Performing Principles

PERFORMING PRINCIPLES Different to the majority of mechanical filters that simply capture la rge airborne particles, ZEPPELIN electronic purifiers attract and retain pollutants like strong magnets. Millions of dust, smoke and pollen particles travel through the air towards the purifier. The electrostatic precipitator filters are based on the fact that electrically charged particles that are subject to an electric field are attracted towards the electrodes that create such field and deposited upon them.

ELECTROSTATIC FILTER CONTAMINATED AIR

PURIFICATED AIR

PERFORMING SCHEME The separation of the particles that are suspended in the air requires three fundamental stages: 1) The ionization of the particles that are suspended in air is achieved by making them pass through a system of electrodes constructed with strings of tungsten that is 0.2 mm of Ø and aluminium plates.

2) Capture of particles: once the charged particles pass through an electrostatic field, created by aluminium plates that are connected alternately to different potentials, they move towards the electrodes where they are deposited. In the case of liquid drops, coalescence is produced and the liquid film slides through the length of the aluminium plates. 3) Elimination of the material deposited in the electrodes by cleansing them. The efficie ncy of this type of filter is related to the passing velocity of the contaminated air. If velocity of contaminated air in the first stage is high, the ionization process becomes difficult and some particles may not be ionized. In the second stage, that velocity makes it so that the inertia of the ion cannot be counterbuffed by the force of the electrostatic field and the ion will not be retained in the collection plates. The migration velocity, that is to say, the velocity with which the ion is directed toward the collection plates depends on the force of the electric field. Because of this, it is important to work at the maximum voltage of the electrode. Although, there is a moment in which the voltage should not be increased because frequent discharges between the plates can be produced making the precipitator non-efficient. Taking in consideration that the velocity of the migration increases with the size of the particles, some could think that the efficiency would be reduced for small particles, but in practice this reduction is not remarkable. Other variables that affect the process of electrostatic precipitation are the viscosity of the particles that may produce a decrease of efficiency. The efficiency increases when the air temperature decreases.

Page 1

BASE DESIGN For a correct dimensioning of our equipment, we start from the following formulas of electrostatic precipitation, Force that acts upon a particle into an electric field:

F = q ⋅Eρ Where: F = force. q = charge upon particle. Eρ = force of electric field. If ‘q’ is the limit charge, then

q = ρ ⋅Ec ⋅a2 F = ρ ⋅ Ec ⋅ E ρ ⋅ a 2 Where: a = radius of the particle. Ec = force of crown field. ρ = constant, varies with the particle properties and it is 3 for the conductors and from 1,5 to 3,0 for particles with an average dielectric constant.

Motion of a particle in a resistant medium: Stokes Law. F = 6π ⋅ d ⋅ µ ⋅ ω Where: µ = gas viscosity. ω = velocity of the particle where its electrostatic precipitation is its migration velocity. Therefore,

ω=

ρ ⋅ Ec ⋅ Eρ ⋅ a 6⋅π⋅µ

In our precipitators, the values of Ec and Eρ are practically the same. Efficiency of the dust collection of an electrostatic precipitator: It is defined by the amount of dust (kg) eliminated by the purifier divided by the amount of dust (kg) that enters the equipment expressed in a percentage. The following expression is used:

ε =1−

1 eλ

Where: ε = collection efficiency. e = constant exponential λ = depend on various factors related to the design of the precipitator and the physical properties of the dust.

Page 2

Deutsch Law

ε =1−

1 e

A⋅S / Q

Where: A = area of the receptor electrodes. Q = flow of gas. S = effective migration velocity. The following draw represents the average efficiency of the precipitator according to tests that were conducted with similar equipment. ASHRAE EFFICIENCY 100 % 95

Working point

90 85 80 75 70 0,6 1,0 1,2

1,6 2,0

2,3 2,6 3,0

3,3

v (m/s) ∆p (mm.wc.)

10

12

14

16

18

20

22

24

26

28

30

Where: v = velocity of air flow (m/s). ∆p = pressure drop (mm.wc.). mm.wc = millimetres of water column (1 mm.wc. ≈ 10 Pa).

