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food production hoods: air filtration & heat recovery solutions

BUILDING SERVICESdesigning

Paul O’Sullivane: pauld.osullivan@pmg.ie

Commercial catering facilitiescan be energy-intensivedepartments for anybusiness/facility. They areextremely demanding on indoorclimate systems with largevariations on temperature andhumidity requirementsthroughout the various differentcooking and preparationprocesses. For the HVACengineer they can presentcomplex design challenges.

This article looks at the potentialheat recovery benefitsassociated with ventilationcooking canopies/hoods as aresult of the increasing use of UVfiltration and subsequently,cleaner exhaust air streams. Itconcentrates specifically on thecooking ventilation hood, thisbeing the central artery of atotal kitchen ventilation system.

In exploring the possible benefitsassociated with heat recoverysolutions for kitchen ventilationshoods and how this may befeasible due to improvements inefficiencies of grease filtration,designers should first consider thedifferent types of greaseextraction filtration.

Mechanical Grease ExtractionFiltersTraditionally, grease extractionhas been by the use of

mechanical grease filters such asthe baffle and multi-cyclonetypes highlighted in Figure 1.These have generally been ableto extract up to a maximum of85% of airborne grease in theform of both particulate andvapour.

ASHRAE completed a study ofthe different mechanical greaseextraction filters and their abilityto eliminate grease in theexhaust airflow [The Facts,Mechanical Grease, June 2003].It found that the type of cookingprocess and cooking applianceutilised is important as thecontent ratio of particulate/vapour will have an impact onthe max efficiency of greaseextraction from the multi-cycloneand baffle type filters.

It is important to note that noneof the mechanical filters testedby ASHRAE captured grease at aparticle size less than 2.5_m.Figures 2 and 3 highlight the

differences in exhaust emissionsat different cooking processes. Itis important for the HVACengineer to understand thecooking configuration whenselecting a suitable ventilationcanopy design. Reference toHVCA DW172 will highlight thedifferent selection procedures.

ASHRAE also found that: —— Cyclonic filters are far more

efficient than baffle types at grease extraction;

— Higher airflow results in higher pressure drop, meaning more entrainment of grease particulates. Efficiencies improved with increasing air volumes;

Figure 1 shows the baffle and multi-cyclone type mechanical grease filters

Figure 2 (above) and Figure 3 (below)highlight the differences in exhaustemissions at different cooking processes

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food production hoods: air filtration & heat recovery solutions

— Total filter efficiency, even forthe best grease extractors, does not reach 90% because mechanical filters cannot trap vapor effluents.

Even at this high level of filtrationthere can still be problemsassociated with grease build inexhaust systems, namelyincreased maintenance costsand fire safety issues. In anattempt to alleviate these issuesfurther improvements in filtrationhave been developed, mostnotably UV filtration.

UV grease filtrationThe introduction of integratedultra violet light to deal with theremaining small particles andvapour grease has become acommon theme in recent years.The result of their introductionhas undoubtedly increased boththe capital and operational costof commercial cookingcanopies, (UV lamp life is approx8,000hrs) but they havefacilitated improvements in bothfire safety, fire-rated ductworkrequirements, and net energyconsumption for kitchenventilation systems in general.

While UV filtration in general isnot a new science and has beenwidely used in other filtrationapplications, this recentventilation filtration concept usesspecial UV lamps for thetreatment of organiccompounds, principally grease-laden and odorous extract air,by the processes of photolysisand ozonolysis.

There are two primary chemicalreactions that take place in theUV oxidation process. The UV

lights emit radiation in the UV-Cband, the shortest of the threebands (UV-A, UV-B & UV-C) andalso create ozone in the vicinityimmediately surrounding thelamps. The chemical processtaking place when UV-C lightdirectly hits molecular chainsand breaks them into smallercompounds is called photolysis.

The photolysis reaction is mosteffective on small greaseparticles (especially vapor) sincethe light can only break thechemical bonds on the outersurface of the grease particle if itis large and has a double bondstructure.

The second chemical processthat takes place is when theozone, created from theinteraction of the UV light withthe oxygen molecules in the air,continues to react with thegrease molecules as they movethrough the exhaust ducts to theoutside. This process is calledozonolysis.

The repeated reaction of theozone with the grease vapourwill eventually break them downsmall enough to create the by-products CO2 and H2O. Theseby-products of the oxidationprocess do not adhere to theduct surfaces and will be carriedaway by exhaust airflow.

The UV-C filtration process, takingplace at the light and in theexhaust duct continuing to theexternal, facilitatesimprovements in greaseextraction above the 90% level,making the air stream sufficiently

clean and more suitably-available for energy recovery.

Ventilation canopy and ductedsystem solutionsUV filtration is normally suppliedas an integral component of aventilation canopy. Figure 4highlights the main componentsof an integrated supply andexhaust hood with multi cyclonemechanical filters, and UV filtersin the exhaust plenum behind. The components of a ventilationhood supply and extract systemcan comprise the canopy withintegrated filters andsupply/exhaust plenums; thededicated extract system fromthe canopy, which can beprovided in stainless steel ductwork to overcome issues such asmoisture-laden air and thecorrosive effects this can have;dedicated supply air to thecanopy itself, supplied through alow velocity discharge face; andan extract fan and supply airhandling unit.

