V ARIABLE VOLUME / V ARIABLE SPEED PUMPING SYSTEMS APPLICATIONS Bell & Gossett Bulletin D-120Bw REPLACES BULLETIN D-120A UNIT RESPONSIBILITY WITH Packaged Systems Group Equipment Selection Program Part of the TERMINALS ZONE A TERTIARY CS OR VS PUMP(S) VARIABLE SPEED PUMP(S) AUTOMATIC AFD BYPASS TECHNOLOGIC CONTROLLER PRIMARY CHILLER PUMPS CONSTANT SPEED ROLAIRTROL ™ & COMP. TANK BALANCING VALVE (CIRCUIT SETTER ® ) BV BV TDV TDV TDV CHILLER TWO WAY ZONE VALVE V-A COMMON COMMON (PRIMARY- SECONDARY) CHILLER TERMINALS ZONE B TERTIARY CS OR VS PUMP(S) BV BV V-B COMMON TERMINALS ZONE C TERTIARY CS OR VS PUMP(S) BV BV BV V-C COMMON BYPASS (optional) SUPPLY SUPPLY RETURN POWERSAV ™ ADJUSTABLE FREQUENCY DRIVE(S) RETURN ∆P SENSOR/ TRANSMITTER OTHER ZONES
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VARIABLE VOLUME / VARIABLE SPEEDPUMPING SYSTEMS APPLICATIONS
VARIABLE SPEED CHILLED WATER PUMPCONTROL PRIMARY-SECONDARY-TERTIARY SYSTEMWITH REVERSE SECONDARY DISTRIBUTION
DESCRIPTION:The volume of water supplied to the system is variedin response to the cooling load by the two-way zonevalves. The basis of Variable Speed Pumping is tosave pumping cost by optimizing the pump motorhorsepower input. The pump produces only the flowand head required at any time. This is performed byregulation of the pump speed by an AdjustableFrequency Drive and Technologic™ Pump Controller.The controller regulates the pump speed in responseto the system load conditions.
DESIGN PHILOSOPHY:The concept of Variable Volume-Variable Speedpumping offers several advantages:
The ability to supply the cooling required to meetthe demand.The two-way control valve properly sized and selectedassures the zone control system with the flow requiredat all loads. The Chillers are sequenced by theircontrol system to maintain their temperature.
The flexibility to expand the system easily–The primary-secondary-tertiary system shown hasultimate capabilities with little modifications required.Additional chillers can be added in parallel along withexpansion of distribution pumps to meet the new loadconditions.
Accurate comfort control–By maintaining the system control at the design tem-perature drops, the complete system operates at itsdesign conditions and comfort levels.
Minimum energy consumption–Util izing the Variable Volume system (two-wayterminal control valves), the pumping energy can bereduced by more than two-thirds that of a constantspeed pumping system riding its pump curve. Inaddition the primary-secondary design decouples thechiller pumps from the distribution pumps enablingthem to be cycled, staged and/or loaded to meet theload requirements as they occur.
APPLICATION COMMENTS:• The Chillers are placed in a primary-secondary rela-
tionship with the distribution load allowing constantspeed pumping for the chillers which are sized for thechiller pressure drop and the chiller related piping. Theparallel chiller arrangement provides more efficientstaging by chiller efficiency and its design permitsfuture expansion.
• When reverse return distribution piping is employed,the piping pressure drop differential from zone to zoneis minimized however it must be proportionally bal-anced to maintain correct distribution and the ∆P sensor/transmitter should be placed centrally as shown; fordirect return systems the importance of proportionalbalancing is essential and the ∆P sensor/transmittershould be located near or across the last zone’s risers.
• Improperly located ∆P sensor/transmitters may greatlydiminish, or even negate, the savings potential of theVariable Speed Pumping System. Conversely, properlocation and a practical number of ∆P sensor/trans-mitters will maximize the savings by optimizing thesystem operation.
• A primary-secondary-tertiary should be consideredwhen:- applied to a large system (large cooling or heating
loads or long piping runs)- high diversity requirements- high system loss on only 1 or 2 zones (out of many)- variable pressure drops (∆P) on different zones if
any of the above conditions are present the tertiaryVariable Speed pumping concept will save signifi-cant operating cost over primary-secondary. TheBell & Gossett ESP PLUS™ computer programshould be consulted for a detailed load analysis.
