Buffer Tank Assisted Defrost

[mad fridgie]

This is quite a funny read, I have long subscribed to the theory that the North America is about 10 years behind the rest of the world when it comes to refrig and air and this proves it!

I can't believe it's that hard a concept to grasp! The rest of the world caught on and designed R/C systems the same way for the last 15 to 20 years but no, good old US of A reckons it can't be done!

Good on your mate!

[pilko]

Not to good then!
We measure pressures and temp around the comp (as the temps you are reading are the process variables (after the fact) not the system variables (driving force)

Are you refering to my lack of an accumulator.
Excuse my ignorance --- I am an electronics man and sadly lacking in Fluid and vapours.

pilko

[MikeHolm]

@ mad fridge,
"I get the impression you do not even have temp defrost termination!"
Are you refering yo me or the expert on the other site.(this is all getting very confusing)

pilko

Guys, maybe i can shine a bit of light on the current north American technology (LOL). Dealing with this york unit, with a johnson controls board, the defrost is 1) based on 6 hrs comp run time, 2) has a liquid line sensor and an ambient sensor. The sensor only allows defrost at temps from -5C to 10C and will de-energise the fan, energise the crankcase heater (if there is one), energise the RV, energise stage two if there is one, start a max 4.5min cycle.

There is a defrost curve for the 4 different HP outputs available by moving a jumper. With this one, the defrost is terminated by both liquid line temp (24C) and/or time.

[MikeHolm]

So Pilko, to both really, tried to post of hvac-talk, sort got kicked off,
I question and gave answer to your questions, but they have been deleted.
So I have tried another approach, lets see what happens.
No kicked of that way as well?
So sorry can not help on the other forum.

Mad, i have also done as you did and got kicked off as well. You have to post in a newbe forum thread for a while until you can prove you are in the trade. I haven't bothered.

[mad fridgie]

You are starting to get, some better answers on HVAC Talk. (not right but moving in the direction)
Defrost on demand, the way to go!
The bit others are mis-understanding is the amount of energy removed from your home during defrost. Which has to be replaced, and the cost of this replacement.
Also understanding of the non steady state nature of refrigeration.
You do need to measure pressures. (this determines comp performance)
The carrier may well not be designed to have the fan off, this could just be mechanics, nothing to do with energy.

[pilko]

The answer lies in these images. The first image shows 2 defrosts, the first at 1000 CFM and the second at 500 CFM. When I zoom and put them together in image two, the area under the 20* return temp (-dT x Time) is only 29% larger with a 500 CFM defrost BUT considering the CFM is only half, the heat loss is only 64%.
It is logical to conclude then that heat loss at 0 CFM will be a small fraction of heat loss at 1000 CFM
I will do the complete calcs using air density and specific heat.
Anyway the chart appears to prove your theory.

pilko

[mad fridgie]

Nice info, with good calculations (on HVAC Talk). It always difficult to know what somebody really knows or not.
You seem to have a good handle on this side.

[pilko]

The Calcs:-

At 1000 CFM Average dT of the air = -9.04
Time = 138 seconds
Heat Loss = CFM/60 x dT x Density of air x Specific heat of air
= 1000/60 x 138 x 9.04 x 0.075 x 0.2375
= 370 BTU
At 500 CFMAverage dT of the air = -10.3
Time = 156 seconds
Heat Loss = CFM/60 x dT x Density of air x Specific heat of air
= 500/60 x 156 x 10.3 x 0.075 x 0.2375
= 238 BTU
It should follow that the heat loss at 0 CFM will be very low

pilko

[mad fridgie]

Just seen the answer HVAC Talk,
with a blocked filter, you get poor refrigeration performance which leads to irregular ice build up ( this comes down to distribution, vapour liquid fraction "x"), and can mislead the defrost sensor.
When ice is heavily formed (due to faults) normal defrost (time limited) will not clear the coil.
You can post this reply!

[mad fridgie]

HVAC Talk
Well the answer you would expect, from someone who does not understand the principles of refrigeration.(cause and effect)
No wonder heat pumps have a bad name when the wrong advice is given.
I will defend though, a standard unit may not have the smarts or the mechanical components to ensure correct operation and reliability to run with out a fan, that does not mean that is correct engineering, or the reason they state are correct, they are not!

