Hot Water Boilers and ControlsWhy Condensing Boilers are “Different”

Presented Oct. 14, 2008Long Island Chapter, ASHRAE

H.W. Boilers and ControlsMajor types of boilers

Advantages and disadvantagesResistance to thermal shockFiretube vs. Watertube

Minimizing thermal shockPiping arrangementsControl systems, boiler and buildingOperations– ΔT

Boiler efficiencyRelevant factors

Condensing boilersHow efficient are they?Major typesApplications

Piping arrangementsPrimary-SecondarySingle LoopHybrid systems

Control SystemsConventional boilersCondensing boilers

Hot Water Boiler Types

Firebox Sectional Cast IronVertical Tubeless Scotch MarineFlexible WatertubeFinned Copper TubeCondensingIndustrial Watertube

Boiler Types NOT Recommended

FireboxGeometry makes uniform circulation difficultMany right angle welds concentrate stress

Sectional Cast IronLess efficient than other typesLarger units are easily damaged by thermal shockEutectic cast iron boilers are an exception

Thermal Shock

Resistant boilersCopper fin tubeFlexible Watertube boilersMost condensing boilersEutectic cast iron boilers

Shock prone boilersConventional Cast IronScotch MarineFirebox Boilers

Thermal ShockThermal Shock results from -

rapid temperature changes in the boileruneven temperature changes to boiler vesselparts of boiler expanding (or contracting) more rapidly than other partsrigidity in boiler constructioncontinuous “flexing” of rigid partscan be caused by frequent cyclingfor example: shutting plant down at night

Thermal Shock results in -leaking tubescracked tube sheetscracked sections in cast iron boilers

Causes of Thermal Shock

Return of cold water to a hot boilersystem piping in building cools down overnightboiler is kept hotsecondary pumps over pump the primary pumps

Return of hot water to a cold boilercold boiler is started after being isolated from flow

Failure to bring a cold boiler up to temp slowlycold boiler should stay at low fire until up to temperature, at least for 30 minutes

Four pass water backed firetubeFour pass water backed firetube

9

Firetube Hot Water Boiler Design

Scotch marine firetube boilerAdvantages

Very efficientSizes up to 800 HPBurn any fuelLow waterside Δ PEasy to cleanEasy to maintain

Replace or plug tubesClean tubes

DisadvantagesProne to thermal shock

Slow warm-upMaintain temperature in standby boilers

Floor space requirementsTube pull area

Must not condenseTypical 20 - 30ºF ΔT limit

On many large projects, firetube hot water boilers are extremely efficient and reliable, but hydronic system and control system design must be adapted to the boilers needs

Copper Fin Tube Boilers

•Fan assisted – sealed combustion•Low emissions•Medium efficiency -- 80 to 84%•Staged Combustion

•Atmospheric•Draft hood•Modulating combustion

•Return water temp down to 105ºF•Can be stacked two high to conserve floor space (no offset)•From 122 MBH to 4000 MBH•Condensing heat exchanger can be added

For Hydronic Heating andDomestic Hot Water

Indoor and Outdoor

Copper Finned Tube Heat Exchanger

Hydronic Systems for Fin Tube Boilers

Generally use primary-secondary schemePrimary pump is supplied on the boiler

Primary pump sized to match the boiler requirements

Too much flow can cause erosion of boiler tubesToo little flow can cause local overheating

Boiler mounted pumps run with the boilerShut down when boiler is off line

Secondary pumps run via BMS control

Finned Copper Tube SummaryPro

High efficiencyLow standby lossesLow costNo thermal shockLow water temperaturesSealed combustionDirect heating of DHWSimple to maintainLow costCan offer Condensing Operation

ConGas fired onlyFlow sensitive

Use primary-secondary systemsExcess flow – erosionLow flow – scale formationMust have flow to operate

Beware of rated efficienciesCondensing versions use secondary heat exchangers

Flexible Watertube Boilers

Small footprint143 HP unit is 47.5”w by 160”long by 86” high34” tube pull to each side

Can be field erectedno welding or tube rollingbuild in one week

Guaranteed against Thermal ShockRequires a minimum flow

Flextube Boilers

Water in the Tubes/Exhaust Gases Pass Around the TubesUp to 12 MMBtu/hr InputMultiple PassesHot Water/Low Pressure Steam Boilers -High Resistance to Thermal StressHeating Applications

Industrial Watertube BoilersUsed for low, medium and high temperature hot water and high pressure steam

Sizes from 15 to 100 million BTU/hrTubes are tangent allowing for individual expansion and contraction

