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View Full Version : O/T Would like an opinion, fact. Full frig or empty. Whats more efficient?



JRouche
01-14-2016, 10:44 PM
The reason I am askin is cause we have a small frig in the backyard, rarely used but convenient all the same, so it is gonna stay lit.

The issue is my Wife says it should be run empty until needed (fill with same temp products), less to keep cool.

I think a full box with beverages (Its a bev. frig after-all) would be better. That is if H2O based liquids are better at keeping the box at a certain temp verses just an empty refrigerator box??

What is your take folks??? JR

portlandRon
01-14-2016, 10:54 PM
Full is more efficient once the contents are cooled down.

Evan
01-14-2016, 11:12 PM
The less often the fridge must start the less energy is wasted during the start cycle. The fuller it is the less often it must start. I keep a couple of gallons of plain water in my apartment freezer so it starts far less often. I also use about 20 to 30% less electricity than the average of users in this apartment building and that is including the 400 watts per hour each night for my oxy concentrator. However, that is just heat and this building is full electric so it really doesn't matter where the heat comes from, all devices are 100% efficient for electric heat generation in winter. In summer it matters.

Glug
01-14-2016, 11:12 PM
More efficient, as in requires less electricity?
If it was a furnace, and you wanted to keep it at 300F, then it seems intuitive that the more stuff that's in it, the more energy it would require. Why would it be different for a fridge?

Evan
01-14-2016, 11:15 PM
The starting cycle is big energy waster in a fridge. It's drawing power but not cooling for the first few minutes. The fridge warms up faster the less there is in it so the more often it starts.

A.K. Boomer
01-14-2016, 11:29 PM
The less often the fridge must start the less energy is wasted during the start cycle. The fuller it is the less often it must start. I keep a couple of gallons of plain water in my apartment freezer so it starts far less often. I also use about 20 to 30% less electricity than the average of users in this apartment building and that is including the 400 watts per hour each night for my oxy concentrator. However, that is just heat and this building is full electric so it really doesn't matter where the heat comes from, all devices are 100% efficient for electric heat generation in winter. In summer it matters.

It's good to have you back Evan!

Glug --- maybe it's a trick question --- if all systems are equalized and then you start to take one month readings on consumption then look at Evans answer cuz it's spot on --- start caps in the motor not to mention all the losses just in transfer delays and such eat it --- the full fridge and freezer win - less overall run time and less start cap engagement --- so if you got a frozen turkey from the store and cold milk and brought it immediately home you will be ahead of the game from the get go,,, maybe not so if you had to freeze a thawed bird or make warm milk cold, but make no mistake there's a time frame where the pay off would start rewarding in favor of the full fridge once again...

use the same theory of operation with the furnace --- in the long haul more crap in your house is actually energy efficient due to thermal storage and the furnace not having to kick on - on a whim,,, longer coast times and yes longer run times - but over all shorter run times for both fan blower and furnace and gas usage due to more efficient transfer --- everything having to do with heating already up to temps and not having to cycle so much and heat up things that do not have much to do with transferring till the snap discs kick in... yet gas is being burnt...

Evan
01-14-2016, 11:41 PM
Here is my power consumption compared to others in this building and this is for the middle of summer when the fridge and freezer really matter. It is also when I do not win from running the oxy machine. I still do far better than the others. I use LED lighting, turn down the hot water to 120F and keep the fridge and freezer full.

http://ixian.ca/pics12/hydro1.png

A.K. Boomer
01-14-2016, 11:44 PM
Don't forget also the power consumption from running the printing machine almost 24/7 :p


There is not just an efficiency benefit from a full fridge and freezer - there's also the increased life to the appliance to consider...

darryl
01-15-2016, 12:32 AM
It's going to use more energy to get a full fridge down to temperature, but then it uses energy based partly on how well it keeps the cold in. That's not a function of the contents, but of the insulation, seal effectiveness, and design. What Evan said is true- the more often it has to go through starting and establishing the cooling process, the more energy is wasted.

I have another take on it as well- an empty fridge is full of cooled air. When you open the door, cool air drops out and is replaced by warm room air. That air now needs to be cooled. If the fridge is mostly full, there's less air for one thing, plus lots of obstacles to slow down the air flow out of the fridge- so less loss of cool.

It is interesting that all the energy being used by an electric appliance of any kind is lost into the room, where it supplements the heat being produced by the furnace or electric heating. If you use electric heat, then in essence anytime you need to heat you could consider that all of your electric appliances are being run for no cost- if not for them running your electric heaters would have to draw more juice. Same thing goes for the number of people in the house- the more there are, the more heat goes into the room from the metabolic process of living. Of course this is offset by how many times they open the fridge and open outside doors.

A.K. Boomer
01-15-2016, 01:00 AM
.


It is interesting that all the energy being used by an electric appliance of any kind is lost into the room, where it supplements the heat being produced by the furnace or electric heating. If you use electric heat, then in essence anytime you need to heat you could consider that all of your electric appliances are being run for no cost- if not for them running your electric heaters would have to draw more juice.


Spoken from somebody who lives far north --- summer time hits around here and if somebody is using AC then it's a double wammy in the inefficiency department...

J Tiers
01-15-2016, 01:25 AM
Overall, it should not really matter.

The inside is at temperature "X", the outside at temperature "Y".

With a given type and amount of insulation, there is a certain flow of heat energy through that insulation per hour. That heat energy must be extracted from the inside about as fast as it comes in, so that the inside does not warm up too much. But the "leaking" heat energy must be pumped out or the inside will change temperature. If it stays at a given temp, there is a balance between the rate of heat leaking in, and heat being pumped out.

The flow of heat is no different whether the thing is full or empty, it is only the difference in temperatures that counts. Perhaps there is a temporary difference if the thing is packed full, simply due to the ability (or not) of heat energy to get into the area of the thermostat. But the balance must be kept, or the inside changes temp.

Then also, the temperatures count in a different way when you talk of efficiency. The heat pump used to cool it becomes less efficient as the difference in temperature is larger, and also is to some extent dependent on the absolute temperature into which heat must be rejected.

With a lot of high heat capacity stuff, like water, in it, the fridge turns on less often, simply because a given amount of heat energy changes the temperature less, so the fridge runs less often. But when it does run, it will have to run longer, to get the heat out of that mass of material.

If there is any difference in efficiency, it can only come from a difference in run time. The actual heat energy being moved around is the same whichever way you load it.

