Refrigeration?

[CTho9305]

Can someone explain who is right in this thread, and exactly why? I dont see how having food or no food in a fridge would affect the amount of energy coming through the fridge walls.

 

[drag]

It takes less energy to maintain a tempurature in a large mass than a small mass. A mouse has to work a lot harder (pound for pound) than a elephant to stay warm on a cool night. So a refrigerator with more mass and will stay cool longer than a identical refridgerator with nothing inside.

 

[CTho9305]

No offense intended, but what kind of physics knowledge do you have?

There are MANY reasons small animals eat more of their body weight than large animals. One follows: The volume of a sphere is is 4/3 pi*r^3. the surface area is 4 pi*r^2. Note that as size increases, volume grows faster than surface area. This means somehting small will have relatively large surface area compared to something bigger. Thus it loses more heat per volume and (per mass) so each "unit" of mouse has to be more active. Smaller animals generally also have higher body temperatures (cats are ~104F IIRC) and if you have a hotter body, you will radiate heat faster than a cooler one.

 

[Evadman]

It should be the same eiter way, empty or full. The energy expended by leaking though walls and seals will be the same empty or full.

It will stay cooler longer if the reefer turned off and there was mass in the fridge.

 

[Fatt]

You forgot the variables...

If you keep the fridge closed forever it makes no difference.

If, as in real life, the door is open and closed all day long, the more stuff the frideg has in it the less enegry the fridge uses.

Why?

Because when you open the door the cold air spills out onto the floor and warm air rushes in that has to be cooled down.

If the fride is empty and holds say, 20 cubic feet of air, that's 20 cubic feet that has to be cooled down when you close the door.

If the fridge has 15 cubic feet of stuff in it, which wont spill out when you open the door, then when you close it again it only has to cool down 5 cubic feet of air.

That's kind of oversimplified but it lays out the basic concept.

 

[Sahakiel]

I'm guessing it's the same reason you use less fuel driving at a constant speed as opposed to accelerating then coasting.

 

[Machupo]

Originally posted by: Fatt
You forgot the variables...

If you keep the fridge closed forever it makes no difference.

If, as in real life, the door is open and closed all day long, the more stuff the frideg has in it the less enegry the fridge uses.

Why?

Because when you open the door the cold air spills out onto the floor and warm air rushes in that has to be cooled down.

If the fride is empty and holds say, 20 cubic feet of air, that's 20 cubic feet that has to be cooled down when you close the door.

If the fridge has 15 cubic feet of stuff in it, which wont spill out when you open the door, then when you close it again it only has to cool down 5 cubic feet of air.

That's kind of oversimplified but it lays out the basic concept.

and chris, if you want to get really tongue-in-cheek about it, if there's food in the refrigerator, you're more likely to open it up and get something out than if it's empty...

feel free to smack me next time you see me

 

[PowerEngineer]

Fatt is correct. It's the same reason that so-called chest freezers are supposedly more efficient that up-right ones -- when you open their lid, the cold air stays in the freezer rather than spilling out on the floor.

 

[Beeker25]

Here goes;
I was a Refrigeration Service Technicain for 15 years. The concept of refridgeration isn't putting cold air into a area, it is infact, just the opposite, removing heat from an area. The Refrigerant in it's low pressure, low temperature vapor state moves through the evaporator-which is located inside of the refrigerator or area such as walk-in cooler or other similar type area, and picks up the heat-removes the heat- from the area. The removal of heat from the area is what gives you the cold temperature. The more heat in the area, the more energy it takes to remove that heat, so there are quite a few variables needed to answer your question correctly, such as, is the beer warm or cold when put into the fridge? What is the outside temperature, how well is the box sealed, ect. If the beer was already cold-the same temperature as the refrigerator is when it is running, it would take the same amount of energy to keep the temperature constant whether the refrigerator was full or empty. Once the desired temperature is reached, if the box is kept closed and sealed, it would take the SAME amount of energy to keep the refrigerator at the desired temperature whether it was full or empty. If you filled the chilled refrigerator with warm beer, it would take more energy to run. Hope that this info helps you out, LMK if you have any other questions.

 

[kylebisme]

i am by no means an expert on refrigerators but would like to add fairly obvious but often overlooked science behind what Beeker35 just explaind. heat is simply a form of energy, visable in the movement particals, wich the faster they move the "hotter" it feels. the "refrigerant" that runs though the pipes is a of course a gas, but also wich has less moleculor mass and a looser moleculor structure than the air and prety much anything in the refrigerator. because of this it is extreamly susceptible to the heat in the fridge and, with the aid the fans and the movemnt of the refrgerent itself that help it along, the heat transfers from the inside the refrigerator to the gas and then from the surface area of the pipes that contain the gas to the outside envorment. as the the first law of thermal dynamics explains that energy cannot be distroyed, the only option when wanting make a "refigerator" is to push the heat away.

 

[nachocheese]

To further elaborate on Snowman's post, here is a basic explanation on a standard refrigeration cycle:

A refrigerant (such as R-134a), also known as the working fluid, is pumped in a closed system. Starting at the compressor, the fluid goes from a vapor phase to a liquid phase due to greatly increased pressure from the compressor (this region is commonly known as the condenser). From thermodynamics, the fluid temperature will rise (similar to pumping your tires with air). This heated liquid goes thru something similar to a radiator to dissipate the heat (hence the hot air streaming out of an AC or fridge). This cooler liquid then goes thru a diffuser (throttling valve) which rapidly decreases the pressure. Due to the pressure drop, the liquid turn into a vapor (this region is commonly known as the evaporator). In addition to the phase change, the temperature of the fluid also drops considerably. Therefore, you now have a region around the evaporator which is COOLER than ambient, while the region around the condenser is HOTTER than ambient.

Back to your first question. The heat transfer rate from the inside to the outside of the fridge is independent of what is actually inside the fridge. If the door is open, then you have mass transport in addition to heat transfer, which complicates the problem. However, if you assume the door is closed (ie. the system is closed so no mass goes in or out), then the heat transfer rate is fixed.

On the other hand, it will take MORE energy to change the temperature of something with MORE mass, assuming the specific heat stays constant. This is why boiling 1 gallon of water takes a LOT longer than boiling 1 ounce of water using the same stove.

Hope this helps,

 

[kylebisme]

nachocheese, thanks for the deeper explanation there, as i said i am by no means an expert on refrigerators and just wanted to explain the basic science behind them. however it is always cool to get a beter understanding of how stuff works. also woops i ment "thermodynamics" not "thermal dynamics." i was a near sleep when i wrote that, but we are talking about the physics of energy here not weather patrens.

 

[RossGr]

Seems pretty straight forward that the energy lost from the fridge is related to the temperature differential between the interior and exterior, the contents do not figure in. The internal temperature, though, is dependent on the thermal mass inside the fridge so for a given change in energy the temperature of a full fridge will change less then that of a empty fridge. So to maintain a constant temperature inside the fridge over a period of time requires the same amount of energy full or empty, what changes is the number and length of the compressor/motor cycles required to maintain the temperature. The full fridge will require fewer longer cycles. Now there is a certian amount of overhead associtated with every cycle of the motor/compressor this is energy lost to the system and will be greater for the empty fridge since it cycles more. So the empty fridge will use more energy to maintian the same temperature. This assumes that the box is not opened, also does not include the energy required to initally cool the contents of the fridge.

 

[Howard]

Welcome to Anandtech, nachocheese.