Condenser Water Limitations
Trane CenTraVac™ chillers start and operate over a range of load conditions with controlled water temperatures. Reducing the condenser water temperature is an effective method of lowering the chiller power input. However, the effect of lowering the condenser water temperature may cause an increase in system power consumption.
In many applications Trane CenTraVac chillers can start and operate without control of the condenser water temperature. However, for optimum system power consumption, and for any applications with multiple chillers, control of the condenser water circuit is recommended. Integrated control of the chillers, pumps and towers is easily accomplished with Trane’s UCP2 and/or Tracer system.
Water Treatment
The use of untreated or improperly treated water in a chiller may result in scaling, erosion, corrosion, algae or slime. It is recommended that the services of a qualified water treatment specialist be used to determine what treatment, if any, is advisable. The Trane Company assumes no responsibility for the results of untreated, or improperly treated water.
Water Pumps
Avoid specifying or using 3600 rpm condenser and chilled water pumps. Such pumps may operate with objectionable noises and vibrations. In addition, a low frequency beat may occur due to the slight difference in operating rpm between water pumps and CenTraVac motors. Where noise and vibration-free operation are important, The Trane Company encourages the use of 1750 rpm pumps.
Chillers are designed to ARI conditions of 85°F, but Trane CenTraVac chillers can operate to a 3 psig pressure differential between the condenser and evaporator at any steady state load without oil loss, oil return, motor cooling, refrigerant hang-up or purge problems. And this differential can equate to safe minimum entering condenser water temperatures at or below 55°F, dependent on a variety
of factors such as load, leaving evaporator temperature and component combinations. Start-up below this differential is possible as well, especially with UCP2 soft start features
Water Flow
Today’s technology challenges ARI’s traditional design of three gpm per ton through the condenser. Reduced condenser flows are a simple and effective way to reduce both first and operating costs for the entire chiller plant. This design strategy will require more effort from the chiller. But pump and tower savings will typically offset any penalty. This is especially true when the plant is partially loaded or condenser relief is available.
In new systems, the benefits can include dramatic savings with:
•Size and cost for condenser lines and valves
•Size and cost of the cooling tower.
•Size and cost of the water pumps.
•Pump energy (30 to 35% reduction).
•Tower fan energy (30 to 35% reduction).
Replacement chiller plants can reap even greater benefits from low flow condensers. Because the water lines and tower are already in place, reduced flows would offer a tremendous energy advantage. Theoretically, a 2 GPM/ton design applied to a system that originally used 3 GPM/ton would offer a 70% reduction in pump energy. At the same time, the original tower would require a nozzle change but would then be able to produce about two degrees colder condenser water than before. These two benefits would again typically offset any extra effort required by the chiller.
Contact your local Trane Sales Office for information regarding optimum condenser water temperatures and flow rates for a specific application.
Electrical Information
Minimum Circuit Ampacity
To properly size field electrical wiring, the electrical engineer or contractor on a project needs to know the minimum circuit ampacity of the CenTraVac™ machine. The National Electrical Code (NEC), in Article 440-33, defines the method of calculating the minimum
circuit ampacity. The minimum circuit ampacity is defined as the sum of two amperages: 125 percent of the compressor motor Rated Load Amps (RLA), plus the Full Load Amps (FLA) of all remaining loads on the same circuit. For starter to motor wiring, there are no other remaining loads. For main power supply to the starter, there is a remaining load consisting of the 4 KVA control power transformer which supplies power to the controls, the oil pump motor, oil sump heater and the purge unit motor. Therefore, the remaining load FLA equals 4000 divided by the unit design voltage.
As an example, calculate the minimum circuit ampacity of a machine which has a design RLA of 350 amps and is to be operated on a 460 volt power supply:
Minimum Circuit Ampacity =
(125% x 350 Amps) + 4000 VA
460 V
=437.5 Amps + 8.7 Amps
=446.2 Amps
After the minimum circuit ampacity has been determined, the electrical engineer or contractor will refer to the appropriate conductor sizing table in the NEC to determine the exact conductors required. A typical table for 75°F conductors is included in the Trane submittal. The selection of conductors is based on a number of jobsite conditions (i.e. type of conductor, number of conductors, length of conductors, ambient temperature rating of conductors).
Branch-Circuit Short-Circuit and Ground Fault Protection
Circuit breakers and fused disconnects should be sized by the electrical engineer or contractor in strict accordance with NEC Article 440-21 and in accordance with all local codes. This protection should be for motor type loads and should not be less than 150 percent of the compressor motor rated load amps (RLA).
