fundamentals of heat load calculations part 1. what causes heat loss 23.8.4 temperature difference...
TRANSCRIPT
FUNDAMENTALS OF HEAT LOAD
CALCULATIONS
Part 1
What Causes Heat Loss
23.8.4 Temperature difference
Outside Inside
Overview Of Heat Loss
23.8.4
1. What is heat loss?2. What causes heat loss?3. How is heat loss measured?4. What affects the rate of heat loss?
What Is Heat Loss?
23.8.4
Heat Loss:
1. Transmission losses
2. Infiltration losses
What Is Heat Loss
23.8.5
Transmission Losses
1. Heat transferred through confining walls, glass, ceiling, floor or other surfaces.
- ASHRAE Terminology of HVAC&R
What Is Heat Loss?
23.8.5
Infiltration Losses
2. Energy required to warm outdoor air leaking in through cracks and crevices around doors and windows, or through open doors and windows, and through porous building materials.
- ASHRAE Terminology of HVAC&R
The Human Heat Source
23.8.5
Why do you wear a coat, scarf, gloves, long pants in winter and shorts and sandals in
summer?
The Human Heat Source
23.8.5
How much heat?
Where does it go?
The human body creates heat.
23.8.5How much heat does the human body generate
It Depends.
What Activity?
The activity engaged changed the “comfort level” in the conditioned environment. For example a basketball player in a gym vs. a researcher at a library.
The Human Heat Source
The Human Heat Source
23.8.5
An office worker is estimated to give off 500 BTU/hr.
The Human Heat Source
23.8.5 The human body at rest roughly produces the same heat as a 100-watt light bulb.
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Winter In The North
23.8.5
No protection.
Any heat your body generates is transferred immediately to the surrounding COLD.
IT’S SO COLD!!!
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Winter In The North
23.8.5
0
Ok. It’s getting a bit
warmer.
Building a little box around yourself
provides some insulation from the outside.
The heat from your
body slowly warms the air in your space.
EXCEPT…
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Winter In The North
23.8.5
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DRAT!*%$#@#$
Since Nature hates imbalance, and since you don’t have enough insulation to sufficiently slow down the heat transfer, the heat leaves faster than your body’s ability to generate it.
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Winter In The North
23.8.5
10
There! That Should Work.
You add a heat source to your space.
The temperature starts to go up.
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Winter In The North
23.8.5
70
I just can’t afford this much
energy.
The cost of a bigger fire starts to hit the pocketbook.
What can we do?
Out-dated construction methods were based on cheap fuel.With energy costs going up, this method isn’t the smartest design.
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Winter In The North
23.8.5
70
Awww. The thicker wall should slow Nature down!
Using an appropriate combination of insulating materials and heat sources, we can finally achieve the desired temperature.
Winter In The North
23.8.5The point of maximizing the R-value of the shell is to slow down Nature’s tendency to equalize imbalance. If you can retain the heat, then you can reach your set-point without having to have an unnecessarily large heat source.
The more insulation, the slower the heat loss and the smaller the heat source required at design temperatures.
When IN = OUT the temperature remains constant.
Nature Hates Temperature Imbalance
23.8.5
Example #1: Set hot cup of coffee
on a desk in a comfortable room. Come back in few hours. Is it still as hot as you left? Why not?
What happened to the “hot”?
23.8.5
Nature Hates Temperature Imbalance
Example #2: Set a glass of water
with lots of ice on a desk in a comfortable room. Come back in few hours. Is it still as cold as you left?
Why not?
What happened to the “cold”?
23.8.5
Nature Hates Temperature Imbalance
As the temperature of the contents gets closer to room temperature, the rate of heat transfer slows down.
Why? The rate of transfer from hot to cold is relative to the temperature difference. As the two temperatures get closer to the same, the difference (∆T) is less, therefore the rate slows.
4 Things To Remember
23.8.6
1. Hot goes to cold!!!!!!!!2. Hot goes to cold at a rate relative to the
temperature difference. 3. Radiant heat goes an infinite number of
directions from the source.4. Heat is lazy. It will follow the line of least
resistance.