A
CU MANUAL
IOM-14
16 of 46
Feb 06 R4
gas line is piped into the suction line at the compressor. This configuration requires the following
components.
a.
Hot gas bypass valve.
b.
Hot gas solenoid valve.
c.
Desuperheating TX valve.
d.
Desuperheating liquid solenoid.
e.
Suction accumulator.
Internal hot gas bypass may be ordered as a factory option (recommended) or is field installed as required.
A suction accumulator is recommended to ensure good mixing of the desuperheating liquid and hot gas.
Internal hot gas bypass is limited to a maximum of 50% of the minimum compressor capacity due to oil
return issues in the suction line. Hot gas bypass piping may be sized for a relatively high-pressure drop to
encourage oil return at lower levels of bypass.
Suction line velocities must be checked for proper oil
return at low loads with allowances made for the hot gas bypass capacity.
Example:
With a 10 ton operating capacity per circuit, if the internal hot gas is bypassing 3 tons, the
capacity flowing through the suction line is:
10 tons – 3 tons internal hot gas bypass = 7 tons suction gas line flow.
The suction line must be checked for oil return at 7 tons. If the compressor is equipped with unloaders or
has a tandem compressor set, the unloaded capacity minus the hot gas is checked.
LOW-AMBIENT OPERATION (optional)
Several low-ambient packages are available for cold weather operation. Standard condensing units are
designed for operation above 50°F (10°C) ambient.
Above 20°F (-7°C) Package
This package will have condenser fan motors operating at extended pressures. An Engineered Air
Intercooler is included, and is installed between the TX valve and the distributor to allow ‘just in time’
subcooling at the evaporator.
All Ambient -40°F (-40°C) Package
The minus forty system is equipped with a flooded condenser style of head pressure control. The package
will include head pressure valves, receivers and all required controls.
Piping For Low Ambient Systems
There are several concerns that must be addressed when piping low ambient systems.
•
Hot gas bypass is not recommended. If hot gas bypass is used, it must be piped internal to the
condensing unit with an accumulator and desuperheating valve.
•
The maximum amount of hot gas to be bypassed is 50% of the compressor unloaded capacity.
•
Keep the overall length of outdoor suction piping to a minimum. When the ambient temperature is
colder than the saturated suction temperature (most of the winter), gas in the suction line may be
cooled and condense to a liquid before entering the compressor. Suction accumulators are required.
•
All exposed refrigerant piping must be insulated.
