Flat plate heat exchanger size?

[AirConditioner]

I want to convert a 6500 BTU window air conditioner into a "water source" air conditioner. I've looked at various threads showing homemade heat exchangers, but I think I'd like to just use a flat plate heat exchanger from ebay. I want to plumb it so that both the evaporator and condenser on the original window unit become an evaporator and cold air comes out both sides.

How do I know what size heat exchanger I need for the condenser? Most of them do not have BTU or ton ratings. I can't post a link because I don't have enough posts, but I'm looking at a 14 plate SS heat exchanger on ebay for $48. It's rated for 725 PSI but I don't know if it would be big enough? Dimensions are 2.83"x7.36".

[AC_Hacker]

Originally Posted by AirConditioner

I want to convert a 6500 BTU window air conditioner into a "water source" air conditioner. I've looked at various threads showing homemade heat exchangers, but I think I'd like to just use a flat plate heat exchanger from ebay. I want to plumb it so that both the evaporator and condenser on the original window unit become an evaporator and cold air comes out both sides.

How do I know what size heat exchanger I need for the condenser? Most of them do not have BTU or ton ratings. I can't post a link because I don't have enough posts, but I'm looking at a 14 plate SS heat exchanger on ebay for $48. It's rated for 725 PSI but I don't know if it would be big enough? Dimensions are 2.83"x7.36".

Welcome to hell... (just kidding)

This is a great project that you want to do, and I hope that you completely succeed, and that your share your success (and errors/mishaps) with us through descriptions and photos that you record along the way.

Having said all that, your question is a frequently encountered problem for doing similar projects.

Part of the problem is that many cheap HXs found on ebay are meant for transferring heat from water-to-water, and the transfer capacities that they may list are completely different for refrigerants.

There are also differences in heat transfer rates amongst various refrigerants (thus the capacity of a particular HX) , but those differences are not as dramatic as it is for refrigerant-to-water. So, if you can find data that indicates what the transfer is for one refrigerant, you will be in the ball park.

I did a web search for "brazed plate exchanger sizing water refrigerant", and I turned up a number of promising links, THIS_ONE in particular looked good.

So look through this area and the PDFs until you zero in on a unit that looks good.

It is a good idea to over-size your HX by about 150% to 200% , it will improve efficiency.

You will also want to be sure that your refrigerant-side fittings are for brazing, rather that any kind of threaded fitting. In the industry, various terms are used, but "sweat fitting" seems to be the most often used.

Lastly, you need to know that soldering refrigerant tubing and components is not strong enough to survive refrigeration use. You will want to braze your tubing and fittings. The process is almost identical to soldering, but higher temperatures and stronger bonding alloy is used.

And just to make you life more confusing, technicians and repairmen in the refrigeration field often say, "solder up the system", when they actually mean to braze the system.

Let us know what you find.

Best,

-AC_Hacker

[AirConditioner]

Originally Posted by AC_Hacker

You will also want to be sure that your refrigerant-side fittings are for brazing, rather that any kind of threaded fitting.

Why is that? The one I was looking at on ebay was rated at 725 PSI with 1/2" NPT female threads. I was hoping I could assemble the whole thing with threaded and flare fittings so that I don't have to braze anything. I know compression fittings aren't good for A/C work, but what's wrong with normal threads?

[AirConditioner]

Another question, what is the difference between more plates, or a larger heat exchanger? Some aren't very large, but have a lot of plates. Others have fewer plates, but are larger. Is one better than the other?

[NiHaoMike]

You might want to consider a "coax coil" exchanger. It's nowhere as compact but the bigger internal volume makes it a lot easier to get the charge right. If possible, get one with about twice the capacity of the compressor (more than that is OK but well into diminishing returns) for the best efficiency.

[AirConditioner]

Originally Posted by NiHaoMike

It's nowhere as compact but the bigger internal volume makes it a lot easier to get the charge right.

