Design of a feasible Corona wind air purifier to guarantee the capture and termination of hazardous fine bioaerosols for use in surgery and operation rooms in hospitals

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TechnIsche Universiteit Delft

CPDNR3333

Conceputual Product Design

Process / Product System Engineering

Delft ChemTech - Faculty of applied Science

Delft university of Technology

Appendices

Subject:

Design of a feasible Corona wind air purifier to guarantee

the capture and termination of hazardous fine bioaerosols

for use in surgery and operation rooms in hospitals

Author

Student Number

Telephone

Eliane Khoury

1207423

06-28129012

Keywords

Bioaerosols, HVAC (Heating, Ventilation Air Conditioning),

Operation room, Air Purifier, Corona discharge, Electric

(corona) wind, UV irradiation.

Assigment Issued

Report Issued

Review date B.O.D

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Appendixes tor Final Report CPD - 3333

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Appendix 8.3: Data Summary ... 72

Appendix 9: DESIGN APPROACH ... 75

Appendix 10: INTGRATED PRODUCT CONCEPT (I.P.C) ... 77

Appendix 10.1: Electrostatic Forces ... 77

Appendix 10.2: Conversion Factors for Electrostatics ... 79

Appendix 10.3: Electrostatic Precipitators ... 79

Appendix 10.4: Force on Airborne Particles in Still Air ... 79

Appendix 10.5: Force on Airborne Particles in Flowing Air ... 81

Appendix 10.6: Air breakdown ... 82

Appendix 10.7: Electric Mobility Values ... 89

Appendix 10.8: Required UV Dose ... 90

Appendix 10.9: Cylinder Choice ... 92

Appendix 10.10: Cylinder Prices ... 94

Appendix 10.11: Electrodes Choice ... 95

Appendix 10.12: High Voltage Power Supply Choice ... 95

APPENDIX 11: PROCESS FLOW SCHEME (PFS) ... 98

APPENDIX 12: DETAILED DESIGN ... 99

Appendix 12.1: Determining Potential, Electrode Geometry and Wind Velocity ... 99

Appendix 12.2: UV Lamp Design ... 113

Appendix 12.3: UV Intensity Required ... 114

APPENDIX 13: TARGET SPECIFICATIONS (T.S.) ... 115

Appendix 13.1:Threshold and Boundary Conditions ... 115

Appendix 13.2: General Mass Balance Due to Ventilation ... 116

Appendix 13.3: Design Hypothesis ... 120

Appendix 13.4: Mass Balances for Existing Situation ... 122

Appendix 13.6: Mass Balance for Series of System ... 127

APPENDIX 14: COST CALCULATIONS ... 128

Appendix 14.1: Comparison of annual maintenance for air purification systems ... 128

Appendix 14.2: Annual Operating Costs ... 128

Appendix 14.3: Discount cash Flow Return ... 129

Appendix 15: ECONOMIC FEASIBILI1Y ... 130

Appendix 15.1: Ingredients Costs ... 130

Appendix 15.2: Manufacturer Costs ... 131

Appendix 15.3: Income Factors ... 131

15.4: cash Flow for Possible Strategies ... 131

Appendix 16: Waste / Safety ... 134

Appendix 16.1: Risk Models ... 134

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Appendix 1: TYPES OF AIR CLEANING SYSTEMS

Air cleaners are generally classified according to the technology employed to remove various sized particles andjor gases from the air. The general types of technologies available for use in air cleaners include mechanical filters, electron ic air cleaners, and hybrid filters for the capture of particles, and gas phase filters to control odors. Air cleaners, which operate by chemical process, such as ozonation, also exist.

Appendix

1.1:

Mechanical Filters

Mechanical filters may be used in centra I filtration systems as weil as in portable units using a fan to force air through the filter. Mechanical filters capture particles by several physical mechanisms. Larger particles such as lint and fibers impact or "impinge" upon the filtration medium. Smaller particles are strained out of the air stream by increasingly smaller openings in the filter pack. Finally, very small submicron-sized particles are

captured by diffusion toward the surfaces of the filtration medium (independent of airflow) where they are captured by electrostatic interaction between surface charges of particles and the filtration medium. This latter mechanism is the predominant factor in the

effectiveness of the highest efficiency mechanica I filters' removal of submicron-sized particles. Mechanical filters are of three major types:

a. Flat Filters

Flat or panel filters usually contain a low packing density fibreus medium that can be dry or coated with a viscous substance such as oil to increase particIe adhesion. Dry-type filter media may consist of open-cell foams, non-woven textile cloths, paper-like mats of glass or cellulose fibers, wood fill, animal hair or synthetic fibers. They mayalso consist of slit and expanded aluminium. Media filters of various materials are available in a wide range of sizes and thicknesses. The typical, low-efficiency furnace filter in many residential HVAC systems is a flat filter, one-half-inch to one-inch thick, th at is efficient in collecting large particles, but removes a negligible percentage of smaller, respirable-size particles. Figure 4 demonstrates how the effiCiency of the typical furnace filter over the 0.01 - 10 micron diameter size-range compares to other types of air cleaners which are described.

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b. Pleated Filters

One of the most effective ways to increase the particIe collection efficiency of mechanical filters is to increase the filter media density using small denier fibers. This causes smaller media penetrations and increases the screening or straining mesh size. However, any increase in filter density significantly increases resistance to airflow, causing decreased airflow through the filter. The most effective approach to overcoming this problem is to extend the surface area by pleating the filter medium. This lowers the airflow velocity through the filter and decreases overall resistance to airflow such that pressure drop is

reduced. Additionally, pleating of filter media increases the total area available for

filtration and, thus, extends the u sefu I life of the filter. The efficiency of extended-surface (pleated) media filters is much higher than for other dry-type filters. For example, Figure 1.2 demonstrates the efficiency of a pleated paper filter over the 0.3 - 10 microns diameter size-range compared to a typical flat furnace filter.

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Figure 1.2: Fractional Efficiency of Several Air Cleaners Over the 0.3 -- 10 Wm Diameter Size Range [HanleyJ.

c. High-Efficiency Particulate Air Filters

High-efficiency particulate air (HEPA) filters, formerly called high-efficiency particulate arrestors, are a further extension of extended-surface media filters. HEPA filters were originally developed during World War II to prevent discharge of radioactive particles from nuclear reactor facility exhausts. They have since become a vital technology in industrial, medical, and military clean rooms and have grown in popularity for use in portable residential air cleaners.

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microns. While still very good filters when compared to conventional panel type and even extended-media pocket filters, these versions of the original HEPA filter have higher airflow, lower efficiency, and lower cost than their original version. The true HEPA has very high pressure drop performance and both versions require prefiltration for maximum life cycle. Also, HEPA filters are generally not applied to residential HVAC systems due to their size and horsepower requirements. A disadvantage of HEPA filters is that the need for a powerful fan leads to increased energy costs compared to less efficient filtration systems, and replacement filters are generally quite expensive (retail prices range from $50 to $100, depending on size). The major advantages of the original HEPA filters, however, include high efficiency, which aetually increases with use, and a long

maintenance-free life cycle of up to five years wh en used with a prefilter. Nelson, et al. (1988) state that: "Because the designation of a filter as HEPA ensures a high degree of

Figure 1.3: Inside a HEPA Filter

efficiency, it should be sought if a mechanical filter is to be used." Additionally, the 1990 review of indoor air pollutants and environmental controls published by the American Thoracic Society (1990) concludes that: "High-efficiency particulate filters (HEPA) are highly efficient in removing particles of a wide range of size. A room-size unit will control particles in th at room, and a central unit will remove particles from the air of the building when the ventilation system is operating" [62].

Appendix

1.2:

Electronie Air Cleaners

Eleetronic filters are generally marketed as eleetronic air cleaners and formerly referred to as eleetrostatic precipitators; they employ an eleetrical field to trap particles. Similarly to mechanical filters, they may be installed in centra

I

filtration systems as weil as in portable units with fans.

The simplest form of eleetronic air cleaner is the negative ion generator. A variety of negative ion generator-type air cleaners is available. The simplest types use static charges to remove particles from indoor air. They operate by charging the particles in a room, which become attraeted to and deposit on walls, floors, table tops, curtains, occupants, etc., where they may cause soiling problems.

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Electrostatic precipitators are the more common type of electron ic air cleaner. They employa one-stage or a two-stage design for particIe collection. In the less expensive but less effective single-stage design, a charged medium acts to both charge and collect airborne particles. A two-stage design employs a high-voltage electrode or wire, which places a charge on the incoming airborne particles. In the second stage, the charged airborne particles are drawn between a series of oppositely charged metal plates, which attract the charged particles from the air causing them to precipitate onto the metal plates. Collection efficiency is a function of the area of the collecting plates, the flow rate, and the strength of the electrical field. The airflow remains constant with use; while the particIe capture efficiency declines rapidly as the charged collector plates become coated with particles. Cleaning the plates restores the initial efficiency and must be done regularly (at least every few months) to maintain adequate performance.

The advantages of electronic filters are that they generally have low energy costs because of low pressure drop. The airflow through the units remains constant with use, and the precipitating cell is reusabie, avoiding long-term filter replacement costs. The major disadvantages are that:

(1) They become less efficient with use.

(2) Precipitating cells require frequent cleaning.

(3) They can produce ozone, either as a by-product of use or intentionally (4) Those installed into HVAC systems have a relatively high initial cost including

expensive installation because of the size of the unit and it is related wiring cost. Additionally, the charged particles produced by negative ion generators can sometimes soil room walls and furnishings.

Appendix

1.3:

Hybrid Filters

Hybrid filters incorporate two or more of the filter control technologies discussed above. One such approach uses one or more types of mechanical filters combined with an electrostatic precipitator or an ion generator in an integrated system or single self-contained device.

An example of a hybrid filter is the "electret" media filter, which uses permanently

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Figure 1.4: The Effect of Dust Loading on the Fractional Efficiency of a Charged-Fiber Filter [Hanleyl

Ionizing charged-media type filters also exist. In this type of electron ic air cleaner, dust in the air is initially charged and then collected on a charged-media filter. Several versions of this type of filter exist. They operate by charging the particles in the air with negative or alternating negative and positive charges, which enhances their deposition in conventional extended-media high-efficiency filters. Theoretically, the ions flow into the occupied space, causing particles to become charged and are then drawn back to the central air handier where they are collected. The claimed advantages of such systems is th at they enhance the performance of the particulate filters, reduce particulate counts in the occupied space, and reduce the housekeeping costs of particIe soiling in the space. The disadvantage of the technology is that it lacks definitive performance documentation and represents very high initial equipment cost over and above the cost of conventional high-efficiency filters. Another category of hybrid filters, although not vet available commercially, is

electrostatically enhanced filters. In this type of interaction, an electric field is actively superimposed on fibrous, media-based air filters. The principle underlying this technology is electrostatic precipitation superimposed on other capture mechanisms such as

impaction, sedimentation, or diffusion. Under experimental conditions, this technology generally leads to increased filtration efficiency, relative to media-based filters alone, especially under low-f1ow velocity conditions. Experimental data have been obtained for different pollutants such as latex aerosols, dioctylphthalate (DOP) smoke, and two different kinds of laboratory generated dust (Kao, et al., 1987).

Appendix

1.4:

Gas Phase Filters

Compared to particulate control, gas phase pollution control is a relatively new and complex field. Neither mechanical nor electronic filters effectively re move gases and associated odors. Air cleaning units are often equipped with a chemica I filter designed to remove pollutant gases from the air. Two types of gas phase capture and control are physical adsorption and absorption (also called chemisorption). Both are used for

removing certain solvent vapours, odours, and low concentrations of gases and vapours in indoor air. Physical adsorption results from the electrostatic interaction between a

molecule of gas or vapour and a surface. For example, in the adsorption of air, N2 is

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Appendixes for Final Report CPD - 3333 .

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activated alumina, zeolites, porous clay minerais, and molecular sieves are useful as adsorbents due to their large internal surface area, stability, and low cost.

