Has anyone piped in outside air to use for the exhaust inlet?

[Bert McMahan]

Pretty much the title. I run my laser in an air conditioned area, and though it's only hobby use of a few hours per week I really hate the idea that I'm pumping out all of my air conditioned air when I have my exhaust going.

Has anyone ever ran a second line in from outdoors to provide the "inlet" air to their laser, so they're cycling outdoor air through their machine? It seems like the tube itself would be fine since it's being temperature regulated by the chiller. Of course if it was SUPER cold outside I'd run the risk of freezing things, but other than that... any downsides?

[Doug Fisher]

On my laser, there are so many ways air can enter the machine through gaps around many of the doors on the machine. Additionally, to get good crossflow across the work surface you want air coming in through the front vents on the door/lid of the machine. I have a seen people build systems that direct flow via tubes and snorkles/scoops that attach to the front of the machine's lid but they are a bit awkward in terms of space needed and use. If you have your laser in a room that can be closed off/isolated AND has a window, cracking the window open while shutting the HVAC vents off is a viable option if the weather isn't too extreme but you need to make sure you are not sucking just-vented exhaust. I mention this because many of us have inserts that fit into our windows that mount the exhaust outlet right out the other side.

[Rich Harman]

If you are exhausting air outdoors then you are already, by necessity, bringing in new air from outside. If that wasn't happening you would implode the room you were working in.

Might as well bring that new air into the room right next to the laser so that as much as possible gets exhausted back outdoors through the machine. I don't think it would be worth the trouble to make all of the outside air go through the laser, but you should be able to direct the make-up air so that a large percentage of it gets pulled through the machine, and that might be worth the trouble.

[Keith Downing]

As the others mentioned, my concern would be that it's important to pull a vacuum on the laser with the exhaust motor (suction); so that you are removing as much of the smoke and fumes as possible from the machine as it runs. Improperly placed air inlets could completely negate that effect; and even properly placed inlets might have undesired effects.

In addition, I don't know what climate you live in, but here it's very hot and humid this time of year. I would not want massive amounts of hot/humid air coming directly into my machine unchecked. In fact I want the opposite: air with little humidity and with a very minimal difference in air temperature to avoid buildup of moisture in the machine or on the work.

[Bert McMahan]

Keith, it's hot/humid where I am too. Fortunately for me I do mostly vector cutting so my workpieces shouldn't care anything about heat or humidity.

Hopefully I can do some fancy ducting with my new shop. Right now I'm going through a wall so I know I'm not getting any exhaust pulled back in anywhere, but I wouldn't be able to put in any other pipes.

I am imagining the hood of the laser having its own duct attached. My laser has a homemade frame, and I'm thinking about adding an inlet to the v2 design so I can decide where my makeup air comes from. It would certainly put more strain on my blower though, so I may end up needing to add more power.

[Kev Williams]

Best way I can think of is what Doug suggested, put the laser in its own room with positive in and out ventilation...

However, the 'hot & humid' thing you speak of will become your X factor, because your chiller will have to work harder to keep the water cool...

The air replacing your blower's output will likely take less extra energy for your AC to cool than the chiller will use to keep the now-warmer water cool.

Net benefit: IMO, somewhere between not worth the trouble and less than zero ;)

[Jerome Stanek]

Years ago the builder I worked for used to make a fresh air plenum for the furnace so it wouln't draw heated air from the house. worked very well as the furnace was in its own closet.

[John Lifer]

There is a guy I think on the RDworks facebook that did just what you are talking about. Ran a second duct from window to the side of the laser. I will probably do the same before the winter. My machine has a door on the side that would work well. And I could get a side draft going. It wouldn't be nearly 100%, but I think I could close off more gaps and get 60 to 75% reduction in heat loss. And it makes a difference in my basement as I have limited heat.

Secondarily, if I'm using my Wood burning fireplace, I get a backdraft if I'm running several hours at a time unless I open a window.

[Rich Harman]

... because your chiller will have to work harder to keep the water cool...

The air replacing your blower's output will likely take less extra energy for your AC to cool than the chiller will use to keep the now-warmer water cool.

The chiller would still be operating in the air conditioned room, the warmer make-up air would be moving through the laser, not the chiller. Net effect - room stays cooler, chiller works no harder at cooling the tube. In winter time the extra work required for the chiller to cool the tube results in more heat put into the room, so it is still a win.

