1.08 fudge factor is density of air in lbs/cu-ft * specific heat of air * 60 minutes in hour. 0.075 lbs/cu-ft * 0.24 BTU/lb * 60 mins/hr. = 1.08. It is temperature and pressure dependent. Formula works for heating where there is not any phase change occurring. Air conditioning cooling has phase change (latent heat) of humidity in air, turned to condensed water which consumes some of the air conditioner's cooling btu's capacity. Direct electric heater input power is very near 100% efficiency on power transfer to air flow, where air conditioning cooling from evaporator has a lot of other factors impacting heat absorption by evaporator coil.
question how do you find the delta T? do you run the furnace and take the split temp between the supply and return duct? or do you go,by this being a constant split. I think i remember from school it was rule of thumb and it was 17 degree f and between 22 degree f. would have the same question for finding current cfm
You are correct, you would find the actual delta T by taking the difference of the return air and the supply air. A typical value that I have always used for a "typical system in ideal conditions" is 20 degrees F.
Great question! It has to do with the delivery of energy. A lower delta T means that there is less heat transfer so you need more movement of that lower energy. Let’s say that we have to heat a volume of air in a room. To do that we have to add energy, in this case BTUH’s. If we added very hot air (therefore with a high Delta T) we don’t need to move the air as fast because the room will come up to temperature quickly because of all of the energy that we have at our disposal. Now if we take the same room and blow in air that is just a little bit warmer than the room air (low delta T), we don’t have as much energy to bring the room up to temperature so we need to move more of that air lower energy air to add up to the total energy that the room needs to keep temperature. Another way to think of this is with a bicycle. Let’s say that you want to ride your bike up a hill in 2 minutes exactly and there are two scenarios. The first scenario is that you have super strong legs (high delta T). The second scenario is that you skipped leg day (low delta T). Both require you to pedal at a certain speed to get up the hill in 2 minutes; pedal speed in RPM (CFM air flow). With super strong legs we can use a harder gear and pedal up the hill in two minutes using less RPM’s. With weaker legs we have to use an easier gear which will require us to pedal faster but with less applied force to make it up the hill in 2 minutes. Same hill, same time, different pedal speed based on the amount of strength (energy) that we have. More strength = slower pedal required. Less strength - faster pedal. Hope this helps. If not please let me know!
Great question. Altitude does come to play with a precise calculation. The 1.08 has become a "convenience" and is based on standard conditions (at sea level). For true calcs, air density at the blower is needed. Here is a handy PDF from Rupp Air with the formula to account for air density. www.ruppair.com/documents/white-papers/Actual%20Air%20Density%20BTU%20Calculation.pdf
if I have a target ACH of 5, a garage size of 30x20x9, I get a CFM of 450... Living in Florida, I have aa deltaT of 30F... (target indoor temp 70F when outdoor is 100F) So 450 x 30 x 1.08 = 14,580; rounding up I need a 1.5Ton unit? (Mini Split, no duct work). CFM = 20 x 30 x 9 x 5 / 60 = 450. How's my math?
Thanks for watching! While air flow is certainly a useful too calculate BTUH, when sizing equipment the building itself needs to be taken into account. This is done by way of an ACCA Manual J load calculation. Factors like insulation values, building materials, building orientation, and weather data all come into account. How's this, shoot me an email with your contact and I'll let you know what info is needed and I can work up a Manual J for you and give you some equipment recommendations. kingenuitytech@gmail.com
6:34 - “I’m a pepper, he’s a pepper, she’s a pepper. Wouldn’t you like to be a pepper, too?!” BTUs?! Shouldn’t this be called ATUs? Pint is American; kg is British!
@@KingenuityLLC Thanks. I like to put just a little more thought into my comments than _many_ posters do (btw, I guess I’m showing my age with that jingle allusion.) I’ve only been in refrigeration for a couple of years, so I really appreciate your video. Now, I need to come up with a way to calculate the American Thermal Unit!
Hello, can you please tell me if you know why there are no metal-plastic windows in the US like in Germany? US windows mainly have an aluminum frame and one glass, the thermal conductivity coefficient of such windows is three times higher than that of metal-plastic ones.
1.08 fudge factor is density of air in lbs/cu-ft * specific heat of air * 60 minutes in hour.
0.075 lbs/cu-ft * 0.24 BTU/lb * 60 mins/hr. = 1.08. It is temperature and pressure dependent.
Formula works for heating where there is not any phase change occurring. Air conditioning cooling has phase change (latent heat) of humidity in air, turned to condensed water which consumes some of the air conditioner's cooling btu's capacity.
Direct electric heater input power is very near 100% efficiency on power transfer to air flow, where air conditioning cooling from evaporator has a lot of other factors impacting heat absorption by evaporator coil.
