Avoid Mistakes When Specifying Spraybooths by Ron Joseph

I t is surprising how many spraybooths are inade- quately equipped, either because the vendor has not properly pointed out the advantages and dis-

advantages of a particular feature or because the customer is loath to spend the extra money to do the job right.

During recent weeks I visited two separate spray- booths, both lacking some major essentials.

SCENARIO #I

The first was a very large, enclosed cross-draft spraybooth as depicted in Figure 1. The customer wanted the option to divide the booth into two sec- tions with a moveable partition and operate each side separately and also wanted the ability of remov- ing the partition to operate the booth as a single unit. A large access door in the partition allows painters to walk from one side to the other. In fact this is the only manner in which painters can get to the left section, unless they walk all the way around the outside of the facility and access it from the outside.

One might think that this is not asking too much of the vendor until one understands the complexity of maintaining a balanced air flow.

Suppose the customer wants to run only the left side of the booth. The blower for the air makeup system must be balanced with the exhaust fan so that the maximum pressure differential across the booth (not across the filters) is 0.03 to 0.07 in. W.C., either positive or negative.

As the dry filters start to get clogged with over- spray the pressure across the filters will increase and impede the air flow through the booth. Unless the exhaust blower is fitted with a variable fre- quency drive or an automatic baffle, either of which is designed to maintain the air flow (measured in ft/min), the pressure inside the booth will become progressively more positive. Air turbulence will de- velop and overspray will tend to deposit on the walls, lights, and ceiling of the booth. The loose, dry overspray will in turn cause contamination of freshly painted surfaces, reworks, more air and

Ron Joseph is an independent coating consultant in San Jose, Calif.

Moveable Partition

Left Section Right Section

Figure 1. Cross-draft spraybooth with a movable partition.

waste pollution, and higher operating costs. This topic was dealt with in detail in one of my earlier columns.

The booth I visited had not yet been properly balanced. During my visit only small parts were being painted and it was clear that the booth was operating under a strong negative pressure. Al- though I didn’t have a manometer with me it was easy to make this determination because the inter- leading access door slammed shut every time a painter walked through. When the blower was off, the door remained steady on its hinge.

Suppose that the customer has only sufficient work to justify using the left side of the booth, every time someone opens the access door dust and dirt from the right section will rush in and spoil perfectly beautiful high-gloss paint finishes. During my visit I didn’t see a single finishing component that wasn’t marred with some dust spots-a great disappoint- ment to the painters who made every effort to turn out an automotive-quality part.

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lncomlng Air

Exhaust Air Exhaust Air

- ExhaustPlenums -

Figure 2. Semidowndraft spraybooth balanced air flow.

It is likely that when both the right and left sides of the booth are used (with the position still in place) the dynamics will change again because the two partitioned booths have not been balanced against each other. You can imagine that when the filters in one booth plug up more than in the other not only will the balance within a single partition change, but so will the balance across the two booths. Hence, when the access door is opened dust from one side will blow into the other, and this dynamic can change frequently depending on the size of the parts that are being painted in each section.

My visit didn’t extend long enough for me to witness the painting operation when the partition was removed, but I can imagine that with two huge air makeup blowers and two equally large exhaust blowers, each operating individually, maintaining a balance might be a nightmare.

Should you want to install a new booth or if you are currently experiencing difficulties with your ex- isting booth I strongly recommend that you speak to your vendor and have him/her check the air balance and tell you what needs to be done to keep the air flow constant at all times, not just when the booth is empty or the filters are brand new.

SCENARIO #2

In this scenario the customer had specified a semi- downdraft booth as shown in Figure 2. Such booths are often specified because they share some of the air flow advantages of downdraft booths such as allowing the painter to walk around a component

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without getting overspray to blow back on him/her. A major advantage is that a pit is not required under the floor and this considerably lowers the cost of the installation.

Although the booth was still under construction and I didn’t have the opportunity to evaluate it under working conditions, I made the following ob- servations:

l One blower appeared to supply air makeup to both exhaust plenums.

l There were two exhaust blowers, one for each plenum.

l Neither of the exhaust blowers was fitted with a variable frequency drive nor with an adjustable baffle.

l The compressed air plumbing leading to the booth had so many 90 and 45” bends that I lost count.

l The customer had asked the vendor to install some old regulators that had been used in a previous spraybooth.

l Neither regulator was fitted with a pressure gauge nor an air filter.

l The piping leading from the compressor to the booth went through several reductions, starting at l%in. nominal ID and ending at the booth with a G-in. quick disconnect.

Even before the customer paints a single item, I foresee the following potential problems:

l Since the exhaust blowers have fixed through- put, it is highly probable that the booth will seldom be balanced. If the painters tend to put components on one side of the booth in prefer- ence to the other the two filter banks will be- come clogged with overspray at different rates and eventually the air flow will pull predomi- nantly to one side.

l If the booth goes out of balance, as it is bound to do, turbulence will result. The consequences have already been discussed under Scenario #l.

l The spraybooth is located in a humid environ-

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90 Metal Finishing

ment; therefore, unless air filters are installed it is likely that moisture will get into the com- pressed air line and this can lead to curing problems, especially when polyurethanes are used.

l The compressed air piping layout encourages large pressure drops. Hence, it is likely that when one or more painters use HELP guns, they will not have sufficient air volume to properly atomize the high-solids coatings.

l Because of the manner in which the piping has been fitted, it is highly probable that rust from the inside of the pipes will eventually collect at

the lowest point; namely at the regulator on the spraybooth wall.

Installations of spraybooths are expensive and time consuming. Often such installations are carried out during production downtime or during an an- nual plant shutdown. In my opinion is it counter- productive to save on cost by omitting important component options, such as variable frequency drives, from the specification. When it comes to spraybooths, which will remain in operation for many years, it is worth paying a little more and doing it right the first time. MF

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