Mike Amick
01-15-2016, 01:34 AM
The flow of heat is no different whether the thing is full or empty

I don't think this is right. if the fridge is totally empty and you leave the door open, it is going to warm up
in there much fast than if every square inch was stuffed with very cold anything else.

Evan
01-15-2016, 02:43 AM
"The flow of heat is no different whether the thing is full or empty,"

Nope, there is a very big difference. To get warmer with a full fridge a lot more stuff must warm up including the two gallons of water in my freezer.

becksmachine
01-15-2016, 03:21 AM
"

Nope, there is a very big difference. To get warmer with a full fridge a lot more stuff must warm up including the two gallons of water in my freezer.

Yes, given any consistent temperature differential, the TEMPERATURE of that mass will increase faster with less mass to warm up.

But this is only because it takes less ENERGY (power) to raise the temperature of less mass.

Which means it took less energy to extract that heat in the first place. Consequently, the flow of ENERGY through a given barrier (insulation) is consistent with a consistent temperature differential.

Temperature is not, by itself, a measure of energy.

Dave

Black_Moons
01-15-2016, 04:04 AM
It's going to use more energy to get a full fridge down to temperature, but then it uses energy based partly on how well it keeps the cold in. That's not a function of the contents, but of the insulation, seal effectiveness, and design. What Evan said is true- the more often it has to go through starting and establishing the cooling process, the more energy is wasted.

I have another take on it as well- an empty fridge is full of cooled air. When you open the door, cool air drops out and is replaced by warm room air. That air now needs to be cooled. If the fridge is mostly full, there's less air for one thing, plus lots of obstacles to slow down the air flow out of the fridge- so less loss of cool.

...

Exactly my thoughts, a fridge filled even with just empty jugs is going to consume less energy to recool itself after you open and close the door.

Evan has a point with how long the fridge takes to cycle however, if the fridge is not opened very much that effect is likely to dominate.

Of course, Both effects say that a full fridge is going to take less energy to maintain.

Black_Moons
01-15-2016, 04:10 AM
Yes, given any consistent temperature differential, the TEMPERATURE of that mass will increase faster with less mass to warm up.

But this is only because it takes less ENERGY (power) to raise the temperature of less mass.

Which means it took less energy to extract that heat in the first place. Consequently, the flow of ENERGY through a given barrier (insulation) is consistent with a consistent temperature differential.

Temperature is not, by itself, a measure of energy.

Dave

Evans point is not that the mass keeps his fridge cool, its that the mass lowers the cycle frequency of the compressor.

Where as an empty fridge might spend 10 minutes on, 90 minutes off, a full fridge will spend 20 minutes on, 180 minutes off. Right there you don't see any actual savings.

The savings Evan is talking about is that for the first few minutes, the system is pressuring up and is wasting a lot of energy not being at running pressures and actually cooling the fridge.
Hence its more like the full fridge will spend 18 minutes on, 180 minutes off, because the first 2 minutes on is 'wasted' bringing the system up to pressure, so that is 2 minutes of running it can save every 200 minutes by cycling only half as often. Not 50% savings by a long shot, but some savings none the less, and OP's question was what was more efficient, full or empty.

Evan
01-15-2016, 04:18 AM
The issue is the inefficiency of the cooling machine. The more often it must start the more energy that is wasted. At the beginning of the cooling cycle the inefficiency is 100% and slowly improves as it runs. It also does not run until the entire mass is fully cooled. It only runs until the measurement device is sufficiently cooled. Very big difference there because the heat in the items in the insulated volume do not conduct heat very well, usually. So, the run time with a very full cooler is not directly proportional to how full it is, not even close. That is especially the case because of the on/off differential. The empty fridge will warm the thermostat very quickly and start far more often. I am not guessing this, I measured it when I bought the freezer last year. I also measured my power use very closely on a daily basis for months, possible because of the "smart" meters. My much lower use of power than the neighbours is no accident. The freezer played a significant part and almost none of the neighbours even have one, yet I still use less power even with my oxy concentrator as well. I also have a high powered computer that is on at least 16 hours per day and sometimes 24/7. I also run a lot of lights, I very much like it bright. The one thing I do not run is my TV. I rarely turn it on, maybe for an hour per week. Loading up the freezer cut the power use very measurably. It also reduced the run time, both number of starts and the total run time. I also placed a little computer fan that blows behind the fridge. That makes a very big difference. These are things I have found over many years. The house I lived in drew about half the average energy for similar houses.

BTW, if you are wondering how the total run time for a full freezer can be less than an empty one that is simple: The average temperature in a full one is a little higher with the same setting. I have a computer connected type K thermistor on a long wire. <grin>

strokersix
01-15-2016, 04:55 AM
You guys are not answering the question directly. Or perhaps the question poorly posed.

Does the refrigerator use more electricity when it is full or empty is the real question and the answers posted are toward this question.

Question as written was about efficiency. If you define efficiency as electricity used per beverage kept cold then obviously the empty fridge is the worst because it is keeping zero beverages cold. :D

Georgineer
01-15-2016, 05:25 AM
... Same thing goes for the number of people in the house- the more there are, the more heat goes into the room from the metabolic process of living. ...

Not strictly relevant to the thread, I know, but in my science teaching days the temperature of the lab rose perceptibly when the kids came in. Unfortunately I never got round to measuring temperatures, so I can't quote figures.

I read somewhere (I can't remember where) that a resting human has, on average, a specific heat output of 1 watt per kilogram. Take a class of 30 fairly active sixteen year olds of between sixty and seventy kilos each, and that's quite a number of watts in a closed room.

George

John Stevenson
01-15-2016, 05:43 AM
Commercial butcher against us has one of these big massive walk in fridges. At the side is a small shed full of bundled packages of newspaper, as the fridge empties he fills it up with newspaper and when new supplies come in he transfers it back into the storage shed.
He reckons it cost money to freeze air.

flylo
01-15-2016, 06:20 AM
Full

Tom S
01-15-2016, 06:45 AM
The full fridge will be more efficient.

Its because of what I call thermal mass, which to Evan's point allows the compressor to run at and efficient level for a longer period of time. J Tiers has pointed out that the infiltration load will be the same whether the fridge is full or not - a given amount of heat will enter that space based on insulation values and air leakage. If the fridge is empty all of that infiltrating heat is going to go towards heating the air inside the box. Air is quick to gain heat, quick to lose heat - so you end up with short on-off cycles. If the fridge is full the infiltrating heat will be trying to heat up the air, but will also be transferred to the product inside the fridge via the air. 100lbs of wet or solid food can absorb a whole lot more energy with a minimal temperature change compared to air. So it takes longer for the compressor to turn back on, and it takes longer for the compressor to take that heat back out (which also means that the air falling out of a full fridge has less of an effect then the air falling out of an empty fridge). The amount of time that a compressor is on during the day may be the same, but with the longer cycle it's spending more time at an efficient range.