• Where there are multiple significant loads that havepeaks at different times, multiple sensor/transmitterslocations should be considered. The TechnologicPump Controller will automatically monitor all sensorswith reference to each set point and control to thelargest deviation from set point, thus satisfying allloads.
• Where extended loads are light or where a system hasweek-end shut-downs a small bypass with a balanc-ing valve set for low flow around the zone two-wayvalve might be a design consideration to reducethermal stratification and allows a quicker start-upafter the shut-down period.
• A good understanding of set point is required tooptimize the operation of the variable speed pumpingsystem. If it is set too high, the pumping system willnot slow down and the predicted savings in power willnot be realized.
VARIABLE SPEED CHILLED WATER PUMPING CONTROLTWO-WAY VALVE REVERSE RETURN PIPING SYSTEM
APPLICATION COMMENTS:• Primary-Secondary pump system for maximum
energy savings and control. For further informationsee Design Philosophy #1 (Drawing D120-102).
• The Variable Speed secondary system responds to adifferential pressure sensor/transmitter which issensing across typical supply-return branch orterminal and two-way control valve. This allows forenergy savings as the terminals respond to a lighterload and the two-way valves control at a lower flowposition. This increased pressure drop signals thecontroller to operate the pump(s) at a lower speed tomaintain its set point.
• In a system where there are major load differencesfrom one riser to another, multiple ∆P sensor/trans-mitters can be applied to each major load with uniqueset points for each load. The controller will control thepump(s) speed for the worst condition, providing evengreater energy savings.
• Single ∆P sensor/transmitters should be placed cen-trally in a reverse return piping system to provide anaverage indication of load for the entire system.
• Reverse return piping design is usually selected tominimize pressure drop from zone to zone. However,it must be proportionally balanced to maintain uniformdistribution.
• The greater the system head loss (terminal coils,piping, fittings) the greater the potential for pumpingenergy savings.
• An optional bypass with a 2-3% maximum flowsetting may be considered for zones with week-endshut down for even pick-up and for more stability forlight load conditions. In lieu of bypass on last zone, a3-way valve may be employed with balancing valve inits bypass.
VARIABLE SPEED CHILLED WATER PUMPING CONTROLTWO-WAY VALVE DIRECT RETURN PIPING SYSTEM
APPLICATION COMMENTS:• Primary-Secondary pump system for maximum
energy savings and control. For further informationsee Design Philosophy #1 (Drawing D120-102).
• The Variable Speed secondary system responds to adifferential pressure sensor/transmitter which issensing across typical supply-return branch orterminal and two-way control valve. This allows forenergy savings as the terminals respond to a lighterload and the two-way valves control at a lower flowposition. This increased pressure drop signals thecontroller to operate the pump(s) at a lower speed tomaintain its set point.
• In a system where there are major load differencesfrom one riser to another, multiple ∆P sensor/trans-mitters can be applied to each major load with uniqueset points for each load. The controller will control thepump(s) speed for the worst condition, providing evengreater energy savings.
• Single ∆P sensor/transmitters should be placedacross the farthest and most resistant load in a directreturn piping system to provide a signal for the entiresystem.
• Direct return piping design is usually selected as alower first cost alternative. However, balancing ismore difficult since the pressure drop across the firstzones will be much greater and proportional balancingwill be required to prevent overflow conditions.
• The greater the system head loss (terminal coils,piping, fittings) the greater the potential for pumpingenergy savings.
• An optional bypass with a 2-3% maximum flowsetting may be considered for zones with week-endshut down for even pick-up and for more stability forlight load conditions. In lieu of bypass on last zone, a3-way valve may be employed with balancing valve in
CONVERSION OF CONSTANT SPEED PUMPING & 3-WAY VALVESTO VARIABLE SPEED CHILLED WATER PUMP CONTROL
APPLICATION COMMENTS:• Primary-Secondary pump system for maximum
energy savings and control. For further informationsee Design Philosophy #1 (Drawing D120-102).
