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

  1. #16
    Join Date
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    Quote Originally Posted by Matt McCoy View Post
    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.
    Shenhui 1440x850, 130 Watt Reci Z6
    Gerber Sabre 408

  2. #17
    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.

  3. #18
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    Quote Originally Posted by Matt McCoy View Post
    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.
    Last edited by Rich Harman; 07-01-2018 at 4:22 AM.
    Shenhui 1440x850, 130 Watt Reci Z6
    Gerber Sabre 408

  4. #19
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    Seems like a lot of work to save a nickel a day. If you had 5 machines it might be worth it, maybe.

  5. #20
    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...

  6. #21
    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.
    Last edited by Bert McMahan; 07-12-2018 at 11:49 AM.

  7. #22
    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.

  8. #23
    Join Date
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    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.
    Last edited by Bill George; 07-12-2018 at 1:07 PM.
    Retired Guy- Central Iowa.HVAC/R , Cloudray Galvo Fiber , -Windows 10

  9. #24
    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.

  10. #25
    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...
    w3.JPG
    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...

    --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.
    ========================================
    ELEVEN - rotary cutter tool machines
    FOUR - CO2 lasers
    THREE- make that FOUR now - fiber lasers
    ONE - vinyl cutter
    CASmate, Corel, Gravostyle


  11. #26
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    Quote Originally Posted by Bert McMahan View Post
    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.
    Retired Guy- Central Iowa.HVAC/R , Cloudray Galvo Fiber , -Windows 10

  12. #27
    Join Date
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    Quote Originally Posted by Kev Williams View Post
    ...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.
    Shenhui 1440x850, 130 Watt Reci Z6
    Gerber Sabre 408

  13. #28
    Quote Originally Posted by Rich Harman View Post
    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!)
    ========================================
    ELEVEN - rotary cutter tool machines
    FOUR - CO2 lasers
    THREE- make that FOUR now - fiber lasers
    ONE - vinyl cutter
    CASmate, Corel, Gravostyle


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