FILTERS FOR AIR CONDITIONING INSTALLATIONS Mechanical filtration system is essential in any air treatment system, but very fine particles are not retained. Particles smaller than 5 microns (µm) constitute a problem for our health. Once they settle in our lungs, it is impossible to get rid of them. In fact these are the most contaminating particles because they are the majority of particles and can remain floating in air for a long period of time. To eliminate these particles, the equipment that works with electrostatic precipitation, offers the highest efficiency. Also, by having a high tension stage, it destroys all micro-organisms. By being an open filtration medium, it has a small drop of pressure for which it can be intercalated in existing air conditioning installations without any problem. The elimination of these particles by means of conventional extracting equipment requires a large increase of the dimensioning in the installation of air as well as the high cost of performing. As a consequence, we can assure a fast amortization of it.

Page 3

In this figure, we have a scheme of an installation for air treatment with two electrostatic filters. One of them purifies the air that is sent out to the exterior and the other one purifies the air that is introduced into the interior.

Optional outside air outlet

Recirculation air

Extraction and return fan

Return air from air-conditioned area

ELECTROSTATIC FILTERS Air driven to air-conditioned area

Optional outside air inlet

Air-conditioner

The use of this type of equipment is recommended for the following pollutants as well as the applications shown below:

POLLUTANTS -

Viruses. Bacteria. Pollen. Plant spores. Tobacco smoke. Soldering smoke. Atmospheric dust. Escape gasses. Soot. Oil gasses. Industrial processing smoke. Cement industry dust.

APPLICATIONS Among others: - Bars. - Dancing clubs. - Offices. - Day care centres. - Schools. - Meeting rooms. - Laboratories. - Operation rooms. - Old age homes. - Computer rooms. - Hospitals. - Pharmacies. - Workshops. - Sick building syndrome. - Etc.

Page 4

Its usage in hospitals is considered of vital importance. It should be used as a pre-filter to the absolute filters to extend their duration. These absolute filters, as we know, are not able to regenerate. Used as a post-filter to the absolute filters, we can guarantee the destruction of all micro-organisms. The absolute filters, as the name indicates, are capable of guaranteeing an absolute retention of particles and bacteria but not their destruction. This can be reached by electrostatic filters.

ASHRAE EFFICIENCY 100 % 95 % 90 % 85 % 80 % 75 % 40

70 %

∆p (mmwc)

30 20 10

TV 212/50 MODEL 0,64

MODEL

DIMENSIONS L x H x W (mm)

1,28

1,92

FILTRATING SURFACE (mm2 )

2,56

3,20

v (m/s)

FLOW (m3 /h)

TV 120/36

520 x 410 x 190

470 x 360

1900

TV 214/36

700 x 410 x 190

680 x 360

2700

TV 220/36

985 x 410 x 190

970 x 360

3800

TV 420/36

985 x 820 x 190

970 x 770

7600

TV 120/50

520 x 525 x 190

470 x 480

2500

TV 212/50

620 x 525 x 190

600 x 480

3250

TV 220/50

985 x 525 x 190

970 x 480

5000

TV 420/50

985 x 1050 x 190

970 x 960

10000

TV 620/50

985 x 1575 x 190

970 x 1.340

15000

TV 820/50

1.970 x 1050 x 190

1920 x 960

20000

TV 1220/50

1.970 x 1575 x 190

1920 x 1.340

30000

TV 1620/50

1.970 x 2100 x 190

1920 x 1.900

40000

NOTE: this equipment is fabricated with white metal sheet 1 mm thick, treated with epoxy -polyester with RAL 5000 colour textured. The inspection and cell extraction can be done without the use of tools. Page 5

SEMI-INDUSTRIAL ELECTROSTATIC FILTERS These filters are created to work with air not very contaminated, for instance, the air that can be evacuated from the interior of a hotel business establishment.

“FE MODEL”

FLOW (m3 /h)

POWER (W)

DIMENSIONS L x H x W (mm)

FE-2500

2500

563

1055 x 545 x 500

96 %

FE-5000

5000

1470

1145 x 545 x 990

96 %

MODEL

EFFICIENCY

Page 6

INDUSTRIAL ELECTROSTATIC FILTERS In every industrial process, for instance, cooking of food in bars and restaurants, it is generated a series of sub-products that have been proven to be harmful to our health and well-being, besides they deteriorate the atmosphere as well. These filters are created to palliate these effects. We have created two versions of these filters to be used depending on the amount of contamination. The difference between them lies in the width of the collecting plate. The normal version is a plate that has a width of 200 mm. and it is named with initials AV. The special version is a plate that has a width of 250 mm. and it is known with the initials AVL. The dimensions of the filtration units, as well as the capacity of the power equipment, let us to offer these two versions with double ionizing cells. The standard presentation of this equipment is done with RAL-9002 textured paint. It is constructed with a metal sheet 1.5 mm. thick.

