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Why does the fridge take so long to get cold ?

Started by Captain Cook, Feb 21, 2006, 11:43 AM

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Captain Cook

It does work and Ive checked the settings but it takes a good day and a half sometimes 2 days before it gets cold enough to put stuff in it.  I have automotive A/C experience (certified a long time ago) but still cant figure it out. Thanks again in advance for any ideas !

wavery

Quote from: Captain CookIt does work and Ive checked the settings but it takes a good day and a half sometimes 2 days before it gets cold enough to put stuff in it.  I have automotive A/C experience (certified a long time ago) but still cant figure it out. Thanks again in advance for any ideas !
The fridge in your camper does not operate like an automotive A/C or conventional refrigerator.

It does not have a compressor. It works off of heat conduction plates. You are right, they take a long time to cool down and are not designed to cool food. They are designed to keep, already cooled food, cool.

It is best too turn your camper fridge on a day or two before you are ready to use it. Put food in it that has already been cooled by your home fridge. Also, try not to open your fridge very often. Remember, cold air falls fairly rapidly. It is replaced by room temperature air. Each time that you open your fridge, you have to re-cool the air inside.

Cheryl

Hi There,
We found that if we turned the fridge on using the propane that it cooled much quicker. After it was cold and we loaded it with already cool and/or frozen items, then we used electric to run it. Openning the door as little as possible helps also.
Good luck,
Cheryl

wavery

I found this on a website:
http://www.websitea.com/mai/teca/pg06.htm
It's hard to read because another ad keeps popping in, so I copied it for you:
-------------------------------------------
Thermoelectric cooling (conductive cooling), or as it is sometimes called, "the Peltier Effect", is a phenomenon discovered by a French watchmaker during the 19th century. It is described as a solid-state method of heat transfer generated primarily through the use of dissimilar semiconductor materials. To understand the cooling method, it is first necessary to know how thermoelectric cooling systems differ from their conventional refrigeration counterparts.

Like conventional refrigeration, thermoelectrics obey the basic laws of thermodynamics. Both in result and principle, then, thermoelectric cooling has much in common with conventional refrigeration methods - only the actual system for cooling is different.

Perhaps the best way to show the differences in the two refrigeration methods is to describe the systems themselves. In a conventional refrigeration system, the main working parts are the evaporator, condenser, and compressor. The evaporator surface is where the liquid refrigerant boils, changes to vapor and absorbs heat energy. The compressor circulates the refrigerant and applies enough pressure to increase the temperature above ambient level. The condenser helps discharge the absorbed heat into the ambient air.

In thermoelectric refrigeration, essentially nothing has changed. The refrigerant in both liquid and vapor form is replaced by two dissimilar conductors. The cold junction (evaporator surface) becomes cold through absorption of energy by the electrons as they pass from one semiconductor to another, instead of energy absorption by the refrigerant as it changes from liquid to vapor. The compressor is replaced by a DC power source which pumps the electrons from one semiconductor to another. A heat sink replaces the conventional condensor fins, discharging the accumulated heat energy from the system.

The difference between the two refrigeration methods, then, is that a thermoelectric cooling system refrigerates without the use of mechanical devices, except in the auxiliary sense, and without refrigerant.

hermoelectric (Def): Semiconductor materials with dissimilar characteristics are connected electrically in series and thermally in parallel, so that two junctions are created.
The semiconductor materials are N and P type, and are so named because either they have more electrons than necessary to complete a perfect molecular lattice structure (N-type) or not enough electrons to complete a lattice structure (P-type). The extra electrons in the N-type material and the holes left in the P-type material are called "carriers" and they are the agents that move the heat energy from the cold to the hot junction.

Heat absorbed at the cold junction is pumped to the hot junction at a rate proportional to carrier current passing through the circuit and the number of couples. Good thermoelectric semiconductor materials such as bismuth telluride greatly impede conventional heat conduction from hot to cold areas, yet provide an easy flow for the carriers. in addition, these materials have carriers with a capacity for carrying more heat.

 
Heat Sinks
The design of the heat exchanger is a very important aspect of a good thermoelectric system.

The upper part of the diagram illustrates the steady-state temperature profile across a typical thermoelectric device from the load side to the ambient. In the temperature graph, the total steady-state heat which must be rejected by the heat sink to the ambient may be expressed as follows:

 
If the heat sink is not capable of rejecting the required Qs from the the given system, the temperature of the entire system will rise and the cold junction temperature will increase. If the thermoelectric current is increased to maintain the the load temperature, the COP (Coefficient of Performance) tends to decrease. Thus, a good heat sink contributes to improved COP.

Energy may be transferred to or from the thermoelectric system by three basic modules: conduction, convection, and radiation. The values of Qc and Q1 may be easily estimated; their total along with the power input gives Qs, the energy the hot-junctHeat Sinks
The design of the heat exchanger is a very important aspect of a good thermoelectric system.

The upper part of the diagram illustrates the steady-state temperature profile across a typical thermoelectric device from the load side to the ambient. In the temperature graph, the total steady-state heat which must be rejected by the heat sink to the ambient may be expressed as follows:

 
If the heat sink is not capable of rejecting the required Qs from the the given system, the temperature of the entire system will rise and the cold junction temperature will increase. If the thermoelectric current is increased to maintain the the load temperature, the COP (Coefficient of Performance) tends to decrease. Thus, a good heat sink contributes to improved COP.

Energy may be transferred to or from the thermoelectric system by three basic modules: conduction, convection, and radiation. The values of Qc and Q1 may be easily estimated; their total along with the power input gives Qs, the energy the hot-junction heat sink must dissipate.

