to the cold air causing the gas to condense (latent for the gas, and sensible for the air). The result is a flow of air at approximately the same enthalpy (plus the heat provided by the work of the compressor, depen -dent on the efficiency of the compressor), with a lower humidity and higher DBT.

Types of DEHUMIDIFICATION . Controlling units and systems.4.

Fig. 9: Condensation dehumidification.

4

21

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8 7

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C1. Process air2. Dry air3. Gaseous coolant4. Liquid coolant5. Condensation coil6. Evaporator coil7. Compressor8. Condensed liquid output

On some occasions a second evaporator is installed with an external condensation coil or vice versa, to treat dry bulb temperature after drying.

- Performance: The absolute humidity of outgo -ing air depends on the dew point the coil is able to reach, which in turn depends on the pressure of the coolant gas, optimized by the manufacturer for each application. The incom -ing and outgoing air conditions must be speci -fied for the design (DBT-X-Q-Wunit). The sen -sible heat ratio normally falls between 0.4 and

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%001 %08

%04

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%06

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Fig. 10: Optimum air conditions for condensation dehumidifiers.


Fig. 11: Desiccant rotor dehumidifier operating principles.

1. Process air2. Dry air3. Reactivation air4. Wet air

Fig. 12: Desiccant rotor dehumidification process.

0.7 in contrast to cooling units for A/C, which are normally over 0.8. RHS = (sensible heat eliminated /total heat eliminated).

- Components: In contrast to cooling units, evap -orative coils are deeper and work at a lower frontal speed, and larger gaps are left between flaps. They are normally short. Everything is geared towards facilitating the condensation and subsequent precipitate falling into a water collection tray. Careful cleaning is required.

- Control: There are a number of different ways we will not go into here.

Its main advantages are lower consumption in hu -mid/hot air conditions and the good understanding refrigeration engineers have of them.

B) DESICCANT ROTOR DEHUMIDIFIERS:

A design based on the difference between vapour pressures. Humid air has a high vapour pressure. When they are dry, desiccant materials such as lithium chloride and silica gel, which can cover large matrix surface areas, have very low vapour pressures, so water molecules leave the air and are trapped by the desiccant material. The honeycomb matrix (large surface area in a small volume) is im -pregnated with the desiccant material and given a round or rotor-like shape. It is placed between two separate air flows. The wheel is then turned slowly (Fig. 11).

The incoming air flow (process air) passes through the wheel and gives up humidity to the desiccant material. It then joins the propulsion flow of the sys -tem. As the desiccant material turns through the process area it becomes saturated, so in order for it to be able to retain humidity once again, the rotation continues and it passes through another air flow, referred as the reactivation air. This air, normally ex -terior, is preheated to a high temperature in order to open the pores and reverse the desiccant function, so previously retained humidity is transferred to this extraction air flow (Fig.12).

There are two process types for retaining humidity:

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1

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1- Absorption: The desiccant material retains hu -midity by means of a chemical process whereby its chemical structure is changed through the inclusion of ‘n’ molecules of water vapour. The problem that can arise involves the precipitation of the desiccant material out off the rotor matrix when it is reactivated and completely saturated. Over time, this can make it necessary to change the wheel because of a significant reduction in its efficiency. Lithium chloride is an example.

2- Adsorption: This process is physical. The desic -cant material does not change its chemical struc -ture. Rather, the water vapour is retained by an attraction exerted by the vapour pressure forces, and the way in which the desiccant is synthesized. An example of a material of this type is silica gel.

.% 001=

X

t


Types of dehumidification. Controlling units and systems

There is another type of wheel with non-active des -iccant materials such as aluminium coated in oxide, which retains a small quantity of humidity and does not need to be reactivated with external energy: Ro -tary enthalpy recovery.

FEATURES OF ROTARY DEHUMIDIFICATION

- Performance: A function of the quantity of desic -cant material, the rotation speed, and the adsorp -tion capacities of the desiccant material in the manufacturer’s setup (Fig. 13).

- Increase in the temperature of the process air: When the water vapour is expelled from the pro -cess, the air temperature increases. This is due to the transfer of the heat the water absorbed during evaporation (latent heat). This is easier to under -stand in terms of evaporative cooling: When hot, bodies sweat in order regulate their temperature by cooling through the evaporation of the sweat on our skin. Humid heat is more difficult to bear because vapour pressures are more similar, so it is more difficult for sweat to evaporate and the same effect is not achieved. The sensible tem -perature of air also increases as a result of the thermal inertia dragged by the wheel from its reactivation side. This delta T does not however normally represent more than 10% of the total. It depends on the Tr, of the desiccant material and the setup of the unit. This temperature rise can be reduced by setting up the wheel flow in differ -ent ways, such as with an intermediate regenera -tion flow, or an intermediate bleed-off sector to remove this residual heat.

