What Do Ice, Physics and Water Have to Do with Air Conditioning?



Physics of indoor cooling

April showers may bring May flowers, but to those who live in mild areas of the country, it can also mark the beginning of air conditioning season! Birds are chirping, flowers are blooming and warm sunshine may be in your near future.  What a perfect time to share some fun facts about air conditioners!  (Cue the sounds of spring)


TONS of Cooling!

 

In school, kids learn that one ton is equal to 2,000 pounds. Logically, if you had a 4-ton air conditioner or heat pump, you would expect your 8,000-pound piece of equipment to arrive on a flatbed truck!  But thank goodness that is not the case! The tonnage or weight assigned to air conditioners and heat pumps has to do with the fact that people used to cool spaces with blocks of ice.


Before modern air conditioning, people rated their capacity to cool indoor spaces by the amount of ice that melted. When ice melts, it pulls heat energy from its surroundings.  It takes 143 BTUs (British Thermal Units) to melt one pound of ice.  The heat is transferred to the ice, which causes it to melt. In order to melt one “ton” of ice, you need approximately 12,000BTUs/hr.  Do you see the ice/air conditioner connection yet?

 

Here are the numbers:

     

  • One British Thermal Unit (BTU) = the amount of heat you get from burning one kitchen match all the way down to the end.
  • One ton = the ability to cool 12,000 BTUs in an hour

 

Your professional or licensed HVAC dealer should be able to determine the correct “tons” air conditioner or heat pump required to cool your home efficiently.  It’s important that the proper size of an air conditioner or heat pump is matched to your specific home. And thanks to modern marvels, the tons of cooling have nothing to do with massive blocks of ice.


Physics Can be Cool!

 

Air conditioners and heat pumps use the basic laws of physics and the refrigeration cycle to maintain a comfortable indoor temperature when the outdoors heat up.  The refrigeration cycle is based on the physical principle that a liquid expanding into a gas extracts or pulls heat from its surroundings. You can test out this basic concept for yourself with a water faucet and your hand!


  • Put one hand in lukewarm water and then hold it up in the air.
  • It will feel cooler than the dry hand, especially if there is some air movement.
  • Why? Because the water is evaporating! 
  • As the water evaporates, it pulls heat away from your hand.

 

The refrigeration cycle removes heat from one area and relocates it to another. To cool your indoor spaces, your air conditioner’s or heat pump’s refrigerant is pumped through a closed refrigeration system. The same refrigerant is continuously used over and over as it passes through the cycle! With induced pressure changes from the condenser coil, compressor, evaporator coil and the expansion valve, the state of the refrigerant is forced to fluctuate between a liquid and gas. It’s like a teeter-totter on a playground that doesn’t stop: liquid, gas, liquid, gas, etc.! This continuous cycle allows the heat to be transferred from inside your home to the exterior.

Here’s how it works:1


  • The refrigerant comes into the compressor as a low-pressure gas. It is then “compressed” to become a high-pressure gas.
  • The gas then flows through the condenser coil . Here the gas “condenses” to a liquid, and gives off its heat to the outside air.
  • The liquid then moves to the expansion valve under high pressure. This valve restricts the flow of the fluid and lowers its pressure as it leaves the expansion valve.
  • The low-pressure liquid then moves to the evaporator coil, where heat from the inside air is absorbed and changes it from a liquid to a gas.
  • As a hot low-pressure gas, the refrigerant moves to the compressor where the entire cycle is repeated.

Heat and Humidity!

 

Believe it or not, there is water in the air even when it’s not raining! Humidity is the amount of water vapor in the air. The more water vapor in the air, the higher the humidity. People may be able to feel when the humidity level is high, especially when it’s combined with high temperatures.

Air with higher humidity has an increased amount of water vapor. As a result, sweat may not evaporate as quickly as it would with a lower humidity level. This phenomenon may make us feel hotter than the actual temperature because evaporation is slowed.

