Suction line heat exchanger
 

Ok, so the window AC unit I turned into a heat pump is doing such an awesome job that I have decided to build another, similar unit. The first unit and this unit will have similarly sized heat exchange coils, only this one will be based off a 9k btu compressor vs a 15k btu in the first. So in theory, I should be able to squeeze more COP out of this one. which leads me to this thread...

While researching propane-based refrigeration cycles, I kept coming upon mentions of using suction-line heat exchangers (SLHX). The research and application papers I read said that considerable performance and efficiency gains could and/or should be had by using them in the refrigeration cycles of propane-based setups. Somewhere along the line I decided that I would try one in action and see if it would help. Before I do, though, I need to become more familiar with them and gain some insight and confidence.

I have a ton of questions about these gadgets. The main idea behind these things is that they take waste heat from the liquid line and warm the cold suction line with it. This is supposed to subcool the liquid line (a good thing?) and superheat the vapor line (another good thing?) while increasing suction pressure (i know this is good.) and mass flow (definitely good.). This sounds like some sort of free lunch to me.

I will be running a TXV on this new unit (harvested from the first unit), so it alone should net me some performance vs. the cap tube running in it now. Keeping this in mind, will the txv work with or against the hx? Should I put the sensor bulb on the evap or compressor side of the slhx? Would it make much difference?

Next come the plumbing questions. Should I extend the liquid line or the suction line? Vertical or horizontal orientation? How big is too big?

I know I will probably learn more than I accomplish from this endeavor... that's kind of the point. I hope somebody can help clear up the muddy water a little before I jump in.

jeff

Can you cite links to the literature that you've come across?

I think I may have seen such a device in a photo of a Heat Pump that Piwoslaw posted a long time ago.

I'm pretty sure BradC has some information on what you're trying to do... maybe Vlad, too.

Also, it's not clear from your description if you plan to separate your compressor function from your condenser/fan function, but I think it will make your life a lot more comfortable if you do. There's a reason they call air conditioners "window shakers". I really think that they're best left outside, bolted to a concrete pad.

Best,

-AC

Do you mean the HX "W", called "ekonomizer"?

Some quick papers:

http://www.irc.wisc.edu/file.php?id=49

LCYE - Suction line accumulators, with heat exchanger

Numerical simulation of the effects of a suction line heat exchanger on vapor compression refrigeration cycle performance - Springer

I have lots of theory SLHX, but no experience at all. Therefore I'm watching eagerly!

Piwoslaw,

It would be useful to English-only readers if you edited in to your diagram post what all the labels are.

Best,

-AC

Did you have to spoil the fun? :p
Changed it. Sorry about that.

Piwoslaw,

Yes, the part labeled "w".

I had written a descriptive post, but the site ate it when I tried to post it.

I use them all the time, in 2 ways. The simplest is as the diagram shows and could be fine provided you do not wish to have a reversing system to do cooling as well (or defrost, but there are other ways of doing that). The second one is to get a suction accumulator with a pancake HX inside it. The added benefit of this system is to take up the extra refrigerant when in reverse mode so the pot doesn't get flooded.

The pic I showed on the other thread shows my HP with both types on it. The main reason I am using it is that it will extend the operating range down another 5-8C

So what is your rationale for using the size that you chose to use?

Did you just eyeball it, or was there some design criterion that you used.

-AC

Everything I have read says to size the accumulator to contain the entire charge. I can't tell you how to size the suction line HX only that the 42mbtu unit I am making now has one about 8" long. It is a 3/4" pipe inside a 7/8" pipe, IIRC. I got some of the parts from an old westinghouse unit which was working well after 30 years but the case rotted out.

Mikesolar,

Yours looks to be the type of HX i plan on using. I'm with AC Hacker on this one, it seems MASSIVE! How is the heat transfer on that behemoth? It's half the size of your compressor... I understand about the accumulator, but are your coils that mismatched? Or is it sized for max heat transfer? Or do you pump down every cycle?

If I could get another 8 deg c out of my unit, that would get me to -13 c (8 deg F). This would cover my heating needs in Kentucky for 363 days a year!

I have been reading up on the research surrounding these units, which claims that a 10-15% gain in capacity and efficiency is possible, as well as increased system stability. Also, they seem to perform the best at high temperature spreads. Combined with a TXV to keep the evaporator near saturation, it would seem to be a winning combination.

Here's another paper, which looks very similar to Mike's rig:

http://www.annex32.net/pdf/presentat...008_Stene2.pdf

The unit I envision will be functionally plumbed like this:

Take cover off of 1982 Window shaker A/C, chop compressor suction line. Add 2 meters of suction line, coiling excess suction line tightly along length of liquid line in counter-flow. Braze. Recharge, replace cover. Run unit with performance gain.

In actuality, it will most likely be much more complicated.

The difference between that one and mine is that the old westinghouse unit is that the TXV actually meters the liquid line to the condenser NOT the evap so is called a subcooling valve. The evap becomes a flooded Evap which took me some time to get my head around but my mentor has been making them this way for 25 years and they work very well.

Other than that, the idea of the the SLHX is a good one and should be combined with a bit larger Evap and condenser but sizing those, especially when they are air coils is a real art. Even the manufacturers do more trial and error than you would expect and they don't like to give away their design ideas. I've tried to get it out of them without success.

Jeff, how long of a HX would you put in? I don't think there is much advantage to anything over 6-8".

Aha, so that's why you're running such a large accumulator. It's also a flash tank, or a "slobber box". It took me a while to decipher your plumbing, the reversing valve is the same color as the compressor shell! That and your full-wave bridge rectifier check valve setup threw me off for a minute. I'm somewhat confused with your TXV description, however. The condensor is not fed liquid.

The metering device in flooded systems is usually a float in the flash tank. When the refrigerant in the flash tank drops below a certain level, the valve opens to maintain that level. When the flash tank fills past this level, the valve closes. Kind of like a toilet. The flash tank sits above the evaporator, and gravity keeps fluid in the evaporator. This is obviously not what you have.

I found info about the westinghouse valves here:
http://sporlanonline.com/literature/10/210-60.pdf
http://www.refrigeration-engineer.co...bcooling-valve

A subcooling valve is the opposite of a TXV. Instead of measuring superheat against suction pressure, it measures subcooling against head pressure. Instead of regulating the amount of liquid in the evaporator, it regulates the amount of liquid in the condensor. Since you have a flooded evaporator, this makes perfect sense.

Your system is plumbed closely like what I am planning. The main differences being:

I will have only one SLHX, the big can in yours.
TXV bulb will be on the suction line just prior to entering the big can.
Two metering devices, two check valves.

Yup, I made a booboo. It is metering liquid which goes to the evap and the bulb is measuring the temp on the discharge side of the condenser before the two HXs. The desired subcooling is around 15C or even more. superheat is only a couple of deg.

I think I see what you are doing. In this way, you have a different TXV for each direction.

New Regenerative Cycle for Vapor Compression Refrigeration
 

Just came across this juicy paper, courtesy of Uncle Sam:

New Regenerative Cycle for Vapor Compression Refrigeration

-AC

Yes, I will have separate metering devices for heating vs. cooling. I may use two txv's in this one, or I might keep the cap tube for cooling and for a method of easily charging the unit. The unit will essentially function as a larger R22 type split heat pump with the Hx in the suction accumulator.

This guy calls it a liquid suction heat interchanger, why I don't know.
Liquid-suction heat interchanger

The main purpose of the exchanger will be as an accumulator. With the unit I have now, I can only push the superheat down to about 15 deg F before the evap starts burping foam. It seems that oil dissolves really well in R290. With nearly no distance between the evap and the cxr, the foam was going straight to the pot.

