Refrigerant Piping, Direct Expansion; (Dx) Coils - Carrier 39SH Installation, Start-Up And Service Instructions Manual

Refrigerant Piping, Direct-Expansion (DX)
Direct-expansion coils are divided into 1 or 2 splits
Coils —
depending upon the unit size and coil circuiting. Each split re-
quires its own distributor nozzle, expansion valve, and suction
piping. Suction connections are on the air entering side when
the coil is properly installed. Matching distributor connections
for each coil split are on the air leaving side. See unit label or
certified drawing to assure connection to matching suction and
liquid connections.
The lower split of face split coils should be first on, last off.
Row split coils utilize special intertwined circuits; either
split of these row split coils can be first on, last off.
CAUTION
Direct-expansion coils are shipped pressurized with dry
nitrogen. Release pressure from each coil split through
valves in protective caps before removing caps.
Do not leave piping open to the atmosphere unnecessar-
ily. 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 workday. Clean all
piping connections before soldering joints.
Failure to follow these procedures could result in personal
injury or equipment damage.
SUCTION PIPING — Connect suction piping as shown in
Fig. 57 for face split coil.
TXV — Thermostatic Expansion Valve
Fig. 57 — Face Split Coil Suction Line Piping
Suction line from coil connection to end of the 15-diameter-
long riser should be same tube size as coil connection to ensure
proper refrigerant velocity.
Refer to Carrier System Design Manual, Part 3, and size re-
maining suction line to compressor for a pressure drop equiva-
lent to 2.0 F. This will provide a total suction line header pres-
sure drop equivalent to approximately 2.5 F. Refer to Fig. 58
for piping risers to the compressor.
To minimize the possibility of flooded starts and compres-
sor damage during prolonged light load operation, install an
accumulator in the suction line or a solenoid in the liquid line
of last-on, first off split in row-split applications.
EXPANSION VALVE PIPING — Distributor nozzles and
expansion valves sized for acceptable performance for a range
of conditions are factory supplied. Use the AHU (air-handling
unit) selection program in the electronic catalog to select opti-
mal nozzle sizes.
Circuiting selection should result in a circuit loading of 0.8
to 2.0 tons per circuit at design load. Circuit loading must be
evaluated at minimum load to ensure that it does not drop
below 0.6 tons per circuit. Solenoid valves may be used, if nec-
essary, to shut off the refrigerant supply to individual expansion
valves to maintain adequate coil circuit loading.
Compressor minimum unloading and TXV quantity is nec-
essary to determine minimum tonnage per circuit.
Minimum Unloading Equation:
(Tons per Circuit) x (Minimum Unloading)
Example:
Condensing Unit:
Minimum Unloading:33%
Coil:
Coil Tons per Circuit:1.68
Total TXVs:
In the first example we will determine the tons per circuit
when both TXVs are active and the compressor is unloaded to
its minimum of 33%.
=
a39-139.tif
=
=
UNACCEPTABLE
If we install a liquid line solenoid valve before one of the
TXVs and close it so that only one TXV is active when the
compressor is unloaded to its minimum of 33%, we see the
following:
=
=
= 1.10 tons per circuit at minimum unloading ACCEPTABLE
32
a39-516tf.tif
Fig. 58 — Suction Line Riser Piping
x (Total no. of TXVs)
no. of TXVs Active
38ARS012
6 row, 11 FPI, Half Circuit
2
(1.68 Tons per Circuit) x (33% Minimum Unloading)
x (2 TXVs)
2 TXVs Active
(1.68) x (.33) x (2)
2
.55 tons per circuit
(1.68 Tons per Circuit) x (33% Minimum Unloading)
x (2 TXVs)
1 TXV Active
(1.68) x (.33) x (2)
1
at minimum unloading