Consulting-Specifying Engineer - January 2009 - (Page 60) Equipment Lifecycles 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 cant 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 prounit 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. Energy costs 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 Munters. He has more than 20 years of experience in the HVAC industry and 10 years in desiccant dehumidification technology. 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. 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 of ±2% RH is about the limit of affordable technology. (Lower cost equipment, with ±3% to ±5% accuracy, seldom operates reliably in the humidity range of the dry air leaving the unit, which is well below 15% at 70 to 125 F.) Also, allow adequate time for any humidity sensor to acclimate to the leaving 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 desicvides 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 60 Consulting-Specifying Engineer • JANUARY 2009
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