Interesting. This is an R22 unit, and I was hoping when I was done to be able to recover and reuse the original charge. I'm going to charge with propane for testing, but if it works I want to go back to R22.

If I used a flat plate exchanger, will it hold less R22 than the original condenser on the air conditioner? I plan on storing the R22 in a converted paintball CO2 tank, and I imagine I will lose just a little bit in the process. I don't want to have to buy more R22.

This whole thing is kind of a test of concept for me, if it works I want to go bigger. I don't want to invest a ton of money in this particular unit. That's also why I would like to try and use threaded fittings, so I can modify my plumbing easier if I need to.

[AC_Hacker]

Originally Posted by AirConditioner

Why is that? The one I was looking at on ebay was rated at 725 PSI with 1/2" NPT female threads. I was hoping I could assemble the whole thing with threaded and flare fittings so that I don't have to braze anything. I know compression fittings aren't good for A/C work, but what's wrong with normal threads?

My thinking on my first system was exactly the same as yours. But I realized along the way that I would need to join copper tubing. I had all the equipment and supplies, propane torch, flux, solder, etc. Even though I had heard that brazing should be used I decided tried soldering since I was only doing an experimental system. My initial failures convinced me that using plumbing solder on the copper tubing which was subject to severe temperature cycling and constant vibration was absolutely not going to work.

So I bit the bullet and bought a small MAPPS gas brazing torch and a bottle of MAPPS gas and a small amount of Sil-Fos® 5 (I hate to use brand names, but this stuff is awesome), and if your tubing is nice and clean, you will not need flux. The equipment looks very much like a propane torch, but it will get to the temperature you need. I even tried to use a propane torch and failed to get the tubing hot enough.

Here is some advice from the Homemade Heat Pump Manifesto thread

HERE

Here is some information regarding brazing:

HERE

I tried avoiding brazing to the BPHX also and wanted to use threaded connectors but the system it slowly leaked refrigerant and oil.

When you braze copper tubing to Brazed Plate heat exchangers, you are "joining dissimilar metals" so you will need to use silver brazing flux, as discussed in the Homemade Heat Pump Manifesto thread..

HERE

When you braze, you will need a purging gas, like dry nitrogen or argon, etc. I used propane, which worked just fine, but I would not ever recommend that anyone do anything as dangerous and foolish.

Brazing is not very difficult, and the tools and supplies are not so expensive. Having that simple skill opens up a large range of possibilities.

%%%%%%

Also, I think you are on the right track in regard to your choice of BPHX. Don't get deflected by too many options.

Sincerely,

-AC_Hacker

[AirConditioner]

Originally Posted by AC_Hacker

I tried avoiding brazing to the BPHX also and wanted to use threaded connectors but the system it slowly leaked refrigerant and oil.

Not trying to offend you or anything, but are you sure you sealed the threads properly? (Maybe there is a special way it needs to be done for such high pressure?) I thought threaded flare connections were pretty standard in A/C stuff? I even found access valves for sale with 1/8" NPT threads. (still can't post a link)

Originally Posted by AC_Hacker

When you braze, you will need a purging gas, like dry nitrogen or argon, etc. I used propane, which worked just fine, but I would not ever recommend that anyone do anything as dangerous and foolish.

This is one of the biggest reasons I don't want to braze. Eventually I'd like to braze a finished "contraption", but I would much prefer to thread my test system so that I can change parts easier. I also want to experiment with a thermal expansion valve, it might take more than one configuration before I'm happy.

I'd really be cranky if I spent $50+ on a heat exchanger, brazed it in, and then changed my mind about how I want this to work. (or ruined it in the process!)

I did read here about somebody's experience with compression fittings, and I can see why they might fail. I'm just not sure why threads would leak? I also wanted to store my R22 charge in a (new) paintball CO2 tank, which is threaded for 3/8" flare threads. How would I store the R22 if I can't seal the threads?

