JOHNSON CONTROLS
83
SECTION 3 - HANDLING, STORAGE, AND INSTALLATION
FORM 102.20-N1
ISSUE DATE: 7/06/2016
3
REFRIGERATION
DX Direct Expansion Coils
DX coils are often divided into splits, depending upon
the AHU size and coil circuiting. Each split requires
its own distributor nozzle, expansion valve and suc-
tion piping. Suction headers are on the air entering side
with suction connection at the bottom end of the head-
ers when the coil is properly installed. Matching dis-
tributor connections for each coil refrigeration circuit
are on the air leaving side. Refer to the drawing and/or
connection labeling to ensure the suction and distribu-
tor connections are matched correctly.
Direct expansion coils are shipped
charged with nitrogen.
Do not leave the piping open to the atmosphere unnec-
essarily. Water and water vapor are detrimental to the
refrigerant system. Until the piping is complete, recap
the system and charge with nitrogen at the end of each
day. Clean all piping connections before brazing the
joints.
The orientation of the refrigerant distributor is not criti-
cal, but the distributor tubes must not be kinked or bent
in a non-uniform configuration. For other piping and
DX Coil Types
Three types of coil arrangements are used in field
mounted split systems: interlaced, row split and face
split.
Interlaced
- These coils are the most desirable type of
coil field mounted designs. Interlaced coils ensure the
entire face of the coil is active with any number of op-
erating compressors. Interlaced circuitry interweaves
the coil tubing in both circuits across the entire face
of the coil, assuring uniform cooling of the air by the
refrigerant. This type of coil also allows one circuit to
operate while the other circuit is turned off. Interlaced
coils provide excellent temperature control at full and
part loads as well as good thermal expansion valve
(TXV) superheat control, which is essential for com-
pressor reliability.
Row Split
- Row split coils arrangements place coils
back-to-back in the air stream. Air passes through one
coil before passing through the next coil. Generally, the
last coil in the air stream is activated first. Each circuit
may be controlled independently in this arrangement.
When both coils are operating, the coil closest to the
leaving air will operate at a lower temperature. This
type of coil may not permit lead lag of the circuits and
it may be difficult to balance the capacity between the
coils.
Face Split
- On this coil, the circuiting is divided be-
tween two separate coils. In field mounted systems,
this arrangement may suffer from TXV superheat con-
trol problems and compressor reliability. At low air-
flow, low load situations, the TXV may have difficulty
controlling the system's superheat.
Air stratification, poor humidity control, and con-
densation on downstream components can also occur
when using face split coils. One way to address TXV
control at part load is to provide a face damper to shut
off airflow when a coil face is inactive.
Combined Coil Types
Coil types may be combined in some systems, which
requires special care. Control sequences and piping ty-
ing the multiple systems and coils together should be
well thought out. Advice from an experienced design
engineer may be necessary.
DX Coil Circuiting
On many coil banks, two, or all three circuiting meth-
ods may be combined, depending upon the cooling ca-
pacity, and the level of control required. However, coil
sections must be combined so that they provide full-
face operation as shown in
Refer to
for the available
standard and special quote arrangements from the
Johnson Controls coil marketing group.
