Heat Transfer
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| Illustration 1 | g01075578 |
Many know what air conditioning does, but very few understand how air conditioning works. An air conditioner evaporator works similarly to a pot of boiling water on a stove. The reason why an air conditioner can continue to cool the air is because a liquid called the refrigerant is boiling within the evaporator coil. A boiling pot is hot and an air conditioner is cold. A cold substance that boils is usually quite confusing.
Cold is a definite condition. The condition regarded as cold does not exist. Cold can only be defined in a negative way by saying cold is the absence of heat. When heat is removed from a substance, the substance becomes cold as a result. The pot of boiling water and the air conditioner are devices for removing heat.
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| Illustration 2 | g01075579 |
The basis of all air conditioning systems is, that heat flows from a warmer object to a cooler object. All substances contain some heat. Theoretically, the lowest temperature that can be obtained is 459° below 0°F. No one has reached that temperature. Anything warmer than 459° below 0°F contains heat. When an object is made cold, the heat in the object that is being made cold is transferred to another object. Heat always flows from a warm object to a colder object.
There are three ways in which heat is transferred:
- Conduction is heat that travels through a solid object. (Illustration 1)
- Convection is heat that travels through a substance such as water, steam or air. (Illustration 2)
- Radiation is when the increase in the temperature of a substance allows a measurable amount of heat to escape. (Illustration 3)
Measurement of Heat
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| Illustration 3 | g01075580 |
Heat is measured by intensity and by quantity. Place a pot of water over a flame on a stove. The water gets hotter and hotter until the water boils. A thermometer in the water shows the temperature. The thermometer shows the intensity of heat. The thermometer does not show the quantity of heat that is present.
The unit that is used for measuring the quantity of heat is called a British Thermal Unit (BTU). One BTU is specified as that amount of heat that is necessary to raise 1 pound of water 1°F (473.6 mL of water .55°C).
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| Illustration 4 | g01075582 |
The quantity of heat can best be explained by thinking of heat as drops of red coloring dye. Each drop of dye corresponds to 1 BTU. If one drop of red dye is added to a cup of water, the water will turn slightly pink. Two drops will turn the water reddish in color. Adding more drops will turn the water to deeper shades of red.
Adding more BTUs to the water increases the temperature.
Sensible Heat
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| Illustration 5 | g01075583 |
Two types of heat also exist: sensible heat and latent heat.
Heat that is measured with a thermometer is called sensible heat. Sensible heat can be felt. Another explanation for sensible heat is the amount of heat needed to raise 1 pound of water from 0 °C (32 °F) to 100 °C (212 °F).
Latent Heat
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| Illustration 6 | g01075584 |
The second type of heat is called latent heat. Latent heat is hidden heat. (Latent is the Latin word for hidden.) Latent heat cannot be felt. Latent heat cannot be measured with a thermometer.
Latent heat can best be explained by inserting a thermometer into a block of ice. The thermometer reads 0 °C (32 °F). Allow the block of ice to melt. Collect the melting water in a container. When the block of ice is checked a few hours later, the block of ice is smaller because some of the ice has melted away. However, the thermometer reads 0 °C (32 °F). Where did the heat go that caused the ice to melt? The added heat was in the water that melted from the ice. However, checking the water temperature as the water melts from the ice shows the water temperature to be only slightly higher than the temperature of the ice.
The slight increase in the water temperature does not account for all of the heat that the ice has absorbed. The only answer left is, that the latent heat has been used up to change the ice from a solid to a liquid.
All solids soak up large amounts of heat when solids change from a solid to a liquid.
Latent Heat of Fusion and Latent Heat of Vaporization
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| Illustration 7 | g01075587 |
Water changes into ice or ice changes into water at 0 °C (32 °F) sensible heat. The process of changing ice into water or water into ice is called latent heat of fusion. 144 BTUs of latent heat is added to change 1 pound of ice into 1 pound of water. Therefore, the ice must absorb 144 BTUs of latent heat. To change 1 pound of water into 1 pound of ice, 144 BTUs of latent heat is removed from the water.
Water changes into steam or steam changes into water at 100 °C (212 °F). The process of changing water into steam or steam into water is called latent heat of vaporization. 970 BTUs of latent heat is added to change 1 pound of water into steam. Therefore, 970 BTUs of latent heat is absorbed into 1 pound of water before all of the water is turned into steam.
Just as all solids soak up large amounts of heat when solids change to a liquid, liquids soak up large amounts of heat when liquids change to a gas.
Put some water in a pot. Place a mercury thermometer in the water. Place the pot over a flame. As the water heats, the thermometer reading will rise. At atmospheric pressure, the water boils when the thermometer reaches 100 °C (212 °F) sensible heat. The water will boil faster when the flame is increased.
However, the thermometer reading will not increase above 100 °C (212 °F). What happens to the additional heat from the increased flame? The additional heat is used to change the water from a liquid to a gas. Since the temperature of the boiling water does not increase above 100 °C (212 °F), the boiling must be a natural means for the water to cool itself.
Effects of Pressure
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| Illustration 8 | g01075588 |
At atmospheric pressure, water boils at 100 °C (212 °F). What is atmospheric pressure?
Atmospheric pressure can be defined as the weight of the atmosphere upon an object. Pressure is measured in pounds per square inch (psi). At sea level, atmospheric pressure is 14.7 psi. Any pressure that is less than sea level (14.7 psi), is known as a partial vacuum or commonly called a vacuum. Vacuum is measured in inches of mercury (in hg). A perfect vacuum (0 psi) has never been produced. No one has been able to mechanically obtain ZERO pressure.
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| Illustration 9 | g01075590 |
There is a direct relationship between a liquid's boiling point and the pressure on the liquid's surface.
Illustration 99 shows three pots of boiling water. The pot on the left has a pressure of 14.7 psi and the water boils at 100 °C (212 °F). Increasing the pressure inside the pot causes the water to boil at a higher temperature. Decreasing the pressure inside the pot causes the water to boil at a lower temperature. The pressure can be decreased to a point where the water boils without the flame.
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| Illustration 10 | g01075591 |
There is a direct relationship between the temperature of a vapor and the amount of pressure on the vapor.
When the pressure on the vapor is increased, the temperature of the vapor also increases.
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| Illustration 11 | g01075592 |
There is a direct relationship between a vacuum, the ambient temperature and the boiling point of a liquid.
Shown is a manifold gauge set connected to a vacuum pump and a flask with water. The vacuum pump lowers the pressure in the flask thus creating a vacuum. At a room temperature of 21.1 °C (70 °F), water boils with a vacuum of 28.2 in Hg. (.7 psi).
Boiling water is a natural cooling process. The boiling water removes the same amount of latent heat at 21.1 °C (70 °F) as boiling water at 100 °C (212 °F).
Substances other than water react in the same manner but at different temperatures.