[AirConditioner]

Looks like I can finally post a link. This is the heat exchanger I was looking at on ebay, it seems to be a pretty good deal:
Sweden's Swep 14 Plate 316L SS Heat Exchanger UL Listed | eBay

Many of the other ones are not rated for such high pressure. (725 PSI) Anybody think that is big enough for a 6,500 BTU system?

[AC_Hacker]

Originally Posted by AirConditioner

Not trying to offend you or anything, but are you sure you sealed the threads properly? (Maybe there is a special way it needs to be done for such high pressure?) I thought threaded flare connections were pretty standard in A/C stuff? I even found access valves for sale with 1/8" NPT threads. (still can't post a link)

Well, it's clear that you are too hard-headed to take advice, but hey.. we can still be friends.

In terms of sealing it properly, I'm pretty sure I did everything that science or magic could provide... and it still leaked.

Just remember the key words, "high pressure", "severe temperature cycling" and "relentless vibration". That is what makes refrigeration different, that's why brazing is superior.

Originally Posted by AirConditioner

This is one of the biggest reasons I don't want to braze. Eventually I'd like to braze a finished "contraption", but I would much prefer to thread my test system so that I can change parts easier. I also want to experiment with a thermal expansion valve, it might take more than one configuration before I'm happy.

Well, OK... as long as you realize the temporary nature of your 1st project.

Originally Posted by AirConditioner

I'd really be cranky if I spent $50+ on a heat exchanger, brazed it in, and then changed my mind about how I want this to work. (or ruined it in the process!)

OK, now we're getting to the heart of the issue. You aren't confidant of your skill level. That's quite natural, and prudent. So, the secret is to get some tools, some lengths of differing diameter copper tubing, maybe some kind of dissimilar metal also to practice on.

Practice off and on for a few days until you get the feel of how the process works.

That Sil-Fos® 5 is really wonderful stuff. You need proper heat, clean fittings and a bit of sensitivity to the process, and you will succeed.

Proper heat is critical, they say "cherry red"... you really need to see it, and then you will understand what that lovely color means.

Try brazing tube-to-tube... You'll need to flare one of the tubes a bit and the other tube will go inside, doesn't need to be a tight fit, the Sil-Fos® 5 will flow around and make everything pretty... just gotta get to "cherry red".

Also try brazing smaller tube to larger tube, use the 'pinch' trick and braze as you usually would.

Originally Posted by AirConditioner

I did read here about somebody's experience with compression fittings, and I can see why they might fail. I'm just not sure why threads would leak? I also wanted to store my R22 charge in a (new) paintball CO2 tank, which is threaded for 3/8" flare threads. How would I store the R22 if I can't seal the threads?

I store mine in empty green camping propane bottles which I have vacuumed to within an inch of their life.

I made a simple tool from an extra propane torch that I modified. If I didn't include that in the links, do a search on this site.

Originally Posted by AirConditioner

Looks like I can finally post a link. This is the heat exchanger I was looking at on ebay, it seems to be a pretty good deal:
Sweden's Swep 14 Plate 316L SS Heat Exchanger UL Listed | eBay

Many of the other ones are not rated for such high pressure. (725 PSI) Anybody think that is big enough for a 6,500 BTU system?

Looks like a reasonable HX. But if you're still asking if it's big enough, you didn't look through the Brazed Plate manuals I suggested...

So, if you don't want to do your own research, my friend, you'll just have to guess.

By the way, if (when) the threaded connectors leak, you might be able to braze to the threaded area on the HX. It won't be as easy as finding a HX with sweat fittings.

Oh, and here's one that someone did all kinds of bizarre things to... Make him a too-low offer, see what happens.

Alfa Laval CB26 24H 1" NPT Brazed Plate Heat Exchanger P N 3080079604 | eBay

And try this guy, you can get your threaded connector and your sweat fitting in most any size in the world...

http://www.ebay.com/itm/Stainless-St...item906f6f3789



Best of luck,

-AC