Water Heat Transfer

[psoundappan]

I am looking for some information regarding how the water hardness affect it's heat transfer rate. We are using water to quench the heat treated parts and I wanted to know how the hardness will affect the rate of cooling(parts).

 

[Montemayor]

psoundappan:

Water hardness, per se, has no effect on the rate of heat transfer when the water is used as a coolant. The damned precipitates[/] it causes (and resultant corrosion & scaling) is what renders the application as one with a limited life if the water outlet temperature is allowed to get above approximately 105 - 120 oF. Therefore, this pretty well limits "hard" water as a coolant. The hardness of the water is due to its dissolved solids - and the eventual precipitation of these when the water is heated creates a nullification of the effective cooling surface.

If the coating of your parts and the reduced cooling rate is OK with you, then use hard water to quench. Otherwise, be prepared for downstream consequences.

Art Montemayor
Spring, TX

 

[athomas236]

psoundappan,

I am no great expert on these things but as far as I know when the results of actual operating experience are compared with theoretical calculations there is usually some correction factor.

To be honest few folks know what is covered by this. All that is known is that using suc corrections does not cause any problems with actual performance being different to predicted.

athomas236

 

[25362]

You didn't specify the metals parts being quenched.

Anyway, depending on the temperature of the quenching water, its hardness and saltiness, adhering bubbles (water and dissolved air), released on boiling, and deposits with their intrinsic undesirable properties concerning heat transfer and corrosivity, may be minimized.

One procedure used for water quenching I know of, is to keep either a strong circulation in the tank or by fast moving of the metal part to be quenched, providing good agitation. Daily water replacements and rapid rinsing of the quenched metal surface are recommended.

 

[25362]

A bit more of information:

The thermal conductivity (T.C.) of hardness salts (mainly CaCO3) varies with porosity between 0.3 and 2.6 W/(m.K), approximately, on the average, as a firebrick does.

Water dissolves air 0.6% by volume even at 80 deg C.
To give you an idea, and for the sake of comparison, the T.C. of an air/steam bubble would be about only 0.025 W/(m.K), that for clean water about 0.6 W/(m.K), and that of iron ~ 75 W/(m.K).

Off-hand, it appears that air/steam bubbles would be a larger heat transfer resistance than hardness deposits.

For rapid quenching as that attained by immersion in water, I've once read an article recommending the use of salty water to keep surfaces relatively free of adhering bubbles and precipitated salts. Since salty water is very corrosive a thorough rinse of the quenched surfaces is highly recommended.