Page 463 - Mechanical Engineers' Handbook (Volume 4)

452 Refrigeration

. These are extremely corrosive to many exchanger materials, yet difficult to detect
and CO 2
in sampling. Pump power, water treatment, and special condenser material should be eval-
uated when considering costs.
Allowances must be made in heat-transfer calculations for fouling or scaling of ex-
changer surfaces during operation. This ensures sufficient surface to maintain rated perform-
ance over a reasonable interval of time between cleanings. Scale-factor allowances are
2
expressed in m K/kW as additional thermal resistance.
2
Commercial practice normally includes a scale-factor allowance of 0.088 m K/kW.
The long hours of operation usually associated with chemical-plant service and the type of
cooling water frequently encountered generally justify a greater allowance to minimize the
frequency of downtime for cleaning. Depending on these conditions, an allowance of 0.18
2
or 0.35 m K/kW is recommended for chemical-plant service. Scale allowance can be re-
flected in system designs in two ways: as more heat-exchanger surface or as higher design
condensing temperatures with attendant increase in compressor power. Generally, a compro-
mise between these two approaches is most economical. For extremely bad water, parallel
condensers, each with 60–100% capacity, may provide a more economical selection and
permit cleaning one exchanger while the system is operating.
Air-cooled condensing equipment can also be used in refrigeration systems. With tighter
restrictions on the use of water, air-cooled equipment is used even on larger centrifugal-type
refrigeration plants, although it requires more physical space than cooling towers. Larger
condensers include an array of propeller fans located at the top of the condenser that pull
air over the condensing coil. Circulating fans and exchanger surface are usually selected to
provide design condensing temperatures of 49–60 C when design ambient dry bulb temper-
atures range between 35 and 38 C.
The design dry bulb temperature should be carefully considered since most weather data
reflect an average or mean maximum temperature. If full load operation must be maintained
at all times, care should be taken to provide sufficient condenser capacity for the maximum
recorded temperature. This is particularly important when the compressor is centrifugal be-
cause of its flat head characteristics and the need for adequate speed. Multiple-circuit or
parallel air-cooled condensers must be provided with traps to prevent liquid backup into the
idle circuit at light load. Pressure drop through the condenser coil must also be considered
in establishing the compressor discharge pressure.
The condensing temperature and pressure must be controlled for the refrigeration system
to function optimally. Too high a condensing temperature results in increased power and
reduced capacity. Too low a condensing temperature can result in poor performance of the
expansion device. Air-cooled condensers often employ fan cycling, modulating dampers, or
18
fan speed control to maintain proper refrigerant condensing temperature and pressure. On
condensers with multiple fans, one or more of the fans can each be cycled on and off to
maintain refrigerant conditions. When modulating dampers are used, the airflow through the
condenser can be controlled from 0 to 100%. Variable-speed drives can also be used to
control fan speed and airflow through the condenser.
In comparing water-cooled and air-cooled condensers, the compression power at design
conditions is usually higher with air-cooled condensing, because of the larger temperature
differential required in air-cooled condensers. However, ambient air temperatures are con-
siderably below the design temperature most of the time, and operating costs frequently
compare favorably over a full year. In addition, air-cooled condensers usually require less
maintenance, although dirty or dusty atmospheres may affect performance.

7.3 Evaporators
There are special requirements for evaporators in refrigeration service that are not always
present in other types of heat-exchanger design. These include problems of oil return, flash-
gas distribution, gas–liquid separation, and submergence effects.