condenser water temperature control

November 2009 CTV-PRB006-EN

Engineering Bulletin

Condenser Water Temperature ControlFor CenTraVac Centrifugal Chiller Systems with Tracer AdaptiView Controls

© 2009 Trane All rights reserved CTV-PRB006-EN

Introduction

Intended specifically for HVAC system designers and Trane field sales engineers, this engineering bulletin provides information on the effect of condenser water temperature on Trane centrifugal chillers with Tracer AdaptiView™ generation controls. It discusses various condenser water temperature control strategies for designing efficient systems and provides operating recommendations for CenTraVac™ chillers.

Trademarks

CenTraVac, System Analyzer, TRACE, Tracer AdaptiView, Trane, and the Trane logo are trademarks of Trane in the United States and other countries.

Table of Contents

CTV-PRB006-EN 3

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Optimizing the System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Condenser Flow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Condenser Water Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Economize with Outdoor Air or Free Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . 5Outdoor Air Economizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Free Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Guidelines for CenTraVac Chillers with Tracer AdaptiView Controls . . . . . . . . . 7

Operating Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Direct Tracer AdaptiView Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Maintaining The Minimum Refrigerant Pressure Differential . . . . . . . . . . 8Cooling Tower Fan Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Cooling Tower Bypass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Chiller Bypass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Throttling Valve(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Variable-Speed Condenser Water Pump . . . . . . . . . . . . . . . . . . . . . . . . . . 10Control Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Chiller System Differential Pressure Logic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Sequence of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Hardware Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Other Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Indirect Tracer AdaptiView Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Maintaining The Minimum Refrigerant Pressure Differential . . . . . . . . . 17System Design Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Cooling Tower Fan Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Cooling Tower Bypass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Chiller Bypass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Throttling Valve(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Variable-Speed Condenser Water Pump . . . . . . . . . . . . . . . . . . . . . . . . . . 20Control Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Sensing Condenser Refrigerant Pressure . . . . . . . . . . . . . . . . . . . . . . . . . 20Sensing Refrigerant Pressure Differential . . . . . . . . . . . . . . . . . . . . . . . . . 21

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

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Optimizing the System

Condenser Flow Rate

“System power”—i.e., energy consumed by the chiller plant—is the combined power used by the chiller(s), cooling tower, and the pumps that circulate evaporator and condenser water.

While lowering the flow rate through the condenser increases chiller power consumption slightly, it allows the tower to operate more efficiently and significantly reduces condenser pumping power. Generally, this reduction more than offsets the small increase in chiller consumption; that means a lower system operating cost, particularly at part-load conditions.

Other benefits attributable to a lower-than-”normal” condenser flow rate are listed below. Since some of these benefits are mutually exclusive, clearly define the design goals of the application before conducting an analysis to discover the optimal flow rate.

Consider these factors when determining the optimal condenser flow rate for your application:

• Pressure drop—i.e., high condenser system pressure drops favor low condenser water flow

• Chiller efficiency—i.e., more efficient selections favor low condenser water flow

• Load profile—i.e., more hours at part load favor low condenser water flow

Energy and economic analysis tools like Trane’s System Analyzer™ and TRACE™ software can help you determine the right condenser flow rate for your application. For more information on this subject, review Trane Engineers Newsletter, “How Low-Flow Systems Can Help You Give Your Customers What They Want” (1997—Vol. 26, No. 2).

Potential low-flow benefits for . . .

Existing facilities:

• Lower pumping costs

• Lower leaving-tower/entering condenser water temperature

• Lower tower operating costs

• Increased chiller capacity when replacing a chiller without replacing the tower

And new facilities:

• Lower pumping costs

• Smaller, less expensive condenser pump

• Smaller, less expensive condenser water piping

• Smaller, less expensive cooling tower

• Lower tower operating costs

• Lower leaving-tower/entering condenser water temperature

Condenser Water Temperature

Cooling towers are generally selected to supply 85°F water to the chiller condenser at design conditions—i.e., when the ambient wet-bulb temperature equals design and the chiller is operating at full load. But these conditions rarely occur. Usually, the ambient wet-bulb temperature is below design and the chiller is running at less than full capacity. The tower can typically provide a lower condenser water temperature at these “off-design” conditions.

