Friday, March 06, 2015

Case Study: Variable Frequency Drive Installation For 150 HP Blower Motor


Introduction

  • Variable Frequency Drive is an electrical device that can reduce the power consumption  as a result of reduction of the motor speed. 
  • Centrifugal loads are not only for blower but it also includes pumps and other.
  • In this case study the application is for the blower fan motor that would supply hot air to the drying bin.
  • The problem comes when the product to be dried is less than the capacity of the the drying bin since the drier operator would evacuate excess hot air to compensate the required temperature.
  • The evacuation of hot air in the drying bin manifests waste of energy since the electrical energy requirement of the motor is constant at the given load.
Objectives
  • The objective of this study is to reduced the energy consumption of the 150 HP, 440 volts, 3 phase motor that drives the hot air blower fan in the drying bin.
  • Determine the payback period for the total installation cost of Variable Frequency Drive 


Energy Cost Computation  

KWH= (Pt) x (To)                                                                              <FMLA.1>
TOTAL COST= KWH x (COST/ kwh)                                              <FMLA.2>

                  Pt= total power consumed
                  To= operating hours

¨      @ 15 hp, 220 volts, 3 phase motor:

Pt= 150 hp * .746
   = 112 kW
           
¨      @ 24 hour daily cyclic duty:

KWHD= 112 kW * 24 H= 2688 KWH

Where: KWHD= daily KWH consumption

¨      @ cost of P 5.00 / KWH:

TOTAL COST= 2688 KWH * P 5.00/ kwh

TOTAL COST= P 13,440 (daily energy cost)
           
¨      @ 1 month:

TOTAL COST= P 13,440.00 * 26 (based on 26 operating days per month)

              TOTAL COST= P 349,440 (monthly energy cost)

Energy Computation For Centrifugal Loads

Pt= kN3  * HP

            Where: k= proportionality constant
                        N= motor speed
                        Pt= total power consumed

¨      @ 150 HP motor running at 100 % speed

Pt= k (100%)* 150 HP
Pt= 150k

¨      Energy computation w/ daily cyclic duty of 24 hours per day at 100% speed

KWH= (Pt) x (To)                                                    from <FMLA.1>

Therefore:

KWHD= [k (100%)3 * 150] * 24h

KWHD= 3600k

¨      @ P 5.00 / kwh

TOTAL COST= 3600k*5

TOTAL COSTFS= P 18,000.00 k

(total cost expression w/ other values held at proportional constant “k” for centrifugal loads running at full speed)
ENERGY COMPUTATION FOR CENTRIFUGAL LOADS W/ VARIABLE SPEED DRIVE.


POWER REQUIREMENT OF FANS AND BLOWERS ARE DIRECTLY PROPORTIONAL TO VOLUME FLOW RATE DESCRIBED IN THE FORMULA.


 P= QwH/ 33,000

Where: Q= volume flow rate of fluid in cubic feet per minute.
              W= density of fluid  
              H= total head in ft.
              P= power in horsepower

 POWER IN RELATION TO SPEED OF CENTRIFUGAL LOADS.

Pt= kN3  * HP
KWH= (Pt) x (To)                                                                            

            Where: k= proportionality constant
                        N= motor speed
                        Pt= total power consumed

THEREFORE POWER AND VOLUME FLOW RATE HAS A LINEAR RELATIONSHIP AND COULD BE RELATED DIRECTLY TO COST.

example: 150 hp motor with daily cyclic duty of 12 hours running at 100% speed, 8 hours at 80% speed and 6 hours at 60% speed.

KWH= (Pt) x (To)             

                Therefore:
KWHD=     [k(100%)3  * 150HP * 12h]
               [k(80%)3  * 150HP * 8h]
               [k(60%)3  * 150HP * 6h]
                                                              
KWHD= 1,800k + 614.4k + 194.4 k
KWHD= 2,608.8 k

¨      @ P 5.00/ kwh

TOTAL COST= 2,608.8 k * 5

TOTAL COST VS = 13, 044.00 k

(total cost expression w/ other values held at proportional constant “k” for centrifugal loads using variable speed drive and cyclic duty described in the above example)


Conclusion



 by ratio

TOTAL COSTVS     =     13,044.00 k
          TOTAL COSTFS           18,000.00 k

 relating total cost obtained w/ variable speed drive to the total cost obtained w/o variable speed drive:

TOTAL COSTVS= .724 * TOTAL COSTFS

Cost Analysis

from general energy cost computation as shown above a 150 hp motor running at full speed for 24 hour has a total monthly energy cost of:

TOTAL COST=  P 349,440.00

from the relation obtained by adopting the cyclic duty example describe above:

TOTAL COSTVS= .724* 349, 440.00   

TOTAL COSTVS= P 253,227.52 (total monthly cost w/ VFD)

 savings

SAVINGS= P 349, 440.00 - P 253, 227.52

SAVINGS= P 96, 212.48 (monthly savings)

return of investment

total investment (1 unit) = P 435,000.00

payback period= P 435,000.00     =    4.52 months
                                             P  96, 212.48                             

Reference:
  1. A case Study for Project Proposal of Using Variable Frequency Drive to Blower Motors at Pioneer Hi-Breed Seeds, by: Celestino Cole, 2007 
  2. Schneider Electric Manual, Altivar 61  

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