Looking at some data sheets, when the 1/2hp unit we use at work starts up it draws 1,173 watts and it may take 2 minutes from it to drop from that to the 770 watts it draws when running at ideal pressures. Having a full fridge will reduce the amount of time spent up at the higher power consumption levels and allow the unit to be more efficient.

vpt
01-15-2016, 07:08 AM
Our walk in cooler is filled with "stuff" to take up the extra room (excess room needs to be cooled too ;) ) and to hold a more steady and longer temp.


If you want to use the most efficient "filler" use blocks of ice, they take up the room and help cool! :D

Tom S
01-15-2016, 07:37 AM
Our walk in cooler is filled with "stuff" to take up the extra room (excess room needs to be cooled too ;) ) and to hold a more steady and longer temp.


If you want to use the most efficient "filler" use blocks of ice, they take up the room and help cool! :D

Ice won't stick around at refrigerator temperatures.... you'll get an initial boost of efficiency from latent heat until the ice is melted, then you'll have water all over the floor.

1-800miner
01-15-2016, 08:38 AM
[QUOTE=Tom S;

Looking at some data sheets, when the 1/2hp unit we use at work starts up it draws 1,173 watts and it may take 2 minutes from it to drop from that to the 770 watts it draws when running at ideal pressures. Having a full fridge will reduce the amount of time spent up at the higher power consumption levels and allow the unit to be more efficient.[/QUOTE]

Now you have me confused; I would think that a compressor would draw less juice starting out at no pressure, and use more as the pressure increased towards operating range.

Forestgnome
01-15-2016, 09:14 AM
IMO it's too fine a line to differentiate with theory and conjecture. I think the differences would be miniscule. You would have to do a controlled study to really know. The fact that Evan's fridge uses less energy than his neighbor's could be due to many factors, including the temperature at which his living space is maintained. According to the prevailing theory on this board, the most efficient fridge would run continuously. I sure wouldn't want to pay for compressor replacements on that fridge!

vpt
01-15-2016, 09:14 AM
Ice won't stick around at refrigerator temperatures.... you'll get an initial boost of efficiency from latent heat until the ice is melted, then you'll have water all over the floor.

Just turn the thermostat down more in the fridge to keep the ice cool.

A.K. Boomer
01-15-2016, 09:16 AM
Kinda an expensive route to take - but you can stock it full of 80lbs of sockeye and halibut and it will kick on allot less, although you might want to put a padlock on the freezer part because it's got a street value of over 1500 bucks...

Seastar
01-15-2016, 09:57 AM
Knowing Evans penchant for accuracy I am sure his energy consumption numbers are correct.
However, I suspect his use of less energy than his neighbors has much more to do with his lifestyle and attention to detail than a full or empty freezer.
JT is correct, thermodynamics says there should not be much difference unless you are dumping the cold inside fairly often.
Bill

A.K. Boomer
01-15-2016, 10:17 AM
I believe where allot of people get confused is in the laws of thermal dynamics and thinking your somehow "tricking" them --- your not --- excuse the expression but it's a dead heat either way --- in the long haul and after everything is stabilized there is simply no difference that way what-so-ever.

That being said once the electro-mechanical complications of the fridge itself then have to correspond to the obvious variations of the two different "bleedown rates" of the examples given then there are obvious differences - plain and simple - all kinds of other laws of physics involved there that really cannot be argued with.

How much of a difference? im not even going to guess at a fixed percentage rating but I would try to be realistic and keep it in the single digits --- but again so many variables and most all being fridge design and where the temp sensors are and in comparison to uniform (or not) insulation properties --- it's a clusterfuque at best, but in general - and without being heavily manipulated to create an uneven temperature within the unit itself --- I can tell you that it's absolutely in favor of the fully stocked box,,, just simple rock solid theory of operation and consumption involved...

Lu47Dan
01-15-2016, 10:26 AM
I would say that the full one is more efficient than an empty one would be but the only way to be sure would be run a test on energy consumed in both states. Get a watt meter (http://www.amazon.com/s/?ie=UTF8&keywords=p3+power+meter&tag=googhydr-20&index=aps&hvadid=86274271825&hvpos=1t2&hvexid=&hvnetw=g&hvrand=11244616350383094354&hvpone=&hvptwo=&hvqmt=b&hvdev=c&ref=pd_sl_1891dq2gjx_b) and run your own test to see who's correct.
Dan

MikeL46
01-15-2016, 10:41 AM
strokersix got it right.

An empty fridge is the epitome of inefficiency.

Power is used, essentially nothing being cooled. At least the full one is doing something useful.

Mike

A.K. Boomer
01-15-2016, 10:47 AM
the trade offs the same --- the cycle time isn't - that's all you need to know, the empty fridge ends up with more run time due to useless cycling with nothing happening for a brief while, and if the compressor has start caps then there's another unwanted consumption up against the one that's cycling less...

Tom S
01-15-2016, 10:55 AM
Now you have me confused; I would think that a compressor would draw less juice starting out at no pressure, and use more as the pressure increased towards operating range.

Depending on the device giving the pressure drop (cap tube, AXV, TXV), the compressor valving, and the system you're usually starting against a significant load caused by the head pressure. Plus when the unit is running it's pulling in less dense refrigerant from the suction line (as compared to just starting), meaning when the piston is at TDC there is less refrigerant in a given area then when the unit just starts pumping. Less work required by the motor.

hitnmiss
01-15-2016, 11:47 AM
Having a second fridge in the backyard for drinks and worrying about it's efficiency full or empty... Kinda like buying a 1 ton pickup for commuting then lowering the tailgate to improve mileage.

My sister replaced her kitchen fridge in part because the new ones use less electricity, then promptly parked the old one out in the hot garage for drinks. Usually has a about 3 six packs in it.

J Tiers
01-15-2016, 11:53 AM
most any cooler or a/c has a timer to prevent starting against backpressure. So that idea of starting so often that it doesn't leak down is not going to happen.

The idea of how fast it warms up with the door open...... That seems to be off topic for the question, really, but if its totally empty and you open the door a lot, that will likely waste more energy than if its full and does not exchange hot air for cold. That really depends on how long the door is open.

You could equally well argue that when full, the door is open a long time as you need to pull out stuff and stack it on the counter just to find what you want, so in that way its a LOT LESS EFFICIENT WHEN FULL.

