AAON CF Series Installation Operation & Maintenance

Also see for CF Series: Installation operation & maintenance

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AAON CF Series Installation Operation & Maintenance Manual pdf 25 page image

25to carry oil up both risers, and it collects inthe trap, effectively closing off the largerdiameter riser, and diverting refrigerant upthe small riser where velocity of therefrigerant is sufficient to maintain oil flow.At full load, the mass flow clears the trap ofoil, and refrigerant is carried through bothrisers. The smaller diameter pipe should besized to return oil at minimum load, whilethe larger diameter pipe should be sized sothat flow through both pipes providesacceptable pressure drop at full load.Figure 7 - Double Suction RiserConstructionA double riser can also be used for heatpump operation. The specific volume(ft3/lb) of refrigerant at the dischargetemperature and pressure (heating mode lineconditions) is significantly lower than thespecific volume at the suction temperatureand pressure (cooling mode line conditions).To compound the issue, the capacity inheating mode is lower than the capacity incooling mode. The discharge velocity in theriser during heating mode is much lowerthan the suction velocity during coolingmode. Often, a double riser is necessary toget acceptable velocities for the dischargemode and acceptable velocities for thesuction mode. In the example diagrams (SeeFigure 8 & Figure 9), the cooling mode willuse both lines, and the heating mode will useonly one.Suction Line TrapsInclude a trap immediately after theevaporator coil outlet to protect the TXVbulb from liquid refrigerant.Include traps every 20 feet in verticalsuction riser sections.Suction Line InsulationThe entire suction line should be insulatedwith a minimum 1 inch thick Armaflexinsulation. This prevents condensation fromforming on the line, and reduces anypotential loss in capacity associated withheat gain placing additional load on thesystem. This line should still be insulated inheat pump systems even though it acts asboth a discharge and suction line.Suction Line GuidelinesFor proper performance, suction linevelocities less than a 4,000 fpm maximumare recommended. The minimum velocityrequired to return oil is dependent on thepipe diameter, however, a general guidelineof 1,000 fpm minimum may be applied.When suction flow is up, variable capacitycompressors require a minimum velocity of1,500 fpm at full load.Tandem compressors must be considered forfull load operation (both compressorsoperating) and at partial load (only onecompressor operating). When suction flowis up, and the tandem has a variable capacitycompressor, the velocity for only onecompressor in operation must be greaterthan 1,500 fpm. For on/off compressors, thevelocity must be greater than the minimumvelocity required to return oil.Heat pump vapor lines must be checked forsuction flow (cooling mode operation) anddischarge flow (heating mode operation).