- Pre-cooling: For applications that dry exterior air with a high humidity content, it is usual to add dehumidification with a pre-heating coil to dehu -midify the air with coolant water or gas, in order to reduce the size of the desiccant unit.

- Post-cooling: Fisair normally offers additional de -humidifying devices for after the rotor on the pro -cess side to dissipate the increase in temperature if this is undesirable. These may be air-to-air heat exchangers or water or gas coils integrated into the unit.

Fig. 13: Operating outline of dehumidification with heat recovery.

- Control: Usually undertaken by way of one of the two means described below:

a) a) Reactivation heater on/off or stage con -trol: a) Valid for exterior air (if not required, the drying is not activated) or applications en -abling longer response times.

b) b) Proportional regulation of the heating: Us -ing triacs, solid state relays, vapour or natural gas control valves.

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THE ADVANTAGES OF THIS SYSTEM:

- You can dry to practically any humidity content (dew point temperature from -20ºC to – 70ºC), so a small percentage of the propulsion flow is treated: lowering costs.

- Easy maintenance and operational simplicity.

- Despite greater energy consumption, most is heat energy so excess vapour from the system or natural gas can be used: very economical op -tions with a high degree of availability in systems during wet times of year.

It is important to properly regulate the flows (and keep filters clean) when rolling the systems out, be -cause the drying capacity depends on this.

SIMPLE MAINTENANCE

The life of the rotor is almost unlimited and it is advisable to take special care of the filtering levels and the maintenance of the filters. In addition it can be washed in the event it comes into contact with particles of dirt.

The other maintenance required is very simple: Checking the voltage of the traction system and correctly adjusting the flows.

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Example of a FISAIR DH system designed to meet maximum energy efficiency standards in a famous

lithium battery research centre.


CLIMATIC NEEDS FOR PRODUCTION PROCESSES

Temperature and relative humidity values applicable to Industrial Air Conditioning.

Types of dehumidification. Controlling units and systems

.T(ºC) R.H.(%)

Bakeries… storage of:

Sugar 25-27 40-50

Dry raw materials 21 55-65

Flour 16-26 55-65

Milk 7-13 55-65

Canned goods:- Fresh fruit 1-3 55-65- Eggs 1-3 45-55- Premises 21-26 40-70

Breweries… storage of:

Malt max. 27 max. 60

Hops 0-1 55-60

Confectionary

Melted chocolate 18 50-60

Cold chocolate 18 40-50

Cocoa powder:- Cooked 26-28 30-50- In packets 16-18 40-50

Production of cakes 18-20 40-50

Production of marzipan 18-20 -

Packaging room 18-20 40-50

Warehouses 16-18 40-50

Ceramics

Storage 16 -26 35-65

Painting 24-26 40-50

Clean rooms

Manufacture 20-25 40-55

Control 22 45

Tolerance:

- General manufacturing ±1 ±5- Specific manufacturing ±0.5 ±2

Industrial liqueur production

Storage of barley 15 35-40


Electrical goods

Devices and machines 20-22 30-40

Measuring instruments 22-24 40-60

Furs

Storage <5 50-65

Leather goods

Vegetable tanning:

- The first 10 days 20 75- From then onwards 30 35


T(ºC) R.H.(%)

Optical goods

Manufacture 24 45

Pharmaceutical laboratories

Storage of powdered substances prior to transformation 21-27 30-35

Storage and packaging zones for powdered products 24-27 15-35

Milling room 26-28 30-40

Pressing room and coating of pills 20-26 20-26

Manufacture of vials 25-28 30-50

Preparation of medicines 26-28 35-48

Biological manufacture 26-28 30-40

Serums 24-26 45-55

Storage 23-25 35-40

Microanalysis 26-28 45-55

Photographic materials

Manufacture of:

- Normal films 23-24 40-65- Non-inflammable films 15-27 45-50

Packaging 18-24 40-60

Storage of negatives and colour film:

- 1 month <18 25-60- 6 months <10 25-60- 12 months <5 25-60

Positive film

- 1 month 2 25-60- 6 months 18 25-60- 12 months 10 25-60

Storage of developed film:

- Non-inflammable film 15-27 25-60- Film containing nitrate <10 25-60

Plastics

Hardening 26-28 25-30


Rubber

Storage 16-24 40-50

Vulcanization 26-28 25-30

Cabinetmaking

Cabinets and carpentry 16 50

Workshops

Precision mechanics:

- Precision work 20-22 50-55- Control and calibration 24 45-50

Manufacture of calibration tools 20-24 45-50

Manufacture of machinery 22 42-50

Calibration of clock springs 24.5 45

Other premises

Telecommunications centres 18-20 40-60

DehumidificationBasic Technical Information

dehumidiﬁcation - humidity · pdf file10 dehumidiﬁcation and controlling units/systems...

Documents