Captain Cook

awesome info everyone !  

wavery, thanks for the info from your copy and paste !  Thats really cool !

mike4947

As an addition, the average absorption fridge in a PU has a BTU rating of 600-700 on propane or 120 volt. Takes a long time for that small amount of BTU's to transfer any heat in the air and fridge walls to the outside.

The fridge will actually cool down much faster if you toss in a frozen gallon jug of water when you start it up.
The small battery operated inside the fridge fans also help to keep the air circulating over the cooling plate and helps maintain an even inside temp as well.

wavery

I thought that article was a good lesson in theory for everyone.

What most people don't understand about A/C and refrigeration is that refrigeration units do not produce cold air. Their basic function is the removal of hot air (Therms).

You will notice that all of these units are rated in BTU (British Thermal Units). That is a measurement of HEAT. If a unit is rated at 13,000BTU, that means that it will remove 13,000 units of heat in a measured amount of time.

The reason that I bring this up, in conjunction with this topic is, when you open the fridge, most (if not all) of the cooled air (air that has had the heat removed) is lost and replaced with warmer air. That is a basic design flaw in these fridges. IMHO, these fridges should be top opening. That way, you loose little if any cold air when the door is opened because the cold air is heavy and stays low in the contained fridge.  

If you have a 2 cubic ft fridge and only have 1 cubic ft of food and drinks inside, each time you open the door, you loose 1 cubic ft of cold air and it is replaced by 1 cubic ft of warm air. If you have 1.5 cubic ft of food and drinks in the fridge, only .5 cubic ft of air must have the heat removed. Therefore, the fridge is twice as efficient.

The front opening fridge unit has to go to work to remove the newly introduced heat, every time the door is opened. The trick is to keep the fridge as full of cold items as possible. The more air space that is in the fridge, the more warm air is introduced when the door is opened. If the fridge is full, very little warm air is introduced. A half full fridge is much harder to keep cool then a full fridge.

chasd60

The article about the Peltier effect cooling has some good info. It mentions Peltier as compared to "conventional" cooling. There is the regular refrigeration method of using a refrigerant that removes heat by changing phases from a liquid to a gas. This is like in home and automotive A/C units.
 
 
The refrigerator in your camper is the absorption type which primarily uses ammonia as a refrigerant.
Good info here on absorption type from the "Ammonia Refrigeration Technicians Association", ARTA, INC.
http://www.nh3tech.org/absorption.html
 
A little off topic but I am looking into an absorption type A/C chiller unit that will by powered by solar thermal power.

wavery

Quote from: chasd60The article about the Peltier effect cooling has some good info. It mentions Peltier as compared to "conventional" cooling. There is the regular refrigeration method of using a refrigerant that removes heat by changing phases from a liquid to a gas. This is like in home and automotive A/C units.
 
 
The refrigerator in your camper is the absorption type which primarily uses ammonia as a refrigerant.
Good info here on absorption type from the "Ammonia Refrigeration Technicians Association", ARTA, INC.
http://www.nh3tech.org/absorption.html
 
A little off topic but I am looking into an absorption type A/C chiller unit that will by powered by solar thermal power.
That's a more appropriate article. I tried to find one like that but couldn't. The conductive theory is the important part.

Thanks for the link, I saved it :D .

skoryaro

Hook-up a small 12v or 120v fan that you can turn on and off and mount it on the backside of the fridge (not inside the fridege) so that it blows across the heat heat fins.  This helps disipate the heat quicker and makes quite a difference.  

Quite a few RV owners mod their fridge in this fashion to improve performance.

AustinBoston

First, are you aware that the fridge is adjustable?  There should be a setting that has 5 positions on it.  Good luck finding that narrow band halfway between "too warm" and "frozen solid".  The adjuster doesn't work on 12V, and may not apply to propane either.

In most parts of the country, you should be able to get it to freeze solid in 24 hours.  They start to loose capacity quickly at some point (95

SpeakEasy

Quote from: waveryIf you have a 2 cubic ft fridge and only have 1 cubic ft of food and drinks inside, each time you open the door, you loose 1 cubic ft of cold air and it is replaced by 1 cubic ft of warm air. If you have 1.5 cubic ft of food and drinks in the fridge, only .5 cubic ft of air must have the heat removed. Therefore, the fridge is twice as efficient.

Or in the case of my DW, if you put 2.5 cubic ft of food in the 2 cubic foot fridge, the d**n thing actually cools down when you open the door! Talk about efficiency!

 :D  :D  :D  :D

mike4947

Just to add, our small fridges do not have a true thermostatic contol. Lager absorption fridges have a control board powered by 12 volt that provides a themostat. What we have is an "amount of cooling" control.

Problem is variations in outside temperatures affect the inside fridge temperatures. Set the fridge for the afternoon high temp and at night when the temp cools off the internal fridge temp drops.
Ask anyone who's woken up to "extra crispy" lettuce and other "crunchy" things that aren't supposed to be crunchy.
We learned that an outside themometer probe stuck in the fridge really helps us control the inside the fridge temps without having to open the door. Loosing all that hard earned cold air.

AustinBoston

Quote from: mike4947Ask anyone who's woken up to "extra crispy" lettuce and other "crunchy" things that aren't supposed to be crunchy.

Try frying an egg that is frozen like a rock some time ;)

Austin

wavery

Quote from: AustinBostonTry frying an egg that is frozen like a rock some time ;)

Austin
Try putting it in the microwave :D

j/k