ASHRAE Journal - August 2008 - (Page 39)

ERV is implemented with an air-to-air enthalpy exchanger, as a component of an Energy Recovery Heat Exchanger T3, H3 T4, H4 engineered air handler, integrated with a Return Air unitary air conditioner, or as a component Exhaust of a makeup air unit. As illustrated in Figure 1, an air-to-air enthalpy exchanger transfers sensible and latent heat (i.e., T2, H2 T1, H1 heat and water vapor, or total enthalpy) Outdoor Air Supply Air between conditioned air being exhausted from the building (at T3 and H3) and outside air being supplied to the building (at T1 and H1). During the cooling season, Duct Wall (typ) the warm, humid outside air is precooled Figure 1: Generic conﬁguration of an enthalpy exchanger. by transferring heat and moisture to the cooler, drier air being exhausted from the building, so that T2 Myth of Declining Latent Cooling Capacity of Unitary ACs and H2 are lower than T1 and H1. The opposite is true in heating When air-conditioning equipment cools air, it reduces the mode (i.e., building exhaust air preheats the cooler incoming temperature of the air (sensible cooling) and it reduces the outdoor air via the enthalpy exchanger), but the primary focus moisture content of the air (dehumidiﬁes the air) by causing of this article is the cooling season. a portion of the water vapor in the air to condense into liquid Rotary wheel type enthalpy exchangers have a heat and mass water (latent cooling, so-called because the latent heat of transfer core material that is coated with a desiccant. The core moisture condensation from the air is the portion of the cooling alternately passes through the outdoor supply air and exhaust air- load associated with dehumidiﬁcation). The dehumidiﬁcation streams as the wheel rotates, absorbing heat and moisture from the effectiveness of air-conditioning equipment is commonly charwarmer, more humid airstream, and desorbing heat and moisture acterized by the sensible heat ratio (SHR), which is the ratio to the cooler, drier airstream. Heat and moisture are stored in the of the sensible cooling capacity to the total (sensible + latent) core material as the core passes from one airstream to the other. cooling capacity. Reducing SHR increases the portion of the In many types of commercial buildings—e.g., ofﬁces, retail, total cooling capacity that is providing dehumidiﬁcation. hotels, hospitals—the main design humidity load is the moisture It has become a matter of common wisdom that increases in brought into the occupied space of the building with ventilation the energy-efﬁciency ratio (EER) of unitary air conditioners that air at hot, humid ambient conditions. The reason why ERV has a have occurred since the early 1980s have been accompanied by positive impact on building humidity control is that an ERV will a decrease in the latent cooling capacity as a fraction of the total remove a large portion of the moisture from the ventilation air cooling capacity (i.e., increased SHR). The wisdom explains before it enters the building by transferring it to the less humid that among the ways that efﬁciency is increased, increasing the exhaust air from the building. To appreciate why this matters, a evaporator surface area results in a higher evaporator temperareview of how sensible and latent cooling loads in commercial ture and, therefore, less moisture removal capacity. buildings have evolved provides useful perspective. In the real world of commercially available air-conditioning In recent years, an increasing number of buildings and equipment, no such direct relationship exists. ARI1 conducted their occupants have experienced serious moisture problems, an extensive review of the relationship between EER and SHR, including so-called sick building syndrome, mold growth, and for current equipment and equipment going back to the 1970s. occupant discomfort due to high humidity. These problems arise The consistent ﬁnding is that the SHR of individual unitary air from poor interior humidity control and liquid water sources conditioner models has varied between 0.65 and 0.80 at all EER such as plumbing leaks and rain water leakage (Serious cases levels, from 1970 to the present, with no statistically signiﬁcant of liquid water cannot be remediated by dehumidiﬁcation alone. correlation between either the EER and the SHR or between The water leaks need to be addressed). Moisture and mold the year of manufacture and SHR. problems can cause serious health problems among occupants and reduce productivity. In extreme cases, buildings have been Ratio of Sensible and Latent Cooling Loads Has Changed rendered uninhabitable. The ﬁnancial consequences of these Although the SHR of unitary air-conditioning equipment has situations can be substantial and are reﬂected in rapidly escalat- not changed, building loads have changed substantially. The ing liability insurance rates. These impacts have been widely energy-efﬁciency improvement measures such as better wall and roof insulation, reduced window U-values, increased solar reported and documented by research and in the press. Poor humidity control often is attributed to the inability of shading, more energy-efﬁcient lighting, which have been driven conventional unitary air conditioners to handle the moisture by Standard 90.1 have almost exclusively reduced sensible loads. A common conception is that the increased efﬁciency of modern air-conditioning equipment has resulted in reduced About the Author John Dieckmann is a principal with TIAX LLC in Cambridge, Mass. latent (moisture removal) capacity. August 2008 ASHRAE Journal 39

Table of Contents Feed for the Digital Edition of ASHRAE Journal - August 2008

ASHRAE Journal - August 2008 Contents Commentary Industry News Letters Meetings and Shows Maintain to Sustain—Delivering ASHRAE’s Sustainability Promise Ultraviolet Germicidal Irradiation: Current Best Practices Improving Humidity Control With Energy Recovery Ventilation Single- or Two-Stage Compression Data Center Cooling: Using Wet-Bulb Economizers Building Sciences InfoCenter Practical Pointers Products Emerging Technologies Washington Report People Special Products Classified Advertising Advertising Index

ASHRAE Journal - August 2008

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