• Convert 3-Way valves to Two-way operation to providevariable terminal flow in relation to load and maximizepotential pump energy savings by only circulating whatis actually required. Check the valve operator for ade-quate closing power since it will now operate in a two-way mode and must be able to close against the high-est expected differential pressure or that determinedby the pump variable speed controller ∆P setting.
• Primary-secondary pump chillers are needed becauseof variable system flow in secondary.
• The balancing valve (BV) in the return line should bere-balanced to proportionally set the design flow to thisterminal in proportion to the other valves in this zone.
• ∆P sensor/transmitter taps to be located at end ofdirect-return main, or centrally for reverse return main.
• For multi-distribution mains, suggest ∆P sensor/trans-mitter for end of each direct/return main with uniqueset points and one Technologic Pump Controller.
• The valve size and plug characteristic should also bechecked for two-way operation. Valve size should bebased upon the terminal design flow (gpm) at anadequate pressure drop. Calculate its Cv factor andcompare against the existing valve’s value. (Note:Cv=GPM/√∆P). If more than 25% oversize is foundthen a smaller valve should be installed. (See drawingD120-105-1.)
MODIFY EXISTING3-WAY TO 2-WAYBY CLOSING BYPASSBALANCE VALVE ORBLIND FLANGINGVALVE BOTTOMPORT.
BV
AB
A
B
CONVERSION OF CONSTANT VOLUME PUMPINGWITH 3-WAY VALVES TO VARIABLE SPEED-
VARIABLE VOLUME PUMPING SYSTEM
APPLICATION COMMENTS:• Convert 3-Way valves to Two-way operation to provide
variable terminal flow in relation to load and maximizepotential pump energy savings by only circulating whatis actually required. Check the valve operator for ade-quate closing power since it will now operate in a two-way mode and must be able to close against the high-est expected differential pressure or that determinedby the pump variable speed controller ∆P setting.
• The valve size and plug characteristic should also bechecked for two-way operation. Valve size should bebased upon the terminal design flow (gpm) at anadequate pressure drop. Calculate its Cv factor andcompare against the existing valve’s value. (Note:
Cv=GPM/√∆P). If more than 25% oversize is foundthen a smaller valve should be installed. The valveplug characteristic should also be checked. Forproportional operation of a hydronic coil consider anequal-percentage valve plug shape to provide amodifying effect to the non-linear hydronic coilrelationship so that the ratio of heat transfer versusvalve position gives a linear change with the valvecontrol signal.
• The balancing valve (BV) in the return line should bere-balanced to proportionally set the design flow to thisterminal in proportion to the other valves in this zone.
VARIABLE SPEED HOT WATER HEATING PUMP CONTROLTWO-WAY VALVE REVERSE RETURN PIPING SYSTEM
APPLICATION COMMENTS:• Primary-Secondary pump system for maximum
energy savings and control. For further informationsee Design Philosophy #1 (Drawing D120-102).
• The Variable Speed secondary system responds to adifferential pressure sensor/transmitter which is sens-ing across typical supply-return branch or terminal andtwo-way control valve. This allows for energy savingsas the terminals respond to a lighter load and the two-way valves control at a lower flow position. Thisincreased pressure drop signals the controller tooperate the pump(s) at a lower speed to maintain itsset point.
• In a system where there are major load differencesfrom one riser to another, multiple ∆P sensor/trans-mitters can be applied to each major load with uniqueset points for each load. The controller will control thepump(s) speed for the worst condition, providing evengreater energy savings.
• Single ∆P sensor/transmitters on reverse return systemshown should be placed across typical terminal
supply-return in middle of piping run to provide anaverage signal for the entire system.
• Primary-secondary pumped boilers recommendedbecause of variable system flow in secondary. Pumpsplaced on discharge side of boilers to reduce boilerpressure requirements. Air separator on hot watersupply, downstream of common, to maximize airremoval.
• Each terminal should be proportionally balanced witha balancing valve. Terminal two-way control valve tobe equal-percentage characteristic to linearize heatoutput versus flow of terminal to minimize controlhunting.
• For multi-distribution mains, suggest ∆P sensor/trans-mitter for each main with unique set points for each onthe Technologic Pump Controller.
