How It Ticks

Around the house · Exhibit 12

How does a refrigerator work?

Updated

Put your hand on the back or the sides of a running fridge and they feel warm. That warmth is the heat that used to be inside, in the milk and the leftovers. A fridge moves it out with a liquid that boils inside the box and turns back into liquid outside it, around and around. Your kitchen fridge drives that loop with a compressor. A gas fridge in an RV drives it with a small flame, has no compressor or pump, and wants to stand level.

Below are both, cut open side by side. Let more heat in with the first slider, then tilt the gas one with the second.

A compressor fridge and a gas absorption fridge. The blue arrow is heat taken from inside the box. The upright red one is what the compressor (the circle) or the flame (the red triangle) adds: thin for the compressor, nearly four times the blue one for the flame. The red one at the top is everything that ends up in the kitchen.

How does a refrigerator keep food cold?

Inside the box, usually in the back wall or behind a panel, runs a pipe with a liquid in it. In many home fridges that liquid is isobutane, which the industry calls R600a; the Beko manual I quote below names it. When a liquid boils it soaks up heat, the way water in a pan takes heat from the stove without getting hotter than its boiling point. If the liquid in that pipe boils at a temperature below the inside of the fridge, it pulls heat out of the air and the food around it. That pipe is the cold coil, the evaporator in a repair manual.

The boiling point depends on pressure. Squeeze the gas, and it turns back into liquid at a temperature above your kitchen’s, so even a warm room is cold enough to make it condense, and as it condenses it gives its heat away. That second pipe is the warm coil on the back, the condenser. Pressure here is in bar: 1 bar is roughly the air pressure around you. The graph uses measured values for isobutane from NIST, the US standards lab:

Isobutane boiling at different pressures. Move the slider and see where the dot sits against the blue band and the red lines.

At 0.6 bar isobutane boils at about −24 °C. At 5.3 bar it turns back into liquid at about 40 °C. So the compressor, the black lump at the bottom of the back, sucks the gas out of the cold coil at low pressure and pushes it into the warm coil at high pressure. A thin tube, the capillary tube, then lets the liquid back into the cold coil, the pressure drops, and it boils again: compressor, condenser, capillary tube, evaporator, and round. Physicists call any machine like this a heat pump.

The heat that leaves at the back is more than the heat that came out of the box, because the compressor’s own work ends up in the kitchen too. That’s why the big red arrow in the first toy is always the blue one plus the small one, and why leaving the fridge door open warms a closed kitchen. The other box in the kitchen that moves energy around, the microwave, does the opposite job.

Which number on a fridge dial is coldest?

On the fridges I could check, the higher number. Electrolux says on its support site: “Level 1 means the warmest level”, “Level 6 or 7 means the coldest level”, and “Level 0 means turning the fridge off”. On a Beko fridge-freezer the dial goes from min through 1 to 4, “4 being the coldest position”.

The number on the dial isn’t a temperature. It only says how hard you want the fridge to try, and in Electrolux’s words the numbers “don’t correspond to exact temperatures, because the actual cooling level can vary depending on things like the room temperature and how full the fridge is.” Both makers tell you to start in the middle; Electrolux adds a thermometer in a glass of water and wants the inside “between + 2°C and + 8°C”. Beko wants the average “no higher than +5°C (+41°F)” 24 hours after first switching on.

Does a fridge work harder when it’s hot, or when it’s full?

When it’s hot, yes. Heat leaks in through the walls and the door seal faster when the kitchen is warmer, and the thermostat answers by running the compressor longer. In the toy, the “hot room” setting grows every arrow, the compressor’s and the flame’s included. For the EU energy label, most fridges are measured at two room temperatures, 16 °C and 32 °C.

The same rules sort fridges into climate classes, and the manual lists the class in its spec table: SN is for rooms from 10 to 32 °C, N from 16 to 32 °C, ST from 16 to 38 °C, and T from 16 to 43 °C. The Beko I mentioned is SN-ST, “designed to operate in ambient temperatures up to 38 degrees C”.

