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Digester Heating 101

Tom Mossinger – Carollo Engineers PNCWA 2012

2012 Annual Conference Boise, ID

October 23, 2012

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Agenda

• Review hot water system theory and operation

• Discussion on typical components

• Highlight typical problems

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Heating System - Considerations

• Move heat from sources to uses

• Hydraulics

•  Allow for future expansion and/or cogeneration

•  Space requirements

• Complexity of system

•  Flexibility

• Operations and maintenance

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Heating System Types

•  Primary loop

•  Primary-secondary loop

• Constant flow

•  Variable flow

• Hybrid systems There  are  

Literally  100’s  of  configura7ons!  

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Simplified Hot Water System Schematic

Cogeneration System

Main Loop

180°F

TIC TIT

BLR ~180°F

TIC

TIT

150° MAX

TIT

98°F

Sludge

Softened makeup

water

Expansion tank

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Typical Cogeneration System

Engine Generator

Waste Heat

HW System

Exhaust

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Typical Boiler System

Boiler

Exhaust TIC TIT

180°F

Internal Controls

On 150°F

Set Point 190-195°F Off 195-210°F

(Typical)

Main HW Loop

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Typical Digester Heating System

TIC

TIT

150° MAX

TIT

98°F

Sludge

Main HW Loop

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Hot Water System Pressure Control

Softened makeup

water

Expansion tank

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Typical Hot Water System Components

•  Pumps

•  Air separator

•  Expansion tank

•  Makeup water connection

•  Chemical feeder

•  Air relief valves

•  Control valves

•  Balancing Valves

•  Strainers

•  Pressure relief valves

•  Boilers

•  Valves

•  Heat exchangers

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Digester Heat Exchangers

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Shell and Tube Sludge Heater

• Combined boiler and heat exchanger

• Hard to incorporate future hot water systems

•  Least efficient

•  Less flexible

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Tube-in-Tube Heat Exchangers

•  Primarily used for thicker sludge or where plugging is likely

• More expensive

•  Larger footprint

• Hard to predict heat transfer

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Spiral Heat Exchangers

• Most efficient

•  Predictable heat transfer

•  Less expensive

•  Smallest footprint

•  Easy to clean

• More likely to plug with thicker sludges or poor screening

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Typical Problems

• Corrosion –  Air intrusion

–  Water treatment

•  Flow rate control –  Balancing

• HW system expansion after expansion after expansion

• Complication!

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Heat Exchanger Scale – Vivianite, Baked on Sludge

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Ductile Iron is an Excellent Heat Conductor

F Temp

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Scaling Decreases Heat Transfer

F Temp

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Thermal imaging provides a quick non-destructive method to determine the

extent of scaling

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What is the extent of scaling in the digester piping?

100°  F                                                                      125°  F  Temperature  

 

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Prevention and planning can prevent scaling problems

•  Tools available for cost-effective detection

• Mitigation involves removing conditions that favor scaling: –  Remove phosphorus

–  Prevent large temperature changes

–  Prevent large pH changes

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Water Treatment

• Companies such as Nalco provide sampling and treatment chemical packages

• Chemicals typically include: –  Scale inhibitor

–  Oxygen scavenger

–  Biocide

• Use water softener for all make-up

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Questions?