Handbook of air conditioning and refrigeration / Shan K

is 77°F (25°C) and the supply temperature differential is 10°F (5.6 °C), calculate the effectiveness
required for this indirect evaporative cooler.
Solution If outdoor air is used as the cooled air, from Eq. (27.8),
� in � T ca,e � T� ca,l
T ca,e � T� wet,e
A cooling effectiveness of 0.76 is required for this indirect evaporative cooler.
27.4 INDIRECT-DIRECT TWO-STAGE EVAPORATIVE
COOLING SYSTEMS
Indirect-Direct Two-Stage Evaporative Cooler
�
AC SYSTEMS: EVAPORATIVE COOLING 27.13
80 � (77 � 10)
80 � 63
� 0.76
When the cooled air leaves an indirect evaporative cooler during a hot summer, its dry-bulb temperature
may be still above 70°F (21.1°C) with a relative humidity between 60 and 80 percent. It
is beneficial to add a direct evaporative cooler so that the temperature of the cooled air can be reduced
further with an increase in relative humidity. A higher relative humidity of cooled supply air
is often acceptable when the supply air absorbs the space sensible load and maintains a desirable
space relative humidity during hot summer. Compared with a single-stage indirect evaporative
cooler, an indirect-direct two-stage evaporative cooler significantly lowers the space temperature
it serves.
In an indirect-direct two-stage evaporative cooling system (TSECS), a direct evaporative cooler
is always connected in series after an indirect evaporative cooler to form an indirect-direct evaporative
cooler. If an indirect evaporative cooler is connected in series after a direct evaporative
cooler, such a direct-indirect evaporative cooler is actually a single-stage direct evaporative cooler
with a considerably higher discharge cooled air temperature than that of an indirect-direct cooler.
An indirect-direct two-stage cooler can provide cooled air with a dry-bulb temperature of 67°F
(19.4°C) and a relative humidity of about 95 percent when the entering outdoor air is at a dry-bulb
temperature of 93°F (33.9°C) and a wet-bulb of 70°F (21.1°C). The temperature of this cooled air
may increase to an indoor temperature of 78° F (25.5°C) with an indoor relative humidity of 66 percent
after absorbing the space cooling load during a hot summer. Indirect and direct evaporative
cooling processes connected in series are illustrated in Figs. 27.1c and 27.4a along with a psychrometric
chart.
For an indirect-direct two-stage cooler at part-load operation, one of the stages, either direct or
indirect, can be turned off based on the indoor and outdoor conditions. In an indirect-direct twostage
cooler, plate heat exchangers are widely used in the indirect cooler. On the wet air side, either
a water spray or a wetted surface treated with water-absorbent material is used. In the direct cooler,
a rigid medium with impregnated cellulose is often used.
Compared with central systems using a chilled water cooling coil or a DX packaged system using
a DX coil, an indirect-direct two-stage evaporative cooling system with or without a DX coil refrigeration
has the following advantages:
● Evaporative cooling replaces all the refrigeration power required and maintains an indoor thermal
environment within the ASHRAE summer comfort zone when the outdoor air condition is favorable.
● More outdoor ventilation air can be extracted so that IAQ will be improved.
● Better indoor relative humidity control is possible in winter heating when the indoor relative
humidity is very low.