Handbook of air conditioning and refrigeration / Shan K

15.44 CHAPTER FIFTEEN
Heating Coils
After calculation of ε of the sensible cooling coil from Eq. (15.39), the sensible cooling coil load
can be found as
(15.41)
From the energy balances between the moist air and chilled water, the temperature of conditioned
air and chilled water leaving the coil T al and T wl, in °F (°C), can be calculated as
Tal � Tae � ε(Tae � Twe) Twl � Twe � C(Tae � Tal) (15.42)
In heating coils, only sensible heat change occurs during the heating process. The humidity ratio w
remains constant unless humidifiers are installed before or after the heating coil. For steam heating
coils, the steam temperature T steam �� T ae. At the same time, T steam remains constant during the
heating process. Therefore, C max equals infinity, and C min/C max � 0.
The system performance of both water heating and steam heating coils can also be determined from
Eqs. (15.36) through (15.42), in which the hot fluids are hot water or steam, and moist air is cold fluid.
Fluid Velocity and Pressure Drop
Air velocity calculated based on the face area of a finned-tube coil va is a primary factor that determines
the effectiveness of heat transfer, carryover of droplets of condensate in wet coils, air-side
pressure drop, and energy consumption of the system. For dry coils, there is no danger of carryover.
Their maximum face velocity is usually limited to a less than 800 fpm (4 m/s). The maximum air
velocity calculated based on the minimum free flow area may be as high as 1400 fpm (7 m/s). For
coils used in terminal units such as floor-mounted fan coil units, the face velocity is usually around
200 fpm (1 m/s) so that the pressure drop across the coil is low.
In addition to the face velocity va, the air-side pressure drop �pa depends also on fin-and-tube
configuration. For dry coils with a fin spacing of 12 fins/in. (2.1-mm fins), and at a face velocity
va � 600 fpm (3 m/s), �pa may vary from 0.1 to 0.2 in. WC (25 to 50 Pa) per row depth.
Selection of water velocity and , in gpm (L/s), for a finned-tube coil of a given tube inner diameter
and number of water circuits is closely related to the temperature rise or drop. For a finnedtube
coil with a face area of 1 ft2 (m2 V˙ gal
), a full serpentine four-row coil may need 4.8 gpm (0.303 L/s)
1
of chilled water at a water temperature rise around 10°F (5.6°C), whereas a 2 serpentine coil needs
only 2.4 gpm (0.151 L/s) and has a temperature rise of 20°F (11.1°C). Heat transfer, pressure drop,
erosion, noise, energy, maintenance space, and initial cost should be considered during the selection
of the temperature rise or drop and its corresponding water volume flow V˙
gal and water velocity. For
finned-tube coils, a temperature rise of 10 to 20°F (5.6 to 11.1°C) is generally used. Water-side
pressure drop is usually limited to 10 psi, about 22.5 ft WC (69 kPa). A water velocity between 2
and 6 ft/s (0.6 and 1.8 m/s) and a pressure drop of 10 ft WC (30 kPa) should be maintained for reasonable
pump power consumption.
Dry Coil at Part-Load Operation
Q cs � 60V˙ a� ac pa(T ae � T we)�
Because the sensible cooling and heating processes are horizontal lines on the psychrometric chart,
if the entering chilled water temperature or hot water temperature remains the same at part-load as
in full-load operation, then the reduction of water flow rates at part-load operation tends to shorten
the horizontal lines from el to elp (as shown in Fig. 15.29) to maintain the required space temperature
at part-load operation.