VARIABLE SPEED HOT WATER PUMP CONTROLTWO-WAY VALVE DIRECT RETURN PIPING SYSTEM
APPLICATION COMMENTS:• Primary-Secondary pump system for maximum
energy savings and control. For further informationsee Design Philosophy #1 (Drawing D120-102).
• The Variable Speed secondary system responds to adifferential pressure sensor/transmitter which issensing across typical supply-return branch orterminal and two-way control valve. This allows forenergy savings as the terminals respond to a lighterload and the two-way valves control at a lower flowposition. This increased pressure drop signals thecontroller to operate the pump(s) at a lower speed tomaintain its set point.
• In a system where there are major load differencesfrom one riser to another, multiple ∆P sensor/trans-mitters can be applied to each major load with uniqueset points for each load. The controller will control thepump(s) speed for the worst condition, providing evengreater energy savings.
• Single ∆P sensor/transmitters should be placedacross the farthest and most resistant load in a directreturn piping system to provide a signal for the entiresystem. For multi-distribution mains suggest ∆Psensor/transmitter for each main with unique set pointfor each on the Technologic Pump Controller.
• Direct return piping design is usually selected as alower first cost alternative. However, balancing ismore difficult since the pressure drop across the firstzones will be much greater and proportional balancingwill be required to prevent overflow conditions.
• Primary-secondary pumped boilers are recommendedbecause of variable speed flow in secondary. Pumpsplaced in discharge side of boilers to reduce boilerpressure requirements. Air separator on hot watersupply to maximize air removal.
• Each terminal should be proportionally balanced witha balancing valve. Terminal two-way control valve tobe equal-percentage characteristic to linearize heatoutput versus flow of terminal to minimize controlhunting.
• The greater the system head loss (terminal coils,piping, fittings) the greater the potential for pumpingenergy savings.
• An optional bypass with a 2-3% maximum flowsetting may be considered for zones with week-endshut down for even pick-up and for more stability forlight load conditions. In lieu of bypass on last zone, a3-way valve may be employed with balancing valve inits bypass.
VARIABLE SPEED BUILDING PUMP CONTROL FORDISTRICT HEATING SYSTEM-DIRECT RETURN PIPING
APPLICATION COMMENTS:• The building variable speed pumping system responds
to a differential pressure sensor/transmitter sensingacross the end of the supply-return terminal risers.This provides energy savings as the terminals respondto a lighter load and the two-way valves control at alower flow position. This increased pressure dropsignals the controller to operate the pump(s) at a lowerspeed to maintain its set point. The greater the systemhead loss (terminal coils, piping, fittings) the greaterthe potential for pumping energy savings.
• In a building system with multiple risers and there aremajor load differences from one riser to another,multiple ∆P sensor/transmitters can be applied to eachmajor load with unique set points for each load. Thecontroller will regulate the pump(s) speed for the worstcondition, providing even greater energy savings.
• ∆P sensor/transmitters should be placed across theend of each distribution loop in a direct return districtpiping system to provide an adequate ∆P for eachsystem loop.
• Direct return design is usually selected for the buildingpiping as a lower first cost alternative. However,balancing is more difficult since the pressure dropacross the first zones will be much greater and pro-portional balancing will be required to prevent over-flow conditions.
• Reset of each building’s hot water supply temperaturecan be customized with O.A. reset by employing areset controller positioning the building’s heat ex-changer two-way control valve. A temperature sensoris also provided in the return water to the district returnto insure a required maximum return temperature, ifrequired for the district system.
ZONED VARIABLE SPEED/VARIABLE VOLUME CHILLEDWATER PUMPING CONTROL & TWO-WAY VALVE SYSTEM
APPLICATION COMMENTS:• Each zone pump must be carefully sized for the
supply-return distribution main in addition to it’s zonerequirements without overpressuring adjacent zone’sreturn. The zone’s variable speed/variable volumepump control automatically compensates for it’s loadrequirements by maintaining a zone differentialpressure as the load changes. Two-way valve controlpermits diversity control within the zone or building.
• Single ∆P sensor/transmitters should be placedacross the farthest and most resistant load in a directreturn piping system to provide a signal for the zonesystem. In a system where there are major load differ-ences from one riser to another, multiple ∆P sensor/transmitters can be applied to major loads with uniqueset points.
