Phase Changes – Like Changing Your Clothes, Only Cooler… or Maybe Hotter.

We were all first introduced to different phases of matter in elementary school science classes where we were asked to identify the examples of solids, liquids, and gases. I’m sure that I am not the only one whose teacher used an empty mason jar to exemplify “gases.” That elementary-level lesson really ends up devolving into a focus on water. It’s a natural progression, being as water has so many useful properties, but if the focus is only on water, we end up missing so many interesting details about other materials. There are other ways that we experience phase changes without giving it much thought.

A lighted candle is an example of demonstrating all three phases in one place. The bulk of the candle wax stays in a solid phase, or close enough to it; the wax near a lit wick is liquid, and it evaporates into a gas before being consumed in the combustion process. Candles made from paraffin wax will have a melting point of ~120 degrees Fahrenheit (°F) and a boiling point (transition from liquid to gas) at ~700°F. The range of temperature that paraffin exists in liquid phase is much broader than water, but all three phases are still observable at the same moment. 

The phase of any chemical depends upon its temperature and the pressure acting on it. Every chemical will have a freezing/melting temperature and a boiling/condensation temperature for a given pressure. Below is a list of commonly found chemicals and their comparative phase change temperatures at sea level. This table helps to highlight that water in the liquid phase only exists in a very narrow temperature range in comparison to other common-use liquids. Other common fluids that we use can stay liquid at temperatures much lower and/or much higher than water can. It is important to note here that “fluids,” in an engineering sense, encompasses both liquids and gases.

What this table doesn’t illustrate is how variations in ambient pressure will influence these numbers. Anyone who has tried to cook dinner while vacationing at a ski resort has experienced the effects of pressure on the phase change temperatures of liquids. Water serves as the best usable example here. If you are cooking at an elevation of 11,000 feet in Cusco, Peru, for example, you are dealing with water that boils at 191°F. Cooking rice will take quite a bit longer there because the water temperature will never rise above 191°F. This is all because the increased elevation results in lower ambient pressure.

The energy required to raise the temperature of a liquid to its boiling point is relatively low and happens at a constant rate of energy consumption. For every unit of heat energy absorbed by a fluid, a consistent increase in the temperature of the fluid is observable.  However, once the temperature of a liquid is increased to its boiling point, it can go no further. Energy can still be absorbed by the liquid, but that energy will be used to convert the liquid into a gas rather than to increase the temperature of the liquid. In the case of water, raising the temperature from 34°F to 212° takes just one-fifth of the heat input (i.e., energy) that is required to take water already at 212°F and evaporate it into water vapor (steam). Likewise, upon cooling, that same quantity of energy must be removed from the water vapor to condense it back into liquid phase water.

Even though liquids can be used as a very effective means to keep equipment cool, you have the potential to unlock much more cooling capacity if you plan for the liquid to change phases into gas form, i.e., evaporate. It’s the phase change, not the temperature changes, that will allow for large amounts of heat transfer. Various liquids require different amounts of energy to convert them from a liquid to a gas, then back to a liquid.  The amount of energy that it takes to make the phase change is specific to the liquid; it is a special property called the enthalpy of vaporization (aka latent heat of vaporization, or heat of evaporation). This property of liquids is the key factor in making our beloved HVAC systems operate and it refers to the amount of energy that must be put into a fluid to complete the transformation from liquid to gas phase.

If you want to take some of the heat out of the air in your house, the concept of phase change is employed (using a refrigerant in this case). HVAC systems work by drawing in air from inside the house through the return air ducts. That air is then blown past, or through, something that is much cooler, such as a heat exchange coil. Heat is exchanged between the air and the much cooler coil and so the temperature of the air decreases. The chilled air is now blown back into the house through the air registers dispersed throughout the house. All that this process requires is a cold object to pass the air over/through. To accomplish this, we utilize refrigerant fluids that boil at very low temperatures and use the properties of phase change to maximize the amount of heat/energy that the fluid will absorb from the air in the home. Making use of phase changes from a liquid into a gas and eventually back to a liquid is the cornerstone of HVAC systems.

Phase changes are also used for cooling purposes in the chemical industry. High-temperature gases from chemical processes are effectively cooled by a device known as a waste heat boiler. Inside the waste heat boiler, water is converted to steam by absorbing heat from the incoming hot gas.  

An understanding of the concept of phase changes and the role they play in heat transfer is often relied upon by EDT engineers to assess damage and determine the root cause of problematic HVAC systems/equipment.

About the Author

Lucas Brown, P.E. is a Consulting Engineer in our Birmingham Office. Mr. Brown provides consultation related to mechanical systems and machinery, mechanical design, failure analysis, damage assessment, interpretation of codes and standards, HVAC systems, plumbing, and evaluation of fire and explosion origin and cause. You may contact Lucas for your forensic engineering needs at lbrown@edtengineers.com or (205)-838-1040.

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