The pros all told me that this always happens with room ac units if the superheat at the cxr inlet goes below 30 deg F. BTW, I find that the forum experts will tell you anything you want to know if you stay IN THE BOX. This includes making up a story about a generic something you are working on somewhere. It also means talking in their language. In your case, I would reference the westinghouse unit at a brokedown palace. Homeowner Joe Dirt inherited the place and can't afford a new unit. Only then will the experts bare their souls on your problem.

Jeff, what was the original refrigerant? If it was R410 then the oil needs to be changed to mineral oil from POE. The foaming is one symptom, moisture in the system is another. POE is very hydroscopic.

Are you referring to that US based HVAC talk forum? 30F is a lot of superheat, 20F being what Copeland uses in their specs and typical is way less.

Do you know of another one?

-AC

Mikesolar and/or AC_Hacker,

Yes, the two main places I look are the hvac talk forum and refrigeration engineers forum. The pros in the know all design the PTAC and RAC setups for 30 degrees superheat at the inlet line to the cxr at design conditions (85 deg F/30 deg C ambient). Asked about SLHX, they said it's just another part that could make these things fail. In walk-in freezer systems, though, they are commonly used since they save money. Their main focus is on reliability, then cost, efficiency comes in a distant third. Unless it's a big system that costs a lot to run.

I read in an article I can't find now that a designer noticed the "foam burp" effect. What the pros know is that it is a well known condition and design for it not to happen. This one briefly discusses it in section 4.2

http://docs.lib.purdue.edu/cgi/viewc...8&context=icec

The main idea is that at "high" evap temps (>-10 deg c) and below about 15 k of superheat, the propane boils so rapidly in the evap that the oil doesn't fully separate. The obvious solution is to keep superheat above 15 k. You can do this by either raising the superheat setting on the TXV or by adding SOMETHING in between the evap and cxr. This something will be the SLHX in my unit. With it, I will be able to set the TXV to a lower superheat setting (than 15K).

For an insight into the theoretical arguments surrounding this subject, just google 'R290 suction superheat'. The pros will argue this to death because it depends so much on specific situations. When narrowed to R22a (refrigeration engineer grade propane), the topic quickly devolves into scare tactics and warnings that propane is explosive. They are all against drop-in replacement of r12/r134/r22.

All my experimental systems have been made from old R22-based units. They are plentiful and dirt cheap. Mineral oil, 10 SEER type units.

AC,
I just read the paper you presented. Is this awesome or what? Exploiting supersonic evaporator flow for efficiency gain! The author admitted he shoulda used a larger accumulator because he slugged his compressor! Priceless...

I hope the next version will support sustained operation. It would be a tremendous innovation.

I don't know if my unit was producing supersonic flow, I highly doubt it, but it was doing the same thing as the author described in his paper. Right at the point where efficiency was greatest, the evap would burp foam. I wasn't slugging my cxr, I was foaming it. I believe it was at around 10 deg F of superheat at the bulb, 15 deg f at the cxr inlet. from this effect, I learned how hard I could push my evaporator.

So, you had a sight glass, right?

-AC

It is interesting that, at the end of the first paragraph of section 2, they want to insult the compressor. Not sure how that will help:D

Doh!
 

Well,err,hmm,uh...no. That would have made my cheap reverse a/c unit not so cheap.:o

In all honesty, I have no way of knowing whether I was slugging my compressor or not. But I do know I was running my evap too hard.

I wonder if the viscosity issues of R290 and mineral oil are the same with a scroll as they appear to be with the recip? Are you really getting the foaming or is there some other issue. BTW, a sight glass is not very expensive.

OK, OK, you win. In my next unit, I'll put a sight glass in the suction line right before the compressor. When and if I test the envelope, I'll know what actually got through the line. If I have more trouble with the existing unit, I'll stick one in it and revisit this issue. For now, the 30 degF superheat at the suction inlet will remain.

Mikesolar,
It seems to be an issue at high flow, high temp (>-10 degC), low superheat conditions. With r290 or r600, POE oil is the worst, then PAG, then mineral oil. The mfrs are recommending Shell clavus g68 or suniso 4gs oil in r290 systems due to their higher viscosity, where in r22 systems they recommend clavus g32 or suniso 3gs. Kind of like putting racing oil in your car if you like to rev the engine. If the oil thins out, it will start from a higher initial viscosity and won't get too thin or foam up.

