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The Basic Refrigeration Circuit Oct 05, 2016

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In this episode of HVAC School, we discuss the entire basic refrigerant/compression refrigeration circuit.

We are in the business of moving heat. Heat refers to motion in the molecules. Temperature is the average velocity of those molecules. Heat needs a temperature differential to move. So, HVAC systems absorb heat when the refrigerant is colder than the ambient temperature. They reject heat when the refrigerant is hotter than the ambient temperature.

Remember the components and their functions in the following order:

Compressor: increases the vapor refrigerant's temperature and pressure.
Discharge line: carries hot, high-pressure, superheated vapor to the condenser.
Condenser: changes the vapor to a liquid.
Liquid line: moves the subcooled (high-pressure) liquid to the metering device.
Metering device: drops the liquid's pressure (creates some flash gas).
Expansion line: leads the low-pressure liquid/vapor mixture to the evaporator.
Evaporator: changes the liquid/vapor mix to a vapor.
Suction line: moves superheated vapor to the compressor.

Note: Heat pumps can shake things up a bit; the suction line becomes the discharge line (and vice versa), and the condenser becomes the evaporator (and vice versa). However, heat pumps have two metering devices and a bi-flow liquid line drier, so the liquid line stays the same. So, watch out for heat pump systems with that tricky little reversing valve.

We also elaborate on some fancy accessories. These include accumulators, discharge line mufflers, receivers, and more.

And we discuss much more...

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Solder Rings, Thermal Imaging and My Grouchy Brother Sep 29, 2016

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In this episode of HVAC School, I talk with my brother Nathan... and he whines a lot about cool tools I like.

As you might have guessed, Nathan is in the camp of people who believe that proper training promotes good practices; fancy tools won't make an outstanding tech. Even though I respectfully disagree with him on some things, he has a point. Tools will only be useful if a tech knows how to use them. They should make your life easier, but they shouldn't have much bearing on performance. He doesn't like wasting money or time on tools that probably won't help him. So, he doesn't like solder rings or thermal imaging cameras.

On the other hand, I'm in the "tool nerd" camp. I love new technology and think tools can make us do much better work if we learn how to use them properly. On the job and in my spare time, I enjoy reading up on the latest technology and trying out the newest tools. Missing out on the newest "wow!" tools is a fear of mine. Spending money on a tool that will help me do better, more efficient work is ALWAYS worth it. I think solder rings and thermal imaging cameras are cool and can be put to good use.

We talk about how efficiency, organization, training, job performance, and customer trust relate to tools. We can find some common ground in some areas, even if we have fundamentally different attitudes toward tools.

Oh, and we talk about digital gauges and the jumping spider that lives in Nathan's van.

—Bryan

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The Basics of Moving Heat Sep 22, 2016

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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.

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Why a TXV instead of a TEV?.. or a CSV? Sep 16, 2016

Show notes

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

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Thanks to Daniel Anderson for making this his first episode suggestion.

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