autojet sprayng system

8/14/2019 Autojet Sprayng System

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Equipment Maintenance

38 www.cepmagazine.org December 2005 C E P

Spray nozzles are carefully engineered to deliver specificperformance under certain operating conditions. T h e i r

performance is affected by the nozzle type, spray pattern,

c a p a c i t y, operating pressure, material of construction, droplet

v e l o c i t y, and spray distribution, angle and impact. This article

provides an overview of nozzle selection considerations and

discusses troubleshooting options and maintenance techniques

to help engineers maximize the productivity of their spray sys-

t e m s . Table 1 (p. 39) summarizes the different nozzle types and

the spray performance characteristics.

In the chemical process industries, droplet size is often the

most critical performance parameter. For instance, in gas coolingapplications, droplets must be small enough so that complete

evaporation occurs quickly without wetting the walls and duct-

work. For gas absorption and chemical injection, the surface area

of the droplet controls the rate and extent of the reaction, and

must be optimized for the given process. Application of a coat-

ing to a substrate is another area where spray nozzles are used.

Coatings are created when individual droplets coalesce and buildup in layers on a substrate. To achieve a uniform coating, the

droplet size and distribution must be exact. In spray drying, noz-

zles must produce droplets of a consistent size and surface area,

otherwise, the quality of the powder will be poor.

Table 2 (see p. 40) illustrates the wide range of droplet sizes

produced by different spray nozzles at various pressures and

capacities. Droplet sizes are expressed in terms of the volumemedian diameter (V M D), a widely accepted reference parameter

defined as the value where 50% of the total volume (or mass) of

the liquid sprayed is made up of droplets with diameters larg e r

than the median value and 50% smaller than the median value.

Another term used when referring to the V M D isDV0 . 5.

Specific droplet size information, such asDV0 . 1, D3 2,DV0 . 9,

Dm a x, droplet size distributions, etc., should be obtained from the

nozzle manufacturer.DV0 . 1 is where 10% of the total volume (or

mass) of the liquid sprayed is made up of drops with diameters

smaller or equal to this value.D3 2 or the Sauter mean diameter

expresses the fineness of a spray in terms of the surface area it

produces.Dm a x denotes the maximum droplet size by volume

or mass present in the spray. This diameter is used when com-

plete evaporation of the spray is required. The same drop size

measurement should be used when comparing different nozzles

(e . g .,DV0 . 5 from one nozzle should not be compared withD3 2from another nozzle).

Nozzle selection for gas conditioningA comprehensive explanation of performance standards and

nozzle selection considerations for all chemical spray applicationsis beyond the scope of this article. Instead, the focus is on the sig-

nificance of nozzle selection in gas-conditioning applications.

Although it is a broad term, gas conditioning generally refers to

controlling the properties of a gas by injecting water and/or

reagents to cool the gas to certain temperatures, change its humid-

ity or scrub the gas of unacceptable components. Gases that

reside in cooling towers, ductwork and dryers are conditioned

using desuperheating, gas absorption and chemical injection.

Desuperheating involves spraying water into superheated

steam. The water evaporates by absorbing heat, reduces the tem-

perature of the steam and restores it to its saturated state. Fine-

spray hydraulic nozzles, air-atomizing nozzles and special desu-

perheating lances are used because of the small droplets they

generate. If the droplets are too large, the saturated steambecomes excessively wet and additional drying through a separa-

tor is required.

Absorption is the process by which contaminants are

removed from a gas via vigorous gas-liquid interaction. T h e

Environmental Protection Agency has identified many extremely

hazardous air pollutants that must be controlled to emission lim-

its. Gas absorption, or scrubbing, provides an efficient means for

compliance. The scrubbing liquid, which exhibits a high solubili-

ty for the gas contaminants, absorbs target components from the

Use these techniques to select,troubleshoot and maintain spray nozzles

systems for optimum performance.

Christine Pagcatipunan

Rudi Schick

Spraying Systems Co.

Maximize

the Performance ofSpray Nozzle Systems


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Table 1. Basic nozzle and spray characteristics.