When not packed solid, you can open, grab, and close again before much cold air leaks out. More efficient.

Other than that silly stuff, its just plain no different full or empty. Same heat energy flow, long term same efficiency. Only difference is going to be run time.

Because the timer forces a start from low pressure, the longer the run the more time is spent moving heat energy, and less re-compressing gas to operating condition. Likely is a fairly tiny difference there, a hair better if full enough to run longer when it runs. But if the insulation is decent, it won't run much either way

gellfex
01-15-2016, 12:00 PM
Not strictly relevant to the thread, I know, but in my science teaching days the temperature of the lab rose perceptibly when the kids came in. Unfortunately I never got round to measuring temperatures, so I can't quote figures.

I read somewhere (I can't remember where) that a resting human has, on average, a specific heat output of 1 watt per kilogram. Take a class of 30 fairly active sixteen year olds of between sixty and seventy kilos each, and that's quite a number of watts in a closed room.

George

I've got a gadget you would have liked. It's a cordless USB logging thermometer the size of a big lipstick. You can set the data point interval and then plug it in and download to it's graphing program or export the data. I use it evaluate how a steam heating system is doing. It's a nice waveform with a 1.5 hr frequency and a 2 degree amplitude.

Re zero sum heating: I justify heating my basement shop in that all the heat goes upstairs to the house and reduces heat calls there. But I've yet to study it using the above mentioned device, I take it on faith! Besides, I'd have to download a daily temp file in order to compare similar days.

Forestgnome
01-15-2016, 12:56 PM
Having a second fridge in the backyard for drinks and worrying about it's efficiency full or empty... Kinda like buying a 1 ton pickup for commuting then lowering the tailgate to improve mileage.

My sister replaced her kitchen fridge in part because the new ones use less electricity, then promptly parked the old one out in the hot garage for drinks. Usually has a about 3 six packs in it.

Funny, I was going to use the tailgate analogy. "Common sense" would say a truck is more aerodynamic with the tailgate down, but computer modeling and real-world tests say the opposite. That's why I say the only way to know is through testing. Otherwise it's a toss-up.

Forestgnome
01-15-2016, 12:57 PM
most any cooler or a/c has a timer to prevent starting against backpressure. So that idea of starting so often that it doesn't leak down is not going to happen.

The idea of how fast it warms up with the door open...... That seems to be off topic for the question, really, but if its totally empty and you open the door a lot, that will likely waste more energy than if its full and does not exchange hot air for cold. That really depends on how long the door is open.

You could equally well argue that when full, the door is open a long time as you need to pull out stuff and stack it on the counter just to find what you want, so in that way its a LOT LESS EFFICIENT WHEN FULL.

When not packed solid, you can open, grab, and close again before much cold air leaks out. More efficient.

Other than that silly stuff, its just plain no different full or empty. Same heat energy flow, long term same efficiency. Only difference is going to be run time.

Because the timer forces a start from low pressure, the longer the run the more time is spent moving heat energy, and less re-compressing gas to operating condition. Likely is a fairly tiny difference there, a hair better if full enough to run longer when it runs. But if the insulation is decent, it won't run much either way

And why would you open an empty fridge in the first place?

Evan
01-15-2016, 01:24 PM
I just have to laugh because if I don't I will cry, thanks to a problem I have called Pseudobulbar Affect. I do have a recording wattmeter and a recording thermistor that I can put in and on the freezer. The blocks of ice are in bags so they can be easily removed. I guess I should settle this the real way, by running an experiment with actual numbers recorded. As I said, with the ice in the freezer is not only starts less often but it also runs less total time. As I also said that is because the ice doesn't get cooled as much as air will be since it has a slow heat transfer rate when being cooled. The temp switch in the freezer is quick to cool so the freezer turns off long before the ice is fully cooled during the cooling cycle. The average temperature is then a little higher with the ice in the freezer so not only does it run less often is also just plain runs less for the same setting as when it is empty. Clearly, the freezer will draw less power when full.

The fridge or freezer doesn't measure the temperature of what is in it, just the air that surrounds the temp switch.

Paul Alciatore
01-15-2016, 03:03 PM
As has been stated, a refrigerator runs to remove heat that has gotten inside it.

As I see it from a scientific/engineering point of view, there are three main factors involved here. 1. The heat can "leak" in through the walls, door, seals, etc. 2. When the door is opened, a rush of room temperature air mixes with the colder(?) air inside the refrigerator. 3. Warmer objects can be placed inside it, bringing that heat with them.

There is not much you can do about #3. You bought the refrigerator to cool these things (food) and keep them cool.

Number 1, the efficiency of the box itself is important but is established by the manufacturer and you can only seek to maintain it in proper working order. This will primarily be replacing faulty door gaskets.

Notice that neither #1 nor #3 is effected by how full the refrigerator is.

That leaves #2, opening the door. Every time the door is opened warm air rushes in. That is, unless your outdoor refrigerator is in a winter temperature that is actually lower than the set point for the refrigerator: this is distinctly possible in northern climates. But in summer or if the refrigerator is indoors, then warmer air rushes in. This is going to be a highly variable process. How many times is the door opened? How long is it left open? How fast do you open it? Etc, etc, etc. So any study of this must take this high level of variability in mind.

It is easy to see that if the door is opened for a short time that some air will be replaced with warmer air while the items in the refrigerator will not be immediately warmed. If the door is opened only once or twice a day there would be a lot less loss than if it were opened many times (typical family). So, yes, a full refrigerator will be somewhat more efficient (use less energy) than an empty one. But just how much more efficient is hard to determine.

The "through the door" ice and cold water dispensers are probably a very good idea for a family with many members who frequently want ice or cold water. This keeps the door closed and no (or a lot less) warm air can rush in.

Evan
01-15-2016, 03:26 PM
"Number 1, the efficiency of the box itself is important but is established by the manufacturer and you can only seek to maintain it in proper working order. This will primarily be replacing faulty door gaskets. "

Not so. Putting a little fan that blows over the back of my fridge greatly improves the efficiency. I have been doing that for many years. I also have fitted sheets of Styrofoam that the freezer was shipped in and I have left them on top of the freezer. It greatly adds to the insulation on top.

http://ixian.ca/pics12/freeze1.jpg

Forestgnome
01-15-2016, 03:37 PM
If the amount of air exchanged when the door is opened is a major factor, then it would seem that it would be more efficient to take up extra space with styrofoam blocks, since they don't have much thermal mass. The arguments seem to be in favor of larger thermal mass taking up space.

softtail
01-15-2016, 04:07 PM
Probably beyond the scope of worrying about, but conventional wisdom says a fridge with stuff in it is 'more efficient'. Of course a fridge with stuff in it necessarily means someone has opened the door, and hints at someone opening it again. I would worry more about keeping good airflow through the cooling 'fins' and keeping them clean. -ST-

A.K. Boomer
01-15-2016, 04:27 PM
I would worry more about keeping good airflow through the cooling 'fins' and keeping them clean. -ST-

Well said - probably one of the most neglected things on a fridge and can make a HUGE difference in run time...

