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Thread: Evap fin spacing and freezeup
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09-01-2011, 02:31 AM #1
Evap fin spacing and freezeup
There are a lot of complaints on this forum about frozen up evaporators and some comments about fin spacing as one issue. I understand that a closer fin spacing means a higher capacity in a smaller package but there is obviously a point where freezing really becomes a problem. The question is ... is there a rule of thumb spacing to avoid this problem and what would trigger my sixth sense when looking at a outdoor unit while it is still at the wholesaler? Hope I asked this properly
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09-01-2011, 04:27 AM #2
Re: Evap fin spacing and freezeup
MikeHolm,
I assume you are speaking coldroom evaporators.
Usually here i would use 6fpi(fins per inch) for temps 0c and above and for Freezer applications the same if it's a low usage room or if it's in a supermarket the general rule is 4fpi
LawrieRemember we are here for a good time, not a long time.
Trust me i'm a Fridgy.
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09-01-2011, 05:25 AM #3
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09-01-2011, 06:29 AM #4
Re: Evap fin spacing and freezeup
Mike,it's still p,,,ing down rain and i'm over it.
Remember we are here for a good time, not a long time.
Trust me i'm a Fridgy.
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09-01-2011, 12:34 PM #5
Re: Evap fin spacing and freezeup
(I assume you are speaking coldroom evaporators.
Usually here i would use 6fpi(fins per inch) for temps 0c and above and for Freezer applications the same if it's a low usage room or if it's in a supermarket the general rule is 4fpi)
Nope, heat pump doing house heating, most of the complaints seem to come from England during the cold snap. Lots of pics of frozen up Mits or Daikins or some such. When it cannot be traced to a blockage in the drain or water from the roof etc, there must be a way to keep the freezing to a minimum and allow the defrost to work properly.
First time using the quote feature, hope it works.Last edited by Brian_UK; 13-01-2011 at 07:32 PM. Reason: didn't come through as quote/That better ?, Brian
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09-01-2011, 12:38 PM #6
Re: Evap fin spacing and freezeup
You must be having a lovely summer.....(not) service work on a jetski
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09-01-2011, 05:57 PM #7
Re: Evap fin spacing and freezeup
There are a number of issues at play when it comes to defrosting, the fin spacing being one. The fin spacing will help to reduce the amount of times you need to defrost as the coil will take longer to block up.
Another aspect that can help is the angle of the coil. A coil set vertically is the worst to defrost as the defrosted water must make its way from the top of the coil to the drip tray before the unit restarts as this will refreeze any water sitting on the coil. A horizontal coil is easier to defrost and less susceptible to snow ingress, while a coil mounted at an angle is actually the best solution when it comes to defrost.
Probably the most important part of the defrost process is actually the hydronic circuit. You must ensure that you have sufficient flow rate passing through your heat exchanger to ensure the water temp stays high and allows the unit to defrost for as long as it needs without defrost shutting down due to a poor flow rate.
The modified AC units fall down on all the above points and that is why they defrost poorly and cannot work effectively at low temps.
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09-01-2011, 06:12 PM #8
Re: Evap fin spacing and freezeup
^ Some very interesting observations. Thank you.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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09-01-2011, 08:19 PM #9
Re: Evap fin spacing and freezeup
Would it make sense to say that the colder (and/or perhaps, more humid) the climate the wider the fin spacing should be? Assuming all other aspects of the design are optimized for the conditions.
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09-01-2011, 08:57 PM #10
Re: Evap fin spacing and freezeup
Humidity is the cause of your frosting problems and cold means a drop in COP. Countries with very cold winters tend to have low levels of humidity (Canada/Central Europe), whereas countries with milder climes (Ireland, UK) tend to have higher humidity in winter. So fin spacing is actually more critical in Ireland and the uk than most countries where apart from snow, which can be sheltered against, blocking of the coil is less frequent
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09-01-2011, 09:15 PM #11
Re: Evap fin spacing and freezeup
Thanks Guys,
I will have a chat with a couple of coil manufacturers here and see what they can do. The enclosure should be built to keep the rain and snow out. Viessmann makes one like that, A350 I believe.
