Drying Tips Page 2  Courtesy: http://www.p1m.com

7.10.a) Psychrometric Chart: With the aid of a thermo - hygrometer, the psychrometric chart becomes a valuable tool.  With the know temperature and relative humidity, the psychrometric chart is used to obtain the dew point, specific humidity, and grains of moisture per lb. of dry air and vapor pressure.

The psychometric chart contains four varying lines that run vertically, horizontally, diagonally and in an upward motion as illustrated in Figure 7-10 thru 7-10 (d) below.

Psychrometric Chart
Figure 7-10

Dry Bulb - lines run vertically in 10°
increments across the chart
Figure 7-10 (a)

Wet Bulb - lines run diagonally from left to right in
10° increments across the left side of the chart
Figure 7-10 (b)

Dew Point - lines run from left to right horizontally
10° increments across the left side of the chart
Figure 7-10 (c)

Relative Humidity - lines run from left to right
in an upward curve in 10° increments
Figure 7-10 (d)

7.11) Readings: Temperature, humidity and dew point readings should be taken during the loss site evaluation, at the on-set of the drying process, and on an ongoing basis, and all readings should be recorded, and readings should be performed using a properly calibrated - thermo - hygrometer.

The use of third-party consultants for loss site monitoring and report preparation on mid-sized and large losses, quantifies the processes, moreover, reduces risk and liability.

7.11.a) Temperature: Temperature should be considered the balance point of the drying process, since the temperature affects the dew point, vapor pressure, relative humidity and specific humidity.

The following affect temperature:

   - The temperature, where the air becomes saturated, is the
      dew point.
    - Water vapor forms when the temperature is below the dew
      point, causing condensation.
    - As the temperature doubles, the vapor pressure doubles,
      when heated at a constant temperature.
    - Heat causes water molecules to vibrate, resulting
      in the molecules to expand farther apart, or become larger.
    - The temperature of a fluid will determine how strong its
       molecules interact.
    - The boiling point is the temperature when a liquid bubbles
      and changes into vapors, and the boiling point is the
      temperature when the vapor pressure liquid equals
      the atmospheric pressure.
    - The temperature of a liquid does not have to reach its
      boiling point to evaporate completely.
    - The pressure exerted from vapor pressure depends on
      the temperature, as long as there is liquid present.
    - As the temperature changes, the relative humidity will
      change due to expansion and contraction of air volume.
    - The occurrence of dew is a result of rapid cooling.
    - Evaporating molecules absorb heat from there surroundings,
      acting as a cooling process.
    - Water vapors in the air can hold latent heat.
    - Vapor pressure increases as substance gains heat.

7.11.b) Relative Humidity: Relative humidity is the percentage of moisture in a volume of air, and its density is measured using a hygrometer.  Within the exterior atmosphere, the warmer the air is, the more moisture it will hold.  However, for interior drying purposes, the warmer the air is, the quicker it will release moisture when the appropriate number of air changes or  the appropriate amount of dehumidification equipment is used.

As the temperature changes (increase or decrease), the relative humidity will change due to the expansion and contraction of the air volume.

Humidity levels affect a persons comfort level:
- Increased humidity in warm climates will decrease
  temperature comfort rates.
- Increased humidity in cold climates will increase
  temperature comfort rates.

7.11.c) Dew Point: The dew point is the temperature where the air becomes saturated, resulting in water vapor to form, which causes condensation and secondary damages.

When the dew point is reached, the relative humidity of the air is considered 100% saturated at the surface.

An improperly balanced drying system; airmovers to dehumidifiers or airmovers to air changes could result in secondary damages.

7.11.d) Specific Humidity: Specific humidity is the actual quantity or weight of moisture in air volume, and varies according to the temperature and pressure in the air.  The specific humidity can be determined using the psychometric chart once the temperature and relative humidity are known. 

In relation to specific humidity, the density of air would weigh 12.387 pounds per cubic foot or (0.08073 pounds per cubic foot) at 32º F with an atmospheric pressure of 29.92 inches of mercury.  

The specific humidity (weight of water) will only change during dehumidification in a closed drying process or during evaporation in an open drying process.  While the specific humidity (weight of water) will only fluctuate by a tenth of a percent from 0º to 102º F and for all intentional purposes could be considered constant during temperature changes. 

When the vapor pressure or specific humidity increases, the absorption or repelling rate of materials will increase.

The specific humidity level is a determining factor when drying.  The lower the specific humidity, the more efficient the drying process becomes.  This is based on the number of grains in a pound of dry air as described in Section 7.11.e, and that moist air will travel to dry air --- when it can.

7.11.e) Grains of Moisture: The grains of moisture as found in a pound of dry air would have to be reduced before the relative humidity can be reduced.

