UV Light box fan placement?

[Passage]

I'm building a UV light box (tube type) using eight 24" BL tubes to make 8X10 pt/pd prints. I've gleened construction instructions from several sources, however, all directions and photos of the boxes are a little vague on fan placement. Some appear centered in a way that would cool both above and below the shelf supporting the ballasts. Others seem to be blowing across the tubes only. Can ya help a guy here? Thanks

 

[Donald Miller]

The greatest source of heat and the most prone to heat induced failure is the ballasts.

 

[Allen Friday]

On my Edwards Engineering Light Box, the fan is centered to draw heat from the bulbs and the ballasts. When I build my own, larger light box, I also centered the fans. The one statement that you made which bothers me, however, is "Others seem to be blowing across the tubes only." My fans are set to draw the air out of the box, not to blow air across the tubes. If I place my hand next to the fan on the outside of the box, I can feel the air flowing out, not in.

 

[sanking]

On my Edwards Engineering Light Box, the fan is centered to draw heat from the bulbs and the ballasts. When I build my own, larger light box, I also centered the fans. The one statement that you made which bothers me, however, is "Others seem to be blowing across the tubes only." My fans are set to draw the air out of the box, not to blow air across the tubes. If I place my hand next to the fan on the outside of the box, I can feel the air flowing out, not in.

I don't believe it matters whether the fan pulls the air out of the box or blows it in. Either way you will get the air exchange, because if you blow it in it has to come out.

My unit of twelve 48" BL tubes has one 4" diameter fan on the side that blows air over the tubes. I know that this is efficient because I expose with a light integrator and the units of exposure remain constant in terms of one second = one unit, even with very long exposures of 15-30 minutes. If the system were not working the tubes would over-heat and the units would b longer.

Sandy King

 

[bruce terry]

I don't believe it matters whether the fan pulls the air out of the box or blows it in. Either way you will get the air exchange, because if you blow it in it has to come out.

Yeah, air is air I suspect. The fan mounted on the flank of my (homemade to Edwards specs) EELB w/12 24" tubes blows into the box and out the opposing vent cut-outs. Bottom quarter of the 4" blade-arc cools across the tubes, second quarter pretty-much wasted against the 3/4 plywood mounting platform, top 50% cooling the six hotter ballasts. Never a problem even running for hours. Superfluous info, just adding to what's been said.

Bruce

 

[Allen Friday]

Thanks for the info Sandy and Bruce. I didn't know if it would make a difference or not. I was wondering if a fan blowing directly on two or four of the tubes would cause some kind of temperature fluctuation that might lead to a difference in the output of the bulbs. Apparantly that is not the case.

Allen

 

[donbga]

Thanks for the info Sandy and Bruce. I didn't know if it would make a difference or not. I was wondering if a fan blowing directly on two or four of the tubes would cause some kind of temperature fluctuation that might lead to a difference in the output of the bulbs. Apparantly that is not the case.

Allen

I've made several 12 tube UV units and I've set the exhaust fan up so that it draws air across the tubes. The sides are ventilated to allow cool air to enter through the sides of the exposure unit.

I also ventilate the top compartment that contains the ballasts, although the air vents there are passive but large.

 

[Jan Pietrzak]

UV light box

I have not put fans in any of the ones I have built, but the top of the box is covered with peg board, so that the ballast area vents out the top.

Part 2 is that my print drying screens sit on top of the box, starting at 4"s up prints dry in about 30min from the riseing heat of the box. Like AB says it a multi tasker.

Jan Pietrzak

I've made several 12 tube UV units and I've set the exhaust fan up so that it draws air across the tubes. The sides are ventilated to allow cool air to enter through the sides of the exposure unit.

I also ventilate the top compartment that contains the ballasts, although the air vents there are passive but large.

 

[bobherbst]

Fan Cooling of Fluorescent Tube Light Sources

OK, the engineering side of me can't pass up this one up. Take a look at Jon Edwards light source plans on his web site for an excellent deisgn example - www.eepjon.com. I've been using his units in workshops and classes at local universities for 5 years. The single box fan in the 11x14 unit draws air over both the tubes and ballasts through holes cut in the opposite side. The internal shelf to which both the ballasts and the lamps are fastened creates a baffle separating the two chambers on three sides. The fourth side is open from top to bottom which is where the box fan resides thereby allowing air draw through both top and bottom chambers. The top chamber has three holes and the lamp chamber has 5 holes giving a ratioed air flow between them. For this design to provide proper ventilation across both tubes and ballasts, you do need the hinged flip down door at the front where you insert the contact printing frame. By closing the door, the chambers are sealed and air flows across the box from side to side cooling all of the components. If no door is used or it is left open, the open door becomes the primary inflow bypassing the tubes and ballasts.

The ballasts should be ventilated in addition to the tubes. Sandy is correct that by giving good ventilation across the tubes, variation in light output is reduced by keeping the tubes cooled. But a ballast will fail prematurely if it is not also cooled. The operating life of the ballast will be extended if it is also ventilated. I built a large light source years ago for the University of Akron. Ventilation was by convection only through holes drilled in the plywood top. One of ther ballasts failed 3 weeks into the class due to excessive heat build-up over the day.

