Air cutains to make walls around an exhaust hood?? 1

[lci]

I have an exhaust hood that is located over a hot coil at 1000F. The coil is a 6000lb of steel wire bundle approximately 4ft ind dia and 6ft long. The hood is to exhaust the heat from the area. Unfortunately the hood is 6 ft above the coil.

I would like to use air curtains to set up walls on three sides surronding the coil to direct the exhaust air over the coil from one direction only thus increasing the air velocity in the one side left open.

Has anyone ever used an air curtain to improve the draft into an exhaust hood?

 

[fredb]

Hello

I have added curtains of which you speak to mfg. operations in the past. One caution is that the suction you create beneath the hood is not so strong that it draws the bottom and sides of the curtains inward and interferes with your process.

 

[sameyer626]

I do not think the response to air curtains is correct. There will be nothing to draw into the airstream of the return air. Forgo the expense of those blowers and increase your fan speed or size.

 

[lci]

samayer626,

The hood is almost six feet above the coil and 12 above the working floor. To get an adequate air stream over a line worker, it is impossible due to the large exposed area. To have the exhaust air cool the line worker we would need to increase the cfm by a factor of at least ten which will increase the hood face velocity. The higher velocity may lead to other problems such as noise and vibration.

 

[fredb]

Hello Again all,

The equation for calculating the volume of air for a canopy type hood is: Q = 1.4PDV

where: Q = Exhaust volume, cfm
P = Perimeter of hood face, feet
D = Hood distance above the work, feet
V = Capture velocity, feet/min.

If Q, and D, remain the same as Ici tells us they do, then as the perimeter P of the hood is reduced by closing off three sides, then V, the capture velocity must increase significantly, just as Ici desires.

Futher, Industrial Ventilation, a Manual of Recommended Practice, 10th Ed. by the American Conference of Governmental Industrial Hygienists, tells us on page 4-17, under the title "Hood Design Data" fig. 4-14,.......Side curtains on two or three sides to create a semi-booth or booth are desirable........

 

[lci]

fredB,

Thank you for the response. Do to the nature of how the coil moves down the line, I can not phisically isolate or block three of the sides from the canopy hood to the floor. Since I can not mechanically put a wall up I am proposing to place three air curtains around the perimeter of the hood effectively leaving one side open. My first concern is, will the hood just pull in all the air from the air curtains? My belief is that the momentum force from the air curtains will be significantly higher due to the higher velocity and the draw created by air entering the hood. There will be some air from the curtain, but most of the air will enter through the open side. The actual physics to prove this is where I fall a bit short. Any reference?

 

[lilliput1]

I think air curtains will only interfere with the hood operation. Blowing makeup air towards the exhaust push-pull arrangement would help particularly if cool air is used. Probably not much because of radiant heat. Can the process be done without people? Are they suited in reflective heat protective clothing? You may want to also look into evaporation cooling. Stutz make piezoelectric humidifiers. They aggitate the water molucules to for cloud of water that absorbs heat from ambient to turn to steam. You must use RO or deionized water, unless you can accept the resultant calcium silicate residue. You also have to make sure the steam do not obstruct visibility but it is a thought anyway.

 

[HighPotential]

Why don't you introduce air vertically, underneath the coil so that it flows up and around it, to the hood? An upward curtain, if you will.