Minimize lifecycle cost of desiccant dehumidifiers
Energy is the most costly item to consider when looking at the lifecycle of HVAC equipment.
By Mike Hayes, PE, Munters Corp., Atlanta
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Lifecycle cost of any type of HVAC equipment consists of the installed cost of the equipment, including its commissioning, combined with the sum of its annual maintenance and energy costs over its lifetime. Energy is by far the most costly of these components. The other issues should not be overlooked because failure to install or maintain a piece of equipment can lead to its failure to perform the desired task efficiently.
When purchasing a desiccant dehumidifier, the key factor is how much energy it will consume to achieve the desired dehumidification effect. According to exceptions accepted to ASHRAE 90.1-2004, desiccant dehumidifiers have been proven to be the most efficient dehumidifiers commercially available today. By reviewing ASHRAE 90.1 2004-3 at www.ashrae.org/technology/page/64, the specifier will note that the condenser reactivated (site recovered heat) desiccant system is more efficient than energy recovery or a heat exchanger enhanced system at the vast majority of entering conditions and certainly at any condition a normal dehumidifier would see. Hourly analysis will be needed to estimate the energy usage of the equipment alternatives. This allows comparison of the alternatives at part load conditions as well as peak loads. By definition, part load is what the equipment will see more than 99.6% of its operating hours. Clearly, hourly analysis is necessary to quantify that operating expense.
Another reason that space humidity should be evaluated at part load conditions is the industry has recognized that many traditional systems fail to provide adequate humidity control during those hours. The environmental impact of the energy used, as well as its cost, impacts the sustainability of the project.
Desiccant dehumidifiers use a desiccant wheel to dehumidify the supply air. A dry desiccant wheel absorbs moisture from the supply airstream. The heat of vaporization of the moisture as it is absorbed increases the air temperature in a constant enthalpy process. Once the wheel has absorbed the moisture, the moisture must be driven off of the wheel during a “reactivation process.” Generally, this is performed by having a second airstream that has been heated to provide a low relative humidity. The majority of the energy consumed by such units is in reactivation: the energy needed to raise the reactivation air temperature high enough to remove moisture from the desiccant. The reactivation air is typically outside air, but exhaust air can be used as well.
Installation of desiccant dehumidifiers is very similar to that of any other air handling equipment. There are equipment designs for both indoor and outdoor installations. The principal difference between desiccant and other HVAC equipment is that desiccant units have a reactivation airstream that carries the moisture from the wheel outdoors to the atmosphere. In an outdoor installation, this is easy. Most units are designed to take this air from the outside and discharge it to the outside. The only caution is to take care that the discharge air (humid air) does not short cycle into any outdoor air intakes of the unit or other adjacent equipment.
In indoor installations, reactivation air (from building exhaust or from outdoors) must be brought to the unit and then discharged from the building. Reactivation fans need to generate sufficient external static pressure to overcome the duct runs required. (Note that the reactivation air leaving the unit has high moisture content and therefore a high dew point.) Make sure that moisture will not condense in ductwork, or that the design allows condensed water to drain out of the duct appropriately. In some cases insulation can be added to the reactivation discharge duct to eliminate condensation. In other cases where the reactivation ductwork egresses through cold environments, drainage or sloping may need to be added to control the flow of water. Ice rinks are a typical environment where indoor temperatures are low and condensation will form inside the reactivation air ductwork leaving the unit.
Most desiccant dehumidifiers are designed to allow removal of the desiccant wheel if that should ever be necessary. Manufacturers recommend a wheel pull area adjacent to the unit similar to the distance needed to pull a cooling coil. Service technicians and building owners will thank the designer who provides reasonable service access beside the unit.
Controls are another important aspect of installing a dehumidifier for efficient and effective operation. Moisture migrates quickly to obtain equilibrium. So in most buildings, the dew point—the absolute humidity-—stays rather consistent compared to the relative humidity (RH), which varies with changing air temperatures as well as changing absolute humidity. While RH sensors are inexpensive, they often provide inadequate control and generally are not very accurate. As temperature fluctuates in the space, the RH changes while the absolute humidity remains constant. RH control can allow for large swings in the absolute humidity level providing for inefficient operation and unintended consequences.