• The constant speed chiller pumps are sized to deliverdesign flow to the system common and it’s return.
• Direct return piping design is usually selected as alower first cost alternative. However, balancing ismore difficult since the pressure drop across the initialterminals within a zone/building will be much greaterand proportional balancing will be required. Balancingof the zone returns is required to minimize reverseflow and over-pressuring in adjoining zones.
• A primary-secondary variable speed distributionpumping system will show a lower first cost andcomparable operating cost with less complexity thanthe direct zone pumping method shown. In addition,∆P sensor/transmitters can be located to insuredesign flows for critical zones.
COOLING TOWER WITH VARIABLE VOLUME-VARIABLE SPEED CONDENSER PUMP CONTROL
APPLICATION COMMENTS:• Variable pumping flow rate is controlled by the con-
denser leaving water temperature which is affected bythe refrigeration load.
• An alternate sensor location is to measure differentialtemperature across the condenser or sense the refrig-erant head pressure (care must be exercised to insureagainst refrigerant leakage).
• Pumping energy is directly influenced by the pressuredrop through the condenser tubes and the towernozzles. Variable pump speed provides operatingpower saving. Variable speed pump control willautomatically maintain flow as the condenser tubesand tower sprays foul due to water conditions.
• Minimum condenser flow is mandatory and is set onthe pump control to reduce condenser tubes foulingwith extremely low flows and reduce freeze-up withoperation near freezing conditions. Differential tem-
perature sensing may be required for minimum flowoperation.
• Maximum pump speed may be set on the pumpcontrol to prevent over-pressure tower nozzles andreduce misting.
• Tower bypass piping may be required for “free-cooling” or cold weather operation with tower basininstalled inside the building and proper freeze-detectioncontrols. Balancing valve in the tower-bypass is set tomaintain positive head in the riser to the sprays toprevent air introduction into condenser water from thespray heads due to negative pressure.
• Where variable speed fan control is utilized on thetower, the variable speed pumping system will compli-ment it. Both will reduce total power consumption forthe condensing water system.
DOMESTIC WATER SUPPLY PRESSURE BOOSTERWITH VARIABLE SPEED PUMPING
APPLICATION COMMENTS:• The variable speed/variable volume pump is sized for
fixture full load capacity. As the building demand isreduced, the variable speed pump is reduced in speedto meet the load indicated by the pressure at the pres-sure sensor/transmitter compared to the controller’sset point. The Technologic Pump Controller has anadjustable set point and control that automaticallyregulates the pump speed to meet the load.
• Variable speed pumping is justified on basis ofchanges of city water pressure (pump suction), loadchanges and overheaded constant speed pumpselection. Overheading is caused by a pump selectedfor a greater delivery and head than actually requiredand may also result from a higher city water pressure.
• The constant volume (standby) pump is automaticallybrought on-line by the Automatic Bypass if a failureoccurs at the variable/speed (duty) pump drive, Techno-logic pump controller, or the pressure sensor/transmitter.
• Down-stream pressure control is essential to protectthe system in high head conditions.
• The proper pressure sensor location is very important.It should be away from the immediate pump locationso that it will sense “typical” or average fixture supplypressures; usually a location near the top of thefarthest riser will provide this. Alternate locations maybe dictated by job conditions but care should beexercised so that it’s location does not cause short-cycling of the pumps and starvation of farthest loads.
APPLICATION COMMENTS:• Variable speed pumping is justified by the resulting
process control accuracy; with resulting every savingsin large systems having substantial flow variations.
• The liquid level transmitter can be different analogtypes to suit process, materials, etc., such as floattype transmitter, bubbler-pressure transmitter, or non-intrusive types such as ultra-sonic, etc.
• In this application the typical “on-off” cycling of theconstant speed pump is eliminated and the pump lifewill be extended at a lower power consumption.
BATCH PROCESS HEAT TRANSFER WITH CONSTANTTEMPERATURE CONTROL USING VARIABLE SPEED PUMPING
APPLICATION COMMENTS:• Variable speed pumping is justified by providing
accurate process control and fast response to systemload changes by monitoring temperature of batchprocess resulting in uniform product production. Inaddition, electrical power reduction via variable speedpumpingh at part load conditions is achieved.