Chemisorption, on the other hand, occurs when the sorbent attracts gas molecules onto the surface of the sorbent. Chemisorption involves electron transfer and is essentially a bond-forming chemical reaction between the adsorbing surface and the adsorbed molecule. Chemica I reaction can occur when the molecules absorb, or go into solution with elements of the substrate or with other reactive reagents, which are manufactured into the sorbate. This enables the sorbent to form chemical bonds with the contaminant molecule, which binds it to the sorbent substrate or converts it into more benign chemica I compounds. For example, one common chemisorbant employs potassium permanganate as an active oxidating reagent impregnated into an alumina or silica substrate. This chemisorbant will convert formaldehyde, for example, into benign water and carbon dioxide, which is desorbed back into the air stream. Other more complex reactions result in compounds that bind to the sorbent substrate. Once bound, the contaminant is chemically altered and cannot escape back into the air stream. Chemisorption is usually slower than physical adsorption because of the complexity of the process. It is also not reversible as the active reagent component is consumed through the chemisorption process.

Activated charcoal is a widely used adsorbent. The activation process etches the surface of the carbon to produce submicroscopic pores and channels where adsorption can occur. These pores provide the high surface area-to-volume ratio necessary for a good sorbent. Another advantage of charcoal is th at it is non-polar, permitting adsorption of organic gases from air with a high moisture content.

There are several disadvantages to the use of activated charcoal. Although relatively small quantities of activated charcoal have been reported to reduce odours in residences, many pollutants affect health at levels below odours thresholds. Activated carbon adsorbs some gaseous indoor air pollutants, especially volatile organic compounds, sulphur dioxide, and ozone, but it does not efficiently adsorb volatiIe, low molecular weight gases such as formaldehyde and ammonia. Because the rate of adsorption (i.e., the efficiency) decreases with the amount of pollutant captured, gaseous pollutant air cleaners are generally rated in terms of the adsorption capacity (i.e., the total amount of the chemical that can be captured). All adsorbents have limited adsorption capacities and thus require frequent maintenance. Another problem with the use of traditional adsorption beds is that there is no means to determine the effective residual capacity of activated carbon while it is in use. Additionally, there is concern that sorbent filters, when saturated, may re-emit trapped pollutants.

Recent developments in the gas phase sorbent filter field have yielded advanced products for use in residential HVAC systems as weil as in portable air cleaners. This technology utilizes smaller, more active sorbent particles of carbon, permanganatejalumina, or zeolite which are incorporated into a fabric matte. The resulting matrix of fibre and active sorbent particles combines particulate filtration and gas phase filtration into one filter. The

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One manufacturer reports substantial sorption capacity increase over a similar weight of larger pelletized charcoal. The whole filter cartridge is disposable to facilitate servicing. Like other gas phase sorbent filters, their useful service life varies according to indoor pollution concentrations. Unlike the more bulky traditional sorbents, they are considerably more economical. Also, because they have particulate arrestance capability comparable to the generic pleated particulate filter, no prefilter is required and the cartridge can be changed based upon static pressure increase.

Most sorbents are manufactured and applied in pellet form. This makes it possible to create gas phase filters which are sorbent mixtures of two or more materiais. Usually, compound mixtures more effectively remove odours and gases than charcoal alone. Additionally, filter manufacturers can include in their products specific adsorbents to target particular odours or gases. The recent "hi-tech" matrix-type sorbent can employ mixtures of sorbent types, which allows more effective removal of a much broader range of pollutants than is possible with a single type of sorbent.

Appendix

1.5:

Ozone Generators

These air cleaners utilize a chemical modification process instead of mechanicalor electronic filters to "clean" the air. Ozone (referred to as trivalent oxygen or saturated oxygen by some manufacturers) has been used in water purification since 1893. When used in water solutions such as cooling towers, ozone generators have demonstrated good control of reactive contaminants without creating negative side effects. Introducing ozone into the air stream can have beneficial effects under controlled conditions where humans are not exposed. For example, high concentrations of ozone are used to retard microbial growth in meat storage, and to control and counteract microbial growth and odors from fire and flood damaged buildings. However, ozone is of concern when considering spa ces for human occupancy. The high concentration levels required for contaminant control are in conflict with potential health effects as established by

authorities including the National Institute of Occupational Safety and Health (NIOSH), the

u.s.

EPA (1995), and the U.S. Food and Drug Administration (FDA). Appliance-sized ozone generating units have typically been marketed in the United States and branded as air cleaners by several manufacturers. These air cleaners are marketed with the claim that ozone re moves air contaminants from indoor air by oxidizing airborne gases, and even particulates, to carbon dioxide and water vapour. Further data is available on:

http://www.sfdph.org/php/tbITBControI4HomelessShelters0127200S.pdf

The operation mechanism, according to ozone generators manufacturers, is as follows: ozone generators convert oxygen molecules into ozone or "activated oxygen" by high voltage electricity. The outflow of the generator is ozone molecules th at physically contact the pollutants, odours, germs and viruses' particles, break them down, disinfect the air and turn back to a stabie molecule of oxygen. [24]

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contaminants. Wh en considering bio-agents, the data indicates th at as long ozone is used at concentrations that do not exceed public health standards, ozone applied to indoor air does not effectively remove viruses, bacteria, mold, or other biologica I pollutants.

Same data suggest th at low levels of ozone may reduce airborne concentrations and inhibit the growth of some biological organisms while ozone is present, but ozone concentrations would have to be 5 - 10 times higher than public health standards allow before the ozone could decontaminate the air sufficiently to prevent survival and regeneration of the organisms once the ozone is removed [26]. Even at high

concentrations, ozone may have no effect on biologica

I

contaminants embedded in porous material such as duct lining or ceiling tiles [27]. In other words, ozone produced by ozone generators may inhibit the growth of some biologica I agents while it is present, but it is unlikely to fully decontaminate the air unless concentrations are high enough to be a health concern if people are present. Even with high levels of ozone, contaminants embedded in porous material may not be affected at all.

However, some ozone generators are manufactured with an "ion generatorHor ''ionizerHin the same unit. An ionizer is a device that disperses negatively (andjor positively) charged ions into the air (similar to the electric precipitation principle). These ions attach to particles in the air giving them a negative (or positive) charge 50 that the particles may attach to nearby surfaces such as walls or furniture, or attach to one another and settle out of the air. In recent experiments, ionizers were found to be less effective in removing particles of dust, tobacco smoke, pollen or fungal spores than either high efficiency particIe filters or electrostatic precipitators.

