1. Regular Guest
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Jun 2018
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Originally Posted by hvacker
Just adding as far as checking the formula, ASHRAE has a correction sheet in their books for corrections after publication. Not sayin the Gods have failed, probably the publisher.

Hi hvacker,
Have you noticed this correction in Ashrae handbook?

2. Professional Member
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Jul 2014
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Newton's Law of Cooling says that the rate of thermal energy is proportional to the temp difference - that's seems right - like what we'd expect.

With that Energy equation (not rate), it's got the product of HDD (Days x Temp) times the inverse of Temp (1/Temp) - so it just the # of days that relates to the energy consumption - that's seem right. Yeah, using an equation(s) and not getting 'fooled' by it can be a challenge.

In regards to fluid dynamics, I found it hard to grasp the math behind the pump laws - that the pressure goes up 4 times if you
double the flow rate. Sure, you see the equation. So you got to go to the basic physics - that Force is proportional to the change
in momentum (mass x velocity). If we double the flow rate we intuitively see that the flow rate should double. The tricky part is
to realize that by doubling the flow rate we've also double the mass rate (eg, more water molecules per time). So the change
in momentum goes up four times. Change is momentum relates to Force which relates to Pressure (force per area).

This is exactly related to the question: "If a 30mph wind is hitting the side of a barn with a force X, how much force at 60mph?"
The force on the barn is proportional to how much momentum hits it per second (or minute, hr). So force (pressure) up 4X. Understanding the effects between momentum and energy, in my opinion, is the 'magic key' to understanding fluid mechanics.

After struggling with all that, I came to one definition of heat: Heat is Energy without momentum.

Time for a 'cold one'...

3. Originally Posted by Malmo
Hi hvacker,
Have you noticed this correction in Ashrae handbook?

I Haven't seen this particular formula corrected but have seen correction sheets in one of mine but they are probably older than yours.