[Nick Hale]

I thought of this exact thing. It's a 110°F outside and I am pumping all that nice 70°F air out and bringing hot air in. I wasn't sure how I would plumb it in. I'm not quite ready to put an 8 in inlet in the side of my laser. Lol

[John Lifer]

I don't worry about it in the summer, but like I mentioned as secondary affect is that you are creating a room (building) with a negative pressure as compared to outside. Air is trying to come in to equalize. It comes back in through my chimney. Not a good thing. It is slow enough when laser is off not to cause issue, but at 300-400 cfms exhausting, I can really pull smoke back in. UNLESS I open window. And that is cold air.

And your equations are faulty. Heat generated by laser that is removed from the tube by the chiller is NOT equal to the heat removed by exhausting warm air (or cold air in the summer) to the outside. Sorry, Apples and ORANGES.

WAY more BTUs are going out. 80 watt laser should be dumping about 272 BTUs per hour, just say it is 10 times that or 2700 BTUs per hour. If I'm bringing in 32 degree F air and wanting 72, that is 40 degree difference, and 300 cfm exhaust dumps roughly 324 BTU per hour per degree out the door. 324*40 = 12000 BTUs going out. (yes, last calculation is not entirely accurate, BTU to CFM is estimate. No direct conversion. But 12000 vs 2700 is a big difference, say even half that.... and I already said heat by laser x 10. Well worth controlling in my opinion.

[Bill George]

In the commercial HVAC world its called Make Up Air. Restaurants with high volume exhaust fans like over the grill and oven area dump the MUA right near the fan intakes within a few feet anyway. That way usually not much conditioned air gets removed from the area. Some restaurants try to use AC for the comfort of the kitchen workers.... pretty expensive.
Usually its 20% added to the Btu's handled by the compressor, and that 20% is the heat of compression. Even air moved thru a fan has some heat added from compression.

[Kev Williams]

My above post was assuming just using outside air vented into a dedicated room with the laser... for those who've never used a simple 3000 air-to-water chiller, you would be very surprised at how fast an 80w tube will raise the water temp 5°c, and warmer ambient air temps just add to the temp rise, which will make a refer chiller work harder. How much harder? Hell, I don't know, how efficient are refer chillers actually?

But that's all moot if you're only adding air to the laser cabinet. So then, to move outside air ONLY into the cabinet is going to require fabrication of a plenum to effectively enclose the machine's air intake AND transitions into a inlet to connect your duct work. Assuming doing this to my machines, my LS900 the plenum would take up the entire bottom of the lower door (IF it was still on ;) ), for my GCC it would cover most of the 10" x 36" front door, and for my Triumph it would be the 1-1/2" x 52" pass-thru slot on the back of the machine. Any such plenum would just be in my way...

And when a laser is removing inside air, the air replacing it comes from outside at whatever its temp is. Doesn't matter from where... :)

And I don't need BTU figures to assume cost savings. Since I made the garage a workshop, it was easy to track how much extra it cost me to heat the garage: About $30 per month. A whole $1 per day. And this is a poorly insulated garage at best. Haven't tracked my cooling bills because I changed out my AC unit shortly after the garage became a shop, but I do know my cooling unit's power usage: The portable cooler is 10.8 amps during refer use, deduct for cycling lets say 1kw per hour, and it's on maybe 12 hours a day. My swamp cooler adds 300watts. So 1.3kw x 12 hours x 8c per kw comes to: about $1.28 per day. This includes the fact that I work over 15 hours a day 7 days a week, there's 2 blowers in the garage running probably 12 hours each per day, plus the other 2 blowers for the basement machines that also run several hours a day (the house and garage share the same air space)--bear in mind these aren't the cost differences between running blowers and not running blowers, this is the complete total cost to heat or cool the garage space! The extra cost associated with running the blowers is only a small portion of the total...

Therefore- given that any of my lasers (as should everyone's :D ) generates over $1 per minute in revenue when it's running, this means that the first laser that I fire up each day will pay for the total cost of heating or cooling the garage within the first minute it runs. To pay for the extra cost of re-heating or re-cooling blower air probably takes 5 seconds...

Not much incentive to double up the vent plumbing. And if nothing else, this place is ventilating well! I'm definitely not breathing stagnate indoor air!


.

[Matt McCoy]

My thought is that the higher temp/humidity on the internals of the cabinet (e.g., gantry, optics, electronics, tube, material) might be a penalty against any HVAC efficiency.

