Handbook of air conditioning and refrigeration / Shan K

Water Circuits
Contact conductance is directly proportional to the magnitude of interference. Interference
determines the quality of mechanical bonding during manufacturing. A higher interference means a
better quality of mechanical bonding. Contact conductance increases as the thickness of the fin
increases. It also increases as fin density (fins per inch) increases. Contact conductance decreases
as tube diameter increases. It also decreases as the hardness of the copper tube increases in an
aluminum fin and copper tube coil.
In a water cooling or water heating coil, tube feeds or water circuits determine the number of water
flow passages. The greater the finned width, the greater the number of tube feeds and thus the
greater the number of flow passages. For two finned-tube coils of the same finned width, a difference
in the number of tube feeds or water circuits means that the water flow rate, in gpm (L/s),
number of passes, and pressure drop of the chilled water, in ft WC (Pa), inside the two coils are different.
One pass means that water flows through the coil’s finned length once.
The number of serpentines ( , , , 1, 1 , or 2) of a water cooling or a water heating coil
1 1 1 3
�4 �2 �4 �2 indicates its water flow arrangement. The greater the number of serpentines, the larger the total
cross-sectional area of the water circuits and the greater the water volume flow rate. Figure 15.28
shows five water cooling coils, each specified by the number of serpentines, water circuits, passes,
1 and rows, all made by the same manufacturer. In Fig. 15.28, �2 serpentine means that at the first
row, there are eight tubes across the finned width, but only four of them are tube feeds that are connected
to the return header. For a full serpentine coil, all eight tubes in the first row are tube feeds
and connect to the return header.
15.9 SENSIBLE COOLING AND SENSIBLE HEATING
COILS—DRY COILS
Heat Transfer in a Sensible Cooling Process
AIR SYSTEMS: COMPONENTS—FANS, COILS, FILTERS, AND HUMIDIFIERS 15.39
In a sensible cooling process, the humidity ratio w is always constant. A sensible cooling process
only exists when the outer surface temperature of the coil T s, in °F (°C), is equal to or higher than
the dew point of the entering air T ae�, in °F (°C), that is, T s � T ae�. A sensible cooling process is
indicated by a horizontal line toward the saturation curve on the psychrometric chart, as shown in
Fig. 15.29.
If heat conduction through the coil framework is negligible, for a sensible cooling process
occurring at steady state within a certain time interval, the rate of sensible heat transfer from the
conditioned air Q cs (in Btu/h or W) must be equal to the heat absorbed by the chilled water in the
tubes. This relationship can be expressed as
Q cs � 60V˙ a� ac pa(T ae � T al)
� A oU o �T m � F s A a N rU o �T m
� 60m˙ w c pw(T wl � T we)
(15.24)
where � volume flow rate of conditioned air, cfm [m/(60 s)]
�a � air density, lb/ft3 (kg/m 3 )
Tae, Tal � temperature of conditioning air entering and leaving sensible cooling coil, °F (°C)
cpa, cpw � specific heat of moist air and water, Btu/lb�°F (J/kg�°C)
Ao � total outside surface area of coil, ft2 (m2 )
Uo � overall heat-transfer coefficient based on outside surface area of coil, Btu/h�ft2�°F (W/m2 V˙ a
�°C)