Author Topic: A very silly question about heating water  (Read 6921 times)

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Online IconicPCB

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Re: A very silly question about heating water
« Reply #25 on: May 18, 2015, 03:33:00 am »
it takes as much energy to heat as it does to cool ( on the understanding of direction of flow of energy ) a bunch of water.

It is the phase changes which tend to be energy hogs.
 

Offline Rick Law

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Re: A very silly question about heating water
« Reply #26 on: May 18, 2015, 04:32:08 am »
it takes as much energy to heat as it does to cool ( on the understanding of direction of flow of energy ) a bunch of water.

It is the phase changes which tend to be energy hogs.

Doubt he wanted to turn it from ice to water or from water to steam.

As to the amount of energy, it really doesn't matter in a sense.  If he is considering waste heat recovery, x BTU from waste heat is x BTU he doesn't have to buy regardless of how small X is.  That is money he doesn't have to spend - as long as ROI (Return On Investment) is positive.

I think pre-heating the water with a heat-exchange with the used-water is probably easiest and biggest bang for the buck.  Even with heat-exchange, getting a positive ROI is probably going to be difficult.
 

Offline mtdoc

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Re: A very silly question about heating water
« Reply #27 on: May 18, 2015, 04:53:07 am »
Cogeneration is a mature technology that captures waste heat from an internal combustion engine or boiler that is generating electricity. I believe Honda even makes a home sized unit.  But I've never heard of using waste heat from an electric motor. Seems doubtful that it would be worth the effort - they're too efficient.
 

Online IconicPCB

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Re: A very silly question about heating water
« Reply #28 on: May 18, 2015, 05:58:59 am »
The larger the motor.. the more efficient it is.

Many a small motor would be better for heating the water.
 

Online Mechatrommer

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Re: A very silly question about heating water
« Reply #29 on: May 18, 2015, 06:06:47 am »
you havent used a motor for an air compressor, it got fin installed and if it runs for 10 minutes you are lucky if you can survive touching it for 5 seconds.
A regular induction motor? Did some logging on my old compressor, it never came above 40C at full load. Haven't tested on my "new" (used) one, but i think it will be the same.
i dont know induction or what. whats certain its AC mains motor connected right to the compressor in a single body, not with that rubber band type. to be precise, i'm not sure if the motor get hot or the compressor, maybe both, whats certain both single piston compressor and the motor wil get very very hot in 10-20 minutes of operation, havent timed that precisely.

btw, if the heated water is for drinking purpose, piping, coupling and sealing will be something. but if not for drinking, it will still be something, but much easier than drinking purpose i guess.
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline eas

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Re: A very silly question about heating water
« Reply #30 on: May 22, 2015, 07:08:04 pm »
In the US, or my part of it, at least, automatic car washes have become continuous engineering projects, with ongoing optimization of capital expenses and operating expenses. It seems like they are constantly tweaking them. I'm sure they reuse water multiple times, and in so doing, reuse the heat. If significant waste water goes to the sewer, they must use counter-current heat-exchangers to recover the heat for new supply.

Recovering waste heat from the electric motors may make sense at some point, but I'm sure there are many optimizations with better ROI.
 

Offline eas

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Re: A very silly question about heating water
« Reply #31 on: May 22, 2015, 08:16:10 pm »
Nothing inconsistent here.  Compressing air is less efficient than running an electric motor.  Don't know the number but suspect it is not that far above 50%.   Using 50% and 90% as bounds on the number says that a 1 HP compressor (~750 W) will be dumping between 75W and 325 W as heat.  Don't know anyone here that would want to grab either an incandescent light bulb or a soldering iron of those wattages.  But as the prior poster says, it won't heat much water.

I'm pretty sure that most of the heat produced by an air compressor is due to the thermodynamics of compressing a gas, rather than mechanical inefficiency due to friction. My "hybrid" hot water heater, which uses and air-source heat pump meets our hot water needs by 1/4-13rd the electricity of our old resistive electric HWH. Probably still not enough for a car wash, but nothing to sneeze at, and even more useful if you can put the cold air to good use too.
 

Online CatalinaWOW

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Re: A very silly question about heating water
« Reply #32 on: May 22, 2015, 09:04:32 pm »
Nothing inconsistent here.  Compressing air is less efficient than running an electric motor.  Don't know the number but suspect it is not that far above 50%.   Using 50% and 90% as bounds on the number says that a 1 HP compressor (~750 W) will be dumping between 75W and 325 W as heat.  Don't know anyone here that would want to grab either an incandescent light bulb or a soldering iron of those wattages.  But as the prior poster says, it won't heat much water.

I'm pretty sure that most of the heat produced by an air compressor is due to the thermodynamics of compressing a gas, rather than mechanical inefficiency due to friction. My "hybrid" hot water heater, which uses and air-source heat pump meets our hot water needs by 1/4-13rd the electricity of our old resistive electric HWH. Probably still not enough for a car wash, but nothing to sneeze at, and even more useful if you can put the cold air to good use too.

I agree, it is gas thermodynamics.  But that is where the heat comes from when you burn yourself on a compressor, whether it is your bicycle pump or the big one that drives the jackhammers.  And it is part of the overall efficiency of a compressor.   It is also one of the fundamental flaws of the compressed air cars that you see from time to time touted as a solution to the transportation problem du jour.  That energy that went out as heat isn't recoverable for propulsion.
 


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