heat pick-up from the fan

[henry_4]

Is anybody know how to calculate the air heat pick-up resulting from the fan (not the engine)?

[john dalton]

Dear Henry_4,

In order to do so, we will need the following information:

Motor FLA or RLA:

Motor voltage and phase:

Motor efficientency rating:

From the above information we will be able to give you the maximum heat load output of this motor, if you would like to know the actual heat load output of this motor as it is running now, we will need the following additional information:

Motor total running watts or amp draw from each leg and actual voltage from all legs:

Respectfully Submitted,
John J. Dalton

[willf650]

Don't forget you also have to take in account the friction of the fan itself. I will add I have no clue how to calculate that but in my expiriences you generally get around 3 deg and have seen as high as 7deg. Funning thing is I remember the unit I saw 7 degrees on had the motor located outside the unit.

[LGM]

If you will go to any manufacturers air handler catalog they will have a table showing heat gains for motors connected inside the air stream or outside the air stream.

[john dalton]

Dear LGM,

Not all equipment manufactures have that information in their engineering literature, and custom built-up or specialty systems almost never have this information available. Although the information is easily obtainable with the above information.

Respectfully Submitted,
John J. Dalton

[john dalton]

Dear Willf650,

If you are referring to the heat generated by the blower wheel bearings, I have to say, that if the bearings were selected and installed correctly, this heat is negligible and is never quantified unless the blower wheel is extremely large in capacity(namely 1,000 HP or above). Under this capacity, normally operating bearings generate an extremely small percentage of heat compared to the in-stream motor itself.

Respectfully Submitted,
John J. Dalton

[service guy]

What type of Fan? Is it a dust collector?

[willf650]

I'm actually talking about the heat produced from the friction of the fan in the air, nothing to do with beariings.

[john dalton]

Dear Willf650,

Alright I’ll bite, what is the Manufacturer, Model number, size, HP, type, RPM, application, and drive configuration of this system?

Respectfully Submitted,
John J. Dalton

[john dalton]

Dear Willf650,

Regarding your previous post:

“Don't forget you also have to take in account the friction of the fan itself. I will add I have no clue how to calculate that but in my expiriences you generally get around 3 deg and have seen as high as 7deg. Funning thing is I remember the unit I saw 7 degrees on had the motor located outside the unit.”

Are you stating that an air handler with no heating or cooling being called for can produce a 3 to 7 degree temperature rise by the “friction” of the fan blade or wheel rotating in the air itself?

Unless the blower wheel or blade is turning incredibly fast (ie >15,000 RPM), this is highly improvable friend.

I worked on a seventy ton cooling system a couple of years ago that had a 56,000 CFM blower transferring all the air through a 4 inch round utility hose to provide cooling and pressurization on the inside of the new C17 transport aircraft in Long Beach, CA and the friction of the blower wheel still had negligible effect on the temperature rise across the blower and system itself.

But please, give us the information on these air handlers you worked on that would produce such a large sensible temperature increase.

Respectfully Submitted,
John J. Dalton

[marauderx]

Originally posted by john dalton

Unless the blower wheel or blade is turning incredibly fast (ie >15,000 RPM), this is highly improvable friend.


I think you meant to say "highly improbable." Either way, I agree.

The fan blades are going to add negligible heat to the air when compared to the motor that turns them. With the motor in most air handlers an estimate of 2 degrees F rise is common, and 3 degrees F is conservative. This was mainly due to the motor; if the motor is outside of the airstream the temperature rise is virtually 0. It would be a rare manufacturer that produced a 1 degree F rise over the blades, no matter what the CFM.

[john dalton]

Dear Marauderx,

“Improbable” or “implausible” would have been the word(s), it seems my head was working ahead of my fingers, not a very safe thing to do either in the office, nor in the field. Thank you friend.

But regarding your previous post:

“…With the motor in most air handlers an estimate of 2 degrees F rise is common, and 3 degrees F is conservative….”

A typical 5 ton air handler producing 2,000 CFM with a 1.0 HP 208/230 V – 3 phase 87.5% efficient motor pulling full load amps of 3.1 will only produce an approximate 0.219 degree temperature rise over the motor.

Even if the above same 5 ton air handler had an optional 3.0 HP 208/230 V – 3 phase 87.5% efficient motor pulling full load amps of 9.3, it would only produce a temperature rise of approximately 0.657 degrees over the motor.

Therefore, I’m not following your estimation of a 2 to 3 degree rise for MOST air handlers.

I do acquiesce with your statement that the heat produced from the blower wheel or fan blade rotating through the air would produce effectively no heat under almost all circumstances.

Respectfully Submitted,
John J. Dalton

PS: “Acquiesce”…must have seen Pirates of the Caribbean one too many times…lol

[Edited by john dalton on 06-14-2005 at 03:41 PM]

[willf650]

John I don't have and model # of units I've worked on but let me give you a little of back ground as to why I say what I say. 90% of my work now a days is doing controls and I generally have at least a mixed air sensor and a supply air sensor on the units I work on. Many times I have preheat and cooling coil discharge sensors as well. My sensors are supposed to be 1/10 deg acuracy but I would say 1/2 deg is more probable. I calibrate all my sensors to read the same with a base temperature reading as a Visala thermometer I have before I start a unit. As I start the unit I notice a 1 to 3 degree rise 95% of the time after the sensors settle out. This is with no cooling or heating happening. Granted this is an unscientific approach but it happens every time with out exception. Today for instance I was working in an air house which is a common comon plenum return for 3 units in it. I imaginge I have about 150000 or better cfm running thru this room and am reading 77 + or - 3/4 a degree on the three mixed air sensors on the units. I had the one multi zone off while we were upgrading controls on it and after I start it I'm reading 79 + or - a 1/4 degree on both the hot deck and cold deck sensors still 77 on the mixed air. This was the only unit I didn't have in operation so it was the only one with temp rise readings unaffected by the the DX units running. This particular unit has the motor and belt drive outside the units so motor heat was not an issue. If you really want the M# to prove me wrong I will get it when I go back Friday.

