Laboratory Evaluation of the OASys Indirect/Direct Evaporative

System Description
Figure 1 shows two images of the OASys system (Model CRS1000), provided by Speakman CRS. As
shown in the right-hand diagram, outside air is drawn in by a single centrifugal fan, which forces all of
the air through the dry side of the indirect cooling module. As the air exits the first stage, it splits in two
directions. The supply airflow passes forward through a direct evaporative cooler module on its way to
the conditioned space. A smaller exhaust air stream reverses direction to pass through the wetted side of
the indirect cooling module, and create the cooling medium for the intake air. The point at which this
split occurs is the most significant change from the earlier IDAC system, which split the streams upstream
of the indirect module as they exited the fan. This change enables a significant improvement in system
performance, which will be discussed in more detail later. The level of water in the reservoir at the
bottom of the unit is maintained by a float valve, and a single pump is used to circulate water from the
common reservoir to the indirect and direct modules. The reservoir also has a flush pump, which is
operated periodically to control the buildup of minerals as water is evaporated, and to drain the system
when not in use. The water supply line also has a solenoid valve to shut off the supply when the system
needs to be drained.
Figure 1: OASys System
EXPERIMENTAL DESIGN AND PROCEDURE
Process Description and Performance Characteristics
Performance data are required in order to document the ability of evaporative systems to maintain
comfort under various conditions. The data collected are intended to provide enough information to
adequately model their performance, and thus to perform further analysis to determine the annual energy
usage and peak demand for different climates. The results may be disseminated through Emerging
Technologies program information transfer activities, and may be used to develop marketing materials for
future rebate or incentive programs.
The performance of an evaporative cooler is best described using a psychrometric chart, which displays
moisture content (humidity ratio in mass of water vapor per mass of dry air) against temperature. Figure
2 shows a simplified psychrometric chart with some of the basic concepts and terms identified. When dry
air is exposed to liquid water, some of the heat contained in the air will be absorbed through the
evaporation of the water, causing a decrease in the air temperature. (Hot, dry air is converted to cool,
humid air.) If continued long enough, air will become saturated with water vapor (100% relative
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