A well-designed dehumidification system will improve product quality, save energy and protect process equipment.

Table 1. The median number of air changes (AC) due to infiltration and permeation per hour is 0.5. The most dominant influence on the actual number of air changes is room size.
Engineers working with cooling equipment should be familiar with sensible temperatures and heating and cooling loads. But, the psychrometric chart serves as a reminder that the condition of air involves a second dimension: moisture.

While some level of moisture may be considered desirable, when moisture is excessive, potential negative effects can occur. In some circumstances, the moisture content of air has no ill effects. In other cases, moisture levels may be moderated sufficiently by air conditioning - cooling - so that little harm results. With process parameters tightening, demand for ventilation air increasing and the drive for quality and productivity accelerating, engineers are finding it necessary to design for both dimensions of air: temperature and moisture.

Potential sources of moisture in an enclosed industrial space are numerous (see sidebar). In a typical setting, excessive moisture may ruin both raw materials and finished goods - powders coagulate, papers curl, metals rust or moisture is absorbed. Chilled process lines will sweat when high humidity air cools to its dewpoint, causing rust and leaks and ultimately destroying process equipment.

Molds, which thrive on moisture and feed on many materials, are another enemy. Worker productivity also can slip significantly due to the discomfort of high humidity.



Table 2. Construction factor (CF) takes into account the effect that good vapor barriers and construction materials will have on moisture migration.

Set Design Goals

A critical step when designing a moisture removal system is to establish clear design goals. Design goals should take into consideration why moisture is a concern and what the system needs to accomplish with regard to moisture. Avoiding condensation means keeping a close eye on dewpoint. Keeping workers comfortable implies relative humidity must be maintained in a certain range. Preventing moisture regain may require very low humidity levels to be maintained. Whatever the issue, a set of control parameters must be established to address the potential problem directly.

To ensure appropriate parameters have been specified, map temperature and humidity design conditions on the psychrometric chart. The goal is to avoid the problems you have been asked to address when the air in the building space stays within the confines of the area mapped on the psychrometric chart.

Only by properly estimating the amount of moisture generated by various sources can moisture removal equipment be selected correctly. Typically, pound per hour units are used to express moisture load.

Infiltration and Permeation. Infiltration and permeation are closely related. Infiltration is the movement of water vapor through cracks, joints and seals. Permeation is the migration of water vapor through materials such as brick and wood.

One physical law of nature states that all conditions must be balanced. In dehumidification applications, this law of nature means the partial pressure of water vapor must be the same on either side of a barrier. Water vapor will migrate through brick walls to get to the less humid side. In effect, it will search out a path to attempt to balance partial pressures.

Specific moisture load due to infiltration and permeation is not easily measured. Actual moisture deviation, materials of construction, vapor barrier and room size have an effect on vapor migration. Combined infiltration and permeation load can be approximated from the following equation:

lb of moisture/hr =
V x AC x Gr x MF x CF
7,000 x 13.5

Where:
  • V is volume of room (ft3).
  • AC is air change factor from table 1.
  • Gr is the difference in moisture outside less inside (gr/lb).
  • MF is the migration factor, which is equal to Gr ÷ 30 (Its minimum value is 1.0).
  • CF is construction factor from table 2.
  • 13.5 is a conversion factor used to calculate the change from cfm to lb/hr.
  • 7,000 is a conversion factor used to calculate the change from grains to lb.

According to American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE), the median number of air changes (AC) due to infiltration and permeation per hour is 0.5. The actual number of air changes is influenced by several factors, the most dominant being room size. The larger the room, the longer it takes to convert one volume. Table 1 demonstrates the compensation necessary for the reduction in infiltration and permeation on larger or smaller volumes.

Infiltration rate is a function of the magnitude of imbalance between the outside absolute humidity and that inside the conditioned space. The greater the difference, the greater the driving force to make the vapor pressures equal. This is attributed to the migration factor.

The moisture difference in grains of moisture per pound of air (grains/lb) may be obtained from the psychrometric chart. By locating the outside and inside conditions on the chart, absolute moisture in grains/lb can be obtained for each. To obtain the absolute moisture values for specific cities, consult the ASHRAE Hand-book, a commercially available software program or the psychrometric chart.

Another primary influence on the number of air changes is the amount of moisture that is allowed to permeate through the walls, floor and roof. The construction factor (CF) takes into account the effect that good vapor barriers and construction materials will have on moisture migration. Table 2 provides CFs for common construction materials. The CF will vary between 0.3 and 1.0.

Doors and Windows. Additional sources of moisture include door and window openings and architectural voids such as conveyor passages. The moisture amount is directly proportional to the frequency of the opening, the difference in indoor and outdoor moisture content and the wind velocity at the opening.

