×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Calculate RTU kW?

Calculate RTU kW?

Calculate RTU kW?

(OP)
Hi folks,

I was hoping someone could help me a little. Namely, what considerations should be made when calculating the kW total of a rooftop unit? I have been using the following equation: I*V*1.73/1000=kW

I have been using that formula for the compressor(s) and each fan. If a fan HP is known, I use the following instead: HP*0.746=kW

Is this fair? Are RTU compressors typically cycled, or run simultaneously? Any information would be helpful. Thank you kindly.

 

RE: Calculate RTU kW?

RTU compressors vary, you may have one that cycles, two that stage, or even four to eight that stage depending on capacity of course.  If you know the make and model#/serial# you can contact the vendor and get a kW for the unit.  I have tended to assume around 1kW per ton for an average RTU when I'm estimating kW for a quick ballpark figure.  I would first consult with the manufacturer.

RE: Calculate RTU kW?

(OP)
MechEngNCPE,

Thanks for the response. I have attached an example nameplate.

From the data on the nameplate--and the equations I listed above--I have calculated a total of 21.0 kW for the two compressors, two outdoor fans, the supply fan, and the combustion air blower. It is a 15-ton unit. The nameplate shows an output capactiy between 40-55 kW.

Would you say my calculations are incorrect? Again, any and all help is greatly appreciated.

RE: Calculate RTU kW?

The full load amps (FLA) is 31, now 31*460V = 14,260 W = 14.26 kW, so around 1kW/ton as I mentioned above, more like 0.95kW per ton.  The output capacity is for your natural gas burner and refers to the BTU/hr of heat provided by the combustion of the input of natural gas.  Hope this helps.

RE: Calculate RTU kW?

You forget your power factor in your very first equation.  

RE: Calculate RTU kW?

(OP)
Chicopee, thanks for your input.

Not to sound blunt, but could the PF be calculated from the information given in my previous attachment? As far as I know, PF = real power / apparent power

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources