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Buying Power: Downdraft Spraybooths

Thinking about purchasing a new downdraft spraybooth or prep station? If you’ve got questions, we’ve got answers.

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If you’re planning a shop expansion or need to increase workflow
in your current shop, you may be considering the purchase of a
new downdraft spraybooth or prep station. But which one do you
choose? And which options do you select?

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Spraybooths are a lot like automobiles. The basic function of
an automobile is to get you from point A to point B, but vehicles
come with a wealth of features in a wide price range. The task
is to decide which features will provide you with the benefits
you desire.

Similarly, most of the questions surrounding downdraft-spraybooth
and prep-station technologies arise because people don’t fully
understand the different features. Without enough knowledge, it’s
impossible to decide if a given feature will be beneficial.

To help with future purchasing decisions, I’ve collected some
of the most frequently asked questions about downdraft spraybooths
and prep stations and filled in the answer blanks with sound knowledge.

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Q: What can I expect my new downdraft spraybooth to
do?

A: Simply put, your spraybooth should increase productivity
by providing clean paint finishes and accelerated paint-curing times.

Q: What’s the difference between a dry-filter exhaust
system and a water-wash exhaust system?

A: The exhaust filters in a dry system are designed to
clog up. They’re paint-stop filters, and this is their sole purpose. A water-wash
exhaust system has no filters in the exhaust air stream. Filtration
is achieved as air passes through water spray jets or a water
curtain and baffle arrangement. The water is filtered to prevent
paint solids from clogging pipes and pumps. Once booth pressure
has been set, it will usually remain steady for longer periods
than with a dry system.

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As the filters in a dry system clog, they allow a smaller volume
of air to pass through. So unless you can control the booth’s
incoming air supply, the cabin begins to develop a positive pressure.
The air that can’t exit through the floor filters has to be displaced
because more air is continuously entering the booth.

Under these conditions, a smoke test would show the excess air
swirling across the booth floor, climbing the booth walls and
rejoining the downdraft air stream. When this swirling air contains
particles of paint overspray, which are passed over the wet paint,
the surface becomes contaminated.

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Regardless of the width of the ceiling filter or the shape of
the booth ceiling, blocked filters will create a swirling of air.
And though the pattern may be different, the result will be the
same. The only way to avoid this is to balance the booth pressure
again.

The booth pressure can be balanced either by changing filters
or reducing the incoming air supply. Please note that reducing
the air input can, if cut too much, result in other problems,
so this should only be done if the system is designed for it.
Some booths offer automatic pressure control that measures cabin
pressure against the surrounding shop and automatically compensates
for the difference. This system will maintain the correct booth
pressure as long as the filters have usable life.

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Based on normal production rates of four to five jobs per day,
dry filters would require changing about once a week at a cost
of around $200. (They’ll last longer if you use an HVLP spray
gun.) These contaminated filters may be considered hazardous waste
and, as such, must be handled correctly.

Water-wash systems require frequent maintenance and incur high
disposal costs because the contaminated water is often considered
hazardous waste. If the water-wash system isn’t maintained correctly,
it will begin to let paint overspray through the water curtain,
causing a build-up on the exhaust fan and ducting. Once this has
occurred, cleanup is a major job.

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However, there’s little actual maintenance involved in keeping
the system operating. The pump filter must be cleaned once a week,
but it takes no more than 10 minutes to do; the water filters
will need changed about once a month. The system also collects
paint sludge, which must be cleaned out. This will need done once
or twice a year, depending on the particular booth and the workload.
Like contaminated water, sludge is hazardous waste and must be
disposed of correctly. In areas where the hazardous-waste regulations
are enforced, waste material is collected in barrels, and you
pay for handling on a per-barrel basis. Depending on the size
and thickness of the material, one change of dry filters can fill
a barrel; the same barrel can take up to six changes of water-wash
filters.

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Generally, water-wash booths are initially more expensive to purchase
than dry-filter booths because there’s more construction and machinery
involved. Running costs should, however, be lower because there’s
no need to continually change exhaust filters. Both types of exhaust
systems, if maintained properly, will achieve clean paint finishes.

Q: What’s the difference between a direct-fired air-makeup
unit and an indirect-fired unit?

A: In a direct-fired heater, the gas flame of the burner
fires directly into the air that enters the booth. An indirect-fired
heater has a separate chamber into which the burner fires; the
air passes over the outer surface of this chamber to collect its
heat.