Thank you for explaining the factor. Just add F to specific heat units.😊
thanks for imp inform , next week i hve exams and i was very confused.. now i am clar.. after the eam i wil lsure remember your guide . thanks
Thank you so much for sharing. Have a nice Christmas
Glad you liked it. Same to you!! :-)
Thank you for your clear explanation. Please make more videos.
Thanks so much! If there are any specific topics that you would like videos for, please let me know!
@@KingenuityLLC static pressures, and duct design
question how do you find the delta T? do you run the furnace and take the split temp between the supply and return duct? or do you go,by this being a constant split. I think i remember from school it was rule of thumb and it was 17 degree f and between 22 degree f. would have the same question for finding current cfm
You are correct, you would find the actual delta T by taking the difference of the return air and the supply air. A typical value that I have always used for a "typical system in ideal conditions" is 20 degrees F.
Thanks for important information.
How come the needed CFM increases with decreasing delta T?
Great question! It has to do with the delivery of energy. A lower delta T means that there is less heat transfer so you need more movement of that lower energy. Let’s say that we have to heat a volume of air in a room. To do that we have to add energy, in this case BTUH’s. If we added very hot air (therefore with a high Delta T) we don’t need to move the air as fast because the room will come up to temperature quickly because of all of the energy that we have at our disposal. Now if we take the same room and blow in air that is just a little bit warmer than the room air (low delta T), we don’t have as much energy to bring the room up to temperature so we need to move more of that air lower energy air to add up to the total energy that the room needs to keep temperature.
Another way to think of this is with a bicycle. Let’s say that you want to ride your bike up a hill in 2 minutes exactly and there are two scenarios. The first scenario is that you have super strong legs (high delta T). The second scenario is that you skipped leg day (low delta T). Both require you to pedal at a certain speed to get up the hill in 2 minutes; pedal speed in RPM (CFM air flow). With super strong legs we can use a harder gear and pedal up the hill in two minutes using less RPM’s. With weaker legs we have to use an easier gear which will require us to pedal faster but with less applied force to make it up the hill in 2 minutes. Same hill, same time, different pedal speed based on the amount of strength (energy) that we have. More strength = slower pedal required. Less strength - faster pedal.
Hope this helps. If not please let me know!
outstanding explanation @@KingenuityLLC
It was helpful for me
Speaking on the convince factor. I know it changes with the starting temperature. How does altitude affect it?
Great question. Altitude does come to play with a precise calculation. The 1.08 has become a "convenience" and is based on standard conditions (at sea level). For true calcs, air density at the blower is needed.
Here is a handy PDF from Rupp Air with the formula to account for air density. www.ruppair.com/documents/white-papers/Actual%20Air%20Density%20BTU%20Calculation.pdf
Hi, when calculating the air flow formula, do I need to use °F for your delta-T? Will the formula be diffrent when using °C for delta T.?
Yes, this formula will only work if the delta-T is in °F.
if I have a target ACH of 5, a garage size of 30x20x9, I get a CFM of 450... Living in Florida, I have aa deltaT of 30F... (target indoor temp 70F when outdoor is 100F) So 450 x 30 x 1.08 = 14,580; rounding up I need a 1.5Ton unit? (Mini Split, no duct work). CFM = 20 x 30 x 9 x 5 / 60 = 450. How's my math?
Thanks for watching! While air flow is certainly a useful too calculate BTUH, when sizing equipment the building itself needs to be taken into account. This is done by way of an ACCA Manual J load calculation. Factors like insulation values, building materials, building orientation, and weather data all come into account. How's this, shoot me an email with your contact and I'll let you know what info is needed and I can work up a Manual J for you and give you some equipment recommendations. kingenuitytech@gmail.com
@@KingenuityLLC wow, that's incredibly generous offer. I'll shoot you an email. Thank you.
6:34 - “I’m a pepper, he’s a pepper, she’s a pepper. Wouldn’t you like to be a pepper, too?!”
BTUs?! Shouldn’t this be called ATUs? Pint is American; kg is British!
I absolutely love this comment!
@@KingenuityLLC Thanks. I like to put just a little more thought into my comments than _many_ posters do (btw, I guess I’m showing my age with that jingle allusion.) I’ve only been in refrigeration for a couple of years, so I really appreciate your video. Now, I need to come up with a way to calculate the American Thermal Unit!
Hello, can you please tell me if you know why there are no metal-plastic windows in the US like in Germany? US windows mainly have an aluminum frame and one glass, the thermal conductivity coefficient of such windows is three times higher than that of metal-plastic ones.
how to compute cfm aircon 3tr & 1.5 hp..
Thank you very much those❤
You're welcome 😊 Thanks for watching!
Gonzalez Charles Davis Karen Williams Sandra