Handbook of air conditioning and refrigeration / Shan K

Types of Coils
Fins
AIR SYSTEMS: COMPONENTS—FANS, COILS, FILTERS, AND HUMIDIFIERS 15.33
combination thereof. Chilled water and refrigerants used to cool and dehumidify moist air are
called coolants. Coils consist of tubes and external fins arranged in rows along the airflow to
provide greater surface contact. Coils are arranged in circuits to create a shorter path length. In a
coil, water and refrigerant flow inside the tubes, and air flows over the outside surface of the tubes
and fins.
Coils can be classified into four categories according to the medium used.
Water Cooling Coil. A water cooling coil uses chilled water as the coolant inside the tubes. The
chilled water cools or cools and dehumidifies the moist air that flows over the external surface of
the tubes and fins, as shown in Fig. 15.25a. To maintain a higher rate of heat transfer, the air and
water normally follow a counterflow arrangement; i.e., coldest air meets the coldest water and the
warmest air meets the warmest water.
1 5 The water tubes are usually copper tubes of �2 - to �8-in.
(13- to 16-mm) diameter with a thick-
ness of 0.01 to 0.02 in. (0.25 to 0.5 mm). They are spaced at a center-to-center distance of 0.75 to
1.25 in. (19 to 31 mm) longitudinally and 1 to 1.5 in. (25 to 38 mm) transversely. The tubes may be
arranged along the airflows in 2, 3, 4, 6, or 8 rows, in either staggered or aligned form. The staggered
arrangement provides better heat transfer and a higher air pressure drop. Chilled water coils
are commonly rated at a pressure of 175 to 300 psig (1205 to 2070 kPag). Figure 15.26 shows the
structure of a water cooling coil and a DX coil.
Direct-Expansion (DX) Coil. In a direct-expansion coil, the refrigerant (usually HCFC-22, HFC-
134a, HFC-404A, HFC-410A, HFC-407A, or HFC-407C) is evaporated and expanded directly inside
the tubes to cool and dehumidify the air flowing over it, and condensation occurs on the outer
surface of the DX coil, as shown in Fig. 15.25b. This is why it is called a DX coil or wet coil. A DX
coil acts as the evaporator in a refrigerating system. In a DX coil, coolant or the refrigerant is fed into
a distributor and is evenly distributed to various tube circuits, which are made of copper tubes, typically
0.5 in. (13 mm) in diameter. Refrigerant distribution and loading to various circuits are critical
to the performance of a DX coil. After evaporation, the vapor refrigerant is discharged from the
header to the suction line. The characteristics and performance of a DX coil are covered in Sec. 10.2.
Water Heating Coil. The water heating coil shown in Fig. 15.25c is similar in construction to the
water cooling coil. There are two main differences between them. Hot water, instead of chilled water,
is used as the heating medium in a water heating coil. Also, there are fewer rows in the water
heating coil than in the water cooling coil. Generally, only two-, three-, or four-row water heating
coils are available on the market. Water heating coils are rated at pressures of 175 to 300 psig (1205
to 2070 kPag) at temperatures up to 250°F (120°C).
Steam Heating Coil. Steam heating coils use the latent heat of condensation released by steam
inside the tubes to heat outside and recirculating air, as shown in Fig. 15.25d. In a standard steam
heating coil, steam enters one end of the coil and condensate comes out the other end. For more
even distribution of steam, a baffle plate is often installed just after the inlet.
In a steam heating coil, it is important that the coil core inside the casing expand and contract
freely. The coil core is also pitched toward the return connection to facilitate drainage of the condensate.
Steam heating coils usually have a rating of 100 to 200 psig (690 to 1380 kPag) at 400°F
(205°C). Steam heating coil tubes are usually made of copper, steel, or stainless steel.
Fins are extended surfaces that are often called the coil’s secondary surface (the outer surface of
the tubes is called the primary surface of the coil). Fins are often made of aluminum, with a fin