[MikeHolm]

Mad, I cannot tell if some of the conversation above is you or "HVAC talk" talking. Uggg

Regardless of the math, any object in still air will react faster to internal heat in still air than in moving ambient air. Pure common sense. Don't you freeze faster when its windy out?

[mad fridgie]

It is me, replying to "hvac talk"
I might know a bit about refrigeration, but I am bloody useless when it comes to computers
I think the OP understands where I coming from??????????????

[pilko]

It is time for me to make a decision.
Mad fridgie You have provided valuable technical information, the other forum has not.The only usefull information the other forum has provided is obvious, that the heat pump will run longer to defrost the coil. My graph already showed that ( and it is not much longer)
I am still not sure that it is safe to do it as the consequences are great (it is my money)
I will program my HP to reduce fan speed during DF, for a number of times at 50% CFM then reduce to 40% and so on.
During this testing It would prudent to monitor important parameters, please advise which and I will set it up.
Thank you everyone for your help. Especially mad fridgie-- for all your time and frustation.

regards

pilko

[mad fridgie]

I will now talk about the possible risk.
When the fan is not running the amount of energy pick up is very low (this you understand), what can happen, instead of the liquid being boiled off, some liquid may leave the indoor coil. (this very specific to your machine, and i can not help here) the liquid can not be compressed, so could hydraulic your comp internals (highly unlikely), but it could dilute your oil, which will reduce the lubricating effect. (additional wear on bearings ect.) The amount of refrigerant flow is determined by the outdoor coil pressure, which should be low, but (not in your case) if timed defrost only this pressure could rise, forcing more flow through the indoor coil, and the type and size of the expansion device. (TEV, Capillary, EEV, fixed orifice)
The accumulator that you do not have, traps any liquid prior to entering the compressor.(this the mechanical limits of your system)
OK so far!

[pilko]

I'm Listening and understanding.

[mad fridgie]

It is possible that when the defrost is complete, (starting to pre -heat, still no fan) then when the fan starts a puff of stream my come out, not that this really a problem, just people maybe unaware and think it is smoke.

[pilko]

Now that I have a better understanding, can I get back to my original question, phrased in a different way?
Proposal:-
--Install tank in vapour line as shown.
--Tank water collects heat during heating mode (I realize that is heat not going into living area.)
--Tank will eventually reach close to vapour temp.
--During defrost, stop the indoor fan the buffer coil will now replace the indoor coil and become the evaporator.
--The compressor will be happy.
--No more cold air into my living area.

pilko

[mad fridgie]

Reply to latest "Hvac-Talk" (please forward)
Carrier.
if one brand can defrost with out a fan running, then this means the "energy mass balance" would work for all.
So the "Pros", are all wrong for the reasons given.
Mechanically the USA brands may have problem, due to chance of liquid entrapment in suction line (Accumulators do not seem to be standard)
Controls are also likely to be very poor.
Tech was limited to a level which should be understandable.
The USA has had it to good for too long, installing 15Kw heater bank to cover defrost, yes it works, but masks basic fundamental flaws. Only now are is the common man looking into energy saving. You are behind most of the world in this area. Learn from those with more experience and knowledge in this subject.

[mad fridgie]

Now that I have a better understanding, can I get back to my original question, phrased in a different way?
Proposal:-
--Install tank in vapour line as shown.
--Tank water collects heat during heating mode (I realize that is heat not going into living area.)
--Tank will eventually reach close to vapour temp.
--During defrost, stop the indoor fan the buffer coil will now replace the indoor coil and become the evaporator.
--The compressor will be happy.
--No more cold air into my living area.

pilko

No many other issues to contend with.

If you want to change your system, go to hot gas defrost with a buffer in the comp discharge line.
You still need an accumulator!
Nothing to do with the indoor unit.

[pilko]

Can you explain in simple terms why the heat in the buffer tank, transferred to the buffer tank coil cannot do the job of the "A" coil during defrost.

pilko

[MikeHolm]

OK, here is a pic of the coil. I don't know if "A" coils are used outside North America.

coil-evap-pupa-lg.jpg

[pilko]

Thanks Mike. I know what an "A" coil looks like.
I think I am not explaining myself properly.-- my proposed is a tank of water with a heat exchanger in it.
The Heat exchanger/coil will do the job of the "A" coil during defrost, since the "A" coil cannot satisfy the heat requirement during defrost.

regards

pilko

[mad fridgie]

In simple terms, the vapour will be very highly superheated, which in turn cause very high discharge temps (damage to compressor)

[mad fridgie]

"A" coil can satisfy the defrost requirement.

[MikeHolm]

Thanks Mike. I know what an "A" coil looks like.
I think I am not explaining myself properly.-- my proposed is a tank of water with a heat exchanger in it.
The Heat exchanger/coil will do the job of the "A" coil during defrost, since the "A" coil cannot satisfy the heat requirement during defrost.

regards

pilko

The coil pic was for Mads info, not sure if they use ones shaped like that down there.

[MikeHolm]

"A" coil can satisfy the defrost requirement.

It is the interior coil

[mad fridgie]

Thanks Mike,

This design of coil is not common in this neck of the woods.
Generally more long and skinny (long and no so high) and usually comes as package (fan included in the housing)

[MikeHolm]

These ones are meant to sit above a forced air heat exchanger. Pretty archaic in my opinion. (the furnace, not the coil)

[pilko]

Just calculated the area under the hot vapour line temperature curve for the two defrost conditions,930
and 590 CFM.(blue lines)
Since the areas under under the curves are proportional to heat (dT x Time) I thought it would be interesting
to compare the two quantities of heat that were transferred from the "A" coil to the compressor.
At 930 CFM, heat transferred = 641 CFM
At 590 CFM, heat transferred = 632 CFM
In other words, just as much heat is transferred in both cases.
This is further confirmation of your argument.
My concirn is the final temperatures at the end of defrost -- 10*C at 930 CFM and 4*C at 590 CFM.
What Is the minimum safe temperature for the compressor???

pilko

[pilko]

Cannot figure out how to edit my post.
The heat transferred units should be *C.Minutes not CFM.

[mad fridgie]

What we really need to know is the saturated temps (pressure converted into temp),
It is the differences between the saturated temps and the real temps that determine some of limits.
Without the sat temps, actual temps have little meaning, for the refrigeration system.

[mad fridgie]

Ref Eng.pdf

This how you do it if want to shorten defrost time, and have energy benefits from the defrost.

[mad fridgie]

I better explain.

the tank is heated ( in heating mode), by the liquid refrigerant leaving your indoor coil, so there is no loss of energy from your home. This will also benefit the outdoor energy absorption (better cooling), as we have sub cooled the liquid, reducing the vapor fraction leaving the TEV, in cold conditions the outdoor coil refrig pressure drop will reduce and increasing the LMTD (temp across the coil will be more even) better distribution is likely to occur and more than likely lifting the comp suction pressure (exact performance is equipment specific)
We now have an energy source, that has no negative effects on your internal heating. (even some positives),no losses of heat or any extra energy introduced.
We go into defrost, ( I would turn the fan off only to reduce the wind chill factor because no heat is being produced) The boilng refrigerant, passes through the warm tank, absorbing lots of energy (cooling the tank down), because we are absorbing this energy defrost will be quicker. Compression ratios will be smaller, so more efficient. But the biggy is that we are not removing any energy from the home at all due to the defrost.
Because the tank is warmed (close to saturation temperatures, not compressor discharge temp), you are not going to get very excessive suction superheat temps (as you would with your original drawing).
On completion of defrost leave fan off until the indoor coil reaches 40C + (ramp slowly)
So all issues are resolved, and large energy savings to boot.
You can post on HVAC TALK along withe drawing.

[pilko]

Thanks mad fridgie,
I am now really confused and frustrated.
The only difference between your proposal and my original proposal back in post #1 is
that you are bypassing the indoor coil in defrost mode. (and bypassing the tank in cooling
mode but that was not being discussed)
When I made my original proposals the following replies were given:-

big Freeze---
"Not really feasible as yours is an air to air unit and therefore the unit is designed
to heat air and not water."

Nevgee---
"Have you considered the amount of superheat and what effect that would have on the
compressor?"

you
"In simple terms, the vapour will be very highly superheated, which in turn cause very
high discharge temps (damage to compressor)"

So to summarize, my original proposal in post #1 would work.

pilko

[mad fridgie]

The devil is in the detail. And is not the same
Me was referring to post 67
lets put it this way.
1000KVa, what size cable at?
a; 1000v
b; 10V

are they the same?
If it was sized for 1000V, what would happen if you then used 10V

[pilko]

"the tank is heated ( in heating mode), by the liquid refrigerant leaving your indoor coil"

Same with mine.