[john dalton]

Dear Willf650,

Regarding your previous post:

“…If you really want the M# to prove me wrong I will get it when I go back Friday.”

I do not want the Model number to prove you wrong; I would like the drive motor information on a typical in-stream motor to show you what I am speaking of though. This forum is not about “proving someone right or wrong”, but merely about the sharing of our experiences, knowledge, preferences, options, and basic refrigeration and physics facts.

The heat generated by an in-stream motor can be calculated very precisely based upon a combination of the motor manufacture’s information, and the operating data of the system itself. That is not an option, a guess, or even a preference; it’s a cold hard engineering fact.

Respectfully Submitted,
John J. Dalton

[john dalton]

Dear Willf650,

In regards to your experiences in the measured temperature rise across the in-stream motor systems, I would not contest that experience. But I would ask you, if possibly another phenomenon, such as inaccurate temperature sensor comparisons between one another, ambient temperature influences between the interior and exterior of the air stream itself, or any combination of these or other factors could possibility account for your experienced temperature rise across the air handler, and could that be a more plausible explanation to what you are experiencing.

Respectfully Submitted,
John J. Dalton

[ps]

Just to expound on what John has mentioned, other factors that could be contributing to the differences in temperature readings:

1)Type of sensors (ie - MAT = averaging, h/c decks = duct)

2)Location of sensors (ie - MAT = improper mixing box location, h/c decks = flow and temperature stratification)

3)Controller input circuitry

Where are you actually reading these temperatures from(BAS, controller, resistance (ohm) readings)?

[john dalton]

Dear Ps,

I concur completely with your expanded explanation of my previous post in terms of addition items that could have very easily affected the readings that Willf650 was experiencing at the jobsite.

We all have to stop sometimes and give some thought to the measured responses, and their possible meanings, that our test instruments are giving us in this day and age of digital displayed high resolution test equipment.

It probably isn’t applicable, but that remind me of the difference of what a salesman and an engineer knows:

A salesman knows nothing about everything, and an engineer knows everything about nothing.

As an engineer, I can identify with this statement.

Respectfully Submitted,
John J. Dalton

[chill factor]

2545 x bhp= btuh
TD= btu/1.08 x cfm

[john dalton]

Dear Chill factor,

That’s a good “generic” equation, but unfortunately it doesn’t take the different manufacture’s individual motor wattages or motor efficiencies into account.

Respectfully Submitted,
John J. Dalton

[chill factor]

Tell us John,
How would you figure it out and package your formulas to get the answer?

[john dalton]

Dear Chill Factor,

The formula:


Delta T = (W) (100% - E) (3.4129) / 1.08 x CFM


W = Rated wattage of the motor / actual running wattage of the motor*

E = Rated efficientency of the motor

*Toggling between the full rated wattage of the motor and the actual operating wattage of the motor will furnish both the maximum, and the actual delta T of the motor.

Friend, Did I sense a bit of sarcasm in you question, or am I reading too much into your posted question?

Respectfully Submitted,
John J. Dalton

[chill factor]

Johnny Boy,
I know all that we can see are words. We dont have the luxury to hear how it is being said. Be rest assured that I was not being rude to you, I just wanted to post short and sweet.

[RichardL]

Not to muddy the water an more, but I have several squirrel cage fans we use when in a very hot and confined area..
They are extremely high speed blowers on a double shaft motor that we Lovingly refer to as "The Piranah"....
Should one make the mistake of picking it up from the ends while running you would understand why the name...

Anyhow...With a 100 deg/f ambient, the measured air temp at the fan inlet is around 85 deg..at the fan discharge it runs around 115 deg/f.

The fans were once used on a very compact A/C unit for a specific computer...Lots of power in a small package.

[john dalton]

Dear Chill factor,

Short, sweet, and to the point, I can identify with that one friend. I guess its time to get the windex and towel out to clean my monitor, sorry for the clarification needed.

Respectfully Submitted,
John J. Dalton

[john dalton]

Dear RichardL,

Regarding your previous post:

“…Anyhow...With a 100 deg/f ambient, the measured air temp at the fan inlet is around 85 deg..at the fan discharge it runs around 115 deg/f.

The fans were once used on a very compact A/C unit for a specific computer...Lots of power in a small package….”

Are you stating that the delta T of 30 degrees F is the temperature rise of this “blower only”, and that this temperature rise is from the blower motor only?

Respectfully Submitted,
John J. Dalton

[john dalton]

Dear RichardL,

If you answered yes to the above two questions, would you be so kind as to post the wattage (or amps, volts, and phase), the manufacture’s rated efficientency, and the total CFM of this killer blower?

Respectfully Submitted,
John J. Dalton