ReplyDefrost without the fan?
Where's the heat to melt the ice supposed to come from?
The heat absorbed by the refrigerant in the compressor isn't sufficient.
Professional Member*
Hi I have been assisting Pilko on setting up his defrost system
http://hvac-talk.com/vbb/showthread.php?t=978262
There seems to be a lot of resistance to the "no fan method."
There was a suggestion that Pilko should come out of retirement to teach you engineers.
If you want to be taught then I am willing to teach those who are willing to learn.
barbar (aka, mad fridgie, Refrigeration Engineer.com)
Regular Guest
Defrost without the fan?
Where's the heat to melt the ice supposed to come from?
The heat absorbed by the refrigerant in the compressor isn't sufficient.
Professional Member*
It is my understanding that the system in north america are "reverse cycle" and not "hot gas" defrost, there fore your total energy required for defrost comes from a combination of residual energy from the components of the system (primarily the indoor coil, and the compressor) and the energy drawn by the compressor during defrost. There may not be much heat (temperature) but there is sufficient energy above the freezing point of water to defrost.Originally Posted by amd
(check results on http://hvac-talk.com/vbb/showthread.php?t=978262 )
Global Moderator
Look at the 4 page thread on this subject in AOP residential. No need to start a new one here.
Professional Member*
I was unable to comment on that thread, It is very basic in technical information. (as comment was made) The OP of that thread is not a refrigeration engineer.
Most replies from the pros, simply had no technical back up around the comments and in my opinion where totally wrong on a fundamental level, however your systems may have practical limitations that do not allow for this type of defrost. If so what are they?
I opened this thread up so a technical discussion could be made to all that are interested and all could comment to what ever level they are up to.
This industry is not black or white, it is grey. so to encourage learning differing opinions have to be aired.
Professional Member
barbar,
technical discussions here take place in the Pro's Technical forums.
You seem like a knowledgeable guy and one that I'd like to see as a Pro here.
The requirements are in my signature. I'm sure you meet them but for the post count, which you could easily remedy.
Professional Member*
Thanks JP,
when post are met, we will see if you want me on board as pro.
cheers
barbar
Professional Member*
I don't understand how limiting the heat gain from the indoor coil during defrost would help?
Professional Member*
That is a good and fair question, if we looked at the defrost part only, then reducing the energy gain from the indoor coil, does reduce the effectiveness of the defrost (Speed or length of defrost), how ever if the fan is on, you are removing energy from the home, (and a cold draft), this energy does need to be replaced, I believe it is common that you have a heater bank, that over comes this problem. But is this efficient? "No"
Why is it not?
During defrost and the fan is running, your cooling system COP is very high (little power draw, for lots of cooling), this is due to the fact the condenser saturated pressures are very low (defrosting the outdoor coil). When you revert back to your heating cycle, the heating COP is not has high,as it was in the defrost cycle. So the net effect is that you need to introduce more electrical energy, to recover the losses made during defrost.
So here is a very simple example (no too techie)
Your heating COP is 3, or in other words you introduce 3kws of energy into your home by using 1Kw of electrical power
On Defrost your cooling COP is 6, or in other words, we remove 6Kws of energy from your home using 1kw of electrical power.
So keeping the maths simple, we have removed 6Kw for 1Kw, which has to be replace which is 6Kw for using 2Kw, we have wasted 1Kw.
Practically defrost will be a bit long, and the thermal mass of the indoor unit will be colder, so the actual saving would not be 1Kw, but slightly less.
Regular Guest
Melting ice a fairly energy intensive. (144 btus per pound) I still don't know where the heat is supposed to come from if the fan is off.
Compressors produce heat due to compression and motor losses, but the cop is 1. (total lifespan of compressor might be reduced due to wear of defrosting like that, longer defrost cycles also reduce the total amount of heat the heatpump can capture, increasing heat strip use)
Heat strips also have a cop 1.
I also don't know where you got the 6 COP figure during defrost cycles; with a very cold outdoor coil, the odds of actually have a solid liquid column of refrigerant feeding the indoor coil are low.
I don't think there's a point of playing silly games:
There are much better ways to reduce energy losses associated with defrost cycles:
- Install a demand defrost board to eliminate unnecessary cycles
- Install an outdoor stat to keep the second stage of heat strips off above a certain outdoor temp) -->Temper the air, don't heat it.
- Keep the outdoor coil clean so it doesn't run as cold in heating mode -->less frosting
Professional Member*
Ok good points.
YOU pick a standard unit used in north america, but you must know how the system works and not rely on the sales BS, pressures, temps, air flows, current draw power factor, compressor displacement.
We will work out firstly your ice production (what is required to be defrosted)
I use Copeland Select7, so any figures I quote as being technical will be referring to this.
Do you think this is a new concept? It is being used just about in every other part of the world, for at least 20 years, and for temperatures down to -15C (more modern ones down to -20C)
You do not understand the process, so you think it is a silly game!
I do agree that smart defrost control is important "defrost on demand" and good service!
