Application Considerations
point and adequate ethylene glycol concentration for safe operation.
The maximum catalog leaving solution temperature from the evaporator is 65°F for outdoor ambients up to 115°F. High leaving water temperatures exceeding this may result in excessive suction temperatures and, therefore, inadequate motor cooling. For applications requiring high leaving water temperatures, contact your localTrane sales office for suggested alternatives.
The maximum water temperature that can be circulated through an evaporator, when the unit is not operating, is 108°F (100°F for CGA 8, 10, 12½ and 15 ton chillers). The evaporator becomes thermal stress limited at these temperatures.
2
SupplyWaterTemperature Drop
The performance data forTrane chillers is based on a chilled water temperature drop of 10°F.Temperature drops outside this range will result in unit performance that differs from that cataloged. For performance data outside the 10°F range see the “Performance Adjustment Factors” section in this catalog. Chilled water temperature drops from 6 to 18°F (8 to 12°F in CGA units) may be used as long as minimum and maximum water temperature and minimum and maximum flow rates are not violated.
Temperature drops outside 6 to 18°F (8 to 12°F in CGA units) are beyond the optimum range for control and may adversely affect the controller’s capability to maintain an acceptable supply water temperature range.
Further, temperature drops of less than 6°F may result in inadequate refrigerant superheat. Sufficient superheat is always a primary concern in any direct expansion refrigeration system and is especially important in a package chiller where the evaporator is closely coupled to the compressor.When temperature drops are less than 6°F, an evaporator runaround loop may be required.
TYPICALWATER PIPING
All building water piping must be flushed prior to making final connections to the chiller.To reduce heat loss and prevent condensation, insulation should be applied. Expansion tanks are also usually required so that chilled water volume changes can be accommodated. A typical piping arrangement is shown on the following page.
WATERVOLUME IN THE LOOP (MINIMUM LOOPTIME)
The volume of water in the loop is critical to the stability of system operation.The minimum required water volume is dependant on the chiller controller and system GPM.Water volumes less than the minimum required for the system can cause nuisance problems including low pressure trips and freezestat trips. The cause of these trips is “ShortWater Loops”.
The minimum required water volume (as a function of loop time and GPM) is as follows:
CGAF: Minimum LoopVolume = GPM x 3 Minute LoopTime
CGA: Minimum LoopVolume = GPM X 5 Minute LoopTime
If the loop piping does not contain enough volume, then a tank should be added so that the equations hold true. Generally, the more the loop volume the greater the system stability and controllability.
EXAMPLE: CGAFC50 with 100 gpm. Minimum LoopVolume =
GPM x 3 Minute LoopTime Minimum LoopVolume = 100 x 3
300 Gallon Minimum LoopVolume
If a chiller is attached to an on/off load such as a process load, it may be difficult for the controller to respond quickly enough to the very rapid change in return solution temperature.This condition may result in freezestat or low temperature trips. In this case, it may be necessary to add a mixing tank in the return line.
MULTIPLE UNIT OPERATION
Whenever two or more units are used on one chilled water loop,Trane recommends that their operation be controlled from a single control device, such as aTraneTracer system.The
1
Series Operation
Some systems require large chilled water temperature drops (16 to 24°F). For those installations, two units with their evaporators in series are usually required. Control of the units should be from a common temperature sensor to prevent the separate unit controls from fighting one another and continually hunting. It is possible to control water temperature from the two individual unit controls, but a common temperature controller provides a positive method for preventing control overlap, more closely matching system load and simplifying compressor
2
Parallel Operation
Some systems require more capacity or standby capability than a single machine can provide. For those installations, two units with their evaporators in a parallel configuration are typical.The only effective way of controlling two units in parallel is with a single temperature controller. For further information, please contactTrane Applications.
12 |