• In this application the two-way or three-way tempera-ture control valve pressure drop on the tank and heater
coil is eliminated, resulting in further optimum energyreduction. The small circulation pump promotes effec-tive thermal mixing and expedient response to loadchanges.
• This is one example of many process control applica-tions showing how variable volume/variable speedpumping and heat transfer equipment can be employed.
VARIABLE SPEED PRIMARY CHILLED WATER PUMP CONTROL WITH TWO WAY VALVE DIRECT RETURN SYSTEM
DESCRIPTION:The volume of water supplied to the system is varied inresponse to the cooling load by the two-way zonevalves. The basis of variable speed pumping is to savepumping cost by optimizing the pump motor horse-power input. The pump produces only the flow and headrequired at any time. This is performed by regulation ofthe pump speed/adjustable frequency drive andmodulation of a bypass valve through the use of theTechnologic™ Controller. The controller regulates thepump speed and valve position in response to thesystem load conditions and in order to protect thepumps and chillers.
DESIGN PHILOSOPHY:The concept of variable speed primary pumping offersseveral advantages:
The ability to supply the cooling required to meet thedemand.The two-way control valve properly sized and selectedassures the zone control system with the flow required atall loads. Two-way valve control permits diversity controlwithin the zone or building. The Technologic™ providesoptimal chiller/pump sequencing commands.
The flexibility to expand the system easily.Additional identical chillers and pumps can be added inparallel to meet the new load conditions.
Reduce energy consumption.Utilizing the variable volume system (two-way terminalcontrol valves), the pumping energy can be reduced bymore than two-thirds that of a constant speed pumpingsystem riding its pump curve.
Protection features built into B&G algorithms.B&G developed and fully tested software eliminatesnuisance chiller trips due to lack of flow and preventsoperation of chiller beyond maximum flow. End of curveprotection ensures that pumps do not run off theiroperating curve and any and all speeds.
SYSTEM CONTROL:Pump and system controls should utilize dynamicsequencing that provides most efficient, energy-savingscontrol while furnishing appropriate protection of chillersand pumps.
Priority #1 Energy SavingsThe variable speed system responds to a differentialpressure sensor/transmitter, which is sensing across aterminal and two-way valve. This allows for energysavings as the terminal responds to a lighter load andthe two-way valves control at a lower flow position.This increased pressure drop signals the controller tooperate the pump(s) at a lower speed to maintain itsset point.
Priority #2 Minimize Tube FoulingThe variable speed system responds to ensure mini-mum flow requirements are being met for all runningchillers. When necessary, the minimum speed and the
valve position shall be adjusted in order to meet theoperating chillers’ flow requirements. Maintainingminimum chiller flow is mandatory and is set on thepump controller to reduce chiller tube fouling withextremely low flows.
Priority #3 Prevent Premature Tube ErosionThe third priority is to monitor system flow rate toprevent operation above the maximum flow for thechillers and the pumps. Additional chiller(s) andpump(s) shall be signaled to run upon detection of ahigh flow condition in order to avoid pump operationbeyond end of curve and to protect chiller from highvelocity/tube erosion conditions. Maximum flow mustbe constantly monitored independent of drive speedto guarantee that at any speed, these damagingconditions do not occur.
APPLICATION COMMENTS:
• Single differential pressure sensor/transmitters should beplaced across the appropriate load in a direct returnpiping system to provide a signal for the zone system. Ina system where there are major load differences fromone riser to another, multiple differential pressuresensor/transmitters can be applied to major loads withunique set points. The Technologic™ Pump Controller willautomatically monitor all sensors with reference to eachset point and control to the largest deviation from setpoint, thus satisfying all loads.
• A good understanding of set point is required to optimizethe operation of the variable speed pumping system. If itis set too high, the pumping system will not slow downand the predicted savings in power will not be realized.
• Constant monitoring of chiller flow is essential in order toprotect both the chillers and the pumps. This can beaccomplished by utilizing either differential pressuresensors or flow sensors piped across each chiller. Ifdifferential pressure is used, size the differential pressuresensor for minimum chiller flow.
• Theoretically, the bypass should be sized for the smallestchiller, but since maintenance schedule may take thatchiller out of service, the bypass needs to be sized forthe minimum flow of the largest chiller.
