Air-to-air heat exchanger and venting dust outside

[Jim N Burke]

Given that venting dust outside is better than using a filter and that having a 6-inch or greater hole from the outside to for allow return air is not really practical for those of us in the north country (I spent a lot of time, effort and money to seal and insulate my house thank you very much.) I wonder if anyone here has used an air-to-air heat exchanger in this type of situation.
For anyone unfamiliar with these, they use the air being exhausted to warm the replacement air being brought in from the outside. This cuts your energy loses significantly when you need to bring in fresh air.
I suspect that there would be a lot of practical problems to overcome; the dust itself plugging up the works would be one. Another issue I could see would be the cost of one that could handle the amount of air we would need to pull through it - even small DCs are pushing 1500 cfm or more. A quick google search doesn't any prices for units this size. I expect they are beyond the means of most of us and even for the professionals the cost may be prohibitive but I thought the question worth asking.

Jim

[Jim Andrew]

A system with filter doesn't suck the heat out of your shop in winter, and air to air heat exchanger averages the temperature between the interior of shop and the air coming in. For example, if your shop is 60 degrees and the outside is 20 degrees, the air you bring in using the air to air heat exchanger will be 40 degrees. So you are still shooting your heat outside. These filters are designed to clean the air as clean as or cleaner than air from outside. Only downside is cost of filters and having to blow it out occasionally.

[Dan Hintz]

As Jim said, it's simply not practical due to the sheer volume of air being moved. Vent inside via a good quality filter element (like a nano or HEPA filter) and be done with it. I created a filtering system for my laser that includes a HEPA filter as well as a charcoal bed for nasty fume capture for under $200 using a plastic trashcan. You can do similar with a larger barrel for wood chip capture.

[Bob Riefer]

Another thing to consider is that a DC system need not be only one or the other (vent inside through filters, or vent outside). You can use filters during the coldest months, and vent outside the rest of the year. Theoretically, this approach would lengthen the life of your filters.

Then again, for hobby use, the DC may not be running for hours on end anyways. With all the objects in the room holding their own heat (floor, tools, tables, walls etc.) the air in the room will heat up quickly and easily when the DC is shut off.

As an example... When I'm heating up my shop from ice cold, it takes at least an hour to reach temperature since all the objects in the room AND the air must be warmed. If I open the overhead garage door after my shop has reached steady heated temp, I definitely get a blast of cold air and the temp in the shop decreases. But, as soon as I shut the door, I'm back up to proper temperature in just a couple of minutes because all the objects do not need to be heated this time around.

[Leigh Betsch]

I wonder how well radiant heat would work in a small shop. That would heat the equipment and structure without heating the air, so if you blow the air outside you aren't blowing heated air out. Although I'm sure the air does heat up do to convection so there would still be losses.

[Alan Schaffter]

I posted some of this info in other threads so will repeat only some of it here-

Venting a DC outside is best for CFM at the source (at the machine) where you want it, because you are eliminating a source of static pressure (SP) resistance (or two if you omit both separator and filter). Everything you add to a DC - machines, blast gates, ducting, fittings, separators (cyclone, Thien, etc.), filters, and yes, heat exchangers will add SP resistance and reduce CFM. To be effective a heat exchanger (like a car radiator) needs a lot of surface area to transfer the heat from and to the air traveling through it- and that means a lot of friction and SP resistance. And, that resistance is double what you might think- not only does it affect the outgoing air, it affects the incoming (takeup) air sucked up by the DC as well. The volume and velocity of air moving through a DC is typically much higher than through a typical heat exchanger which can afford to run at low volumes since it is serving no other purpose (like collecting dust). Also, even if your DC system could live with the increased SP resistance, the heat transfer in the exchanger from air flowing at typical DC system CFMs and velocities would not be very good.

As to heat loss- the shop structure and contents have nearly 1000 times more thermal mass than the air which is why your shop air warms up rather quickly after the garage door has been cycled (the same way the cool air replaced by warm room air when a refrigerator door is opened cools almost immediately). Unless you run your DC continuously which typically only factories or large commercial shops do, the affect on comfort will be minimal and the effects on heating/cooling costs only slightly more (unless you live in areas of extreme temperature).

[Steve Rozmiarek]

Restaurant kitchen hoods move a lot of air, so they often use a make up air system rather than a heat exchanger. The system I was around heated and cooled. I'd think that may be the best way to do this if you really want to vent outside.

[Victor Stearns]

I have my DC outside of workshop and venting to outside also. I did this for the noise and no longer have to worry about filters. The shop is heated by radiant heat. When I do use the DC, I just crack open the service door. I do have a thermometer mounted next to this door. The temperature will drop at the thermometer while the door is open, but after closing the door the temp returns to normal fairly quick. Granted I am not running the DC continuously. One issue with the air-to-air heat exchangers are they do not flow enough air in comparison to the air flow of a DC. Thus I would suspect that they would be restricted and create a negative pressure with the workshop. I have considered installing a vent on the far side of the workshop that would open and close based on the DC operation. Still this is a consideration and not a requirement for me at this time.
Victor

[Rick Potter]

Not living in cold country I only have a couple months a year that I run my 220V radiant heater while I am in the shop. When I ran the 3HP cyclone to the outside, I HAVE TO open a window, or the DC sucks enough air out of the room that the dust collection actually diminishes noticeably. Just today, I have been thinking of installing a vent in the wall that would only open under negative pressure. Seems like a slatted exhaust vent mouted backwards would do the job? Maybe. Any thoughts on that?

Rick Potter