Figure 4 highlights the main componentsof an integrated supply and exhausthood with multi cyclone mechanicalfilters, and UV filters in the exhaustplenum behind.

1. Supply plenum

2. Mechanical grease filters.

3. Grease collection.

4. Testing and balancing port.

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food production hoods: air filtration & heat recovery solutions

Chapter 30 of the 1999 ASHRAEHVAC Applications Handbookand HVCA DW172 providesadditional guidance on differentventilation canopy types.

Energy recovery optionsWhere heat recovery is beingconsidered there are differentsolutions available to the HVACengineer. In terms of energyrecovery the most efficientsolution can be a hygroscopicadiabatic thermal wheel (Figure 5).

Performance of a rotatingadiabatic thermal wheel willdepend on the characteristics ofthe system, i.e., whether the inletand outlet flows vary or areequal, whether the processtemperatures are consistent ornot. Efficiencies of 75% for no-hygroscopic wheel and 90% forlithium chloride coveredhygroscopic wheels are possible.

Utilising a piped run-around coilsolution is a proven energyrecovery solution on ventilationsystems and can give sufficientefficiencies (50/60%) to temperincoming air. Due to theparticular conditions of the

supply and exhaust air moreenergy is available thanrequired. For this reasondesigning for efficiency is notnecessarily the most suitableselection criteria.

Extract temperatures can varyfrom between 30/50°C. With theapplication the concern is thatthe exhaust air volume will tendto be greater than the supply,generally around 80/85%following DW172 guidelines.

Generally, a supply airtemperature of 18/20°C willprovide good cooling effect atthe canopy due to theimmediately-surrounding airtemperatures at around 25/30°Cand the radiant heat emittedfrom the equipment.

This means that in summer fullfresh air can be effective toprovide cooling for all but a fewof the warmer design days.Therefore, a decision to providemechanical cooling should becarefully considered with theclient and end-users as it mayonly operate for a few days ofthe total year.

Theoretical ExampleAn example of the profile ofenergy recovery for varying oncoil conditions is outlined asfollows. The following systemcharacteristics are taken from aparticular case study (Table 1):

Utilising the base data aboveand assessing the performanceof a run-around coil (RAC)solution over a winter externaltemperature range of -3˚C to10˚C, the graphical data shownin Figure 6 highlights the leavingair temperature for both supplyand exhaust air streams. Figure 7highlights the kW capacityrecoverable for eachcorresponding condition.

It is evident that from 5°Cexternal temperature upwardsthere is sufficient capacity in theRAC selection to provide the fullheating complement for thesupply air system to theventilation hood. Even thoughthe temperature efficiency ofthe energy recovery system isaround 52%, this is still more thanadequate for the required off-coil conditions.

Figure 5 — Typical heat recoverysolution: Adiabatic thermal wheel

Element ConditionsExtract AC

Supply AC

Heat RecoveryOption

Temp Efficiency

5.5 m3/s @ 30˚C

3.5 m3/s @ 18˚C

2 Pipe Run Around Coil

50 - 60%

Table 1: Example ventilation hood conditions

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food production hoods: air filtration & heat recovery solutions

Even at -3°C supply air enteringtemperature, there is stillapproximately 83% of theheating requirements satisfied bythe energy recovery system. Thisis clear evidence that integratedUV ventilation hoods are

capable of being almost stand-alone systems and relativelyindependent of the energyneeds of the commercialkitchen.

Looking at the off-coil

temperature for the exhauststream, it is also evident thatthere is still available energydischarging to atmosphere,approximately 19/22°C. Aconsideration at the designstage may be to increase thecapacity of the exhaust air coilto recover additional energy tosupply a combination of supplyair coils in different applicationsof the project.

The EconomicsProviding an integratedventilation canopy with heatrecovery and UV filtration canalmost double the capitalinvestment of the specificinstallation, though with thecorrect data and equipmentselections, paybacks of four to sixyears are very achievable. Witha typical canopy installationlifetime of 10 to15 years there areappreciable economic andenvironmental benefits toconsidering heat recovery andUV filtration in commercialcatering facilities.

Paul O’Sullivan is a LeadEngineer with PM Group based inCork. He holds a Diploma inBuilding Services Engineering, anHonours Degree in MechanicalEngineering, and a MastersDegree in Building ServicesEngineering Management. Paulhas seven years industryexperience (in Ireland andFrance) primarily in thehealthcare sector, along withthe commercial and industrialsectors. He is a member ofEngineers Ireland, has lecturedin building services design atCork Institute of Technology.

Figure 6 — The graphical data shown here highlights the leaving air temperature forboth supply and exhaust air streams.

Figure 7 highlights the kW capacity recoverable for each