Figure 1.5: Inside Ozone Generator

The following table summarizes validity and efficiency of available technologies that may principally compete with our product as an air purifier. However, it is important to indicate that all data above concerning existent filtrations

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Appendix 2: OPTIONS AND SELECTION

Appendix

2.1:

Alternatives for functions Combination

Alternatives inc/uding monitoring stage:

-I Monitoring I

-I Collection I

Purified

(1)

Feed

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_(Clean)

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-I Monitoring I

-IEliminatiOn I

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(3)

Monitoring

Collection

Elimination

(4)

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Collection

Monitoring

Elimination

Purified

(Clean)

Air

Purified

(Clean)

Air

Alternatives excluding monitoring stage while concerning direct treatment:

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Appendix

2.2:

Impactor for Partieles Concentration

A conventional impactor operates by accelerating particles through particles though a nozzle. The air stream is directed at an impaction plate a certain distance from the nozzle that deflects the flow. Due to large inertia, the larger particles (Particles larger than a certain threshold size-known as the cutpoint or cut-off) are unable to follow the deflected air and thus impact the plate. Smaller particles follow the fluid streamlines and exit the impactor [17].

A virtual impactor is similar to a conventional impactor but uses a different impaction mechanism. A tube with a larger diameter than the nozzle containing stagnant or slow moving air replaces impaction plate. In this case, the larger particles enter the collection tube due to their greater inertia and the smaller particIe follow the diverted major flow around the collection tube. The air moving though the tube (the micro flow) becomes more concentrated with the large particles than the original flow. The smaller particles are flushed out of the virtual impactor just as in the conventional impactor. By properly

controlling the flows of the impactor, it is possible to adjust the cut-off si ze of the particles collected. Additionally, the porti on of the total airflow that passes through the collection probe represents a smaller percentage of the total flow (10% to 30%), 50 the virtual impactor is also concentrating the particles into the collection probe airflow. [17] The use of impactor can be beneficia I for air sampling, when the aerosols have to be maintained and preserved with no harm for further investigation and sampling.

Note: The term "sampling" is usually taken to mean the collection of a large volume of air and concentrating the particulate matter in either an air or fluid medium 50 as to prepare a "sample" for further investigation and analysis. (19 NATIBO).

By cascading a series of probes, each taking the flow from the preceding probe, particles can be concentrated to many times the original air concentration before collection.

However, the results obtained for particIe size classification by cascade impactors can be distorted because of the particles failing to adhere to the correct stage. This may be caused by the particles bouncing off, or being blown off by the gas stream.

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Addition of a thing disc of substrate (cellulose,

glass fiber, stainless steal, etc.) improves the accuracy of weightings, and a thin layer of grease

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Eguations

tor

Impactors [33, 341:

ParticIe classification according to aerodynamic size utilizes elutriators, cyclones, centrifuges and impactors. Inertial impactors have been widely used for particIe collection, mainly because of their sharp cut-oü' characteristics. Due to the extensive theoretical work their performance has become weil understood and their charaeteristics can be predicted. The most important limitations of these instruments are the following:

1) Particles may bounce from the collection surface upon impaction. 2) Collected particles may re-entrain.

3) Wall losses between the impactor stages may be considerable.

4) Very large particles may break-up upon impaction, especially at high impaction velocities.

The particIe bounce problem has been traditionally encountered by coating the impaction plates with a sticky material. However, for certain analytical techniques, carbon-containing coatings, which are typically used, may interfere with the measurement of carbonic compounds. The virtual impactor provides an alternative solution to the particIe bounce and re-entrainment problems associated with inertial impactors. Similar to inertial impactors, it classifies particles according to aerodynamic size.

The collection efficiency of impaetors is normally measured experimentally while depending on primarily the Stokes number

S,

defined as:

T S=2..

Tt

Where the stopping time of the particles 'rp can be expressed in terms of their mobility Z, mass mp and number of elementary charges n as:

Under the usual conditions when the distance L between the impactor nozzle and the collector, the nozzle diameter

_dn,

and the collectors diameter

de

are such that

_{de >L> dn}

Then a good measure of charaeteristic time 'tf for fluid deceleration is:

d

Tt =_n

U

Where the average jet velocity U is written in terms of the mass flow rate

m

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4co~

and the fluid density p based on stagnation conditions upstream tha '}111rdn Po

impactor nozzle is:

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Where the global Mach number

M,

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Where:

y,

m, and Il are the heat capacity ration, molecule mass and viscosity coefficient of the carrier gas (generally air), respectively.

Po and Co are the stagnation pressure and sonic speed based on the stagnation conditions in the chamber preceding the nozzle. D is diffusivity of the particles.

For a nit density sphere, Fd(dp) is a linear function

of dp independently from the fluid and geometric

parameters which affect only S, Mand Re. [35]

Fiqure 2.2: Typical Impactor Efficiency Curve [35]

Appendix

2.3:

Cye/one for Partie/es Coneentration

Cyclone is the generic name for collection systems where particles are removed from spinning gasses by centrifugal forces. Cyclones have a cylindrical body with a conical base, and are usually elevated 50 that a collection hopper can be placed below the base. In the conventional (reverse flow) cyclone, gases enter the cyclone tangentially. The gases then spi ral down towards the apex of the cone and then are reversed up again through the exit, a centrally placed exhaust pipe in the top of the cyclone. The gas flow is shown below in figure 2.3.

Cyclones are used as particles separators, but by extracting a minor flow of the concentrated particles, they become

... concentrators. Acyclone having a minor flow is referred to as a virtual cyclone.

"" Another type of cyclone flow is demonstrated in a straight-through cyclone concentrator, figure 2.3, this device imparts a swirling motion to the entering gas stream by means of fixed blad es with a specific curve placed at the cyclone entrance. The tangential velocity causes the separation, while the axial

component ensures that the gas will flow through the device and exit at the other end.

Fiqure 2.3: Conventional Cyclones

At the exit, the gas near the outer wall of the tube, which contains a concentrated

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cleaned gas near the centre that now contains the smaller particles continues on through the tube [17, 21- Crawford].