"We go into defrost, ( I would turn the fan off only to reduce the wind chill factor because no heat is being produced) The boilng refrigerant, passes through the warm tank, absorbing lots of energy (cooling the tank down), because we are absorbing this energy defrost will be quicker. Compression ratios will be smaller, so more efficient. But the biggy is that we are not removing any energy from the home at all due to the defrost.
Because the tank is warmed (close to saturation temperatures, not compressor discharge temp), you are not going to get very excessive suction superheat temps (as you would with your original drawing)"

same with mine.

Why will my system not work

pilko

[mad fridgie]

NO it is not.
I am going to put 100amps at 10V, down a 0.5mm wire, or 1 amp at 1000V down the same wire, same result!
Is there going to be "any" difference between the 2

[pilko]

I Think I understand you now.
Are you saying that the tank requires two coils of different specs. because the flow through them
is different in heating mode and defrost mode?

pilko

[mad fridgie]

Yes i am talking about pressure drops, because at each stage you are different states and densities.
Look at your evap inlet (cooling, defrost mode) small pipes, designed for primarily for liquid flow (high voltage), if you use your system vapour will pass through (low voltage), massive pressure drop, causing high superheat (not high temp) and very low comp inlet pressure. You could use one coil in the tank, but valving would be bigger more expensive and also has other issues.

[pilko]

Thanks mad fridgie,
Based on hopefully increased knowledge, would this work as it would be simple for me to build:-

Heating mode.

B closed A open till water in tank reaches X*C.

Then A closed B open.


Defrost mode.

B closed A open.


Cooling mode.

A closed B open

pilko

[mad fridgie]

Yes it would work, but is not energy efficient! You are stealing energy from the house, worse than turning the fan off.

[pilko]

In both your case and my case the heat is being generated by the heat pump.
My tank would heat much faster as it is heated by hot vapour."Short time not heating the house"
your tank heats much slower as it is heated by the cooler fluid. "heating tha house at a lower temperature"

pilko

[mad fridgie]

No it is not,
in your case the heat is produced by the compressor and is useful to your house heating , in my case the heat made by the outside ambient.
(I use heat because you do, I prefer the word "energy") This is not an element! You can not think of it as an element. This your mistake!
The energy in mine has no use in the house.
Speed is not important. As long as the energy introduced into my tank is equal to what is required for a fast defrost.
On this side of the equation, what i am doing does NOT effect the energy rejected into the home!

[mad fridgie]

In simple terms (and wrong terminology)
You have 20Kw energy leaving your compressor, 10Kw is taken by the house, normally the remaining 10kw is pushed back outside, where energy renters the refrigerant 7kw, the comp compresses 3Kw, back to 20Kw.
So there was 10kw of energy that goes round and round, in my system I steal some of this say 1Kw, this is replaced for outside, so the 7Kw goes to 8Kw.
Yours steals the 1kw from the house heating you now have 9kw for house heating. the outlet of the indoor coil does not change. so the 10Kw still just flows round and round.

[pilko]

In both cases the energy is produced by the uotside ambient heating the outdoor coil and then compression.
In your case, some of that energy is transferred to the tank before the energy transfer medium is returned to the outdoor coil.
In my case all of the energy is transferred to the tank for a shorter time then it is all transferred to the indoor coil before the energy transfer medium is returned to the outdoor coil.

I do not see the net difference.

[MikeHolm]

Good explanation Mad, 1 pint for you.

The big question for Pilko is to figure out how to make a tank with two HX that works for him.

[richardb14]

Thermia have a system not to far off this for defrosting the outdoor unit of an Air/Water heat pump. It utilises some of the stored energy as the heat source (indirect) to remove the ice in reverse cycle.

[richardb14]

I should also point out that Danfoss now own thermia!

[pilko]

@ Richard --- thanks for the info.

@ Mad --- Thank you for all of your support and a special thanks for being so patient with me.
Most people would have given up on me by now.
It is obvious that I need to study the thermodynamics of the refrigeration process.

Regards

pilko

[Emmett]

This is quite a funny read, I have long subscribed to the theory that the North America is about 10 years behind the rest of the world when it comes to refrig and air and this proves it!

I can't believe it's that hard a concept to grasp! The rest of the world caught on and designed R/C systems the same way for the last 15 to 20 years but no, good old US of A reckons it can't be done!

Whoa Cowboy, the gent pilko is north of the border not one of ours!!!