Evap fin spacing and freezeup

[MikeHolm]

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:confused:

[lawrence1]

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
Lawrie

[mikeref]

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
LawrieHey Lawrie, not much to do on a sunday arvo either huh? Think o.p. is talking about air-con condenser in reverse cycle.

[lawrence1]

Mike,it's still p,,,ing down rain and i'm over it.

[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).... supposed to be a quote from above but it didn't come through that way.


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.

[MikeHolm]

You must be having a lovely summer.....(not) service work on a jetski

[Bigfreeze]

(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).... supposed to be a quote from above but it didn't come through that way.


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.

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.

[desA]

^ Some very interesting observations. Thank you.

[MikeHolm]

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.

[Bigfreeze]

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.

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

[MikeHolm]

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.

[Oakgreen16v]

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.

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 !

[Bigfreeze]

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 !

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.

[Oakgreen16v]

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.

[Bigfreeze]

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.

[MikeHolm]

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.

[mad fridgie]

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 bottom

[MikeHolm]

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:confused: 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?

[MikeHolm]

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...

[desA]

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...

Can you explain more about the 'slanted fin', you're looking for? Perhaps a simple sketch? Could be very interesting.

[Bigfreeze]

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.

[mad fridgie]

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.

[mad fridgie]

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.

[Bigfreeze]

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.

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

[MikeHolm]

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 :off topic:.

[MikeHolm]

MF, what do you mean by the split fin design?

[mad fridgie]

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
Rad systems are not that common in NZ, mainly deep embeded underfloor,
I wonder if it would not be cheaper, on defrost to overide your stats, to ensure correct flows and loads

[mad fridgie]

MF, what do you mean by the split fin design?
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

[MikeHolm]

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.

[MikeHolm]

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.

[mad fridgie]

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.
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)

[Bigfreeze]

Rad systems are not that common in NZ, mainly deep embeded underfloor,
I wonder if it would not be cheaper, on defrost to overide your stats, to ensure correct flows and loads

I hate the bloody things. Drag down the efficiency of most systems. We install 95% without stats and never on rads.

I think its better to educate people on the benefits of weather compensation than try to engineer around stats.

[Bigfreeze]

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 :off topic:.

You need your products to be tested to european standards here too. Its not cheap. In the middle of that very process myself. Developing a tank to work in conjunction with heat pumps. A steel pressurised tank would be expected to be rated a 6 bar at 70C

[Bigfreeze]

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?

[mad fridgie]

I hate the bloody things. Drag down the efficiency of most systems. We install 95% without stats and never on rads.

I think its better to educate people on the benefits of weather compensation than try to engineer around stats.
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.

[Bigfreeze]

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.

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.

[mad fridgie]

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.
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

[MikeHolm]

Man, I wrote a big response but I got an error 500

[MikeHolm]

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.

[desA]

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

Surely the ice will begin forming where the off air is coldest? The airflow direction would then be important in your coil design.

[desA]

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.

Have you considered the effects of the DX coil, in tank, rupturing under pressure?

[mad fridgie]

Plan
____________
__________
____________
__________ airflow <---
____________

This is not how a draw it, so I have changed the airflow arrow

[desA]

Plan
____________
__________
____________
__________ airflow <---
____________

Refrigerant counter-cross flow, or parallel-cross flow?

[mad fridgie]

Refrigerant counter-cross flow, or parallel-cross flow?
Hi Des please note, what you see is not what I had drawn, (spaces had been removed so air flow is opposite direction, have rectified in my thread)
Flows are normally counter cross, especially if your superheat is produced here.

[desA]

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?

[MikeHolm]

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.

[MikeHolm]

Sorry, I meant that HX wall is around 2mm not the tank wall.

[desA]

http://i53.tinypic.com/27ymro4.png

Counter-cross

http://i52.tinypic.com/1zlt8qv.png

Parallel-cross

Now, perhaps MH can overlay his fin arrangement? Then we can discuss the ice formation & progression through the coil.

