Capture Velocity For Type II Hoods 1

[supaman79]

Guys,

I'm redesigning a ventilation system for a type II hood (bread only, non-portable unit). Originally an engineering firm designed a national system for Subway that absolutely did not work, so Subway hired us to redesign it to remove the bread odor and heat generated from the equipment. The million dollar question is, what is ASHRAE's recommended CAPTURE VELOCITY for this type of application. Also NUVU Engineering (the firm that originally designed this), couldn't give me a CFM that would successfully remove the odors...so now I'm stuck with a ridiculously easy design, but know concrete data to start with. Any answers guys. 300-500 fpm capture velocity sound right, with a transport velocity @ 1,000-2000 fpm?

 

[CRG]

supaman79

Depending on the code your jurisdiction requires, there may be design minimums. Note: that the mechanical code does not guarantee that the hood you install will work; however, it does give minimum guidelines. I have designed hoods using both IMC and UMC and both codes that define the cfm by the area of the hood. Also, to ensure that a hood will work properly there must be sufficient makeup air for the space.

CRG

 

[supaman79]

Thanks CRG.....That'll give me a good start

 

[friartuck]

Hi supaman

I'm not familiar with US codes, but I do design a lot of canopies. In the UK, a velocity of 0.35m/sec-0.40m/sec would cope with this situation, but the inflow of air will also greatly assist the removal and capture of fumes. Try an induction canopy which has integral supply grilles with nozzles pointing into the canopy



Friar Tuck of Sherwood

 

[friartuck]

PS where do you get 300 to 500fpm from...thats pretty high I think. Shouldn't it be more like 30-50fpm?

I also have a theory on the ductwork velocities. For years I was told to keep them high so that the grease (if the canopy extracted from that type of process) could be carried away.

In practice, all that happens is that the grease (assuming your filtration isn't great) will simply carry over even further up the duct. This makes it more difficult to clean. If you keep the velocity lower, the carry over is limited to the first bend and a metre or so. Which makes cleaning easier.
Any one got any comments on the best duct velocities?

Friar Tuck of Sherwood

 

[supaman79]

Friartuck,

Upon further research, you are right on a couple of points:

1.) Last year in Engineering Systems Magazine they pointed out that 2,000 FPM (transport velocity) for grease exhaust systems a little to high for a number of reasons...the first being the point that you pointed out...and the second for cross drafts across the space
1,000-1,500 is suitable.

2.) As far as capture velocity....from looking into it a little bit deeper, I seen for a type II hood, non grease, 50-75 fpm is suitable

 

[CRG]

IMC (International Mechanical Code) requires not less than 1500 fpm for ducts serving a Type I hood. Original question is about Type II hood; however, comment above discusses grease exhaust systems. Life is usually simpler in this litigious world if your design meets or exceeds the minimum code requirements for your jurisdiction unless there is a compelling reason to deviate from the design standard. Note: because a design meets the minimum code requirements does not ensure that the design will meet your client’s expectation. So, I am not giving a blanket recommendation for using the codes as prescriptive design manual; instead, I only mention that 1000 fpm for Type I grease duct does not meet some code requirements.

 

[friartuck]

Supaman

Dis a bit of digging for yah. Today a rep came in from a kitchen canopy manufacturer who have installed canopies at Canary Wharf London. The strict guidelines imposed by the London Borough Council requires the use of such canopies that removes all odours. The original method was either by AAF Ltd cyclone or carbon filtration. The new method is by use of Ultra Violet 'C' (as opposed to A or B.

The units have standard filters to reomve the grease and UV to get rid of the smell.

There are a number who do these inc. Haltons, Brittania etc.

Hope this helps




Friar Tuck of Sherwood

 

[quark]

There are two excellent papers in ASHRAE Journal (Oct. 2001 and Nov. 2002 issues)on kitchen exhaust. Very difficult to condense them here but I recommend a reading of those papers.

Regards,