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Modern downdraft spraybooths use heating units that produce from
1 to 1.5 million BTUs. (Note: In some warmer areas of the country,
heaters may only be required to produce 750,000 BTUs.) Generally,
spray temperature is between 68 degrees F and 72 degrees F, so
the heating unit must be capable of maintaining the booth at this
temperature, regardless of the outside air temperature. Typically,
air-handling machinery will provide a minimum of three volume
air changes per minute – if the booth volume is 3,000 to 3,500
cubic feet, a total capacity of approximately 10,000 cubic feet
per minute will enter the booth.

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A consensus is that the heater should provide a minimum of 90
degrees F of temperature rise on the spray cycle, which means
the air intake temperature can be as low as -18 degrees F for
a spraying temperature in the booth of 72 degrees F. During the
spray cycle, no air is recirculated. It’s during the baking or
curing operation that recirculation can take place if the heater
system is designed for it. If the paint needs a curing temperature
of 140 degrees F and the heater gives us 90 degrees F of rise,
the outside air must be at least 50 degrees F (140 – 90 = 50).

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If the outside air is less than 50 degrees F, one of two things
must be done to increase the temperature rise. One is to recirculate
by taking the air from the booth, continually passing the same
air through the heater and discharging it back into the booth.
The second method is to slow down the air speed through the heater
so the air collects more heat. Certain direct-fired heaters use
a system of dampers and air-speed control for recirculation of
the air within the heater enclosure.

Both direct- and indirect-fired systems are perfectly acceptable
from a code standpoint. And both units, if correctly designed
for the climate, will perform equally. When considering running
costs, you should take into account fuel and maintenance costs,
including intake filters, routine servicing and parts replacement.

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Q: What are my spraybooth lighting options?

A: Spraybooth lighting is a much-discussed but little-understood
subject. This is because the science of lighting is extremely
complex, and there’s a wealth of options out there.

The object of spraybooth lighting is to provide a high level of
light evenly spread over the surface of a vehicle. Quite simply,
the more light you have, the better you can see things. To achieve
this, spraybooth manufacturers use fluorescent or metal halide
lamps. Standard car-sized booths are generally equipped with between
3,000 and 4,000 watts of light, regardless of the type of lamp.

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In the spraybooth industry, there’s also a great deal of talk
about color rendering index (CRI) lamps. CRI lamps with a high
index, 100 being the highest, are better for seeing a wider spectrum
of colors. If you’re color matching in a spraybooth, you’d want
lamps with the highest CRI. Manufacturers can provide lamps with
index ratings in the 90s for both fluorescent and metal halide
lights. Note: A high output of light and a high CRI don’t necessarily
go together, but your booth manufacturer certainly will have chosen
the correct lamps or lamp options for the particular booth you’ve
selected.

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When CRI is an important consideration – as it is in color matching
– correct maintenance is vital. In lighting terms, this is what
the lamp manufacturers call "planned group relamping."
In plain English, this means you shouldn’t wait for a single lamp
to fail before replacing it because then you’re continually working
with lamps of different ages. As lamps age, their light output
and CRI tend to shift. To maintain uniformity, renew all the lamps
at the same time according to the manufacturer’s schedule.

Hazardous-waste disposal should also be considered here. Items
such as fluorescent tubes have to be disposed of correctly, which
may involve some additional costs.

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Q: Should I consider purchasing a prep station?

A: Anyone who’s planning a new body shop in North America
will almost certainly consider including one or more prep stations.
These units, in their most basic form, draw dust-laden air from
a sanding or grinding operation through a filtration system and
return the clean air to the work area. This helps to keep the
remainder of the shop a great deal cleaner.

Although no codes specifically address prep stations presently,
they have been proposed and will, when finally in print, provide
guidelines to manufacturers and users. Until then, you should
be aware that installing and using a prep station for spraying
may elicit some questions from the electrical inspector or fire
chief. At the moment, the existing codes defining open-shop spraying
are in effect; the addition of a new prep station won’t change
these requirements. And remember, the station itself is another
electrical appliance in the spray area and must be engineered
to comply with the codes.

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I’ve heard a prep station described as a spraybooth without walls,
though some do come with curtains. With or without curtains, prep
stations are a very useful addition to the shop. They’re an extremely
flexible and productive part of the refinishing process. Although
manufacturers don’t actually recommend it, I’ve seen three cars
on a single prep station with just the necessary repair areas
exposed to the airflow for priming.

