Handbook of air conditioning and refrigeration / Shan K

AIR SYSTEMS: COMPONENTS—FANS, COILS, FILTERS, AND HUMIDIFIERS 15.43
FIGURE 15.30 Maximum fin resistance number � fmax of rectangular
fins. (Source: ASHRAE Handbook 1989, Fundamentals. Reprinted with
permission.)
The heat capacity of the cold fluid (chilled water) C w, in Btu/h�°F (W/°C), is
C w � 60m˙ w c pw � 500V˙ galc pw
(15.37)
In Eq. (15.35), Cmin is the smaller value of Ca and Cw, in Btu/h�°F (W/°C).
In a sensible cooling coil using chilled water as coolant, m˙ acpa is usually smaller than m˙ wcpw. Then
(15.38)
In Eq. (15.38), T al is usually the required off-coil (leaving coil) temperature during the design of a
sensible cooling system. Most finned coils are actually a combination of counterflow and crossflow
arrangements. Because the effectiveness of a finned coil with a counterflow arrangement is nearly
the same as that of a crossflow arrangement, the effectiveness can be calculated as
� dry �
1 � exp[-NTU(1 � C)]
1 � C exp[-NTU(1 � C)]
where C � C min/C max
C max � greater value of C a and C w,Btu/h�°F (W/°C)
(15.39)
The number of transfer units (NTU), dimensionless, actually indicates the size of the coil and can
be calculated as
NTU � UA
C min
� dry � T ae � T al
T ae � T we
� U oA o
C a
� U iA i
C a
(15.40)