Electroplating Fume Suction System Design

[Mechaniac]

I've recently been put in- charge of reviving a Fume Suction System installed at the electroplating section of factory. After my stint in the army my engineering skills leave a lot to be desired, even though i am a Mechanical Engineer (by education not profession) so it is my request to PFers to help me out.

The Electroplating Tanks are of dimensions 1510mmX750mm on top, they are meant for electroplating with chromium, lead, nickel and various other toxic chemicals that emit highly toxic fumes when the dc current is switched on. The shop floor has been arranged in two parallel section with nickel and allied tanks numbering 1 to 4 on one side(Side A). Chromium, lead and other tanks numbering a 1 to 7 on the other side(Side B). Im attaching a layout and a few pics.

On Side A On side A, a common duct of dia 300mm is fitted in such a way that it runs paralled to the breadth of all four tanks(for about 35 Metres) and it is at height of about 3 metres from ground. Smaller ducts descend from the common duct in pairs and are fitted to the sides of the tanks along it's length. These smaller ducts have slits in them for suction. The bigger common duct is connected at it's terminal end to an Induced Draft fan for suction, driven by a 10 HP motor.

Side B Side B is similar to side A, only difference is that it has Seven tanks and the common duct has a telecscopic dia with a dia of about 450 mm for two tanks and from thereon 300 mm dia for the remaining 5 tanks. Induced draft at it's terminal end is driven by a 15 HP motor.

Now the exhaust of both Sides A and B is connected to a common duct which then goes into a Fume Scrubber.

The problem is that inspite of powerful motors for the suction, the suction produced at the tanks is very miniscule.
i) How can I Calculate the amount of suction I need to produce at each tank powerful enough to suck all the air and fumes surrounding the tanks.
ii) How do i measure the amount of suction actually being produced at the tanks right now.
iii) I need to understand weather the system designed and in place right now has been correctly designed or it just a stab in the dark.

 

[SteamKing]

To do a technical analysis, you would need to make measurements of the piping system and dig up performance data on the fans you are using. For example, your main fan is labeled 'capacity 10000 cu.m.' What does this mean? Is this the capacity for an hour, a day, a week? The two smaller fans are labeled 10 HP and 15 HP, but who knows what their capacities might be. For all of these fans, you would need to locate their performance curves in order to analyze the system.

You say this system is being 'revived'. Does this mean it has lain idle for a long period? If so, has anyone inspected the inside of the lines to determine if there are no obstructions? That might be why there is insufficient draft being produced by the fans. Make sure the lines are clear and that all valves and dampers are functioning properly.

You can poke and prod this problem, but I think you should consult with a mechanical engineer to inspect the system, test it, and let him make recommendations about getting it up and running. I think it would be quicker and cheaper in the long run. You don't need an inspection from OSHA, the EPA or some other workplace safety group shutting down your plant because this system isn't operating.

 

[Mechaniac]

@SteamKing
Hey my fault, the diagram has been labelled incorrectly, the main fan you mentioned is actually the scrubber system. The 10 Hp and 15 HP fans are the only ones. And their capacities are slightly more than 10,000 CFM and 17500 CFM respectively.

I got hold of some design calculations that were used to design the system.

Size of the electrolyte tanks = 1510mmX750mm(i.e 4.9ft X 2.5ft )

(Now from some table), minimum ventilation rate required for a tank 2.5 ft in width is 500 CFM per FOOT of length of the tank.

Therefore for our tanks that are 4.9 ft ~ 5 ft in length Ventilation Rate (Vt)= 5x500= 2500 CFM per electrolyte tank.

Therefore Amount of air to be exhausted from Side A (with 4 tanks)= 2500 X4= 10000 CFM
Amount of air to be exhausted from Side B (7 Tanks) = 2500 X7 = 17,500 CFM

Total Amount of Ventialtion Required = 27500 CFM = 46,610 M^3/Hr

 

[Mechaniac]

When i say the system is being revived i meant, that the contractor who has installed this system has not ensured proper functioning even though the system was installed fairly recently. So I've been put in charge of making it work and calling guy from outside is not an option as the obtaining the requisite clearance is a long drawn out procedure. I'm ready to slog it out and learn, time is not an issue here, just need to know what all needs to be done.

The diagram contains the measurements of a few aspects of the system like diameter of the circular HDPE duct pipes(indicated as ∅)the ducts, and it's length(35 M), the width of the slots on the electrolyte tanks are 1.25 inches thick and 1 Ft feet wide(3 slots on either sides of the tanks as despicted in the diagram).

(1) If you could mention what all other measurements are required i can make them and i'll plot it on the diagram again.
(2) And how do i go about plotting the performance curves of the ID Fans ? I need to be pointed in the right direction.

Thanks for the comments so far.

Regards

 

[SteamKing]

If you had a contract with the vendor to install this system, you need to enforce its terms, especially before any money changes hands. If someone installs something for me and it doesn't work, it's not up to me to fix it. They either make it right or they can come and remove it That's crazy and the management of your firm should realize that. And if you don't have the necessary experience and qualifications to work with ventilation systems, it should not be your responsibility to fix it either. Have you considered what could happen to you if you can't fix it, or worse, your fixes cause the system to fail down the road? What if someone gets hurt and claims that their injury was due to some malfunction of the ventilation system? I see little upside to your position and a lot of potential downside.

 

[Mechaniac]

@Steamking your concerns are justified,
but i don't really intend to carryout any major repairs myself, but what i do intend to do is analyze the system, find out what is wrong with it and report it. I want to anaylse it on papers and if possible learn to do it on a computer. I want to see Whether the current design in itself is sufficient to produce the suction required, if the hardware installed is of the correct configuration, whether any additions are required.

or rather i should restate my problem: If i had to design such a system from scratch what would be the various consideration i would have to take into account, how would i go about designing it. What are the various calculations i would have to perform.

Guess

 

[SteamKing]

If you've received received education as a ME, then you should have been exposed to basic flow of fluids. Bernoulli's equation, Darcy-Weisbach, pressure drop, and all that. That's what is needed to to analyze your system. Start by familiarizing yourself with all of that again. You'll also need to review the fan laws so that you can understand how the blowers move air around the system. I'm not a ventilation professional, so I can't advise you on the equipment you'll need and the tests you should perform. I still say your company is penny wise and pound foolish for not engaging a professional to study this system and make recommendations.

 

[Mechaniac]

Im using a vane probe anemometer at various points on the electroplating baths to determine the face suction velocities at the vents. Attaching the readings here.

I'm just going to think aloud and bounce ideas off the thread.