The separation efficiency of the cyclone is calculated according to the following equation, which assumes laminar particIe motion in the radial direction and turbulent flow of the gas. It is assumed that the effect of the turbulent eddies is to distribute the particles uniformly over the cross section at any given angle 8. However, this assumption is not necessarily valid for cyclone flow and may be too conservative since the centrifugal forces effects may serve to damp out the turbulent eddies, which naturally occur in turbulent duct flow [21- Crawford]

100 - High efficiency Where: o 10 20 4·l'm

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Pp Q do.s

8

1 IJ

r2

r1 W Efficiency ParticIe Density (kgjm3)

Inlet Air Flow Rate (m3jsec) ParticIe Cut Size (m) Total turning angle (rad) Viscosity of Air (kgjm sec) Radius of the cyclone (m) Radius of plate Hub (m) Turning Parameter (mjrad)

Figure 2.4: Generalized efficiency curves for these the three Cyclone types

In general, operating costs increase with efficiency (higher efficiency requires higher inflow pressure), and three categories of cyclones are available: high efficiency,

conventional, and high throughput. Generalized efficiency curves for these three types of cyclones are presented in the side figure [50].

Note: Efficiency versus size curves present broad

100 generalizations, not exact relationships.

5 .... · .... · .... lr · ·+··· .. ···;·· .. ··· .. · .... ··· .. ···1

0.5 1.0 2.0 3.0 4.0 ~.O

p _anicle-SIZ _eratio. _"<\.

The ParticIe collection efficiency versus particIe size ratio for standard conventional cyclones are indicated in the

following figure, which indicate relatively low efficiency at small particIe size.

While the efficiency for conventional cyclones is calculated based on:

d

=

9flW

[

_J

II2

pc 27r N.

V;

(p_p- Pg) Figure 2.5: ParticIe colleetion efficiency versus

(17)

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Where dpc

=

diameter of particIe collected with 50% efficiency.

1

'1i

=

collection efficiency of particles in thejth si ze range (0

< 1lJ<

1)

dpj

=

characteristic diameter of the jth particIe size range (in \Jm).

The overall efficiency of the cyclone is a weighted average of the collection efficiencies for the various size ranges, namely

LT/m

1]

=

J J

11

=

overall collection efficiency (0

<

11

<

1)

mj

=

mass of particles in the jth size range

M

=

total mass of particles.

M

Appendix

2.4:

Electrostatic Precipitator (ESP)

Electronic-precipitator air cleaners impart an electrical charge to particles flowing through them, and then collect the particles on oppositely charged metal plates or filters. These more elaborate systems must be fitted into ductwork and wired into the house's current. Most have a collector-plate assembly th at must be removed and washed every one to two months. Most available indoors precipitator are designed to work in two stages is

presented in the following figure:

Charged palticle

-r

~

• •

+é~

1

particle-laden air + , o ..0 . :

~

A

S +

+

.t

~

.

.+ + I!> + + -e ~ e ~

Corona plasma

GO:.

e +

/i"

-e + ·

Positive ion ~"%il

=

"

e e I I + Cl 0

Charging section Collection section Figure 2.6: Two-stage indoor electrostatic precipitator

Clean air

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cleaned periodically. Also, these devices are often inserted into airstreams without regard to residence time or air velocities, and hence efficiencies can be much lower than those used in industrial applications. A well-designed electronic air cleaner for home or office building applications would not only be relatively large and have a high-energy demand, but it would also generate ozone at potentially hazardous levels [29, 30].

In other words, ESP removes particulate matter from a gas stream by creating a high voltage drop between electrodes. A gas stream carrying particles flows into the ESP and between sets of large plate electrodes; gas molecules are ionized, the resulting ions stick to the particles, and the particles acquire a charge. The charged particles are attracted to and collected on the oppositely charged plates while the cleaned gas flows through the device. While the gas flows between the plates at velocities in the range of 1 to 3 meters per second, the particles move tO\'Jards the plates at a velocity (ca lied the drift velocity) that is in the range of 1 to 10 meters per minute. During the operation of the device, the plates are rapped periodically to knock off the layer of dust that builds up. The dust is collected dry and can be disposed of or recycled in some industrial applications.

ESPs are large and expensive to buy, but have the important advantage that they collect particles with very high efficiencies. Another major advantage is that they present very little resistance to gas flow therefore cause only a slight pressure drop even when operating on flows as large as a million cubic feet per minute. Therefore their operating costs are not as large as one might expect. Many coal-fired power plants use ESPs [23].

Appendix

2.5:

Corona Discharge

In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor, which occurs when the potential gradient exceeds a certain value, in situations where sparking is not favoured.

It

is a commonly used method for controlled surface charging in a defined way. This provides ions of single polarity determined by the potential applied, which charge the surface to a limit that can be controlled.

Sigmond and Goldman [27] define corona discharge as a "self-sustained electrical gas discharge where the Laplacian (geometry determined) electric field confines the primary ionization processes to regions close to high-field electrodes or insulators".

Corona discharge involves two asymmetric electrodes, one highly curved (such as the tip of a needie, or a narrow wire) and one of low curvature (such as a flat plate, or a cylinder, is electrically grounded [28].

The high curvature ensures a high potential gradient around one electrode, for the generation of a plasma.

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Appen Ixes lor Fmal Report CPD - 3333

_ _ _ _ _ d_·_& _ _ · _ _ _ _ _ _ _ _ _ _ ~.SE·

..

accelerated, 50 that by the time it collides with an atom it has sufficient energy to detach an electron, leaving a positive ion and an additional electron, which is accelerated

producing its own electron / ion pair and the result is an avalanche of electrons formed close to the point [Cross].

Typical corona geometries are a point and a plane as iIIustrated (figure 2.7), or a cylinder with a wire at its axis (Figure 2.8). While light is emitted during the ionization process and, in air, a bluish glow can be seen in darkness around the harp electrode, which gives the discharge the name "corona".

Furthermore, corona discharges are categorised according to the applied voltage frequency into: Direct Current (DC) corona, Alternating Current (AC) corona, High Frequency (HF) corona, and the combinations of the above types.