• Size the bypass modulating valve for zone differentialpressure.
• Chiller isolation valve control, linked to each chiller, isneeded to guarantee most efficient operation and to onlyallow flow through operating equipment.
• In the event that no chiller is running, the bypass valvecontrol will function to protect the pumps from operatingat deadhead pressures in the event all of the zone controlvalves are closed.
• Locate the air separator in the return piping prior to thebypass pipe to maximize air removal due to the relativelyhigher temperature of the water.
SYSTEMS EXHIBITING ALL OF THESECHARACTERISTICS ARE MOST LIKELY TORESULT IN SUCCESSFUL APPLICATION OFVARIABLE SPEED PRIMARY PUMPING
System flow can be reduced by 30%.If system flow can be reduced by 30% from design,the potential for energy savings and reasonable pay-back period is likely. Systems whose load does notvary significantly will not reap the benefits of thissophisticated control scheme.
System can tolerate modest change in water temperature.As with primary-secondary piping arrangement,systems that can tolerate moderate changes in watertemperature are better candidates for variable primarypumping, Allowing temperature to rise before stagingon another chiller could result in greater cost savings.
Operators are well trained.If the operator is not familiar with the equipment andthe control scheme, he is unlikely to operate asdesigned thus the system will not operate efficiently.Initial periodic training is mandatory.
Demonstrates a greater cost savings.First costs are straightforward to determine, sincethese are comprised of equipment and installationcosts at the time of design or bid. Operating costs canbe more difficult to determine. Software is available.(ESP-Plus) Life cycle cost should be the cost method-ology used to ensure that the building owner’s totalout-of-pocket expenses are minimized. Comparisonbetween variable primary pumping and primary-secondary designs is the most common assessmentto investigate during the design phase. Keep in mindthat variable primary systems require one set of largervariable speed pumps as compared to secondaryvariable speed pumps. Primary-secondary does re-quire two sets of pumps, but the cost of larger primarypump adjustable frequency drives may offset the costof starters, with an added benefit of reduced systemcomplexity to benefit the untrained owner.
High % of hours is at part load or low returnwater temperature.Applications that operate for a high percentage of timeat part load will utilize the variable speed, energy sav-ings potential of variable primary systems by loweringspeed, thus consuming less energy during lighterloads. Those that operate at full load but with lowerthat design return water temperature benefit in a similarmanner by slowing down the pumps to an adequaterate, thus maintaining optimal return temperature,allowing the chiller(s) to operate more efficiently.
AVOID THE USE OF VARIABLE SPEED PRIMARYPUMPING IN THESE APPLICATIONS
Supply temperature is critical.Depending upon the volume of the system and therate of change, the supply water temperature mayvary. If temperature is critical, primary-secondarypiping will allow for more evenly maintained tempera-ture. Only those systems that can tolerate variablesupply water temperatures should be considered.
System volume.Constant volume results in little energy savings. Theamount of fluid contained in the system can dictatethe rate of change of temperature through the chiller.Without adequate system volume, the temperature ofthe return fluid can be too low creating a low tempera-ture trip in the chiller or a freeze up condition.
Existing controls are old or inaccurate.If existing controls are old or inaccurate, system willbe difficult to control. Variable speed primary pumpingrequires additional controls for the chillers and thepumps. The system relies on these controls for properregulation. Pumps will operate based on flow and tem-perature, and ∆P requirements. Sensors must be main-tained and periodically calibrated to ensure that thesequencing that utilizes the sensor inputs is accurate.
Limited operator knowledge.Care must be taken to keep the complexity of thesystem within the operator’s ability. If the operatordoes not understand the control sequencing andcomplexity, he is unlikely to operate the system asdesigned. Systems too complex for the operator areoften turned to manual, bypassing the energy savingfunctionality of the control system.
Unequal sized chillers.For redundancy, similar in parts, and maintenanceissues, chillers are chosen to be equal in size. Thisredundancy is compromised when chillers of differentsizes or types are utilized. Unequally sized chillerspresent flow concerns. Individual pumps per chillerneed to be replaced with headered pumps, See pagesD120-116 and 117. In addition, flow meters or differ-ential pressure sensors along with two position controlvalves need to be provided for each chiller. The controlvalves should be sized per chiller manufacturer’srecommendation. The control algorithm must compen-sate for the minimum flow for each chiller as well asthe proportion of flow for each chiller when the systemis flowing at less than full chiller capacity.
