Determining Moisture Loads or Latent Loads

Moisture load can come from many sources, which provide the data needed to calculate the total latent load on any air conditioning or drying system. The total latent load equals the sum of applicable individual loads.

Outside design level. Bry-Air Dehumidifier performance characteristics are expressed in terms of specific humidity or grains per pound of air. To determine the outside design moisture level, use the standard design dry-bulb and wet-bulb conditions because this value measures the design total heat (wet-bulb) occurring with the highest practical dry-bulb. The design moisture level will exist when a lower dry-bulb occurs with the design wet­bulb. This condition represents the same total heat, but a higher specific humidity. The table below lists the recommended design specific humidity for various design wet-bulb temperatures. Use the standard accepted design wet-bulb for your locality.

Example: If the accepted design level for your city is 95°db (dry-bulb temperature) and 76°wb (wet-bulb temperature), this condition equals 104 gr/lb. But there will be many days when 76°wb will occur at a lower dry-bulb temperature. From the table below, the proper design specific humidity for comfort would be 115 gr/lb; for industrial work it would be 125 gr/lb. Figures below assume that these levels will be reached or exceeded on 30 percent of summer days for comfort work and 10 percent of days for process work.

Ventilation latent load. Determine the latent load equivalent to the outside air by subtracting the indoor or maintained specific humidity from the outdoor specific humidity and multiplying that amount by the pounds of outside air brought into the system.

EXAMPLE: If 1,000 cfm ventilation air is at 125 gr/lb. design and the design inside condition is 70 gr/lb., what is the ventilation latent load?

1000 × (125 - 70) = 3930 gr/min or 235,800 gr/hr. 
  14

The average density of air is given as 14 cu. ft. per pound of air and is used regardless of the actual density at design conditions.

Recommended Design Outside Moisture Level

Latent Heat Dissipated by Adult Occupants

Evaporation from a wetted surface. Determine the amount of moisture evaporation from a pan, tank or other wetted surface into a space using the following calculations:

Where:
Gr. = moisture evaporated in grs/hr.
Vel = air velocity in F.P.M.
VL = vapor pressure equivalent to temperature of surface water - inches of mercury.
VA = vapor pressure equivalent to dew point temperature of air over surface - inches of mercury.

Vapor Pressures of Water

Miscellaneous Moisture Loads

Drying hygroscopic materials. The calculations shown above apply only to evaporation of free water from a surface. When hygroscopic materials are in the first stages of drying-when the surface is actually wet­then the above relationship may exist. But after surface drying is complete, further drying will occur at a rate that depends on the rate of diffusion within the material; the rate varies with the degree of dryness within the material and is based on expected structural changes that occur during the drying process.

Establish the drying rate of hygroscopic materials in order to establish the hourly moisture load. Unfortunately these rates must be determined experimentally in each situation.

Usually, the desired outcome with hygroscopic drying is to improve drying rate or degree of dryness in the final product within an existing set up or with the addition of a dehumidifier. In doing so, the desired drying period is generally included with the total weight of material to be handled.

One caution here: the drying period cannot be arbitrarily assumed; it must be realistic. For example, if dry air circulates in a dehumidifier and cannot dry a material to totally dry within 2 hours, and then 2 hours will be neither a possible nor a realistic desired drying time.

Storage of hygroscopic materials. When hygroscopic materials enter a dry storage space, even for a short time, they contribute a moisture load that must be absorbed by the dehumidifier. The table below lists the moisture holding capacity of various materials in equilibrium with air at the relative humidities shown. The percentages compare the moisture to the substance's totally dry weight.

If the incoming material has unknown moisture content, assume that it is in equilibrium with 60 percent rh air. In winter, the materials will likely come into a room in equilibrium with 90 percent rh air. However, in winter most other sources of rh are lower, so the summer figure (60 percent) can be used all year, unless the product loads makes up most of the entire total and the permeation load is minor by comparison.

Regain of Hygroscopic Materials

Moisture Content Expressed in Percent of Dry Weight of the Substance at Various Relative Humidities – Temperature, 75°F.