Economical Refrigeration

[44'cruisingcat]

When you say "condenser in the cool box", do you mean the evaporator?

The condenser is the "radiator" that is outside the coolbox - hot refrigerant gas from the compressor is condensed there, so any heat lost from that, the "hot side" of the TX valve is fine.

A simple rule of thumb - (if you can safely access the plumbing when the fridge is running) If a pipe feels cold when the fridge is operating normally, it's worth insulating, if it feels warm then leave it open.

[hellosailor]

Wheels-
"The copper line that runs from the compressor unit to the condenser in the cool box, " it's the evaporator that's inside the cool box, isn't it? But whatever we call "that coil in the box" yes, insulating the line that runs to it will ensure the cold liquified gas doesn't absorb heat until it gets into the box, where you want to remove heat.

Mike-
On an ice box, I've seen and used a manual galley pump that pumps the melt water out of that drain and into the sink. If you place some bottled drinks, etc. in the sink (or a bowl in it) and use that meltwater to keep them cold--it makes a great way to have cold drinks at hand without needing to open the ice box. Keeps the bilge dry too.<G>

[Alan Wheeler]

Yeah I always get confused with which one is which. But yes it is the cold pipe I am talking about, so I will insulate it.
Hey by the way, I got the car AC going. I was given a bottle of R12 by one of our wonderful Kiwi posters here. I'm not sure if I should tell you what I did, but then again it could make a really funny story and certainly will make eyes roll. It's one of those "he knows just enough to be dangerouse" type situations.

Chris had suggested I use the gas as a bartering tool for getting the AC fixed. But I figured if I did that I lose a bottle of valusable gas that I could use ont the boat in the future. So having basic knowledge of how a fridge system works, (I mean basic) I decided I would try this myself. I knwe I couldn't make it worse, the AC didn't work. So I made up a brass fitting to fit the connector on the AC line. There were two of these fittings. One on the side of the pump that pumped hot gas to the condensor at the radiator and one on the cold side that came from the evaporator back to the pump. the one on the "hot" side had instructions that that one was to be used for filling the AC unit. I figured that I would simply pour in some gas and see if the thing worked. I firstly worked out which way the bottle should be, which was upside down. But nothing happend, so I tried the other fitting. Still nothing. So then I figured I would start the car and run the AC. I tried the the "hot" side again. Still nothing. Then I tried the "cold" side and to my surprise, the AC started cooling down. I then thought, ummm, how much am I supposed to put in? So I just waited for the system to feel nice and cold and then disconnected it all.
I drove home and it worked great, but I was also expecting a huge explosion and gas billowing out over the road from maybe having put too much gas in the sytem and blowing a radiator or something. I drove down our main town street waiting for the imbarresing bang, but to myu relief nothing happend. I have gone two weeks and a 1000Kms now and it all seems OK.

OK, so don't go raining on my parade OK. Ignorance can be bliss. ;-) :-)

[Steve Kidson]

If you are going to put bottles of water in the fridge to use up space to lessen the area for cooling, why not consider the following:

1. Pre-freeze the water bottles at home before that week end cruise.

2. Water bottles will take up space for the important provisions such as beer and wine pre-cooled at home of course

Fair winds

Steve

[Alan Wheeler]

Sooo, what the better suggestion would be Steve, is to pre cool or even freeze cans/bottles or beer and wine. As you drink them you create space to place food. If you need space to place food, you have to drink. Far better than wasting water.

[hellosailor]

Wheels-
If your, ah, casual fill worked, it worked. That's the good news. Doing it again might be tempting the fates though. If you attach an R12 bottle upside down, and allow liquid Freon to flow, it can create excess pressure or freezing in the system and, let's it it a "cyrotechnics display" as parts of the system get upset about that.
The usual way to fill is indeed what you did, letting the compressor run to suck the gas in and compress it. And, absent pressure gauges, you'd just run it till it is cold, that may not be perfect but it usually will come workably close.
If the gas goes away again...you might want to consider bartering the rest of the bottle for a professional job, locating the leak, sealing it, and repressurizing to the right pressure. Much better chance it will work better & longer that way.

[Alan Wheeler]

Yes it may be the better way, but the car ain't worth it.