Danl
01-15-2016, 05:06 PM
It will be interesting to see some actual numbers and analysis on this.
I did note the fact that Evan had a lot going on on July 19th, and that he was probably away from home a week later.

:)

Dan L

PeteM
01-15-2016, 08:05 PM
Given that you and your wife both think the refrigerator can be left empty for long periods, it stands to reason it could also be off for those same long periods. Would have to have the door securely blocked partly open to keep it from molding and make sure kids couldn't lock themselves inside. But turn it off or way down, and your wife is right.

Some older refrigerators have such crappy insulation that the full refrigerator might actually be less efficient if in the sun, as well. The question becomes one of the loss from dumping the cold air in an empty refrigerator versus the loss of continually cooling a much larger mass.

The real solution here might come from the Brits. Keep the refrigerator stocked completelly full of beer. But don't cool it quite so much as the Yanks.

J Tiers
01-15-2016, 10:45 PM
And why would you open an empty fridge in the first place?

To put something in it, of course! :D

more to the point, why would you even have it running?

I assume that "empty" does not mean "without contents", but rather that there is considerably more empty space than stored material. A freezer or especially a fridge can be pretty full, with no more shelf space open, and yet not be so full volumetrically. Freezers do better since they can usually have more things stacked.

bborr01
01-15-2016, 11:39 PM
This discussion reminds me of something I read in a forum a while back, probably on this forum. Someone was talking about how they have 10 tons of machinery in their shop so when they turn the heat down it takes a long time for the shop temperature to drop. This is true. But it also means that when you decide that you want to heat the same shop, you have 10 tons of machinery to warm back up.

The extra thermal mass helps to steady the temperature, whether it is in a refrigerator or a shop.

I'd say that if the refrigerator door is never opened, it won't make any difference what is in it. Or for that matter if there is anything at all in it. The difference is when the door is opened. In a room temperature surrounding like most refrigerators operate in, warmer air rushes in when the door is opened. If there is something to capture that air space, warm air can't get in.

Which brings us to efficiency. How long does the door stay open each time it is opened. I have seen refrigerators that are well organized and refrigerators that lack any organization whatsoever. As was mentioned earlier in this thread, if you have the door open for several minutes looking for something, it would probably pay to organize things.

Brian

Evan
01-16-2016, 12:02 AM
It makes a difference even if the door isn't opened. The insulation is not perfect, far from it.

"I did note the fact that Evan had a lot going on on July 19th, and that he was probably away from home a week later."

Nope, I didn't go anywhere. I did have a day where I didn't turn much of anything on and may have only opened the fridge a couple of times. I never leave it open. Open, grab, close. It matters. I don't like my soda pop cold and all I drink is warm water, warm pop and a little milk. I cook foods that do not take long at all except when I bake cupcakes. Much is cooked in the microwave and that is much cheaper than an electric stove. I have been using a microwave since they first were commonly available for home use. I do everything I can to lighten my environmental load while still playing with my toys. It always amazes me how much water people use on average. I probably use about 1/10 of the average. That comes in part from nearly always having lived on a deep, low rate well. I just don't like wasting things, whatever it is. As a world we cannot afford to.

I can't thank some people here enough for helping me to avoid the waste of some of my prized possessions that could have ended up scrapped.

danlb
01-16-2016, 12:26 AM
The reason I am askin is cause we have a small frig in the backyard, rarely used but convenient all the same, so it is gonna stay lit.

The issue is my Wife says it should be run empty until needed (fill with same temp products), less to keep cool.


I'd store the drinks in the main fridge until needed, then throw a six pack in the small one and turn it on. Turn it off when the six pack is gone. A fridge that's needed once a week is way more efficient if it's turned off 6 days a week.

Dan

J Tiers
01-16-2016, 01:36 AM
....
I'd say that if the refrigerator door is never opened, it won't make any difference what is in it. Or for that matter if there is anything at all in it. The difference is when the door is opened. In a room temperature surrounding like most refrigerators operate in, warmer air rushes in when the door is opened. If there is something to capture that air space, warm air can't get in.

Which brings us to efficiency. How long does the door stay open each time it is opened. I have seen refrigerators that are well organized and refrigerators that lack any organization whatsoever. As was mentioned earlier in this thread, if you have the door open for several minutes looking for something, it would probably pay to organize things.

Brian

Yep, that's the way it is, folks.

Heat energy leaks in, and has to be removed. The goodness or badness of insulation only affects how fast that happens. The removal has to average out to be the same amount per hour that leaks in, or the average temperature will trend up or down. The problem posed assumes that the idea is to maintain a reasonable constant temperature over the long term.

The only effect which varies the heat flow, and so the energy required to remove it, is opening the door. That's almost outside the discussion, as somewhat of a "reality-based social effect", not a technical one, but since it's part of how the fridge is used, it counts.

A full fridge loses less cold air, and has to cool off less warm air. But then, a full fridge is open a good deal longer as you dig for what you want. That "reality-based social effect" probably totally offsets the other one, so I call it a wash. In reality it's probably worse, since it likely gives time for all the cold air to drain, while opening to get the orange juice takes only seconds.

And then the folks who leave it open as they pour their juice, that's another "reality based social effect" that zaps the other ones.

The only fair comparison is if you eliminate opening the door, or possibly if you have "X" number of 4 second door opening cycles per hour. That's enough time to open, grab a pitcher, and set it on the counter as you close the door.

In either case, unless "X" is rather high, I'd bet the net effect is minimal. To estimate it, you can check the heat capacity of air, the temperature change, estimate the air volume exchange per door opening, and and compare that heat energy "load" to the normal leakage of heat energy through the insulation.

bborr01
01-16-2016, 08:45 AM
It doesn't seem like extra insulation on the top of a cooler would make much difference. Heat rises so it would keep heat from escaping but it seems like for cold it wouldn't have much benefit.