CEP www.cepmagazine.org December 2005 39

Nozzle Type Spray Spray Characteristics Spray Angle, deg Typical ApplicationsPattern

Hollow cone: Provides a good interface between 40165 Air, gas and water coolingwhirl-chamber air and droplet surfaces; large, Flue-gas desulfurization (FGD)type open passages resist clogging Gas absorption, scrubbing and stripping

Hollow cone: Provides small droplet sizes, 100180 Water curtaindeflected type but is prone to clogging and Dust suppression

erosion of deflector surface Tube, pipe and small tank cleaning

Hollow cone: Droplets are slightly coarser 50180 FGDspiral type than other hollow-cone sprays; Gas cooling

high flowrate for a given pipe size; Evaporative coolinglarge, open passages resist clogging

Full cone Uniform coverage with medium- 15125 Washing and rinsingto-large droplets; most flexible Chemical injectionand widely used in industry; Dust suppressionfree passage of droplets limitedby internal vane

Full cone: Coarse droplets; minimal flow 50170 Gas absorption, scrubbing and strippingspiral type obstruction due to lack of internal FGD

vanes; coverage is not as uniform as Dust suppressioninternal vane-type nozzles

Flat: Designed for use on a spray 15110 Conveyor coatingtapered edge manifold or header section for Product cooling

uniform overall coverage across theimpact area

Flat: Even distribution; medium-sized 2565 High-pressure washingeven edge droplets; high-impact spray

Flat: Medium-sized droplets; large, free 15150 Product washingdeflected type openings reduce clogging

Solid stream High-impact spray effectively 0 Tank washingremoves persistent residue

Atomizing: Low-capacity f low; hollow-cone spray 35165 Evaporative coolinghydraulic, is used when compressed air Spray dryingfine mist is not desirable Humidification

Desuperheating

Air-atomizing: Gas cooling and conditioninginternal mixing Produces smallest, most-uniformly Wide variety Spray drying

sized droplets; high turndown ratios Humidification Desuperheating Chemical injection

Air-atomizing: Most often used when Wide variety Product coatingexternal mixing spraying viscous liquids


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gas, solubilizes them, and then disposes of them or converts

them into useful byproducts. For example, ammonia gas, hydro-

gen chloride and hydrogen fluoride can be scrubbed by water.

Chlorine, hydrogen sulfide and sulfur dioxide can be scrubbedby caustic solution. In flue-gas desulfurization, limestone is used

to scrub sulfur dioxide.

Absorption is usually performed in packed-bed scrubbers,

Venturi scrubbers or open-spray towers. The type of scrubber

dictates the nozzle selection and performance considerations.

Packing that is contained in bed scrubbers increases the sur-

face area and contact between the liquid and gas. The gas enters

through the bottom of the column and flows through the packing

where it contacts the liquid. The liquid is sprayed through noz-

zles at the top of the column countercurrently to the gas. Full-

cone nozzles provide good spray coverage and distribute the liq-

uid evenly through the packing.

Venturi scrubbers entrain large volumes of gas. Scrubbingliquid is sprayed through a nozzle and creates a draft that draws

the gas into the moving stream. Because droplets will be

entrained with the gas, different types of nozzles can be used.

Hollow-cone nozzles are preferred because of the droplet size

and resistance to clogging.

Open-spray towers contain several levels of spray nozzles,

which should be strategically placed so that they uniformly

cover the tower cross-sectional area while creating dense spray

zones though which the gas must pass. Hollow-cone nozzles are

typically used because the droplets are small enough to create

adequate surface area, but large enough so that they will notbecome entrained in the gas stream. Their clog-resistant design

can be manufactured in custom sizes to meet specific liquid

flowrate and spray angle requirements. Special materials, such asabrasion-resistant silicon carbide for lime slurry injection, are

often available for difficult applications.

During chemical injection, a chemical is sprayed into a gas

to induce a reaction. One example is NOx control, where urea or

aqueous ammonia is injected into a gas stream to limit the

amount of NOx that is emitted into the atmosphere. A q u e o u s

ammonia or urea can be injected directly into the flame by utiliz-

ing full-cone nozzle injectors. Selective catalytic reduction

(SCR) typically uses two-fluid nozzles because of the demand

for very small droplets. Complete evaporation of the droplets

within a defined reaction zone is necessary to avoid damaging

the catalyst and gas mixer. If droplets are too large, then unreact-

ed ammonia will accumulate. If they are too small, the ammonia

will not be carried far enough to evenly distribute with the gas.In gas cooling, or quenching, the temperature of the gas is

reduced by spraying liquid to induce heat transfer. In the petro-

chemical and power industries, gas is quenched as it exits a

process through a duct.

