Chapter hvac engineering Fundamentals: Part 1 Introduction

9.7.2
Coil thermodynamics
The thermodynamics and psychrometrics involved in the cooling-coil-
to-air heat transfer process include (1) the psychrometric analysis of
the cooling and dehumidifying process, (2) the effect on the heat trans-
Equipment: Part 1
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308
Chapter Nine
Figure 9.21
Cooling and dehumidifying process on the psychrometric chart.
fer process of the air and water velocities, and (3) the effect of parallel
flow and counterflow on coil heat transfer capacity.
Most air-cooling processes involve some dehumidification; this is the
natural psychrometric response of the system if the air is cooled to
near its saturation condition. The process is usually shown as a
straight line on the psychrometric chart (Fig. 9.21) from some enter-
ing-air condition to the saturation curve (line
ABC
). The real process
follows the curved line
AC.
The air cools sensibly to saturation, then
subcools and dehumidifies down the saturation curve to point
C.
Point
C
is called the
apparatus dew point
(ADP) and is determined primarily
by the temperature of the refrigerant or chilled water in the coil. Some
of the air passes through the coil without being cooled, remaining at
the condition of point
A.
The resulting mixture is at a condition such
as point
B.
The reason for this is found in the geometry of the coil
(Fig. 9.22). As air flows through the coil, some of the air impinges on
the tubes and fins, but some air does not touch any part of the coil—it
is bypassed. The percentage of air which remains at condition
A
is
known as the
bypass factor.
Typically it is on the order of 5 to 10
percent for a four-row coil. This means that for design purposes there
is a limited area in which point
B
can fall, which is defined by the
system thermodynamics. The arbitrary selection of a point outside
Equipment: Part 1
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Equipment: Part 1
309
Figure 9.22