Going back to the wet hand experiment - if the humidity level is high, it may take longer for the air to absorb the extra moisture on your wet hand. That means that your hand wouldn’t feel as cool because evaporation is slowed. This explains why it feels hotter when the humidity levels are high. Likewise, very low humidity can make us feel cooler than the actual temperature. This happens because the dry air helps moisture evaporate more quickly than usual.


So how can an air conditioning help you feel more comfortable in your home? When warm air comes in contact with your air conditioner’s or heat pump’s cold evaporator coil, some moisture may be condensed out of the air, making your home feel less humid. The moisture collected by the evaporator coil goes to a drain and then it is sent outside, away from your home. But air conditioners and heat pumps are not meant to control humidity independently.  It just happens to be an incidental by-product of the refrigeration cycle! 

Who Named Air Conditioning?

 

Humidity also has something to do with the origin of the term “air conditioning.” Stuart W. Cramer coined the term “air conditioning” in his opening remarks to the American Cotton Manufacturers Association convention in 1906.  Cramer, a leading figure in the textile industry, installed the first air conditioning system in the south. His intent was to create ideal humidity levels at his Chronicle Cotton Mills in Belmont, NC in order to ease the process of spinning yarn.

 

The business owner and trained engineer was credited with designing and equipping over 150 cotton mills, held 60 patents, and pioneered humidity control and ventilating equipment.2  While he didn’t invent the air conditioner, in his speech Cramer refers to his new “Automatic Regulator, which is the automatic control of humidifying and heating systems.”3 But Cramer wanted a more general term for the indoor air controlling technology. Cramer opened the conference by stating, “I have used the term “Air Conditioning” to include humidifying and air-cleansing, and heating and ventilation.”4 Thus, the word “air conditioning” was used and the rest is history…

 

The Growing Number of Air Conditioners!

 

Air conditioning is continuously named as one of the top inventions in modern history. Most warm-weather states can give the nod to the air conditioner for their increasing populations. Can you imagine what life must have been like during those southern summer days? It was certainly different without an air conditioner!

 

In the south, single-family homes were typically built with high ceilings to let the heat rise, tall windows to catch the occasional cross breeze, and large porches to shade windows from the sun’s heat.  Old photographs often depicted people sitting on those expanded porches, hoping to catch a cool evening breeze. Outdoor living became part of the culture. However, people started to retreat indoors once air conditioning technology became more common.

 

Air conditioning’s popularity began to increase when the equipment became more affordable to the common homeowner. By the 1950s, more than one million room air conditioning units had been sold. However, it wasn’t until 1977 that newly constructed homes tipped the scales of residential air conditioning. In 1977, 54% of newly constructed single-family homes had air conditioning compared to the 46% that weren’t built with air conditioning.5 The upward trend continued for decades.

 

By 2015, the U.S. Census revealed that nearly 93% of newly constructed homes in the United States were built with air conditioning. However, the numbers also acknowledged that between 1997 and 2015, over 99% of newly constructed homes in the “south” were built with air conditioning.5 It appears that indoor comfort, whether for personal or economic reasons, will continue to advance and thrive! Thank goodness!

 

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1 Southwest Wisconsin Technical College. Basic Refrigeration Cycle. n.d. https://www.swtc.edu/Ag_Power/air_conditioning/lecture/basic_cycle.htm. 7 April 2017.

2 Hill, Michael. NCpedia. 2006. http://www.ncpedia.org/air-conditioning. 5 April 2017.

3, 4 American Cotton Manufacturers Association. "Proceedings of the ... Annual Convention of the American Cotton Manufacturers Association." American Cotton Manufacturers Association. Charlotte: American Cotton Manufacturers Association, 1897-1944. 182. https://babel.hathitrust.org/cgi/pt?id=nyp.33433066400650;view=1up;seq=210.

5 U.S. Census Bureau. "Presence of Air-Conditioning in New Single-Family Houses Completed." U.S. Census Bureau - Air Conditioning. Washington D.C.: U.S. Department of Commerce, 2015. 1-20. https://www.census.gov/construction/chars/pdf/aircond.pdf.