Nozzle selection for gas coolingSince gas cooling is used widely in the process industries, the

authors have chosen to present a step-by-step approach to nozzle

selection for such applications.

Step 1. Gather accurate process data for critical variables.

Evaluate the composition of the gas to calculate its

molecular weight.

Measure the gas inlet flowrate at a specified inlet tempera-

ture. Check the accuracy of the thermocouples and other meas-uring devices. Accurate readings must be obtained for the tem-

perature of the gas at the inlet and outlet. Temperature aff e c t s

the density of the gas at the inlet, which in turn affects the vol-

ume of cooling water and the droplet size required for suff i c i e n t

heat removal.

Determine the temperature of the cooling liquid being

sprayed. Do not assume that the temperature is ambient; some-

times the liquid that cools the gas is taken from another process

stream in the plant.



N o m e n c l a t u r e

A = cross sectional area of tower or duct, ft2

D = diameter of the droplet, m

f = fraction of liquid evaporated, dimensionless

h = specific enthalpy, Btu/lbF

Hv = latent heat of vaporization of the liquid, Btu/lb

K = thermal conductivity, Btu/h-ft-F

Qgas = volumetric flowrate of gas, ft3/min (actual)

Qliq = volumetric flowrate of liquid, gal/min

t = residence time, s

T = temperature of the gas, F

Twb = wet bulb temperature of the system, F

u = fluid velocity into or out of the duct, ft/s

Greek Letters

= density, lb/ft3

= evaporation constant, ft2/h

Subscripts

gas = process gas to be cooled

in = point at inlet of duct or spray tower

liq = cooling liquid

out = point at outlet of duct or spray tower

vap = vapor produced when liquid evaporates upon cooling

the process gas

Table 2. Spray droplet size during atomization.

Air Atomizing 0.005 20 0.008 15

0.02 100 8 200 12 400

Hydraulic 0.03 110 0.05 110

Fine Spray 0.22 375 0.43 330 0.69 290

Hollow cone 0.05 360 0.10 300 0.16 200

12 3,400 24 1,900 38 1,260

Flat Fan 0.05 260 0.10 220 0.16 190

5 4,300 10 2,500 15.8 1,400

Full Cone 0.10 1,140 0.19 850 0.30 500

12 4,300 23 2,800 35 1,720

*Qliq = volumetric flowrate of liquid

**VMD = volume median diameter

Spray Pattern 10 psi 40 psi 100 psi

Qliq*, VMD,

** Qliq, VMD, Qliq, VMD,

gal/min m gal/min m gal/min m


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Determine the gas velocity and residence time using the

duct or tower diameter and length. Adimensional drawing of the

duct or tower should be referenced to determine the most eff i-

cient placement of the nozzles.

Step 2. Determine the volumetric flowrate of liquid, Q,

required to cool the gas. To calculate Q, assume the following:

the duct walls are adiabatic; all heat lost by the gas is absorbedby the liquid; and liquid volume is negligible compared with

vapor volume.

The equations used to calculate Q are based on the laws of

conservation of mass and energ y. For the process gas, the law of

conservation of mass is:

(g a sA ug a s)i n = (g a sA ug a s)o u t ( 1 )

whereA is the cross-sectional area of the duct or tower in ft2,

g a s is the density of the gas to be cooled in lb/ft3 and ugas is the

velocity of the gas in ft/s. All parameters are measured at the

system inlet and outlet, as denoted by the subscripts.

For the cooling liquid, the law of conservation of mass is:

f Qliq = (v a pA uv a p)o u t ( 2 )

wherefis the fraction of liquid that evaporates, dimensionless,

Ql i q is the volumetric flowrate of the cooling liquid in gal/min,

and v a p and uv a p are, respectively, the density and velocity ofthe vapor that is produced when the liquid evaporates upon cool-ing the gas, measured at the outlet of the system.