When reducing the atmospheric grains of moisture within a room or space, the potential release of the grains of moisture from structural components and content items would result in a higher than desired atmospheric relative humidity due to lateen time.  This is due to the amount of moisture grains within structural and content materials, and the surfaces permeance ratings, which would replenish the atmospheric grains as they are removed through wicking and dehumidification.

When removing grains of moisture to achieve an overall satisfactory humidity level, the grains per pound (gpp) should be monitored.  Moreover, the overall moisture content; atmospheric, structural components and content items should be considered to produce positive results.

When the drying process is completed, the desired relative humidity should have a have a near-constant parallel rating with the specific humidity as produced by the buildings operating HVAC system.  Or the relative humidity should have a near-constant rating with the infiltrated outdoor atmosphere, when the structure does not have an operable HVAC system.

When the moisture content of building components and content items have reached the desired levels described in section 7.15 and Table 7-B.  And when the relative humidity is within ASHRAE levels of 30% to 60% rh at 70º F for a near-constant time frame, the drying process could be considered complete. 

 7.11.f) Vapor Pressure: Vapor pressure, specific humidity and grains of moisture per pound of dry air are related in that, as the vapor pressure increases or decreases, the grains of moisture and the specific moisture will remain constant with the vapor pressure and vice versa.

Vapor is the gaseous state that solids and liquids pass when heated.  Vapor pressure as related to gaseous matter are not the same thing, for vapor to be a true gas its temperature would have to be 705º F.

Vapor pressure with respect to drying would be the pressure produced by vapor molecules escaping to and from the surface of a liquid.

The porosity of structural and contents’ surfaces, and the surface tension of the liquid (water) play an important role in the drying process, since water has less friction than most liquids.  The greater the porosity of the substance or vapor pressure, the faster the evaporation rate.  This is important considering hotter liquid molecules have less viscosity than colder liquid molecules, and during an accelerated drying process could increase secondary damages when  the room temperature is at or above the comfort level.   

When performing restorative drying, the reduction in vapor pressure will decrease the pounds per square inch (PSI) that water molecules exert on the surrounding building components and contents.

7.12) Equilibrium: Two states of equilibrium will be encountered during the drying process:

    - Interior atmosphere-to-exterior atmosphere
    - Interior atmosphere-to-material saturation 

Equilibrium exists when two states of matter are equal with each other, meaning the decreasing rate of the forward action becomes equal with the increasing rate of the reversing action as shown in Figure 7-12.

Equilibrium Reactant Chart
Source: Microsoft Encarta Encyclopedia
Figure 7-12

The temperature would be the balance point of equilibrium since the temperature is the shared property. (ref. 7.11.a)

When the temperature is raised during equilibrium, it causes endothermic reactions to occur, while lowering the temperature causes an exothermic reaction, resulting in the equilibrium to react to minimize the change. 

An equilibrium state with respect to high humidity would result in the natural occurrence of secondary damages (fungi).

7.13) Material Permeance: Material surfaces of have pores or capillaries between its fibers that will absorb water by capillary.

Capillary is when liquids move into or out of passageways and capillary occurs when the liquid within a capillary is in contact with the air.  The smaller and more regular a capillary is, the greater its ability to absorb or repeal moisture.

The surface tension of a substance plays an important role in the absorption and evaporation of a materials surface --- since surface tension will draw water within a capillary when the walls of the capillary attract the molecules of the liquid surface.  While surface tension will repel or push the water out of the capillary walls when water molecules are attracted to each other.

The Institute of Inspection Cleaning and Restoration Certification (IICRC) has rated surfaces as highly porous, semi-porous and non-porous based on their permeance or magnetic reaction:

    - Highly porous:  (Permeance Factor >10), would
      include carpet and padding, tactless strips,
      mattresses/box springs, cardboard, wicker
      and stuffed upholstered furniture
    - Semi-porous:  (Permeance Factor > 1 - 10), would
      include linoleum, vinyl wall covering, vinyl
      upholstery, hardboard furniture, structural and
      trim woods, painted drywall and plaster
    - Non-porous:  (Permeance Factor - 1), would
      include laminates, heavy vinyl, plastic, glass,
      tiles or any durable materials

The IICRC also states that organic components and certain organic soils can absorb and release water readily.  This is due to the number of capillaries within an organic substance, as an example paper products can contains millions of capillaries, and the most effective capillaries are micro in size.

Permeance (perm) is the measure of water vapor flow through materials of specific thickness.  A perm measures at 73.4º F or 23º C at the number of grains of water vapor that passes through a square foot of material in 1 hour with a differential vapor pressure equal to 1 inch of mercury.  The lower the materials permeance, the more effectively the material retards water.

Table 7-A represents a brief listing of material permeance, which should be used when calculating a drying process. 
 

Vapor Permeance Chart
Source: Information Technology Specialist, Inc. *
Masterchem Industries **
ASTM E96-95 ***
Table 7-A