An expanation of the differences between desiccant, sensible and enthalpy wheels.

The rapid growth of wheel technology in air treatment applications has brought with it confusion about the differences in wheels. It is not uncommon for desiccant or enthalpy wheel salespeople to have a customer tell him that he's currently using desiccant wheel technology only to discover that the customer is, in fact, using an energy recovery ventilator (sensible wheel). And a sensible wheel company routinely gets questions from customers about their "desiccant wheels."

Often the attempt at resolution for contractors and/or engineers is to pull out and analyze with the customer a psychrometric chart, which customers typically view as a foreign language they can't understand. And rarely does the psychrometric chart easily and completely demonstrate how the wheels work to improve conditions and/or save energy.

The decision about which system is best suited usually depends on the intended application. To install the right system and achieve the best performance, a number of factors have to be taken into consideration, including internal moisture content, outside air required, amount of exhaust and desired results. On occasion, a combination of wheel technologies will provide the best solution. To help simplify wheel technology, the following describes the functions of and takes an in-depth look at the construction and operation of each type of wheel.

Different wheels, different functions

The three types of wheels perform three distinctively different functions involving sensible heat, latent heat and total heat. Sensible heat can be defined solely as the temperature of the air. The warmer the air, the greater the sensible heat. Latent heat can be defined by the humidity or the amount of moisture in the air. the moister the air, the greater the latent heat. Total heat is simply the sum of sensible and latent heat.

Desiccant wheels remove latent heat. Sensible wheels remove sensible heat. Enthalpy wheels remove total heat (sensible and latent). Sometimes the term "heat wheel" is applied either sensible or enthalpy wheels. There is a big difference.

The vast majority of desiccant wheels are made up of silica gel or molecular sieve bonded to a substrate of fiberglass paper or sometimes aluminum. When viewed from the air path the wheel takes on the appearance of finely meshed honeycomb material with many small flutes. the flutes, like the fins on a coil, force the air path along the concentrated desiccant. The desiccant provides an attraction for water molecules through microporosity or essentially based on the size of its pores. In this manner, silica gel and molecular sieve desiccants are adsorbents or mechanical attractors of water like a sponge.

In a typical application, 75% of a desiccant wheel is in the target air path with remaining pie shaped 25% in a regeneration chamber. Regeneration is accomplished by passing extremely hot air, about 250°F, through the wheel and into an exhaust. The hot air provides greater attraction for water molecules than the desiccant and the wheel is thus regenerated. The hot air may be generated by a number of strategies, the most common being a direct gas furnace.

Desiccant wheels by nature are removers of moisture (latent heat) whether from outside air or internal load. They do not by themselves reduce the energy load. They simply replace latent (moisture) load with increased sensible load. As in the opposite of cooling something by putting water on it, they increase sensible heat (temperature) by removing moisture from the air.

Common desiccant wheel applications

The three most common uses of desiccant wheels are reducing an internal moisture load, holding or lowering a specific relative humidity (RH) or specific dew point and processing high latent outside air. Many times, more than once of these purposes is being served by the same wheel. Commercially, desiccant systems are most common in supermarket applications. The coils in supermarket refrigerated and freezer cases have low dew points. When warmer, moisture-laden air contacts the lower dew point of the refrigerated cases, condensation takes place (much like when two air fronts collide, making rain).

Condensation creates several problems for supermarkets. Frost or ice on coils lessons energy efficiency and forces the refrigeration system into defrost cycles which rapidly increases energy usage both in operation and recovery. Condensation and frost on case doors and product also reduce the marketability of product.

Sensible wheels are essentially sensible heat (temperature) collectors acting as sensible heat exchangers in two air streams. They are most commonly made from aluminum, although there are also mylar and polypropylene versions. When viewed from the air stream, they share the honeycomb appearance of the desiccant wheels with numerous small flutes to channel air across the collecting material.

Unlike desiccant wheels, sensible wheels require both an exhaust and a supply air path. In the stationary position, a portion of the wheel is in both air streams. When operating, the wheel simply rotates between air streams. In summer, the wheel can pick up sensible heat from the supply and exchange it in the exhaust air path, thus lowering the temperature of the supply. The process is reversed in winter. In spring and fall, the wheel is self-adjusting so long as the exhaust air path conditions are more desirable than the supply. The wheel would typically be shut down in favor of an economizer if conditions dictate.

While a lot of complex arguments are made about the effectiveness and efficiency of sensible wheels in different conditions, there is a simple principle that can be applied.

The greater the sensible temperature difference between the two air streams, the more work the wheel will do. The equation is generally the sensible temperature difference times wheel efficiency equals temperature change in each air stream.

Enthalpy wheels combine strategies

Enthalpy wheels are a combination of sensible and latent strategies on the same wheel. Their physical appearance is very similar to sensible wheels. A desiccant is bonded to the wheel's aluminum collection system. Air is channeled through multiple flutes across both the desiccant and the sensible collector.

Like sensible wheels, enthalpy wheels straddle both air streams and a desirable exhaust air stream is required. Also like sensible wheels, the greater the latent and sensible difference in the two air streams, the more work the wheel will do. The efficiency equation is generally identical to sensible wheels. The sensible temperature difference and the latent grain difference times wheel efficiency equals sensible temperature and latent grain change in each air stream.

Many industry insiders believe the enthalpy wheels could be applied almost anywhere a sensible wheel is applied. The cost difference between the two is not significant. The sensible performance is almost identical. So long as the internal latent load is already being addressed (as in many retrofit applications), enthalpy wheel performance on outside air latent loads can be dynamic.

It is important to remember that wheel applications are not limited to an either/or choice. Many of the manufacturers in the industry offer both single and dual wheel systems utilizing all the possible wheel combinations. These systems are available as both make up air and complete package systems including options like DX cooling, chiller coils, waste heat systems and dozens of other variations.

The best source of applications technology remains the systems manufacturers themselves. Most of them have applications and sales engineering teams specifically available help engineers, contractors and end users.