18

Boiler Water Flow versus ΔT

BoilerHP

BoilerOutput(x1000)BTU/HR

SYSTEM TEMPERATURE DROP - DEGREES F

MAXIMUM CIRCULATION RATE - GPM10 20 30 40 50 60 70 80 90 100

6070

5040302015

80100125150200250300350400500600700800

500670

10051340167520102345268033504185502566958370

10045117201340016740200802345026780

100134200268335402470536670836

1005134016752010235026803350402046905360 2680

2345201016751340117510058386705034183352682352011681341006750 33 25 20 17

456789112134157179223279335447558670784895

1120134015651785 1340

1175100583867058750341933525120916813411810184675033 27

4054678094107134168201268335402470535670805940

1075 89578567055844739233528022416814011290786756453322

14 12 11 101929384858677796

120144192240287336383479575670765

17 15 1325 22 20334250596784

30 273745526075

3340475467

105126168210251294335419502585670 595

52044837229826122318614911293 84

100134167201236268335402470535

A brief discussion of boiler efficiency

Boiler efficiency depends on many factorsBoiler designPercent load (firing rate)Fuel being firedTemperature of fluid (water or steam) in boiler

Always try to obtain efficiency guaranties:Based on fuel being firedBased on actual design water temperatures

Don’t believe everything you read

Boilers are really heat exchangers

The lower the stack temperature the higher the efficiencyThe lower the fluid temperature the lower the stack temperatureHeat recovery exchangers can be used to recover energy in flue gassesScotch Marine boilers are extremely efficient heat exchangersConventional boilers capture sensible heatCondensing boilers capture sensible AND latent heat

Efficiency by LossesFuel energy in = heat energy outEnergy leaves in hot water (or steam) or as a lossEfficiency = 100% minus lossesGreatest loss is stack loss (100% minus stack loss = “Combustion Efficiency”

Typically 15% to 20% including latent and sensible heatWith natural gas, 10% of energy in fuel is lost as latent heat of vaporizationWith fuel oil, 4% of energy in fuel is lost as latent heatRemainder of stack loss is sensible heatSensible heat loss increases with excess air

Second greatest loss is radiation lossTypically ½ to 3% of energy input at maximum loadRadiation loss is a constant BTU loss, not a constant %

Typically other losses from boilers are insignificant.

Condensing BoilersOne ft3 natural gas yields two ft3 water vapor.Two ft3 water vapor condenses to one ounce waterAbout 9% of the BTU content in each ft3 natural gas burned leaves the stack as latent heat of vaporization in this water vaporBy condensing this water and lowering the stack temperature, 98% efficiency can be reached. Some heat pump supplement boilers can achieve this.A 1 million BTU/hr boiler will produce 6 gallons/hr liquid water when fully condensingThis water will only condense at gas temp <135ºFNo manufacturer’s boiler can take full advantage of typical 160 to 180F hydronic applications

Cautions !!!!!!!!!!!!!!!Any boiler can be a condensing boiler

Just return water cooler than 130°FWater will condense somewhere in the boiler

Conventional boilers will be damagedBy corrosion or failed refractory from condensationBy sooting due to blocked fins (copper fin tube)

Condensing boilers are specialThey can handle flue gas condensate safelyThey can also run at non-condensing temperatures

Condensing boilers need special flue materialAL29-4C or PVC (316L is used in Europe)

Types of Condensing BoilersFiretube – high water volume

Can be used in variable flow single loop systemsForgiving of low or no water flowExamples are Pulse, Benchmark, VantageThe higher the water volume, the lower the flow can be with boiler firing

Watertube – low water volumeMust be used in primary – secondary systemsRequire a minimum water flow through boilerExamples are Wall mounted European design, Copper Fin, Aluminum heat exchanger design

Examples of firetube condensing boilers

Vertical Extended Surface Firetube

High water volumeTubes have internal “fins”Full condensing OperationHigh water volumeNo minimum water flowNo minimum water temperatureEfficiency up to 98%Sealed Combustion availableScreen type low NOx burner

Extended Heating Surface Tubes

Stainless steel tubesAlloy finned inserts

Exceptional heat transfer

Down firedEfficiencies up to 98%Extremely quiet

Efficiency as a function of percent firing rateand return temperature

Pulse Boilers

Use fuel energy to pull in combustion airNo combustion air blower motor (except for start)

High water volume designRequire vibration isolation on mount and pipingSpecial designs available for low emitted noiseExtremely compact

Internals of Pulse Boilers

Is not sensitive to water flowUses very little electrical energy – 0.25 amps, 120V

Pulse Boiler internal construction

Benchmark SeriesLow mass firetube design

Oil as a back-up fuel

NYC does not allow use of propaneMost condensing boilers also burn propaneMany designs allow for automatic switch to propane

Very few condensing boilers can burn oil#2 oil is used as a back-up in some designs

Boiler is prevented from condensing when on oilWater temperature is automatically raised

Dual Vessel Condensing BoilerDual Fuel Capable

Fully condensing on gasBurns #2 oil as backupSizes 2, 3 and 4 million BTU

Cut away view of dual fuel condensing boiler

Compact watertube condensing boiler

Compact watertube condensing boiler

Premix Burner 316L SS Heat Exchanger

Hydro-Formed water-tubes

Efficiency of European design boiler

Very low electrical consumptionExtremely efficient heat exchanger

Flue gas temperature is 20ºF above inlet waterCompact316L heat exchanger allows one unit to be applied for hydronic or domestic hot water

Condensing boilers – Special concernsFlue gasses will condense in flue piping

Need constant pitch TOWARD boilerNeed drain sections

Condensate is acidicNeutralize with limestone chipsUse trap to keep flue gasses out of neutralizer and out of boiler room

Follow manufacturers instructions for ventingWatch differential between comb. air and exhaust.