Brian


"Number 1, the efficiency of the box itself is important but is established by the manufacturer and you can only seek to maintain it in proper working order. This will primarily be replacing faulty door gaskets. "

Not so. Putting a little fan that blows over the back of my fridge greatly improves the efficiency. I have been doing that for many years. I also have fitted sheets of Styrofoam that the freezer was shipped in and I have left them on top of the freezer. It greatly adds to the insulation on top.

http://ixian.ca/pics12/freeze1.jpg

Paul Alciatore
01-16-2016, 03:20 PM
If the refrigerator is empty, then there is no reason to open the door. The greater efficiency of a filled refrigerator is due to the amount of hot air that gets inside it when the door is opened, but if the door is not opened, then that effect can not work. So, a completely empty refrigerator, which is not opened, will be just as efficient, NO, ACTUALLY MORE EFFICIENT than a completely full one which is opened often to put things in and out of it.

If there is only one or two things in it and they are only used infrequently, then the efficiency will also be rather high, probably as good as a full one. But if you open it often them the efficiency will go down in proportion to the number of times it is opened.

This is not a simple yes/no question. As I said above, there are many factors involved. And those factors are not independent of each other. The number of items in a refrigerator influences the number of times the door is opened. And the number of times the door is opened influences the efficiency. In general use, a full refrigerator is probably a bit more efficient than an less full one. But this is not an iron clad rule and your mileage may vary depending on any combination of other factors.

If you want a rule to follow for a more efficient refrigerator, the number of times the door is opened is a better thing to control than the number of items in it.

boslab
01-16-2016, 04:42 PM
My kids inspect the refrigerator regularly to see what's in there, I rekon they have the memory of a goldfish myself, I haven't been shopping and they still check at regular intervals, my assumption is they think the fridge is a portal through which food arrives, the portal only works so long as it's switched on.
Things that are gone bad are absorbed by the portal, no action on thier part is required.
Also if they have seen somthing tasty in there it should always be there on demand, gnashing of teeth etc if It disappears back through the portal
Mark

Mike Burch
01-16-2016, 05:03 PM
It doesn't seem like extra insulation on the top of a cooler would make much difference. Heat rises so it would keep heat from escaping but it seems like for cold it wouldn't have much benefit.

Brian
No, heat itself does not rise.
Heat can be transferred by conduction, convection, or radiation.
Heated gas is less dense than cool gas, so warm air will indeed rise with respect to cooler air - thus transferring heat energy by convection.
But that's not how heat gets into a freezer.
The outer case of the freezer can be heated by all three methods, but heat gets inside the freezer by conduction (and only conduction) from the outer case through the insulation. This can happen in any direction. The rate at which it happens depends on the temperature difference, the thickness of the insulation, and the type of insulation (urethane being very much more efficient than polystyrene).
So ugly and inconvenient though it is, leaving a sheet of insulation on top of a freezer will indeed slow the leakage of heat into the thing.
Mind you, siting the freezer further away from the oven (or putting a reflective sheet of something in the air space) might improve matters, by preventing the raised temperature of the oven case from radiating heat which raises the temperature of the side wall of the freezer case.
Modern fridges no longer have the radiator at the back, separate from the body of the fridge, but rather inside the case, thus utilising the whole cae to radiate the extracted heat. Raising the case temperature like this is, of course, a bloody stupid idea, driven by the triumph (inevitable in any corporation run by suits) of ignorant stylists over sensible engineers. It should be outlawed in the interests of increasing efficiency and reducing wastage of electricity.

JRouche
01-16-2016, 11:04 PM
Love it. I knew you guys were a lil off the wall. Yup, thank you for showing up. What a great exchange of opinions and facts to enjoy. Bunch of brains here, its a good thing.

Thanks for participating. JR

bborr01
01-17-2016, 03:57 PM
I have a reasonable understanding of thermodynamics but it still seems like the least effective place to use insulation on a freezer is on the top of it. I think that barring something like the sun shining on the top of a freezer, the sides would yield more benefit.

Brian


No, heat itself does not rise.
Heat can be transferred by conduction, convection, or radiation.
Heated gas is less dense than cool gas, so warm air will indeed rise with respect to cooler air - thus transferring heat energy by convection.
But that's not how heat gets into a freezer.
The outer case of the freezer can be heated by all three methods, but heat gets inside the freezer by conduction (and only conduction) from the outer case through the insulation. This can happen in any direction. The rate at which it happens depends on the temperature difference, the thickness of the insulation, and the type of insulation (urethane being very much more efficient than polystyrene).
So ugly and inconvenient though it is, leaving a sheet of insulation on top of a freezer will indeed slow the leakage of heat into the thing.
Mind you, siting the freezer further away from the oven (or putting a reflective sheet of something in the air space) might improve matters, by preventing the raised temperature of the oven case from radiating heat which raises the temperature of the side wall of the freezer case.
Modern fridges no longer have the radiator at the back, separate from the body of the fridge, but rather inside the case, thus utilising the whole cae to radiate the extracted heat. Raising the case temperature like this is, of course, a bloody stupid idea, driven by the triumph (inevitable in any corporation run by suits) of ignorant stylists over sensible engineers. It should be outlawed in the interests of increasing efficiency and reducing wastage of electricity.

J Tiers
01-17-2016, 05:55 PM
I have a reasonable understanding of thermodynamics but it still seems like the least effective place to use insulation on a freezer is on the top of it. I think that barring something like the sun shining on the top of a freezer, the sides would yield more benefit.

Brian

That depends on the way heat leaks in.....

If heat is radiating from elsewhere, all around the 'fridge, then insulation is pretty much effective anywhere on it.

If there are convection losses, from air passing along the surface, then in otherwise still air, the top (and bottom) might be less effective than the sides, which would be expected to have better flow.

If there are air currents other than convection, then any side may be effective.

In most cases it will be a combo of all possibilities, and insulation all over will work fine and be effective. It's mostly special cases where one surface is much more (or less) effective. In the general case where you don't know the specific most lossy part, all sides are effective.

Rosco-P
01-18-2016, 07:17 AM
I'd store the drinks in the main fridge until needed, then throw a six pack in the small one and turn it on. Turn it off when the six pack is gone. A fridge that's needed once a week is way more efficient if it's turned off 6 days a week.

Dan

+1 to the above.

However it seems that the convenience (for you), outweighs the energy costs of running the mini-fridge 24x7.

Evan
01-18-2016, 07:49 PM
"It doesn't seem like extra insulation on the top of a cooler would make much difference. Heat rises so it would keep heat from escaping but it seems like for cold it wouldn't have much benefit."