I started with an R410 pot, drained the POE and put in MO. The fridgie boys say that 5% POE won't be a problem in running the system so i am not concerned about getting it all out BUT, I still haven't turned it on yet. There may be a host of issues most of which, I am guessing, will revolve around sizing Evaps and condensers properly.

I don't know if the oils noted by the authors are available here or are they European only.

From what I know about compressors (not much), the R410 types are set up for a higher compression ratio and higher ultimate pressures (r22 or 290 high side runs in the 200's psi range, r410a runs in the 400's psi range). I have heard that you can run them at lower pressures, though.

The oils I posted are worldwide standards made by American companies. Suniso is made by Sunoco, same guys that supply gas to NASCAR. 3gs is 30 weight, 4gs is 40 weight. Shell Clavus is made by Shell oil, the number is the weight. Any Hvac supplier will know what they are and have something equivalent.

Mostly right. Compression ratios remain the same, it's just the low and high side pressures are higher in an absolute sense. The other thing about 410A vs 22 or 290 is it has quite a high latent heat capacity so you need less mass flow for the same refrigerating capacity.
This means 410 compressors have a smaller displacement than their R22 counterparts, so a 18KW compressor for 410A will provide a significantly reduced capacity with a lower pressure refrigerant.

I can dig up the figures and give you a calc if you are really interested (or you could run a quick simulation in coolpack with a fixed mass flow compressor and see the difference in Qc when you switch refrigerants)

I'm a bit confused. Can you help clear some things up for me?

I thought that r22/propane and r12/isobutane were kind of equal opposites. The hc's have large latent heat but small density, the fc's have small latent heat but high density, when compared to each other. R410a plays the middle, having high density and medium latent heat. When the terms are combined, R410a comes out a winner at medium-high latent heat-density.

Does this make sense?

Also, you stated the r290 running in the 410a compressor would have reduced capacity versus the nameplate. Would the compressor also be consuming less power in this condition? What ill effects would the compressor running out of its range have besides being underemployed?

I'm speaking ideally here, not practically. Just trying to gain some clarity and straighten out my thoughts on the different refrigerants. The research on the subject isn't exactly in common english.

Brad, I have Coolpack as well but it is really hard to get my head around mostly because they assume you know all the acronyms and what the proper ranges should be which takes more time than my ADHD brain will allow, haha.

I was looking for, and asking others, on RE about the performance difference between R410 and R22 (or R290) on the same pot but didn't get good answe r so if you have any insight, it would be welcome.

Did you see the 25 second Coolpack walkthrough I put in my Gas and gear thread?

Nope, probably my ADHD, haha. I will look at it.

I haven't forgotten these questions, I've just not had time to sit down and think about how to answer them yet. All great questions, it just takes me a while to come up with a way of communicating the answers that don't need my wife to interpret for you.

I found another article which describes the effect I am trying to achieve:

http://www.pattonnz.com/site/patton/...Exchangers.pdf

It suggests that with R290, a 40% in capacity can be theoretically achieved (ignoring changes in mass flow) and realistically 12% (considering mass flow) can be had with a SLHX that is 100% effective. Similar increases in COP are also suggested.

The charts the author provided show the effectiveness vs. gain is close to linear. I've been told the reduction in gain isn't as much from ideal as the author suggests, due to R290's inherent high latent heat. The liquid subcooling has more to do with heat flow than the vapor superheating, the mass flow doesn't decrease as much as expected.

Yet another article that ties things together a bit better:

http://www.guentner.eu/know-how/tech...chash=d9123254

Actually, the site has a number of energy-saving design articles in the know-how section.

http://www.guentner.eu/know-how/

BradC,

Take all the time you need. I've found coolpack and downloaded it, but have no idea how to use it to any useful extent yet... time will tell if this will change. I'll ask questions about it on the new coolpack thread. If I can figure out how to use it, the program seems robust and precise. It could be a godsend, preventing a lot of wasted trial and error.