[Rich Harman]

My above post was assuming just using outside air vented into a dedicated room with the laser...

Outside air will find a way into the room wether or not you have a dedicated vent for it. Wether you bring in the outside air through one opening or it comes through hundreds of cracks - the effect is the same - the volume of air coming in is the same, it just gets mixed with the room air before it reaches the laser. The cost of cooling or warming that air will be the same regardless of how it is brought into the room. However, if you can redirect some or all of it right back outside via the laser then you do not have to expend extra energy warming or cooling it.

[Rich Harman]

My thought is that the higher temp/humidity on the internals of the cabinet (e.g., gantry, optics, electronics, tube, material) might be a penalty against any HVAC efficiency.

I doubt this is really much of an issue. We have members here using lasers in all sorts of environments, including hot, high humidity. Humidity should not be that important, wether or not it is condensing is. When first starting up for the day you would want to run the blower for enough time to stabilize the temperature of the machine, after that you should be good to go.

There are probably some extreme cases where you would have to take extra steps, like not using the outside air system for the day, but for the vast majority it would work out fine.

*note that I have Chinese machines in mind for all these opinions. Western style machines with air cooled tubes may be more sensitive, or at the minimum piping in outside air would probably void a warranty.

[Matt McCoy]

Humidity is one of the ingredients for condensation on a glass tube, so maybe a consideration for some. There is no free lunch, so the possibility of reduced efficiency elsewhere against any potential HVAC efficiency should be weighed, IMHO.

[Rich Harman]

Humidity is one of the ingredients for condensation on a glass tube, so maybe a consideration for some. There is no free lunch, so the possibility of reduced efficiency elsewhere against any potential HVAC efficiency should be weighed, IMHO.

Sure, but in a machine like mine (I assume many are similar) the make up air would not be passing over the tube so it would be unaffected.

Also, it is not a free lunch. In order to get the benefit you need to acquire the materials and put in the effort of installing it.

[Glenn Norton]

Seems like a lot of work to save a nickel a day. If you had 5 machines it might be worth it, maybe.

[Bert McMahan]

Cost isn't the issue, it's physical capability. Your air conditioner can't keep up with a 400 CFM blower dumping hot, humid air into your room continuously, so the room will heat up and your AC will run constantly. I'll do the math here in a minute, be back soon...

[Bert McMahan]

I ran some numbers. I'm no A/C engineer but I think my reasoning is sound. Assuming you live in a hot/humid area (outdoor temp 90 F, 70% RH) and your shop is at normal indoor levels (70 F, 45% RH), I calculated the power needed to cool 400 CFM of outdoor air to indoor comfort levels in real time.

I can share my calcs if you want, but the short version is this: you'd need a 38,000 BTU air conditioner (or around 3.2 tons) to handle the cooling needs. That's literally the entire capacity of my entire house's air conditioner (I have a 3.5 ton unit) just to cool the incoming air.

Cost wise I calculated a conservative $0.34 an hour- but I'd have to purchase an entire house sized dedicated air conditioner to keep up with my exhaust blower.

[Bert McMahan]

Boring calculation details:

-I calculated the energy required to convert dry air from 90 to 70 at 400 CFM. Total: 2.69 kW
-I calculated the total amount of water in 90F/70% RH.
-All of this water had to be cooled to room temperature. Cooling water vapor down to room temp requires 0.2 kW
-I calculated the amount of water that must be condensed out of the air to change the RH from 90/70 to 70/45. This is where the real killer was, it takes 8.35 kW to get all that water out of the air.

If you live in a dry climate the numbers will be FAR better for you.

[Bill George]

400 cfm is roughly about 12,000 Btu's of AC or one ton. Would take about a 1 Hp unit, again rough numbers. Pipe the MUA directly to the air intake Area of the machine, do not let it mix with the room air IF you can. Have a motorized damper open the fresh air only when the Exhaust fan is running. This is from the commercial HVAC world.

[Bert McMahan]

Bill, is that 400 CFM the rating of the internal blower fan for the unit? Because if so, that would be what it's cycling through- not what it can cool to 70C in one "pass" through the system. I doubt that you'd be able to cool 90F, high humidity air from the outside at that rate, but I could be wrong. And again, the real killer isn't the air it's the humidity; you have to condense out just under 30 pounds of water an hour, which takes far more energy than simply cooling the air.

