DETERMINING MOISTURE 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 wet-bulb. 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°F db (dry-bulb temperature) and 76°F wb (wet-bulb temperature), this condition equals 104 gr/lb.

But there will be many days when 76°F 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 process work.

Ventilation latent load. Determining the latent load equivalent to the outside air by subtracting the indoor or maintained 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?

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

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 Design Outside Wet Bulb Design Specific Humidity Comfort Work Process Work F (gr/lb) (gr/lb) 81o 139 149 80o 130 143 79o 125 139 78o 120 134 77o 118 130 76o 115 125 75o 112 121 74o 108 117 73o 105 113 72o 100 109 71o 95 106 70o 90 102 LATENT HEAT DISSIPATED BY ADULT OCCUPANTS Dry Bulb Temperature Occupants At Rest Occupant Doing Light Physical Exertion* Occupant Doing Heavy Physical Exertion** F (gr/hr) (gr/hr) (gr/hr) 60° 400 1300 1960 65° 530 1630 2400 70° 670 2060 2920 75° 900 2540 3450 80° 1180 3040 3950 85° 1525 3550 4450 90° 1870 4000 5000

* Examples – Waiters, dinner dancing, light factory assembly work
** Examples – Factory machine operator, continuous dancing

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

VAPOR PRESSURES OVER WATER

MISCELLANEOUS MOISTURE LOADS      

Moisture permeation. This is discussed in detail in calculating loads.

Moisture loads in the table above represent unvented appliances. Although personal judgement is used to determine vent or hood efficiency, the hood efficiency should never be higher than 50 percent.

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 wet-then 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.

Wt. of material entering minus

Average        ₌     Wt. of material leaving
drying rate                Drying time (hrs)

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 totally within 2 hours, 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 an 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.

MOISTURE REGAIN OF VARIOUS HYGROSCOPIC MATERIALS
Moisture Content Expressed in Per Cent to Dry Weight of the Substance at
Various Relative Humidities - Temperature, 75°F  (24°C)

Used by permission for Chapter 24, 1964/1965 ASHRAE guide and Data Book-Applications

Dehumidifiers & Dryers
Plastic Auxiliary Equipment