The law of conservation of energy states:

(gasAugashgas + 0.5(gasugas2) + Qliqhliq)in =

((gasAugas(hgas + hvap)+ 0.5(vapuvap2))out (3)

where h is the specific enthalpy in Btu/lbF.Step 3. Determine the optimal droplet size required for

complete evaporation. Droplet size is a function of the resi-

dence time and should be very small so that evaporation occurs

q u i c k l y. Smaller droplets provide more total surface area for the

gas and liquid molecules to interact and effect heat transfer. If

droplet sizes are too large, they may not completely evaporate,

thereby resulting in wall wetting, sludge buildup and problems

downstream with the refractory lining. The process of determin-ing the maximum allowable droplet size is governed by the fol-

lowing equation, which calculatesD, the droplet diameter, as a

function of residence time, t, in seconds:

D(t) = (D02 t)1 / 2 ( 4 )

where is the evaporation constant in ft2/ h :

= 8K(T Tw b) / (Hvl i q) ( 5 )

andD0 is the initial diameter of the droplet in m, Kis the

thermal conductivity of the gas in Btu/h-ft-F, Tis the temper-

ature of the gas in F, Twb is the wet bulb temperature of the sys-

tem in F, andHv is the latent heat of vaporization of the sys-

tem in Btu/lb.

Step 4. Select nozzles and lances based on the specific gas-

cooling application. Fine-spray hydraulic nozzles and air- a t o m-izing nozzles are recommended due to their small droplet sizes.

Although fine-spray hydraulic nozzles produce very small

droplets, large-volume, two-fluid atomizers are preferred

because the droplet sizes are even smaller. A i r-atomizing noz-zles break-up the fluid in multiple stages as it passes through

the nozzle body and then use additional energy from com-

pressed air to break-up the droplets even further as they exit

the air cap (Figure 1).

Step 5. Material selection is key when choosing the optimal

nozzles and lances. Unwanted reactions can take place betweenhigh-temperature gases and nozzle materials. Thus, material

selection should be based on the temperature and composition of

the gas. Common materials are high-temperature and corrosion-

resistant 310SS and Hastelloy C, or abrasion-resistant materials,like silicon carbide and Stellite. Lances are integral spray-system

components that determine the efficiency of liquid delivery to

the nozzles. Custom lances can be manufactured to specified

lengths, materials, and connections. Recirculating lances, air

p u rge and water-cooling jackets can improve the process.

Step 6. Locate optimal positioning for nozzles and lances.

Consider these factors:

Lance spacing. Make sure the cooling liquid sprayed is dis-tributed evenly with the gas flow. Special lances can be made to

specific lengths.

Direction of gas flow. Position nozzles to spray co-current

with the gas flow. Counter-current systems create a spray pattern

that is less predictable.

Duct orientation. Vertical orientation is preferred.Horizontal ducts have the added problem of gravity. The spray

will reach the bottom wall if droplet sizes are too large or if gas

velocity is not sufficient to carry off the spray.

Spray angle. The spray angle must be small enough to

prevent the spray from touching the walls and causing

sludge buildup.Step 7. After completing the calculations for Q andD u s e

computational fluid dynamics (CFD) to validate the results.

The following CFD example, illustrated in Figures 25, simu-

Figure 1. Two-fluid atomizer operating principle.


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lates the flow field in the vicinity of the atomizer and at vari-

ous locations in the duct. The gas is air and the cooling liquid

is water. Information is predicted for the gas temperature,

vapor-concentration variations, droplet concentration in the

duct and droplet trajectory in the duct.

The general flow field is simulated as a multiphase flow prob-lem. The cooling liquid is supplied to the atomizer at a set pres-

sure. The surroundings of the atomizer are introduced to a mov-

ing gas at the process gas velocity (ui n _ g a s). The pressure in the

duct is 17.4 psi. The duct length and area are 25 ft and 50 ft2,

r e s p e c t i v e l y. Qg a s = 234,560 ft3/min (actual), Tin_gas = 500F,

Tout_gas ( d e s i r e d ) = 350F, i n _ g a s = 0.0143 lb/ft3, out_gas =

0.0572 lb/ft3, Qliq = 75.2 gal/min, Ti n _ l i q = 68F,D0 = 8 0m,

t= 0.4 s. The initial vapor mass in the duct is zero. The gas veloc-

ity is ui n _ g a s = 66 ft/s.

