Sheave re-size verification

[neonhomer]

I just want to run this by someone.... The new sheave I have to get is going to be around $450 after everything is said and done...

Unit is a Trane M series MCCB040UA CHW air handler... Constant Volume. Spec'd for 17,000 CFM (approx). Unit is actually putting out 22,000 CFM. I figure it to be 135% over spec. (Airflow was measured by duct traverse using a 48" AirFoil and ADM870c Airdata Multimeter.)

Motor sheave is a 5.30", fan sheave is a 14". Since a 4" motor sheave is not available in the configuration I need, I requested a 19" fan sheave. (Sheaves are 4 groove, 3VX type, with QD bushings.) Motor speed is 1760, 25HP.

I figure the new sheave size by taking the size and multiplying by 1.35 to get the 19" sheave (comes out to 18.9). I had figured the original motor sheave by taking 1.35 1/x which gives me .74.... .74 * 5.3 = 3.9


So, am I right, or did I just make a $450 mistake? Have to do this sheave change because the unit is over air, and the returns are rattling from the airflow. (This unit serves a gym and a couple of small rooms....)

[airdata870]

Fan law #1 CFM old/CFM new x PD old = PD new
22000/17000=1.294 x 14= 18.11"
hope you remembered a new belt good luck

[codgy]

put a vfd on motor and forget it. Original design was for a specific static presure and with minimal ducting you probably are under that pressure. It would be difficult to know what you are operating on and you could vary speed w/ cooling stages.

[flyrfan]

I came up w/ 18.11" sheave as well. Fan should be spinning at 515 RPM.

Sheave change is the way I would go too.

[neonhomer]

Well, since the job is still under construction, the mechanical contractor is taking care of sheave changes. I just had to recommend the new size.

There is sufficient slide in the motor to keep the same belts.

VFD is out of the question. I suggested that right off, and so did the guy from Trane. The mechanical guy said no.... they would rather pay $500 for the sheave than a couple of grand for a VFD.

[flange]

If you are going to do this type of work, and stick your neck out, copy down the above formula and know where to find it. you will amaze people with your knowledge and skills.

[neonhomer]

Going to write that on the back of my clipboard....

[neonhomer]

Going to write that on the back of my clipboard....

[flyrfan]

If you have an adjustable pitch pully, always put a tach to the fan shaft to verify RPM, that's what your going for. Here the fan law:

Q1/Q2*R2=R1

where Q1 = New CFM, Q2=old CFM; R1=new RPM; R2=old RPM