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Chilled-Water-Plant Basicsconditions. Contact the manufacturer for these limits. Some chillers may allowextended operation below the selected flow rates.If water velocity through the condenser tubes is too low for significant periodsof time and the water is extremely hard, long-term fouling of the tubes mayalso occur. Webb and Li3 tested a number of internally-enhanced condensertubes at low velocity (3.51 ft/s [1.07 m/s]) and high water hardness. While theyfound that some of the internally-enhanced tubes fouled, in the long term theyconcluded:Because of the high hardness and low water velocity used in thesetests, we do not believe that the fouling experienced is typical of thatexpected in commercial installations. With use of good maintenancepractices and water quality control, all of the tubes tested are probablysuitable for long-term-fouling applications.It is important to remember that a chiller selected for low flow (as discussed inthe next section Chilled-Water-System Options) does not necessarily have lowvelocity through its tubes. If tube fouling is a major concern, consider the use ofsmooth, rather than internally-enhanced, tubes in the condenser for ease ofcleaning.Air-cooled condenserObviously, air-cooled chillers do not use condenser-water, since they reject theirheat by having ambient air passed across refrigerant-to-air heat exchangers. Inpackaged air-cooled chillers, the manufacturers attempt to provide optimalperformance by staging fans in response to chiller load and ambient, dry-bulbtemperature.LoadsIn comfort-cooling applications, loads are usually satisfied by air handlersequipped with coils to transfer heat from conditioned space air to circulatingchilled-water. Air is thus cooled and dehumidified as it passes across the finnedsurface of the cooling coils. Since the psychrometric process of conditioning airtakes place at the coils, selection of the optimum coil size and type from thewide variety available is important for proper system performance.SYS-APM001-EN5

Chilled-Water-Plant BasicsSome specialized process loads do not involve cooling air. Instead, they mayinvolve heat transfer directly within a piece of process equipment, such as thecooling jacket of an injection-molding machine. Heat transferred from the loadscan be controlled in a number of ways:nnnnThree-way valveTwo-way valveVariable-speed pumpUncontrolled coilsThree-way valve load controlA three-way control valve regulates the amount of water passing through a coilin response to loads. The valve bypasses unused water around the coil andrequires a constant flow of water in the system, regardless of load. A drawbackof this bypass is that the temperature of the water leaving the three-way valveis reduced at part-load conditions. This can be a major cause of so-called “low T syndrome” discussed on page 63 in the section Chilled-Water-SystemIssues. Three-way valves are used in many existing systems.Figure 2Valve-controlled loadsThree-Way ValveCooled AirTwo-Way ValveCooled AirTwo-way valve load controlA two-way, water modulating valve at the coil performs the same waterthrottling function as the three-way valve. The coil sees no difference betweenthese two methods. The chilled-water system, however, sees a great difference.In the case of the two-way valve, all flow in the coil circuit is throttled. No wateris bypassed. Consequently, a system using two-way valves is a variable-flowchilled-water system. The temperature of the water leaving the coil is notdiluted by bypass water so at part-load conditions, the system return-watertemperature is higher than with three-way valve control.6SYS-APM001-EN

Chilled-Water-Plant BasicsVariable-speed pumping load controlBy using a pump for each coil, the flow may be controlled by varying the pumpspeed. In such systems, there may be no control valves at the coil. This canreduce both the valve and the valve installation costs.Uncontrolled coilsFigure 3 shows a control variation using an uncontrolled or “wild”coil. In thissystem, control of the conditioned air supply is executed by face-and-bypassdampers that permit a portion of the air to bypass the coil surface. Advantagesof the strategy are the elimination of control valves and improved part-loaddehumidification. A disadvantage is that all the water is pumped all the time;however, in systems with very small water pressure drops, this systemarrangement may work economically.Figure 3Uncontrolled coilBypass DamperAir HandlerCooled Air MixtureFace DamperChilled-water distribution systemChilled water is circulated through fixed piping—most commonly steel, copper,or plastic—that connects the chiller with various load terminals. Piping is sizedto meet a project’s pressure loss, water velocity, and construction costparameters. Pressure drop is overcome by the use of a chilled-water pump.SYS-APM001-EN7

Chilled-Water-Plant BasicsChilled-water pumpThe purpose of the chilled-water pump is to circulate chilled water within theloop. Generally, the pump must overcome the frictional pressure losses causedby the piping, coils, and chiller and the pressure differential across open controlvalves in the system. The pump, while working at the system static pressure,does not need to overcome this static pressure. For example, in a forty-storybuilding the pump need not overcome the static pressure due to those fortystories.The pump is typically located upstream of the chiller, however it may beanywhere in the system, provided that the pump:nnnmeets the minimum pump net positive suction-head requirements. That is,the system pressure at the pump inlet must be both positive and highenough to allow the pump to operate properly;maintains the minimum dynamic pressure head at critical systemcomponents (usually the chiller). If the dynamic pressure head is not highenough at these components, proper flow will not be established throughthem;accommodates the total pressure (static head plus dynamic head) onsystem components such as the chiller’s evaporator, valves, etc.Note that the pump heat is added to the water and must be absorbed by thechiller. Generally, this represents a very small temperature increase.Multiple pumps are often used for redundancy. Depending on the terminalcontrol devices and system configurations, the chilled-water pumps may beeither constant or variable-flow.8SYS-APM001-EN