[Bigfreeze]

http://i53.tinypic.com/27ymro4.png

Counter-cross

http://i52.tinypic.com/1zlt8qv.png

Parallel-cross

Now, perhaps MH can overlay his fin arrangement? Then we can discuss the ice formation & progression through the coil.

Surely you'd run with counter cross as you'd want the warmest air to meet the warmest refrigerant first for maximum heat transfer across the coil?

[desA]

Where does the ice first begin to build up in the coil? For each configuration.

[Bigfreeze]

Where does the ice first begin to build up in the coil? For each configuration.

TBH i'm not sure, I'd have to look into it further, I was hoping you'd answer that for me :D

[mad fridgie]

Evaps are very rarely piped so simply.
I am one who very rarely uses the evap tp produce the TEV required superheat, So just for ease we will say the refrigerant and the fins are the same temperature, and heat transfer is a constant (knowing that this is not true)
So ice build up, and this depends upon so many factors, coil depth, evap temp, air temp , humidity, velocity! but generally the majority of the ice is going to form on the inlet face of the coil. So on split fin the mass ice forms on the fins with the greatest gap, whilst still allowing high levels of air flow.

[Bigfreeze]

Evaps are very rarely piped so simply.
I am one who very rarely uses the evap tp produce the TEV required superheat, So just for ease we will say the refrigerant and the fins are the same temperature, and heat transfer is a constant (knowing that this is not true)
So ice build up, and this depends upon so many factors, coil depth, evap temp, air temp , humidity, velocity! but generally the majority of the ice is going to form on the inlet face of the coil. So on split fin the mass ice forms on the fins with the greatest gap, whilst still allowing high levels of air flow.

I was thinking along those lines, coil depth, air velocity etc. Obviously the moisture would freeze quickest at the coldest point but whether the majority would reach that point is the question in my mind as it would depend on how quickly the moisture would condense upon impact with the coil and therefore where in the coil it would deposit. Anyway, I like your idea on the split fin design

[DTLarca]

I was thinking along those lines, coil depth, air velocity etc. Obviously the moisture would freeze quickest at the coldest point but whether the majority would reach that point is the question in my mind as it would depend on how quickly the moisture would condense upon impact with the coil and therefore where in the coil it would deposit. Anyway, I like your idea on the split fin design

For cross counter flow parallel circuits with negligible pressure drop the earlier circuits after the expansion device, post epi-saturation, are the coldest - they have greatest refrigerant wetting than the pipe run approaching the superheating dry-runs.

Am I wrong?

[Bigfreeze]

For cross counter flow parallel circuits with negligible pressure drop the earlier circuits after the expansion device, post epi-saturation, are the coldest - they have greatest refrigerant wetting than the pipe run approaching the superheating dry-runs.

Am I wrong?

No, you're right. Isn't that what I alluded to when I mentioned the warmest air would hit the warmest refrigerant in the counter cross flow configuration, as the refrigerant flows in from the air off side?

[DTLarca]

No, you're right. Isn't that what I alluded to when I mentioned the warmest air would hit the warmest refrigerant in the counter cross flow configuration, as the refrigerant flows in from the air off side?

Yes, we're talking about the same thing - I started reading at only the last few recent posts in the thread. I'm just wondering then how reliable is a statement that the ice starts forming on the coil inlet face - under what conditions is it true and under what conditions is it not true - I was thinking along those lines.

[Bigfreeze]

Yes, we're talking about the same thing - I started reading at only the last few recent posts in the thread. I'm just wondering then how reliable is a statement that the ice starts forming on the coil inlet face - under what conditions is it true and under what conditions is it not true - I was thinking along those lines.

I think as was mentioned above that the factors are so innumerous that its very hard to say.
Fin spacing - Icing would not form as rapidly at the air on contact point and would be drawn further into the coil.
Air velocity - The faster the air is moving the more likely the moisture will travel into the coil before condensing will take place.
Coil Depth - The deeper the coil the more your temp drop across your evap, the greater amount of moisture deposited
Humidity - The higher the humidity, the quicker it will condense and therefore ice closer to the air on side - the converse is also true
Etc Etc

Change any one of those factors in your coil design and I'd imagine you'd come up with a vastly different outcome.
The best solution as I see it is to examine the market and prevailing weather conditions you are planning to sell to - in Mikes case canada for a start - then adapt your coil to suit the prevailing weather conditions.
So he'll need to prevent snow ingress - Horizontal or angled coil, low humidity, and a short but intense heating season.