DC coronas are divided according to the polarity of the high-field electrode(s), into

negative -, positive -, and bipolar corona. The feedback processes in positive and negative coronas must be very different. In the case of negative corona, the cathode borders directly to the ionization region. Therefore, the cathode y-processes (Yic, Yinc, Ypc) are fast and efficient. In the case of positive corona, the cathode is separated from the ionization reg ion by the drift reg ion, in which the photons can be absorbed and secondary electrons originating from the cathode can be attached to atoms and molecules. Thus in positive coronas, processes at cathode play the minor role and photoionization as a y-process (Ypg)

often predominates [27].

DC Coronas may be

positive,

or

negative.

This is determined by the polarity of the voltage applied on the highly- curved electrode. If the curved electrode is positive with respect to

Figure 2.7: Corona Discharge

the flat electrode it's called a

positive corona,

if

negative, it's called a

negative corona.

The physics and behaviour of positive and negative coronas are strikingly different. This asymmetry is a result of the great

difference in mass between electrons and positively charged ions, when the electron mass is 9.10938188 x

10'31 kilograms while the positive ion mass depend on

the ions themselves. Therefore, only the electron having the ability to undergo a significant degree of ionizing inelastic collision at common temperatures and pressures will be involved.

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few electrode diameters away from the discharge electrode, the electric field is not strong enough to sustain ionization. Unipolar ions of the same polarity as the discharge electrode drift into this reg ion and fill the interelectrode space.

Corona plasma region + +

Unipolar ion region

High voltage

discharge electrode

Grounded electrode

Ionization boundal)'

Figure 2.8: Sketch of a positive

De

corona discharge in a wire-cylinder electrode geometrv. eThe sketch is not to scale: the corona plasma region occupies only a verv smal! portion of the interelectrode spacing.) The dashed line indicates the outer boundarv of the ionization reg ion

Appendix

2.6:

Electron Avalanches

Both positive and negative coronas rely on a process known as the electron avalanche. A corona begins with a rare natural 'background' ionisation event of a neutra I air molecule, perhaps as the result of photo-excitation or background radiation. This

ionisation creates a positive ion, and a free electron. If this event occurs in an area with a high potential gradient, the positive ion wil I be strongly attracted toward, or repelled away from, the curved electrode (depending on the polarity of the corona), whereas the

electron will be attracted in the opposite direction. This will, occasionally, prevent the recombination of electron and positive ion.

These high-energy electrons, accelerated by the field, (whichever their direction of travel) often collide with neutral atoms inelastically, potentially ionizing those atoms. In a chain-reaction or "electron avalanche" those additional electrons are also separated from their positive ions by the strong potential gradient, causing a large cloud of electrons and positive ions to be momentarily generated by just a single initial event.

A number of mechanisms can sustain this process, creating avalanche after avalanche, to create a constant corona current. A secondary source of corona electrons is required as the electrons are always accelerated by the field in one direction, meaning th at

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Appendix

2.7:

Corona Wind Theory

The electric charge on a conductor rests entirely on the outside surface. Moreover, electric

t; harge gathers

nea.ra $harp point Olla

Sou<tuctQr

charge tends to concentrate more heavily on sharp points and edges than on broad surfaces. Therefore, the electric field near a sharp point on a conductor is stronger than near a broad surface. Sometimes this field can be

extremely strong that it ionizes the air around the tip. The tip then attracts and neutralizes ions th at have an opposite charge as that of the conductor. In addition, the tip repels like-charged ions. This repulsion of ions creates an electric "wind" that emanates from the tip.

Figure: 2.9: Accumulation of charges at sharo edges of a conductor

In corona discharge, the ions originated in the localized high-field reg ion in the immediate vicinity of the point, and, under the action of the field, gives rise to the corona wind, which diverges from the point. Therefore, the velocity profile of the wind in the plane perpendicular to the electrode axis will be bell shaped and symmetrical. Hence, most technologies and techniques available to investigate and analyse the wind velocity of the formed non-uniform field, restrict measurements to a localized region where spatial variation of wind velocity can be assumed negligible.

For both positive and negative discharges the majority of the interelectrode spacing contains slow-moving ions of a single polarity (that of that sharp electrode). These make

Negative ions ncutralized at Ihe tip

Sharp, charged mctal tip

Po

s

itive

IOll

w

l

nQ

frequent collisions with neutral molecules, which are ions of the same size to the ions. This results in transfer of the momentum from the ions to the molecules and there is a bulk move ment of the gas in the direction of the electric field, which is known as the ion wind or electric wind or even corona wind.

Figure 2.10: Ionic wind in case of Positive

Corona discharge (Positive High Potential Aoplied)

This is generated by the ions and powered by the energy these gain from the electric field alone; in other words, it is called ionic wind, not because of movement of the ions, but because it is generated by energy transferred by ions. Typically, ionic wind has an initial speed of the order of 1 m/sec. [Cross; "The

Electric wind", P.H.W. Vercoulen, Tudelft, 1990]

Appendix

2.8:

Ultraviolet (UV) germicidal irradiation (UVGI):

Germicidal lam ps, or UVGI, are used to: disinfect air and water; cure inks and coatings, disinfect foods and destroy pollutants in water and air through UV-based "advanced oxidation". UVGI represents the shortest wavelength portion of the ultraviolet (UV-C). Germicidallamps are available in ozone and non-ozone producing types.

(22)

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Appendixes for Final Report CPD - 3333

"'El:)

mobile room air cleaning units and in special fixtures mounted toward the ceiling in rooms. The most common applications are in hospita I j health care facilities, food processing plants, shelters, prisons, and other commercial uses where it is important to eliminate biological contaminants.

The process is known as simpie, reliable, economical and is employed either as a stand-alone solution or in combination with other methods such as filters, ozone and chemicais.

[44, 45]

The target wavelength for most ozone-free air purification applications is

254

nanometres, while for water purification, ozone-producing wavelengths are preferred and the target wavelength is

180

nanometres (which generates ozone also). i

~~aI

The disinfection process and treatment occurs when UV initiates a photochemical reaction, which

effectively damages the DNA (deoxyribonucleic acid) molecule to such an extent that cell division (breaks the C=C bond), and thus multiplication, can no longer occur due to cellular death.

