Why does an egg boiler require more water to cook fewer eggs?

I got an egg boiler machine, on the instructions is stated:

Less water is used when cooking more eggs.

My thermodynamics understanding cannot figure this out yet. Why would I need less water for more eggs?

The machine beeps when the water has evaporated, so the eggs are ready. In that case, why will less water cook more eggs?

• There is an answer on Quora. Maybe it helps. Sep 18, 2018 at 0:10
• @J...The eggs are not touching the water. Sep 18, 2018 at 12:19
• @Taemyr they do in the egg boiler I have. If they don't touch the water then it's a steamer rather than a boiler, I suppose? Sep 18, 2018 at 12:50
• @AaronF 25 ml seems to be very little to cover 7 eggs. Also the amount of water varies by 5 ml per egg. Sep 18, 2018 at 13:15
• @Taemyr oh the water doesn't cover them. It's just enough that the bases of the eggs are sitting in the water. 5ml seems a reasonable amount of displacement per egg, but I haven't done any calculations :-) Sep 18, 2018 at 13:24

Presumably, the rate of the steam escaping the cooker depends on the "resistance" of the steam path: from the opening in the bottom, where the steam enters the dome, to the opening on the side of the dome, from where the steam escapes.

The more eggs in the cooker, the narrower the path, the slower the flow. Also, as relatively slowly moving steam makes contact with more eggs, it is more likely to condense and make its way back to the water at the bottom of the cooker, which further reduces its escape rate.

So, with more eggs in the cooker, a smaller amount of water will last about as long as a greater amount of water with fewer eggs, resulting in a similar degree of cooking.

• Could a slower flow also mean that more water would result in a higher pressure, possibly damaging the eggs or the cooker itself? Sep 18, 2018 at 23:38
• Not a fizzix expert, but perhaps related to total volume at start of process? Or if it is steaming the eggs and not a traditional boil (like another answer/comment suggests) perhaps having to do with the space for steam the volume of the eggs are displacing? But then we are back to related to total volume... Sep 19, 2018 at 0:19
• @jpmc26 I don't see anything in the manual that suggests this is a pressure cooker (e.g., any mention of a safety valve), so the pressure shouldn't get significantly above atmospheric. Sep 19, 2018 at 9:31
• The condensation part makes a lot more sense than obstructing gas flow. Thank you, I didn't think of that. Sep 19, 2018 at 10:02
• I wish someone would test these theories instead of just guessing an answer. Sep 19, 2018 at 17:44

In boiling eggs, we note that each egg displaces a certain amount of water volume in the pot.

To cook eggs, we want them to be immersed in boiling water- immersed, but no more, because the excess water does not assist in the boiling process.

Why is this so? Because the water serves only as a heat transfer medium which completely surrounds the egg, and conveys thermal energy from the bottom of the pot to the eggs at a convenient temperature- that of boiling water.

This means that 1) we want the eggs to be just immersed in the water- but no more, 2) the more eggs we put in the pot, the less water is required to just immerse them- at least up to the limit of having the bottom of the pot completely populated with eggs.

• This seems to miss the implications of "The machine beeps when the water has evaporated, so the eggs are ready." I think the point of confusion is that if there is less water then it will take less time to evaporate so you are cooking the eggs for a smaller amount of time. This answer makes a lot of sense for the normal way of cooking eggs in which they start and end in boiling water but less so for this machine's apparent mode of operation... Sep 18, 2018 at 9:19
• Also following the link to the instructions the amount of water for 7 eggs is 25ml. Even one egg at 55ml you are not covering the eggs at all... Sep 18, 2018 at 9:23
• Downvoted because even though the question refers to an "egg boiler machine", such machines aren't really egg boilers so much as egg steamers. In particular, the eggs are not immersed in the water in such machines. Sep 18, 2018 at 10:32
• The eggs are not submerged in this device. They supported above the water and cooked by steam. -1.
– MPW
Sep 18, 2018 at 13:39

The reason is, that your egg boiler recycles the water a couple of times.

You say that the process is finished when the water is gone, so you are talking about an egg boiler that cooks the eggs with steam. In these egg boilers you have a shallow, heated pan at the bottom into which you fill the water. Then the egg boiler boils this water at a constant rate (the heat input is constant, as the pan will remain at 100°C for the entire time and is electrically heated). So a constant amount of water is boiled off per second.

Now, above the water you have the eggs. And here comes the difference: If you put a single egg in, it has a certain surface on which the water condenses. This condensing is slower than the steam production, the additional steam just escapes through the hole in the top of the cover. The more eggs you put into the steam, the larger their surface, and the more water will condense on them. This condensed water then drips down back into the pan, and is evaporated another time.

As the steam production remains constant when you increase the number of eggs while the water condensation grows, less steam escapes through the hole in the cover. Thus, the water loss is slower with more eggs.

Now, to get well-cooked eggs, you need to heat them for a specific time. And because more eggs keep the water inside the boiler longer, you must reduce the starting amount of water to get the same cooking time. That's it: You reduce the amount of water to keep the cooking time constant.

I confess, I simplified a bit: I ignored the condensation on the inside of the cover itself. I did this, because that condensation is independent of the number of eggs. The condensation on the cover simply serves to quickly heat up the cover to some temperature slightly below 100°C, and then just supplies the heat necessary to keep it at that temperature.

• This makes far more sense to me than the eggs restricting the steam flow enough to make it that significant. Great first answer here.
– JMac
Sep 19, 2018 at 13:05
• I agree with this answer: the eggs are a heat sink. More eggs are a more efficient heat sink and will more effectively recirculate the steam into water. Sep 19, 2018 at 15:17

Egg boiling machines are designed to cook eggs that have only a small part submerged in the water.

To ensure that the same volume is submerged regardless of the number of eggs, you need to add more water as you reduce the number of eggs.

This is because of displacement rather than thermodynamics. Fewer eggs displace less water, so more water is required.

• Actually, the eggs in such an egg cooker never touch the water at all. So eggs displacing the water isn't the answer, as discussed in the comments under one of the other answers. Sep 18, 2018 at 11:04
• With the one I have - bosch-home.co.uk/supportdetail/product/TEK1101GB/01#/… - the eggs are definitely partly submerged in the water. Perhaps other egg cookers (specifically egg steamers ) work slightly differently to the one I have? Sep 18, 2018 at 12:48
• The one i had when I grew up didn't. But fair enough. Sep 18, 2018 at 12:53
• Does your boiler also requires <10 ml of water per egg? Sep 18, 2018 at 13:26
• @DmitryGrigoryev yes, more or less. The problem I have is that I lost the measuring device years ago, and now I just estimate the quantity. As long as I don't put too much water in then the eggs come out fine. Here's an image of the measuring cup that I lost media3.bosch-home.com/Product_Shots/2000x2000/… - you can see the different amount required for hard and soft eggs, and that more water's required for fewer eggs. Sep 18, 2018 at 13:29

I propose that the answer is found in the equation:

$$PV=nRT$$

This sounds like an egg steamer, as many others have noted. As such, what's cooking the egg is the heat transfer from the steam under pressure. It's not apparent whether this is a sealed pressure cooker or not (probably not, given the lack of warnings); but it certainly creates some pressure.

Having more eggs in the container means that you have a smaller interior volume, and thus a higher pressure, or higher temperature for the same pressure, so less water is required.