Handbook of air conditioning and refrigeration / Shan K

27.24 CHAPTER TWENTY-SEVEN
warm and cold liquids at a pressure drop of about 14.7 psi (101 kPa). The maximum working pressure
for a high pressure model can be as high as 350 psig (2412 kPa).
The plate-and-frame heat exchanger has a high heat-transfer coefficient on the surfaces of its
corrugated plates, a smaller temperature difference between the fluids on the two sides of each
plate, and a compact size. It is easily dismantled for cleaning and routine maintenance.
27.7 DESIGN CONSIDERATIONS
Scope of Applications
In locations where the outdoor dry-bulb temperature T o � 95°F (35°C) and outdoor wet-bulb temperature
T o��62°F (16.7°C), a stand-alone direct-indirect two-stage evaporative cooling system
plus DX coil refrigeration can provide a comfortable indoor environment for occupants at a space
temperature of 75°F (23.9°C) and a relative humidity of 45 to 50 percent, using evaporating cooling
most of the time in the cooling season.
In locations where the outdoor dry-bulb temperature T o � 95°F (35°C) and outdoor wet-bulb temperature
T o��65°F (18.3°C), a stand-alone indirect-direct two-stage evaporating cooling system can
maintain a comfortable indoor environment for occupants at a space temperature of 75°F (23.9°C) and
a relative humidity of 55 to 65 percent. In locations where the outdoor dry-bulb temperature T o �
95°F (35°C) and the outdoor wet-bulb temperature T o��70°F (21.1°C), a stand-alone indirect-direct
two-stage evaporating system can maintain an indoor environment within the summer comfort zones
for occupants at a space temperature of 78°F (25.6°C) and a relative humidity of 55 to 66 percent.
In locations where the outdoor wet-bulb temperature T o��72°F (22.2°C), an add-on evaporative
cooler or an add-on evaporative cooler plus DX coil refrigeration save energy and is often economical.
Life-cycle cost analysis is recommended, especially for small and medium-size systems or
systems with a low number of operating hours during the cooling season.
In addition to the comfort cooling in residential and commercial buildings, evaporative cooling
has been used in process cooling in industrial applications such as spot cooling, cooling of large
motors, cooling of gas turbines and generators, textile mills, cooling of wood and paper products,
laundries, animal barns and houses, and product storage cooling.
Evaporative cooling is best suited to applications where both summer cooling and winter humidification
are required.
Beware of Dampness, Sump Maintenance, and Water Leakage
In direct evaporative coolers, cooled air contacts directly with liquid water. Select rigid wetted
media instead of atomizing humidifiers in which water droplets may cause dampness or wetted
surfaces at a certain distance downstream of the atomizing humidifiers. Sumps must be investigated
and cleaned regularly. Water bleedoffs and necessary water treatments are essential to maintain a
clean and efficient evaporative cooling system. The quantity of bleedoff should be no greater than
the rate of evaporation. The water sump must be properly sealed so that water leaks are prevented.
These considerations should be taken into account during system design.
Selection of Summer Outdoor Design Conditions
During the design of an evaporating cooling system, outdoor conditions should be selected to calculate
the performance of the evaporative coolers; dry-bulb, wet-bulb,or dew point temperature; and
whether this corresponds to 0.4, 1, or 2.5 percent annual accumulative frequency of occurrence and
the mean coincident dry-bulb, wet-bulb, or dew point temperature.