Dew point control or calculated dew point from temperature and RH measurements will provide better results in terms of both stability of the humidity level and the lifecycle cost to maintain that stability. This is particularly the case when the building is unoccupied. For those hours overnight, on weekends, and during school vacations, dehumidification is required while temperature during the unoccupied period can be allowed to float. Keeping the dew point constant saves energy by avoiding needless heating and cooling when the real goal during unoccupied hours is simply to avoid moisture accumulation, condensation, and mold.
Commissioning and O&M
Wheel-based desiccant dehumidification commissioning requirements are also similar to standard air handling products. Common tasks like inspecting fan operation and rotational direction, belt tension, and filter installation must be performed before startup. Wheel rotation (direction as well as rotation) must be confirmed, but keep in mind that 0.1 rpm is a very slow rotational speed and should not be mistaken for malfunction. The reactivation heat source easily can be verified as functional and its leaving air temperature verified to be that as specified by the designer or manufacturer.
Wheel performance easily can be verified by measuring temperature rise in the supply air across the desiccant wheel. As the dehumidification process heats the airstream, a significant increase in temperature indicates that the wheel is dehumidifying. Quantifying that performance requires a little more effort. The manufacturer should be able to provide a performance estimation for the wheel at the actual entering conditions, if they are not at the design maximum specified by the contract. Note that measuring humidity at low dew points and low relative humidity leaving desiccant wheels requires an accurate portable humidity sensor. At a probable cost of $500 to $2,000, an accuracy ofng air condition to provide accurate readings (several minutes, not several seconds).
The assembled wheel system is relatively simple and straightforward. The wheel turns quite slowly at approximately 0.1 rpm. Reactivation heating sources can be direct or indirect fired gas, hot water, steam, electric resistance heat, and refrigeration condenser heat, to name the more common sources. Supply and reactivation air fans are required as well. These components are well known to service technicians and require only normal maintenance efforts.
The vast majority of modern desiccant wheels are low maintenance. Most desiccant materials are adsorbent types that do not chemically bond with the moisture and therefore do not have the ability to “wash out” or lose their capacity over time. The slow rotational speed provides for low maintenance of the wheel's mechanical operation, and failures of the drive mechanism or belts are rare.
The wheel itself is a laminar flow device. Air traveling through the wheel passes through flutes in the wheel that are large enough to allow dirt to pass. Filtration is required to prevent build up on the wheel, but a desiccant wheel is slightly less susceptible to dirt buildup than a cooling coil, because air flows through the wheel in opposite directions, blowing off accumulated dust, six to ten times every hour.
In addition, cleaning of the wheel is easier than cleaning a coil. It is fairly simple to remove the wheel from a unit for cleaning; no strong detergents or acids are required. Finally, the second reactivation airstream often requires access to both sides of the unit for fan or other component maintenance. Filter changes are all that is required from a maintenance perspective to keep most desiccant wheels operating efficiently for a long life. (Many have had 25-year life spans.)
Some industrial applications use adsorption-type desiccant wheels for low dew point applications. These types of desiccant dehumidification wheels require more care. Allowing the wheel to become too saturated can result in a loss of the desiccant material. These units need to have safety cycles to allow the desiccant to be dried, to be “reactivated” continuously, or to keep moisture from being attracted to the wheel even when dehumidification is not required or the unit is not operating. In addition, these types of wheels can dehumidify only through a limited number of sorption/desorption cycles (the wheel absorbing moisture and releasing it). The wheel's capacity will degrade over time, and eventually it will need to be replaced to provide the required capacity. Consultation with the dehumidifier manufacturer is recommended when this type of desiccant dehumidifier is applied.
The vast majority of desiccant dehumidifiers provide low lifecycle cost if the equipment is maintained correctly. These efforts are not extensive, nor are they extraordinary compared to common air handling systems. With a little care desiccant dehumidifiers can provide dry, efficient buildings for many years.
The desiccant dehumidification cycle is a constant enthalpy heating and dehumidification process. The heat of vaporization that is released from the water as it changes state heats the air. This desiccant process needs heat to drive the moisture from the desiccant material. Traditionally, this heat source has been a gas-fired burner, steam heat exchanger, or electric resistance heater.
Often, site-recovered heat is used to make the desiccant cycle very efficient. This can be waste heat from any source and often is condenser heat from refrigeration or air conditioning. Products are available that package air conditioning as part of the unit and use the condenser heat to provide desiccant dehumidification with no additional energy requirement. These reactivation energy costs must be calculated and evaluated as an important part of the lifecycle cost analysis.
Hayes is commercial product manager with