What is desiccant wheel technology? In what environments are they most efficient and in what environments are they least efficient? Do they really work well?
Desiccant wheels are also known as heat recovery wheels. The main purpose of a desiccant wheel is dehumidification without the use of refrigeration. A typical packaged air conditioning system removes humidity from the air by lowering the air temperature below the dew point and allowing moisture to condense onto the fins of the indoor coil. As shown by the latent heat removal capacities mentioned in the answer to question #6, only 20% to 30% of the cooling capacity of a packaged unit operating at full capacity is latent. Up until the late 1980’s, this limited latent capacity was sufficient to remove the required quantity of moisture from the air required to ventilate most buildings in most geographic areas of the United States.
Two events have occurred that changed this situation:
- The first event was the realization by our industry that if the quantity of ventilation air introduced into a building was insufficient to dilute the contaminants produced within the building, the quality of the indoor air became unacceptable. It didn’t matter if the contaminant was carbon dioxide, caused by human respiration, or formaldehyde, due to the out-gassing of pressed-board furniture. Indoor air quality issues were exacerbated by the fact that many buildings had tightened their envelopes and reduced their ventilation due to the energy shortage of the 1970’s and the ensuing energy saving initiatives. In most cases, the solution to poor indoor air quality is to introduce additional outdoor air into the building to dilute the contaminants. It was recognized that current codes did not require sufficient outdoor air for ventilation in many cases. Therefore, the codes were amended to require significantly greater outdoor air quantities to be introduced into new buildings. ASHRAE 62-1989 is the standard that forms the basis for most of the new ventilation codes. As we introduce additional outdoor air into a building, we increase the dehumidification load on our HVAC equipment. As the load exceeds the latent capacity of the equipment, the humidity level within the structure increases.
- The next issue working against our packaged HVAC unit is the current mold phobia we are experiencing. We all know that increased humidity levels within a structure lead to an environment that encourages mold growth. To make matters worse, some engineers were attempting to deal with the increased loads that increased ventilation put on the HVAC systems by increasing the total capacity of the systems they designed.
Unfortunately, these systems were designed for a full load day, which very rarely occurs, and so the amount of time at which the units operated continuously at full capacity was minimal. When a unit is oversized in this manner, it tends to run so little at full capacity that it has minimal ability to remove latent heat. This causes the humidity level within the building to increase, further increasing the possibility of mold growth.
But what has this to do with desiccant wheels?
The answer is that if you are operating a building that requires a great deal of ventilation air, chances are that packaged HVAC equipment will not meet the moisture removal requirements. There are several mechanisms being added to packaged equipment today as factory installed options to allow these units to remove significantly more moisture from the air than a typical packaged unit. One of these mechanisms is a desiccant wheel.
One manufacturer claims that its desiccant wheel has the ability to reclaim up to 83% of the energy lost due to ventilation. If one assumes that 20% of a building’s HVAC energy usage is used to heat and cool ventilation air, we are still looking at a 16.6% possible energy savings using a desiccant wheel. This is without the added advantage of possibly reducing the total installed tonnage in a building and reducing the relative humidity levels to an acceptable range.
What does a desiccant wheel look like?
A desiccant wheel is basically a circular plastic or styrene disk with an extended surface that is impregnated with silica gel. Silica gel is the stuff that’s inside the little white bag that comes packed with cameras and electronic equipment. It’s placed there to absorb any moisture that enters the box. Obviously, if it removes moisture from the air in the box your camera was packed in, it can remove moisture from the air passing through your air conditioner.
How are desiccant wheels positioned and why?
The desiccant impregnated wheel is normally positioned so that half of the wheel is in the stream of outdoor air being introduced into the building, and the other half of the wheel is in the stream of exhaust air being removed from the building. The wheel is slowly rotated so that any spot on the wheel rotates through the outdoor air, then through the exhaust air, and back into the outdoor air. As the outdoor air hits the silica gel, moisture is absorbed by the gel. As the heel rotates, the moisture laden gel is placed in the exhaust air stream. The air being exhausted is conditioned air, so it has relatively low moisture content. This air regenerates the silica gel by absorbing the moisture from the gel. The regenerated gel is now ready to absorb moisture again as it rotates into the outdoor air stream. In some cases, the gel is regenerated by raising its temperature with a heater.
What environment(s) are desiccant wheels most efficient and do they really work?
Desiccant wheels do work. They are best applied where one must treat large quantities of clean, extremely humid, outdoor air. A school in a humid climate would be a perfect application. Large quantities of ventilation air are required. The occupancy is consistent, and the outdoor air stream, as well as the exhaust air stream, is accessible. Big box, retail, warehouse type stores are also a good candidate for desiccant wheels when they are located in humid climates. In this situation, a typical strategy would be to have one or two 40-ton packaged rooftop units with heat recovery wheels handle the complete ventilation of the space. The additional rooftop units could be ordered without any outdoor air options, assuming that the climate was such that economizer operation was not realistic. The heat recovery wheels would probably reduce the overall installed tonnage and electrical service requirements as well as provide dehumidification of the ventilation air.
Like most innovations, there is a downside. There is an increased first cost and space requirement for equipment containing a desiccant wheel. There is energy expended by the motor that rotates the wheel and by the increased resistance that the wheel and filters add to the air passing through the wheel. There is also the cost of maintenance, which can be considerable. There is usually additional filtering required. The wheel must be kept extremely clean. The seals that separate the wheel into the exhaust and the outdoor air sections must be kept in good condition. Requiring monthly inspection and cleaning of a heat recovery wheel is a real possibility.
We believe that the best strategy for dealing with ventilation air is:
- Start with a team comprised of your design people, your mechanical engineer and your service company.
- Discuss – prototypically – how much outdoor air is being introduced.
- How much is being heated and cooled.
- How much is exhausted clean.
- Then work on a strategy to reduce the amount of air that is heated and cooled.
One example of reducing the required quantity of conditioned ventilation air is the use of kitchen hoods that use unconditioned make-up air ducted to the perimeter of the hood rather than conditioned air dumped into the kitchen, in front of the hood. Another possibility is demand ventilation where the quantity of outdoor air introduced into a space close to zero and is varied based on the carbon dioxide levels in the space or the number of people present. Then, after minimizing the quantity of conditioned outdoor air you are handling, speak with manufacturers about heat recovery wheels and other hardware, and the projected ROI.