Basic Heat Transfer: Conduction Thru a Wall

This is a very basic heat transfer question for any technical-minded person: One of the latest things with today’s dishwashers is to have a stainless steel lining (forming the inside of the dishwasher). The salesmen claim it holds in heat better than the old plastic-lined tub design. Does this make sense? Let’s assume everything else is equal, such as insulation behind the inner wall forming the tub (i.e., the inside of the dishwasher).

To me, it’s as obvious as this: Metal is an excellent conductor of heat whereas plastic is not as great. Plastic offers resistance. I think the industry is just trying to push another gimmick over the people in this regard. It may, however, help keep the inside of the dishwasher clean, including the inside of the door (where, at least in our case, we have found grime tends to build).

What do the SDopers say about this metal vs. plastic argument as a better material by which to RETAIN heat in a closed chamber?

Is “everything else is equal” a reasonable assumption?

I’m not a dishwasher designer, but a stainless steel liner by itself retaining more heat sounds unreasonable. I’ve never seen a dishwasher that didn’t have a stainless steel liner here in Norway, but that choice may very well have other reasons than heat retention.

Metal conducts heat well, but what’s it going to conduct it to? It’s not connected to any metal outside of the dishwasher.

On the other hand, a shiny surface like polished steel will absorb and radiate very little radiation (it’ll reflect it instead). The ideal insulation consists of two metal layers facing each other without touching, and vacuum in between (this is approximately how Thermoses and other dewars are made).

On the gripping hand, though, I expect that radiation is going to be a very small portion of the heat transfer from the inside of a dishwasher to its walls. You’ve got a lot of liquid water and very moist air moving around at high speed, which tells me that convection is going to be extremely high. So even if the polished metal eliminates radiative transfer entirely, it’s not going to make much difference, since the convection is still there.

I too have never seen a plastic lined dishwasher in the UK and I certainly wouldn’t buy one. SS just seems better suited to deal with the temperature and harsh chemicals in there.

My fairly new Miele machine is very much better insulated than its much cheaper predecessor. The outside barely gets warm to the touch, whereas the old one got quite hot. Nothing to do with the liner though. When I was doing the research, I found a wide variation in price. I decided that the main difference would be in the quality of the motors and component parts as well as things like door hinges. What swung it for me finally was a separate tray for cutlery instead of those baskets that I never really liked.

“Holds in heat” may be referring to the heat capacity of the steel, rather than its heat conductivity. The stainless steel lining is probably a lot denser than the plastic lining, and once it gets heated up it stays heated up more easily than plastic.

I’ve never seen that as a selling point of a dishwasher, but I know for ovens the heat capacity of the walls of the oven contribute quite a bit to the stability of the temperatures within.

To the specific statement, " holds in heat better ", likely not. Metal conducts heat better than plastic. The insulation behind the walls will be exposed to the conducted heat quicker and at a higher rate of conduction. If all things aside from the liner material are equal, then the salesman is incorrect.
Consider that metal hot water pipe loses heat faster than plastic. Metal solar heat collectors are more efficient than plastic at transferring heat to the liquid medium inside. Heat radiators are metal.

Sounds dumb to me – and I’m a physicist who specializes in heat transfer and temperature in industry.

The lining of the dishwasher contributes a very small fraction of the total thermal resistance between the inside and outside of the machine. It’s thin, and pretty conductive. Practically all the thermal resistance is coming from the insulation and any air gaps.

On a volume basis, pretty nearly everything solid has a heat capacity of around two million joules per cubic meter kelvin. Some things are a bit higher, some a bit lower, but the inner lining isn’t all that massive, and the machine’s loaded with dishes and water too. I bet plastic is thicker than metal in this application. So that’s, well… a wash.

It doesn’t matter what the emissivity of this lining is, or at least not much. Looking inward, the surface stays wet, and water has a very high emissivity – it might as well be black paint sloshing around in there, to use a visible light metaphor. Looking outward, there are a great many reradiating surfaces implied by any path through the insulation, so this is but one of many.

I think the salesmen in question suffer from some combination of over enthusiasm, genuine ignorance, and perhaps even cynical profiteering.

Why would you want to retain heat inside a dishwasher?

The hot water has already sprayed onto the dishes and done its work of removing dirt/food particles.
Any heat escaping from the dishwasher into the house just reduces the work the furnace has to do, vs. just being wasted when the hot water goes down the drain.

The issue here is drying! Many dishwashers are lousy at drying dishes, and if the inside is stainless steel, then I believe it is just going to make the existing situation worse…allowing the heat to escape through the walls via the high conduction of a metal.

So, from your opening paragraph (shown here before the ellipsis), it sounds like either liner (plastic or metal) is too thin to really matter by itself. What matters is how things are insulated behind the wall, including any air gaps. We can only hope the dishwasher mfgs know their heat transfer, but I have little confidence. Every model seems like a crap shoot - and the few that rely on indirect heating (i.e., heat source hidden beneath the dishwasher tub) is even less likely to be effective!

The point about insulating a dishwasher is to maintain the wash temperature with minimal input of energy. If you are trying to maintain a 55C wash in an ambient 20C the heater will be on more with poor insulation. A good machine will come with a sensor-assisted wash cycle to adjust the wash duration according to how dirty the dishes are. These days, we all want to use minimal amounts of energy and water.

At the end of the wash, the temperature is boosted to 80C to aid drying. High end washers will open the door and blow the dishes dry with a fan.

See, that’s the thing. There’s a market force driving manufacturers toward less energy consumption. They may do badly at understanding how it all works, at least at first, but as long as there are objective tests happening to document their success, the manufacturers will eventually find their way there.

Salesmen may misrepresent things, knowingly or not. I think that’s part of the reason there are formal testing standards and rules about putting the results on the product. You can trust those test results for comparison purposes way better than you can trust advice from the salesmen.

Nice save. I’ve never seen it. I can tell it’s purpose, but I have no idea why that adjective. Any help?

It is from a story by Niven and Pournelle “The Mote in God’s Eye”. The aliens had three arms - two small and one big (the gripping hand)



And there’s a separate market force that wants the cheapest possible dishwashers. Plenty of people have only experience with cheapness-first designs. I’m going out on a limb here to put our esteemed **Jinx **in that camp.

Nice dishwashers in good working order leave dishes absolutely dry. Some do it more efficiently than others, but they all do it.

What is this “furnace” of which you speak? Flushing waste heat outside the residence as efficiently and thoroughly as possible is a year-round effort for some of us. :smiley:

Not all metals conduct heat well. Stainless steel is one of the worst; depending on the type of stainless steel, its thermal conductivity can be 1/10 that of aluminum, and 1/20 of copper. Though it’s still a better conductor than most types of plastic.

In this case, I suspect any inner lining is very thin, and has negligible effect on the overall thermal insulation of the dishwasher.

What kind of plastics are comparable to stainless? Not many are even 1/10 as conductive as the common stainlesses such as 304 at about 16 W/(m K). Are you thinking of heavily filled plastics with, say, boron nitride or aluminum flake or something?

Did they build dishwashers with plastic tubs? Every one I have ever worked on was enameled steel.

And Stainless would insulate quite a bit better than mild steel, and not require a failure prone enamel finish.

I can think of many reasons to avoid a plastic tubbed dishwasher, but heat conductivity doesn’t figure into it.

Just to throw a wrench into things, if I were designing a dishwasher door where the priority was to minimize heat loss through the door, I would use a vacuum insulated door. To maintain its form factor (ie. not super thick), I would use stainless steel or another suitable metal.

However, I doubt that the dishwasher manufacturers go to these lengths, so this likely doesn’t apply to your original question.