Random Ideas about HVAC Contracting Success

My second grade granddaughter, Taylor, asked me to answer a tough homework question last night. She asked, “What am I? I’m a gas; I don’t smell like anything; I’m everywhere; you can’t see me, and people love me.” Taylor was impressed when I replied, “I am the sky.” She did not know what my passionate interest in the subject of air was.

Let’s take a closer look at the fascinating properties of air from an HVAC perspective.

What’s in the air?
Several people answered Taylor’s question with “oxygen”. She suspected this was the wrong answer and was delighted to learn why.

Air consists of 21% oxygen, 78% nitrogen and about 1% other gases. I was delighted to report how Mother Earth is maintaining these percentages around the world. She thought that was cool, and so did I.

Air consists of 21% oxygen, 78% nitrogen and about 1% other gases.Air consists of 21% oxygen, 78% nitrogen and about 1% other gases.petrroudny / iStock / GettyAir also contains water vapor. Under comfortable conditions, water vapor or moisture makes up less than 2% of the air you breathe.

Of course, depending on your current location and activity, other substances, such as pleasant smells, easily float in the air. During a typical day, you may also encounter unwanted elements in the sky.

Most importantly, we can only live without air for about three minutes. This fact fascinated my granddaughter. It was something she hadn’t thought of before.

Standard air
Air properties are constantly changing. They depend on temperature, altitude and humidity. Standard air is the baseline and is defined as dry air at 70 degrees Fahrenheit, at sea level.

Most HVAC formulas use standard air as the basis. So if you are measuring or calculating air under conditions very different from this standard, you must adjust the air properties of the formula to ensure accurate results.

Air takes up space. It consists of atoms and molecules. A pound of air at 70 degrees fills a 29 inches. x 29 in. x 29 in. cube at sea level.

Air is heavy
The next time you drive down the road, open the window, put out your arm, and get your hand to the wind. You feel the force of the air pressing against your hand and arm. The faster you drive, the more air hits your arm. Air IS heavy. Let’s calculate how heavy. One cubic foot of air weighs 0.075 pounds or about the weight of a pencil. The area of ​​your hand and arm facing the wind is about a square foot. Moving at 35 miles per hour converts that speed to 0.58The faster you drive, the more air hits your arm.  Air IS heavy.The faster you drive, the more air hits your arm. Air IS heavy.AscentXMedia / iStock / Getty miles per minute or 3,062 feet per minute.

Multiply 3,062 feet per minute by 0.075 pounds to find that 230 pounds of air hits your hand and arm per minute. Yes, the air is heavy.

To get a better idea of ​​the weight of air, consider the amount of air a fan moves in an hour. Thirty pounds of air per minute per ton equals 1800 pounds per hour. (400 cfm = 30 pounds x 60 minutes per hour = 1,800 pounds) That’s almost one ton of air to move one ton of refrigeration per hour!

An air conditioning system must move about 400 cubic feet of air per minute (cfm) per ton of refrigeration. Four hundred cubic feet of air weighs 30 pounds (400 x 0.075 pounds = 30 pounds)

To get a better idea of ​​the weight of air, consider the amount of air a fan moves in an hour. Thirty pounds of air per minute per ton equals 1800 pounds per hour. (400 cfm = 30 pounds x 60 minutes per hour = 1,800 pounds) That’s almost one ton of air to move one ton of refrigeration per hour!

This fact provides additional insight when diagnosing restrictive air filters, coils, or duct systems. Imagine the resistance to airflow caused by trying to pull nearly 10,000 pounds of air per hour through a highly efficient air filter in a five-ton system.

Altitude affects airtightness

At higher altitudes, the airtightness decreases or becomes thinner. I used this excuse to slow down after a few hours trying to keep up with Taylor and her family as we played, hiked, and rock climbed on the beach at Lake Tahoe (elevation over 6,000 feet).

When designing or testing HVAC systems at higher altitudes, you must make adjustments to compensate for the lower airtightness at higher altitudes.

Changes in air density are easy to understand when examining the airflow required at different altitudes. At sea level, most refrigeration systems require 400 cfm per ton. Cooling systems at 5,000 feet above sea level require an air flow of 465 cfm per ton. A system operating at 10,000 feet requires 580 cfm per ton.

As the height increases, each cubic foot of air contains less mass or contains less stuff. Heating equipment adds heat to the air, which is essentially transferred from the heat exchanger to the stuff in the air. Getting the same amount of stuff to a higher altitude requires more airflow.

Temperature affects air
Hot air rises. Just as the air density decreases at higher altitudes, the air density and weight of air decreases when heated. Hot air balloons are an excellent example of this principle. The lighter air weight in the balloon compared to the air outside the balloon causes it to rise.

Air density changes when air flows through a heating system. For example, if air enters a system at 70 degrees and leaves the system at 135 degrees, the air is 10% less close to exiting than when it enters the system.

In a refrigeration system, air can enter at 75 degrees and leave at 55 degrees. The air is denser when it leaves the indoor air coil.

Technical data from the supply register confirms that air at 55 degrees is nearly 15% heavier than air at 135 degrees. A supply register will throw air 15% further in heating mode than in cooling mode.

Hot goes to cold, warm air rises and cool air falls
As a contractor, your job is to transfer air and heat from one place to another to make homes and buildings more comfortable. To do this, you use the second law of thermodynamics that teaches hot movements to cold.

A gas oven burns fuel. The burning fuel produces heat. Hot air rises when cooler air falls. Decades ago, we used gravity furnaces to heat houses where the heat rose through large channels from a furnace in the basement. We did not use fans.

In the refrigeration cycle, heat in the air is attracted to the cool refrigerant in the coil. The heat is conducted to the condensing unit and dissipated outside.

Today, most systems use a mechanical fan to mix cooler air with warmer air and force it into every room to create even temperatures throughout the house.

In the refrigeration cycle, heat in the air is attracted to the cool refrigerant in the coil. The heat is conducted to the condensing unit and dissipated outside.

The laws of heat transfer are all around us, whether we are aware of it or not. If you are in a warmer climate, consider the hot air you exhaled in a building yesterday. Your hot breath went through the building with a fan, over the indoor battery, into the refrigerant and was discharged from the condensing unit outside. The wind then shifted the heat from your breath, perhaps at 10 miles per hour to the northeast. Today, the warmth of your breath is 240 miles northeast of where you exhaled it.

Air properties produce fascinating results, don’t they?

Tip of the iceberg
This short article, prompted by a second grader’s homework question, only addressed some of the many air properties we use to harness and control air. Maybe we can look at how high pressure goes to low pressure in another article. We can also discuss what determines the rate of heat transfer from hot to cold and provide several useful calculations to predict changes in air properties.

Your opportunity is to continue to learn, observe and monitor aerial properties to better serve your customer and expand your career.

Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you are an HVAC professional and are interested in a free airtightness correction table for altitude and temperature, please contact Doc at ncilink.com/ContactMe or call him at 800-633-7058. Go to the NCI website at nationalcomfortinstitute.com for free information, articles, downloads and current training opportunities

Comments are closed.