Trane CGAX/CXAX Manual page 8

Application Considerations
Supply Water Temperature Drop
Full load chilled water temperature drops from 3.3 °C to
10°C may be used as long as minimum and maximum
water temperature and minimum and maximum fl ow
rates are not violated.
Temperature drops outside this range at full load
conditions are beyond the optimum range for control
and may adversely affect the microcomputer's ability to
maintain an acceptable supply water temperature range.
Furthermore, full load temperature drops of less than
3.3°C may result in inadequate refrigerant superheat
which is critical to long term effi cient and reliable
operation.
Suffi cient superheat is always a primary concern in
any refrigerant system and is especially important in a
packaged chiller where the evaporator is closely coupled
to the compressor.
Parameters which infl uence the water temperature
stability:
• Ambient temperature and water temperature (modify
cooling capacity)
• Number of capacity steps
• Minimum time between starts of a compressor
• Control dead band
• Water loop volume
• Load fl uctuations
• Fluid type or percentage of glycol
Typical Water Piping
All building water piping must be fl ushed prior to
making fi nal 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.
8
Avoidance of Short Water Loops
Adequate chilled water system water volume is an
important system design parameter because it provides
for stable chilled water temperature control and helps
limit unacceptable short cycling of chiller compressors.
The Conquest Air Cooled chiller's temperature control
sensor is located in the supply (outlet) water connection
or pipe. This location allows the building to act as a
buffer to slow the rate of change of the system water
temperature. If there is not a suffi cient volume of water
in the system to provide an adequate buffer, temperature
control can suffer, resulting in erratic system operation
and excessive compressor cycling.
Typically, a two-minute water loop circulation time is
suffi cient to prevent short water loop issues. Therefore,
as a guideline, ensure the volume of water in the chilled
water loop equals or exceeds two times the evaporator
fl ow rate. For systems with a rapidly changing load
profi le the amount of volume should be increased.
If the installed system volume does not meet the above
recommendations, the following items should be given
careful consideration to increase the volume of water in
the system and, therefore, reduce the rate of change of
the return water temperature.
• A volume buffer tank located in the return water piping.
• Larger system supply and return header piping (which
also reduces system pressure drop and pump energy
use).
An optional factory-installed buffer tank is designed
to meet the minimum two minute loop time without
additional job site piping. The buffer tank can also be
used on jobs that already meet or exceed the minimum
loop time to further reduce the potential for compressor
cycling, increasing the compressor life span, and
reducing system temperature fl uctuations.
CG-PRC026C-GB