View Full Version : Theoretical question-Air flow
05-08-2005, 10:54 PM
The thumb of rule for air flow is 400 CFM/ton cooling and 200-300 CFM for heating (with exception of refrigerant heating cylce with heat pump, which requires high CFM), but what is the theoretical ground for this. A few thoughts:
1. Different heat exchange properties when air flowing through cooling coils and combustion heat exchangers.
2. Occupants' Comfort.
3. Different gravities of heated and cooled air.
05-08-2005, 11:12 PM
....RELEASE THE HOUNDS!!!
05-09-2005, 01:31 PM
Regarding your post:
Based upon your fundamental airflow questions, I offer you this simple and quick answer to your questions:
Don’t try to reinvent the wheel my friend, the HVAC equipment manufacturer you are working on, or are installing will state in his installation and operating literature what CFM, static pressure, and temperature delta Ts the equipment is to operate at. Never go by rule of thumbs unless there are no operating conditions available by the original equipment manufacturer, rare, but on occasion, it does happen.
John J. Dalton
05-09-2005, 02:03 PM
More important than heat exchange properties, density differences, etc. is the temperature difference between the refrigerant and the conditioned air and phase changes that take place by conditioning the air. For heating air, there aren't any phase changes. But when the air is cooled water vapor starts condensing out of the conditioned air (latent load) than potentially ice starts forming on the coil. Ice formation tends to screw everything up on comfort cooling systems because those systems aren't designed to operate that way.
IMHO, John is right. These things are best left to the manufacturers who design, build, and test the systems. Follow their guidance and you and your customers will be much happier.
dan sw fl
05-09-2005, 08:10 PM
... DELTA T ______ ENOUGH SAID!.
05-09-2005, 08:16 PM
Actually, felix presents a good question and what you can learn in the process of getting the answer is highly practical and answers many more questions than this one.
The answer lies in learning to read and apply the psychrometric chart as well as a few basic heat transfer and airflow equations.
I spend two days in one of my seminars answering this and a host of other similar questions and use practical and real life examples to illustrate the concepts.
05-09-2005, 08:38 PM
Dear Norm Chris,
Regarding your previous post:
“I spend two days in one of my seminars answering this and a host of other similar questions and use practical and real life examples to illustrate the concepts.”
It took five and a half years of college to get my BS in mechanical engineering because I was working full time in the HVAC/R industry, I’ve spent my last 31 years honing my craft, taking scores of seminars, evening, and weekend classes, and have attended countless factory training sessions from almost every major manufacture known in the industry, have studied countless hours over the years to get my CM in RSES, have studied more hours than I’d like to admit to my family for my CMS in the upcoming heat pump category, but sir, I have to say, your above statement is understated to say the least.
My brother, who has also been in the industry for the last thirty years, and I took a two day course of yours in psychometrics back in 1991 in Santa Clara, and I have to say to this day it has been the most comprehensive, practical, and interesting class in HVAC/R that I have ever had the pleasure to take.
After all these years I can say….thank you Norm from the both of us. I know this sounds like an ad for your seminars, but its not, it’s just a warm felt thank you for a job you did well so long ago.
John J. Dalton
05-09-2005, 08:48 PM
Thanks for the kind remarks. I am now a factory instructor for York International and have improved that seminar and I teach it through York's training department.
I just happen to be in Montana right now and will be teaching a one day seminar in the morning on "Preventing Compressor Failures". Then I will drive to the other end of the state and put it on again. I now have an entire series of one and two day seminars that I give through York and they always get positive feedback.
Applied Psychrometrics and Airflow is on of the most useful of the seminars and is fun to teach. It is great to see the lights go on when guys who have been in the trade for many years hear and understand this material for the first time.
05-09-2005, 09:02 PM
Regarding your previous posted question:
“Applied Psychrometrics and Airflow is on of the most useful of the seminars and is fun to teach. It is great to see the lights go on when guys who have been in the trade for many years hear and understand this material for the first time.”
Well my friend, back in the winter of 1991, after already seventeen years in the field, those “lights going on” would have been from my brother and I.
Thank you again, and I’m glad you have found a place to teach future generations of service technicians about things they already “thought” they knew. Good luck in the future and I look forward to seeing you on more threads my friend.
John J. Dalton
05-09-2005, 09:07 PM
You know, that was a special weekend after all, that’s also the weekend I first meet, and had the pleasure of having a conversation over breakfast with “Shorty” of RSES’s educational committee. Good memories.
John J. Dalton
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