Handbook of air conditioning and refrigeration / Shan K

Coil Cleanliness
Drain and Isolating Valves. Drain valves should be provided for each coil for maintenance
and repair. Hose-end type of drain valves may be used for remote drains. Isolating valves
should be installed at both supply and return piping to cut off the water supply during repair and
maintenance.
Even Distributing Baffles. When heating coils are stacked above cooling coils, usually perforated
baffles should be installed in the path with less flow resistance to balance the difference in pressure
losses between two parallel flow paths. Heating coils usually have fewer rows and less flow resistance
than cooling coils.
Coil cleanliness is important for proper maintenance and operation. Coil cleanliness includes outer
surface cleanliness and internal coil care. If an air filter of a minimum of 25 percent dust-spot efficiency
is installed upstream from the coil without leakage air bypassing the air filter, then dirt will
not accumulate on the outer surface of the coil. If an air filter has efficiency lower than 25 percent
dust-spot rating or with many air leaks bypassing the air filter, dirt accumulation may eventually
block the air paths in a coil and significantly increase the pressure drop across the coil. In air
systems, coils usually should be inspected and cleaned every 3 months in urban areas. Blowing out
contaminants with compressed air and flushing with chemical detergent are the two cleaning methods
often used.
A chilled water system is usually a closed water system. Most of the makeup water taken from
the city water supply in the United States is well treated. If the water strainer is periodically inspected
and works properly, the inner surface of the copper tubes of a water cooling coil is usually
not contaminated.
Symonds (1997) pointed out that the internal coil care is needed after a burnout of a compressor
or a compressor motor in a packaged system. In such a condition, the DX coil is often dismantled
into segments. It is preferable to use chemical agent to flush the inner surface of the coil to clean
the existing acid, sludge, and solid particles.
Condensate Collection and Drain System
AIR SYSTEMS: COMPONENTS—FANS, COILS, FILTERS, AND HUMIDIFIERS 15.57
In DX coils and water cooling coils, air is often cooled and dehumidified in summer. Moisture is
removed from air and forms condensate on the outer surface of the DX coil and the wet part of the
water cooling coil. A condensate collection device must be installed, and a condensate drain line
should be designed. This is to prevent the leakage of condensate and the stagnancy of the condensate
flow which often cause the indoor air quality problems due to the growth of the bacteria and
microorganisms inside the the drain pans. Trent and Trent (1997b) discussed the problems involved
in condensate control.
Condensate Flow. The amount of condensate removed from the conditioned air depends mainly
on the type of air system (whether it is a comfort system, a processing system, a makeup system—
all outdoor air—or a recirculating system) and the location of the air system. If air enters the coil
at 80°F (26.7°C) dry-bulb temperature, 67°F (19.4°C) wet-bulb temperature and a humidity ratio
we � 0.0116 lb/lb (0.0116 kg/kg) and leaving the coil at 57°F (13.9°C) dry-bulb temperature, 56°F
(13.3°C) wet-bulb temperature, and a humidity ratio wl � 0.00905 lb/lb (0.00905 kg/kg), for a
cooling and dehumidifying capacity of 1 refrigeration ton (400 cfm or 189 L/s supply air) in a
comfort system using recirculating air, the mass flow rate of condensate m˙ con can be calculated as
m˙ con � V˙ a� a (w e � w l) � 400 � 0.075(0.0116 � 0.0095)
� 0.063 lb/min or 0.0075 gpm (0.00047 L/s)