While chiller efficiency generally improves as the condenser water temperature decreases, the lowest tower water temperature may not be the most economical system choice. Colder tower water isn’t free ... it requires additional fan energy. Consequently, the optimum condenser water temperature (i.e. the temperature that minimizes system power) is an intermediate temperature between design and “as cold as possible.”

CTV-PRB006-EN 5


Note: For a more detailed discussion of condenser optimization strategies, review these Trane Engineers Newsletters: “Tower Water Temperature ... Control It How?” (1995—Vol. 24, No. 1) and “Chiller Plant System Performance” (1989—Vol. 18, No. 2).

Economize with Outdoor Air or Free Cooling

The best ways to economize are often the simplest. If the building load analysis reveals that cooling is needed when ambient temperatures are low, consider adding an outdoor air economizer or free cooling to the system. Both options provide inexpensive cooling and may reduce chiller operation and minimize system energy usage.

Outdoor Air Economizer

In many systems, outdoor air can be used for direct space cooling, provided the sensible temperature is below 55°F and the dew point is low enough to assure that the space humidity level remains below 60 percent RH. To apply this option successfully, the outdoor- and exhaust-air sections of the air handling equipment must be sized to handle the increased volume of outdoor air. Equally important are a system design and control strategy that promotes proper humidity management.

Free Cooling

Chiller-mounted refrigerant migration: As an option, free cooling can be fully integrated into the chiller. The benefit to this option is that no additional pumps or piping are necessary.

Another means of free cooling is a plate-and-frame heat exchanger installed in a “sidecar” arrangement; see Figure 1, p. 6. Positioning the heat exchanger in series with the chiller(s) exposes it to the warmest water in the system, extending its operating hours and usefulness.

Note: “Sidecar” free cooling is practical when the tower sump temperature is at least 20°F less than the return chilled water temperature. For more information about this design option, see Trane Engineers Newsletter, “A New Era of Free Cooling” (1991—Vol. 20, No. 3).

6 CTV-PRB006-EN


Regardless of the method used, combining chillers with free cooling requires consideration of the chiller manufacturer’s condenser water limits. For more information, see “Operating Recommendations,” p. 7 in this bulletin.

Figure 1. “Sidecar” free cooling

CTV-PRB006-EN 7

Guidelines for CenTraVac Chillers with Tracer

AdaptiView Controls

Operating Recommendations

All chillers require a minimum pressure difference between the condenser and evaporator refrigerant circuits to assure proper management of oil and refrigerant, as well as hermetic motor cooling (when applicable).

Following are specific guidelines for CenTraVac™ condenser refrigerant pressure with Tracer AdaptiView controls.

At start-up ...

• The chiller should reach the required minimum pressure differential within 15 minutes of starting the chiller. This means that the entering condenser water can be very cold at the start (e.g., ~40°F–50°F). The chiller can start in an inverted mode where the condenser water is colder than the evaporator chilled water temperature.

• Running the chiller continuously for 30 minutes at the required minimum refrigerant pressure differential will assure that the oil returns to the oil tank via the oil reclaim system.

When running ...

• The chiller can run steady state with very cold entering tower water. It is important to maintain a minimum 3 psid (20.7 kPaD) pressure differential as shown in Figure 2. The 3 psid (20.7 kPaD) minimum at all loads is nominally equivalent to a 15°F differential between leaving chilled water temperature and leaving condenser water temperature. For example, a chiller running at 40°F chilled water could operate with an entering condenser water temperature below 50°F, as long as the leaving condenser water temperature is greater than or equal to 55°F. If you have chillers with older controls, refer to previous revisions of this bulletin for pressure differential minimums.

An optional signal is available from the chiller that can be used to maintain the 3 psig pressure differential. Once the signal option is chosen, there are two different control signal pathways available for use. The first pathway is the direct option. In this case, the signal from the chiller can be set up to go directly to the bypass valve or variable speed drive (see “Direct Tracer AdaptiView Signal,” p. 8). The second pathway is the indirect option. In this case, a raw signal from the chiller is provided and an intermediate controller must be used prior to the external bypass valve or variable speed drive (see “Indirect Tracer AdaptiView Signal,” p. 17).

