DESICCANT vs. REFRIGERATION
We do not feel that we are experts in all phases of all environmental control technologies.
The information you will find here is meant to refer to the way the capabilities of refrigeration and desiccant dehumidifiers are applied to the drying procedures in our industry only.
We feel that the principal market segments are fire and water damage restoration, the petro chemical industry, potable water tanks, construction and temporary humidity control. We will address each of these segments with the hopes of covering some of the drying problems that are found with workable solutions.
The use of dehumidification has become a no-brainer; it makes you money and takes a lot of ”worry” out of your project. In the petro chemical industry, for example, the use of dehumidification will reduce costs up to 35% over the old systems of "same day blast/coat".
In the water damage restoration market segment, the only way we had to remove moisture was with fans and vacuum extractors. This system eventually worked but it took four times longer than it would have taken with dehumidifiers.
The general characteristics of each of the two systems should be understood in order to make an accurate assessment of the logic behind the decision to use one or the other.
Methods of removing moisture from air are typically compression, the use of desiccants, refrigeration or a combination of these systems. From a practical standpoint, compression is very difficult to use in the field. We will therefore analyze only the refrigeration and desiccant systems in our discussions.
It is our opinion, from a financial point of view, that if you use a dehumidifier more than four times a year, you should own it. Today’s dehumidifiers are constructed with aluminum or stainless steel, the electrical circuitry is solid state and all components are greatly improved to such a degree that the average trouble-free life span of these units is over 10 years.
In laymen terms the desiccant dehumidification systems, using the solid sorption method, normally use a fixed desiccant structure, or rotor. The desiccant used is either silica gel or lithium chloride.
Humid air is drawn into the dehumidifier through a filter and a plenum which has an internal partition to keep the humid (process) air from mixing with the reactivation air. The air passes through the rotor where moisture is adsorbed from the air by uniquely synthesized silica gel desiccant contained within a corrugated flute shaped rotor. As the vapor is adsorbed within the desiccant, energy is given off to the process air stream. This energy pick-up causes the "process" air stream sensible temperature to increase by the amount of latent conversion occurring. The dehumidified air is then drawn through a blower and discharged from the unit into the controlled space to be dried.
As the air to be dehumidified passes through the flutes of the desiccant rotor, the rotor is turning at a pre-determined rate and carries the adsorbed moisture to the reactivation zone.
Here the moisture is subjected to a counter flow heated air stream and discharged back into the atmosphere. The air is heated to 250-275°F.
Reactivation air which is drawn through a filter is heated to 250 - 275°F also. This heat changes the relative humidity (moisture holding capacity) of the reactivation air which allows it to desorb and remove moisture from the desiccant. Heat in the reactivation air stream provides the energy necessary to change the adsorbed moisture to vapor.
This process, or exothermic hydration reaction, normally raises the temperature of the process air stream by 10-15°F. If ambient temperatures are already at 90°F, the temperature entering the area to be dried is therefore above 100°F. In these cases refrigeration type dehumidifiers are used in combination with desiccant dehumidifier to cool the processed air enough to pass the requirement of "creature comfort" inside the area to be dried.
The controlling factor in using refrigeration dehumidification is to pass the humid ambient air over a series of refrigeration coils. The temperature of the coils is purposely set to be very much lower than the temperature of the incoming ambient air. As the air cools, it reaches saturation, and condensation forms. This condensation is removed from the system. The air comes out of the cool coil section of the dehumidification unit at a lower temperature, dew point, and absolute humidity. The cooler air, which has a very low dew point, can then be heated to lower the relative humidity. This air is then moved into the area that is to be dried.
The amount of dry air to be used is measured in cubic feet per minute (cfm). The humid air in the space to be dried is exchanged for dry or processed air from the dehumidifier (either refrigerant or desiccant). This system is referred to as a purge since the dry air is forced into the space under fan pressure replacing the moist air so many times each hour.
More or less dry air can be used depending on ambient conditions, the degree of wetness, availability of electrical power and time constraints.
It is important to note that one ton of refrigerant air is equal to 500 cfm of desiccant air. More refrigerant air is required for drying since desiccant processed air will be close to 10% relative humidity and refrigerant processed air will be close to 40% relative humidity.
WHICH SYSTEM TO USE AND COST INVOLVED.
In reference to which dehumidification system is the best, the answer is easy…they both work but they might not work well enough. It all depends on the ambient conditions. If the ambient is at 50°F the refrigeration (DX) system will freeze up and shut down. This is because the Delta "T" (for temperature reduction) is approximately 35°F. The machine will then reach 35°F below the ambient or 15°F at the condensers which will turn the condensate into ice and shut down the unit.
The desiccant (DH) unit will work well at all temperatures but OSHA has a rule that says a mass or stream of air may not be injected into an area where people are working if the temperature of the air is 100°F or more.
The air temperature of a processed stream is 15-20°F above ambient, therefore, if ambient is 85°F or more you may be shut down by OSHA.
Desiccant dehumidifiers (DH) are also more expensive than refrigeration (DX) units when both are to be purchased or rented for the same size tank. The difference in the purchase price of the desiccant unit is as much as 60% higher and approximately $500 more to rent per month than a refrigeration unit.
Therefore, to cover your ass-pirations under all conditions, the only solution is to own both types of dehumidification units.