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13-01-2011, 10:45 AM #12
Re: Evap fin spacing and freezeup
Surely the above is a function of system design and application not an inherrent issue with any particular unit, flow rate is very important to efficient defrost your correct but if the installer/designer does not consider this then it is clearly not an equipment issue.
Check the seasonal CoPs for the recent EST report with regards to Jap heat pumps not working effectivly at low temps....... Guess where 9 out of the top 10 performing ASHPs were manufactured !
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13-01-2011, 12:12 PM #13
Re: Evap fin spacing and freezeup
No, because the Jap manufacturers endorse the installation of their units on rad systems for example without the use of a buffer store. A rad system will you 3/4 main lines 90% of the time. It is impossible to attain sufficient flow through that sort of pipe for either heating or defrost.
They also incorporate the use of a bypass valve to keep flow rates up when actuator valves are closing. Therefore if one loop remains open, the vast majority of the water returning to the unit is water it has just heated along with 3-4 litres of water that has been through the UFH. Its patently stupid designed that tries to be all things to all men.
Heat pumps cannot and should not be designed to operate in the manners which the japanese manufacturers endorse, just so they can offload a few more units into situations where they should never have even been suggested. Don't get me wrong, Mits and Daikin do wonderful AC equipment but if they wanted to tap the heat pump market they should have done it properly and designed and unit from the ground up besides worrying about economies of scale.
I have been trying to access the EST report and when I get a chance to look at it I Will come back to you.
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13-01-2011, 12:46 PM #14
Re: Evap fin spacing and freezeup
I think the buffer vessel issue is more to do with modulating output and the need to fit one being negated with inverter controls on the units which have derived from air con backgrounds. There are some situations where a buffer may be advantageous but generally speaking they are not necessary unless you are fitting an old fixed speed heat pump.
Flow rates depend on the application and vary a large amount, we monitor the flow rate on every system installed and even with microbore pipework feeding the radiators you can still achieve the required flow rate range (10 - 25 L/M depending on the ASHP), heat the house and defrost effectively, again it comes down to design. The majority of retro fits we come across use 15mm pipework to the radiators and on newbuild you wouldnt specify 10mm pipe if designing for a heat pump system in the first place so either way the buffer/ flow rate arguement dosent work.
The autobypass is to prevent temperature/pressure related faults and protect circulators in the event all TRVs are shut. It is a slightly unecessary feature i agree as if your TRVs were all shut why would you want the heating on in the first place, why not just turn it off? Simple solution, dont fit TRVs to the bathroom radiator and use that as the bypass.
I think the most influential factor with any type of ASHP in terms of its performance will always come down to the quality of system design and installation, people will always have preferances as to which brand to use and as with all things there are differences in equipment quality, this is really the underlying message behind the EST report.
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13-01-2011, 12:50 PM #15
Re: Evap fin spacing and freezeup
Can you give me a link to the test results. I downloaded the field report in full which gave me very little info apart from the fact there was a very limited range of heat pumps tested. None of which i would put in my top 10 brands. The fact that none of the units broke a COP of 3.5 is horrendous as far as I'm concerned.
Unfortunately the UK seems to be lagging behind the rest of Europe when it comes to heatpumps. The selection there is pretty poor compared to other EU countries. The main reason being they won't accept test results from European test houses such as Arsenal research and insist on independently testing the units in the UK. It can cost up to 10K per unit to have your system tested so alot of the smaller more progressive manufacturers have opted out rather than fork out fortunes of money for double testing of equipment.
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14-01-2011, 12:26 AM #16
Re: Evap fin spacing and freezeup
Well, for us in the new world where we don't get half the good stuff you get, I don't think the big guys want to bother with the onerous process of a CSA or UL certification as it is hugely expensive and there is nothing like a CE designation here. Either you put out the big money or you don't make the product.