Figure 2. Minimum condenser-evaporator refrigerant pressure differential

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Guidelines for CenTraVac Chillers with Tracer AdaptiView Controls

Direct Tracer AdaptiView Signal

Maintaining The Minimum Refrigerant Pressure Differential

In this section, we will be making use of differential pressure (evaporator-condenser) directly from the Tracer AdaptiView™ control panel. This may not work for all chiller applications. It will depend on chilled water configuration. If necessary, use the indirect control option (see “Indirect Tracer AdaptiView Signal,” p. 17).

System Design Options

Regulating the refrigerant pressure difference between the condenser and evaporator typically means controlling the condenser refrigerant pressure when necessary. There are essentially five different ways to accomplish this through system design:

• cooling tower fan control

• cooling tower bypass

• chiller bypass

• throttling valve(s)

• variable-speed condenser water pump

Brief descriptions of these methods follow, along with the primary advantages and disadvantages of each.

1. Cooling Tower Fan Control

One way to increase condenser refrigerant pressure is to cycle on and off, or modulate the speed of, the cooling tower fans. Tower fan operation is usually based on the water temperature of the tower sump/basin. This strategy allows a single control system to furnish properly controlled water to more than one chiller.

Advantages ...

• Low-cost controls.

• Better system efficiency.

Disadvantages ...

• Control system may not be appropriate for the application (e.g. those that use river water, or with a tower that serves other systems).

• Many weather conditions can prevent cooling tower fan control from maintaining the leaving-tower water at or above the minimum temperature needed for proper chiller operation.

• If the tower sump contains a great deal of water, it may not be possible to comply with the CenTraVac™ “Operating Recommendations,” p. 7.

• Fan cycling may result in wide condenser-to-evaporator pressure swings.

2. Cooling Tower Bypass

This design option, shown in Figure 3, p. 9, elevates the condenser refrigerant pressure by mixing leaving condenser water with entering-condenser water from the cooling tower. A suitable bypass piping arrangement for this purpose connects two butterfly valves with a common actuator linkage (or a single three-way valve) to a flanged tee.

CTV-PRB006-EN 9


Advantage ...

• Good control.

Disadvantages ...

• A valved bypass may be more expensive than other system design options.

• Requires a “dedicated” condenser water pump.

• May vary cooling tower flow below the tower flow limit.

3. Chiller Bypass

Figure 4, p. 9 illustrates this system design option. Using the chiller bypass to reduce condenser water flow through the chiller raises the temperature differential (ΔT) across the condenser which, in turn, maintains the condenser refrigerant pressure.

Advantages ...

• Excellent control.

• Maintains a constant cooling tower flow rate.

• Does not require a “dedicated” condenser water pump on the tower.

• The system bypass can be provided at the tower rather than at each chiller, reducing this option’s first cost.

Disadvantage ...

• Like the cooling tower bypass, a valved bypass may be more expensive than other system design options.

4. Throttling Valve(s)

Throttling valves offer another means for reducing condenser water flow to increase refrigerant pressure by creating a greater temperature differential across the condenser. There are two

Figure 3. Cooling tower bypass

Condenser Pressure or Pressure2–10 Vdc Signal (Optional Signal)

ΔAdaptiView

ChillerControlPanel

ChillerCondenser

2 ButterflyValves

CondenserWater Pump

Electric or PneumaticValve Actuator

To/FromCooling Tower

Figure 4. Chiller bypass


ΔAdaptiView

ChillerControlPanel

ChillerCondenser


CondenserWater Pump

2 ButterflyValves


10 CTV-PRB006-EN


common variations of this design option. The first, shown in Figure 5, p. 10, requires only one butterfly valve.

Note: Be sure to select a valve with a modulating range that can maintain the minimum refrigerant pressure differential (ΔP) when the chiller is running at minimum load and at a minimum tower sump temperature.

The nonlinear flow characteristics of a butterfly valve can cause unstable control at low flow rates. To avoid this instability, consider adding a small globe valve in parallel with the butterfly valve as shown in Figure 6, p. 10. Operate the valves in sequence so that the globe valve opens over the first half of the signal range and the butterfly valve begins to modulate when the globe valve is fully open. The globe valve should be large enough to prevent butterfly valve operation in an unstable region.