A cold room is a problem as well. Beko warns that “if the ambient temperature is below 0°C, the food in the fridge compartment will freeze.” It also built in what it calls Freezer Guard, so that at the advised setting the frozen food “will not defrost even if the ambient temperature falls as low as -15 °C”. Beko adds that a new fridge “MUST NOT be placed in low ambient temperatures” when first installed, because the freezer won’t get down to temperature. My own conclusion: without a feature like Freezer Guard, a cold garage is a risk for the freezer too.

Full or empty is less clear. Dometic lists “the amount of food to be refrigerated” and “how often you open the door” among the things that affect cooling, and Electrolux names “how full the fridge is”. I found no maker’s figure for how much a full fridge saves or costs, so I won’t give one.

How does a gas fridge work without a compressor?

RV and camper fridges like Dometic’s run on “liquefied gas (propane or butane)”, on 12 V or on mains, with a loop that has no compressor and no pump. It’s called an absorption fridge because water soaks up (absorbs) the refrigerant, ammonia, and lets it go again. Hydrogen fills the rest of the pipes. Baltzar von Platen and Carl Munters first filed the idea in Sweden in 1922.

The flame heats the ammonia solution until ammonia gas bubbles out. The bubbles carry slugs of liquid up a narrow tube, like a coffee percolator. From the top, everything moves on its own. The ammonia gas rises to a warm coil and turns to liquid. That liquid runs down into the cold coil and evaporates into the hydrogen, taking heat from the box as it goes. Further down, in a pipe called the absorber, the water soaks the ammonia back up. On 12 V or mains an electric heating element does the flame’s job: Dometic’s troubleshooting table lists “the AC heating element” and “the DC heating element”.

Dometic says the unit “operates silently”. Once the bubbles have lifted the liquid to the top, nothing but gravity moves it on.

Why does an RV fridge have to be level?

Because the liquid finds its own way downhill through the pipes, and a tilted fridge changes which way is downhill. Norcold gives the limits in its manual for the N400, N500 and N510: the fridge “is made to operate within 3° off level side-to-side and 6° off level front-to-back (as looking at the front of the refrigerator)”, and “operating it at more than these limits can cause damage to the cooling system and create a risk of personal injury or property damage.” So side to side means across the fridge door, which may run along the van, not across it. The second slider in the first toy tilts the gas fridge sideways and marks when it goes past 3°.

Driving is different from parking on a slope. Norcold again: “While the refrigerator should be level when the vehicle is stopped, performance during travel is not usually effected.” Dometic says to “park the vehicle so that it is level, especially when starting up and filling the refrigerator before setting off on a journey”, and when the fridge reports no cooling, the first thing its error table says is: “Check if the device is at an angle and adjust it if necessary.”

Can you run an RV fridge on gas while driving?

Only if the van’s maker allows the gas system to run on the road, Dometic says, and only where local rules allow it too. It also says never to run the fridge on gas at a gas station, in a parking garage, on a ferry or while the van is on a transporter. Dometic’s RMD10 fridges have a tank stop mode: switch off the ignition and gas is blocked for 15 minutes, then the fridge goes back to automatic mode, so if refueling takes longer than that, turn it off or pick another mode. If you run it on gas from its own battery packs, which Dometic calls stand-alone gas mode, nothing is blocked, and Dometic says you must switch the fridge off by hand every time you refuel. Many fridges have no tank stop mode at all; on those, the gas is off at the pump only if you turn it off.

What do the toys leave out?

Real fridges hide their pipes inside foam walls and run them in long loops; mine are boxes and zigzags. The first toy’s flame arrow uses the best figure measured on a test absorption unit in 2019: it took out of the box about a quarter of the heat it was given (COP 0.26), so the flame adds nearly four times what it removes, and at other settings up to nine times. For the compressor I have no measured figure, so its arrow is my illustration. Its tilt only checks Norcold’s limit; what exactly goes wrong past it, the manual doesn’t say, so the toy doesn’t guess. The hydrogen, the ammonia and the water aren’t drawn separately. The graph is real data for isobutane but leaves out the capillary tube, and other fridges use other gases with other curves. Frost, defrost heaters, fans and the freezer’s own coil are left out.

Sources