[DTLarca]

I think as was mentioned above that the factors are so innumerous that its very hard to say.
Fin spacing - Icing would not form as rapidly at the air on contact point and would be drawn further into the coil.
Air velocity - The faster the air is moving the more likely the moisture will travel into the coil before condensing will take place.
Coil Depth - The deeper the coil the more your temp drop across your evap, the greater amount of moisture deposited
Humidity - The higher the humidity, the quicker it will condense and therefore ice closer to the air on side - the converse is also true
Etc Etc

Change any one of those factors in your coil design and I'd imagine you'd come up with a vastly different outcome.
The best solution as I see it is to examine the market and prevailing weather conditions you are planning to sell to - in Mikes case canada for a start - then adapt your coil to suit the prevailing weather conditions.
So he'll need to prevent snow ingress - Horizontal or angled coil, low humidity, and a short but intense heating season.

Ah okay.

I just had a browse on the first page of this thread - my settings divide the threads into 50 posts per page. I see Des was already asking the same question.

And now you've given us an exploded view of the points Mad Fridgie was making/listing.

I agree with the fin spacing supposition.

I agree with the velocity supposition - increased velocity = increased bypass factor.

I agree with the coil depth supposition.

I'm wondering about the humidity supposition. The higher the humidity the higher the air enthalpy and so the longer it will take to cool and so the further it will go through the coil before dropping its moisture. Seems to me that maybe a higher humidity will simply increase the rate of deposition without adjusting the location of deposition?

[Bigfreeze]

Ah okay.

I just had a browse on the first page of this thread - my settings divide the threads into 50 posts per page. I see Des was already asking the same question.

And now you've given us an exploded view of the points Mad Fridgie was making/listing.

I agree with the fin spacing supposition.

I agree with the velocity supposition - increased velocity = increased bypass factor.

I agree with the coil depth supposition.

I'm wondering about the humidity supposition. The higher the humidity the higher the air enthalpy and so the longer it will take to cool and so the further it will go through the coil before dropping its moisture. Seems to me that maybe a higher humidity will simply increase the rate of deposition without adjusting the location of deposition?

That I don't know. i will leave that to a brighter mind than mine :D
I'd imagine, once at this stage, that the only way to be certain of anything would be field testing.

[DTLarca]

That I don't know. i will leave that to a brighter mind than mine :D
I'd imagine, once at this stage, that the only way to be certain of anything would be field testing.

Indeed, hypothesis, experimentation, modify hypothesis, more experimentation then finally we have a theory or a bunch of competing theories to describe the facts seen during experimentation :)

[desA]

Evaps are very rarely piped so simply.

Not really. Most practical coil lacing will follow a progressive trend through the coil - even though they may follow inverted v, or alternative - to manage oil flow effectively. Some coil makers, however, simply don't have a clue & basically take horizontal coil designs & stand them vertically.

In general, most - or at least a large portion - of the last coil row ends up devoted to superheating the evaporated fluid.


I am one who very rarely uses the evap tp produce the TEV required superheat, So just for ease we will say the refrigerant and the fins are the same temperature, and heat transfer is a constant (knowing that this is not true)

Fins will be at some temp between air local temp & refrigerant inside the tubes - depending on how close to the tube surface you measure.


So ice build up, and this depends upon so many factors, coil depth, evap temp, air temp , humidity, velocity! but generally the majority of the ice is going to form on the inlet face of the coil. So on split fin the mass ice forms on the fins with the greatest gap, whilst still allowing high levels of air flow.

Can you explain this further? Why would it be the inlet face of the coil & not the middle, or exit?

[DTLarca]

Can you explain this further? Why would it be the inlet face of the coil & not the middle, or exit?

With Heat Pump AC units some manufacturers configure the indoor coil for cross counter flow in cooling which results in cross parallel flow in heating. Others prefer to maximise indoor coil capacities for heating and so they use cross parallel flow configurations for cooling.