Advantages ...

• Provides good control at relatively low cost if the valves are properly sized.

• May reduce system pumping costs.

Disadvantages ...

• Requires a pump that can accommodate variable flow.

• Using a single butterfly valve (without a globe valve piped in parallel) may cause erratic control at low condenser flow rates.


5. Variable-Speed Condenser Water Pump

This system option also modulates water flow through the condenser, increasing the temperature difference between the entering and leaving water and, in turn, raising the condenser refrigerant pressure. As shown in Figure 7, p. 11, it requires one variable-speed condenser water pump (an inverter-duty motor may be necessary, depending on the turn-down).

Figure 5. Throttling valve

Figure 6. Throttling valve with globe valve piped in parallel


ΔAdaptiView

ChillerControlPanel

ChillerCondenser


CondenserWater Pump

ButterflyValve



ΔAdaptiView

ChillerControlPanel

ChillerCondenser

Globe Valve

CondenserWater Pump


ButterflyValve


CTV-PRB006-EN 11


Advantages ...

• Good control at relatively low cost.

• Can reduce pumping costs.

Disadvantages ...

• Requires a suitable pump, motor, and drive combination.


Control Strategies

The system design options described in the preceding section offer ways to maintain the necessary condenser-evaporator refrigerant pressure differential by regulating condenser refrigerant pressure. Successfully implementing any of these options requires a control system that measures the refrigerant pressure differential at the chiller. Direct measurement of the condenser-evaporator refrigerant pressure differential provides the most reliable operation.

Chiller System Differential Pressure Logic

The following control sequence is meant to be adaptable to either a VFD or a modulating electronic control valve that will accept a 0–10 Vdc input and that can vary flow in the condenser water loop. Tracer AdaptiView™ embedded head pressure control does not actually sense or control condenser water flow directly. Instead, the chiller’s refrigerant system differential pressure is measured and the flow device is modulated to maintain a minimum required refrigerant differential pressure for the CenTraVac™ chiller.

Sequence of Operation

In general, the following control states, chiller modes, delays, set points, and functions only exist if the Refrigerant Pressure Output Type in the Tracer™ TU Configuration menu is set to Condenser Head Pressure Control.

At Tracer AdaptiView™ power-up or reset, and after the compressor is stopped, the Condenser Head Pressure Control output is initialized to the voltage defined by the Off State Output Command setting.

Upon recognition of a call for cooling and a corresponding call for the condenser water pump, the Condenser Head Pressure Control output is commanded to a value that is 50 percent of the maximum flow position (50 percent of the Output Voltage @ Desired Maximum Flow). The condenser water pump is then started.

Once the condenser water flow is proven and all other pre-start conditions (pre-lube, etc.) are met, the compressor is commanded on. Once compressor operation is confirmed, the head pressure control will begin running closed-loop control for head pressure per the internal setpoint.

Figure 7. Variable-speed condenser water pump


ΔAdaptiView

ChillerControlPanel

ChillerCondenser

Variable-SpeedCondenser Water Pump


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During normal operation with condenser water conditions providing a refrigerant differential pressure greater than the Head Pressure Control Setpoint, the control output will increase the condenser water flow, up to the maximum allowed output.

During operation with cool condenser water conditions and a resulting refrigerant differential pressure less than the Head Pressure Control Setpoint, the control output will decrease the condenser water flow, down to the minimum allowed output.

Hardware Requirements

In order to provide the 0–10 Vdc head pressure control output signal, a CenTraVac™ chiller with Tracer AdaptiView™ controls must be equipped with the optional dual analog “%RLA and Condenser Pressure Output” LLID. If factory-installed, this is shown on wiring diagrams as LLID 1A15. The 0–10 Vdc output signal will be available on LLID 1A15 terminals J2-4 (+) and J2-6 (-).

On Tracer AdaptiView units not factory-equipped with LLID 1A15, and if applying condenser head pressure control is desirable, it will be necessary to purchase and install the correct dual analog output LLID. Contact Trane Aftermarket for parts identification and pricing.

