Packaged Unit Good Performance Range

[EnOm]

Hi

When it comes to packaged units with belt driven drives, what are there any recommendations for where to choose the design operating point? Some of the units I have have an airflow range from 2400-4100 CFM.
One of the unit's I'm looking at will satisfy the required cooling load at 4000 CFM. However, I'm thinking the point seems to close to the edge of the performance envelope since the highest volumetric flow rate listed for this unit is 4100 CFM @ 964 RPM/2.13 BHP

I'm considering selecting a larger unit that can meet the cooling load requirement at an operating point that is not so close to its limits. Am I thinking in the right direction? Or it's perfectly fine to go with the first unit?

Best Regards

 

[willard3]

What units are you asking about, air handling units?

If it's an air handling unit, fan type will effect your choice.

 

[chicopee]

Plus or minus 10% should be a good range. Yes, I would look for the next larger size unit. You should also consider future requirements.

 

[EnOm]

Willard3: I am asking about rooftop packaged air handling units, cooling only.

chicopee: Thank you.


Best Regards.

 

[KiwiMace]

The flow rate of any fan is only relevant when considered with the system pressure drop. The upper limit of airflow is most likely linked to the lowest pressure drop.

 

[willard3]

I'll ask again about the fan and its pressure/flow characteristics and system pressure drop.

 

[EnOm]

KiwiMace: Thank you for your response. Yes true. But in this particular case it is not. These packaged AC units are belt driven and can provide the required airflow at a wide range of system pressure drops. I'm not really worried about system pressure drop because the motor RPM will be adjusted to provide the required airflow rate at the outlets. The ducting is quite short, so it very unlikely that the system pressure drop will fall outside the fan's performance envelope.

Willard3: Thank you for the response. I don't fully understand what you mean, but maybe if you look at my reply to Kiwimace you will better understand what I am trying to discuss here.

Best Regards.

 

[DrRTU]

For plain small tonnage package RTUs, w/ belt dive used in a CV application, without looking at a fan table. I would use +5% and -20% as a rule of thumb. The +5 is very hard to do - you run short of static. The -20% helps with higher OA loads but suppressed SAT could be an issue under full cooling. Look for the “greyed” sections in the tables for high speed drives and oversized idf. Carrier will always be very deep into the service factor with odd name plate ratings.

 

[willard3]

Have you measured the system pressure drop? Does it fall within the non-overloading area of the fan curve?

Eyeball pressure drop calculations will bite you in the butt.

 

[EnOm]

DrRTU:
I don't have a lot of experience yet, but I'll look into that. Thank you.

willard3:
I have not calculated it exactly, but I'm quite sure there won't be a problem. The spaces are some (largest about 45ft x 45ft) large halls, each with its own packaged unit located on the hall's roof. Duct work should be relatively short.