Unless I am mistaken we cannot get inverter compressors unless it is a replacement for a Mits or something. The domestic makers don't use them and digital scrolls are somewhat rare. The upshot is that our stuff is almost all single speed so a buffer is needed. Almost none of the makers here make an air to water HP only air to air (so far).
I still haven't got a good idea from this forum whether the digital scroll has a good reliability anyway so I will stick to working with single speed units. That said, the low temp performance of the heat pumps needs a re-think. Does anyone know of the idea of having a second evap and solenoid which would switch on as the temp gets quite low? Just a thought and I wonder how it would effect defrost.
Bit if a rambling note, Sorry.
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14-01-2011, 01:23 AM #17
Re: Evap fin spacing and freezeup
I do not think inverters/digital compressors are required for small hydronic systems (air to air yes), But i do like the softstart!
The reaction time is so slow in side your house with a low temp water base system (not air), that simple on and off controls are more than acceptable (with anti cycling off course). I also think to save any issues that the heat pump water pump should be dedicated soley for the heat pump and how the water is to be applied via a second pump. Re Buffer tanks, if they are used to act as a water balancing device then OK, or sized to ensure defrost OK, but as a thermal storage device (i am talking about those who install a few hundred litres) what a waste of time.
I use a piece of 50 -150mm dia, about 1000mm high.
out put from heat pump in at the top, take for heating out of the top.
Return from heating into the bottom
To the heat pump from the bottom.
Heat pump control sensor at the bottomLast edited by mad fridgie; 14-01-2011 at 01:26 AM.
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14-01-2011, 11:46 AM #18
Re: Evap fin spacing and freezeup
You appear to be describing a standard low loss header that we use in boilers all the time to allow the boiler which has min and max flow rates to work within its best range. The output to the tank or other load can vary as needed.
I can see your substituting this for the tank as long as it is a question of flow rate matching. Yes, my desire to use a tank is both flow matching and limiting the cycling of the HP. Although I like to do heating systems with only one zone, my clients often want 3 or more t-stats in a 160m2 home and one time i had 12 of them (NUTS they have seen too many TV renovation shows) so a buffer is necessary.
Working with solar (multiple sources)most of the time I try to stratify the tank as much as possible. In Europe you can get custom tanks with ports anywhere you want but in N.A. every custom tank must have an approval which means a licensed and inspected welding job and the cost is prohibitive. We then must either use an imported double coil stainless tank or jury rig a standard glas lined steel tank with diffuser pipes and other devices. It is hard to control different sources into one tank.
Our electric water heaters here have 2 elements with the bottom one on most of the time and the top one only comes on for peaking use only. I had thought to run the HP to the bottom of the tank and de-superheater to the top but I have yet to try it. Others here are using two fuels (propane or NG) for peaking but to have natural gas here means a meter charge of $30-40/mo whether you use it or not so on a cost basis I would rather not have it.
This tank should allow for proper de-frosting without taking heat from the floor but do you feel that that is a moot point?
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14-01-2011, 01:14 PM #19
Re: Evap fin spacing and freezeup
Getting back to the original point about maximizing defrost effectiveness and reducing freeze up. I talked to a couple of coil manufacturers here and they say that the standard technology only allows for the fins to be perpendicular to the copper tubing so I guess if we wanted to have a slanted fin, the whole coil will have to be on a slope. They will have to make expensive changes to the stamping equipment to make the slanted fin.
OH well...
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14-01-2011, 01:45 PM #20
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14-01-2011, 06:47 PM #21
Re: Evap fin spacing and freezeup
If you are refering to the coil at an angle as I mentioned Mike, I meant the whole coil would be at a 30-34 degree slant.
If you manufacture the tank with a baffle in the middle you could use it as both a buffer and a dhw tank although its not a system I particularly favour it does help with stratification.
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14-01-2011, 08:26 PM #22
Re: Evap fin spacing and freezeup
If i was designing and building from scratch (as you seem to be doing),I would look a split fin design, giving you the best of both worlds, high efficiency and high frost load capabilities.