The Tracer AdaptiView/UC800 platform does not have LLID hardware to provide a 4 mA–20 mA analog output. If a 4 mA–20 mA signal is a requirement of the controlled device, then the use of an external signal converter (not provided by Trane) will be required.

The condenser water flow device to be controlled (electric valve, pump VFD, etc.) is field-provided and -installed.

Setup

The Tracer™ TU laptop service tool must be used to configure and set up the head pressure control output feature of the Tracer AdaptiView™. Start and connect Tracer TU to the UC800.

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1. In Configuration view, select the Options tab. Set %RLA and Condenser Rfgt Pressure Output to Installed.

2. In Configuration view, select the Options Setup tab. Set Rfgt Pressure Output Type to Head Pressure Control.

Note: Older units with retrofit Tracer AdaptiView CVR software will also need to have Pump Control set to Installed, or the Head Pressure Control feature will not appear.

3. Select Save to save the configuration to the UC800.

4. In LLID Binding view, ensure that the %RLA and Condenser Pressure Outputs LLID is properly bound in and communicating.

5. In Field Startup view, select the Head Pressure Control tab. Set the following setpoints:

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• Actuator Stroke Time - Set this value to the actual time that it takes the commanded flow device to “stroke” from the specified minimum flow position to its maximum flow position. This setpoint is adjustable from 1 second to 1000 seconds, with a factory default of 30 seconds.

Note: The factory default setting may not be appropriate for the device being controlled. Measure the actual stroke time of the device being controlled and enter it as the Actuator Stroke Time.

• Desired Minimum Flow - This value sets the desired minimum flow for the application. It is the lowest flow that is commanded while running the pump. This setpoint is adjustable from 0 percent to 100 percent, with a factory default of 20 percent. This value is typically adjusted in the field to result in a lowest flow rate that is just above the point at which the condenser water proof-of-flow device (flow switch) will make/break.

• Head Pressure Control Setpoint - This is the setpoint to which the control algorithm controls. For CVHE, CVHF, or CVHG and CDHF or CDHG chillers, this setpoint is adjustable from 3 psid to 10 psid (20.7 kPaD to 68.0 kPaD), with a factory default of 3 psid (20.7 kPaD).

• Output Voltage at Desired Maximum Flow - This is the voltage for the full desired flow of the device to be controlled, and is the largest flow command that will ever be sent to the device. This setpoint is adjustable from 0 Vdc to 10 Vdc, with a factory default of 10 Vdc.

• Output Voltage at Desired Minimum Flow - This setpoint corresponds to the lowest flow point of the device to be controlled. For example, an electronic valve may accept a 2 Vdc to 10 Vdc signal, with 2 Vdc corresponding to full closed and 10 Vdc corresponding to full open. In this situation, the Output Voltage at Desired Minimum Flow would be set to 2 Vdc. This setpoint is adjustable from 0 Vdc to 10 Vdc, with a factory default of 2 Vdc.

Note: The Desired Minimum Flow setpoint defined earlier is in addition to the value set for the Output Voltage at Desired Minimum Flow. For example, if the Desired Minimum Flow is set to 20 percent, the Output Voltage at Desired Minimum Flow is set to 2 Vdc, and the Output Voltage at Desired Maximum Flow is set to 10 Vdc, the lowest flow command sent during chiller operation will be 3.6 Vdc (i.e., 20 percent of 2 Vdc to 10 Vdc).

• Off State Output Command - This setpoint sets the voltage that the output will assume after initialization and/or after the compressor and condenser water pump have been shut off. This setpoint is adjustable from 0 Vdc to 10 Vdc, with a factory default of 2 Vdc.

• Damping Coefficient - This setpoint can be used to make the control output more or less aggressive for a given system. This setpoint is adjustable from 0.1 to 1.8 in increments of 0.001, with a factory default setting of 0.5.

Setting a larger Damping Coefficient will result in a faster response, and setting a smaller Damping Coefficient will result in a slower response.

Note: The factory default setting of 0.5 should provide an adequate control response for almost all systems. Adjust the Damping Coefficient only if close observation of the system operation indicates the control response is inaccurate.

CTV-PRB006-EN 15


• Select Save to save the setpoints to the UC800.

