Its In the Bag!

I have had an eventful two weeks – first in Colorado Springs at the 2013 HVACR & Mechanical conference and next in Las Vegas at the 2013 HVAC Excellence Educators and Trainers Expo. Even though I am very glad to be home I always am energized by these events. The educational sessions are great, put on by industry professionals who are tops in their field. Although I love ogling the latest techie toys, there are often inexpensive ideas and tips that help demonstrate how systems work. For example: you can use a large plastic garbage bag and a stop watch to demonstrate airflow. You flatten the bag, place it over a register, and time how long it takes to fill up. Then a quick calculation gives an idea of the CFM. The formula works like this – there are approximately 7.5 gallons per cubic foot, so a 55 gallon trash bag = 55 gal/7.5 gal/ft³ = 7.33 ft³. The flow in cubic feet per second is determined by dividing 7.33 ft³ by the seconds it takes to fill the bag. That multiplied by 60 gives you CFM. Written out it looks something like (7.33 / seconds to fill bag) x 60 = CFM. You can even do return air by filling the bag and then holding it over a return grill so that it completely covers it. You time how long it takes to collapse the bag. Now I am not recommending this as a means of checking system performance, but it is great for demonstrating exactly what is meant by cubic feet per minute. What I love about this is it demonstrates both the volume by seeing the bag fill up, and the time by clocking how long it takes. After demonstrating the concept of air flow you can show how to measure it properly with accurate instruments. Hopefully, the students will have a better idea of exactly what is meant by CFM and the measurements will mean more to them.

Delta T vs TD

Once again my good friend Brian Baker is keeping me honest. After my last posting  about Delta T where I said that Delta T was the same thing as Temperature Difference, Brian sent me a copy of a Sporlan Cold War Article by Garth Denison. It is essentially a refrigeration glossary of useful terms, including Delta T and TD – short for Temperature Difference. In this listing, Mr. Denison makes a distinction between Delta T and TD (temperature difference). He defines Delta T as a temperature difference between two points in the same media and TD as temperature difference between two different media. So Delta T is a temperature change in something, while TD (temperature difference) is the difference in temperature between two different things. Although the distinction is subtle, it makes some sense. Remember that the Δ symbol really stands for change. The change is represented by the change in shape of the triangle from top to bottom. In the case of a water cooled condenser, the Delta T would be the change in temperature of the water from 80°F entering the condenser to 90°F leaving the condenser. The TD, or temperature difference, would be the difference between the 90°F water leaving the condenser and the 100°F refrigerant inside the condenser.  

Its All Greek to Me!

Years ago a proper education included the study of Greek. Why? Because so much of our modern language and western culture is rooted in Greek culture. As education has moved away from studying classics and Greek, we have retained many symbols and phrases without really understanding them. Even in HVACR, we use Greek symbols on a regular basis. Ever hear someone refer to taking a “Delta T” measurement? This simply means to measure the temperature difference, or change in temperature. Delta comes from the uppercase of the fourth letter of the Greek alphabet, which looks something like a triangle, Δ. In many math and science fields, Δ is used as shorthand for change, or difference. One way to think about this is to look at the form of the letter Δ. It changes in width from the top to the bottom. An easy way to remember it is simply that Delta starts with “D” and Difference starts with “D.” So Delta T simply means temperature difference. It just sounds so much more educated to be checking out a system’s Δ T. Two more commonly used terms which come from Greek are: zeotrope and azeotrope. Zeo means to boil, Tropos means to turn; thus, a refrigerant which is zeotropic turns, or changes as it boils. “A” used as a prefix means “not.” For example, amoral means not moral. Similarly, azeotrope means a refrigerant that does NOT change when it boils. If you understand where the terms come from, they seem a little less confusing.