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Variable Primary Systems

Richard Roberts – Systems Sales EngineerJanuary 03, 2012

Lower Capital Cost Installed (vs Primary/Secondary)No secondary pumps/piping/valves/electrical to buy and install

Fewer piping and electrical connections – less field labor

No large common pipe, but smaller bypass pipe/valve/flow meter/controls

Lower CHW Pumping Energy

Potential Savings, may reduce pump horsepower and initial cost.Reference ARTI (Air Conditioning & Refrigeration Technology Institute)

Estimated energy savings of 3 to 8 percent

Estimating initial cost saving of 4 to 8 percent

Estimating life cycling saving cost 3 to 5 percent

Variable Primary Flow Only (VPF) Advantages

Smaller Footprint (vs Primary/Secondary)More space available for better access to the other equipment

Higher ReliabilityFewer mechanical components

Any pump can serve any chiller

Variable Primary Flow Only (VPF) Advantages

Potentially higher PSID rated 2-Way valves in systemRequires more robust (complex and calibrated) control systemRequires coordinated control of chillers, isolation valves, and pumpsin sequencingPotentially longer commissioning time and start-up (ref ASHRAETaylor article)Requires greater operator sophisticationSudden flow variation through the chiller

More of an issue when additional machines are staged on

More complexity with bypass control

Variable Primary Flow Only (VPF) Disadvantages

Primary/Secondary System

Constant SpeedPrimary Pumps

Variable SpeedSecondary Pumps

Common Pipe

Typicalloadwithtwowayvalves

Primary/Secondary System at Design

54.0°F

54.0°F

54.0°F

44.0°F

44.0°F

44.0°F

Primary Pumps

360 GPM Each

1080 GPM @ 54.0 °F

Secondary Pumps

1080 GPM @ 44.0 °F

TypicalCoil

No flow

44.0°F

54.0°F

150 ton chillers360 GPM Each54.0-44.0°F

1080 GPM @ 54.0 °F

Primary/Secondary System at Part Load

51.5°F

51.5°F

51.5°F

44.0°F

44.0°F

44.0°F

Primary Pumps

360 GPM Each

1080 GPM @ 51.5 °F

Secondary Pumps

810 GPM @ 44.0 °F

TypicalCoil

44.0°F

54.0°F

270 GPM @ 44.0 °F

810 GPM @ 54.0 °F

75% System Load

Primary/Secondary System

54.0°F

54.0°F

44.0°F

44.0°F

Primary Pumps

360 GPM Each

720 GPM @ 54.0 °F

Secondary Pumps

720 GPM @ 44.0 °F

TypicalCoil

44.0°F

54.0°F

No Flow

720 GPM @ 54.0 °F

OFF 67% System LoadClean Coils

Primary/Secondary System

53.0°F

53.0°F

44.0°F

44.0°F

Primary Pumps

360 GPM Each

720 GPM @ 53.0 °F

Secondary Pumps

792 GPM @ 44.8 °F

TypicalCoil

44.8°F

53.0°F

72 GPM @ 53.0 °F

792 GPM @ 53.0 °F

OFF 67% System LoadDirty Coils

Primary/Secondary System

51.9°F

51.9°F

44.0°F

44.0°F

Primary Pumps

360 GPM Each

720 GPM @ 51.9 °F

Secondary Pumps

900 GPM @ 44.8 °F

TypicalCoil

44.8°F

51.9°F

180 GPM @ 51.9 °F

900 GPM @ 51.9 °F

OFF 67% System LoadDirty Coils

Primary/Secondary System

Variable Primary System

Secondary Pumps

Primary Pumps

Primary Pumps

Flow Meter

BypassValve

Automatic Isolation Valve

Typical load with 2 way valve

Typical load with 2 way valve

Variable Primary System at Design

54.0°F

54.0°F

54.0°F

44.0°F

44.0°F

44.0°F

Primary Pumps

360 GPM Each

1080 GPM @ 56.0 °F

1080 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassClosed

150 ton chillers360 GPM Each54.0-44.0°F

Typicalloadwithtwowayvalves

Variable Primary System – Part Load

54.0°F

54.0°F

54.0°F

44.0°F

44.0°F

44.0°F

Primary Pumps

270 GPM Each

810 GPM @ 54.0 °F

810 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalve

75% System Load

810 GPM @ 54.0 °F

Variable Primary System – Part Load

54.0°F

54.0°F

44.0°F

44.0°F

Primary Pumps

180 GPM Each

540 GPM @ 54.0 °F

540 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalve

50% System LoadChiller off

540 GPM @ 54.0 °F

Variable Primary System – Min Flow (120 gpm each)

51.5°F

44.0°F

Primary Pumps

120 GPM (oneoperating)

120 GPM @ 51.5 °F

90 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassOpen

Typicalloadwithtwowayvalve

System flow below chillerminimum flow

Chiller off

Chiller off

Closed

Closed

30 GPM @ 44.0

90 GPM @ 54.0 °FFlow meter

Pumpsoff

Variable Primary System – Min Flow (120 gpm each)