Other Applications

Reverse Acting

There are some systems or flow devices where it might be desirable to have a reverse acting command provided for the flow control device. An example might be a valve controlling a bypass line, where opening the valve increases the bypass flow and reduces the flow through the condenser of the chiller. In this situation, it is necessary to increase the voltage signal to reduce the condenser flow, and decrease the voltage signal to increase the condenser flow. This can be accomplished by inverting the setpoints for Output Flow at Desired Maximum Flow and Output Voltage at Desired Minimum Flow.

• Output Voltage at Desired Maximum Flow—This is the voltage for the full desired flow of the device to be controlled, and is the largest flow command that will ever be sent to the device. For an application requiring a reverse acting signal, this setpoint is set to the low value, typically 2 Vdc (for a device accepting a 2 Vdc to 10 Vdc signal).

• Output Voltage at Desired Minimum Flow—This setpoint corresponds to the lowest flow point of the device to be controlled. For an application requiring a reverse acting signal, this setpoint is set to the higher value, typically 10 Vdc (for a device accepting a 2 Vdc to 10 Vdc signal).

All other Head Pressure Control setpoints and recommendations remain the same.

Duplex Chiller

CDHF and CDHG Duplex chillers have two refrigeration circuits with two refrigerant differential pressures. When Condenser Head Pressure Control is selected in Configuration view of a Duplex chiller with Tracer AdaptiView, a single analog output will be provided that is based on the lower of the two water flow commands of the two running circuits.

If only one circuit of the Duplex chiller is running Condenser Head Pressure Control, analog output will represent the flow command of just the running circuit.

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If Condenser Head Pressure Control is selected for a Duplex chiller, there will be only a single analog output from a single analog LLID that is located in the circuit 1 panel.

Note: If Condenser Head Pressure Control is not applied, and if it is desirable to instead receive simple traditional DeltaP or %HPC signals, then an analog output LLID for each circuit is required—one in each panel.

CTV-PRB006-EN 17


Indirect Tracer AdaptiView Signal

Maintaining The Minimum Refrigerant Pressure Differential

The following system design options offer the ability to control chiller head pressure; however, they require an intermediate controller. The optional 0–10 Vdc head pressure control output signal must be ordered. This applies when you want to utilize the indirect signal options from the chiller controller:

1. condenser pressure

2. evaporator/condenser differential pressure

System Design Options

Regulating the refrigerant pressure difference between the condenser and evaporator typically means maintaining the condenser refrigerant pressure when necessary. There are essentially five different ways to accomplish this through system design:

• cooling tower fan control

• cooling tower bypass

• chiller bypass

• throttling valve(s)

• variable-speed condenser water pump

Brief descriptions of these methods follow, along with the primary advantages and disadvantages of each.

1. Cooling Tower Fan Control

One way to increase condenser refrigerant pressure is to turn off, or modulate the speed of, the cooling tower fans. Tower fan operation is usually based on the water temperature of the tower sump/basin. This strategy allows a single control system to furnish properly controlled water to more than one chiller.

Advantages ...

• Low-cost controls.

• Better system efficiency.

Disadvantages ...

• Control system may not be appropriate for the application (e.g. those that use river water, or with a tower that serves other systems).

• Many weather conditions can prevent cooling tower fan control from maintaining the leaving-tower water at or above the minimum temperature needed for proper chiller operation.

• If the tower sump contains a great deal of water, it may not be possible to comply with the CenTraVac™ “Operating Recommendations,” p. 7.

• Fan cycling may result in wide condenser-to-evaporator pressure swings.

2. Cooling Tower Bypass

This design option, shown in Figure 8, p. 18, elevates the condenser refrigerant pressure by mixing leaving condenser water with entering-condenser water from the cooling tower. A suitable bypass piping arrangement for this purpose connects two butterfly valves with a common actuator linkage (or a single three-way valve) to a flanged tee.

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Advantage ...

• Excellent control.

Disadvantages ...

• A valved bypass may be more expensive than other system design options.

• Requires a “dedicated” condenser water pump.


3. Chiller Bypass

Figure 9, p. 18 illustrates this system design option. Using the chiller bypass to reduce condenser water flow through the chiller raises the temperature differential (ΔT) across the condenser which, in turn, maintains the condenser refrigerant pressure.

