Question 10.2: Determine the reduction in the annual energy costs due to st......
Determine the reduction in the annual energy costs due to staggered duty
cycling of three identical fan motors (each rated at 30 \ kW 40 \ HP). The motor manufacturer specifies that a cycle has 20 \ min on and 10 \ min off as the minimum duty cycle. The utility monthly demand charge is $10/kW. First, determine the recommended NEMA duty period assuming 2-pole motors.
(a) Using Table 10.2, the allowable number of starts per hour for 40 \ HP, two-pole motors is 3.7 \ starts/h. Thus, each start should last as in the following:
Start period=\frac{60 \ min/h}{3.7 \ stars/h}=14.4 \ min/star
The duty period is thus about 15 \ min. Since the duty period is the sum of the off time and the on time and since the minimum off time is about 2 min (130 s based on Table 10.2), the maximum on time allowed by NEMA standard is 13 \ min.
Thus, the manufacturer’s on time is longer than that recommended by NEMA.
(b) The reduction in the electrical demand peak due to staggered duty cycling approach is 1/3 of the total demand of all the three motors (or 1/3 ∗ 90 kW = 30 \ kW). Indeed, at any given time, only two out of the three motors are operating. Thus, the annual savings in electrical demands charges are given as follows:
\Delta kW=12*30 \ kW=360 \ kW/yearTherefore, the staggered duty cycling control provides an annual energy cost savings of \$3600.
TABLE 10.2
Allowable Number of Starts Per Hour and Minimum Off Time for Motor Loads
| Six-Pole Motors | Four-Pole Motors | Two-Pole Motors | Motor Size HP (kW) |
|||
| Min. Off Time (s) | Max. Starts/h | Min. Off Time (s) | Max. Starts/h | Min. Off Time (s) | Max. Starts/h | |
| 35 | 26.1 | 39 | 23.0 | 77 | 11.5 | 2.0 (1.5) |
| 37 | 18.4 | 42 | 16.3 | 83 | 8.1 | 5.0 (3.75) |
| 39 | 15.8 | 44 | 13.9 | 88 | 7 | 7.5 |
| 44 | 12.1 | 50 | 10.7 | 100 | 5.4 | 15 |
| 48 | 10.9 | 55 | 9.6 | 110 | 4.8 | 20.0 (15.0) |
| 51 | 10.0 | 58 | 8.8 | 115 | 4.4 | 25.0 (18.75) |
| 53 | 9.3 | 60 | 8.2 | 120 | 4.1 | 30.0 (22.5) |
| 57 | 8.4 | 65 | 7.4 | 130 | 3.7 | 40.0 (30.0) |
| 64 | 7.7 | 72 | 6.8 | 145 | 3.4 | 50 |