HVAC School - For Techs, By Techs

In this episode of HVAC School, Bryan talks to some apprentices about basic thermodynamics. That is the fancy scientific way of saying that we're moving heat. The way we think of "hot" and "cold" is relative to our comfort. However, the scientific concepts of "hot" and "cold" are very different from our relative understandings of those qualities. For instance, there is only ONE value of "cold" in the universe: absolute zero (0 kelvins, -460°F). Any temperature above that contains heat. Heat and temperature are NOT synonymous. Instead, heat refers to molecular motion, and temperature is an average measurement of molecular motion. Therefore, not all heat results in a temperature change. For example, adding heat to an ice cube at 32°F (0°C) changes the ice cube from a solid to liquid water. The heat added is called latent heat. Heat cannot move unless there is a differential in temperature, and it always moves from an object with more heat to one with less heat. Everything in nature tends towards equilibrium, and heat is no exception. In those cases, heat transfer will theoretically occur until both objects are at the same temperature. There are three main methods of moving heat: conduction, convection, and radiation. Conduction moves heat when a warmer object touches a cooler one. Convection occurs when heat moves through a fluid. Radiation occurs when heat moves on electromagnetic waves, such as when the sun's heat passes through a window. Join us as we cover: Heat & temperature and the difference Boiling and superheat Fahrenheit, Celsius, and Kelvin scales Absolute zero Molecular motion Hot and cold British Thermal Units (BTUs) Tons of air conditioning (and BTU/ton) Energy conversions Pressure and its effect on temperature Conduction, convection, radiation How heat transfer works in HVAC/R systems   If you want to learn more about heat transfer, check out this article. As always, if you have an iPhone, subscribe HERE, and if you have an Android phone, subscribe HERE.

In this episode of HVAC School, Bryan talks to Leslie about the deceptive TXV. Thermostatic expansion valves, also known as TXVs or TEVs, are metering devices that maintain superheat. They contain an external equalizer. External equalizers give the pressure reading that you would normally take with a suction gauge at the end of the evaporator coil. They supply the closing force to the TXV. TXVs also contain a sensing bulb. The sensing bulb picks up the superheat on the suction line. When a vapor is superheated, its temperature exceeds its saturation temperature. The superheat value indicates how much the vapor temperature exceeds its saturation temperature. The bulb uses that superheat reading to adjust the TXV's opening force. System diagnosis can be tricky with TXVs, and you must set the charge by subcool on TXV systems. (Still check the superheat and follow manufacturer instructions, though.) However, TXV issues are pretty straightforward. Many of their issues deal with an undercharged sensing bulb. In those cases, you will notice issues with the opening force that acts on the valve. When replacing a valve, you typically make a new port for the external equalizer. However, getting solder in the tube can block off the closing force of the equalizer. Restrictions are also common issues for TXVs, and improper superheat is an indicator of a TXV restriction. When we think about the way TXVs manage superheat, "TXV" seems like a misnomer. We might be better off calling them "constant superheat valves" (CSVs). In addition, Bryan and Leslie discuss: Why Bryan doesn't like the name TXV Bulb, external equalizer, and spring forces Superheat and subcool Evaporator load How a TXV is supposed to work and how they fail   As always, if you have an iPhone, subscribe HERE, and if you have an Android phone, subscribe HERE. Thanks to Daniel Anderson for making this his first episode suggestion.