ACHRNEWS

More energy-efficient dehumidification

May 17, 1999
A new dehumidification system that does not use desiccants is staking its claim to a major advantage. Research data show it uses half the energy compared to desiccant dehumidification.

Nautica Dehumidifiers, Inc., Huntington, N.Y., has just completed a 10-year design process to develop its new technology. Called MSP™, it uses multiple-plate air-to-air plate heat exchangers to recover heat from the incoming outside air or recirculated air as an alternative to reheat.

The Basics

Dehumidification is necessary for such applications as supermarkets, indoor swimming pools, and in high-humidity climates. It is a necessity for a number of industrial process applications as well.

Let’s start with a very basic example. Let’s say you have an airstream with a temperature of 82°F which has 130 grains of moisture/lb of air. You wish to provide air at 75° to the space with 35% rh, which corresponds to 44 grains of moisture/lb of air.

Figure 1 shows system operation with a conventional dehumidifier. You would cool the air with a cooling coil to 45° to extract the moisture, and reheat it to 75° with a reheat coil.

With the Nautica technology, you would use an air-to-air plate heat exchanger to precool the air from 82° to 68.5° for the first pass, by regenerative thermal exchange with the cooler, leaving air. The precooled air would flow through a cooling coil to bring it down to 45° to complete the moisture extraction.

The fully dehumidified and cooled air would then be drawn back through the opposite side of the air-to-air plate heat exchanger to be brought up to 75°, then discharged into the space. Figure 2 shows the operation of this technology.

The energy savings come from the elimination of the supplementary energy for the reheat coil, and a partial reduction of the cooling load with the air-to-air plate heat exchanger.

What About Desiccants?

Desiccant-based dehumidification has typically been considered the energy-efficient method.

Desiccant-based systems, however, require a heat source to regenerate the moisture-free desiccant to make the system operate. Also, if the controls drift, performance is reduced while energy use increases.

Desiccant dehumidification systems aren’t always well maintained by their owners. The desiccant wheels have a limited life, and the cost of replacing them is high, since wheels can be costly and the entire unit has to be taken apart to replace the wheel.

Comparative Study

The New York State Energy Research and Development Authority (NYSERDA) performed a comparative study of the performance of air-to-air plate heat exchangers, conventional dehumidification systems, and desiccant-based dehumidification systems.

The study focused on dehumidification for supermarkets. High humidity in supermarkets accelerates the buildup of frost on the refrigeration coils, which increases the frequency of the energy-intensive defrost cycle.

While this, in effect, reduces humidity in a supermarket, it does so with an inefficient process. When humidity is controlled with a separate dehumidifier, the total reduction in energy consumption from reduced defrost cycles (accounting for energy consumed by both the dehumidifier and refrigeration equipment) would be in the range of 5% to 12%.

Reducing Static Losses

As part of its product development, Nautica found that the use of multiple, small-plate, air-to-air plate heat exchangers was more energy efficient, and cost less to construct, than applying larger-sized plate units.

The reason is that the use of multiple small plates provides lower static losses, since they are smaller in size, yet can achieve the same degree of heat exchange.

Also, it was found that smaller systems had more uniform air velocities across the coils, which tended to further improve performance.

The net result is that multiple, small-plate, air-to-air plate heat exchangers are smaller and lighter in weight, with reduced material and shipping costs.

Static losses across the system could be kept as low as 0.2 in. wg. The large-plate air-to-air plate heat exchangers had static losses up to 1.2 in. wg.

Single and Dual Path

In a single-path system, the outside air and the recirculated air are mixed, and then cooled and dehumidified with one set of coils.

In a dual-path system, the latent and sensible loads are separately cooled and/or dehumidified, each with its own dedicated coil.

Using the Nautica technology, one can size a system to dehumidify outside air. The unit is relatively small, as the air volume is lower, and setting up a cooling system in a building in this fashion is energy efficient.

There are various options to these systems, including damperless and dampered variations. In a dampered system, it is possible to vary the air volume in response to varying sensible and latent heat loads of the space.

Into the Pool

Nautica’s systems are ideally suited for dehumidification of natatoriums. The latent heat that is recovered from the humidity in the pool air can be used to heat the pool water.

The energy savings, when compared to other types of dehumidification equipment, can be significant. One computer model showed 60% energy savings.

Systems are constructed to withstand the harsh operating environment of indoor swimming pools. Double-wall construction is used with internal insulation between the walls.

All galvanized steel cabinets are coated with Heresite V-514 air-dried phenolic with plasticizer. Cooling coils are coated with another similar Heresite product.

Figure 3 shows the airflow schematic for dehumidification of a swimming pool. It is also possible to install heating or cooling coils in the supply ductwork.

Daniel Karpen, P.E., is a consulting engineer and certified energy manager. He may be contacted at 3 Harbor Hill Drive, Huntington, N.Y. 11743; 516-427-0723.