It is very easy to test. Place insulation anywhere except where the radiators are placed. Then wait a while and feel under the insulation. If it is warmer then it is a bad place for it. If it is colder then it is a good place for it. On freezers it is always a lot colder under the insulation than it was without it. Outside heat is prevented from having access to the inside. If you have a freezer just put an ordinary blanket folded so it only covers the top and check with your hand in maybe six or more hours. You will be surprised.

Black_Moons
01-19-2016, 07:29 AM
"It doesn't seem like extra insulation on the top of a cooler would make much difference. Heat rises so it would keep heat from escaping but it seems like for cold it wouldn't have much benefit."

It is very easy to test. Place insulation anywhere except where the radiators are placed. Then wait a while and feel under the insulation. If it is warmer then it is a bad place for it. If it is colder then it is a good place for it. On freezers it is always a lot colder under the insulation than it was without it. Outside heat is prevented from having access to the inside. If you have a freezer just put an ordinary blanket folded so it only covers the top and check with your hand in maybe six or more hours. You will be surprised.

"So why don't they make fridges with more insulation?" "Because nobody wants a fridge with 6" thick walls"

I wonder if anyone has bothered to try and make a fridge with vacuum insulation like a thermos?

Mike Amick
01-19-2016, 12:04 PM
I think they should build houses with refrigerator enclosures. This enclosure would be on an outside
wall with highly insulated louvers. In the winter these louvers could be open. Closed in the summer.
Basically the door to the frig would be in the kitchen .. but the rest of the frig would be exposed
to the outside.

Just seems weird putting a cooling device in the house and heating around it. lol.

bruto
01-19-2016, 01:58 PM
I got only about two thirds through this thread before figuring that the thermal efficiency argument will continue until someone buys two refrigerators and races them, and then it will probably still endure until Evan measures them himself. But it seems, along with Evan's observations, that one issue only brushed on may be more important, and that is that the starting load on the compressor itself is sufficiently great, all other things aside, that fewer starts might add up to more saving from the purely electrical standpoint.

I will leave it to someone with more time or mathematical energy than I have to figure out how many extra starting amps for a moment will balance against how much more running time, but I suspect it's an issue.

Here I have a kitchen with a marginal breaker that pops if too many appliances are used at once. We can get away with nearly any combination if the refrigerator is already running, but if it is off, we must be careful because it will pop when it starts.

A.K. Boomer
01-19-2016, 03:57 PM
Well put Bruto but there's also something more to it than just starting load,,, there's "run time VS effective run time" and once again the frequent fire up eats it in this department too...

J Tiers
01-19-2016, 04:17 PM
[QUOTE=bruto;1026354]I got only about two thirds through this thread before figuring that the thermal efficiency argument will continue until someone buys two refrigerators and races them, and then it will probably still endure until Evan measures them himself. But it seems, along with Evan's observations, that one issue only brushed on may be more important, and that is that the starting load on the compressor itself is sufficiently great, all other things aside, that fewer starts might add up to more saving from the purely electrical standpoint.

I will leave it to someone with more time or mathematical energy than I have to figure out how many extra starting amps for a moment will balance against how much more running time, but I suspect it's an issue.
QUOTE]

Not hardly......

A motor that say draws 4 amps at 120V. It will draw about 20 as it starts. It runs 10 min out of every hour, and it starts 10 times per hour, running a minute at a time. A start takes a half second to go from 20A to 4 A, so the average current for the entire start is 12A, because the current is dropping as the motor comes up to full speed. Numbers are reasonable, but not from a particular motor.

For simplicity, assume that all of the current is measured as power by the meter. (In truth, only 60 to 80% is "real" power, the rest is "reactive power".)

So... the motor running 1/6 of an hour at 4A uses 4 x 120 = 480 watts (remember we are ignoring power factor). It runs for 1/6 of an hour, so it uses 80 watt hours per hour.

Now for the starts.

Each start is equal to 12A for a half second. There are 10 starts per hour. so there is a load of 12A for 5 seconds per hour, due to starts. That is 12A x 120V = 1440 watts, neglecting the portion of that which is running power (4 of the 12 amps)

In an hour, there are 60 x 60 seconds, or 3600 seconds. 5 / 3600 is .0014 hour, approximately. 1440W x 0.0014 hour = 2.02 watt hours

So the 10 starts add 2/80, or 2.5% to the total power usage in our assumption, which assumes rather a lot of starting versus the run time. The starts are probably faster, the fridge runs longer between starts, etc, etc. So this is assuming many unfavorable things.

It appears safe to say that the start power is a small portion of the total power usage. Especially since a fridge is often limited in the number of starts by a timer that lets the pressure leak off before allowing another start, that is generally a 5 minute timer when one is used.

Mark Rand
01-19-2016, 05:41 PM
The most efficient case is to keep what's needed in it and nothing extra. When there's nothing in it, turn the damn thing off.

Evan
01-19-2016, 11:59 PM
It isn't just the motor start time that matters. It is the time it takes for the entire refrigeration cycle to get working properly. That is minutes, not seconds and during that time the motor is running at its worst efficiency with no actual heat transfer until the cooler tubing is colder than the inside temp. At the beginning it is warming the inside, not cooling it. That is intentional in most cases as part of the auto defrosting cycle. Fewer starts matters a lot.

Something else to consider is that most new fridges no longer use radiator tubing on most of the back. Many have it completely hidden and attached on the inside of the outer metal walls. That is easy to determine by just placing your hand on the various sides and areas of the fridge. Other have a very small and tight bundle of radiator tubing in the rear underside of the fridge, usually with a fan blowing through it. That can often be significantly improved as I did on the last fridge in the house I lived in. A bigger and quieter fan made a big difference. So did allowing some of the air to blow out the back instead of only from the underside. There is plenty of room for improvement for these types of appliances in nearly all cases. Usually the first thing to do is to allow/provide more air circulation around the entire unit. Pull it out from the wall a few inches.

J Tiers
01-20-2016, 01:38 AM
The longer the compressor need to run to get cooling the fewer starts there will inherently be per hour anyhow, because it is forced to run longer to maintain temperature. Then the minimum off time timer forces an added time between starts. So that issue is self limiting.

The more the door is open the longer it runs, again limiting the number of starts.

Ours is blowing cold air pretty soon after the motor starts... Same with a window A/C. They get to cooling pretty fast. All the pressures will be correct not long after, because the working fluid circulates quickly in a small unit. Not like a house or commercial unit that has a lot of "freon" in it. The bigger the unit, the slower they come up to proper temperatures, of course.

