B y Don B . Shirey III, Member ASHRAE, and Hugh I. Henderson Jr., P.E., Member ASHRAE ngineers and equipment manufacturers need a bet er understanding of de- humidification performance at part-load conditions to evaluate the impacts of their design choices on indoor humidity levels. occupant comfort. and indoor air quality. Data from previous field test studies"? show that the moisture removal capacity of a cooling coil degrades at part-load conditions - especially when the su pply air fan ope rates continuously. Figure 1 illustrates this concept with transient data from a laboratory test. Degradation occurs because a portion of the moisture that condenses on the coil surfaces during the cooling on cycle (blue data) evaporates back into the airstream when the coil is off (green data). The data in the plot shows that the transient off-cycle perfor- mance of the coil IS essentially adiabatic with sensible cooling (red data) provided in conjunc- tion with evaporation of moisture (green data) back nto the irstream, The off-cycle sensible cooling diminishes with time as the amount of available moi ture on the coil surfaces decreases, As a result, a cooling coil that cyc es on and off in response to a control or thermostat signal will provide a smaller fraction of its total cooling capacity as moisture removal when the system spends relatively more time with the coil off. Conversely, the full latent removal capability of the system is only realized when the coil oper- ates continuously, The net impact of this latent degradation phe- nomenon is that dehumidification per ormance depends on the runtime fraction 0(' t he co oli ng coil (load divided by steady-state capacity), Fig- About the Authors Don B. Shirey III is principal resear h engineer at the Florida Solar Energy Center In Cocoa, Fla. Hugh I. Henderson IS Wth Caz enovia , N, Y . April 2004
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8/6/2019 Dehumidification at Part Load_ASHRAE_April2004
cooling equipment and predict that space The latent capacity of a cooling coil
humidity levels are maintained at lower degrades at part-load conditions. This
levels than are observed in practice. Kosar degradation is most significant when the
et al.6 showed that ignoring latent degra- supply air fan operates continuously with
dation causes hourly building simulation a single-stage cooling coil. However,somemodels to underpredict space humidity degradation also occurs with modulated
levels by 5% to 10% RI [ under typical and staged cooling systems as well. Con-
conditions in a small office application. tinuous supply air fan operation is used
The latent degradation model from in nearly all commercial buildings to pro-
Henderson and Rengarajan has already vide the outdoor air ventilation require-
been incorporated into some whole build- mcnts prescribed by ANSliASHRAE
ing hourly simulation models7,~,9 and Standard 62, Ventilationfor Acceptable
will be available soon in other models.l? Indo or A ir Q ualit_v 1 1 and to provide air
This research project has confirmed the circulation for occupant comfort. The im-
validity of the latent degradation model pact of latent degradation must be con-
for single-capacity systems and is work- sidered in these circumstances,
ing to refine the model to consider more A recent utility research project indi-
applications. We are also working to UI1- cated that 2(%of Florida homeowners op-
derstand the moisture-related character- crated the supply air fan continuously. 12
istics of cooling coils so we can develop This fan operation mode may become
guidelines for selecting model parameters more prevalent in residential applications
for an array of cooling coil and equip- if its use is recommended for central air
ment configurations. filtration systems (e.g., uve lamps or
; : : :===============::; ! high-efficiency air filters), ventilation rc-
quiremcnts.l' or occupant comfort. How-
ever, most residential air conditioners
cycle the supply air fan on and off with
the cooling coil in response to the ther-
mostat signal (AUTO fan control), Most
homeowners in humid regions inherently
know this operating mode is preferable
since it provides reasonable moisture re-
moval, However, laboratory data have
confirmed that evenAUTO fan control can
result in significant degradation in dehu-
midification performance at part load.
This study is working to quantity the
equipment characteristics, control
modes, and operating conditions where
latent degradation is a concern. Algo-
rithms to predict latent degradation are
being incorporated into hourly wholebuilding energy analysis tools to allow
building designers and equipment manu-
facturcrs to quantity thc impact as well.
rAir Condit ioning, Inc.
is an Air Conditioning
Equipment provider/manufacturerseeking to expand into the US