Chilled-Water-Plant BasicsDistribution pipingBy itself, the distribution system is easy to understand. Figure 4 shows asimplified distribution system consisting of multiple cooling coils, eachcontrolled by a thermostat that regulates the flow in its respective coil. Thevalves may be either three-way or two-way. As previously discussed, three-wayvalves require constant water flow, while two-way valves allow the water flowin the system to vary. As flow varies, the pump may simply ride its curve or usea method of flow control such as a variable-speed drive. See the section SystemConfigurations, on page 27, for a detailed discussion of distribution-systemoptions.Figure 4Simplified distribution ads2000 ASHRAE HVAC Systems andEquipment Handbook, Chapter 12,Hydronic Heating and Cooling System1Design contains additional referenceinformation on the components of achilled water distribution system.SYS-APM001-ENThe distribution system may contain other components, such as an expansiontank, control valves, balancing valves, check valves, and an air separator, toname a few. The density, and therefore the volume, of the water in a “closed”chilled-water distribution system varies as it undergoes changes intemperature. The expansion tank allows for this expansion and contraction ofwater volume.9

Chilled-Water-Plant BasicsCondenser-water systemAs in chilled-water distribution systems, condenser-water system piping—mostcommonly steel, copper, or plastic—is sized to meet a project’s operatingpressure, pressure loss, water velocity, and construction cost parameters.Pressure drop through piping and the chiller’s condenser, plus the coolingtower static lift, is overcome by use of a condenser-water pump.Cooling towerTo reject heat, water is passed through a cooling tower where a portion of itevaporates, thus cooling the remaining water. A particular cooling tower’seffectiveness at transferring heat depends on water flow rate, watertemperature, and ambient wet bulb. The temperature difference between thewater entering and leaving the cooling tower is the range. The temperaturedifference between the leaving water temperature and the entering wet-bulbtemperature is the approach.Effect of loadAs the building load—or heat rejection—decreases, range and approach alsodecrease. This means that when the building is at part load, the cooling towercan provide colder water at the same ambient wet-bulb temperature.Effect of ambient conditionsAs ambient wet-bulb temperature drops, the approach—at a constant load—increases. This must be considered when cooling-tower-control strategies aredeveloped. Detailed descriptions of these conditions appear in the subsection,Chiller-tower energy balance, page 72. For additional information, refer to 2000ASHRAE HVAC Systems and Equipment Handbook, Chapter 36, “CoolingTowers.”110SYS-APM001-EN

Chilled-Water-Plant BasicsControlsThe chilled-water supply temperature is usually controlled at the chiller. Mostcommonly, supply water temperature is used as the sensed variable to permitcontrol of chiller capacity to meet system load demand. Supply-temperaturecontrol strategies may be used on either constant- or variable-flow systems. Aspreviously discussed, flow control is executed at the load terminals using threeway or two-way valves, or separate pumps for each coil.Control capabilities run the gamut from slow-acting pneumatic controls, toelectromechanical controls, to sophisticated digital controls that use algorithmstuned to give superior performance.Chiller controlToday’s chiller controls are capable of doing more than simply turning thechiller on and off. At a minimum, these controls should monitor:nnnSafety points such as bearing temperatures and electrical points that, whenout of range, may cause motor failure.Data points that may cause operational problems if corrective action is nottaken. An example is low chilled-water or refrigerant temperature, whichmay result in freezing in or around the evaporator tubes.General points that should be logged daily to ensure proper chillerperformance.Table 1 lists the ASHRAE-recommended monitoring points.Table 1 Recommended chiller-monitoring points per ASHRAE Guideline 3-1996FlowChilledWaterInlet PressureInlet TemperatureFlowCondenser Inlet PressureWaterInlet TemperatureOutlet PressureOutlet PressureOutlet TemperatureOutlet TemperatureApproach TemperatureEvaporatorOilRefrigerant PressureRefrigerant PressureRefrigerant TemperatureRefrigerant TemperatureLevelLevelPressureTemperatureAdditional RequiredSYS-APM001-ENApproach TemperatureCondenserRefrigerant Compressor Discharge TemperatureAdditional RequiredVibration Levels11

Chilled-Water-Plant BasicsIn addition to monitoring data, it is vital that the chiller controls alert operatorsto possible problems. Diagnostic messages are necessary for the operator torespond to safety issues and data points that are outside normal operatingranges.While communicating these diagnostic messages is a requirement, some chillercontrols include factory-installed programming that responds to the diagnosticmessages. For example, when the chilled-water temperature nears freezing, thechiller sends a diagnostic message and adapts its operation by reducing thecompressor capacity, raising the chilled-water temperature to a safer condition.Finally, the chiller controls should communicate with a system-level controller.There are many system aspects that are outside the chiller’s direct control, suchas condenser-water temperature and the amount of fluid flowing through theevaporator and condenser. To minimize the system energy costs, the systemcontrols must coordinate chiller, pump, cooling-tower, and terminal-unitcontrols. This can only be done if adequate information is communicated fromeach system component to the system-level controls.Pump controlIn so-called constant flow systems, the pumps are either on or off, providingrelatively constant flow when in the on position. In practice, some flowvariation will occur as system pressure drop changes. In a variable-flow system,pump control is most often performed by maintaining a pressure differential ata selected point in the system. For example, a variable-speed drive will increaseits speed if the sensed pressure differential is too low, or slow down if thepressure differential is too high. The control point is selected to minimize overpressuring the system and to assure adequate flow at all critical loads. Optimalpumping control strategies are addressed in Critical valve reset, page 48.Chapter references1 2000 ASHRAE HVAC Systems and Equipment Handbook, Chapter 12,Hydronic Heating and Cooling System Design and Chapter 36, CoolingTowers, American

HVAC systems designers often use chilled-water systems to provide high- quality, cost-effective air conditioning for building owners. With the advent of more flexible chillers, system-level controls, and software analysis tools, the number of chilled-water-systems options has exploded.