Aux heaters we design for 5% days, which normally equates to a max run time of less than 2% of the heating season, what is the point of designing a heat pump system if much higher than this.
Some data Copeland ZR32 on R22
start of defrost (indoor loaded with liquid refrigerant, outdoor frozen with ice) fan running
27C SCT and 12.5C SST, cooling duty 7.7Kw input power 0.8Kw "COP 9.32"
during defrost
27C SCT and 5C SST, cooling duty 6.1kw, input power 0.9Kw "COP 7.14"
I was pretty conservative with my 6
However I would say the SCT would be lower that 27C, but Copeland indicate that this is out of range, so no data available. i could just go down to fundamentals, but then the info would not be independent.
The lower the SCT the lower the draw, but the SST would track down due the expansion device limitations. COP would still remain high.
Professional Member*
If you took a pretty standard unit heating 3.5Ton 12.3Kw, say an average COP of 3, then the cooling capacity would be around 8.2Kw
If the air ambient was 3C at 100% RH, and chilled to 0C with an evap temp -5C, you would produce 5.2Kgs of ice (presume no by-pass or water drain)per hour (11.5lbs give or take)
If you defrosted every hour, you need 0.5Kw (1650Btu) to defrost this ice. Of course you need some energy to bring up the coil block itself.
If we just for time being forgot all other energy apart from that of the compressor during defrost. This unit normally would draw around 4Kw, but during defrost, the drew would drop (low SCT) by approx 30% (check on compressor charts), so draw would be around 2.8Kw. Therefore we 0.5Kw for defrost we have 2.8kw available defrost will be 0.5/2.8 18% of an hour 10 mins. Of course we do have the residual heat from the system, which will speed it up.
Regular Guest
"You do not understand the process, so you think it is a silly game!"
Tell me where the heat to defrost the coil comes from.
Having the heat come from the compressor isn't more efficient than using heat from the conditioned space and compensating with aux strips.
Is cooling a factor there?Aux heaters we design for 5% days, which normally equates to a max run time of less than 2% of the heating season, what is the point of designing a heat pump system if much higher than this.
In north america, if heatpumps were sized to provide sufficient heat at design, they would short cycle like crazy in cooling mode. The airflow requirements would also be ridiculous.
Even if the strips provide 50% of the heat, running a heatpump with a cop of 2-3 can make sense if there isn't a cheap energy source available (like gas) and cooling is needed anyway.
Maybe north american equipment is obsolete compared to what you have in europe - central models with inverter driven compressors are very new to the market here.
Professional Member*
Interesting method , sounds like you have done a lot of homework too , in systems that use this method , what is the sequence of operation after defrost termination ? (I would imagine that a large pool of very cold air is sitting in the plenum etc)
Professional Member*
Cooling is not a biggy where I live, but in other parts, cooling is as big as heating.
Fixed speed machines will always cycle, it pretty hard to design to cover all eventualities.
As in parts we go from 40C to -10C, so house loads vary dramatically.
For air to air systems, inverters or digital generally can not be beaten, as capacity meets load.
We generally have less airflow for heating than cooling, but this comes down to the wind chill factor, a warm dry wind feels cold, so we tend to have a lower volume at a higher temp.
I am in New Zealand, but our engineering is typically European and Asian.
In my opinion, North america becomes a leader in a field (as it was in the HVACR), then just sits on the technology for way to long, to the point it becomes obsolete as you say.
Professional Member*
Excellent question!
Yes of course the indoor coil and its surrounding will be colder than you would normally expect. After termination, the refrigeration cycle returns back to heating mode, however the indoor fan remains "off", only when the indoor coil reaches a temperature above 40C(upto 50C), then fan is then brought back on, with a ramp if possible. This way you do not get the cold draft.
Professional Member*
I am going out on a limb that for a delay period after termination the indoor fan stays off to reheat the coil,plenum etc, and after a suitable delay (or temp rise) the fan restarts. I get the change in performance due to reduced CR , and am starting to get that the plenum rewarming process also starts at a greatly reduced CR as well , so all in all sounds pretty cool , Trouble is Industry in North America has huge Inertia that resists new methods (not a reason to avoid change, just sayin is all). I think this Idea is worth putting side by side with a conventional system to test different strategies and see what happens . I have the unique situation to be able to both Research and teach at the same time , so even if it was not especially productive , the learning process would still continue.
Professional Member*
Guess it was a sturdy limb....!!
Professional Member*
Hi madblack, i am best known as mad fridgie, good to be "mad" ye!
This came about from one of your country men asking about installing a buffer tank on another site.
(check my first entry and follow the link on this site)
Apart from the backbiting, Pilko has recorded some good data. (not the way i would do it but all the same is excellent)
It is easy to get stuck in our ways and use rules of thumb, but with worldwide changes in energy use, a lot of what was normal is going out of the window, i feel as an industry worldwide that we need to retrain ourselves, to bring us back to the pinochle of the trades field that we once were.
Professional Member*
Very sturdy indeed, I now
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