Wind velocity is the most difficult factor to account for because it varies depending on the location of the opening with respect to the wind source. However, a guideline is 12 cfm of outside air per square foot of opening. Local weather stations can provide details on the normal prevailing direction and speed.

The moisture load resulting from an opening can be estimated by the following formula:

lb of moisture/hr =
Area x Open x Gr x 12
7,000 x 13.5

Where:
  • Area is the surface area of opening (ft2).
  • Open is the number of minutes an area is open per hour.
  • Gr is the difference in moisture outside less inside (gr/lb).
  • 12 is a factor used as an estimated ingress of moisture (cfm/ft2).
  • 13.5 is a conversion factor used to calculate the change from cfm to lb/hr.
  • 7,000 is a conversion factor used to calculate from grains to lb.

Products. The three P's - product, process and people - also must be included in moisture load evaluation. If a product or raw material retains water, it also may release water in a conditioned room. For example, wet wood brought into a conditioned warehouse will release moisture at a specific rate. The release rate is determined by measuring the product weight loss over time.

Processes. Many times, a process itself generates moisture. For example, open water tanks and cooking vessels will add moisture. In these cases, a model must be developed for each process step that affects indoor conditions.

In one example - open water tanks - the evaporation rate can be calculated with the following equation (assuming 10 to 30 ft/min air velocities in the room):

lb of moisture/hr =
0.1 x Area x (VPH2O - VPAIR)

Where:
  • Area is the surface area of room (ft2).
  • VPH2O is vapor pressure of water at water temperature.
  • VPAIR is vapor pressure of air at its corresponding dewpoint.

People. People give off moisture through respiration and perspiration. Moisture load is a function of the number of people and their activity. A worker lifting boxes will generate four to eight times the moisture of a worker at a lab bench. ASHRAE's data on the amount of water added per person per hour of activity is reproduced in table 3.

Ventilation and Makeup Air. Whether replacing air that is exhausted as part of an industrial process or introducing air to meet ventilation requirements, outside air can contribute significantly to moisture load. This is especially important in summer months when high humidity is common.

As with the calculation for infiltration, the difference in absolute humidity must be used, along with the outside air volume controlled by the air-handling system. The formula for calculating moisture load is:

lb of moisture/hr =
cfm x Gr x 60
7,000 x 13.5

Where:
  • cfm is the volume of outside air introduced.
  • Gr is the difference in moisture outside less inside (gr/lb).
  • 60 is a conversion factor used to calculate the change from min to hr.
  • 13.5 is a conversion factor used to calculate the change from cfm to lb/hr.
  • 7,000 is a conversion factor used to calculate the change from grains to lb.



Figure 1. Makeup air can be used to dilute a plant's moisture-laden air with drier air.

Methods for Moisture Control

Once a combined moisture load has been calculated, an appropriate solution can be chosen. Most manufacturers will provide the specifications and assistance needed to select the proper equipment. But, first the general form of solution must be selected.

Several methods of drying air can be used; each has advantages and disadvantages. The common types are:

  • Makeup air.
  • Compression.
  • Refrigerated dehumidification.
  • Desiccant dehumidification.

Makeup air uses the principle of dilution, removing a portion of the moisture-laden air from a space and replacing it with drier air. The net result is lower average moisture content. This method is relatively inexpensive to install but requires drier air. Because the most common source is outside makeup air, this method is difficult to apply in summer months, and it may be expensive to operate in winter due to heating costs (figure 1).

Using compression to dry air is effective when small quantities are needed. When air is compressed, the temperature at which water vapor will condense is raised. This method has high installation and operational costs and is most commonly used when less than 100 cfm of dry air is required.



Figure 2. Refrigeration dehumidifiers remove moisture in the air via condensation.
Refrigeration dehumidifiers reduce moisture in the air by passing the air over a cold surface, removing the moisture by condensation. This method is effective for desired conditions down to 45°F dewpoint for standard applications. It has moderate capital costs and can recover much of the latent energy, thus offsetting operating costs (figure 2).

Desiccant dehumidifiers use special materials that adsorb or hold moisture. Desiccant materials do not change size or shape when acquiring the moisture and can be regenerated by applying heat. This technique is used to dry air in the range of 0 to 45°F dewpoint. It has a relatively high capital expense as well as a high operational cost.

Moisture is an important consideration in industrial settings. A good understanding of moisture sources and issues will lead to appropriate design goals. Meeting these goals entails calculating loads to be handled by moisture removal equipment. The variety of moisture sources, design conditions and ambient conditions will continue to make moisture load calculations a challenge, but the reward can be a well-designed system that protects materials and process equipment, improves quality, saves energy and raises worker productivity.



SIDEBAR:

KNOW YOUR SOURCE

Common sources of moisture for industrial facilities include:

  • Infiltration.
  • Permeation.
  • Door and window openings.
  • People.
  • Ram materials and end products.
  • Processes.
  • Ventilation and makeup air.


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