Month: July 2019

Travel back with me back in time to 2001. I was a young tech, proving my salt out there in the big world of commercial RTU maintenance. One of the steps in the Fall PM list was “Test aux heat” and by golly… that’s what I was going to do!

I was on a roof of a bank and I began testing the heat strips one unit at a time… and sure, I smelled a little something.. but that’s normal right?

I turned around to see the bank manager frantically scrambling up my ladder onto the roof.

Now I have seen many bank managers in my day and this sort of athletic feat was not usual. THERE IS SMOKE EVERYWHERE AND THE ALARM IS GOING OFF! he gasped.

So the fire trucks arrived and my Father also chose to visit that same branch at that exact moment. He looked around and asked “what’s going on here?”….

Oh nothing much Dad… I wouldn’t go in he bank though… it’s a little stinky in there.

So I learned my lesson. Sometimes just “burning off” the heat isn’t the best bet. If the heat hasn’t run much it would be wiser to remove the strips and blow them off with nitrogen or compressed air before you turn them on.

Unless you like watching bank managers do gymnastics.

— Bryan

If you work in large commercial office or mixed use buildings you almost certainly work with VAV systems on a regular basis. If you aren’t familiar with VAV this is a quick intro to the basics so that they won’t seem so overwhelming.

VAV stands for Variable Air Volume, this means that the AHU (Air Handling Unit) can vary amount of air output to suit the amount of air needed for the areas served.

A typical configuration for VAV would be to set the blower to  hit a fixed static pressure target (say 1 to 1.2 inches of water column pressure) and the cooling capacity varies to maintain around a 55° supply air temperature.

Each zone is served by at least one VAV box or more accurately a VAV terminal. The terminal is controlled by a temperature (Generally the BAS system) to open and close to provide more or less air to the zone. As VAV terminals open the static pressure in the supply duct decreases and the blower increases air volume to maintain the static. As the air volume is increased the system cooling capacity also needs to increase to maintain the supply air temperature.

This constant modulation requires variable refrigerant capacity in the form of multi stage or variable refrigerant compression and generally a blower fitted with a VFD (Variable Frequency Drive) although older VAV systems often used a adjustable vane at the blower inlet to vary the air volume.

One of my favorite YouTube channels is THE ENGINEERING MINDSET and the Video below covers VAV in more detail. You can click this LINK to watch it direct on YouTube

Have you ever noticed that the more you’re required to speed up to get all your work done in a day, the more the cleanliness of your work vehicle suffers?

Some techs won’t clean their vans no matter how slow or busy the schedule gets, but for most of us, we prefer a clean and organized vehicle but in the really busy season it can slip a bit.

This is entropy at work

One way to think of entropy is the tendency for energy to be “wasted” or for energy to become more disorganized as it is changed from one form to another. In practical terms the higher the entropy the greater the waste in moving energy around and the lower the entropy the less energy is wasted or unusable.

When your van is clean you waste less energy trying to find what you need but that is easier to do when you aren’t “rushed”.

Don’t confuse entropy with enthalpy. Entropy is the measure of the “availability” of energy to do useful work, enthalpy is a measure of total heat content.

Another way to think of entropy is simply the tendency for everything to go from more organized to less organized over time.

When we talk about entropy in classroom settings when analyzing a pressure-enthalpy diagram of a refrigeration circuit like the one shown above. You will notice the red shape is nearly a rectangle other than the slanted line on the right that shows what happens when the refrigerant is compressed. Rather than going straight up and down it curves to the right to show that additional heat is added to the refrigerant (enthalpy) as it is compressed. This increase in enthalpy follows something called lines of constant entropy, in other words, as more energy is added to a system the faster the molecules move and the less organized they become.

As pressure and temperature increase, so (generally) does entropy, just like when it get’s hot and the dispatcher starts putting the pressure on you your van entropy also increases.

In a refrigeration circuit, the system will work more efficiently when we achieve the desired movement of BTUs with a minimal amount of entropy. Practically speaking this means using lower pressures and temperatures to move heat from one place to another when possible, but we still need to get the job done of moving heat from one place to another and in that process, entropy WILL occur.

We all know the guy who has the PERFECT van, the spotless tools and the clean uniform. His van stays that way because he hardly does anything and then he takes 20 minutes after every call wiping down and oiling his tools. He has very little entropy in his van because there is very little WORK being done.

Absolute zero is the temperature at which no molecular motion exists, in that state there is also no entropy. No heat, no work, no disorganization. More heat, more work, more disorganization (entropy).

So don’t use me as an excuse for a messy van, but if your boss hassles you too much on a busy day you can remind him. Heat and pressure result entropy and the back of that van… that’s entropy baby.

— Bryan

P.S. – For those of you engineering types I know that this article took some liberty with definitions. It’s an analogy, not a doctoral physics thesis.

Most techs know that you shouldn’t fill a recovery tank more than 80% with liquid based. Many know that the WC rating stands for “water capacity” and that you need to adjust for the density of the actual refrigerant rather than just using 80% of WC.

I hope most of you know that the TW marking stands for “Tare Weight” and tells you empty weight of the tank.

But do you know the service pressure your tank is designed for? 

Because R-410a is one of the higher pressure refrigerants in common use today most modern recovery tanks are built to handle R-410a pressures. The standard they are built to is called DOT-4BA400 and will be stamped on the tank collar. The DOT (department of transportation) is the governing body in the USA that has the rule making and enforcement authority on tanks, tank handling and tank shipment.

Tanks listed as DOT-4BA400 are designed for a service pressure of 400 PSIG with a test pressure of 800 PSIG. This means that R410a is safe in the tank at a tank temperature of up to 116°F which equates to 400 PSIG for typical use but that the tank is tested not to fail until over 800 PSIG. 

If a tank has any physical signs of damage it is to be taken out of service and tanks must be re-certified via hydro-static test and visual inspection forma certified facility every 5-years.

— Bryan

 

 

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