[needboat]

Hey guys, you're mixing mushy apples and frozen oranges. it's not just volume or density or bumping random molecules, but one of those things you hated to learn about in high school physics called heat capacity, and it's related cousin specific heat. Simply put, water has a higher heat capacity than air, so it takes more energy to put heat in, and more to take it out. So water will STORE the cold in a box better than air will (hence the cold water jugs) On the other hand, INSULATION in this box is primarily achieved by reducing conduction and convection, and air is a better insulator than water (closed cell styrofoam).
Happy cooling

[hellosailor]

"So water will STORE the cold"
Since there is no such thing as "cold", only an absence of heat energy, perhaps you could rephrase that?

[needboat]

You are technically correct that here is no such thing as cold, but it will be a sad day when a sailor walks into a bar and asks for a heat-depleted beer.

Rephrasing the physics - water will have a slower change in temperature over time as compared to air. We see this this all the time, lakes and ponds do not freeze the first night it dips below 32, nor do they melt in the spring as soon as it rises above 32. The air changes temperature faster than water due to its lower heat capacity. Hence, once a "cold" box has reached its desired temperature, it will remain at that temperature for a longer period of time (less cycling of the compressor) with water jugs inside it, than if the box was empty.

Don't take my word for it, do the experiment with a clean empty box. Bring it to the desired temperature, and then either count the cycling, or turn it off and measure the temperature rise over time. Now do it again with the box full of water jugs. Makes no difference how well you box is insulated, the water will be slower to rise in temperature than the air (and take longer to bring to the lower temperature, too) And for those that think this is only a space filling issue, for a third experiment they can use empty water jugs.

[hellosailor]

Needboat-
"Hence, once a "cold" box has reached its desired temperature, it will remain at that temperature for a longer period of time (less cycling of the compressor) with water jugs inside it, than if the box was empty. "
I understand that. I can't help but wonder, as I said earlier on, if that just means the compressor is cycled less often as opposed to being used for less time all together. If you turn it on, say, once every hour for five minutes, is that any different from turning it on once every fifteen minutes for 1-1/4 minutes? And again, as I said earlier, that may be the significant difference, since every time you turn it on, you have to waste some time bringing the heat pump system out of equillibrium and into operating condition with "fluid here, gas there" in the active mode.

I suspect the systems are designed to run most efficiently when the box is filled with food (water, etc.) so it would be a simple matter of design parameters being matched, rather than any magic of how it is filled. If the actual operation could be "tuned", or made smart enough to compensate for the differing operating conditions....Well, that's where the question of how the box was built and what's really going on crosses from black art to simple science, no?

[northerncat]

it is worse for an electric motor to stop start though
sean

[GordMay]

The greater time period required for the inside temperature to rise to the refrigerator’s set-point, the fewer times the compressor will cycle “on”, over a given time. The run-time, for each “on” cycle, will depend upon:
1. the differential setting of the thermostatic control’s anticipater
2. The specific heat of the contents (thermal inertia of food, water jugs, & air*)

Hence, a fridge with (temperature stabilised) water jugs will run less often, but longer.

The more full refrigerator uses slightly less electricity (over the longer term), primarily due to less frequent compressor start-ups, as hellosailor suggests.
Another significant advantage to a full refrigerator, compared to an emptier one, is more stable interior temperatures.

As I previously indicated, placing pre-frozen water jugs in the fridge, will transfer the energy cost (in freezing) from the recipent fridge, to the donar fridge. This will be significant benefit, if the recipient is a relatively energy-expensive boat fridge, and the donar a cheaper shoreside fridge.

* Since air has a lower specific heat content than liquids & solids, it cools & warms more quickly. Since room-air is almost always warmer than interior fridge temperature, introducing room air into a fridge is very costly. Open the fridge door, or hatch, as infrequently and briefly as possible.

[hellosailor]

So in theory...if you have a reefer with a "tight" thermostat, one that maintains a tightly regulated temperature, and you want to save some compressor cycling, it can actually be "better" to loosen up the thermostat settings and let the box range over a larger temperature shift. (Something that would be hard to sell to the modern houserube.<G>)

Interesting! It means the old fashioned bi-metal thermostats may still have an advantage over electronics, for this application.

[jimisbell]

The new thermostats are designed with a built in historisis (the delay between rising and falling switch opening/closing) that will take care of the delay needed...in fact it is adjustable just as the old ones were. The designers were well aware of the need for historisis to save energy so while they could have made it cycle instantly, they didnt.