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21j
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Join Date: Jun 2010
Posts: 9

Water Heating

02/11/2011 10:12 AM

Hi,

Just wondering if anyone can help with an equation.

I will have a know quantity in a water tank that I will be heating via a electrical element.

Can anybody give me an equation for creating a graph of energy input against time to heat to specified temperature. Also what inputs do I need for the equation.

I had 2 No. equations I found Q=mCpDT and Q=mCpdT/dt

I understand that DT in the first equation is the difference in temperature i.e from 10 degrees C to 50 degrees C DT would be 40. but what about dT/dt in the second equation. This I assume is for the change in heat transfer as the water heats (DT would be getting smaller).

Q= Energy Watts

m=

Cp=

Any help appreciated.
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PWSlack
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#1

Re: Water Heating

02/11/2011 10:28 AM

The first equation should read

E=mCpΔT

where E is the energy needed to achieve a temperature difference of ΔT. This equation is the one needed to drive the graph, and will work up to a point close to the boiling temperature provided the vessel is well-insulated and any vapour well-contained.

Cp, or rather Cp, is the thermal capacity. It varies in the third significant digit for water between 0 and 100degC, and vor highest accuracy it is best to refer to published Steam Tables.

Power is measured in Watts (among other units). Power is a rate of delivery

of energy.

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21j
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Join Date: Jun 2010
Posts: 9
#2
In reply to #1

Re: Water Heating

02/11/2011 10:33 AM

Thanks. The vessel is well insulated and so I cant neglect heat transfer to the surroundings. So E I'm looking for Delta T I have. mCp is ?
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PWSlack
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#3
In reply to #2

Re: Water Heating

02/11/2011 11:07 AM

The mass of whatever is being heated multiplied by its thermal capacity, as stated above.

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dhayanandhan
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Engineering Fields - Mechanical Engineering - New Member

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Location: CHENNAI, TAMIL NADU, INDIA.
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#4

Re: Water Heating

02/12/2011 11:36 AM

Dear 21j

The dT/dt refers to the RATE OF INCREASE OF TEMPERATURE of THE HEATED MEDIUM for the given INPUT ENERGY.

DHAYANANDHAN.S, INDIA
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21j
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Join Date: Jun 2010
Posts: 9
#5
In reply to #4

Re: Water Heating

02/14/2011 4:55 AM

Hi Dhayanandhan,

Thanks for the reply. What inputs do I use for dT and dt?

Thanks
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