Advantages ...

• Maintains a constant cooling tower flow rate.

• Does not require a “dedicated” condenser water pump on the tower.

• The system bypass can be provided at the tower rather than at each chiller, reducing this option’s first cost.

Disadvantage ...

• Like the cooling tower bypass, a valved bypass may be more expensive than other system design options.

Figure 8. Cooling tower bypass

Condenser Pressure or Pressure

2–10 Vdc Signal (Optional Signal)

Δ

AdaptiViewChiller

ControlPanel

ChillerCondenser

2 ButterflyValves

CondenserWater Pump



Field-Provided

Controller

Figure 9. Chiller bypass

Field-Provided

Controller



Δ

AdaptiViewChiller

ControlPanel

ChillerCondenser


CondenserWater Pump

2 ButterflyValves


CTV-PRB006-EN 19


4. Throttling Valve(s)

Throttling valves offer another means for reducing condenser water flow to increase refrigerant pressure by creating a greater temperature differential across the condenser. There are two common variations of this design option. The first, shown in Figure 10, p. 19, requires only one butterfly valve.

Note: Be sure to select a valve with a modulating range that can maintain the minimum refrigerant pressure differential (ΔP) when the chiller is running at minimum load and at a minimum tower sump temperature.

The nonlinear flow characteristics of a butterfly valve can cause unstable control at low flow rates. To avoid this instability, consider adding a small globe valve in parallel with the butterfly valve as shown in Figure 11, p. 19. Operate the valves in sequence so that the globe valve opens over the first half of the signal range and the butterfly valve begins to modulate when the globe valve is fully open. The globe valve should be large enough to prevent butterfly valve operation in an unstable region.

Advantages ...

• Provides good control at relatively low cost if the valves are properly sized.

• May reduce system pumping costs.

Disadvantages ...

• Requires a pump that can accommodate variable flow.

• Using a single butterfly valve (without a globe valve piped in parallel) may cause erratic control at low condenser flow rates.


Figure 10. Throttling valve

Figure 11. Throttling valve with globe valve piped in parallel



Δ

AdaptiViewChiller

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ChillerCondenser


CondenserWater Pump

ButterflyValve


Field-Provided

Controller



Δ

AdaptiViewChiller

ControlPanel

ChillerCondenser

Globe Valve

CondenserWater Pump


ButterflyValve

Electric or PneumaticValve Actuator Field-

ProvidedController

20 CTV-PRB006-EN


5. Variable-Speed Condenser Water Pump

This system option also modulates water flow through the condenser, increasing the temperature difference between the entering and leaving water and, in turn, raising the condenser refrigerant pressure. As shown in Figure 12, p. 20, it requires one variable-speed condenser water pump (an inverter-duty motor may be necessary, depending on the turn-down).

Advantages ...

• Good control at relatively low cost.

• Can reduce pumping costs.

Disadvantages ...

• Requires a suitable pump, motor, and drive combination.


Control Strategies

The system design options described in the preceding section offer ways to maintain the necessary condenser-evaporator refrigerant pressure differential by regulating condenser refrigerant pressure. Successfully implementing any of these options requires a control system that measures the refrigerant pressure differential at the chiller. Direct measurement of the condenser-evaporator refrigerant pressure differential provides the most reliable operation, though sensing condenser pressure for an indirect “measurement” offers a practical alternative.

The control panel on Trane CenTraVac™ chillers can accommodate either control strategy, as described in the following sections.

Sensing Condenser Refrigerant Pressure

If the evaporator pressure is relatively constant, the minimum refrigerant pressure differential can be maintained by sensing and regulating condenser pressure.

Note: Since this control strategy requires a constant evaporator pressure, do not use it in conjunction with chilled water reset or ice storage.

To help implement this indirect control strategy, the control panel can provide a 2- to 10-Vdc signal proportional to the condenser refrigerant pressure (see Figure 13, p. 21). If it detects a condenser pressure of 0 psia, for example, the control will produce a 2- Vdc signal. The control panel generates a 10-Vdc signal if it detects a condenser pressure that corresponds to the chiller’s high-pressure cutout (HPC) setting; i.e., 15 psig for standard chillers. Figure 14, p. 21 illustrates the relationship between the unit control output signal and condenser pressure for a standard chiller.

