An Important Physical RelationshipPhase Change & Heat We are going to delve more into the world of pure physics. To make things flow as smoothly as possible, we will start with some important definitions. Heat- The transfer of energy from one object to another as a result of a difference in temperature between the two. Heat is not a quality that an isolated object can possess; it is a flow of energy between two objects. Specific Heat-The amount of heat required to raise the temperature of a given mass of a given substance by a given amount. Specific heat is a constant property of a substance, and every substance has a unique specific heat. It can be thought of heat capacitance; a high value of specific heat implies a high heat capacity, and a low specific heat a low heat capacity. Think of this: In the spring we often observe lakes that remain frozen long after the ambient air temperature has reached comfortable “tee shirt” temperatures. Ice has a high specific heat, where air has a low specific heat. Therefore, it takes a relatively small amount of heat to warm the air, where it takes an enormous amount of heat to raise the temperature of an equal mass of ice. Phase, Phase Change-There are three distinct phases of matter; liquid, solid, and gas. A phase change is simply the passage from one phase to another. Try this thought experiment: Imagine placing an ice cube on the top of a hot stove. It will melt, and this is a phase change (solid-to-liquid). The melted cube will then boil and turn to steam. This is another phase change (liquid-to-gas). If the steam contacts a cold surface such as a window, it will condense to water which is evident by “fog” on the windows. This also represents a phase change (gas-to-liquid). Latent Heat- Phase changes occur at critical temperatures. For example H2O (commonly known as water) freezes at a temperature of 0 degrees Celsius, and boils at 100 degrees Celsius. When a substance reaches the critical temperature for phase change, it reaches a “limbo stage.” An extra quantity of heat flow is required to fulfill the change and end this period of limbo. This heat quantity is known as the latent heat. Vapor Pressure- All liquids possess an internal pressure that is dependent on temperature. When this internal pressure reaches a critical value, known as the vapor pressure, it boils. When a liquid boils it turns to steam, thus initiating a phase change. CHART temperature-plotted on the vertical axis versus heat energy-on the horizontal axis Consider this process: To start, the working fluid is in the reservoir at the bottom of the thermosyphon. It is substantially colder than the soil around it. Heat flow occurs from the soil to the fluid, the fluid temperature and thus the internal pressure of the fluid rises. Eventually the internal fluid pressure approaches the vapor pressure and the fluid is ready to boil. We are now at point 1. Before the fluid can achieve the phase change to a gas, it must absorb the critical amount of latent heat. Therefore, heat continues to flow into the fluid without a rise in temperature. When the proper amount of heat has been transferred into the fluid the vapor pressure is acheived and it boils and turns to a gaseous state. We are now at point 2. The gas rises to the top of the thermosyphon. The gas is substantially hotter than the surrounding air. Heat flows from the gas to the atmosphere. The amount of heat required for a phase change is eventualy removed from the gas, and it reverts back to a liquid (all of this occurs at the same temperature required to boil the liquid). Before the gas can fulfill the phase change back to a liquid it must surrender the latent heat to the surrounding atmosphere. Once this occurs the gas condenses to a liquid. We are now at point 3. The condensate liquid is cooled further as it drips back down to the reservoir. The cycle starts again. Observe that the same amount of heat required to initiate the liquid-to-gas phase change is liberated into the atmosphere when the gas-to-liquid phase change occurs. This demonstrates a distinct advantage of the two phase versus the single phase thermosyphon. The phase change process is substantially more efficient than a single phase heat removal process. Hopefully this made sense. If it didn’t, the next page will help clear things up. Proceed to “What does this all mean?” Random notes on the two phase thermosyphon.