Blower Motor Heat (net effect on cooling capacity)

[soleowner]

Hi,


ARGUMENT:

The argument is that a 500 watt PSC motor will create 1706 Btu's of heat thus increasing the return air temperature. Since RAT has a significant effect cooling capacity, the assumption is that you must include this heat given off by the blower motor into system load calculation. If true, then the blower motor heat is equal to 6,141,000 Btu/h.

QUESTION: What is the amount of temperature rise that the blower motor heat can give off?

We can assume a test condition of 1600 CFM's at 78 DBT, 63 WBT in a space of 2.7 cu. ft. (20" x20" blower cabinet) at sea level.

OPINION: Given the volume and flow rate, I do not think there is a detectable amount of heat gain to measure a delta T.

Let's try to put this question to bed. The result may change Manual J forever.

 

[CH5OH]

question:Why are you combining metric system (watt) with non metric system (btu,...)?
metric:know your units!
non-metric:know your ratio's!
metric+non-metric:create confusion!
500W motor, 0.85 efficiency=
425W shaft power,85W heat
85W heat=85J/s=306kJ/h
306kJ/h=290BTU/h

 

[AbbyNormal]

blow through fans heat the return air up

draw through fans reheat the air off of the coil.

without going through the math, I would say on a residential system you could have about 1/2 a degree of fan heat

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[soleowner]

Hi CH50H,

Thank you for posting a reply. Please note that the units I am using are customary for my industry and location. Manufacturers typically use these units in there performance tables and technical data.

If I am understanding your formula. The effective heat gain is only 85 Watts. With the volumetric flow, the heat gain would be relatively undetectable. Without a delta T, I cannot answer BTUH = CFM x Delta T

QUESTION: How can I replicate the calculation you did to determine 85 W?

I have to give it to my peers if I am going to write an opinion paper.

BTW: According to Engineeringtoolbox.com tables, the fan efficiency maybe a bit lower.

 

[MintJulep]

Assuming that the motor is in the airstream:

The motor efficiency is irrelevant. The input electrical power is dissipated as heat.

We'll further assume that the motor and fan guys did a half way decent job, and the motor isn't running at its full rated capacity of 500 W, so let's guess that it's drawing 425 W.
425 W converts to about 1450 BTU/hr.

Q (BTU/hr) = 1.1 x cfm x Delta T

Your Delta T will be 0.8 degrees F.

 

[CH5OH]

total electric power=motor shaft power / motor efficiency
motor shaft power=fan power/fan efficiency
motor losses and fan losses are introduced in the air stream as heat.
0.85 motor efficiency I took as a realistic estimate
500W=1706BTU/h
most of the power is used to move the air (and do not produce heat)
the introduced heat is defined by the relevant efficiencies
efficiency of the motor you get from motor characteristics, fan efficiency, you get from the fan characteristics (or you can calculate, if you know delta P produced by the fan and its flow.the increase of temperature you can calculate by the flow and the specific heat of air.Am i talking to students?

 

[soleowner]

Yes, I am a student in the untraditional sense. I work for a mechanical contracting firm who specializes in VRF heat pump technologies and building sciences.

I am working to clear up a prevalent theory within the HVACR industry that is propagated by ACCA (Air Conditioning Contractors of America) building heat load/loss Manual's J and N which is derived from ASHRAE's (American Society of Heating, Refrigeration, and Air Conditioning Engineers) engineering fundamentals handbook.

The theory is that a blower motors radiant heat effects the cooling output enough for it to be added as a loss when designing a cooling system thus needing to be compensated. The theory is based on the notion that there is a direct relationship between the blower motors energy consumption and the heat loss.

Having extensive experience with these systems, my gut reaction was that the delta T is not field measurable. I need to prove whether or not there is
a measurable delta T. Since comfort cooling is not critical, field level measuring tools should be the basis of the challenge to this theory. Laboratory grade measuring devices should not count as valid or practical
foundation for adding such a heat gain.

I really appreciate your clarification of how you deduced the motor's net energy loss.

 

[MintJulep]

most of the power is used to move the air (and do not produce heat)

Sorry, wrong.

 

[Waramanga]

If we assume that the blower motor is inside the conditioned space (in the airstream) then every watt of energy put into that motor ends up as heat which means that the AC system will have to deal with it, full stop. If the motor is external to the system then the motor losses (20% for example) will not end up in the airstream/conditioned space but the other 80% does and again it has to be dealt with. The question of discernable temperature rise is irrelevant.

 

[AbbyNormal]

My load program uses the following to estimate fan heat

Fan Heat (deg F) = 0.363 x (total fan static "WC)/ Eff

The efficiency is the motor efficiency times the fan efficiency

So for a residential unit, perhaps the fan sees an inch of total static and assuming the product of the fan efficiency and the motor are 0.6 then fan heat could be

0.363 x 1 / 0.6 = 0.6F

When the motor is not in the air stream, then you just use the fan efficiency



The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[AbbyNormal]

On commercial equipment, 2 degrees of reheat from the fan is not uncommon on a draw through application.I have measured over three degrees ocurring also.

So when a coil is cooling air by 20 degrees, 2 to 3 degrees of fan reheat robs you of 10% to 15% of your sensible cooling on a draw through application.





The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[CH5OH]

Warramanga,
you're right,when all you're air molecules have come to a stop,all there energy is converted to heat, however the op was after a delta T (I assume this delta t being measured over the fan) so heat going into the air stream=motor heat+friction heat fan blades.

 

[imok2]

I get Full Load Electrical Motor Efficiency of 66% and with Motor and Fan in the Inlet Air Flow or in the Room at 420 watts or 420W/1760 0.81degrees