| Application Considerations 2
Supply Water Temperature Drop
The performance data for Trane™ chillers is based on a chilled water
Application Considerations selections and units with hot gas bypass are shown in the
"General Data" section of this catalog. 13 Application Considerations
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.
TYPICAL WATER 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.
SHORT WATER LOOPS
The proper location of the temperature control sensor is in the supply (outlet) water for 20-60 ton chillers and in the return
(inlet) water for 8, 10, 12½ and 15 ton chillers. This
location allows the building to act as a buffer and
assures a slowly changing return water temperature. If there is not a sufficient volume of water in the system to provide an
adequate buffer, temperature control can be lost,
resulting in erratic system operation and excessive
compressor cycling. A short water loop (less than one
minute in duration) will lead to erratic system operation.
To prevent the effect of a short water loop, the following items should be given careful consideration:
- Add a storage tank or larger header pipe to increase the volume of water in the system and, therefore, reduce the rate of change
of the return water temperature.
- Use three-way modulating valves in lieu of two-position valves with crossover pipes at the airside evaporators. The three-way
valves will reduce the rate of change in the return
water temperature.
- Hot gas bypass to prevent compressor cycling at rapidly changing supply water temperatures at low load conditions.
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 a Trane Tracer® system. The "Stand-alone" alternative is the DDC Chiller Sequencer.
- 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
lead-lag capability.
- 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. Two individual temperature
controllers are not capable of providing reliable
system control and will often result in unsatisfactory operation and possible compressor failure.
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