A big point I gathered from the Guenther article is that too much heat exchange in the SLHX is a bad thing.

Compressors specify a maximum SST (suction socket temperature) which could be exceeded by too much heat transfer into the suction line. This would make your compressor roast itself due to high discharge temps. 99.9% of hermetic compressors use cool suction gas to cool the motor windings, warm suction gas could cause motor overheating. Most likely the refrigerant would be above its critical point also, causing all manner of thermodynamic mayhem in the condenser.
On the other side of the exchanger, too much subcooling causes more problems. Expansion valves are designed to have SOME gas flashing inside the body. Extremely subcooled liquid hardly flashes, causing the valve to eventually lose its seal. In the evaporator, the lack of flash gas causes low quality and velocity (laminar flow pattern). The evaporator heat exchange suffers until the liquid speeds up and starts to churn inside the pipes (turbulent flow). The effective area is greatly reduced, causing reduced capacity and COP.

So now I'm with Mikesolar on the notion that a massive exchanger isn't so useful. In fact, it can be a bad thing. But how big is too big?

You have exellent attitude,jeff5may, approaching this "awkward" matter.
Find below one humble coolpack-wersion (not created originally by me).

Pls,note always the Coolpack knows nothing about lubrication oil behaviour(viscosity/mischibility/dilution etc.) in the refigeration process .These are key factors as well.


One COOLPACK-simulation below for LL/SLHX-solutions :
+7/+50 degC setup
2K subcooling
5K superheating
compressor with isentropic efficiency 0.7 (=very good compressor,usually in heat pump contex ranging between 0.60 - 0.65 appoaching 0.7 when bigger compressors concerned).
Rgd. new PERMAMAGNET-compressors (BLDC) the efficiency is still higher
(heat loss through windings modest).

HEATING COPs through COOLPACK:

R404A: -COP = 4.053
R410A: -COP = 4.193
R407C: -COP = 4.324
R1270 (propylen) -COP: = 4.41
R290 without LL/SLHX: -COP = 4.454
R22: -COP: 4.519
R134a: -COP = 4.519
R290 with LL/SLHX efficiency 50% => 85 degC hotgas: -COP= 4.54
R600a without LL/SLHX: -COP= 4.59
R12 -COP =4.608
DME: -COP = 4.82
R600a with LL/SLHX efficiency 70% => 85 degC hotgas: -COP= 4.92

Coolpack is originally designed for cooling =freezing purposes.
The Guenter article is for R404A and maybe not the best example for heating purposes.But the idea of deviding the circuit into many sections (HX:s for different phase-/physical transitions) is most necessary if highest gain is in sight.

Heating COP to be derived from cooling COP by adding +1 (unit) into the coolpack value.

As you said there is a upper limit in LL/SLHX efficiency you should not exceed
=> CRITICAL temp. of the refigerant (no phase transition!).
Another thing is that HYDROLYSIS of the oil (MO-POE-PAG etc) may start
(hotgas T> +120C /POE ?) if oil contaminated enough by H2O.

The "flash-gas" item you told is something new and strange to me.
I have before always had the idea the expansion valve should be "safe" if no cavitation forces are prevailing in the TXV-body.
Any referenses available on the subject?
Any difference in behaviour between capillary/TEV/EEV-systems ?

PS: You may wonder about the R1270 in my list above.
You have this available at Lowe`s and Walmart at <$ 8 per can (400g).
Guess which stuff is "originally" concerned and for which purpose ?

As US/Canadien citizens you are sitting on a treasure without knowing(?) about the cheap refrigerat resources (even) better than R290 (propane).
As a Finn I feel furious due to non-access to these "crown jewels" of
yours!

Here is the link to a guide for the SAFE changeover to R290 (and maybe R1290) as well as a suitability guide. There are 2 documents but they are too big to post.

GIZ. Publications