At the injection point (Figure 2, Section 1), the gas tem-

perature is 500 F. As the gas moves upward through the duct,its temperature is quickly reduced to the desired outlet tempera-

ture of 350F. CFD simulation predicts a non-uniform tempera-

ture distribution in the duct, with slightly higher temperatures at

the center of the duct due to the higher mass fraction ratio of

H2O at that location.

The droplet size distribution is used by the CFD software to

model the trajectory of droplet size (i . e . , predict the path and

velocity of droplets of various sizes), which enables one to vali-

date the evaporation rate as a function of residence time. T h e

residence time calculation predicts that t= 0.4 s for a droplet

size of 80 m at a gas velocity of 66 ft/s. This data was based

on the duct geometry and the actual gas flowrate.

The CFD-based trajectories for 36- and 74-m droplets are

shown in Figure 3. In both cases, the model predicts a velocitywithin the required velocity for the residence time. Further, all

droplets are fully evaporated within the length of the duct. Wi t h

increasing droplet size, the model predicts higher droplet veloci-ties, which causes droplets to exit the duct prior to their evapo-

ration. This results in inadequate cooling of the gas (Figure 4).

Figure 5 models the droplet concentration as a function of

location in the duct for the 74-m droplets. These droplets are

closest to the size required for full evaporation (or 80 m). T h e

concentration for the 74-m droplets is heaviest at the injection

point. In addition, these droplets are concentrated at the center

of the duct. As the gas moves though the duct, the spray evapo-

rates quickly and the droplet concentration is reduced.

In the last section of the duct,

the model predicts a slight con-

centration of droplets around the

edges of the duct. This is not

considered to be a major con-cern and can be remedied by

realigning the atomizers at the

injection point.

Step 8.Implement control

systems for precise control of

temperature. If the gas-cooling

spray system is manually con-trolled, automation may be the

best strategy for optimizing the


Figure 2 (far left). Temperature profile for inlet gas

based on location in the duct.

Figure 3 (center). Trajectories in the duct for 36-

and 74-m droplets.

Figure 4 (below). Trajectories in the duct for 135-,154- and 174-m droplets.

Figure 6. Good spray tip pattern.

Figure 7. Worn spray tip pattern.


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system. Spray controllers monitor an array of criti-

cal system variables and adjust components to

compensate for fluctuations accordingly (see thesidebar for further details).

Troubleshooting spray nozzlesJust because a nozzle is spraying, one

should not assume it is spraying properly.

Monitoring, evaluating and maintaining spray

systems should be a part of the systems regular

maintenance schedule.

Some spray nozzle problems, like corrosion,

caking and bearding, can be easily detected dur-ing a visual inspection. However, a worn nozzle

is nearly impossible to spot with the human eye.

Spray patterns are much more revealing. T h e

spray tip used to create the profile in Figure 6 is

new and sprays properly. The spray tip used to create the pro-

file in Figure 7 is worn and sprays 30% over its capacity.To troubleshoot spray nozzle problems that are not visually

obvious, look for these process-related changes:

Quality control issues and increased scrap. Worn, clogged

and damaged spray nozzles will not perform per specifica-

tion, and can result in uneven coating, cooling, cleaning,

humidifying and drying.Increased maintenance time. Unscheduled spray system

downtime, or an increase in cleaning frequency, is an indication

of spray nozzle wear.

Flowrate change. The flowrate of a spray nozzle will

increase as the surfaces of the orifice and/or the internal core

begin to deteriorate. In applications using positive-displacement

pumps, the spraying pressure will decrease as the spray nozzleorifice enlarges. Even small changes in flowrate can have a neg-

ative impact on quality.

Deterioration of spray pattern quality. When a nozzle ori-fice wears, spray pattern uniformity is destroyed. Streaks

develop and flowrate will increase in the center or at the edges,

depending on the nozzle type. Effective spray angle coverage

will decrease.

Droplet size increase. As nozzles wear, liquid flow will

increase or spraying pressure will decrease, resulting in larg e r

droplets and less total liquid surface area. If you suspect a prob-

Figure 5. Droplet concentration based on the location in

a duct for the 74-m droplets.