But the worst efficiency will be when it cools very quickly and shuts off. How that affects the starts per hour is determined by the rate of leakage of heat into the cooler. In those cases it may actually overshoot the temperature, and take longer to warm up and run again.

It must be remembered also that the motor is not under full load until the pressures stabilize. At light load the motor is pulling a lot of VA and not so much actual "watt based" power. VA don't come into the efficiency rating, only "watt based" power does.

vincemulhollon
01-20-2016, 09:43 AM
... starts up it draws 1,173 watts and it may take 2 minutes from it to drop from that to the 770 watts it draws when running at ideal pressures..

Somehow this thread has percolated for days and this is the only tangential mention of the dreaded frost-free heating elements. Some fridge and freezer have heating elements in areas where frost might naturally form, and they goose them with a couple hundred watts for a few minutes every time the machine cycles. That's why my $1500 frost free fridge in the kitchen has never been defrosted and the $100 dorm special in my workshop needs manual defrosting every six months or so.

Evan's numbers WRT starting less often would have a huge effect if every run cycle burned half a kilowatt-minute on the anti-frost heaters... Every 120 cycles, thats like 10 cents of electricity at the rates I pay. With casual observation and estimation there are probably two hundred or so cooling cycles per month, or to less than one sig fig of accuracy a full fridge would save about a buck per year just in anti-frost heating element power alone.

Also Black Moons has invented the cryostat that the bio-weirdos used to store "samples" at liq N2 temps in the science building. Sometimes the chemists need liq N2 or expensive liq He for the NMR so we has some minimal contact with them and I believe the bio guys had both giant thermoses and "fridge in a thermos" as per Black Moons. The relationship between the bios and the chemists was funny as they thought the chemists did nothing but set pyrophoric organomercury reagents on fire, and we thought all the bios did was tempt fate with ebola samples, the usual irrational fear of the unknown etc etc.

In my long list of probably dumb ideas to fool with in my infinite spare time I always wanted to make a refrigeration plant that could liquify air. I'm well aware its dangerous and cheaper to buy from a welding supplier, much as the cheap fast and safe way to get a small IC engine is to buy one or cannibalize a leaf blower. But I want the fun of making one. Probably three stages, and unlike the commercial operators I'd be more interested in the refrigerants being non-toxic and non-flammable than I'd be interested in efficiency. I've occasionally looked into buying commercial lab grade freezers on ebay and the problem is all the cryostats out there are either hopelessly broken, contaminated with god knows what, or cost like $5K which is a bit much to indulge casual screwing around. Its like vacuum pump situation times about ten in price. I know about guys who recharge refrigerator systems with propane and get -40F but I want liquid air...

bborr01
01-20-2016, 02:11 PM
Vince,

I have an idea for your dorm fridge that needs to be defrosted regularly.

I have a kegerator in my shop that also needed regular defrosting. The answer that I came up with was to put a timer on it and have it shut down at night for about 4 hours. That is long enough for the cooling coil inside of it to melt off and drain into the drain pan. I haven't had to defrost it in the last couple of years since I put the time on it and it doesn't matter if it warms up a couple of degrees at night when it is not in use.

Brian


Somehow this thread has percolated for days and this is the only tangential mention of the dreaded frost-free heating elements. Some fridge and freezer have heating elements in areas where frost might naturally form, and they goose them with a couple hundred watts for a few minutes every time the machine cycles. That's why my $1500 frost free fridge in the kitchen has never been defrosted and the $100 dorm special in my workshop needs manual defrosting every six months or so.

Evan's numbers WRT starting less often would have a huge effect if every run cycle burned half a kilowatt-minute on the anti-frost heaters... Every 120 cycles, thats like 10 cents of electricity at the rates I pay. With casual observation and estimation there are probably two hundred or so cooling cycles per month, or to less than one sig fig of accuracy a full fridge would save about a buck per year just in anti-frost heating element power alone.

Also Black Moons has invented the cryostat that the bio-weirdos used to store "samples" at liq N2 temps in the science building. Sometimes the chemists need liq N2 or expensive liq He for the NMR so we has some minimal contact with them and I believe the bio guys had both giant thermoses and "fridge in a thermos" as per Black Moons. The relationship between the bios and the chemists was funny as they thought the chemists did nothing but set pyrophoric organomercury reagents on fire, and we thought all the bios did was tempt fate with ebola samples, the usual irrational fear of the unknown etc etc.

In my long list of probably dumb ideas to fool with in my infinite spare time I always wanted to make a refrigeration plant that could liquify air. I'm well aware its dangerous and cheaper to buy from a welding supplier, much as the cheap fast and safe way to get a small IC engine is to buy one or cannibalize a leaf blower. But I want the fun of making one. Probably three stages, and unlike the commercial operators I'd be more interested in the refrigerants being non-toxic and non-flammable than I'd be interested in efficiency. I've occasionally looked into buying commercial lab grade freezers on ebay and the problem is all the cryostats out there are either hopelessly broken, contaminated with god knows what, or cost like $5K which is a bit much to indulge casual screwing around. Its like vacuum pump situation times about ten in price. I know about guys who recharge refrigerator systems with propane and get -40F but I want liquid air...

Black_Moons
01-21-2016, 05:55 AM
...
In my long list of probably dumb ideas to fool with in my infinite spare time I always wanted to make a refrigeration plant that could liquify air. I'm well aware its dangerous and cheaper to buy from a welding supplier, much as the cheap fast and safe way to get a small IC engine is to buy one or cannibalize a leaf blower. But I want the fun of making one. Probably three stages, and unlike the commercial operators I'd be more interested in the refrigerants being non-toxic and non-flammable than I'd be interested in efficiency. I've occasionally looked into buying commercial lab grade freezers on ebay and the problem is all the cryostats out there are either hopelessly broken, contaminated with god knows what, or cost like $5K which is a bit much to indulge casual screwing around. Its like vacuum pump situation times about ten in price. I know about guys who recharge refrigerator systems with propane and get -40F but I want liquid air...

AFAIK, the standard process is not to actually refrigerate air via standard refrigeration systems, but to compress the air to insanely high pressures, cool to room temp (as compression heats the air significantly) and then run it through a turbo-expander to cool it.
The resulting expansion can cool some of the air to cryogenic temperatures and cause some of it to liquefy.

I think my fridge must have been switched over to propane since it hits -40c/f, according to my IR thermometer and the fact that 40% vodka freezes in it. (Its a 'reconditioned' fridge)