When a prep station is used as it’s designed to be, a vehicle
can be fully prepared, primed, basecoated, cleaned and, using
a mobile or track-mounted heater, cured without ever moving. Apart
from the obvious code issues, using a prep station this way will
confine you to smaller repairs because the air supply isn’t guaranteed
to keep out all contamination.

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In an ideal shop, the ratio of prep stations to spraybooths should
be a minimum of 2 to 1; sometimes three or four prep stations
to one spraybooth is more adequate. Some manufacturers even make
double stations with one large air-makeup unit. For the most part,
manufacturers supply dry-filter units, but water-wash prep stations
are available if required.

One word of caution: A prep station isn’t designed to replace
a spraybooth, and the codes specifically won’t allow it.

Q: How should I choose a supplier for my spraybooth
or prep station?

A: Whichever distributor you choose, it’s important to
be sure you receive quality support before the sale takes place,
through delivery and after installation. This should including
product training. Even a reputable manufacturer has good distributors
and better distributors. You may find that you’ve made a preliminary
decision about the booth you’d like to own, only to find the manufacturer’s
representative isn’t able to provide the other services you need.
Spraybooths are expensive and quite sophisticated machines, so
it’s vitally important you can get prompt service when you need
it.

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Q: Exactly what kind of service should a distributor
provide?

A: A distributor should provide information so you’re able
to determine exactly which system and features you want. At some
point, this distributor or salesperson will also need to provide
drawings for contractors to prepare the site for installation.
All manufacturers have standard booth plans, but these must be
turned into working plans for your specific site. These plans
will be used by the contractor and may need to go to the building
department to obtain permits.

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Your distributor should be able to provide a turnkey installation
in which he would be responsible for all the site preparation
and permits, and you simply sign – and hand over – a check. Whichever
method of installation you choose, you should be provided with
a full set of technical specifications or site-preparation requirements,
including blue prints, for the equipment.

I also suggest holding a contractors meeting and bringing the
distributor in to present the specifications and plans, as well
as to answer questions. During the meeting, make sure the booth
distributor asks each contractor to sign a copy of the site-preparation
requirements to show that he fully understands what he’s being
asked to provide. You should also keep copies of these signed
statements. It’s amazing how much more attentive people are when
they know they have to sign a document and take responsibility.

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Today, it seems that a good spraybooth distributor must be part
planning consultant and architect, part contractor and part paint
technician, as well as be knowledgeable about the manufacturer’s
product line. During preinstallation planning, your distributor
should be able to arrange for assistance with shop design and
layout. He should also be well-versed in the code requirements
governing booth installation.

Making a Sound Decision

If you’re thinking about purchasing a new downdraft spraybooth
or prep station, make sure you know exactly what you want – and
need – before making any final decisions. Downdraft spraybooths
come with a wide array of optional features and in a wide range
of prices.

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But without a thorough understanding and knowledge of equipment
technology, you could invest in features not suited for your shop’s
needs. With the right knowledge, however, you can be sure the
money you spend on new equipment will be money well spent.

Writer Dick Bannister has more than 20 years experience in
spraybooth installation and engineering, and was a principal member
of the Committe on Finishing Processes NFPA 33.

Maintaining Balance


One of the two basic requirements of a spraybooth is to provide
clean paint finishes. One cause of dirt in the paint is maintaining
the cabin pressure at too high a level. In fact, the most efficient
way to operate a system is to keep the cabin pressure at zero
rather than at a slight positive pressure.
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It’s long been thought that positive pressure would prevent dust
from entering the booth through cracks in the doors and contaminating
the paint finish. In reality, this is unlikely. If dust particles
were to enter the booth, they would still have to break through
the fast moving layer of downdraft air enveloping and protecting
a car in a downdraft spraybooth. While observing a training class
one day, I witnessed a true test of this fact. During the class,
a group of students was standing in the open doorway of a spraybooth
watching a painter spray a car. With the doors open, the booth
pressure had to be perfectly balanced. To no surprise, the paint
job was as clean as it would have been had the doors been closed.

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Your spraybooth should take in exactly the same amount of air
as it exhausts. This is referred to as balanced pressure. You
should be able to maintain a balanced-pressure system, regardless
of the type or condition of your exhaust or intake filters.

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