1 Ton = 400 CFM??

H_2_O

I don’t understand why 1 ton of cooling approximately requires 400 CFM. I’ve noticed that a lot of engineers use this as a rule of thumb.

pecmsg

400 is a starting point. Adjust accordingly

EdTheHeaterMan

I have adjusted away from the 100 CFM per 3000 BTU of cooling standard on many occasions. Most of my work was done in the southern tip of New Jersey. Cape May County is a peninsula at the southern most part of New Jersey that is surrounded by large bodies of water. The Atlantic Ocean to the east and south, and the Delaware Bay to the west. So the prevailing wind will often blow humid air from the evaporating waters around this area causing an unusually high amounts of relative humidity. The latent heat that must be removed from that air, in order to allow the sensible temperature to drop, often requires a slightly larger piece of equipment for the same exact home that may be located in the central and northern part of the state. This often sets the number at 80 CFM per 3000 BTU of cooling. or 100 CFM per 3750 BTU of cooling.

As @pecmsg so eloquently stated: "it's a starting point".

Do you know why 12,000 is called one ton of cooling?

Back when refrigeration was in the early stages it was determined that it took a certain amount of heat to melt one ton of ice (2000 lbs.) over a 24 hour period. That number was approximately 288,000 BTU. In order to get the BTU per hour, you would divide 288,000 by 24 Hours in order to come up with 12,000 BTU per hour. That number became the standard way to measure refrigeration equipment in 1903 or 1904. Only a few short months after Willis Carrier invented Air Conditioning.

HydronicMike

Depends on where you are. Your probably doing 350 in Florida, Texass or very humid climates, and 450 in a dry climate like Arizona.

bburd

Several factors must be balanced: sensible versus latent heat (humidity) removal, and transferring enough heat through the evaporator coil to make the cooling process efficient. Modern high efficiency equipment needs higher airflow per ton of cooling to achieve that higher efficiency.

DCContrarian

When air is cooled, the heat transfer, in BTU per hour, is equal to the temperature change in F times the flow in CFM times 1.08. So 400 CFM per ton equates to a temperature change of about 28F. If the room is at 74F that means air is leaving the air handler at 46F.

That's a good compromise. With a lower CFM you need a higher temperature change, and vice versa. The colder the air handler, the more humidity is removed for a given amount of cooling. However, if any part of the air handler drops below 32F condensation can freeze and cause all sorts of problems. The warmer the air handler the more efficient the air conditioner will be, but dehumidification may suffer.

EBEBRATT-Ed

28 deg TD with 1 ton and 400cfm will only be with 0 humidity removal and no moisture in the air.

In most (but not all) areas the SHR is 75-80% sensible and 20-25% latent. So 75% of the 28 degrees is around 20-21 degree TD is usually what you see.

bburd

55° F is a typical cooling supply air temperature.

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1862832#Comment_1862832

From what I've seen they use a much larger evaporator but stick with 400 cfm per ton. Some of the Goodman equipment I was looking at recently used lower airflow in their SEER data, something like 360 cfm per ton. I was surprised.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1862834#Comment_1862834

Not no moisture, just a dew point lower than the coil temperature. But yeah, if dehumidification is happening you have to account for that. So in the real world the temperature coming off is going to be warmer.

DCContrarian

Just for fun, I ran some numbers. If your indoor temperature is 74F, your air handler is running at 400 CFM and your AC is delivering 12,000 BTU/hr:

• At a dewpoint of 46F or below (37% RH) the air comes out at 46F
• At a dewpoint of 50F (42% RH) the air comes out at 48F
• At a dewpoint of 60F (63% RH) the air comes out at 55F
• At a dewpoint of 70F (90% RH) the air comes out at 63F

The SHR ratio for those four scenarios is 100%, 92%, 65%, 38%.

AC is kind of magic this way, the more humid it is, the more dehumidification it does.

DCContrarian

At 300 CFM per ton, the SHR for the four scenarios is 100%, 81%, 63%, 38%

At 500 CFM per ton, the SHR's are 100%, 100%, 72% , 40%.

Higher SHR's mean less humidity removal, so higher airflows mean less humidity removal. The higher the humidity the less of a difference you see.

pecmsg

https://forum.heatinghelp.com/discussion/comment/1862954#Comment_1862954

or the higher the Dew Point the higher the latent load.