COWARD ENVIRONMENTAL SYSTEMS, INC.
500 Birchfield Drive,Suite 501,Mt. Laurel, NJ 08054
856-727-4400 -fax 856-727-4344
e-mail: cesi@cowardenvironmental.com

"Moisture"
an Essential Component of Outside Air

Just as proper temperature control of outside air is essential for comfort and correct industrial processing so too is the maintenance of proper moisture levels. Low moisture levels can effect comfort, drying of materials, shrinkage, static electricity problems, reaction rates, product uniformity, etc.

The moisture level of outside air varies widely and in winter could be well below that required for your application. In previous editions of this publication, we have discussed industrial dehumidification. Just as important in dealing with outside air is humidification. Cold air has little capacity for moisture. Saturated air at 0 degrees F has more than 14 times less water than saturated air at 60 degrees.

How then do we best add water to outside air? The first step is to get the air to a condition where psychometrically it can accept additional water. Then water can be added to the air with the aim of achieving a controlled humidity level. The specific method chosen requires careful attention to obtain desired results without undesired side effects.

Some methods and their pros and cons are discussed below:

Heated Pan: Water in a pan is heated with an immersion heater to evaporate water into an air stream passing above. There are many problems with this method. Too rapid boiling will introduce water mist to the air. The large mass of hot water means lots of thermal inertia hindering control.

Very little mixing takes place with the air closest to the pan. It will become saturated and condensing in the duct. The hot water will also introduce some sensible heat. Atomizers: A fine mist of water droplets is generated in the air stream which will evaporate to add moisture to the air. The problems here are as great or greater than the pan. In order for the liquid water to evaporate, we must supply the latent heat of vaporization. This must come from the air stream. This reduces the air temperature as well as the ability of the air to hold moisture. Also, how do you know that the air can hold all the water you spray? And what about mineral fallout from the evaporated water?

Direct Water Vapor Addition: This is usually accomplished by injecting low pressure steam (saturated vapor) into an air stream through dispersion manifolds or cup-type humidifiers at the base of the duct. This can be a very satisfactory method if proper precautions are taken. A dispersion manifold is the method of choice for several reasons, primarily because of better mixing. Care must be taken to assure that only steam and no water droplets are added. This is done with good equipment design, proper valve sizing and correctly applied traps. Correct duct distances must be maintained to allow for the instantaneous condensation and re-evaporation which can occur before equilibrium is established. This is especially true when the humidifier is mounted in an air handling unit. Careful consideration must be given to controls to prevent supersaturation in the duct. If room humidistats are used, provisions should be made for a limiting humidistat in the duct.

Direct Evaporation: This involves the evaporation of water into the air stream from an extended surface. When done with an unheated surface, it has the same problems as the atomizer. When accomplished with a temperature controlled surface, it is the best method for precise control and elimination of secondary problems. .

Proper application involves the use of a sprayed coil. By the design of the coil velocities and depth, a condition of saturation of the air can be achieved which defines the precise amount of water in the airThe extended surface provides uniformity and super saturation is eliminated as long as the air is not subsequently cooled. The humidity level can be raised and lowered by changing the coil temperature. Mist eliminators are used to prevent droplet carryover.

Air Washer: Another excellent method for humidification is the air washer. This system utilizes a set of spray nozzles, collection sump, pump and mist eliminators. Air passing through the spray section can be humidified with a falling water temperature approaching a rising wet bulb temperature in the air. With proper control of entering air and water temperatures, some fine control is possible. Well designed mist eliminators are essential to prevent water carryover. No matter what method is used, keep in mind the limitation of the saturation of air. If your air volume in a duct or air handler is limited, you may not be able to add enough moisture to satisfy the humidity requirements of a larger volume of space. Each situation of humidification has its own best solution. One thing is certain; a satisfactory method can only be accomplished by a thorough analysis of the psychometrics at each step along the way.

by:
Kenneth W. Wicks
ASHRAE Fellow
7-99