Calculate RTU kW?

[freeBODYdiagram]

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.

 

[MechEngNCPE]

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.

 

[freeBODYdiagram]

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.

 

[MechEngNCPE]

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.

 

[chicopee]

You forget your power factor in your very first equation.

 

[freeBODYdiagram]

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