51.5°F

44.0°F

Primary Pumps

120 GPM (oneoperating)

120 GPM @ 51.5 °F

90 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassOpen

Typicalloadwithtwowayvalve

Pumps maintain system pressure

Bypass maintains minimum flow

Chiller off

Chiller off

Closed

Closed

30 GPM @ 44.0

90 GPM @ 54.0 °FFlow meter

Pumpsoff

Designing a VPF Bypass

Size for minimum flow through one chiller

Valve head ratings must be higher than pump dead head

Linear characteristic - valve position % equals flow %

Fast actuator

Valve control range 100:1

Flow meter controls bypass to maintain chiller minimum flow

Selecting a VPF Flow Meter for Small Tonnage Systems

Best accuracy job can afford

Minimum +/- 5% at evaporator minimum flow

Set bypass to open when flow equals minimum plus 2 times flowmeter accuracy rating

Example: 120 gpm minimum, flow meter +/- 5%, set minimum flowat 120 + 120*10% = 132 gpm

Calibrate flow meter at least as often as manufacturer recommends

Buy the best

Selecting VPF AHU Control Valves

All valve pressure ratings greater than pump dead head

Sized by pressure

% valve position equals % coil capacity

Control range 200:1

Selecting VPF Chiller Isolation Valves

All valve pressure ratings greater than pump dead head

Linear characteristic - valve position % equals flow %

Modulating valve with slow actuator

Variable Primary System (1 chiller running)

54.0°F

44.0°F

Primary Pumps

120 GPM Each

360 GPM @ 546.0 °F

360 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalve360 GPM

Variable Primary System (Staging on secondchiller)

54.0°F

45.0°F

Primary Pumps

132 GPM Each

396 GPM @ 55.0 °F

396 GPM @ 45.0 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalve396 GPM

Need to add chiller

Variable Primary System (Open isolation valve)

55.0°F

35.0°F

Primary Pumps

132 GPM Each

396 GPM @ 55.0 °F

396 GPM @ 45.0 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalve198 GPM

198 GPM55.0°F55.0

°F

Variable Primary System (Open isolation valve)

55.0°F

35.0°F

Primary Pumps

132 GPM Each

396 GPM @ 55.0 °F

396 GPM @ 45.0 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalve198 GPM

198 GPM55.0°F55.0

°F

LCHWT approaches 35F

LWT Cutout at 40F

Off goes chiller 1

Variable Primary System (Open isolation valve)

55.0°F

40.0°F

Primary Pumps

132 GPM Each

396 GPM @ 55.0 °F

396 GPM @ 45.0 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalve264 GPM

132 GPM55.0°F55.0

°F

Open slowly

Chiller 2 comes on when flowexceeds minimum

Chiller 1 unloads to maintain setpoint

Variable Primary System (Open isolation valve)

55.0°F

42.0°F

Primary Pumps

132 GPM Each

396 GPM @ 55.0 °F

396 GPM @ 45.9 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalve276 GPM

120 GPM55.0°F55.0

°F

Open to change flow 36gpm/min

Chiller 2 comes on when flow is120gpm – in 3 min 20 sec

Chiller 1 is still producing 100%capacity

Variable Primary System (Open isolation valve)

54.0°F

44.0°F

Primary Pumps

132 GPM Each

396 GPM @ 54.0 °F

396 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalve198 GPM

198 GPM44.0°F54.0

°F

Both valves fully open

Flow equalizes

Chillers modulate to maintainchilled water set point

Staging on Chiller 2 in VPF Systems

Maintain minimum flow through the operating chiller

Avoid sudden changes in flow anywhere in the system

Prefer schemes that work when bringing on any chiller

Setting Flow Rate of Change

Start with 10% of design flow per minute

Most small tonnage chillers can handle faster changes

If the project requires rapid flow changes, contact applicationengineering

If problems occur during operation, adjust the rate

Variable Primary System – Handling RapidlyDecreasing Loads

54.0°F

54.0°F

54.0°F

44.0°F

44.0°F

44.0°F

Primary Pumps

360 GPM Each

1080 GPM @ 56.0 °F

1080 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassClosed

150 ton chillers360 GPM Each54.0-44.0°F

Typicalloadwithtwowayvalves

Variable Primary System – Handling RapidlyDecreasing Loads

54.0°F

54.0°F

54.0°F

44.0°F

44.0°F

44.0°F

Primary Pumps

360 GPM Each

1080 GPM @ 44.0 °F

0 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassOpen

150 ton chillers360 GPM Each54.0-44.0°F

Typicalloadwithtwowayvalves

Variable Primary System – Handling RapidlyDecreasing Loads

54.0°F

54.0°F

54.0°F

44.0°F

44.0°F

44.0°F

Primary Pumps

324 GPM Each

972 GPM @ 44.0 °F

972 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalves

Close the closest valve first

Variable Primary System – Handling RapidlyDecreasing Loads

54.0°F

54.0°F

54.0°F

44.0°F

44.0°F

44.0°F

Primary Pumps

288 GPM Each

864 GPM @ 44.0 °F

864 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalves

Close the next closest valve

Variable Primary System – Handling RapidlyDecreasing Loads

54.0°F

54.0°F

54.0°F

44.0°F

44.0°F

44.0°F

Primary Pumps

252 GPM Each

756 GPM @ 44.0 °F

756 GPM @ 44.0 °F

AutomaticIsolation Valve

BypassClosed

Typicalloadwithtwowayvalves

Close the next closest valve

Questions?