HVAC 102 Scenario Low Evaporator Airflow effects on the refrigeration cycle 

mostly they dont because they dont unserstand it themselves, i have 40 year exp tech that cant explain this :(

@@builderman912which is sooo sad. But I get it

This would be a great topic!

I'm working on the details, still have to finish up fundamentals

I yes I will be covering psychometrics before long and it will be covered in detail.

The same quantity of refregerants is flowing into the evaporator coil so the BTU or heat absorbing capacity is still there. Now that the air is moving slower it has more time to change the temperature of the air. The slower air also has more contact time giving it more time to change the temperature. Since the airflow is less there is overall less heat on the evaporator so the temperature and pressure of the evaporator coil will drop. The lower coil temperature will also mean it is much farther below the due point temperature and dehumidify better. However it won't have enough time to boil all the refregerants to a vapor, dropping superheat and potentially damaging the compressor. Lower airflow = More contact time lower evap temps, shame capacity= larger TD. On a gas furnace more airflow is also a lower TD and less airflow is a greater TD. The furnace makes the same BTU what changes the temp is the airow.

Here is a video that explains it better ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-pPRJqDoVCXc.html

Here is a great article that works together with it hvacrschool.com/an-evaporator-coil-with-no-fins/?fbclid=IwAR2zcS0LxdknvcdLM3ukmlmVPzSOgAKxbH4As4KNk1D3uSxsYI42-oVajew

And more. More that is all connected hvacrschool.com/20-%CE%B4t-delta-t-a-lazy-rule-of-thumb/

Thank you for responding.

The TXV will try to maintain a constant superheat and maintain the correct amount of refrigerant in the coil. Once the air is restriction gets severe the TXV will close to the maximum of its movement and the evaporator will then flood

@@love2hvac Ty, first of all thank you for responding so quickly, I'm a fan of your channel, its in my opinion the best in the industry when it comes to teaching!. That's what I had in the back of my mind. It most be a severity situation. I guess a mild airflow restriction will show signs of TXV overfeeding but only when it starts to get bad is when the suction pressure will begin to drop and liquid starts to go through the evaporator and back into the condenser!.

Great question. In these examples they are all with proper airflow first. This simplifies the foundations and places the importance of airflow first. I believe proper airflow should be established prior to superheat and subcooling. I believe in air conditioning the airflow is essential and an essential part of diagnostics. I will later be doing classes on airflow and multiple ways of checking and verifying it. I will also be doing classes on diagnosing. In that section I will show all the symptoms for each problem such as a dirty evaporator coil and how that effects the entire system It will tie together capacity SH SC compressor capacity static pressure and more.

@@love2hvac thanks for the response, your videos are awesome 👍

less airflow means less heat on the evaporator. Less heat means the saturated or boiling temperature drops and the suction pressure is tied to the saturated so it follows. More airflow means more heat, faster boiling, higher suction pressure and higher saturated temperature.

Get the true flow grid and confirm airflow before doing superheat or subcooling. TXV rarely fails and often misdiagnosed. Bad airflow is the #1 cause for improperly diagnosed TXV. A TXV does not control pressure, it controls Superheat assuming airflow is correct first. If you have low suction pressure, adjusting the TXV will only flood the compressor and cause premature compressor failure.

Pressures domt mean much It's about superheat subcooling, airflow and your metering device. Give me those numbers

@@love2hvac my superheat : 13.5 f, subcool: 47,3 f.. Air : 80,6 f.. And metering is danfoss tex20.. (Double) Airflow in good condition.. Renewed air filter, drier filter.. Problem ( freezing at coil, and low press. Line and compressor after run about 3-4 hours).. My compressor carrier 5h60 .. Condenser cooling media is sea water (30°c) Im in indonesia by the way sir.. Hope u can help me sir..🙏

Excellent information, Thank you! The numbers tell me you are having an indoor airflow problem this could be lack of air across the coil or a dirty coil. The 13.5F superheat tells me the TXV is working correctly The 47 subcooling tells me someone tried adding refrigerant to solve the airflow problem. Alternatively, check the water regulating valve at the condensing coil. If it's letting too much water through it would cause a stable head and a low suction.

@@love2hvac thank you very much.. ..for ur time and advice God bless u sir. ... 🙏

I was debating adding that or not. Since most resi units don't have accumulators or recovers I decided to keep it simple.

@@love2hvac when I think refrigeration I don't think comfort cooling.

There is no set number. It will vary with conditions, and how much airflow the manufacture has designed. Later we will look at how to calculate total BTU with condenser deltaT and CFM.