40
AGR 070A through 100A
Product Manual AGR-3
Series Compared to Parallel Operation
Consider system pressure drop when designing the water piping. Parallel piped systems have half of the
total system flow going through the evaporator of each chiller, reducing the individual unit and total
system pressure drop.
Series piped evaporators require that the total system water flows through both evaporators. Not only is
the pressure drop through each evaporator increased but the pressure drops must be added together to
obtain the total evaporator pressure drop. Series piped evaporators normally require larger circulating
pumps for the chilled water system.
Temperature and Water Flow Limitations
AGR units are designed to operate in temperatures from 40°F (4.5°C) to 115°F (46°C). A low ambient
option with SpeedTrol condenser fan control allows operation down to 0°F (-18°C). The minimum
ambient temperature is based on still conditions where the wind is not greater than five mph. Greater
wind velocities directly into the unit condenser coil will result in reduced discharge pressure, decreasing
the minimum operating ambient temperature. When possible, position the unit on the down wind side
of the building or where the unit will be least affected by wind. Field installed wind baffles are
available. The baffles allow the chiller to operate effectively down to the ambient temperature for
which the unit was designed.
Evaporator flow rates below the minimum values may result in laminar flow causing freeze-up
problems, scaling and poor control. Flow rates above the maximum values will result in unacceptable
pressure drops and may cause excessive nozzle and tube erosion, potentially leading to failure.
Low Temperature Applications
Compressor unloading is acceptable for applications with 35°F or higher leaving fluid temperatures
(LFT). For applications where the (LFT) is less than 35°F, only full load compressor operation is
allowed. Motor cooling can become a problem due to reduced refrigerant flow. If compressor part load
operation is required below 35°F LFT, contact the factory for additional requirements. For applications
with 35 degrees LFT, it is acceptable to use UNT or Microtech controls provided that only one LFT
setpoint is needed.
Ice Storage Applications
Microtech controls must be used for ice storage applications where leaving fluid temperatures are below
32°F. The Microtech has logic to change setpoints from the low ice making mode to higher normal
comfort cooling setpoints. It is important that the Microtech controller receive a signal to convert from
ice mode to normal operating mode. Microtech includes the logic to keep compressors fully loaded
when operating in ice mode. The double insulation thickness option is recommended to prevent
sweating.
Two Pipe Systems
When the same supply and return lines are used for both heating and cooling water, several limitations
should be observed. The maximum allowable temperature to the evaporator should not exceed 100°F
in a non-operating mode. The maximum allowable temperature for system changeover from heating to
cooling is 90°F entering the evaporator. System controls provided by others should prevent chiller
operation until the loop temperature drops to 90 degrees and should prevent water above 100°F from
entering the evaporator under any circumstances. When the loop water temperature ranges between 75
and 90 degrees entering the evaporator, the high return water staging thermostat should be used to
prevent compressor overloading.
System Water Volume Considerations:
All chillers need adequate time to recognize a load change, respond to the change and stabilize without
short cycling the compressor. The water volume in the system and the size of the piping loop is a
critical consideration. Good engineering practice is to have a minimum water volume of eight times the
