A five-ton package direct expansion (dx) air conditioning system is required to provide space cooling for a meeting room. The occupancy of the space requires that 30% of the system capacity be dedicated to conditioning fresh air necessary to satisfy ventilation codes. The high percentage of out side air will cause large fluctuations in load that standard on/off thermostatic controls will be unable to respond accurately to or maintain proper space humidity levels.

The five-ton unit can be modified with an APR-1.4 Control (4 tons capacity control). This will provide 80% capacity control, allowing the system to modulate capacity to match changing load conditions, from 5 tons down to 1 ton and all points in between. Because the APR Control allows the system to operate through 80% of the capacity range, and allow the evaporator coil to stay below dew point in a dehumidifying mode, the system will be able to keep the humidity levels low without the addition of electric reheat.

See the Performance Curve for APR-1.4 in a 5-ton System. 

An existing 20 ton split dx air conditioning system serves an office space. The air cooled condensing unit has 2 - 10 ton tandem compressors together with staging control (50%) and a single set of refrigeration pipes to a single 20 ton evaporator coil air handling unit. The system cycles when the load drops below 10 tons, causing erratic temperature control and an increase in relative humidity due to cycling.

Further investigation reveals that the air cooled condensing unit is located on the roof with the evaporator in a mechanical closet approximately 20' below. The total refrigeration piping run is approximately 60 equivalent feet. The suction line is 1-5/8" OD type L copper. Because of the necessity to maintain a minimum refrigerant velocity of approximately 1100 feet per minute to provide proper oil entrainment with the refrigerant, the suction line size is our limiting factor. A 1-5/8" suction riser requires a minimum of 4.1 tons refrigerant in the suction riser to maintain proper oil entrainment. An APR-2 Control (providing 5.85 tons capacity control) is selected to be installed. The APR-2 will modulate the system capacity on both stages from 20 tons down to 14.15 tons and all points in between and then from 10 tons down to 4.15 tons and all points in between. When the load falls below 14.15 tons the second stage will turn off and when the load falls below 4.15 tons the standard thermostatic controls will become satisfied and shut the system off. Click here to see a performance operation diagram of this system with an APR-2 Control. While the system is in the range that the APR Control will operate, the system will modulate capacity to match changing load conditions while maintaining the evaporator coil in a dehumidifying mode, enabling continuous humidity control. The system will provide a more comfortable environment.

See the Performance Curve for APR-2 in a 20-ton 2-Stage System. 

The new wing of a school is being designed with four classrooms. The school wants independent control of all HVAC systems serving these classrooms and local code requires fresh air ventilation be provided - fully conditioned. Each classroom requires 4 tons of cooling and the outside air requirements will also be 4 tons.

The APR Control allows Multiple Evaporator Zoning using a single compressor air cooled condensing unit. As a result, a single 20 ton air cooled condensing unit modified with an APR-3.5 (16 tons of capacity control) to provide 16 tons of capacity control, modulating the condensing unit from 20 tons down to 4 tons. Refrigeration distribution piping will be specified to serve 5 independently controlled 4 ton dx fan coil units located in the ceiling - taking into account proper pipe sizing procedures to maintain proper oil entrainment back to the compressor. Each of the four 4 ton evaporators serving the individual classrooms will be controlled with a liquid line solenoid valve connected with low voltage wiring to a local thermostat. The evaporators can have whatever air duct distribution is required for the individual classroom configuration. A separate evaporator will be installed to pre-condition the outside air required. Ductwork will then be sized to deliver the conditioned fresh air to the return plenum of each classroom evaporator. This evaporator conditioning the outside air will be controlled with a duct mounted humidistat to assure that the humidity does not rise when the classroom evaporators are not calling for cooling but are still in an occupied mode. When the school is in unoccupied mode a motorized damper will close the outside air intake and shut the fresh air evaporator off.

See the Performance Curve for APR-3.5 in a 20-ton System.

 

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