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14-01-2011, 08:41 PM #23
Re: Evap fin spacing and freezeup
Buffer tanks, unless they are very large or have very high temperature difference compared to the load, then I see no benefit, (unless it full of phase change material, but that is a different argument) A simple anti cycle time is more than acceptable on these slow acting systems.
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14-01-2011, 10:05 PM #24
Re: Evap fin spacing and freezeup
The reason he'll need it MF is because his customers are insisting on stats which will cut his flow rates, affecting heating performance and defrosting.
I, like you, don't deal with buffer tanks unless necessary. They provide no real storage capacity unless your looking at a couple of thousand litres plus (and then you run in to all types of stratification problems) but they are essential in systems where radiators or thermostats are present
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14-01-2011, 10:16 PM #25
Re: Evap fin spacing and freezeup
Bigfreeze, I have always wanted to make my own tanks but we cannot manufacture custom tanks here and have them accepted by authorities unless they meet ASME rules and a 500L tank, for example, would need 1/4" steel and be welded by a certified pressure vessel welder and inspected. This tank would cost 3 times the european version.
Correct me if I am wrong but in Europe you can build a tank, meet the pressure test standards (self tested) and sell the thing (with a rating based on your testing). Over the years the regs there have kept up with the advances in alloys etc so that that same 500L tank that needs 1/4" steel here could be make with 1/8" steel there. Our regs have not kept up with the same advances. Here is a real world example: I visited the Viessmann plant in Germany 10 years ago. They were complaining that the flange for one of condensing boilers needed to be 25mm for North america where in Europe it was 12mm and the max pressure rating for the same boiler was 2 bar here and 5 bar in europe. This mentality is pervasive here and it is one reason why so many companies don't bring all their products here. I know I am a bit .
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14-01-2011, 10:18 PM #26
Re: Evap fin spacing and freezeup
MF, what do you mean by the split fin design?
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14-01-2011, 10:20 PM #27
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14-01-2011, 10:29 PM #28
Re: Evap fin spacing and freezeup
Split is/was common in refrigeration!
For example
Lets say your coil is 10 fins per inch, good heat transfer, but will block with ice easy,
5 fins per inch heat transfer is not as good, but less likely to block with ice.
So split fin, the air entering first enters the fins at 5 fins per inch (row 1 of coil), then passes through 10 fins per inch (following rows)
The majority of the ice forms on the entering fins.
The fin depth, 2 inch, 3inch, 2inch 3 inch etc
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14-01-2011, 10:30 PM #29
Re: Evap fin spacing and freezeup
Long ago I adapted a 4500L tank to have solar fed into the bottom and the backup boiler feed the top but the flow rate necessary through the boiler made mixing of almost all of the water in the tank an inevitability. I couldn't keep the bottom of the tank cool enough to get a good efficiency with the solar. In the end I had to separate the tanks and put them in series. It all worked beautifully after that so I am a bit wary of single tank solutions unless the flow rates are tightly controlled.
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14-01-2011, 10:36 PM #30
Re: Evap fin spacing and freezeup
OK, the split fin is understandable and it is something I can go to the manufacturers with for a discussion. It does make for a bit more expensive HP though. I also like the slanted coil idea. They could be combined. Thanks.
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14-01-2011, 10:43 PM #31
Re: Evap fin spacing and freezeup
When it comes to coil design and cost there are many factors, you need to draw a line in the sand where you want your operation conditions.
So for temperate climates many fins per inch is great (jap made units) for prolonged cold climates, less fins per inch is prefered, But the face area/ no. of rows is that much bigger.
Also I was taught, that for cost purposes longer is cheaper than higher (reduced return bends, and hence less labour and solder)
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14-01-2011, 10:51 PM #32
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14-01-2011, 10:56 PM #33
Re: Evap fin spacing and freezeup
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14-01-2011, 10:59 PM #34
Re: Evap fin spacing and freezeup
Mike, are you going for a package unit outside (evap, compressor etc) or leaning towards a split model with evap outside and the rest inside?
