Economical Refrigeration

[jimisbell]

Actually, I was not asking if the fan on the condenser was of value or not. It seems to be generally accepted that it IS of value and does save electricity.

BUT the question was of what size did the fan need to be to make the maximum impact on the efficiency of the unit.

So far it seems that most agree a small fan moving the air gently will be the most the efficient as a compromise between using battery power to run the fan and battery power to run the compressor.

[Starbuck]

I would think so, jimisbell.

Sorry for the thread drift: as happens, your original post was obscured by "many hands stirring the pot."

[ssullivan]

Originally Posted by jimisbell

So far it seems that most agree a small fan moving the air gently will be the most the efficient as a compromise between using battery power to run the fan and battery power to run the compressor.

Jim: This is the correct answer. May I also add that your fan needs to be the size of (or blow on) all of the fins of our condensor to work most efficiently. You don't want it blowing through a small section, but rather to get air moving across the whole thing if possibe.

Hellosailor: The FRP they sell is some very thin stuff, that is normally used to line home-built showers and bathrooms. It works well in the fridge, as it is waterproof and easy to clean up.

Jeff: Almost... the heat that warms things up inside the icebox comes from the outside through various methods. The vibrations you speak of refer to the vibrations that make up heat itself. Put simply, heat is the vibrating of molecules. These molecules vibrate up against the insulation, which is very good at keeping the vibrations from making it inside. (as a contrast, metal is not so good at it) So, the refrigerators heats up by the vibrations making it through the insulation (conduction/radiation of heat) and also by you opening up the door and spilling already vibrating air into the refrigerator.

I'm going to comment on the jugs of water. They would not save you a bit of energy, other than keeping some of the airspace filled up so that it can't mix with the hotter outside air. No mater how you slice the problem, there is a boundary (the walls of the fridge) and there is heat either coming or going.

So, think of the compressor as a pump that removes heat. That's all it does. It turns on, and moves heat outside the box until your box is cool enough, then shuts down. Heat gets in, as stated above by opening it, or passing through the insulation (or of course by dumping a hot meal or beer in there). Your compressor has to remove the same amount of heat every day (not counting the user putting a lot of hot meals in or something).

So with the water jugs, they would intially cause the refrigerators to run for a longer time to cool them down. After that, they would help the refrigerator not run as soon, by "retaining" some of the cold. (not the right way to phrase it, but it will work here)

After that though, it all goes back to normal and your refrigerator will still need to remove the same amount of heat each day from the box. The heat it removes each day comes in through the insulation and through the user opening the door, etc... So, the water bottle would be as effective as a balloon of air, a brick, or a big piece of steel. None has a long term effect on the refrigerator's efficiency.

Hope that helps.

[Starbuck]

Sean, we agree on almost everything. I understand the energy level of a molucule's vibration being described as heat, and the 'fridge as a heat pump. I also understand why it's convenient to speak of "cold" in these situations, instead of just relative heat, or the absence of heat, and I don't fault you there.

Those aren't my vibrations; they're Hellosailor's (post #9). I've read him carefully several times, and I've come to the conclusion that he must be talking about the vibration (heat) of room-temperature material introduced into the compartment, and not heat generated in the material by an outside mechanical vibration causing the food molecules to bang into each other, increasing their vibration (heat). For a while I was confused, but now I see that if I take the former meaning, we're all on the same page.

[hellosailor]

Jeff-
"The question is, will a gallon jug of water, after... function as a better insulator "
Insulator? Why would you think anything IN the fridge would be an insulator? It might be a heat sink, or a heat source, but it isn't insulating (separating) anything from anything.
Every time the boat rocks, or bounces, or the tidal pull of the moon and sun pull the molecules in that water, aren't they adding motion to them? And isn't molecular motion going to manifest as heat?

I don't see why the thought of adding MORE MASS into a system would make that system produce or contain fewer calories. And we're not really trying to make a frig cold, we're trying to TRANSFER CALORIES out of it. Fewer calories in it, fewer things to produce calories, fewer calories to take out of it.

I admit I'm not up on modern physics but IIRC everything vibrates and produces heat--until you cool it down to absolute zero. Everything in that frig is producing (loosing) heat, until you exhaust the energy by cooling it all down to absolute zero. More mass in the box? More work to do!<G>

Otherwise, a BIG frig would take less energy to run than a small frig--since the big one has more mass in it, and you're suggesting more mass makes it easier to keep cool. No?


