Specifications

2-59
JOHNSON CONTROLS
FORM 102.20-N1 (1109)
2
Thermostatic Expansion Valves (TXV)
Each coil distributor circuit requires its own
Thermostatic Expansion Valve (TXV). Each
condensing unit circuit requires its own liquid line
solenoid valve (LLSV). TXV's are to be equipped with
external equalizer tubes that are eld connected to the
suction line. The valve should be sized in accordance
with the valve manufacturer's recommendations,
allowing approximately 35 PSI pressure drop
throughout the coil and distributor at full load. Do not
oversize the valve. Follow the valve manufacturer's
instructions on the location of the thermostatic bulb.
Proper expansion valve operation is necessary in order
to realize the rated coil capacity.
When a DX type coil is operated with a suction
temperature below 32°F, a buildup of frost will occur
on the nned surface. It is, not recommended therefore,
to operate DX coils for air conditioning purposes at
below freezing 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
When using discharge air temperature control or
systems with outside air economizer cooling, always
include hot gas bypass (HGBP). It is not as critical to
use HGBP with return duct air temperature control, or
suction pressure control, but it provides better capacity
control at low loads.
FIG. 2-89 – SIX COMPRESSOR YCUL
LD09153
DX Coils
Condensing Unit
TXV1
LLSV2
LLSV1
TXV3
TXV2
TXV4
Comp 1 - 3
Comp 4 - 6
Comp 4 - 6
Comp 1 - 3
The venturi type distributor furnished with YORK DX
coils may be ordered for eld application of a hot gas
bypass valve. The connection may be made through a
tee installed in the eld between the expansion valve
and distributor. The system balance point and control
adjustments must assure compressor cooling and avoid
excessive compressor cycling. Refer to Form 50.40-
ES3, Section 3.
Maintaining Adequate Airow
An electrical interlock between the air handler and
the condenser must be included for permissive run
of the condenser. In addition, a differential pressure
switch mounted across the supply fan must always be
included to ensure airow across the coil before the
condensing unit is energized. The condenser must
never be operated unless the air handler fan is operating
and air is owing across the active coil. Insufcient
airow will result in liquid refrigerant returning to the
condensing unit, which could damage the compressors
by liquid slugging or washing oil from the bearing
surfaces.
In variable volume systems, the mini-
mum acceptable airow for xed speed
or VAV systems is 350 FPM face ve-
locity across each DX coil, as applied
to split DX systems. This is critical to
assure that the TXV does not overfeed,
causing compressor failure.
The air velocity owing through chilled water and
direct expansion coils must not exceed specic
recommended values, to prevent water carryover.
VAV Systems
Overhead variable air volume systems have been the
preferred method of air distribution since the early
1970 ’s. Overhead VAV systems offered greater energy
efciency and better control of building diversity than
constant volume systems. Unlike a constant volume
system, in which the leaving air temperature is adjusted
to satisfy the cooling load, in a VAV system the air
temperature remains constant and the air volume is
varied to meet the cooling requirements.