Through research, biologists determined the amount of UV required to destroy different kinds of microorganisms. This amount of UV is referred to as the "dosage", which is determined based on the intensity of the UV (expressed in microwatts) that is delivered for a given period of time (seconds), over a given area (square centimetres).

Power X Time X Area or microwatts-secjcm2 (IlW-secjcm2)

From here it is obvious th at the flow rate should also be adjusted to the power of the UV lamps to guarantee appropriate and sufficient exposure time and verse versa.

The relative effectiveness of UV light wavelengths for the process is known as the

germicidal action spectrum, and it is dependent on the type of microorganisms required to eliminate, which vary in their structure and hence the sensitivity to UV-C irradiation.

[20

-23].

As indicated in the spectrum above, there is a certain range for the UV-C irradiation, however, not all the UV-C are the same and therefore, the right lamp should be chosen. The following figure present and example in the difference between possible UV-C lamps, based on data obtained from:

http://www.negativeiongenerators.com/UV-C spectrum.html

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TyDes of UV lamps [951:

Conventional UV light sources are termed "mercury vapour lamps". These are actually "arc tubes" because they do not have a filament as with household incandescent lamps. The UV lamp is a sealed glass tube that has an electrode at each end. The tube is made of quartz and contains small doses of liquid mercury. The tube is then filled with an inert gas (typically argon) and sealed. When an electric current is applied, an arc is struck between the two electrodes vaporizing the mercury and exciting the electrons in the Hg atoms. As the electrons change orbital states, photon energy is released at specific wavelengths in the electromagnetic spectrum, particularly in the germicidal range.

There are primarily five types of UV wave sources: low pressure, low pressurejhigh

output, low pressurejamalgam, medium pressure, and high pressure arc tubes. "Pressure" actually refers to the internal lamp pressure (Iow pressure is below atmospheric pressure, medium pressure is near atmospheric, and high pressure is above atmospheric pressure). Low pressure, medium pressure, and high pressure each have a different output

spectrum. A low pressure lamp emits two wavelengths at 253.7 nm and 184.9 nm (although these are termed "monochromatic due to the fact th at there is only one wavelength in the germicidal range between 200-310 nm band). The design is similar to common flourescent lamps (without the exterior phosphor coating to filter out UV light). Input power ranges from 8 to 300 watts. The medium and high pressure lamps emit a range of wavelengths from 170 nm to 400 nm in the UV range, but also emit visible and infrared rays. Input power ranges from 250 watts to 30 kilowatts.

Each lamp type has particular advantages relative to UV disinfection or photochemical oxidation. Low pressure lamps are easy to power (by standard or electromagnetic ballasts), have a high germicidal efficiency of 30-40% (germicidal UV output compared to input power), and produce effective UV-C light for 6000 to 10,000 operating hours. Medium pressure lamps produce a much higher germicidal output from a single emission source. Their germicidal efficiency is between 10-20% (the rest is emitted in the visib/e and infrared regions and a/sa be/ow 200 nm). Medium pressure lamps are referred to as

"polychromatic" or "multiwave" lamps. Standard medium pressure lamps have a useful life between 1000 - 4000 operating hours and high efficiency lamp manufacturers can offer lamp life greater than 8000 hours. They are more efficient for many photochemical applications due to the diffuse emission spectrum. They are powered by constant wattage transformers to step-up the power required to operate the lamp. Their efficiency output is not affected by hot or cold water or air temperatures as with low-pressure lamps.

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Which type of lamp andlor system is best suited for my aPO/ication?

• The low pressure and medium pressure spectrums can be compared to the germicidal action curve to produce a UV intensity lamp output. It should be noted that 265 nm is the peak germicidal wavelength and efficiency tapers off on either side to extinction at 200 nm and 310 nm. The low-pressure lamp output at 254 nm is high on the efficiency level in the curve, but the intensity output per lamp is low. • The medium pressure lamp produces a number of peak outputs within the

germicidal band curve. Each wavelength energy and efficiency level can be extrapolated (where 265 nm is 100% and 200 nm is 0% for demonstration

purposes) to obtain a germicidal UV intensity output. As mentioned previously, the multiple wavelengths in the medium pressure band have an added affect by destroying enzymes, proteins, and also damage the cell wall. This provides a high degree of lethality and also protects against a phenomenon known as

"photoreactivation" (or light repair) and enzymatic dark repair where damaged DNA is repaired within the cello Low pressure lamps do not offer this protection. Low pressure lamp systems are best suited for 10w-f1ow processes, higher water/air qualities, and installations where operator expertise is low. A standard 65 watt low pressure lamp can treat up to 40 gallons per minute (gpm) of good quality water (UV transmission greater than 95%). As f10ws increase, or higher UV doses are required, more lamps are needed and contact time must be extended to achieve proper performance. As the number of lamps increase, 50 does the size and complexity of the equipment.

Maintenance and operating costs will also increase.

Medium pressure systems are best suited for high water/air f1ows, where higher UV doses are required, and can be used in a wide range of flow qualities. Medium pressure lamps systems are also more effective for photochemical processes such as dechlorination of water, deozonation, or pollution contaminant removal due to the multiwavelengths produced and high UV doses required. Again, they also offer protection from

photoreactivation and dark repair. They can also be used for high volume air systems (e.g. large HVAC systems) where there must be sufficient UV dose to destroy airborne microorganisms. A single, 5 kW medium pressure lamp can disinfect up to 2300 gpm of high purity water replacing as many as seventy-five low pressure 65 watt lamps and 20 low pressure/highout or amalgam lamps. Power costs are typically higher for medium pressure lamp systems (by 10% to 30% depending on reactor design and the efficiency of the lamps employed), but maintenance, lamp replacement, cleaning, and installation costs are significantly reduced [Aquionics: http://www.aguionics.com/uv.php]

A summary comparison between low and medium pressure UV lamps construction:

Low Pressure UV :Typical Low-Pressure (LP) lamps operate between 120V and 240V,

similar to standard fluorescent bulbs. These lamp types obtain power outputs ranging from 40 to over 100 Watts with current draws of less than 500mA. These low-pressure UV

(25)

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states with subsequent emission of two distinct wavelengths within the UV spectrum, 185nm and 254nm.

