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topic 4955

Push Pull Ventilation


(2000)

Dear Sir,

We are a Hard Chrome Plating company based in India.We intend to replace our existing Pull-Pull Ventilation system,with a Push-Pull Ventilation system.We are looking for guidance/Thumb Rules for Hood design,Capture Velocity of chrome on tank,Hood slot velocity and pressure etc for a Push-Pull Ventilation system.

PARSHVA SHAH
- Mumbai,Maharahtra,India


(2000)

Hi Parshva

Whereas the Ventilation Manual is widely used over here, it takes time before it gets finally updated and 'with it'.

The rule of thumb for a hard chrome system is around 250 CFM for each square foot of tank surface area. This varies up & down depending on the length/width ratio.

Another factor to reduce the airflow is to use floating balls or the 'Indian' floating discs. Yet another, the most popular it to use a proprietary 'coating' which floats on the top of the liquid. This prevents evaporation.

I mention these just in case your system is not working to par. For push pull I will deviate very heavily from the ventilation manual and recommend l00 CFM per square foot for the suction side. For the push side, you could get away with l-l/2 percent of this figure.

An effective push design, using this very low l-l/2 percent (I have seen it used) really does depend on the design and installation of the push system.

Consider using plastic, PVC, piping for the push side. Use unions for rotating. Design the pipe for a transport velocity of around min. 2,000 to 3,000 fpm. Drill holes, all equally spaced, for twice this speed. Use a air source having perhaps at least 2 psi pressure.

Position the pipe header's holes so that they are pointing down by max. 2 degrees. One assumes your tanks have a liquid level not less than 6" from the top. Place the pipe header into the tank and away from the disturbing airflows (or physical impact) at the top of the chrome tank.

For the push 'hood' side, the height of the slot(s), yes, slots, should be at a height of tan l0 times the tank length. This height would be for the major slot. The secondary slot to be in the normal position of at the bottom of the fumehood. Design the hoods according to Hoyle, i.e., one assumes, I assume, you'd know that the hoods should have a design slot of 2,000 fpm, that the plenum is max. l,000 fpm design. However, make both slots somewhat oversize and use adjustable slides to arrive at the 'design' of 2,000 fpm and you'll find that the top slot would have to be more open than the bottom on.

To increase capture, you must know that side drafts (doors etc) must be avoided. That for very long tanks, the hood height has to be increased (often just flat plates will do) to enhance the capture efficiency.

As far as materials of construction are concerned, have a look in the Finishing Com's library, # 7 on recommended materials. OK?

I hope this helps. It is based on simplicity, small sizing of the push system. The comment about very long tanks will probably not apply for chrome applications although I have done this for tanks where the 'push' was over a length of l0 or l2 feet and in which case the push velocity was increased to around 6,000 fpm!

Finally, may I so bold as to suggest that you should really use some type of 'scrubber'. This may be very presumptuous of me as I'm sure you already do. The least expensive and fairly efficient is just a two stage horizontal high efficiency blade type mist eliminator. Have fun! Cheers !

freeman newton portrait
Freeman Newton
White Rock, British Columbia, Canada

(It is our sad duty to
advise that Freeman passed away
April 21, 2012. R.I.P. old friend).




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