Figure 12. Variable-speed condenser water pump



Δ

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ChillerCondenser

Variable-SpeedCondenser Water Pump


Field-Provided

Controller

CTV-PRB006-EN 21


Typically, the unit control’s output signal is sent to a dedicated controller which modulates condenser water flow as necessary-either using valve(s) or a variable-speed drive-to maintain the minimum condenser-evaporator pressure differential (see Figure 9, p. 18 through Figure 12, p. 20).

Important: Do not use this unit control signal to directly control the valve(s) or drive since this may cause unstable operation.

Sensing Refrigerant Pressure Differential

Directly measuring the refrigerant pressure difference that exists between the condenser and evaporator is the most reliable way to maintain the chillers minimum condenser-evaporator refrigerant pressure differential: it accounts for refrigerant pressure changes in the evaporator and the condenser. That makes this control strategy particularly appropriate for systems with chilled water reset.

As Figure 15, p. 22 implies, the unit controller monitors the refrigerant pressures in the condenser and evaporator, and produces a 2- to 10-Vdc signal proportional to the pressure difference between them. This output signal is “scalable”; that means the chiller operator can tailor the pressure differential range for the application; i.e., the 2-Vdc signal can be set to represent any value between 0 and 400 psid, and the 10-Vdc signal to represent any value between 1 and 400 psid. Figure 16, p. 22 illustrates typical settings for the unit controller’s pressure-differential output signal.

Again, this 2- to 10-Vdc signal is usually sent to a dedicated controller that will, in turn, modulate valve(s) or a variable-speed drive to alter the condenser flow rate and maintain the minimum condenser-evaporator refrigerant pressure differential.

Important: Do not use this unit controller signal to directly control the valve(s) or drive since this may cause unstable operation.

Figure 13. Monitoring condenser refrigerant pressure

Figure 14. Refrigerant pressure-to-output-signal relationship (standard chiller)

AdaptiViewChiller

ControlPanel

ChillerCondenser

2–10 VdcProportional Signal

(Option)

Condenser Refrigerant Pressure (psia)

Cont

rol P

anel

Out

put S

igna

l (Vd

c)

22 CTV-PRB006-EN


Figure 15. Monitoring the condenser-evaporator pressure differential

Figure 16. Typical settings for unit control scalable differential pressure output signal

AdaptiViewChiller

ControlPanel

ChillerCondenser

ChillerEvaporator

2–10 VdcProportional Signal

(Option)

Condenser-To-EvaporatorRefrigerant Pressure Differential (psid)

Cont

rol P

anel

Out

put S

igna

l (Vd

c)

0 1.5 3.0 4.5 6 90

2

4

6

8

10

CTV-PRB006-EN 23

Summary

The optimum condenser water temperature for minimizing system power is not the lowest possible temperature (within the manufacturer’s guidelines). Rather, the optimum condenser water temperature will be some intermediate temperature, i.e. between design and “as cold as possible.”

The optimum condenser water flow for minimizing system power is generally lower than 3 gpm/ton. The optimum flow rate will depend on tower performance, the condenser system, pumps and chiller performance. Other influencing factors are certain first-cost considerations such as smaller/less expensive condenser water pumps, smaller/less expensive cooling towers, and condenser water piping.

All chillers must maintain a minimum refrigerant pressure differential between the condenser and evaporator to assure proper oil- and refrigerant-flow management and adequate hermetic motor cooling. This engineering bulletin defines the minimum differential for CenTraVac™ chillers manufactured during or since 1990; it also describes the system design options and unit control strategies that can be used to maintain this minimum differential.

www.trane.com

For more information, contact your local Trane office or e-mail us at [email protected]

Literature Order Number CTV-PRB006-EN

Date November 2009

Supersedes CTV-PRB006-EN (June 2000)

Trane has a policy of continuous product and product data improvement and reserves the right to change design and specifications without notice. Only qualified technicians should perform the installation and servicing of equipment referred to in this literature.

condenser water temperature control

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