Understanding BHP and HP

[joeisme360]

I am in the process of reviewing a shop drawing for an AHU. I have received the attached cut sheet from the manufacture.



When I look at this cut sheet it seems like the motor size they have specified is too small. I took the BHP (4.89) and divided it by the motor efficiency (87.5%) to get a HP (5.6). So to mee a 7.5 HP motor should have been specified NOT a 5 HP motor.

Am I missing something?

 

[tys90]

That looks like a York output, which means it was probably selected using software. I believe the software accounts for the service factors of motors, so technically, this motor can handle a BHP of 5.24 with belt loss without damage. I would still change it to a 7.5 HP motor and review with the electrical engineer.

What did the drawings or specs specify for motor size?

 

[Compositepro]

The motor puts out 5 hp. Due to inefficiency it consumes more than that in electricity.

 

[DrRTU]

I see the fan system as 5.24 bhp on a 5.0 hp motor. You are almost 5% in to the service factor. This leaves no room for fan adjustment if you are low on flow. What is the application and sf of the motor? Carrier Corp. runs fans well into the sf based upon testing and adequate cooling (draw thru).

 

[willard3]

Mark the submittal with the Hp you want for the fan, you are the Engineer, not the supplier. Do not accept bogus marketing arguments.

 

[joeisme360]

tys90:

You are correct this is an output for a York unit and it was selected using software. The supplier told me that there is a service factor on the motor which allows you to run above 5 HP. Would there be any issues of burning out the motor or tripping if you are continuously operating above 5 HP (like at 5.6 HP).

This is the unit that we had used to create the specs/dwgs. Unfortunetly the motor size was not looked at carefully at the time.

 

[joeisme360]

Compositepro:

That was one theory that I had; that the motor HP listed in the shop drawing (5 HP) was the acutal motor output. The inefficiencies would only change the full load amps. ie. a more efficient motor would lower the full load amps and a less efficient motor would raise the full load amps. But both motor regardless of efficiency would output 5 HP thus satisfying the 4.89 HP requirements.

 

[AbbyNormal]

I agree with Dr RTU, they want to give you a motor already in its service factor.

I would have a 7.5 Hp motor in that application.

We pay a premium price per kWh here, the additional cost of going something above a 'standard efficiency industrial motor' like York is offering to a Premium Efficiency motor like a Baldor Super E, pays for itself fast under a year but like I said, we pay well over 30 cents here a kWh.

May not be the case in your area perhaps electricity is cheap.

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

 

[tys90]

A typical motor service factor is 1.15. Say that's what this motor is rated at, that gives it the ability to run up to 5.75 HP. Now say, for some reason in the future they want some extra airflow from this unit. You won't be able to from a motor standpoint because it's already almost maxed out. I would still put in a 7.5 HP motor.

Now you didn't ask this question, but you stated it was a FC fan but make sure you are in a satisfactory area of the curve if it is indeed a FC fan.

 

[Compositepro]

Is a 7.5 hp motor more efficient than a 5 hp motor when both run at 5.1 hp?

 

[CountOlaf]

Like most of the other responders, I agree that 7.5 hp motor can't hurt and will give some future flexibility, but as Tys90 said in his original response, check with the electrical engineer (if there is one) on the job. The increase in supply fan motor could affect wire size, fuse/breaker size, starters, conduit, everything electrical. On these packaged air conditioners, sometimes you're right on the cusp of having to go to quite an expensive "next size up" on an electrical item; or a power panel they had in mind now won't have an adequate position available. If this happens, it's probably better to stick with the 5 hp motor.

 

[willard3]

Operating a motor in the service factor is not a good idea. Wiring/overcurrent can operate at 90C, but that's too hot and heat destroys efficiency.

The service factor is intended for transient events, not constant operation.

 

[CountOlaf]

Let's not forget that the bhp is based on an assumed system static pressure and desired airflow rate...we all know how accurate those calculations are .

 

[ChrisConley]

Actual calculations can be pretty good for assumed system static pressure... assuming the contractor builds the duct the same as what is shown on the plans (hopefully you didn't forget to show all the transitions under beams). I find duct leakage to be playing a larger role than I'd like on projects.

 

[AbbyNormal]

I would say that SMACNA needs to re-write their entire duct leak testing manual, to at least put things in the proper tone or perspective.


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