As far as push vs pull, drawing air into the box through vent holes is a better design than pushing air into the box. I learned this from engineers who design industrial control equipment for 0-60 drgree C environments. Virtually all industrial control cabinets draw air into them and exhaust the warm air. With properly placed inlet holes, this design provides better air flow and fewer eddy currents inside the box. As an aside, ever heard of blowing air into a darkroom for ventilation rather than drawing it out? Use Jon's design as a guideline in your construction.

Bob

 

[Rob Vinnedge]

I have an Edwards print drying box that pushes air into the box rather than pulls it. It seems to work pretty well. Also, having built five darkrooms, each with a different ventilation system, I have found that pushing air into the room and adjusting the passive exit vents maintains a positive pressure in the room and prevents dust from entering - by far the best plan I've been able to come up with.

OK, the engineering side of me can't pass up this one up. Take a look at Jon Edwards light source plans on his web site for an excellent deisgn example - www.eepjon.com. I've been using his units in workshops and classes at local universities for 5 years. The single box fan in the 11x14 unit draws air over both the tubes and ballasts through holes cut in the opposite side. The internal shelf to which both the ballasts and the lamps are fastened creates a baffle separating the two chambers on three sides. The fourth side is open from top to bottom which is where the box fan resides thereby allowing air draw through both top and bottom chambers. The top chamber has three holes and the lamp chamber has 5 holes giving a ratioed air flow between them. For this design to provide proper ventilation across both tubes and ballasts, you do need the hinged flip down door at the front where you insert the contact printing frame. By closing the door, the chambers are sealed and air flows across the box from side to side cooling all of the components. If no door is used or it is left open, the open door becomes the primary inflow bypassing the tubes and ballasts.

The ballasts should be ventilated in addition to the tubes. Sandy is correct that by giving good ventilation across the tubes, variation in light output is reduced by keeping the tubes cooled. But a ballast will fail prematurely if it is not also cooled. The operating life of the ballast will be extended if it is also ventilated. I built a large light source years ago for the University of Akron. Ventilation was by convection only through holes drilled in the plywood top. One of ther ballasts failed 3 weeks into the class due to excessive heat build-up over the day.

As far as push vs pull, drawing air into the box through vent holes is a better design than pushing air into the box. I learned this from engineers who design industrial control equipment for 0-60 drgree C environments. Virtually all industrial control cabinets draw air into them and exhaust the warm air. With properly placed inlet holes, this design provides better air flow and fewer eddy currents inside the box. As an aside, ever heard of blowing air into a darkroom for ventilation rather than drawing it out? Use Jon's design as a guideline in your construction.

Bob

 

[bobherbst]

I have used Jon's print drying units in my workshops. Drying prints and cooling tubes and ballasts are two totally different functions. The purpose of a print drying box is to heat air and exhaust humidity - not give even flow of cooler air around tubes and ballasts for cooling. Air is pushed over a heating element to provide maximum efficiency of the energy used in the heating element. The air is then blown over the coated paper to dry the moist coating. Cooling electronics is a completely different matter. I'll stick with the designs proven out by hundreds of thousands of installations in manufacturing facilities around the world in industrial control applications.

If pushing air into your darkroom works for you, go with it. I'm guessing you still have some form of filtration for dust. Most professional darkrooms use negative pressure designs because the primary purpose is to draw chemical fumes away from where you are working. Jon has some pretty good designs in this area as well and have seen some pretty good designs using standard PVC pipe. Designs for clean rooms in electronics wafer manufacturing are a whole different matter and I won't even go there because the requirements are so much more stringent.

 

[Rob Vinnedge]

Bob,

Yes, I do have heavily filtered, temperature controlled air blown in from each end of my 20 ft. long darkroom by a commercial roof mounted blower unit on a rheostat. The air reaches into all corners of the room and exits through a 20" diameter duct, which is positioned 4' over the center of my 19' sink. As with Edwards vent system positioning, this system is extremely efficient at removing the fumes. The darkroom stays fresh all day (or night) even with the doors closed. And, as I mentioned, the positive air pressure has done wonders to reduce the dust.

I understand your point about the difference between the heating and cooling units.

 

[Neil Poulsen]

My lightbox has a top and bottom on either side of a horizontal 3/4" about square board. I mounted my ballasts above onto this board, and I mounted my lights below onto this board. I drilled small holes, where I needed to thread wires between the top and bottom.

I cut three holes in one side of my UV box for three larger sized computer fans. Half of each of these holes extends to the top portion of my UV souce, and the bottom half of each hole extends to the bottom portion of my uv source. That is to say, the 3x4" horizontal board cuts these three holes in the middle. I cut several holes in the opposite side of my UV source, which although smaller, also extend both into the top and bottom halves of my UV source. These smaller holes allow air into the box, so that it can be extracted on the opposite side by the fans. In this way, I get a nice, even flow of air across both the ballasts above and the lights below.

Whether or not one needs three or one fan, I'm not sure. Three certainly does the job of one. For health concerns, I covered these holes with a simple light trap design painted black on the interior, so that the escaping UV light can't be seen. Let's the air through, but doesn't let the light through.