Trying to think further into it...let's assume for the moment that having more mass IN any given frig, made it appear to be keeping cooler inside. perhaps, because the compressor was cycling on less often? So I asked myself, if that was so, what could cause the compressor to be cycling on less often? And the thought comes that if there is more mass in the frig, and the biggest problem is heat transfering IN from the outside (rather than the heat energy in the stuff inside)...then by putting more mass in the frig we have created a larger heat sink IN the frig, so it can absorb more of that outside heat before it has heated up enough to trigger the compressor again.

Now, would that mean the compressor is doing less work? No, I don't think so. It could mean the compressor is engaging less frequently--but running harder and longer each time it runs. Somewhere on the cusp between the two modes, there's the extra factor that it must take "this long" for the compressor to build enough pressure to get the coolant working well, so there's another factor that maybe it is more efficient to run fewer and longer cycles--because of the time lost getting the system pressurized and into good operation.

I've actually got an accumulating watt meter (Kill-a-watt) and I could plug it into my frig and let it run 24 hours lightly filled, then 24 hours really really filled, and compare the power consumption both ways. But...I'm not that intent on knowing.<G> I trust entropy, I can't see how cooling "more stuff" can take less energy, all else being equal. And the rest, would of course vary greatly from one frig to the next, making the test meaningless except on the frig it was tested on.

[GordMay]

If you put anything into the refrigerator, that is warmer than the set-point,
the reefer', will have to remove that extra heat (thereby working more).
If you put something colder than the set-point, into the refer', that something
will absorb heat from other sources (conduction, convenction, infiltration),
resulting in less work (from that frig), to maintain the set-point .
Hence, putting (already) frozen water-jugs into a refer', results
in less work for that refer'.
It does not result in less "total" work - the extra (heat transfer) work having
been performed in pre-freezing the water elsewhere.

[ssullivan]

Guys... you're all pretty close, but you are over thinking the problem a little bit. What Gord says is true above. I'm not going to repeat it. Just take that for a given.

Forget about adding water jugs for a second and think of this: an empty fridge.

We have an empty fridge and means to remove heat (the refrigeration system).

Now, let's also ignore opening the door up and just say we have a box that has some way to remove heat. This box is insulated. It's also a perfect cube shape.

Ok... so how does this box get warm? Heat comes in through the walls, right through the insulation. When too much heat comes in (and the temperature rises to 40 degrees), the heat pump (refrigeration hardware) kicks in and starts pumping heat out until the refrigerator is back down to say... 34 degrees.

This is exactly how a closed, empty refrigerator works.

Now, let's add your food. When you *first* add your food it might be colder or warmer than the ambient temperature of the fridge. If you add hot soup, it will introduce more heat than the normal heat already passing through the wall, as Gord says above. The refrigerator will have the same amount of work to do as normal that day, except it will have the extra work of removing the heat from the soup. Once the soup is the same temperature as the rest of the fridge, the soup ceases to add heat or subtract heat from the box, since it is inside the box and at the same temperature as the box. Now, the excess heat entering the box is the same amount of heat the box would have if it were empty, since the soup has become the same temperature of the box, not contributing or taking away heat anymore.
So the compressor does the same work it would do if the box were empty.

This basic example applies to all items put in the refrigerator. Frozen items, such as ice, help by not introducing any heat, allowing the compressor to run less that day. BUT... once they become the same temperature as the rest of the refrigerator, the behavior of the system is identical to the empty box example from the beginning of this post.

So... it doesn't matter what you put in the refrigerator or how much of it you put in over a couple of days. It only matters for the first day the stuff is in there. Make sense?

[44'cruisingcat]

What sean says is right. The advantage of keeping the fridge full happens when you open the door - if the fridge is empty, all that cold air will "fall" out (it's a front opening fridge) and be replaced by ambient air. (presumably warmer) If the fridge is full of either food or bottles, less cold air will spill out, so less warm air will get in.

[Starbuck]

Sean & Gord,

I'm being persuaded that total heat removed from the compartment is the total heat removed from the compartment, water jugs or not. I'll think on this further. Thank-you for your painstaking explanations. I often have to re-word how I explain concepts to my students: they all have different ears.

But now back to Hellosailor's contention that new heat is generated in water simply because it's sloshing around:

Hellosailor, he say:
Every time the boat rocks, or bounces, or the tidal pull of the moon and sun pull the molecules in that water, aren't they adding motion to them? And isn't molecular motion going to manifest as heat?

I thought I was reading you wrong, but you've made it clear you believe this is true: that molecules in a liquid generate heat simply by being knocked into each other.

A logical extention of this logic requires breaking waves to be warmer than the surrounding water; or even moreso, the water at the surface of a waterfall to be warmer than the falls water, because the physical energy that the falls water contains as it strikes the surface of the pool is translated into molecular energy (measured as heat) within individual molecules. I don't think these two things are equivalent.

A quick thought experiment:

If I stand at the North Pole & vigorously shake a bottle half-filled with water, I cannot prevent it from freezing in the same amount of time as the second container, similarly filled, standing at my feet. No heat is being generated as a result of my shaking, except for my body temperature as a result of my own physical exertion.

And it's a wonder paint-mixers don't cause explosions.

If any heat at all is being produced by molecular collisions, and I don't think there is, it can't be significant enough to enter into our discussion, or affect the cycling of anyone's Alder-Barber.

What say you (or anyone)?

[hellosailor]

jef-
"No heat is being generated as a result of my shaking, " I'm not sure about that. It might be so little extra heat that you wouldn't notice it, or that you might have to be quick about measuring it, but I'm convinced you could leave a gallon of paint in the shaker for an hour, come back, and find the temp had gone up enough to measure with a digital thermometer.

My point was not whether it made a practical difference, but only to look at the basis for the claim, i.e. that a bottle of water in the frig would somehow keep it cooler than a bottle of air. Since everything in the frig is getting energy "added" to it by gravitational tides...surely that's still a net gain in the energy system?

I still think the empty frig will need less energy to keep it cool than the full one will, and the main "deception" in the picture will be how the thermostat cycles the compressor on and off, with the cycling issues (presurizing the system) being able to help/hurt more than one might think. In the same way that room air conditioners can appear to be more or less efficient, depending on how well they are sized to the room, and how how the compressor is working or cycling.

[jimisbell]

Ooops, now you have gone and done it. You went too far with that one.
Why is it that the turbine water pump on my fire engine heats up to
the point it will boil if the water is not circulated. The turbine
pump does NOT contact the sides so there is no metal to metal
friction, just stiring the water real fast. The reason you cant shake
the bottle to prevent freezing is a mater of the effort it would
take...beyond your strength....while the cold environment will freeze
it much quicker.

In a fire engine there is a valve that allows you to bypass some of
the water back to the tank when the hoses are turned off so that there
is always a circulation of some cooler water from the tank to keep the
pump from overheating.

- Show quoted text -

[44'cruisingcat]

Seriously, how much extra heat do you think the movement of the boat and TIDES (for crying out loud) would add? Enough to be measured? I seriously doubt it. In PRACTICAL terms, what the refrigeration unit has to contend with is:

1, heat passing through the insulation, and

2, cold air lost and replaced by warm air when the fridge is open.

How full or empty the fridge is wont make any difference to 1. Having it full will make a difference to 2 though.

[Alan Wheeler]

Wow! this is made a simple question complex. Lets get the question back on track.
Yes a fan will aid the removal if heat from the radiator on the compressor unit. I have a programme somwhere that actually tells you how much air flow removes, but it is complex and I don't think needed for this. Just a fan of the same physical size of the radiator will be fine. Even a small movement of air via the fan makes a hughe difference in the efficiency.
The big disadvantage of a front opening fridge/freezer is the loss of cold air when you opent he door. That cold air is replaced by warm air that has to be drawn back down to temperature again. That equates to more energy required to cool the fridge.
Using heatsinks like water is a method we use a lot for bringing down food temp quickly. Sea water is better than fresh as it has a lower freezing temp. But storing water in the freezer to save energy is not how it works. It is that you have already imparted the cooling energy (energy made from battery/compressor) into the water which inturn imparts it to the food. The heat energy imparted from the food back to the water must also be taken back out by the compressor. So the result is an initial fast response time "pull down" of temperature followed by a longer pull down time to cyclical temp.
Did that make sense??

[mikereed100]

if you're adding insul. to the inside, you're likely sealing any drainhole, so won't need to worry about losing cold air down the hole and using power to cool the bilge

I introduced a half loop into the drain tube on my ancient cold box. Water accumulates in the loop preventing cold air from flowing out of the box. Don't know if it keeps it any cooler but it makes me feel better.

Mike

[Alan Wheeler]

This brings up a good question. The copper line that runs from the compressor unit to the condenser in the cool box, should that or would that benifit from insulation?? Mine is just the bare copper pipe running to and from the condenser.