Thermostatic Expansion Valves (Txv); Hot Gas Bypass (Hgbp); Dx Coil Circuiting And Staging; Figure 171 - Stacked Coil Circuiting - Pace PAI Installation And Assembly Manual

SECTION 3 - HANDLING, STORAGE, AND INSTALLATION
THERMOSTATIC EXPANSION VALVES
(TXV)
Each coil distributor circuit requires its own thermal ex-
pansion valves (TXV), and condensing AHU circuit re-
quires its own liquid line solenoid valve (LLSV). Equip
TXVs with external equalizer tubes that are field con-
nected to the suction line. Size the valve according to
the valve manufacturer's recommendations, allowing
approximately 35 psi ( throughout the coil and distribu-
tor at full load).
Do not oversize the valve. Follow the valve manufac-
turer's instructions to find the thermostatic bulb. Proper
expansion valve operation is necessary in order to re-
alize the rated coil capacity.
When a DX type coil is operated with a suction tem-
perature below 32°F, a buildup of frost will occur on
the finned surface. It is not recommended to operate
DX coils for air conditioning purposes at below freez-
ing suction temperatures. If the full load operating point
for the coil is selected at a safe temperature, a system
analysis is required to check for the lowest probable
suction temperature at light load conditions.

HOT GAS BYPASS (HGBP)

When using discharge air temperature control or sys-
tems with outside air economizer cooling, always in-
clude hot gas bypass (HGBP). It is not as critical to
use HGBP with return duct air temperature or suction
pressure control, but it provides better capacity control
at low loads.
The Venturi type distributor furnished with the DX coils
may be ordered for field application of an HGPB. The
connection may be made through a tee installed in the
field between the expansion valve and distributor. The
system balance point and control adjustments must
assure compressor cooling and avoid excessive com-
pressor cycling.

DX COIL CIRCUITING AND STAGING

On stacked coils, use a minimum of four coil circuits
to achieve full face control as shown in Figure 171 on
page 105. When the condensing AHU has two com-
pressors per refrigerant circuit, one or two coil circuits
may be used for each refrigerant circuit, depending
upon the cooling capacity.
Form PA102.20-N1
If one coil circuit is used as shown in Figure 172 on
page 105, the LLSV and TXV must be sized to handle
the full capacity of the refrigerant circuit. When two coil
circuits are used per refrigerant circuit as shown in Fig-
ure 173 on page 105, each TXV should be sized to
handle half of the capacity of the refrigerant circuit, and
the LLSV should be sized to handle the full capacity of
the refrigerant circuit.
2
4
1
3
Four Circuits
Recommended

Figure 171 - Stacked Coil Circuiting

100% Capacity
DX Coil

Figure 172 - One Coil Circuit Per Refrigerant Circuit

Compressor #1
TXV
TXV
Compressor #2
DX Coil

Figure 173 - Two Coil Circuit Per Refrigerant Circuit

105
Installation & Assembly Manual │ PACE
2
1
Two Circuits NOT
Recommended
LD09147a
Condensing Unit
LLSV
Condensing Unit
Issue Date: 07/03/2018
3
LD09148
LD09149