Follow NYC codes for spacing and size limitationsSidewall venting limited to 350,000 Btu inputMinimum spacing between ventsNo venting into shaft-ways or small courtyards or over sidewalks

Hot Water System Design

Allow room for expansion of waterProvide constant flow through primary loop

Past all temperature sensorsThrough flow sensitive boilers

Purge all air from systemBalance flow through operating boilersPrevent thermal shock damage to boilersPrevent steaming - maintain water pressureKeep water inlet and outlet temperatures within design limits

41

Three Good HW Operating Practices

Maintain Water QualityPeriodic water analysis to see when treatment is neededMonitor make-up water flow into system

A planned preventative maintenance program.

Burner, Controls, Pressure Vessel, Refractory and circulating pumps and control valves

42

Hot Water Boiler Summary

Provide continuous circulation through the boilerPrevent hot or cold shockPrevent frequent cyclingBalance the flow through boilersProvide proper over-pressureProvide water treatmentCheck for leaks – loss of water treatment

Sequencing multiple conventional boilersKeep as few boilers on line as possibleWarm-up of cold boilersKeep water temperatures above 140ºFKeep warm for stand-by boilersMaintain flow through operating boilersCool-down period for boilers shutting downMinimizing ΔT on operating boilersIsolating boiler water temperature from building system temperatureControl strategy depends on piping arrangement

Basic two boiler system for conventional boilers

Constant and Equal FlowThrough Both Boilers

Primary Pumps setBoiler Flow

Secondary Pumps setBuilding System Flow

Boiler 1

Boiler 2

With two boilers on lineLoad is sharedBoiler ΔT is same as Header ΔT

With one boiler on lineWater at return temp leaves #2ΔT of #1 must double to maintain same Header ΔT

Boiler Return Temperature

Boiler Outlet HeaderTemperature

Water to and fromBuilding SystemCould be 1000’sof Gallons storedin piping

TT

Constant Flow System Notes

Works fine for two boiler systemsTemperature blending is a problem with 3Matches flow to loadAllows one pump to be 100% spareNeed to keep primary pumping rate high enough compared to building loop pumping rate to keep boiler return temperature above manufacturers minimum with cold water returning from building loop.

Three Firetube Boiler SystemArranged for Full Automation

This system shows motorized isolation valves (a) to allow flow to be directed to operating boilers only. Flow control valves (b) are shown bypassing the on-off valves to make certain that there is some flow through the boiler to keep vessel hot.Blend pumps (c ) serve to equalize the temperature within the boiler when in “keep warm” mode. Typically the lead pump(d) would run continuously, and would support the lead boiler. The first lag pump (e) would start and stop with the first lagboiler. A signal from boiler return temperature could be used to cut back on the building pumping rate if the return water temperature fell below the minimum (150 degF for CB firetube boilers. Manual shutoff valves (f) would isolate a boiler from the loop for “cold standby” duty.

(a) (b)

(c ) (d)

(e)

(f)

Primary Loop, Constant Differential

Single Hot Water Loop, Blend Valves

Primary Secondary Loops

Sequencing multiple condensing boilers

Keep as many boilers on line as possibleCondensing boilers are more efficient at low loadsMore residence time allows for more condensation

Modulate operating boilers in parallelReduce or stop flow through standby boilersMaintain minimum flow through operating units

Common header temp sensor needs flowMinimum flow depends on boiler design

Primary Secondary System withOne Dedicated Pump per Boiler

Primary Pumps in Individual Boiler Leads

Manifolded Secondary Pumps

Secondary TapsSpaced as CloseTogether as Possible

Boiler 1

Boiler 2

Boiler 3

TT

Works well with watertube condensing boilers

Advantages of Individual Primary Pumps in Boiler Lead

Flow through operating boilers is “constant”Max Δ T at full fuel input remains constantBoilers have flow for sufficient mixing

No flow through boilers that are off lineNo blending of cool water with heated water in common discharge headerΔ T of operating boilers is reduced

Note: One pump must always runSo that there is flow past the header sensors

Modern control systems for multiple condensing boilersGenerally furnished by boiler manufacturer

Most offer serial communications with individual boilers and with building automation systems

Typical control functions:Sequencing of boilers, pumps, and isolation valvesFull automation of entire primary loopBuilding Management has control of secondary loop

The control sequence for multiple boilers is directly influenced by the way the system is piped. Consult your boiler vendor to make sure his control system matches your piping arrangement!

Thank you!