Desiccant systems in HVAC applications, an alternative or supplement to mechanical refrigeration, are used primarily where simultaneous maintenance of temperature and humidity control is an important benefit to the user.
Desiccants are defined as materials which attract and hold water vapor. With desiccant systems the sensible and latent functions are separated. A desiccant material is used to remove the moisture by absorption or adsorption. Refrigeration is then used to lower the temperature only to the desired level for distribution. This refrigeration is done at a higher temperature than in a typical conventional HVAC thereby achieving a higher operating COP (lower kW/ton). A reduction in energy costs for air conditioning is possible.
They can be used to create very low humidity environments (5-10% relative humidity) which would otherwise be difficult and expensive to maintain using compression-refrigeration equipment. Desiccant-based energy recovery systems basically decouple the latent load from the equipment (typically mechanical cooling) handling the sensible load. It is often used on projects where the latent load is in excess of that which can be done by conventional unitary cooling equipment. They are often considered when controlling relative humidity is essential to avoiding the growth of microbials.
If are properly applied and are correctly designed, desiccant systems can also produce these benefits:
- Independent control of latent loads in the ventilation air.
- Eliminate condensation on cooling coils and in drip pans, and reduce humidity levels in ducts. This will virtually eliminate the growth of mold, mildew, and bacteria. The combination can reduce maintenance and help avoid indoor air quality problems.
- Lower humidity levels in occupied spaces provides equivalent comfort levels at higher ambient temperatures. This could allow chilled water set-points to be raised and there-by save energy and reduce system operating costs.
- Reduce the mechanical cooling load, permitting the use of smaller chillers and possibly even smaller ducting in new construction. These construction cost offsets should be factored into any economic evaluation.
While there are special design situations where desiccant/evaporative chilling systems are potentially economic, the complication of such systems limits their acceptance, especially in situations where there are limited or no on-site qualified operating personnel. Other disadvantages include:
- Increased first cost
- Increased maintenance of the added desiccant equipment
- Cost of energy (usually natural gas) to regenerate the desiccant at a high temperature to drive off the entrained moisture
- In some cases, the need for piping of cooling (typically tower) water to remove the heat of adsorption and precool the heated air off the desiccant units
Changes in building construction and use has caused lower sensible cooling loads due to increased insulation, better windows, and more efficient lighting. At the same time, higher ventilation air requirements, together with higher building occupant densities, result in higher dehumidification loads which means higher moisture (latent) loads.
At the same time the demand for higher efficiency cooling units has manufacturers using larger evaporators operating at higher refrigerant temperatures, resulting in reduced moisture removal ability compared to the sensible capacity...a higher sensible heat ratio of the equipment.
Conventional systems use refrigeration to provide both sensible (lower the air temperature) and latent (dehumidify the air) cooling. To achieve the lower relative humidity desired in some spaces, the air must be cooled below that needed for the sensible load in order to remove sufficient moisture, and then reheated to prevent over-cooling thus increasing energy use. Desiccant systems, coupled with mechanical cooling, can avoid the need for reheat.
Desiccant systems have been commonly used for many years in industrial processes where very low humidity is a must. Other applications include maintaining controlled humidity in warehouse and caves used for storage, preserving ships and other such facilities that would otherwise deteriorate due to moisture build-up, drying air to speed drying of heat-sensitive products. The changing conditions and dehumidification requirements have expanded its use in HVAC applications.
Desiccant systems can be used to create very low humidity environments (5-10% relative humidity) which would otherwise be difficult and expensive to maintain using compression-refrigeration equipment. Pharmaceuticals and others manufacturing hygroscopic products require low humidity operating environments. Candy, seed and photo-film manufacture often require dry air to speed drying of the product. They can be used to retrofit a project where the installed conventional equipment can not maintain the desired humidity levels or where conditions have changed to create a humidity problem (example: original design had too low an outdoor air intake). Heat pipes are often coupled with desiccant systems to improve performance.
Desiccant systems are potentially best applicable where:
- Extremely low humidity (less than 30% relative humidity) is required,
- The latent load is large in comparison with the sensible load,
- The cost of energy to regenerate the desiccant is low when compared with the cost of chilling the air below its dewpoint.
- The air would have to be chilled to a subfreezing dewpoint with mechanical refrigeration.
- The process requires continuous delivery of air at subfreezing temperatures. With desiccant removal of water in the air the defrost problem on the chilling coils is minimized.
The changing requirements for increased ventilation air, increasing the latent load, may increase the application for these systems, particularly as on-peak electric costs rise. With low sensible heat ratios, the relative humidity can rise leading to mold, mildew and fungus along with more uncomfortable space conditions. Desiccant cooling is also considered in all-air systems where 100% outside air is required (clean rooms, hospital operating rooms, laboratories and some restaurants), in supermarkets in humid climates and ice rinks.
