Sizing the Dehumidifier

When deciding what size dehumidifier to use, remember that controlled space requirements sometimes exceed the anticipated design peak load. Unusual and unforeseen humidity loads — such as from abnormal weather conditions or the introduction of high-moisture­ content raw materials — can burden the drying equipment.

Here we present a number of issues that must be considered in approaching and solving specific drying problems. Six typical humidity control examples are presented:

• Food and drug manufacturing, specifically raw materials and processing equipment
  (Production of hard candy)
• Storage or equipment areas (Standby warehouse)
• Product drying
• Controlled humidity and temperature areas
• Specific purposes for dry air production
• Prevention of condensation (Water  treatment plant)

Example 1: Production of Hard Candy
During candy and cough drop production, the material is in a plastic state. It must flow and be shaped by stamping machines. If the presence of excess moisture causes the material to become sticky, it will not flow freely and it will adhere to the stamping machine.  To eliminate this material and equipment problem, dry the surrounding air.  Learn more.

Example 2: Standby Warehouse
Moisture damage in a standby or storage warehouse can be avoided by surrounding the machinery, equipment, or material with dry air. Learn more.

Example 3: Product Drying
Here we have a room used to remove water vapor from such products as cattle feed mixes, nylon or rayon cord for tires, raw plastic material, granular chemicals, raw paper stock, cardboard stock for coatings, or other similar products. In this example, the room is used for drying cattle feed mixes, which are contained on drying carts that stand in the room until the specified level of dryness is attained. Learn more.

Example 4: Controlled Humidity and Temperature Areas
Many air conditioned manufacturing areas often have a required air flow to handle a sensible load in that space. This air quantity requirement and the accompanying dehumidifier size are usually greater than those needed to handle a latent load. By designing a system for the sensible load situation and then determining the appropriate dehumidifier to handle the moisture load, the desired conditions for the space can be maintained. Learn more.

Example 5: Production of Dry Air for a Specific Purpose
Many applications require a specific quantity of outside air to be delivered at a given moisture content and temperature. This requirement may be a need to make up air exhausted from a space or to supply air for a process such as a drying oven. Learn more.

Example 6: Water Treatment Plants
In most water pumping stations, filtration plants, and waste water control plants, control of humidity in the pipe galleries, pump rooms, and control rooms is of prime importance. By reducing the dew-point temperature of the air below the temperature of the piping and walls, sweating and condensation can be eliminated. By circulating warm, dry air through the areas, water accumulation is avoided, maintenance for electrical controls, motors, and instruments is reduced, and paint lasts longer on the pipes, valves, and flanges. Learn more.

Zero Leak System for a Low Humidity Space

In a system where the ductwork and compo­nents are outside the controlled space, dry air leaving the system will induce the flow of humid air into the system. If the humid air is not dehumidified, each cfm will induce a load absorbing the capacity provided by 5 to 10 cfm of the dehumidified air. If all air that enters the system must pass through the dehumidifier, the additional load will be reduced by 50 percent or more. Typical air handling units (AHUs) are not built to be vapor tight. Standard sheet metal type ductwork has lapped seams that allow leakage. However, excellent silicone-based sealants are available; for applications requiring very dry spaces, the seams must be welded shut.

Having a "zero leak" condition means that all air leaving the controlled space is under positive pressure (to minimize infiltration), and all the entering air passes through the dehumidifier. The cost of dehumidification is high when moisture levels must be low. In these situations, even small leaks can double operating costs. Since ducts are a once-only expense, attention to ductwork is vital. The other approach to this situation — a higher capacity dehumidifier — means added costs, year after year.

It is unusual to find air handling units constructed for zero leak performance. Leaks are anticipated at removable panels, bearing flanges, drain pans, and through condensate drains with dry traps. Such units require additional sealing and check valves or positive water seals on the condensate lines. Cooling devices, especially the coils and fan, and the filter box need to be handled properly when cleaned, and they, too, must be tight. Obviously, there are many factors that can restrict the zero leak principle.

One way to maximize the chances for a zero leak system is with air treatment equipment and dehumidifiers designed to fit and operate together effectively. Buying directly from the equipment manufacturer and not mixing sources of various equipment components focuses the accountability for moisture tightness.

Bry-Air constructs custom dehumidifiers and low dew point AHUs for maximum efficiency. This extra care in design and construction means zero leak performance. Proper operating balance compensates for this situation.

When end users, consulting engineers, or other "specifiers" require a system to be substantially air tight, they means there is no leakage at any joint.

Assuring zero leak begins in the fabrication and construction stages. We recommend flanged or gasketed connections with welded seams and joints. As a minimum, all seams and joints should be caulked. Removable panels and access doors should be completely enclosed. (Coils will leak where return bends enter tube sheets).

Prior to insulating, the system should be tested and deficiencies corrected. The best method is to use an open flame, if permissible. This requires candles or other sources of flame. In other situations, smoke devices or soap suds could be used.

When all these precautions in fabrication, construction, and testing are complete, the system will perform as designed. Periodic inspection during operation will allow leaks to be located and corrected.

Next Chapter: Bry-Air Dehumidifers for Product Drying

Previous Chapter: Calculating Moisture Loads