Handbook of air conditioning and refrigeration / Shan K

AC SYSTEMS: EVAPORATIVE COOLING 27.17
The 55°F (12.8°C) all outdoor cooled air from the two-stage evaporative cooler provides cooling
for 95 percent of the cooling hours. However, this cooled air temperature is again lowered to
50°F (10.0°C) by the DX coil so that the space relative humidity in summer can be maintained at
40 to 45 percent. This also reduces the supply volume flow as the air density is increased.
Winter Heating. During winter heating, the heat-pipe heat exchanger uses exhaust heat to raise
the outdoor air temperature from 0°F (�17.8°C) up to 42°F (5.6°C). As the supply volume flow
rate is turned down 50 percent in winter, a DDC controller positions the outdoor and return themselves
to introduce 76 percent outdoor air and 24 percent recirculating air. A 10 percent difference
TABLE 27.2 System Characteristics of Indirect-Direct Two-Stage Evaporative Cooling Systems
System structure
Zone thermal and sound controls
TSECS with or without DX refrigeration
Control zones Single zones or multiple zones
Control method VAV or CV
Control mode DDC, PI control mode, or on-off
Sound control
Indoor air quality
NC 30 to 45
Minimum ventilation control All outdoor air, or 20% to 30% fixed amount of outdoor
ventilation air
Filters
Air systems
Medium efficiency filters (MER 9 to 12)
Types Two-stage evaporative VAV packaged system
(VAV reheat)
Supply fan, types Centrifugal, unhoused or plug fans
Supply fan total pressure 3.0 to 4.0 in. WC
Combined supply fan and motor drive efficiency 0.5
Supply fan volume flow control Inlet vanes, variable-speed drives
Return/relief fan Axial fan, centrifugal, unhoused or plug fans
Return/relief fan total pressure
Return fan volume flow control
0.6 to 1.2 in. WC
Supply-relief fan combination Automatically balanced
Supply-return fan combination Space pressurization control
Heat recovery Heat pipe heat exchanger, 0.65 effectiveness
Operating modes
Cooling systems
Cooling, heating, nighttime setback, purging,
warm-up, optimum start and stop
Indirect evaporative cooler Effectiveness 0.6
Direct evaporative cooler
Refrigeration systems
4- to 12-in. rigid media, saturation efficiency 90%
Compressors Scroll, or reciprocating
Energy use EER 10 to 12
Capacity control Multiple units, staging on/off
Evaporator DX coil
Condensers
Heating systems
Air-cooled or evaporatively-cooled
Type Hot water reheat
Boiler Gas and electric boilers
Maintenance Sumps equipped with side-stream centrifugal slit
separators; periodical flush of sumps