Introduction.

Relative humidity is the actual amount of water vapor in the air as a percentage of the maximum amount of water vapor which the air could hold at a given temperature. When air is warmed, its ability to hold water vapor is increased, and when cooled, it can hold less water vapor. If the amount of water vapor in the air were held constant as the temperature was increased, this would cause the relative humidity to fall, because the warm air would now be able to hold more water vapor then when it was cool. In the winter, as cold, moist outdoor air is brought indoors and heated, it becomes warm, dry air just by being heated.  Air at 20^oF & 70% RH, when heated to 72^oF, will have a relative humidity of just 8%.  Humidifiers bring the relative humidity back to the normal levels we need for comfort, safety & protection from drying.

These two glasses representing the same pound of air at different temperatures are a good visual example of relative humidity. The small glass contains 11/2 ounces of water and is 80% full. This could represent a pound of air at 30^oF and 80% relative humidity. If we pour the 11/2 ounces of water from the small glass into the large glass, we now have only 15% of the glass full. This larger glass would represent our same pound of air heated to 70^oF, but with the same pound of air and the same water vapor content we now have only 15% relative humidity.

Application.

For a humidification application we are basically interested only in how much dry air is entering the space to be humidified. If we have a room in which we wanted to maintain 70^oF and 50% relative humidity, and the room was air tight with a vapor barrier, we would only have to introduce the proper amount of water into the air once. If we maintain a constant 70^oF we would also maintain a constant 50% RH. This would be due to the fact that no dry air could get in to mix with our conditioned air and no moisture could get out. But, even our most modern buildings are not that tight. Outside air enters through open doors, cracks, ventilation, make-up air or exhaust systems. This leakage flow is called infiltration.

Ground Rules for Estimating.

1. Indoor design condition: The desired temperature and relative humidity. For example, 70^oF and 50% RH. The psychrometric chart gives the amount of moisture in the air at these conditions as 55 gr/lb.

2. Outdoor design condition: The given winter temperature and relative humidity for the location. It is the temperature for which heating systems are designed. For example, it may be -10^oF and 40% RH (moisture = 2gr/lb.) in the North, or 35^oF and 60% RH (moisture = 17 gr/lb.) in the South.

3. Volume of outside air entering the space to be humidified.

Calculation

In a residence, outside air enters by natural infiltration, which in turn, depends on tightness of construction. Typically this varies from 1/4 to 1 air volume exchange per hour and may be more with a fireplaces or fresh air exchange devices. In a factory, warehouse or other buildings without air ducts, infiltration, exhaust fans or loading docks are the major sources of fresh air. Infiltration is difficult to calculate and is usually an “engineering estimate” based on a percentage of total volume. Example: A building with 100,000 cubic feet of space. There is no mechanical ventilation or make-up air system. Assume 1 air change per hour. The outdoor heating design temperature is 0^oF and we require 50% RH at 70^oF. The formula for H (lbs/hr) is:

H =   Volume X Air Changes X Grains of Moisture Required
                      Specific Volume X 7000

Grains of Moisture Required From psychrometric chart = 56 grains of moisture per pound of air at 70^oF and 50% RH, minus 9 grains of moisture per pound already in the air (56 - 9 = 47). Specific Volume From psychrometric chart = 13.5 cu. ft./lb. of air at 70^oF, 50% RH and 7,000 = Number of grains per pound of water, a conversion constant.

H =       100,000 cu. ft. X 1 per hr. X 47         =   50 lbs. per hr.
          13.5 cu. ft. per lb. X 7,000 grains/lb

Calculation – Buildings With Ductwork.

In offices and other buildings with air duct systems, the fresh air is deliberately brought in through outdoor air dampers in measured amounts. This makes calculations easier by simply adding the total volume of all fresh air intakes. Using the above example , with identical conditions, but now with ducts bringing in 4000 cfm of outside air, we multiply the 4000 cfm by 60 minutes to get 240,000 cu. ft. per hour. Use this number for volume in the formula below.

H =   Volume Of Air Changes Per Hr. X Grains of Moisture Required
                      Specific Volume X 7000

H =       240,000 cu. ft. X 1 per hr. X 47         =   119 lbs. per hr.
          13.5 cu. ft. per lb. X 7,000 grains/lb

The above examples are for general principles only. Every application is different. Please contact our technical support at any time for specific advice and data sheets about a particular application. For detailed information, you may e-mail Humidity Source , or contact us at 973-916-1001.