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:

1. 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.
2. 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.
3. 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.

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 lead-lag capability.
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. Two individual temperature controllers are not capable of providing reliable system control and will often result in unsatisfactory operation and possible compressor failure.