Medium Pressure UV :Medium Pressure (MP) lamps have higher (nearly atmospheric) operating pressures between 102 and 104 Torr with surface temperatures up to 1500°F. Under these conditions the mercury completely vaporizes creating a plasma with

temperatures that can reach 10,000oF. In this hot plasma, mercury atoms are excited to multiple high orbital levels, which, upon collapse, produce the characteristic broad spectra

I

emission. Many of the performance features of MP lamps are derived from these fundamental differences in operation. Medium pressure lamps are stabie under all temperature conditions. In addition, the broad spectra

I

output results in a diverse range of applications not possible with low pressure.

Appendix

2.9:

Choice of Optional Functions

1. Air Conditioning (HAVC) ~ 1. Whole Hospital

2. One Section

3.

One Room

2. Existent Filtration ~ 1. Yes

2. No

3.

New Purification ~ 1. Monitoring

2. Collection

(26)

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-ru

Conditioning Existent

Filtration New Purification

~

o

Table 2.1: Possible alternatives and version of the product based on relation with existent ventilation system (HAVe) and filtration systems.

From these results it is obvious th at the first priority is for a product that can performed all three suggested functions: monitoring, collection and elimination, disregarding the type of existent ventilation or filters.

(27)

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transferring microorganisms between rooms and patients. Therefore, concerning HAVC, priorities are as follows:

1. Per Room 2. Per Section 3. Whole Hospital

When considering existent flirtation, it is obvious that the rating is always higher once first filtration exists comparing with the lack of it. First stage filtration (existent systems) gives safer feeling and has an acceptable and trusted capacity.

In spite of the results, mainly concerning the type 0 HAVC, the focus will be drawn towards the other two elements, 1 st filtration and 2nd our product. That is due to the differences in installed ventilations between hospitais, based on architectural and economie considerations, etc.

And since we aim to be open to the wide growing market of hospita Is without further limitations, the selected version of our product will consider the existent of the HEPA or UV radiation systems as existent filtration and our product containing all three suggested functions.

Appendix 2.10: Impactor vs. Cyclone

The virtual cyclone offers several advantages. Because centrifugal force can easily be made much higher than the force of gravity, these devices can operate at a higher sampling flow rate, separate smaller particles sizes than the virtual impactor can, and require much less space to handle the same gas volumes [35]. Cyclones are simple to construct, the conventional types have no moving parts, they can be made from a variety of materials (including those with refractory and corrosion resistant properties for special applications), maintenance and costs can be reduced to a minimum, and they can be easily adapted to a wide range of operating conditions such as temperature, pressure and flow rate [36]. Because cyclones use non-impact particIe separation, contamination of solid surface is minimized and particIe properties are not modified by impact. Additionally, the pressure drop is expected to be smalI, since the main flow is turned gradually rather than abruptly, and does not experience prolonged swirling [37].

Cyclones are practical for particIe size of 0.1-15 IJm aerodynamic diameter; and some aerosol centrifuges are high-resolution instruments that can separate particles differing in aerodynamic diameter by only a few percent [35]. With this specific particIe concentration potential (Iow concentration or bioaerosols), virtual cyclones can concentrate

(bio )aerosols, which are usually found in an appropriate size range, and at low

concentration in the air [38]. The use of other samplers and filters can lead to desiccation and shear forces that reduce the efficacy (potency or survival) of the bioaerosols.

(28)

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Appendix

2.11:

Efficiency Curves

An example and indication for fibre filtration efficiency at different velocities [35].

Fiqure 2.12. Filter Efficiency versus ParticIe diameter for face velocities of 0.1 cm

Is

and

lOcmis.

50 ... 'J"Io 10 1 - - -- r - - l - - - I -- - ---j 5

--_

... _-

1-

.-~.l"-,-,-,-,-~

0.5 1.0 2.0 3.04.0 5.0 Partlcle-size ralio ~ q,.,

(29)

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Appendix 3: STRATEGY

Appendix

3.1:

Template for "Porter

5 forces" Model

Supplier Power Similar to Buyer Power issues Threat of Entry • scale economies; absolute cost advantages

• capital requirements

• product differentiation • distribution channels

• govemment and legal barriers

• retaliation by established producers

Internal Rivalry

• Economies of scale

• Industry growth

• Distribution of market shares

• Fixed costs and storage costs

• Differentiation

• Switching costs

• Competitor diversity • Barriers 10 exit

Substitutes

• buyer propensity to substitute

• relative price performance of substitutes

Porter's 5·forces

BuyerPower

Price sensitivity

• cost of product relati ve to total cost

• product differentiation • competition between buyers

Bargaining power

• size and concentration of buyers

relative to suppliers

• buyers' switching costs

• buyers' information

• buyers' ability to backward

integrate

Appendix

3.2:

Template for "SWOT" Framework

Environmental Scan

/

~

Internal Analysis

External Analysis

/~

Strength:

Weaknesses:

firm's resources and

capabilities that can be used as a basis for

developing competitive

advantage.

The absence of certain

strengths may be viewed as weakness.

/~

Opportunities:

New opportunities for

profit and growth.

Design of a feasible Corona wind air purifier to guarantee the capture and termination of hazardous fine bioaerosols for use in surgery and operation rooms in hospitals

~

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Delft

CPDNR3333

Conceputual Product Design

Process / Product System Engineering

Delft ChemTech - Faculty of applied Science

Delft university of Technology

Appendices

Subject:

Design of a feasible Corona wind air purifier to guarantee

the capture and termination of hazardous fine bioaerosols

for use in surgery and operation rooms in hospitals

Author

Student Number

Telephone

Eliane Khoury

1207423

06-28129012

Keywords

Bioaerosols, HVAC (Heating, Ventilation Air Conditioning),

Operation room, Air Purifier, Corona discharge, Electric

(corona) wind, UV irradiation.

Assigment Issued

Report Issued

Review date B.O.D

P~S

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Table of Contents

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Appendix 1: TYPES OF AIR CLEANING SYSTEMS

Appendix

1.1:

Mechanical Filters

':

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Appendix

1.2:

Electronie Air Cleaners

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_(Clean)

_Feed

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