Refrigeration; Dx Direct Expansion Coils - Pace PAI Installation And Assembly Manual

SECTION 3 - HANDLING, STORAGE, AND INSTALLATION

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
headers when the coil is properly installed. Matching
distributor 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
sundry tips, refer to Figure 165 on page 102.
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 oper-
ating 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 refriger-
ant. This type of coil also allows one circuit to oper-
ate 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 compressor
reliability.
Form PA102.20-N1
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 airflow,
low load situations, the TXV may have difficulty control-
ling the system's superheat.
Air stratification, poor humidity control, and condensa-
tion 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 air-
flow 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
tying 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 inFigure 166 on page 102.
Refer to Figure 167 on page 103 for the available
standard and special quote arrangements from the
PACE coil marketing group.
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Installation & Assembly Manual │ PACE
Issue Date: 07/03/2018
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