But what would be the difference in icing patterns on outdoor unit cross parallel versus counter cross?

I use these danfoss video embedded into power point to discuss these concepts with techs.

From the middle of this video to the end you see the air coil is configured for cross counter in heating mode thus cross parallel in cooling mode...

http://www.youtube.com/watch?v=8sCzOgNeN_E

[desA]

http://i54.tinypic.com/21azipg.png

Temp-distance plots for counter-cross & parallel-cross evaporator coils.

[desA]

I was thinking along those lines, coil depth, air velocity etc. Obviously the moisture would freeze quickest at the coldest point but whether the majority would reach that point is the question in my mind as it would depend on how quickly the moisture would condense upon impact with the coil and therefore where in the coil it would deposit.

Based on the TL diagrams above - where would the first ice begin forming?

I do suspect, though, that there will be different weather conditions to consider:
1. Cold air, with moisture;
2. Snow blown into evap inlet;
3. Snow falling onto evap discharge.

The evap/fan coupling & direction of air-flow through the heat-pump will play a part here.

[desA]

For cross counter flow parallel circuits with negligible pressure drop the earlier circuits after the expansion device, post epi-saturation, are the coldest - they have greatest refrigerant wetting than the pipe run approaching the superheating dry-runs.

Am I wrong?

Have a look at this, based on the TL diagrams presented above.

[DTLarca]

I wrote an article on similar matters ( counter versus parallel) a while back - the images are relevant to this topic though

http://fridgetech.com/articles/acrnews/parallelflowchiller/


Counter Flow
http://fridgetech.com/articles/acrnews/parallelflowchiller/counter_flow.gif





Parallel Flow
http://fridgetech.com/articles/acrnews/parallelflowchiller/parallel_flow.gif

[MikeHolm]

Conditions for my part of Canada, indeed for the part of N.A. where 50 million people reside has winter day time temps of +5 to -10 until Jan 1st and -5 to -20 till March. Yes our winters are colder than Ireland and the UK but it must be -20 before the RH gets below 30%. I am being so general here that the statement might be discounted BUT the dewpoint could be anywhere in the coil at different times so under what conditions do you design for. You cannot design for all.

That said, I would instinctively had the hottest refrigerant hitting the coldest part of the coil to get the most heat transfer but does that cool off the refrigerant too much to provide resources for appropriate superheat (and promote the ice buildup farther back in the coil)? Looking at this discussion, that does not appear to be what BF has said: warmest refrigerant to warmest part of coil.

Is this a typical design norm in HP Evap coils?

[desA]

Conditions for my part of Canada, indeed for the part of N.A. where 50 million people reside has winter day time temps of +5 to -10 until Jan 1st and -5 to -20 till March.

What RH% at these temps?

[quote]Yes our winters are colder than Ireland and the UK but it must be -20 before the RH gets below 30%.

So, -20C ; RH 30%

[Bigfreeze]

Conditions for my part of Canada, indeed for the part of N.A. where 50 million people reside has winter day time temps of +5 to -10 until Jan 1st and -5 to -20 till March. Yes our winters are colder than Ireland and the UK but it must be -20 before the RH gets below 30%. I am being so general here that the statement might be discounted BUT the dewpoint could be anywhere in the coil at different times so under what conditions do you design for. You cannot design for all.

That said, I would instinctively had the hottest refrigerant hitting the coldest part of the coil to get the most heat transfer but does that cool off the refrigerant too much to provide resources for appropriate superheat (and promote the ice buildup farther back in the coil)? Looking at this discussion, that does not appear to be what BF has said: warmest refrigerant to warmest part of coil.

Is this a typical design norm in HP Evap coils?

