I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg
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COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106
Tel: 817-624-0820
Liquid line shut off valves and filter driers are factory provided. Filter driers must be field installed on 2-6
ton units. The total length equivalent of pressure losses through valves, elbows and fittings must be
considered when adding additional components in the field. It is a good practice to utilize the fewest
elbows that will allow the mating units to be successfully joined.
Suction Line Sizing
The suction line is more critical than the liquid line from a design and construction standpoint. More
care must be taken to ensure that adequate velocity is achieved to return oil to the compressor at
minimum loading conditions. However, reducing the piping diameter to increase the velocity at minimal
load can result in excessive pressure losses, capacity reduction, and noise at full load.
Suction Line Routing
Pitch the suction line in the direction of flow (about 1 foot per 120 feet of length) to maintain oil flow
towards the compressor, and keep it from flooding back into the evaporator. Crankcase heaters are
provided to keep any condensed refrigerant that collects in the compressor from causing damage or
wear. Make sure to provide support to maintain suction line positioning, and insulate completely
between the evaporator and condensing unit.
It is important to consider part load operation when sizing suction lines. At minimum capacity,
refrigerant velocity may not be adequate to return oil up the vertical riser. Decreasing the diameter of
the vertical riser will increase the velocity, but also the frictional loss.
A double suction riser can be applied to the situation of part load operation with a suction riser. A
double suction riser is designed to return oil at minimum load while not incurring excessive frictional
losses at full load. A double suction riser consists of a small diameter riser in parallel with a larger
diameter riser, and a trap at the base of the large riser. At minimum capacity, refrigerant velocity is not
sufficient to carry oil up both risers, and it collects in the trap, effectively closing off the larger diameter
riser, and diverting refrigerant up the small riser where velocity of the refrigerant is sufficient to
maintain oil flow.
At full load, the mass flow clears the trap of oil, and refrigerant is carried through both risers. The
smaller diameter pipe should be sized to return oil at minimum load, while the larger diameter pipe
should be sized so that flow through both pipes provides acceptable pressure drop at full load.
Suction Line Insulation
The entire suction line should be insulated. This prevents condensation from forming on the line, and
reduces any potential loss in capacity associated with heat gain.
Suction Line Guidelines
For proper performance, suction line velocities less than a 4,000 fpm maximum are recommended. The
minimum velocity required to return oil is dependent on the pipe diameter, however, a general
guideline of 1,000 fpm minimum may be applied.
In a fashion similar to the liquid line, a common guideline to consider is a system design with pressure
losses due to friction through the line not to exceed a corresponding 1-2°F change in saturation
temperature.
At points where small pipe size can be used to provide sufficient velocity to return oil in vertical risers at
part loads, greater pressure losses are incurred at full loads. This can be compensated for by over sizing
the horizontal runs and vertical drop sections. This will however require additional refrigerant charge.
Circuits with variable capacity scroll compressors require suction riser traps every 15 feet.
Suction Line Accessories