Advantages of Automated Gas Cooling Technology

Automated gas-cooling systems can eliminate unnecessary costs by constantly monitoring

gas temperatures in scrubbers, ducts and towers, and then adjusting the liquid and air

flowrates to the nozzles to maintain the desired droplet size and achieve efficient cooling.W h i l emany plants automate their gas-cooling applications by integrating components from numer-

ous manufacturers, purchasing a turnkey system can save on programming and labor costs.

In a typical automated gas-cooling setup, multiple temperature sensors provide gas inlet

and outlet temperature data to a dedicated spray controller that uses closed-loop PID control

to maintain gas temperatures within very tight tolerances (Figure 8). In addition, the spray

controller continually monitors the atomizing-air pressure, nozzle/lance activation, and system

integrity and error handling. If a problem is detected that the controller cannot resolve auto-

m a t i c a l l y, alarms will notify an operator of a system fault.

Liquid lines should feature redundant pumps and double filtration to decrease nozzle wear.

Pumps with variable frequency drive (VFD) control provide proportional liquid regulation and

significant electricity savings by adjusting the speed of the motor to run at the minimum

requirement rather than at a fixed rate.

Air lines should include air filtration and either manual regulation with bypass or propor-tional regulation. Proportional regulation enables a higher turndown ratio and can save energy

by monitoring the air flowrate.

A reputable gas-cooling

system manufacturer will

begin by reviewing a dimen-

sional drawing of your duct or

tower along with key system

performance data (gas vol-

ume, temperature and veloci-

t y, for example). This informa-

tion will enable the supplier to

provide recommendations

regarding optimal droplet size

and flowrate, the number of

nozzles required and the best

possible lance positioning to ensure sufficient cooling of the gas. Systems can be configured

with multiple lance zones to allow increased turndown of flowrate under variable conditions.

Because droplet size is a critical consideration in gas cooling, the manufacturer should be

able to provide comprehensive droplet size data and information about how the droplet size

was obtained, including data measurement techniques, type of size analyzer used and the

reporting methodology (i . e.,V M D, Sauter mean diameter or number mean diameter).

Figure 8. Automated gas-cooling system equipped with closed-loop temperature control.


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lem, arrange for droplet size testing with your nozzle manufac-

t u r e r. A phase-doppler particle analyzer uses the principle of

light-scattering interferometry to measure the size and velocity

of droplets.

Lowered spray impact. Worn spray nozzles operate at lower

pressures, generally resulting in lower spray impact. Ironically, in

applications with centrifugal-type pumps, the impact may actual-

ly increase because of increased flow through the spray nozzle.

Common nozzle problemsSpray nozzles are carefully-engineered instruments that erode

over time and may suffer damage during normal operations and

cleaning procedures. The seven most common problems that

cause sub-standard spray performance are:Erosion. Gradual wear on the nozzle material causes the

spray nozzle orifice and the internal flow passages to enlarg e

and/or become distorted. As a result, flow usually increases,

pressure may decrease, the spray pattern becomes irregular and

droplets become larg e r. Erosion of spray nozzles is a significant

concern in coating applications. Anozzle spraying even slightly

over capacity due to erosion may cause product defects and anincreased scrap rate.

Corrosion. Spray nozzle material can break down due to

cleaning agents, the chemical qualities of a sprayed material or

the environment. The effect is similar to that caused by erosion

and wear, with possible additional damage to the outside sur-

faces of the spray nozzle. Corrosion around the nozzle orifices is

a common challenge in tank washing, spray drying and chemical

injection applications.

High temperatures. Certain liquids must be sprayed at ele-

vated temperatures or in high-temperature environments. Spray

nozzles may soften and break down unless special temperature-resistant materials are used. This is a particular concern for spray

drying applications where ambient temperatures in the dryer can

reach up to 500F (260C).C a k i n g / b e a r d i n g . Build-up of material on the inside, on the

outer edges or near the orifice is caused by liquid evaporation. A

layer of dried solids remains and obstructs the orifice or internal

flow passages. This is a common problem in coating applica-

tions and can often lead to maintenance challenges that cause

significant downtime and decreased productivity.