Hotels have used desiccant systems to provide dry makeup air to maintain a slight positive pressure in corridors. This minimizes humid outdoor air being pulled into the rooms through windows and exterior walls by the toilet exhaust system. The lower humidity air also reduces mold and mildew damage and cost of replacing furniture and wall coverings.
In many cases, other alternatives such as ice storage, ventilation air precooling, and dual path units may produce the desired result at a lower first and operating cost.
Some desiccant system vendors target market these systems to supermarkets and fast food chains. Reciprocating engine driven cogeneration is often included in the design as a way to use engine waste heat in summer. The perceived advantage is that when the supermarket operates at lower humidity (40%, or less) there is a saving on refrigerated food case energy use, due to a reduced need for defrost cycles and to higher efficiency with reduced frost on the evaporator.
Actual financial performance is still unclear, even those projects reported by the Gas Research Institute. Four systems installed in a fast food chain, which were surveyed in detail, failed to provide any significant savings in electric energy and actually may have increased operating costs. In addition, in some situations, the system parasitics (pumps and fans) were unknowingly paid for by these customers as additional electric charges, thereby further reducing actual operating savings. The choice of an induction generation in the cogeneration system also accelerated the failure of other on-site electric equipment. The greatest savings came from coincident fine tuning of existing HVAC system performance.
A superior way to improve system performance can probably be obtained through the use of ice-based thermal storage systems to lower air supply humidity and avail the customer of lower cost off-peak power. Other options include supermarket rooftop units with heat reclaim and subcooling, and with dual path units. In addition, heat pipes have been demonstrated to provide superior economic performance in many applications.
Desiccant units have been used in supermarkets as lower space humidity has been shown to improve the energy performance of case work, increase occupant comfort and may even improve sales. The two key questions are : Which alternative is the most cost effective to install? Which alternative represents the best system to install in a given application? Both of these questions should be addressed by a qualified professional.
Technology Types (Resource)
There are two broad categories -- liquid and solid desiccant systems.
Liquid desiccant systems use spray air washer type equipment with remote regeneration. They are typically used for large applied industrial applications.
Solid, inorganic, crystalline desiccants are impregnated in inert materials and used in a unit with a honeycomb type heat-wheel. They are typically used for commercial HVAC applications.
Solid desiccant systems are available in single-wheel and dual-wheel factory packaged units. The single wheel units include a desiccant wheel, cooling coil and reheat coil (hot gas, electric, steam, hot water, etc.). Outdoor air is dehumidified by the desiccant wheel, a cooling coil reduces the air temperature to the desired dew point and the reheat coil raises the air to the desired dry bulb temperature.
The dual-wheel units use a desiccant-based wheel and a sensible-only heat wheel along with the conventional chilled water or direct expansion coil. During the cooling mode the sensible-only wheel is used as a post-cooler (partially cool hot dry air off the desiccant wheel while preheating regeneration air) or to reheat the over-cooled dehumidified air.
Heat is required to regenerate or reactivate the desiccant material (drive off the absorbed water vapor and discharge it outdoors using a scavenger air stream). Part of the wheel is exposed to the conditioned or process air and the balance to the reactivation air stream. Natural gas is usually used to provide the heat for regeneration in single wheel units; a combination of exhaust air and gas heat may be used on dual-wheel systems.
In addition to the supply air fan, desiccant units require
- a process air fan to pull air through the wheel and over the cooling coil,
- a reactivation air fan
- a fractional hp motor to rotate the wheel
- in some cased, a fractional hp motor driving an inducer fan to push flue gases outdoors.
The combination of desiccant dehumidification and mechanical cooling can allow the chilled water or direct expansion system to operate at a higher evaporator temperature and thus a higher efficiency and reduce operating costs. The amount will depend on site requirements.
Desiccant units typically remove about 4 to 8 pounds or more of moisture per hour for each 1,000 cfm of circulated air. The lower the air flow face velocity through the desiccant and the higher the inlet air humidity the more the moisture that is removed. The addition of a dual wheel reduces the amount of mechanical cooling and reactivation heat required.
While desiccant equipment is durable and efficient where properly applied, it does require special maintenance. Good filtering of both air streams (incoming and outgoing) is vital. The filtering system must be clearly visible to maintenance personnel, it must be easily inspected, removed and replaced, and a maintenance schedule must be established and followed. If a solid desiccant is clogged with particulate or a liquid desiccant properties are changed by entrained particulates, its efficiency drops rapidly and it may have to be prematurely replaced.
The wheel(s) and their rotating mechanisms must be clearly visible to maintenance personnel, they must be easily inspected, maintained and repaired. Care must be taken to not allow holes to be drilled in the casings, access doors to become warped, etc. as if humid air leaks into either the dry air ductwork or into the unit itself, the system efficiency is markedly reduced. The life expectancy of the wheel is also dependent on the proper operation of the regeneration system.
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