If you were at 90 F somewhere with very low outdoor humidity you could do it with a one ton unit easily.

[Kev Williams]

In the 'for what it's worth' department-

for 4 years I cooled my 1390 Triumph with a 3000 'radiator'. In the summer when I can't keep the garage temp below 80 even with a 13kbtu refer AND an evap cooler, I would cool the laser water using adding a 20' coil of copper pipe sitting in ice water...
389522
It works, but I finally broke down and bought a nice 5200 chiller.

and guess what? It works great! -- at doing TWO things:

1- keeping the water almost exactly 20°c at all times, and

2- it's wonderful for heating up the garage... http://www.engraver1.com/gifs/banghead.gif

--not exactly what I need in the summer!

Whereas I used to just leave the 3000 running, I HAVE to turn OFF the 5200 when I'm not using it, because when I don't, it'll raise the garage temp from 80° to 85° within a half hour, while my 2 AC's are running. And the hotter the garage gets, the more the 5200 runs even in 'idle' mode. I have a digital thermometer and have verified the temp rise- and it falling after shutdown- several times.

Ergo, even if I prudently shut the thing off when not using it, it IS heating up the garage when I AM using it. So IMO any outside air that I would pipe to my machine to counteract the blower sucking cool air out of the room will be offset by the chiller heating the air. My recommendation, and I am going to do this: fab up some plumbing that will exit the 5200's hot air outside. :)

[Bill George]

Bill, is that 400 CFM the rating of the internal blower fan for the unit? Because if so, that would be what it's cycling through- not what it can cool to 70C in one "pass" through the system. I doubt that you'd be able to cool 90F, high humidity air from the outside at that rate, but I could be wrong. And again, the real killer isn't the air it's the humidity; you have to condense out just under 30 pounds of water an hour, which takes far more energy than simply cooling the air.

If you were at 90 F somewhere with very low outdoor humidity you could do it with a one ton unit easily.

We size blowers at 400 cfm per ton of AC, 3 ton or 36,000 Btu's will need 1200 Cfm of air moved. Rough numbers and I do not have the time to do calculations. So if you remove 400 Cfm of conditioned room air you will need what I posted above. One Hp is about 800 watts give or take, we pay .09 cent per KwH. It costs to remove air from a building. I have been in commercial buildings that use 100% outside air cooled from 90 to 70 degrees in one pass.

[Rich Harman]

...So IMO any outside air that I would pipe to my machine to counteract the blower sucking cool air out of the room will be offset by the chiller heating the air.

I don't understand your reasoning. The chiller is removing heat from the water/laser tube and adding it to the room regardless if you pipe in make up air or not. Piping in make up air so that it reduces the amount of cooled air you blow outside (room stays cooler) means that your chiller will be adding less heat to the room, not more.

Piping in make up air to the chiller and blowing that outside will also keep the room cooler.

[Kev Williams]

I don't understand your reasoning. The chiller is removing heat from the water/laser tube and adding it to the room regardless if you pipe in make up air or not.

Exactly...

Piping in make up air so that it reduces the amount of cooled air you blow outside (room stays cooler) means that your chiller will be adding less heat to the room, not more.

Not Exactly, because the room doesn't stay cooler, it stays the same temp: the goal is to make the AC work less to KEEP it the same temp. The piped in air is for the laser proper, so that all blower air used is cycled/recycled from outside air only, resulting in a zero net loss of room air due the blower. This is good, but, the heat the chiller is adding to the room is likely to increase rather than decrease, because the warmer outside air WILL heat the laser cabinet slightly, raising the cabinet's ambient air temp, which includes the air surrounding the laser tube, and in turn the water in the laser tube; warmer water = chiller works harder.

Piping in make up air to the chiller and blowing that outside will also keep the room cooler.

You don't need or want to pipe in air to the chiller, you only want to pipe OUT the chiller's hot air.


I have never seen a noticeable temperature rise, or drop in the winter months, in the shop when I fire up one or both blowers due to the outside air coming in...
But I HAVE seen a significant temp rise whenever the chiller and laser is running! Which tells me my chiller is going to make my AC work harder than the blowers will! And if I were to run some new vent plumbing, FROM the chiller's hot air vents TO the laser cabinet to let the blower grab it and push it outside seems the most logical :)

(but I'll welcome that warm air come November!) :D