C l o g g i n g . Unwanted solid particles can block the inside of

the orifice. Flow is restricted and the spray-pattern uniformity is

disturbed. Tank-cleaning nozzles and spray balls can becomeeasily clogged by particles in the liquid, which leads to inade-

quate tank washing and potentially contaminated batches.

Improper re-assembly. Some spray nozzles require careful

re-assembly after cleaning, so that internal gaskets, O-rings and

valves are properly aligned. Improper re-assembly causes leak-ing and inefficient spray performance.

Accidental damage.Damage can occur if a spray nozzle is

dropped or scratched during installation, operation or cleaning.

Preventative measuresThe checklist that follows should become the foundation of

your nozzle maintenance program. Consistent evaluation of

these factors will enable you to detect wear and other problems

before they interrupt production. Each application will determine

how often the individual factor should be checked. The proper

frequency could range from every few months, for a gas-cooling

application, to between shifts, for coating and spray drying.

F l o w r a t e . For centrifugal pumps, monitor flowmeter read-

ings to detect flow increases. Or, collect and measure the spray

from the nozzle for a given period of time at a specific pressure.

Compare these readings to the flowrates listed in the manufac-

t u r e rs catalog or compare them to flowrate readings from new,

unused spray nozzles. For positive-displacement pumps, monitor

the liquid line pressure for pressure decreases. The flowrate

should remain constant.

Spray pressure (in nozzle manifold). For centrifugal

pumps, monitor for increases in the liquid volume sprayed. T h e

spraying pressure is likely to remain the same. This is particu-larly important for coating applications that rely on consistent

spray pressures to maintain uniform coverage. For positive-dis-

placement pumps, monitor the pressure gauge for decreases in

pressure and a reduction in the impact of the spray on the sub-

strate. The liquid volume sprayed is likely to remain the same.

Look for increases in pressure due to clogged spray nozzles. Spray pattern.

Visually inspect the spray pattern forchanges. Check the spray angle with a protractor. Measure the

width of the spray pattern on the sprayed surface. If the spray

nozzle orifice is wearing gradually, you may not detect changes

until there is a significant increase in flowrate. If uniform spray

coverage is critical in your application, request special testing

from your spray nozzle manufacturer.

Droplet size. Droplet size increases cannot be visually

detected in most applications. An increase in flowrate or

decrease in spraying pressure will affect droplet size.

Nozzle alignment. With regard to nozzle alignment, check

the uniformity of spray coverage of flat spray nozzles on a mani-fold. Spray patterns should be parallel to each other. Spray tips

should be rotated 5 deg to 10 deg from the manifold centerline.

Product quality.With regard to product quality/applicationresults, check for uneven coating, cooling, drying, cleaning and

changes in temperature, dust content and humidity.


RUDI SCHICK is the vice president of Sp ray Analysis and Re s e a rch Se rvices, theconsulting, testing and re s e a rch divi sion of Sp raying Systems Co. (P.O. Box

7900, Wheaton, IL 60189; Phone: (630) 665-5000; Email: rudi.schick@spra y.

com; Website: www. s p ra yconsultants.com). Schick serves on the Board of

D i rectors for the Institute of Liquid Atomization and Sp ray Systems (ILASS )

and is active in the American Society of Testing and Materials (ASTM). Wi t h

over 15 years of experience in the area of spray characterization and

re s e a rch, he is a frequent speaker at technical conferences and an invited

l e c t u rer at Carnegie Mellon Univ. s course on atomization and sprays. He

has also authored numerous white papers and articles on spra y

c h a racterization. Schick re c e i ved a bachelors degree in mechanical

engineering from Bradley Univ. and an MBA from DePaul Univ.

CHRISTINE PAGCATIPUNAN is an applications eng ineer at Sp raying Sy s t e m sC o. (P.O. Box 7900, Wheaton, IL 60189; Phone: 630-665-5000; Email:

p a g c a t c h @ s p ra y.com; Website: www. s p ra y.com). As the companys

chemical market expert, she analyzes and customizes spray solutions for

gas conditioning, tank washing, spray drying and other chemical pro c e s s

i n d u s t ry applications. Pagcatipunan is an experienced author an d speaker

in these areas. She earned a bachelors degree in chemical engineering

f rom Illinois Institute of Technology (II T) and a masters degree in marketing

communications from the II T St u a rt Graduate School of Business.

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