“AV“ SERIES

100 %

ASHRAE EFFICIENCY

95 90 85 80 75 70

AV 5000 MODEL

v (m/s)

0,7 1,0 1,2 1,7 2,0 2,3 2,7 3,0 3,3

∆p (mm wc) 10

12

14

16

18

20

22

24

26

28

30

Page 7

To facilitate the comprehension of the following table, there is a drawing that represents one of our models further down.

FLOW (m3 /h)

MODEL AV – AV – AV – AV – AV – AV – AV – AV – AV –

750 1250 2500 5000 10000 15000 20000 25000 30000

6

1

POWER (W) 15 20 25 30 60 90 120 150 180

750 1250 2500 5000 10000 15000 20000 25000 30000

LAMPARA CONTROL FILTRO

DIMENSIONS L x H x W (mm) 450 x 420 x 400 450 x 420 x 500 500 x 548 x 500 500 x 548 x 990 500 x 1096 x 990 500 x 1644 x 990 500 x 2192 x 990 500 x 2740 x 990 500 x 3288 x 990

WEIGHT (kg) 15 20 35 65 130 195 260 325 390

LAMPARA CONTROL FILTRO

2 5

VENTILADOR

3

FILTRO

VENTILADOR

FILTRO

4 7

FRONT VIEW

TOP VIEW

1 2 mesh filters of 490 x 490 x 50 mm. 2 2 fibre filters of 490 x 490 x 48 mm. 3 2 ionizing cells of 475 x 490 x 70 mm. 4 2 collecting cells of 475 x 490 x 225 mm. 5 2 active carbon filters of 490 x 490 x 25 mm. 6 Electric panel. 7

Door electric switch.

Page 8

“AVL“ SERIES

100 % 95

ASHRAE EFFICIENCY

90 85

6

80

1

75

LAMPARACONTROLFILTRO

2

70

FILT RO

5

3

4

7 0,7

10

6

12

1,2

14

16

2,0

18

20

2,7

22

24

3,3

26

28

v (m/s) ∆p (mm.wc) 30

1 4 mesh filters of 490 x 490 x 25 mm. 2 4 fibre filters of 490 x 490 x 48 mm. 3 4 ionizing cells of 475 x 490 x 60 mm.

1

LAMPARACONTROLFILTRO

4 4 collecting cells of 475 x 490 x 275 mm. 5 4 active carbon filters of 490 x 490 x 25 mm.

2 FILT RO

5

3

6 Electric panel. 7

4

Door electric switch.

7

FRONT VIEW

FLOW (m3 /h)

POWER (W)

DIMENSIONS L x H x W (mm)

WEIGHT (kg)

AVL – 2500

2500

20

500 x 548 x 500

37

AVL – 5000

5000

30

500 x 548 x 990

69

AVL – 10000

10000

60

500 x 1096 x 990

138

AVL – 15000

15000

90

500 x 1644 x 990

207

AVL – 20000

20000

120

500 x 2192 x 990

276

AVL – 25000

25000

150

500 x 2740 x 990

345

AVL – 30000

30000

180

500 x 3288 x 990

414

MODEL

Page 9

“FAV” SERIES FAV series are electrostatic filters with a centrifugal fan incorporated. Every unit has a micro-breaker in the three phase items, as well as a contactor and a motor-guard.

100 % 95

ASHRAE EFFICIENCY

90 85 80 75 70

0,7

10

MODEL

FLOW (m3 /h)

12

1,2

14

16

2,0

18

20

2,7

22

24

3,3

26

28

v (m/s) ∆p (mm.wc) 30

POWER (W)

DIMENSIONS L x H x W (mm)

WEIGHT (kg)

115 - 1P 170 - 1P

800 x 420 x 400 800 x 420 x 500

30 40

FAV – 750 FAV – 1250

750 1250

FAV – 2500 FAV – 5000

2500 5000

400 - 1P 1125 - 3P+N

1000 x 548 x 500 1120 x 548 x 990

66 120

FAV – 10000

10000

2320 - 3P+N

1300 x 1096 x 990

200

FAV –15000 FAV –20000

15000 20000

3100 - 3P+N 4260 - 3P+N

1550 x 1644 x 990 1800 x 2192 x 990

280 450

FAV –25000

25000

5500 - 3P+N

2000 x 2740 x 990

520

FAV –30000

30000

5520 - 3P+N

2250 x 3280 x 990

630

NOTE: the FAV-5.000 unit is available: - With direct 12/12 fan of 1.5 HP. - With direct 12/12 fan of 1 HP.

Page 10

“FAVL” SERIES Electrostatic filters with a centrifugal fan. Every unit has a door micro-breaker in the three phase items, as well as a contactor and a motor-guard.

ASHRAE EFFICIENCY 100 % 95 8

90

14 5

85

6 LAMPARA CONTROL FILTRO

1

12 2

V

13

E N TI L A D

14

9

F I L T R

80 75

O

7

3

4

70

10 11

1 Mesh filter of 490 x 490 x 50 mm. 2 Fibre filter of 490 x 490 x 48 mm. 3 Ionizing cell of 475 x 490 x 70 mm.

8 1 HP. 1P motor. 9 Ventilator 9/9.

4 Collecting cell of 475 x 490 x 275 mm.

10 Supports. 11 Shock absorber.

5 Active carbon filter of 490 x 490 x 25 mm.

12 Tightener.

6 Electric panel. 7 Door switch.

14 Pulley.

0,7

13 Transmisión belt.

10

12

1,2

14

16

2,0

18

20

2,7

22

24

3,3

26

28

v (m/s) ∆p (mm.wc) 30

FRONT VIEW

FLOW (m3 /h)

POWER (W)

DIMENSIONS L x H x W (mm)

FAVL-2500

2500

400 - 1P

1000 x 548 x 500

68

FAVL-5000

5000

1125 - 3P+N

1120 x 548 x 990

124

FAVL-10000

10000

2320 - 3P+N

1300 x 1096 x 990

208

FAVL-15000

15000

3100 - 3P+N

1550 x 1644 x 990

292

FAVL-20000

20000

4260 - 3P+N

1800 x 2192 x 990

466

FAVL-25000

25000

5500 - 3P+N

2000 x 2740 x 990

540

FAVL-30000

30000

5520 - 3P+N

2250 x 3280 x 990

654

MODEL

WEIGHT (kg)

NOTE : the FAVL-5000 unit is available: - With direct 12/12 fan of 1.5 HP. - With direct 12/12 fan of 1 HP.

Page 11

“XAV” SERIES They are electrostatic filters with axial fans incorporated. All units have a door micro-breaker.

NUMBER OF FANS

POWER (W)

DIMENSIONS L x H x W (mm)

XAV – 750 XAV – 1250 XAV – 2500

1 1 1

82 - 1P 130 - 1P 150 - 1P

650 x 420 x 400 650 x 420 x 500 800 x 585 x 530

28 38 60

XAV – 5000

2

280 - 1P

800 x 585 x 1020

100

XAV – 10000 XAV – 15000

1 3

580 - 1P 860 - 1P

1000 x 1170 x 1020 1000 x 1755 x 1020

180 240

XAV – 20000

2

1160 - 1P

1100 x 2340 x 1020

380

XAV – 25000 XAV – 30000

4 1

1440 - 1P 2360 - 3P

1100 x 2925 x 1020 1100 x 1755 x 2040

420 530

XAV – 40000

2

3160 - 3P

1300 x 2340 x 2040

700

MODEL

WEIGHT (kg)

Page 12

“XAVL” SERIES They are special electrostatic filters of XAV series. All units have a door micro-breaker.

Collecting cell of 475 x 490 x 270 mm.

Active carbon filter of 490 x 490 x 25 mm.

Axial fan ∅350 mm. Power box Door microbreaker

Filter control lamp

Double breaker with signal lamp

MODEL XAVL – XAVL – XAVL – XAVL – XAVL – XAVL – XAVL – XAVL –

2500 5000 10000 15000 20000 25000 30000 40000

Mesh filter of 490 x 490 x 50 mm.

Ionizing cell of 475 x 490 x 65 mm.

NUMBER OF FANS

POWER (W)

DIMENSIONS L x H x W (mm)

1 2 1 3 2 4 1 2

150 - 1P 280 - 1P 580 - 1P 860 - 1P 1160 - 1P 1440 - 1P 2360 - 3P 3160 - 3P

800 x 585 x 530 800 x 585 x 1020 1000 x 1170 x 1020 1000 x 1755 x 1020 1100 x 2340 x 1020 1100 x 2925 x 1020 1100 x 1755 x 2040 1300 x 2340 x 2040

WEIGHT (kg) 62 105 185 250 390 430 540 720

Page 13

CARBON ACTIVE FILTERS These carbon active filters are designed for the neutralization and absorption of gasses and vapours of corporal origin and those coming from industrial activity.

The performing of these filters is based on absorption by means of active carbon, which is that one that after being conveniently treated, is given great porosity. This results in a great internal surface, a better retention power and a great absorption capacity.

UC - 22 MODEL

FLOW (m3 /h)

VELOCITY AND LOSS OF CHARGE (m/s - mm.wc) 0.30 8 0.31 9

MODEL

DIMENSIONS L x H x W (mm)

WEIGHT (kg)

UC – 750 UC – 1250

470 x 420 x 400 470 x 420 x 500

750 1250

UC – 2500 UC – 5000

570 x 580 x 530 570 x 580 x 1020

2500 5000

0.35 0.35

10 10

68 130

UC – 10000

570 x 1160 x 1020

10000

0.35

10

260

UC – 15000 UC – 20000

570 x 1740 x 1020 570 x 2320 x 1020

15000 20000

0.35 0.35

10 10

390 520

UC – 25000

570 x 2900 x 1020

25000

0.35

10

650

UC – 30000 UC – 40000

570 x 1740 x 2040 570 x 2320 x 2040

30000 40000

0.35 0.35

10 10

780 1040

30 45

Due to its zigzag disposition, the velocity of the contaminated air as it passes through the filters is about 0.35 m/s. This results in a contact time that permits the elimination of smells.

Page 14

ROLLING FILTERS RLUP -21 MODEL. To the basic model, which is represented in the figure, we can add different air treatment systems, in order to comply with the different demands that may be presented. RLUP - 21 is a rolling electrostatic purifier, based on our AV series. It has been created as an ideal solution for the intake and filtration of soldering smoke and oily gasses of production processes. It comprises the following elements:

RLUP - 21

1- A robust piece of furniture of 1350 x 690 x 760 mm. constructed in metal sheet and finished with EPOXI paint. It is easy to move since it has well dimensioned wheels and a handle so that it can be placed over the nearest pollution focus. It disposes of two access doors, one to the filtration system and the other to the fan. 2- A centrifugal fan type 9/9, built in galvanized metal sheet with inlay nerves to avoid deformities. It has a 4-pole closed motor of 1/2 HP, with a 220 V single phase feeding. 3- Air intake arm with 4 joints, a 460 mm. diameter tube with an extractor hood incorporated. 4- Large particle collecting tray. 5- Metallic filter of 490 x 490 x 25 mm, constructed with galvanized metal sheet. The filtration medium consists of 5 mini-waving meshes and a protective grating of electro-soldered string. The efficiency according to CEN-EN G is 75%.

Air intake tube of 160 mm. diameter Purified air output 9/9 extractor with a 12 HP motor Filter control lamp Air intake hood Extractor access door Transporting handle Double electric switch for extractor and filters with signal lights

Room for post-filter Filter access door

Collecting cell of 495 x 475 x 225 mm. Ionizing cell of 495 x 475 x 70 mm. Fibre filter of 490 x 490 x 50 mm. Mesh filter of 490 x 490 x 25 mm. Wheels

Large particle collecting tray

Electrical equipment

Page 15

6- A filter of 490 x 490 x 50 mm. with a filtrating medium made of auto-extinguishable synthetic polyester material in class F1 (according to DIN 53438), constructed with a frame and protective gratings. 7- Ionizing cell of 495 x 475 x 70 mm, constructed of aluminium with insulators of teflon and ionizing strings made of tungsten of 0.2 mm. diameter. 8- Collecting cell of 495 x 475 x 225 mm, constructed in aluminium with separating tubes of 10 mm. diameter and 12 mm. length. 9- Carbon active filters of 490 x 490 x 25 mm. with a galvanized metal sheet frame and protective gratings of perforated metal of 0.2 mm in dia meter and painted in black. 10- Electric system with a single phase power equipment fed at 220 V. with 3 outputs, one for the ionizer at 12.2 kV. DC, one for the collector at 6.2 kV. and another at 220 V. to feed the control lamp. It also has a door button and a double electric switch.

RUP - 21 MODEL The zone that is not identified in the photo is identical to RLUP-21 model.

Double electric switch with signal light

Filter control lamp Door micro-breaker

9/9 fan with 1/2 HP motor Access doors

Transporting handle

Electrical equipment

Carbon active filter of 490 x 490 x 25 mm. Electrostatic cell of 490 x 490 x 136 mm.

Door micro-breaker

Fibre filter of 490 x 490 x 50 mm.

Electrical feeding

Mesh filter of 490 x 490 x 25 mm.

Transporting wheels

Large particle collecting tray

Page 16

The purifying process comprises the following stages: 1- Metallic filter of 490 x 490 x 25 mm, constructed with galvanized metal sheet. The filtration medium consists of 5 mini-waving meshes and a protective grating of electro-soldered strings. The efficiency according to CENEN G is of 75%. 2- A filter of 490 x 490 x 50 mm. with a filtrating medium made of auto-extinguishable synthetic polyester material in class F1 (according to DIN 53438), constructed with a frame and protective grating. 3- Electrostatic cell of 490 x 490 x 136 mm, built in aluminium with separating tubes. It has two stages, an ionization stage and a capturing stage. 4- Carbon active filter of 490 x 490 x 25 mm. with a galvanized metal sheet frame and protective gratings of perforated metal sheet of 0.2 mm. diameter and painted in black colour. 5- Electric system with a single phase power equipment fed at 220 V. with 3 outputs, one for the ionizer at 7.2 kV. DC, another for the collector at 3.8 kV. and another at 220 V. to feed the control lamp. It also has door push buttons and a double electric breaker.

RUPA - 21 MODEL

Absolute filter of 490 x 490 x 292 mm.

Electrostatic cell of 490 x 490 x 136 mm. Fibre filter of 490 x 490 x 50 mm. Mesh filter of 490 x 490 x 25 mm.

The purifying process comprises the following stages: 1- Metallic filter of 490 x 490 x 25 mm, built in galvanized metal sheet. The filtration medium consists of 5 mini-waving meshes and a protective grating of electro-soldered strings. The efficiency according to CENEN G is of 75%. 2- A filter of 490 x 490 x 50 mm. with filtrating medium made of auto-extinguishable synthetic polyester material in class F1 (according to DIN 53438), constructed with a frame and protective grating. 3- Electrostatic cell of 490 x 490 x 136 mm, built in aluminium with separating tubes. It has two stages, an ionization stage and a capturing stage. 4- Absolute filter of very high capacity with a filtrating medium made of hydro-repellent and fire-proof glass fibres, mini-creased at a constant step by distancing thermo-soldered strings and with a bi-component polyurethane sealer. 5- Electric system with a single phase power equipment fed at 220 V. with 3 outputs, one for the ionizer at 7.2 kV. DC, another for the collector at 3.8 kV. and another at 220 V. to feed the control lamp. It also has door push buttons and a double electric breaker.

Page 17

RMP - 21 MODEL

Fibre filter of 491 x 592 x 100 mm.

Multi-dihedral filter of 491 x 493 x 292 mm. Mesh filter of 491 x 593 x 50 mm.

The purifying depends on the following stages: 1- Metallic filter of 491 x 593 x 50 mm., constructed with galvanized metal sheet. The filtration medium consists of 8 mini-waving meshes and protective gratings of electro-soldered string. The efficiency according to CENEN G is of 75%. 2- Absolute filter of very high capacity with a filtrating medium made of hydro-repellent and fire-proof glass fibres, mini-creased at a constant step with distancing thermo-soldered strings and with a bi-component polyurethane sealer. 3- A filter of 491 x 592 x 100 mm. with a filtrating medium made of auto-extinguishable synthetic polyester material in class F1 (according to DIN 53438), constructed with a frame and protective gratings. 4- Electrical system composed of an electric switch with a signal lamp.

Page 18