Microwave-powered home boiler

molerat

In industry time is money so anything that can dry more clothes in the same amount of time using possibly less machinery and manpower is a winner.

coffeehound

Billy_B_North said:

Since the first analogy didn't work . . how about you have a bowl of cold water.  You can heat it up using a 1400 watt microwave oven (700 watts of mw power) or a 1400 watt fan heater.  Which will heat up the water quickest?

In a closed system, neither, and the tumble drier won’t be venting warm air wile it’s getting up to temperature.

You need to remember too that the aim is to dry the clothes, not warm them, if they are losing heat to evaporation then that’s not being inefficient.

I'm sure a mw could heat the mass up more quickly than warm air, though I concede your point that in a closed system, they will eventually end up the same.  As to drying, I can't prove it, but gut instinct says that heating by mw to say 40 or 50 C first and then starting the hot air drying would be more economic overall due to lost air / heat and inefficiency in the condensing system.  Again, heatpump perhaps more efficient overall, but *very* slow.  

Water does evaporate a lot more quickly once it's warm.  You can tell with TDs:  laundry feels wet and heavy until it has reached a certain temperature, and then the drying happens pretty quickly.  Maybe some of the heat produced by generating mw's could be recovered too.  Anyway, you're probably more right than me on the physics.

nottsphil

unforeseen said:

Reed_Richards said:

glennevis said:
I would agree with the poor efficiency. I stuck my 700W microwave oven on a smart power socket with energy monitoring and discovered it uses 1400W.

So where is the other 700 W going?  The energy has to be used in either heating something (thermal energy) or turning the turntable (mechanical energy).

Basing it on a magnetron's average efficiency of 65% then that alone is using just over 1kW to produce 700W of usable energy. Losses are generated in the EHT generation circuitry and the  magnetron itself.
300-400W for fan, turntable, light and other electronics sounds about right.  

You seem to have calculated a sum to make it match a flawed figure. The fact is that the 700W microwave is only using 1400 Watts for a fraction of the time.  You can tell exactly when this is because the noise increases significantly for those periods. 

unforeseen

I don't know about your microwave but my 700W one is continuous at full power. On reduced power then I agree that it only operates for part of the time.