If your warmest air hit your coldest refrigerant first, it will decrease the air temp further, meaning the coils on the air off side with the warmest refrigerant, will have less temperature differential to the air in order to retrieve energy.
Think of it from a heating point of view where you have a heat exchanger on a dhw tank. The warmest water will enter the top of the hx and the coldest water the bottom. As the cold water moves up it gains energy from the cools water first and finally the hottest as it leaves the HX which means the water will leave the temp at the temp closest to the temp of the tank and the water re-entering the tank will be at its coldest

One further thing, even though the RH may seem high at 40-50% at -10 this is still a very small amount of moisture as air at that temp can hold damn all moisture. So while the percentage may seem high the quantity is not. We would have far more problems with humidity here than you will ever have

[desA]

http://www.refrigeration-engineer.com/forums/showpost.php?p=217741&postcount=35

A link to the Mitsi heat-pump thread, where I mention the troublesome temperature range just above freezing.

[Bigfreeze]

http://www.refrigeration-engineer.com/forums/showpost.php?p=217741&postcount=35

A link to the Mitsi heat-pump thread, where I mention the troublesome temperature range just above freezing.

The 0 - +7 range is probably the worst for frosting. The air can hold a decent amount of moisture while the evap temp doesn't get high enough to avoid icing

[mad fridgie]

For this application you have to design a system that either gives you best results for a fixed design condition (its ratings) or one that will work well across a range of conditions (best for the end user), in my opinion these are two completely different units.
If we look at the working conditions, air on could be from 20C to -20C for heating and 40C to -20C for DHW. So when designing the evap you need to calculate to cover all the working conditions.
If we look practically at ice formation, we should consider the difference between ice and snow formation.
Most of these types of units actually reduce airflow by snow formation on the air entering face, whilst we tend to have a more compact/dense ice formation in the interior of the coil (less restriction to air) why is it that snow is formed?

[mad fridgie]

I do not use the evap coil to produce superheat, so excluding pressure drops the refrigerant is the same temperature.
But we should look at pressure drops, if your evap is designed to standard 7Cdb and 6Cwb, for opitium performance the pressure drop is likely to be very slight (SST 1C), but if we look at the same evap when the ambient -20C and when it is this cold humidity is a minor, your SST will be -24C, your pressure drop will increase, for two reasons, a massive reduction in density and massive increase in flash gas (OK this is offset slightly by reduced mass flow)

[MikeHolm]

MF, and if (for the above condition) I bring on another smaller evap coil with solenoid??? What happens then?

Or, size it to 0Cdb and 1C wb...I supposed I will then have pressure issues when ambient is 15C?

[mad fridgie]

MF, and if (for the above condition) I bring on another smaller evap coil with solenoid??? What happens then?

Or, size it to 0Cdb and 1C wb...I supposed I will then have pressure issues when ambient is 15C?
Mike, what we are talking about really is the number of circuits. for the higher ambient you would have fewer circuits, to keep the velocity and heat transfer up, but for lower temps you would have more circuits to keep the velocity/heat transfer the same and the pressure drop low. So when using the higher number of circuits on high ambients, velocity/heat transfer would be lower, than ideal. It would also be likely that a bit of mal-distribution would occur.
Also a little bit of oil entrapment may occur, personally i do not think this would effect the reliabilty of the compressor (as it would return at regular periods) Looking at it practically, as far as i can see the high limit for evaporting on compressors on R410a, is quite low, so loosing efficiency of the evap at the higher ambients is not such a bad thing.
I am no expert on evap design, but know enough to ask the right questions to the evap manufactures, and know enough to know when some try and BS me.
You can have a interwoven coil, where there is 2 pathways for the refrigerant, each having its own expansion device, one could be isolated at the higher ambient conditions.

[MikeHolm]

I think I will pose that question to a couple of manufacturers when I build up some relationships. Are there any examples of the "interwoven coil" that you know of?

I am sure there are ways of minimizing oil trapping as well.

[mad fridgie]

I think I will pose that question to a couple of manufacturers when I build up some relationships. Are there any examples of the "interwoven coil" that you know of?

I am sure there are ways of minimizing oil trapping as well.
I have used interwoven coils many times, most commonly on large de-humidification systems, and generally when i have 2 independent refrigeration circuits.
Due to the market size in NZ a good proportion of the evaps used (even standard ones) are manufactured to order, so having specials does not really effect the cost. (standard materials)