Unequal sized pumps.Like their chiller counterparts, equally sized pumps aredesirable. Operators prefer the simplicity of like com-ponents. For pump protection and to ensure the properflow is passing through each chiller, pumps must beequal in size (flow and head). Dissimilar pumps mayresult in premature pump failure.
VARIABLE SPEED PRIMARY CHILLED WATER PUMP CONTROL WITH TWO WAY VALVE DIRECT RETURN SYSTEM (cont.)
VARIABLE SPEED CHILLED WATER PUMP CONTROLVARIABLE SPEED PRIMARY SYSTEM WITH BYPASS AT THE CHILLERS
APPLICATION COMMENTS:• A variable speed primary pumping system allows for
energy savings and can reduce the floor space re-quirement for HVAC equipment in the mechanicalroom. For further information on design philosophysee page D120-115.
• Where extended loads are light or where a system hasweekend shutdowns, a small bypass (2-3% maximumflow setting) with balancing valve set for low flowaround the zone two-way valve should be a designconsideration to reduce thermal stratification andallow a quicker start-up after shut-down period. In lieuof bypass on last zone, a 3-way valve may beemployed with balancing valve in its bypass.
APPLICATION COMMENTS:• The pressure relief valve mounted in the pump dis-
charge piping will protect against excessive pressureshould all chiller isolation valves be closed.
• The chillers are placed in series with a bank of equallysized (flow and head) parallel pumps to supply chilledwater to the two-way terminal control valves. Thisallows variable speed pumping for the chillers andsystem in which the variable speed chiller pumps aresized for the pressure drop through the chillers, andthe entire network of system piping.
VARIABLE SPEED CHILLED WATER PUMP CONTROLVARIABLE SPEED PRIMARY SYSTEM WITH BYPASS AT THE END OF SYSTEM
APPLICATION COMMENTS:• A variable speed primary pumping system allows for
energy savings and can reduce the floor space re-quirement for HVAC equipment in the mechanicalroom. For further information on design philosophysee page D120-115.
• The parallel chillers are placed in series with a bank ofequally sized (flow and head) parallel pumps to supplychilled water to the two-way terminal control valves.This allows variable speed pumping for the chillersand system in which the vari-able speed chiller pumpsare sized for the pressure drop through the chillers,and the entire network of system piping.
APPLICATION COMMENTS:• Where extended loads are light, and in the event the
lead chiller is off, the bypass valve should not be fullyclosed. This will prevent stagnation of water at thepoint of supply. In addition, the flow through thebypass will protect the pumps from operating atdeadhead pressures in the event all of the controlvalves are closed.
• The pressure relief valve mounted in the pump dis-charge piping will protect against excessive pressureshould all chiller isolation valves be closed.
VARIABLE VOLUME / VARIABLE SPEED SYSTEMS APPLICATIONS
VARIABLE SPEED CHILLED WATER PUMP CONTROLVARIABLE SPEED PRIMARY PUMP PIPED DIRECLY TO CHILLER,
CHILLERS WITH EQUAL RATINGS
APPLICATION COMMENTS:• A variable speed primary pumping system allows for
energy savings and can reduce the floor space re-quirement for HVAC equipment in the mechanicalroom. For further information on design philosophysee page D120-115.
• In this application, dedicated chiller pumps may oper-ate at different rates of speed to minimize energyconsumption.
• The pressure relief valve mounted in the pump dis-charge piping will protect against excessive pressureshould all chiller isolation valves be closed while pumpis running.
APPLICATION COMMENTS:• Where extended loads are light or where a system has
weekend shutdowns, a small bypass (2-3% maximumflow setting) with balancing valve set for low flowaround the zone two-way valve should be a designconsideration to reduce thermal stratification andallow a quicker start-up after shut-down period. In lieuof bypass on last zone, a 3-way valve may beemployed with balancing valve in its bypass.
• The pressure relief valve mounted in the pump dis-charge piping will protect against excessive pressureshould all chiller isolation valves be closed.
