I may get blasted for this but I don't think it get's to technical....this was a reply to a test we had to take for work that had a question with the answer being 20* split....
"Cooling coils are rated with what is known as sensible heat ratio. This basically tells you the amount of heat the coil will remove which can be verified with a dry bulb thermometer.
Cooling coils are responsible for removing more than just sensible heat. They also remove latent heat which would have to be measured with a wet bulb thermometer. When you know the SHR you also the know the latent heat ratio or latent load.
example: SHR for a cooling coil is .75.
That means that 75% of the capacity of the coil is sensible heat and 25% is latent heat.
You may ask why this is important. Typically when your verifying capacity on a HVAC system you would need three measurement the dry bulb and wet bulb and airflow so you could plot the enthalpy change (total heat removal) so capacity can be calculated.
Sometimes you might hear you should have a 20 degree delta T or temperature difference for the system to be operating correctly. This can lead to problems. Cooling coils remove the moisture first...the higher the moisture content the less SHR and more latent heat is removed. So if you walk up to an A/C system on a hot/humid day use a dry bulb thermometer or what you would think as a normal thermometer you will only be detecting 1 of the two heats removed. Since A/C's remove the latent heat first that dry bulb temp. difference across the cooling coil will be lower than what you would expect because of the large latent load.
How can this lead to problems:
You walk up and take DB delta T (temp. across cooling coil) and you get 15*F difference. The person looking at this system may think something is wrong with the system when in fact it is operating normally it just has a large latent load. Which can be verified with a hygrometer or wet bulb thermometer. If the person checking the A/C checks just dry bulb delta T and see 15* he may start trying to fix a problem that isn't there.
This can lead to overcharged systems, setting the fan speed to low, wasted time...ect.
Please don't think in terms of absolute when thinking air conditioning way to many variables to put up one number and call it good.
I hope this helps anyone that might be looking for that magic 20* delta T"