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14-01-2011, 11:02 PM #35
Re: Evap fin spacing and freezeup
With the deep embeded system, reaction time is so slow, temp compensation does not really work, I generally have no zone controls, high flow rates through the floor, split is normally only 2-3 degrees, I control on return water temp normally 28C (supply just over 30C) floor normally stabalises around 24-25C, i do not aim for an air temperature but more I am happy temperature (people are the mystery)
here the new houses tend have bedrooms carpeted and living areas are polished or tiled. The bedrooms tend to be about 3C cooler than the living areas.
being deep embedded we do have a lot of thermal mass, which does cover the peak and troughs.
Houses do over heat, but normally due to very high levels of solar gain.
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14-01-2011, 11:14 PM #36
Re: Evap fin spacing and freezeup
How deep is deep embedded? We're at 80mm, so weather comp works great. We also work at very tight centres, 100mm, and short loops, no longer than 80m so we keep our flow temps as low as possible. How low depends on insulation level but the average would be 26C with 5K heatloss across your loop.
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14-01-2011, 11:24 PM #37
Re: Evap fin spacing and freezeup
About the same 80mm, centres 150-200mm, insulation is not so great here, most houses have heaps of windows, and poor double glazing at that.
Also we do have massive daily temperaure swings, it can drop 20C in just over an hour (not a rareity). So reaction time on tends to be too slow
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15-01-2011, 01:37 AM #38
Re: Evap fin spacing and freezeup
Man, I wrote a big response but I got an error 500
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15-01-2011, 01:41 AM #39
Re: Evap fin spacing and freezeup
I am going for the split with only the evap outdoors, coil hopefully sheltered and If I can do it, a storage tank with a DX coil in it but of course that would mean a custom tank.
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15-01-2011, 05:06 AM #40
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15-01-2011, 05:13 AM #41
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15-01-2011, 05:46 AM #42
Re: Evap fin spacing and freezeup
Plan
____________
__________
____________
__________ airflow <---
____________
This is not how a draw it, so I have changed the airflow arrowLast edited by mad fridgie; 15-01-2011 at 06:21 AM. Reason: bloody computers
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15-01-2011, 06:12 AM #43
Re: Evap fin spacing and freezeup
Last edited by desA; 15-01-2011 at 06:39 AM. Reason: Updated to MF's revised drawing
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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15-01-2011, 06:36 AM #44
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15-01-2011, 06:41 AM #45
Re: Evap fin spacing and freezeup
I've corrected the sketch to suit your updated info.
So, counter-cross. Can you show/describe where the ice begins to form & how it progresses through the coil?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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15-01-2011, 12:24 PM #46
Re: Evap fin spacing and freezeup
Hi all, I can only see one diagram so I am not sure how you would draw the counter -cross flow although I believe I understand the concept but not necessarily why superheat is generated there (but not in a parallel-cross)?
At 100C the pressure ratings for a 1" SS schedule 40 pipe (sorry for imperial notation) is about 180 bar. I will look at pressure ratings for 3/4" but I know it will be 250+ and I need to look at thinner wall material as well but the standard wall thickness for most tanks (and therefore, the most accepted) is about 2mm but this is off the top of my head so I will have to confirm.
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15-01-2011, 12:39 PM #47
Re: Evap fin spacing and freezeup
Sorry, I meant that HX wall is around 2mm not the tank wall.
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16-01-2011, 06:04 AM #48
Re: Evap fin spacing and freezeup
Counter-cross
Parallel-cross
Now, perhaps MH can overlay his fin arrangement? Then we can discuss the ice formation & progression through the coil.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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16-01-2011, 10:14 AM #49
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16-01-2011, 10:20 AM #50
Re: Evap fin spacing and freezeup
Where does the ice first begin to build up in the coil? For each configuration.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )